Are there cases where a double horizontal stabilizer is useful?

The main aerodynamic purpose of the horizontal stab (or certain canards) is to provide longitudinal stability.

If the rear wing with the 5th and 6th engine flies "up," like the main wing, then it will counteract the longitudinal stability of the horizontal stabilizer. If the rear wing flies down, like the h-stab, then it is just extraneous, since the h-stab can be made as large as necessary.

If all you want to do is to add extra engines, they can be mounted on the main wing, like in the eight-engine B-52 bomber:

or the six-engine Antonov An-225 cargo jet:


Why is the delta wing naturally stable in pitch?

This is a very good question, as it shows some of the misunderstandings regarding "tailless" aircraft.  With deltas, one can consider them as blended wing/tails.

The function of a horizontal tail surface is pitch control.  The back of a delta is certainly large enough to do this. So much so, that now one can design
a forward canard to go with it.  This plane should pass the horizontal drop test (nose must go down).

Notice that birds, wanting more straight wing lift,
"unblend" a little and also have a fan tail.  Delta's do not generate as much lift as a comparable straight wing.

So designers can work with what they need, particularly with desired speed range.  Faster, the wings can pull in a little, as with the FB-111 swing wing.


Is the angle of attack equal to zero when the plane is on the ground with wings level?

You're right to be confused. The AoA can be almost anything, depending on wind direction. Normally, a wing has some camber so the lift it creates in flight is possible with an AoA of 0°. Normally, the wing is mounted at an incidence of a few degrees so the fuselage remains horizontal in flight. With this incidence and horizontal wind, the AoA at rest is just this incidence angle. But if the aircraft experiences tailwind, the AoA can be near 180°. If heated air below the wing rises up as a thermal, the AoA can be momentarily around 90°. In almost all cases, flow speed is much lower than that required for flight, but there are exceptions.


Why are bird strikes on approach or landing not as common as on takeoff?

I've heard about lots of bird strike incidents that occured during a takeoff roll or the early phase of takeoff climb but hardly heard of bird strikes on approach or landing.

This is always a dangerous thing. Only because you hear more bird strikes that occur on takeoff, it does not mean there are more at takeoff.
I found, for example, this report about bird strikes from the year 2000. It says:

[...] 38% of the bird strikes occurred during take-off
and climb, and 56% during approach [...]

That means there are more bird strikes during approach than on takeoff. I assume things haven't changed a lot since then.
This question is even answered in the Wildlife FAQ Section of the FAA:

Q: Do most bird strikes occur while in flight, at takeoff, or landing?
A: About 60% of bird strikes with civil aircraft occur during landing phases of flight (descent, approach and landing roll); 37% occur during take-off run and climb; and the remainder occur during the en-route phase.

A more specific statistic:

Source

Possible reason for subjective perception of bird-strikes happening more often during departure
I thought a little about it and came to following possible reason: After a bird-strike on final, the pilots will continue landing. It would be unwise not to land immediately. For the passengers it will look like a normal landing. If the bird damages the engine, there is only a small change of sound, because the engines are running very low.
However, when a bird-strike occurs while taking off, there is immediate action taken by the pilots. And because the engines are at or near full thrust, the sound of a dying engine can be perceived by the passengers. Last but not least, a departing aircraft has a much higher speed than a landing one, which means more damage.
So, for newspapers, a bird-strike at take-off is more dramatic and drama sells.

More damage to the plane than a bird-strike on final
Passengers not reaching their destination
Dramatic passenger interviews about their "near-death-experience"

A newspaper is not interested in a bird-strike on final, where everything went fine and no passenger noticed anything.
To support my theorem, I googled "bird strike" and clicked on the "News"-tab. I looked through every article that had to do something with an bird-strike incident. 5 out of 6 articles were about a bird-strike on take-off. That's over 80%.


Who assigns an arriving aircraft a runway and runway exit to use?

The tower gives the ultimate clearance to land, though the approach controller (if one is available) assigns a specific approach to a runway in the approach clearance. I'm not sure about the game, however in real life if the runway was too short to land on, the pilot would report being "unable" to use that runway and would be assigned another runway instead. It is not the responsibility of ATC to ensure that the runway is long enough to land on.

Unless there is a NOTAM otherwise noting that a particular exit is unavailable for use, the pilot can slow down at whatever rate to vacate at any particular exit. However, once they've slowed down to taxi speed they should vacate as soon as practicable. The tower controller may request that a pilot vacates at a specific exit, but if they are unable to they will go for another exit.
After vacating the runway, the pilot notifies the tower that they are clear of the runway at a particular taxiway, and then will be (depending on the airport) be asked to switch to ground or be given taxi instructions.


Landowner's permission is always required.  Then check the municipal zoning regulations and the like - city, state regs etc.  Then your insurance company.  If all these points come up green, go do it.


How do turbofan internals handle the load from the thrust?

The thrust is generated at the fan and compressor blades, as these are the components which push the air back. It is then carried through their attachment disks to the shafts and from there to what are called thrust bearings. A simple thrust bearing is a fixed flange against which the shaft pushes, but of course they are usually a lot more sophisticated. Thrust can also be generated by careful shaping of the core exhaust duct, but on a subsonic turbofan it can at best be only a small amount. The force is then transferred through the engine's structural framework to its mountings and thence to the airframe itself.

Yes, everything in the engine will deform a bit under load. Luckily the load paths tend to be short, but it still has to be made stiff and strong, with as you say the mounting carrying the full thrust. Extensive use of high-strength materials is made in the engine to keep deformation down to manageable levels. Varying amounts of thermal expansion of different parts also take place. These combined movements necessitate the designing-in of mechanical tolerances, to prevent the binding of moving parts or local build-up of mechanical stresses.


Could someone really die in the way that Goose did in the movie "Top Gun"?

Some sources I found claiming the event was real:

  A flat spin creates a low-pressure area and stalls the canopy when
  ejected. Producers wanted a midair crash but based this accident on a
  real-life incident instead.

(source)

  The way Goose died was actually based on a real life incident that
  happened to a 14 crew in the Navy. Don't remember the specifics, just
  found this out recently while reading an article in one of last months
  flying mags (not Flying)

(source)

  I have a photo of the 'real' Goose and it's true that he died of seat
  ejections but not exactly like the movie.

(source, with pictures)

There's also an interview of Steven C. Schallhorn, who apparently was one of top gun instructors interviewed for the movie:

  Dr. Schallhorn said during his interview for the movie, he explained
  how a flat spin could actually happen with an F-14 and that it
  complicates ejection. "The aerodynamics of the F-14 flat spin affect
  the timing of the ejection sequence," Dr. Schallhorn said. "The canopy
  is jettisoned, followed by the ejection of the back seat, followed by
  the front seat. In a flat spin, the canopy, when it ejects, bobbles
  for an extra few hundredths of a second above the aircraft. That
  upsets the carefully engineered sequence because the guy in the back
  could then hit the canopy. That sequence made it into the movie."

(source)


Difference between FMCF and AIMS on B777?

The B777 uses an Integrated Modular Avionics (IMA) architecture. AIMS is the modular computing platform on the B777. It's basically a bunch of computers in a box with it's own back plane network and operating system. It replaces multiple unique stand alone computing boxes.

FMCF is the software that performs the Flight Management function. It is loaded onto the AIMS platform. In older aircraft architectures (B737, B757, B767) it was contained within a dedicated stand alone Flight Management Computer. In the B787, AIMS was replaced with the CCS (Common Computing System). It also has an FMCF software function.


Which altimeter setting is used to define airspace boundaries?

Airspace boundaries are not defined by altimeter settings or Indicated Altitude. Like the field elevation, they are at True Altitude. This is defined as the actual height above the 19-year average level of the sea surface. Some boundaries are defined by the height above the surface (field elevation) which is in turn at True Altitude. The MSL vertical dimensions of the boundaries do not change based on barometric pressure or altimeter settings. They are relatively constant.
Your Indicated Altitude shown on your Altimeter may be different from the True Altitude due to barometric pressure changes, erroneous settings, or atmospheric anomalies. This does not change the boundary dimensions.
For further clarity, look at how the FAA Pilot’s Handbook of Aeronautical Knowledge defines these terms:

Types of Altitude
Altitude in itself is a relevant term only when it is specifically stated to which type of altitude a pilot is referring. Normally when the term “altitude” is used, it is referring to altitude above sea level since this is the altitude which is used to depict obstacles and airspace, as well as to separate air traffic.
Altitude is vertical distance above some point or level used as a reference. There are as many kinds of altitude as there are reference levels from which altitude is measured, and each may be used for specific reasons. Pilots are mainly concerned with five types of altitudes:

Indicated altitude—read directly from the altimeter (uncorrected) when it is set to the current altimeter setting.
True altitude—the vertical distance of the aircraft above sea level—the actual altitude. It is often expressed as feet above mean sea level (MSL). Airport, terrain, and obstacle elevations on aeronautical charts are true altitudes.

Mean sea level.
The average height of the surface of the sea at a particular location for all stages of the tide over a 19-year period.

If you are on flight following, or an IFR flight plan, it is up to the controller to clear you through an airspace or hand you off to the controller with jurisdiction. If you are not on flight following, or an IFR flight plan, don’t fly so close to the boundary without giving the local tower a courtesy call to “request” transiting their airspace at your desired altitude. It is not a bad idea to do so even if you are well above their airspace. They may know about conflicting traffic of which you are not aware.


Can a wooden/fabric cover design be built of metal?

Ed Swearingen used to say that it made little difference if you build an airplane from wood, aluminum or carbon epoxy - in the end the weight would not be too different if the parts are designed well.

Note that many aircraft in the 1930s used the combination of an aluminum frame and fabric covering. Especially the control surfaces of many aircraft, but also the fuselage, wing and tail surfaces of the Vickers Wellington were built this way. Also, many gliders before the advent of fiber-composite structures used a welded steel-tube frame covered with fabric for their fuselage. Since glueing works better with wood, the fabric cover needs to be sewed to the ribs and stringers, so suction forces will not lift it up over an extensive area.

Key is the limited dynamic pressure: If you do not increase operating speeds, exchanging wood for aluminum will be straightforward. Only the torsion box of the wing would need closer attention, since the low weight per area of plywood cannot be recreated with aluminum sheet - the wall thickness and the resulting buckling strength would be too low. You would be well advised to add more stiffeners and ribs!


Is it a flight violation to mishear ATC instructions for exiting the runway?

I don't think you violated any regulations. It was an ATC instruction, not an ATC clearance.

It was an instruction you were unable to comply with. The instruction was badly timed and was obviously not clear since you did not correctly understand it.  In the busy time during the landing roll, you understood something different, and read back what you understood it to be.

The landing roll-out can be a busy time needing the attention of both pilots.  ATC issuing an instruction while you are basically still in the landing roll should not be expected to be complied with unless it is happens to be convenient for you to do so.

Many airlines SOP say that you do nothing until you are clear of the runway. Both pilots remain heads up and don't touch anything. The pilot in command alone will decide where it is safe to tun off the runway. If ATC makes a "Request", and it can be safely complied with, then the PIC may decide to deviate from SOP and comply.

If you ran off the side of the runway trying to comply with an ATC "request", who do you think will be held responsible? (I'll give you a hint, it won't be ATC)

This is a prime example of why ATC should not be doing this.


Those are the highest obstacle and highest terrain point along the route, respectively. They're both grey because they're well below your planned altitude, so they're no factor.

You can see them by going to "Flight Plan", "Details", "Highest Point".


Materials usually used in dorsals fins specifically in the ATR 72

The loads on section of a dorsal fin are minor, just from the dynamic pressure of airflow that may be striking the panel from the side during yaw excursions in turbulence.  It's a "lifting" surface, insofar as it's contributing to the restorative yaw moment being part of a stabilizing fin, but the loads aren't super high and it may even just be attached like a fairing using dzus fasteners or camlocks or  similar if there is something inside that needs ready access.
Can't speak for the ATR specifically, but the material is probably either 2024 Alclad sheet like the adjacent structure, or if the part has compound curves, you might see a more formable alloy like 6061 or even 5052 in a non-structural part like that (you will see 6-series alloys in heated leading edges for example).  On newer designs it's more likely to be composite, usually Kevlar or a fibreglass/Kevlar blend (fibreglass is better in compression being stiffer, so it is sometimes mixed in with the Kevlar) and most recently carbon fiber.
If I was tasked with redesigning the part to a new material, it's likely because it's aluminum and the goal is weight saving, and I would likely choose carbon fiber as the replacement.  The optimum would be carbon fiber skins sandwiched over a nomex honeycomb core, the way interior floor panels are made.
An important caveat about using CF, is you cannot let it live right beside aluminum where there is a potential for direct contact. They are well apart on the galvanic scale and if there is water present the carbon will destroy the aluminum. If that's a problem it might be better to go with Kevlar composite.


why does the flow need to decelerate (by a thermal throat) in a scramjet?

I found an answer, for those who are curious. The thermal throat described in a scramjet is not an ideal case. Essentially, when the flow is supersonic, heat addition done in the combustion chamber can slow the flow down to subsonic speeds (based off the 1-d heat velocity, Rayleigh Flow). When this happens the flow is considered thermally choked, and is a bad case for this engine. To prevent this thermal choke from happening, the area of the combustion chamber increases with the direction of the flow (area increase increases velocity for supersonic flow).


How safe are commercial airliners from gallium infiltration?

Gallium does not interact strongly with steel so it is unlikely to damage hydraulic pipes (these are made from steel, not aluminium). Same source says it also does not interact much with polymers so unlikely to damage the insulated wiring or rubber hoses or epoxy floor panels. It may damage some sensitive point where important device is attached to the aluminium frame, but hopefully most of such places are not single points of failure.
All it can do is to make a hole in the fuselage under the very rare circumstances. Then the aircraft will be depressurized, descend and land. It is probably still not a good idea to allow gallium in the passenger compartment.


Could the Olympus 593 have been windmill-restarted during supersonic flight?

Yes they could be re-lit without cross-bleed (all engine flame-out above M1.2).

Regarding lack of APU, Concorde has emergency power generation based on hydraulic power (which will be operational above M1.1 due to windmilling and is used to power some or all the fuel pumps for the re-start). (If you're wondering about slower speeds, Concorde would use RAT.)

Regarding the spill-doors, they do not dump all the air out. The automatic operation is a function of inlet pressure (among other parameters). They will open to reduce pressure build-up (and may also open at low power setting above M1.3), but air will still enter the engine and exit the back end.

The all engine flame-out above M1.2 procedure (first 2 pages attached) is simple in nature and does not contain any notes on manual spill door operation. To summarize it:

Page 1:

Fuel transfer pumps will be configured for fwd transfer;
Hydraulic system will be configured for emergency power generation;
Avionics cooling will be configured (anticipating loss of air conditioning);
If no automatic re-light, then thrust levers to idle to permit manual re-light.

Page 2:

HP [fuel] valves (which feed the engines) to be selected off (so not to drown the engines);
Auto ignition off;
Select re-light mode;
Wait 30 seconds from turning off the HP valves (for fuel drainage);
Open HP valves and wait 20 seconds for a re-light.

And below is the optimum re-light envelope.

Source: Concorde FCOM


What types of antennas are used in a Piper Seneca PA34-200?

There can be quite a few antennas:

VHF Comm
VOR
Localizer
Glide slope
DME
ADF
Transponder
Marker beacon
GPS
ELT with integral GPS receiver
ELT
FM Receiver

You can search for antenna at a site like chiefaircraft.com to see what they look like and the frequencies they are designed for. Or go to a manufacturer website, like COMANT Industries, for the same.

Some may have multiple functions in one antenna.

Typical locations:

Top of fuselage/wing, (VHF comm, GPS)
bottom of fuselage (belly), (transponder, marker beacon, VHF comm)
top of tail, (VOR, glide slope)


Why do the Cirrus SR-20 and SR-22 have the CAPS (parachute) system?

Briefly, without the CAPS the aircraft could not be certified because it wouldn't meet the spin recovery requirements of 14 CFR 23.221.

You can read a lot of detail in Cirrus's own CAPS Guide but their design premise is that pilots are bad at recovering from spins, especially close to ground. Cirrus wanted to make a safer aircraft that could recover even if the pilot has no spin training at all, so they implemented two safety features: a "cuffed wing design" from NASA and the CAPS.

Normally, the Cirrus would have to comply with 14 CFR 23.221(a):

  Normal category airplanes. A single-engine, normal category airplane must be able to recover from a one-turn spin or a three-second spin,
  whichever takes longer, in not more than one additional turn after
  initiation of the first control action for recovery, or demonstrate
  compliance with the optional spin resistant requirements of this
  section.

But, rather than comply directly by demonstrating the spin recovery, Cirrus asked the FAA to certify them based on the wing design and CAPS instead, as the CAPS Guide explains:

  Given that Cirrus had demonstrated enhanced low speed handling
  characteristics that will help pilots to  avoid inadvertent spin entry
  and the presence of CAPS, the FAA granted Cirrus an Equivalent Level
  of  Safety (ELOS) for the spin recovery requirement of the
  certification regulations. This ELOS is accepted  by all civil
  aviation authorities that have certified the Cirrus SR20 and SR22

The CAPS also has some other benefits, such as being easy to use for an untrained passenger (think pilot incapacitation). That might be an important "why" reason and selling point from a Marketing perspective but it isn't directly relevant for certification.


Do fly-by-wire flight controls in airliners provide artificial feel?

After some digging, I found an answer for myself: yes, some FBW airliners use artificial feel systems, but not all of them. According to Electronics in the Evolution of Flight (Google Books),

  The [Boeing] 777 fly-by-wire system employs envelope protection. This feature of the artificial-feel system provides increasingly greater force when the aircraft is pushed to its limits.

Conversely, Airbus does not provide any feedback to the pilots as their flight control system enforces control laws that cannot be overridden by inputs (source: Aerospatiale engineering document):

  The positioning of the control surfaces is no longer a simple reflection of the pilot’s control inputs and conversely, the natural aerodynamic characteristics of the aircraft are not fed back directly to the pilot.

Getting regulatory, since I did some further research: 14 CFR 29.395, certification of Transport Category aircraft, only states that the controls are required to resist a certain amount of force (i.e., not break). This differs from how much force is required to "achieve the positive limit maneuvering load factor" or otherwise impart changes to the aircraft's orientation (14 CFR 23.155 / 23.157).


Can I log flight training outside the US in a US-registered aircraft?

Yes. The FAA doesn't care where you fly, and you can even log training time with a foreign instructor. Logging time in a US aircraft overseas is completely fine.


What should I consider when storing a light aircraft outdoors for several months?

Are you in a wet or coastal environment?  You can store a "Cessna type" plane outside for a few months and be ok.  Most flight school planes live outside for years and do just fine.

Don't use a tarp.  You might damage antennas, windows, or paint.  Custom fitted covers are good but they're also expensive.

Don't just start the plane and let it idle.  That hurts the engine more than it helps.  To do any good, the engine must reach and sustain normal operating temperature until the moisture boils off.  A quick flight can accomplish this.


What if all passengers move from one side to the other?

You can get a significant rolling tendency if you shifted cargo or pax all to one side, but not enough to affect controllability; it would just be annoying.  This would apply to most airplanes.

Transport airplanes will have a lateral Center of Gravity range that has to be respected, along with a maximum fuel imbalance limit, to ensure controllability in some critical case.   In the CRJs, which are quite narrow, if everybody piled into the seats in one side, you (or the autopilot) would definitely notice it and have to correct with aileron, although it would be no problem trimming it out.  A really large fuel imbalance would be a much bigger problem however and might use up quite a bit of the available roll authority, so there are fuel imbalance limits in addition to lateral CG limits.

FAR 25 requires that the roll trim system provide the ability to trim out the rolling moment from max lateral CG offset in the most critical case - on takeoff, one engine inoperative, climbing at V2 (engine failure safety speed), with maximum allowable fuel imbalance. On the CRJs this uses up only about a third of the airplane's total roll authority (the RJ's roll trim just offsets the neutral point of the control circuit).


What stops Germanwings 9525 happening again — a suicidal pilot crashing the plane?

Any security precaution will be shared with the pilots. Therefore, a determined pilot will know those and be able to find ways to circumvent them beforehand.
What are some of the possible precautions?

Give both pilots their individual key? Easy - disable the lock before the other pilot leaves the cockpit, maybe by breaking off your own key when it is stuck into the lock.
Use a numeric keypad for unlocking the cockpit door which can only be reset on the ground? After the first terrorist attack where this system is abused, this will be disabled. And it is easy to figure out scenarios where the terrorists will learn the passcode from an unwitting pilot.
Similarly to military ground collision avoidance: Don't let the FCS allow too low flight levels outside of terminal areas? Disable the system by pulling the appropriate circuit breakers.
Let traffic control take over the plane by remote control when limits are exceeded? Easy again - just jam the radio signal. Again, it is easy to imagine scenarios where such a system is abused.

Also, what works for rail-based systems cannot be transferred to aviation. Stopping an airplane in mid-flight does not work as it does with trains.
The pilots have to be able to control the airplane exactly because in an unforeseen emergency they are the last resort which can bring it down in one piece. Taking away this ability does not enhance safety but would be the only basis of really stopping a suicidal pilot.


Why doesn’t runway 31R at JFK have a VGSI?

See this answer.  PAPI's are usually placed at the touchdown point for the runway.  By looking at this satellite photo you can see that the touchdown point is at the intersection of 31R and 22R-04L so there is no room for the PAPI.


What is the meaning of 'stall' in this sentence?

The sentence means the airplane was slowed down (generally) and the nose pulled up until the wing stopped generating lift (stalled) and finally "killed the engine" (typically by reducing the mixture until the engine stops running) and then landed by just gliding down to the airport.  Stalls are not a good thing when low to the ground and maneuvering for a landing, as there is no altitude left to recover, so we practice them to recognize when one is about to occur to avoid them, and to recover quickly at the onset of one.


Could it imply that Wx minimums won't apply?

Yes

  Could it imply that Minimum Altitudes no longer are required?

Yes

Since both of those things are rules under part 91 ("this part") specifically 91.155
and 91.119 you can indeed deviate from them as per 91.3.

The answer to "which circumstances" is far to broad to be defined in an all encompassing way which is why the FAA writes it the way they do.


Non-precision approaches for which constant descents are not approved?

Probably because it's so shallow: you have 8+ NM to descend about 1100'. On a 3 degree angle, you'd typically decsend 1100' in a little over 3 miles.  I'll leave it to good folks with calculators to work out exactly what angle this would be, but it's under 1.5 degrees, so probably below whatever limit TERPS gives for depicting an angle there.

One solution would be to remain at the FAF altitude until reaching a calculated "fly-off" point, at which time you would descend at a 3 degree angle until reaching published MDA + 50', at which point you continue or go around.  But that probably takes specific FMC software & perhaps a tailored approach chart, so with only this one your option pretty much looks to be "dive" (not that far) "and drive" (quite a bit).

Though that gets you to an MDA that is 1700'+ above the TDZE...

ACTUALLY, ON FURTHER CONSIDERATION: It isn't too shallow, it's a steep path from FAF to TDZE, 3000' in 8 NM (would be 9NM at 3 degrees). And the angle to do that probably doesn't quite clear the WAGOK step-down, meaning your vertical path becomes even steeper at that point than it starts out.

So rather than publish gradient***s*** steeper than (whatever the TERPS limit is), and increasing after the step-down fix, they just say NA & let you "make it work" on your own.

I notice that there is no VDP published here, either...


Alpha is also called "Angle of Attack". It is the angle between the relative wind and the chord line of the wing. When the angle of attack is increased to a certain point, the airflow over the wing is no longer smooth. Turbulence is created after the wing, and it loses lift. This is known as a stall.

"Alpha protection" is a system which prevents the angle of attack from getting too high. This is done by software monitoring the plane's angle of attack sensor. When a high alpha situation is detected, the software lowers the nose of the plane to maintain high but still safe angle of attack. It also commands full thrust to all engines. This system is designed to prevent pilots from stalling the aircraft and to allow them to get the best possible performance in emergency e.g. in a wind-shear.

This is not part of the autopilot. It is integrated into the flight control computer and activated at all times. Autopilot are functions which fly the plane in a certain way to reduce the pilot's workload in normal situations (e.g. climb to 13,000 feet, turn left to heading 340).

Pilots can opt to turn off such protection systems should it malfunction, or a very special occasion exists where these protections should be switched off, such as performing at an airshow.

Notable Accidents

In the Airbus, the A320 was the first "fly-by-wire" aircraft. During a demonstration flight AF296 when showing off the alpha-protection system the pilots took the airplane too low and too slow and disabled the part of the system that should have commanded full thrust. The plane didn't stall but was having too little energy to climb anyway. It crashed gently into the trees.

In an acceptance flight, the pilots were testing the alpha-protection at low altitude. The system should have prevented the plane from pitching too high. However, to the surprise of the crew, it failed. The plane stalled with insufficient altitude to recover. The system did not work because water got into the angle-of-attack probes during previous improper washing procedure, froze during the flight and blocked the probes.

In AF447, the system switched to "alternate law" and no longer provided stall protection, because the pitot tubes (airspeed sensors) froze over. The plane stalled from 38,000 feet to the water after the pilots failed to apply correct procedure for unreliable airspeed.


Why is the aerodynamic center for most airfoils located at the quarter chord location?

It has to do with the Birnbaum distribution. The center of its area is at one quarter.
In potential flow theory, lift can be calculated as the linear superposition of a contribution from camber and one from angle of attack. While the camber-related part of lift is constant, the angle-of-attack related part varies linearly with this parameter. This means that changing the angle of attack adds or subtracts circulation, the chordwise distribution of which is described by the Birnbaum distribution. The important part is the self-similarity of the angle of attack dependent lift resulting from this addition or subtraction of the same chordwise distribution.
The center of pressure of the resulting angle-of-attack dependent part is constant and at 25% of chord for 2D flow and wings of large aspect ratio, at least as long as flow remains attached and viscous effects are negligible.
The pictures below (own work) show the circulation over chord produced by camber alone (so AoA is 0°) on the left and the Birnbaum distribution alone on the right (so using a symmetric airfoil). Total circulation is simply the sum of both. Copied over from this answer thanks to the helpful suggestion of ROIMaison.

ultimately the aerodynamic center is determined by how the distribution of lift changes with angle of attack

That is precisely right: The aerodynamic center (a better word for it is "neutral point", btw.) is the point where the additional lift force from a change in angle of attack can be summed up. Do not look at the lift distribution per se (that is what determines the center of pressure), but at its change over angle of attack, because that is what determines the neutral point and what shifts the center of pressure back and forth. The constant part caused by camber does not change with angle of attack and the part caused by angle of attack has its center always at 25% (under the conditions mentioned above) and its magnitude grows with angle of attack. This growing contribution of the angle-of-attack-dependent force is what shifts the center of pressure forward (on airfoils with positive camber) rsp. backwards (on airfoils with negative camber) towards the 25% point.
BTW, I wish we had more people like Aaron Swartz. That article is now 97 years old and these monopolists still want 42 € from me for a copy. Sorry, no details from it – I could not find any online.


Which aircraft had the shortest operational life?

Japanese Ki-100 made its combat debut on the night of 9 March 1945. The last loss of a Ki-100 occurred on 14 August 1945, a day before the surrender of Japan. Hence the  interceptor has been in the service for 158 days, or 5 months 5 days, with 396 units produced.

Looks like nothing bad can be said about this interceptor. It has been recently developed, without major flaws, and, as such, was obviously superior to the older models. It was between the first interceptors to feature the factory-installed armor and self-sealing fuel tanks, and some think it was the best interceptor used by the Army during entire war. But, with the end of the war, it was no longer required.


In the lower picture that accompanies the graphic in your link, focus on the elongated figure eight shaped piece.  The lower portion is attached to a piston extending from the actuator, and is free to pivot about the attaching point.
The upper portion of this link is fixed to the aileron and the entire part acts as a lever arm.  So, when the dark gray piston of the actuator moves left and right, the figure eight lever pivots clockwise or counter clockwise centered on the middle of the upper circle.  Since this part of the link is attached to the aileron, it also moves.
The graphic isn’t great, but try to visualize which parts move from my description and let me know if you are still confused.
You typically won't see the actual control horn on full sized aircraft because it will either be covered be a fairing to streamline it, or it will be contained within the thickness of the wing structure.
For example, in small RC models it is common to have an external horn like this:

However, you will generally NOT see a large external actuator like this on a full sized plane:

There simply isn't a need for such a large mechanism on most airplanes, and it would come with a large drag penalty.  If extra leverage is needed and the mechanism protrudes beyond the thickness of the wing it will be covered by a fairing like this:

Instead, most designers will try to keep actuators contained within the wing, as in the photos below:


Is it possible to have an afterburner in a turbofan with bypass ratio greater than 1:1?

fooot has pointed out the GE F101 (powers the B-1 Lancer) has a BPR (bypass ratio) of >2:1 and comes with an afterburner.

Other examples are the Turbo-Union RB199 (1.1:1) and the Volvo RM8 (0.97:1). Generally, anything below 4:1 is a low BPR, and bypass air is needed for cooling. Pure turbojets resort to other means of cooling, such as the ram air inlet on a J75, or they run inefficiently at low combustion temperatures.

I can't speak for the inner workings of a classified military engine, but an ideal supersonic engine would be a variable cycle engine (VCE). Operating with bypass in the subsonic regime, and without in the supersonic high-power demand regime.

Such theoretical (to the public at least) examples from the 90's are shown (and linked) below. Do note the idea behind the VCE is the elimination of the wasteful and noisy afterburner.

So, it's an either/or situation. A dual operation of afterburner and high bypass in a non-VCE would not have benefits.

(Flight—PDF) Image shows how a VCE routes the bypass air in both modes of operation. Other variations by RR and Snecma can be seen here. Note the novel design of the Snecma with the free-turbine—extra-fan. VCE's would definitely be heavier and more complex engines.

Related: Why do military turbofan engines use a low bypass ratio?


Why are true headings followed above 73N in the northern hemisphere, whereas in the southern hemisphere the change is made at 60S?

Earth's magnetic field is not symmetric north to south.  In particular,  the magnetic declination can get significantly larger at a given southern latitude than it does at the corresponding northern latitude.  This can be seen by looking at a world declination map:

Curves of a given declination reach much lower southern latitudes than they do northern latitudes.  For example, the highlighted curve is 40° west of north declination;  its southernmost point in the northern hemisphere is at about 75°N, but its northernmost point in the southern hemisphere is at 35°S (!).  Another way this manifests is that the South Magnetic Pole is at a significantly lower latitude than the North Magnetic Pole (~65°S vs. ~85°N).
As to why the Earth's magnetic field is so asymmetric, that's a question for Earth Science.SE.  My understanding is that you may not get a good answer over there either;  the behavior of Earth's magnetic field is complex and not fully understood.


Is it possible for more than one airplane intercept the ILS at the same time?

Yes, since the ILS is just a set of radio signals emitted and received by aircraft, there can be more than one aircraft established on localizer and glidepath. The ILS does not lock or tune onto one single aircraft, it continuously broadcasts the localizer and glidepath signal.

Since the localizer and glidepath antennas are located at the end of runway for the localizer and at the side of the runway for the glidepath, you will only need to worry about interference by aircraft on the ground, where they are close to the antennas and can deflect the signal due to proximity. This is why there are protection areas and aircraft need to hold at certain holding points, e.g. CAT II holding points when CAT II approaches are in use.

Aircraft in the air can also interfere and deflect the signal, however the emitted signals spread in a cone, so other aircraft still can receive the correct signals.

To quote KeithS from one of the comments:

  [...] basically, the localizer is projected from the far end of the runway, and the glideslope is projected from the side of the runway at or near the touchdown target. Thus, aircraft in line to touch down on the ideal target will have good line-of-sight to both transmitter pairs (unless there's an aircraft in the ILS Critical Area)

(Image Source: www.aopa.org)

(Image Source: nustyR AirTeamImages (found via LondonReconnections))

(Image Source: When is an aircraft cleared to land?)


How does single-sided deafness affect the different pilot licenses?

If you were willing to travel to the US (which is not all that uncommon to get a pilots license) and get your license under the FAA regulations you could do it as per the regulations,

  §67.305   Ear, nose, throat, and equilibrium. Ear, nose, throat, and
  equilibrium standards for a third-class airman medical certificate
  are:

  (a) The person shall demonstrate acceptable hearing by at least one of
  the following tests:

  (1) Demonstrate an ability to hear an average conversational voice in
  a quiet room, using both ears, at a distance of 6 feet from the
  examiner, with the back turned to the examiner.

  (2) Demonstrate an acceptable understanding of speech as determined by
  audiometric speech discrimination testing to a score of at least 70
  percent obtained in one ear or in a sound field environment.

  (3) Provide acceptable results of pure tone audiometric testing of
  unaided hearing acuity according to the following table of worst
  acceptable thresholds, using the calibration standards of the American
  National Standards Institute, 1969:

The way this is written if you can score at least 70% in one ear on the audiometric speech discrimination test you should be good to go. Note this is for a class 3 medical with will not allow you to fly commercial operations. I am not sure how the new basic med requirements fall into all this but your best if you have specific questions is to contact an AME directly.

From a practical standpoint, the cockpit of most small planes is fairly defining already and the headsets take mono signals from the radios so I don't really see how the lack of binaural hearing would present any issue.


Why are airports like Roswell and Fresno so popular for flight tests?

You can do flight tests from almost any airport, but it helps if:

The weather is dry and stable,
the field is not too far from the factory,
there is a large unpopulated area nearby, and
you have support facilities available.

The support facilities were installed by the Air Force, and the other factors were already taken care of by Mother Nature in the case of places like Fresno or Roswell. Given that the distance from Los Angeles, a major aviation industry hub, to Fresno is only a bit more than 200 miles, flight testing in predictable weather and over the open terrain west of Fresno becomes very attractive. If you look at Lancaster, the next town near the vast Edwards Air Force base, the driving distance to Los Angeles is only 70 miles. This should help to explain why Edwards is today the main Air Force test base. Once you have a long runway and some experienced flight test engineers in place, other manufacturers will also converge on this location in order to make their flight testing more predictable and effective.


Best $L/D$ is short hand for the airspeed where you have the best ratio of Lift to Drag, and is the same as $\textrm{max}\ L/D$.

If you graph Airspeed -vs- the $L/D$ ratio, it will look something like this:

The peak point of the graph would be the "best" $L/D$ and represents the airspeed where the airplane is the most aerodynamically efficient.  For unpowered flight, it is the airspeed which will result in the best glide ratio.


How could the B-29 bomber back up under its own power?

Some B-29s had reversible pitch propellers. All the B-29s used for the Atomic bomb missions had reversible pitch propellers.

Wikipedia B-29 Variants

  Moreover, engine packages changed; including the type of propellers and range of the variable pitch. A notable example were the eventual 65 airframes (up to 1947's end) for the Silverplate and successor-name "Saddletree" specifications; built for the Manhattan Project with Curtiss Electric reversible pitch propellers.

Army Air Forces tests reversible propellers for B-29 bomber


How serious is the problem of "stopping too far" next to the jet bridge?

I can't tell you how often it happens, but the procedure is quite simple: stop the aircraft immediately, and alert the ground crew. It is then up to the ground crew to decide what, if anything, needs to happen. If the aircraft has just slightly passed the intended stop position, maybe it can just stay where it is. If it has overshot significantly, it may be required to push the aircraft back to the correct position with a tug. Worst case, the aircraft might have actually hit some obstacle (building, vehicle, jet bridge) in which case the aircraft or surrounding equipment might have been damaged.


How does the A350/380 operate on Variable Frequency AC generators?

According to Civil Avionics Systems, such system is found on the Airbus A350, A380, Boeing 787, and new business jets. But only certain systems use the variable frequency (VF) AC. Namely motors such as in the fuel and hydraulic systems. They are designed to operate at different speeds with tolerances allowed for the slowest modes of operation.

In short, the motors, heaters, etc., are designed to operate on different loads reliably.

From the 787 manual, items such as cockpit displays and instruments use DC power instead. DC fuel pumps, brakes, and igniters, are also used on the 787.

The reason for it not being implemented decades ago is that motors weren't easily made reliable, especially DC motors. Another reason is the cooling requirements for the motor controllers, on the 787 for example, liquid cooling is used. Solid-state relays replaced most of the traditional circuit breakers, enabling automatic load shedding. One reason for that is to further limit the generated heat.

Snippet from the 787 manual.

The above VF system is by far the lightest, and is less complicated. Between it and the IDG system, there was the short-lived variable-speed constant-frequency (VSCF) used on the McDonnell Douglas MD-90. In its early days it proved troublesome to maintain but eventually it achieved high dispatch reliability. As with the VF, the engine generators outputted VF, but then tap converters (a mid-80's invention) were used to produce constant frequency. Below is the MD-90's air-scoop for the cooling of this system.

(airliners.net) Air-scoop for cooling the VSCF on the MD-90.


If we use upper surface blowing and externally blown flap on the same wing will lift correspondingly be very high?

Two Engines Would Probably Be a Waste

First the basics about lift. Lift can be seen as occurring from one of two effects, a pressure difference across surfaces, or in reaction to the acceleration/deflection of the air (or whatever fluid you are moving through). Any case can be calculated from either method and you will come to the same result. You can't exploit a pressure differential without accelerating air, and you can't accelerate air without creating a pressure differential. Since force in these cases is governed by $F=\dot m\Delta v$ and required power is $P = \frac{1}{2} \dot m (v_{out}^2 - v_{in}^2)$. This means that to maximize efficiency you should try to accelerate the largest quantity of air by the smallest amount.

Now let's talk about flow control. Blown flow control systems increase lift in two ways, first is that they help keep the flow attached, and second, especially in propulsion driven systems like you are describing, is by deflecting part of the thrust downwards. Attaching the flow helps to allow greater angles of attack without stalling, once you have have generated this flow attachment, further actuation of flow control will not help generate any more lift by this method, though it may allow further angles of attack. If you are trying to improve low speed characteristics by allowing extreme angles of attack remember that the increase in lift will approximately follow a sin curve as it results from increasing the angle of deflection for flow across the wing, the first little increase in angle will give lots of lift with little drag, the last will give almost no lift for tons of drag if you want to take it as far as 90 degrees. Also lift is related to speed to a power of two, that means you need a lot more deflection as your speed drops.The second method that blown systems generate lift is by deflection, basically you gain some lift by sacrificing thrust. In the end this is no better than just pointing the engines slightly downwards.

If you want to perform flow control you also have to remember how much area on the wing your exhaust is going to effect. Your jet will likely be small in comparison to the total size of the wing.

At the end of the day, it is probably better to make a system that provides some flow control to the entire wing rather than a massive amount of flow control to a small portion of the wing so spread out those engines rather than trying to stack them; and remember, flow control will give no improvement over simple thrust vectoring at a standstill.


Since jet fuel is cheaper, why do pistons still use av-gas?

First off, there are some light aircraft powered by diesel like the Piper Archer DX and the Diamond DA42. It should be noted both come in a 100LL powered version as well. As it stands now the FAA is looking for a replacement for 100LL but has not yet decided/found one. There are also some airplanes out there that can run on mogas. But anyway, more to the point, you cant simply put diesel into a regular engine as the ignition system and compression requirements are different. Thus to convert a plane to diesel you need a different engine all together. With that in mind most certifying bodies dont just let you swap airplane engines to any engine you see fit. The engine would need to be certified for the air frame which is a costly proposition. Thus it is sensible and economical to keep the current engines running. There are some efforts like those made by piper and diamond to slowly make diesel aircraft available.

As to why 100LL is used, this stems mostly from history. The vast majority of the general aviation fleet both physically and simply in design, stems from the dawn of the jet age. When the lines for the 172 were first drawn there was plenty of 100LL to go around, it was for lack of a better term, the logical choice.


Why aren't passengers trained and evaluated for operating emergency exits in commercial aircraft?

Former TWA here.

Emergency exit seats used to only be given to passengers who are ready, willing and able to assist in case of an evacuation. They could not be assigned in advance, the passenger had to be asked and evaluated in person by the boarding agents, who would have been held responsible if a not-able passenger was to be seated in any of those rows/seats. This used to be the rule, it was deemed that safety was more important than selling seats.

Maybe the rules have changed since?


Are all IFR flights assigned squawk codes in US controlled airspace?

No.
/X (VOR only), /D (VOR/DME only), /Y (LORAN only), and /M (TACAN only) aircraft do not have transponders. Primary-only aircraft can be assigned flight plans, and fly IFR. It's not common, though, because aircraft that will fly within 30 miles of class B airspace, or aircraft that will operate above 10,000 feet MSL are required to be equipped with transponders and Mode C altitude reporting, similar to the ADS-B requirements. (14 CFR § 91.215)


How did this Boeing 787 perform this near vertical takeoff?

First of all, it was nowhere near vertical. It is basically an optical illusion created by the camera angle.

Second, and most importantly, what you are seeing is the pitch angle and not angle of attack.

As it can be readily seen from the following image, the angle of attack and pitch angle can be significantly different.

Image from What is Angle of Attack? by Boeing

Angle of attack (AOA) is the angle between the oncoming air or relative wind and a reference line (usually fuselage centerline or longitudinal axis in commercial aircraft) on the airplane or wing.

Pitch angle (attitude) is the angle between the longitudinal axis (where the airplane is pointed) and the horizon.

Flight path angle is the angle between the flight path vector (where the airplane is going) and the local atmosphere. In other words, it is the angle
between the flight path vector and the horizon, as far as a commercial airliner is concerned.

When referenced to the atmosphere, the angle of attack (AOA) is the difference between pitch angle and flight path angle. Depending on the aircraft atitude, they can be very different. During climb, usually the pitch is increased at constant angle of attack.

For example, in a steep descent, an aircraft could reach very high angle of attack if the nose is below horizon. While flying upside down (not something that happens in an airliner, I might add) , both differ by around 180 $^{\circ}$.

So, the angle of attack in this takeoff is surely well below the stalling angle of 787.


Aside from the Stratolaunch, how many other aircraft have split bodies?

Depends on how you define it but there have been a variety over the years:

The P-38 Lighting has 3 bodies there was also a later experimental version the XP-49 that only saw a single airframe.
The F-82 was a a twin fuselage version of the P-51
The boomerang which was also designed by Burt Rutan (the designer of the stratolaunch)
A one off twin fuselage piper cub was built but never saw production scale.
As @DanPichelman points out in the comments, the Yak-110

  ...although launching spacecraft from them commercially is a new thing

This is not entirely true, piggy backing rocket planes on larger aircraft has been widely done since the 50's


In Polikarpov I-153, Were the gulled wings very important?

The I-153 was a development of the I-15, of which 671 were built. However:

  The upper wing of the I-15 was unpopular with some pilots, as it was felt to restrict visibility, so Polikarpov's design bureau produced a revised version, again powered by the M-25, with a longer span un-gulled upper wing.2 This version, the I-15bis, commenced production in 1937,[7] a total of 2,408 I-15bis' being delivered by the time production finished in 1940.2

So the straight upper wing version was built to improve visibility, and many more units were produced than of the I15.

Structurally and aerodynamically, straight or gulled in this way makes little difference. Structurally, any bend is a weak spot, but the upper wing of biplanes usually had pylons in that position anyway to fix the upper wing to the fuselage. Aerodynamically: the bit missing in between the gull pylons reduces drag a tiny bit - the cross wires have phenominal drag though, and that fat little body also drags a lot of air with it.


What is an NTAP when referenced in a NOTAM?

The Notice to Airmen Publication is an FAA publication, currently available in PDF or print form, published every 28 days. The NTAP contains airway, airport, facility, procedural, general, and international NOTAMs. As voretaq7 notes, this includes long term NOTAMs that a flight service brief won't include if you don't ask. There is also a section with graphic notices, including general notices, special operations, airport and facility notices, and major sporting and entertainment events.

That last section is the one relevant for the NOTAM listed above. This lists information and procedures for any airports affected by special events, which is nice to have in one place rather than looking up long and complicated NOTAMs for each airport. For example, the Masters Golf Tournament referenced in the NOTAM above has a section in the NTAP, affecting airports AGS, AIK, DNL, and HQU. The information includes preferred IFR arrival routings, potential for holds and VFR arrival delays, departure procedures, preferred IFR departure routes, IFR overflights, and VFR arrivals/departures to/from DNL. There is also a special schedule for the tower at AGS, and a temporary tower at DNL.


"0 SMOH Lower End." means just what it says. The lower end was completely rebuilt but there wasn't a complete overhaul of the "top end". Some of the old top end parts must have been re-used without being overhauled.

I would suggest speaking to the person who did the overhaul to get more information on why the top end did not qualify for 0 SMOH as well.


If you depressurize an aircraft at FL340, will the temperature inside fall to match the temperature outside?

This depends entirely on how you depressurized it.

Depressurizing the aircraft by fully opening the outflow valve will lower the temperature somewhat, but will not bring it to that outside.

The expansion process will be closest to free (Joule) expansion, which causes only small decrease in temperature, not an adiabatic one that corresponds to the atmospheric lapse rate. And while some energy will be lost through the air's initial expansion as it escapes, it will quickly be replaced by new warm air coming in through the packs (cabin pressurization and air conditioning system).

That air supply is quite reliable, as it's bled from engines at about $200\ \mathrm{^\circ C}$ – it then needs to be air-conditioned so as not to roast the passengers. The packs will automatically maintain set temperature, putting less effort into cooling the air if the cabin gets too cold.

If you also disable the engines, so no new air can come in, the airplane will lose some heat. Still, in all other regards, the fuselage will remain a well-insulated vessel with some heat sources on the inside. Also, the less-dense air is much less effective at heat transfer, and has much less heat capacity, so it will get chilly, but the passengers aren't going to freeze to death or even get a frostbite.

The heat capacity of air at ambient pressure will be about $280\ \frac{\mathrm{J}}{\mathrm{m}^3\cdot\mathrm{K}}$. A 737 encloses about $200\ \mathrm{m}^3$, giving a total of $56\ \frac{\mathrm{kJ}}{\mathrm{K}}$. For comparison, the plane's passengers have a specific heat capacity of $3.5\ \frac{\mathrm{kJ}}{\mathrm{kg}\cdot\mathrm{K}}$, giving, for 120–160 of them, about $40\ \frac{\mathrm{MJ}}{\mathrm{K}}$. So a $\sim70\ \mathrm{^\circ C}$ one-time drop in air temperature can technically match the outside, but it can only cool the passengers by at most $0.1\ \mathrm{^\circ C}$.

In practice, the cooling won't be instant, and it has to compete with heat exchange with the still-warm interior. Passenger bodies producing $12{-}18\ \mathrm{kW}$ of waste heat (at rest) will also restore cabin air temperature by $\sim1\ \mathrm{^\circ C}$ every 4 seconds. The larger the hole, the lower the temperature minimum that will be reached. With a broken window, the effect of this one-time expansion heat loss will have been mostly undone before anyone notices it, amidst the chaos.

Tearing the top of the fuselage off, on the other hand, will most definitely let the passengers feel the cold outside. But it will be the powerful wind blowing in, not the air leaving the airplane, that will chill its interior.

Breaking a single window or making a lot of bullet holes (really a lot - it will take a crate of ammo to overcome the packs' air inflow) is about the same as fully opening the outflow valve. It will produce some cooling effect, but it will definitely not bring the interior to the outside temperature, as the cooling effect is one-time, and will be rapidly compensated for.

This kind of event has happened a few times in aviation history. It has caused some fatalities – a person sitting right next to the broken window has been killed by the mechanical force of the escaping air. No one has been frozen to death, not even in the Aloha Flight 243, which represents about the worst possible case for depressurization (which still leaves survivors and not just a debris field).


What is the pressure on the pressure side in relation to the freestream?

TL;DR

In most cases the pressure distribution on the pressure-side is higher than on the  suction-side (with respect to the chord-wise location).
There is not general answer to this question since the pressure distribution on the pressure-side is (although mostly close to static ambient pressure) connected to the profile shape.
For low velocities the pressure on the pressure-side is likely to be very close to the ambient static pressure.

Comment on the Figure 2

The depicted airfoil is operating in the transonic-regime. This means the free stream velocity is close to the speed of sound (M=1). Caused by acceleration of the air on the suction side of the profile the air flow will reach supersonic speeds. The rapid increase in $C_p$ at the suction side is a shock.

The following picture shows the Mach-number-distribution around an airfoil in the transonic speed range.

Comment on compressibility

The term $\frac{1}{2}\rho V^2$ is a representation of the dynamic pressure for very low velocities (M<<0.3). Of course this equation can also be used for higher Mach-numbers, however the results (in terms of self-similarity) will contain a large error. Therefore, $p_0 - p_\infty$ should be used to avoid this.

Comment on the pressure distribution on the pressure-side

During level flight (design-speed@design-AoA) there will be a stagnation zone at the leading edge of the airfoil. This results in a pressure higher than the static free-stream pressure. After that the flow will be accelerated resulting in a reduction of the pressure. Often, this reduction will result in a $C_p<0$.
At the trailing edge the pressure of suction- and pressure-side have to be equal, so the pressure rise of the aft-section of the airfoil-pressure side will be adjusted accordingly.

Answer:

For low Mach-numbers (e.i. low free stream velocities) the pressure on the pressure-side is likely to be very close to the ambient static pressure.

Comment on low-pressure-region on the pressure-side

As always with the (elliptic) aerodynamics it's a bit chicken-and-egg. Lets assume (Caution! this is a very simple point of view) there is only one design objective (lift). Then the pressure-distribution on the pressure side needs to fulfil two boundary conditions. 1) high pressure ($C_p$) at the leading edge which is a result of the free stream velocity and 2) the pressure ($C_p$) at the trailing edge which is given by the pressure-distribution on the suction-side. Now, the airfoil-designer needs to find a profile-shape which fulfils both requirements. Whether the $C_p$ becomes negative or not is not of importance, as long as it stays above the $C_p$ of the suction-side and there is no risk of boundary-layer-separation (which is likely to cause missing the pressure at the trailing edge).

  Since there are no simple answers to complex problems, I truly beliefe the best thing would be to install Xfoil (or its successor) and try it out.


What are these fins on the Beechcraft 1900?

Well, those fins were put there to increase yaw and pitch stability on the D version because of the change in the centre of gravity due to a higher fuselage from the C version. This D model also had more powerful engines than the C model. To add on, the strakes at the bottom were added for the same reason too.

More info can be found here.


Are these two aircraft too close to each other?

They're not too close

They (almost certainly) are not closer to each other than is allowed.

It would be quite extraordinary if a pair of airliners at an airport were allowed by air-traffic control to approach each other in violation of limits, which not only include the distances of time and space demanded by safety, by additional margins as well.

If there had been a violation of those limits, it would have recorded as a matter of course and investigated.

On that basis I think it's safe to say that it's extremely unlikely that approach limits were violated.

The camera doesn't lie, but YouTube doesn't tell the truth either

A second point is that it is extremely hard for human beings to judge distances, positions and speed in three-dimensional space.

Frankly, it's a wonder that we're ever able to play table-tennis, but the fact is that when you're looking at a pair of aircraft you have little chance of accurately judging what they are doing in relation to each other.

That's especially so when the aircraft are on YouTube. I bet even a champion wouldn't be able to play table-tennis on YouTube...


What is the Fuel-Oxygen ratio for a large turbofan at cruise conditions?

The GE90-115B has a bypass ratio of 9:1. So if 1000 units [of air] enter the fan, only 100 will make it to the combustion chamber.

Kerosene burns efficiently at 15:1 (air to fuel). But not all the air in the combustion chamber is burned. Only around 22 (average) of the 100 will be burned, the rest will provide cooling, flame stabilization, and dilution.*

So of the 1000, 22 are burned at the 15:1 ratio. Total ratio will be 1000:1.5 (air to fuel). Since oxygen is ~20% of air, then the ratio is 130:1 (oxygen to fuel). (Note: bypass is included.)

To validate that figure:

If the fuel flow (per engine) is 2500 kg/hour (modest value in cruise†), then that means the engine takes in ~1,667,000 kg of air per hour, or 460 kg per second. That agrees (give or take) with the numbers in this post (PDF does not confirm the GE90 version and the fuel flow†).

  According to The GE90 - An Introduction, the GE90 has a mass flow rate‡ of 1,350 kg/s at take-off and 576 kg/s at cruise.

* The Jet Engine - Combustion Process.
† It varies with the airplane weight, cruise altitude, speed, etc. See here (in lb/hr) for the GE90-90B from a 777-200 flight manual, it varies from 2140 to 4440 kg/hr.
‡ Mass flow rate is for the air, as NASA puts it: "The total mass flow rate through the inlet is the sum of the core and fan flows."

FWIW, the -115B1 is the exclusive engine for the 777-300ER. Based on this official data from Boeing (last slide), at a 365-passenger load on a 6,000 NM journey, each engine burns an average of 3790 kg/hr (heavy load; takeoff and climb included), which is within the above -90B range, i.e., 2500 is a correct value to use as the -115B will also have a similar wide range. (Arriving at that figure requires the 777 cruise speed of 892 km/h.)


Does empennage height above the main wing matter

The critical aspect for Vertical stab height is its rolling torque and how it affects the balance of total side slip forces on the aircraft, including wing, fuselage, landing gear, engine mounts.

As viewed from the side, for the same area, a higher aspect Vertical stab will create more rolling torque and be structurally weaker than a more square design.

More important is Vertical stab area, your weather vane, which affects directional stability and spiral stability.  Very important to get this right.  Adequate area is needed to avoid "Dutch Rolling".  Note that a high wing design needs less dihedral, there for can have a smaller Vertical stab.

Also, the tractor propulsion design, with the CG (and wing)  set as far forward as possible, also lowers the Vertical stab and Horizontal stab area requirement, resulting in significant reduction of drag.

But, unless extreme performance is needed, it is best to slightly overdesign the empennage and play it safe.  Study of existing designs past and present will be of great help.


Was the Coanda effect known in 1868 to Matthew Piers Watt Boulton (the one who described the first ailerons)?

The accompanying text clearly describes an application of the Bernoulli effect. However the resultant backwards airflow shown in the drawing is given no explanation, other than that implied by the fact that the device is drawn forwards. So there is no evidence of the Coanda effect, in which the airstream clings to the device skin, being understood.
Would it have worked? Probably, but it would have been pretty inefficient.


What if I fail to receive FAA Medical Certificate via mail?

You're really asking a postal service question (and from my experience with the postal service you're probably out of luck, even if you had the tracking number: Unfortunately when the USPS hands packages off to other nations' postal services that's often the last tracking update - either the item gets to its recipient or not, but it's gone into a black hole).

In any case the answer to the aviation/FAA half of the question ("I haven't gotten my new medical certificate yet - what do I do now?") is much simpler, and involves more helpful people than international mail: "Contact the Medical Certification branch" (see here).

In this particular case you want to actually call them on the phone and speak to a human being to determine (a) if they have a tracking number, and (b) how to proceed to ensure that you have a valid medical certificate in your possession.
I know in cases like this the Medical Certification branch can also fax you a copy of the certificate (I believe these are valid for 60 days) while you try to get things straightened out and get the actual paper certificate in your hands.


Can you fly an airplane at a 90° roll angle without losing altitude?

It depends. As always. If thrust is high enough, why not?
Knife edge flight is a regular part of aerobatic performances, and the fuselage is producing almost* all the needed lift. This requires

Enough speed,
A low ratio of aircraft mass to fuselage side area, and
Enough rudder deflection to trim the aircraft in a sufficiently high sidelip angle.

For horizontal flight your installed thrust must be sufficient to compensate for the high drag in this attitude. This should be no problem with powerful propeller aircraft if their fuel system keeps them supplied at this attitude. Jets must make sure that the sideslip angle can be tolerated by the intake.

The smoke from the exhaust indicates that this biplane flies horizontally. Yes, it is a large model airplane (not man-carrying, but almost big enough for that). But the laws of physics don't change with scale. For an example of a full-size aircraft doing a knife edge, see this video. Thanks go to @romkyns for pointing this out to me!
Note the rudder deflection to keep it at this angle of sideslip. Direction is controlled with the elevator. At this rate of sideslip the propeller thrust contributes to overall lift as well, however, the vertical adds a substantial downforce in order to trim the aircraft and the fuselage has to supply the remaining lift.
* Almost means that the propeller will carry some of the weight, but the large downforce on the vertical needs to be compensated, too. In sum, the lift contributed by the fuselage is very close to the weight of the plane.


Is the air in a jet engine supercritical?

Maybe at takeoff, definitely not in cruise

You have to consider that atmospheric pressure drops with altitude. At FL360 you have ~0.226 bars. Even multiplying with the GE9X's pressure ratio of 60, you get ~13.56 bars, well below the 34 bars needed.

Takeoff might be a different situation, since the engines are operating at full power with ambient pressure ~1 bar, this could lead the high pressure compressor stages to operate in supercritical conditions. Note though that the combustion that immediately follows changes significantly the chemical composition, and thus the critical point might be altered as a consequence.

As engines are optimized for cruise conditions, it is highly unlikely that supercritical conditions have extensive design consequences, unless they would render takeoff impractical.


Is there an FAA ruling about the airworthiness of piston engines with less than 60 on the compression test?

Long story short is that the manufacturers instructions take precedence over all. FAR Part 43 Appendix D requires that at each annual and 100 hr the compression be checked and that if there is "weak" compression further inspection of condition and tolerances of the engine have to be made. Now the term "weak" is kind of broad. FAA Advisory Circular AC43.13-1B states:

  If a cylinder has less than a 60/80 reading on the differential test gauges on a hot engine, and procedures in paragraphs 8-14b(5)(i) and (j) fail to raise the compression reading, the cylinder must be removed and inspected.

However, it also states:

  This advisory circular (AC) contains methods, techniques, and practices acceptable to the Administrator for the inspection and repair of nonpressurized areas of civil aircraft, only when there are no manufacturer repair or maintenance instructions.

So the gist of it seems to be that if the compression is below 60/80 on a hot engine IN GENERAL, then yes it needs to be removed and inspected, UNLESS the manufacturers instructions state otherwise. Keep in mind Advisory Circulars are guidelines and not regulatory in nature although they are usually used as best practice guides. There is a great AVweb article The Savvy Aviator #13: Putting Compression In Context that has more in depth analysis.


What is the difference between ground effect for an infinite wing and for a finite wing?

Ground-effect analysis is generally completed using the method of images, where we imagine an opposite geometry providing a flow that interacts with the original geometry.

The 2D case

Katz and Plotkin have a good explanation; here is their diagram for the 2D case:

For a flat-plate airfoil, the math works out to
\begin{equation}
C_\mathrm{l} =\frac{\pi  \alpha  \left(c^2-8 \alpha  c h+16 h^2\right)}{2 h (4 h-\alpha  c)}
\end{equation}
where $c$ is the chord, $\alpha$ is the angle of attack, and $h$ is the height above the ground.

The 3D case

In the 3D case, we model the entire wing (or aircraft) as mirrored. The math here gets quite a bit more complicated, but I can summarize Phillips and Hunsaker, who provide the following figure and equation:

For an untapered flat wing with small $\alpha$, the lift coefficient in ground effect $C_\mathrm{L_G}$ is about
\begin{equation}
C_\mathrm{L_G}\approx\left(\left(\frac{261.562}{\left(\frac{b}{c}\right)^{0.882}}-\frac{1.944}{\left(\frac{b}{c}\right)^{0.165}}\right) \left(\frac{h}{b}\right)^{0.787} e^{-9.14 \left(\frac{h}{b}\right)^{0.327}}+1\right)C_\mathrm{L}
\end{equation}
where $C_\mathrm{L}$ is the lift coefficient in the absence of ground effect, $h$ is the height above the ground, $c$ is the chord, and $b$ is the wingspan.

Comparing the two

I plotted the 2D equation for an airfoil with $c=1$ along with the 3D equation with $C_\mathrm{l}=C_\mathrm{L}$ for a wing with $c=1$ and $b=10$ (for any reasonable aspect ratio, this trend is similar):

As expected, far away from the ground and very close to it the 2D and 3D cases converge. The 3D case gets a bit more help from the ground in the intermediate range. Because we have only considered inviscid flows in the mathematical analysis, we know that the induced drag is the major difference between the 2D and 3D cases.


Why are most weapons in AC-130 on the left-hand side?

It is not designed to strafe, it is designed to loiter over a target or, to apply suppression fire or support fire for a sustained period of time.

It often circles a target area and a counter clockwise orbit (circling to the left) makes sense since the captain sits on the left and can keep the area in view at all times.

Therefore, the weapons are mounted on the left.


Rib is load tested upside down with weights on its flat bottom side to simulate air deflection during flight,and two ropes holding the rib up at positions where the spars will sit.


What does the SOG part of this METAR mean?

The RMK section:
AO1 = an observation
taken by equipment without a precipitation type discriminator (rain/snow)
SOG 32 = snow on ground, thickness 32 cm
6023 = beats me, I'll keep looking...
SLP176 = sea level pressure: what the current barometric pressure at measurement location would be at sea level, 176 stands for 1017.6 hPa (10 is omitted as persistent)
T10471068 =temperature and dewpoint in two four digit groups: first 0 or 1 for above or below zero degrees, then three digits for tenths of Celcius grades, so in the case above temperature is -4.7 deg C, dewpoint is -6.8 deg C


How is a lead acid battery and a Ni-Cd battery charged on an aircraft?

Airliners like the Airbus A320 use Nickel Cadmium batteries; in case of A320, a pair of main batteries with 23 Ah capacity is used.

While on the aircraft, the batteries are charged using a constant voltage method. As this article notes:

  ... are "constant-voltage chargers" that apply a fixed voltage to the battery as it charges, similar to what the aircraft electrical system does in flight. With a constant-voltage charger, the charging current starts out relatively high and gradually tapers down toward zero as the battery becomes fully charged.

FAA Aircraft Electrical System Handbook also notes:

  The battery charging system in an airplane is of the constant
  voltage type.


Would the front rudder of the Wright brothers really have helped in recovering the glider from a stall?

No, it would not have helped by itself.

What could prevent a stall or recover the aircraft from an incipient stall would be a counteracting movement of that canard surface, but that would need to be commanded by the pilot. It makes little difference if that control surface is ahead or behind the wing as long as it has enough lift potential left to produce the desired pitch down moment.

The way we understand stability today is that the aircraft will correct a deviation from the desired pitch attitude without pilot intervention. Using this definition, the Wright Flyers were unstable. Only by active and permanent corrections could straight flight be maintained. This is rather stressful and makes longer flights very tiring. Flying faster also can be ruled out because the speed of deviations goes up with flight speed, such that the human reaction time delays corrections too much and can even make them counterproductive.

The Wrights were correct that an unloaded surface gives them the highest control authority and, when moved in a correct and timely manner, helps to prevent a stall. However, placing that surface ahead of the wing comes with several disadvantages, loss of stability being only one of several. On the other hand, placing that lightly loaded surface at the rear results in a stable aircraft with better performance.


If you go to www.skyvector.com and zoom in to MRB
(easy to enter a flight from KOKV to KMRB to find it quick)

Select the World VFR view, and you will see no navaids at that location.
Select the Enroute L-29 view, and you will see no navaids at that location.

So no, it is not a type of currently used navigation aid.

If you peruse the Instrument approaches for KMRB at Airnav.com  https://www.airnav.com/airport/KMRB

you will also not see that structure mentioned.

Airport phone numbers are listed, you could call up and ask what it is.


Must an instrument approach plate be visible during the approach?

If you can't find any language for that sort of thing there probably isn't any.  At some point the value of regulations ends and you have to let common sense and good airmanship prevail.


Memorization.  This is the FAA definition:

Vis / RVR
1 SM / 5000
3/4 SM / 4000
1/2 SM / 2400
1/4 SM / 1600

This and a few more are spelled out in the Airman's Information Manual, TBL 5−4−1, RVR Value Conversions


What is the maximum number of infants allowed on a flight?

Since the question was only referring to infants, my answer treats them as a subgroup of SCPs (Special Categories of Passengers). SCPs are Persons with Reduced Mobility (PRMs), infants and unaccompanied children, deportees, inadmissible passengers, or prisoners in custody.
EASA's CRD containing the comments received to NPA 2014-01 has the following statement regarding SCP in chapter 2.5:

The Agency’s NPA did not establish a maximum number of SCPs on board an aircraft. The NPA proposed to clarify the existing AMC (AMC1 CAT.OP.MPA.155(b)) relating to the operator’s procedures that the number and subcategories of SCPs should not exceed the number of passengers capable of assisting them in case of an emergency.
In addition, the NPA proposed a new GM (GM4CAT.OP.MPA.155(b)) that clarifies  that  those passengers, who could assist the SCPs in case of an emergency, should not have another responsibility on board.

It's the closest I could find on this topic. Read the whole NPA, as it is a generally interesting read on SCP considerations (for example if using the word passengers or persons is more adequate to refer to the individuals on a plane).
Essentially, the CRD goes on stating that:

The  AMC  referring to operators’ procedures to establish the maximum number of SCPs has already been adopted by the Agency in 2012 and is based on the JAA Guidance Material.
Following the recommendations from the TÜV Rheinland study, the Agency does not propose any additional limits.
This  CRD  proposes a new  GM  (GM4  CAT.OP.MPA.155(b))stating that  a passenger capable  of assisting in case of an emergency means a passenger who is not an SCP and has no other role or private  responsibility  that  would  prevent  him/her  from  assisting  the SCP.  For  example,  an  adult travelling alone has no other role or private responsibility, unlike a family travelling with children.

In the US, I would assume that due to legal constraints originating on discriminatory aspects, there is also no hard limit as to how many infants are allowed on a flight, and the decision is left with the airline association and the NAA, with existing physical and safety constraints.


How the pitch of a blade must change along the length so that the blade does not stall?

The question should really read, "how the pitch must change along the length to keep lift distribution reasonably constant while respecting stall considerations" or something like that.  Rotor blades twist nose down going outboard, like propellers but with less twist, to account for the higher velocity toward the tip, so that the slower moving root end is doing a decent amount of work making lift.
This tends to make the root end operate closer to stall AOA than the tips.  Whether it actually stalls or not is a more complex function of blade forward velocity (rpm) and vertical circulation velocity down through the rotor disc, whether forward airspeed is involved, whether you want to optimize for hovering or forward flight, and so on.
You're going to have to do some reading.  I found an ASE post from a couple of years ago by @Koyovis, relating to a question about blade twist, that links to a great information source, a book at Google Books that you can read online, called  Helicopter Aerodynamics Vol. 1 by Ray Prouty, who wrote a column for Rotor & Wing for many years.  You should be able to assemble the information you need from that.


Why don't commercial aircraft adopt a slightly more seaplane-like design to allow safer ditching in case of emergency?

Because ditching is extremely, extremely rare, so the costs of redesigning aircraft along with the extra drag and weight (increased fuel burn) it would no doubt add to the airframe far outweighs the potential benefits.

That argument might sound weird to someone, but think about it this way: would it seem reasonable to redesign every single car on the planet so that they can float, just because every year a few people around the world accidentally drive their car into a lake?

A quick search on The Aviation Herald suggests that, since 2005, a total of four flights worldwide have been forced to land on water. To give you an idea of the scale, there are about 100,000 commercial flights worldwide daily. So that's something like 4 flights out of half a billion, which is practically nothing. Also, in those 4 accidents, more than half of the people onboard actually survived. Actually, the increased greenhouse gas emissions caused by adding extra weight, such as floats, to aircraft worldwide would probably kill many more people than such safety equipment would ever save.

It's not that the aviation industry isn't investing heavily to improve safety. It's just that making ditching safer is an area where there is almost nothing to be gained. So the money can be much better spent elsewhere.


Why did this delta wing generate a lower Cl at a higher wind tunnel velocity?

The increase in the lift curve slope at low angle of attack is quite normal. At 10° you see the effect of vortex lift kicking in. Yes, that is specific to delta wings.

Attached and vortex lift over angle of attack (picture source). This is taken from the excellent Cambridge Core article on vortex lift and these particular diagrams are from E.C. Polhamus: A concept of the vortex lift of sharp-edged delta wings based on a leading-edge suction analogy, NASA TN D-3767, December 1966. They show lift over angle of attack, but lift over drag would look quite similar, also with a kink once vortex lift sets in.

The offset between the two speeds is harder to explain. This looks like an error in your angle measurement – there must have been different offsets between the two test runs. That there is an offset is obvious from the nonzero lift at zero angle of attack. Unless your delta wing had lots of camber, there should not be much lift at zero angle of attack.


What is this three-bladed vintage aircraft?

That is a North American T-28 Trojan, a postwar era military aircraft used for basic flight training an the Navy and Marine Corps.

They still remain popular with warbird collectors and enthusiasts.


What is the purpose of these odd "control surfaces" on the BAE Taranis?

They look like spoilers but are more precisely called drag rudders. They are not meant to reduce lift like regular spoilers, but increase drag in order to create a yawing moment.

Regular aircraft have a vertical tail for yaw control. Without this, you need some force acting at a lateral distance from the center of gravity for yaw control, and this is what these speed brakes do. They are opened only on one side, and keeping both slightly open increases control power because then one can be closed while the other can be opened further.

The Northrop B-2 uses split ailerons to the same effect, and the Horten IX had a two-stage wooden speedbrake which could be extended near the wingtips. By raising only the smaller brake with small rudder pedal movements, yaw control could be tailored to flight speed. At high speed only small input was needed, and at low speed both brakes could be fully opened for maximum yaw control.

Horten IX V2 3-side view (picture source). Note the two extended speed brakes near the wingtip in the front view. The real speed brakes were on the lower side of the center wing and right behind the rear landing gear bay (in the drawing called spoilers).

Thanks to @RedGrittyBrick for the hint: Yes, the X-47A and X-47B also seem to have drag rudders. I guess their large chordwise dimension helps to limit the lift loss when they are extended.

Northrop-Grumman X-47B in flight (picture source)


Will Tower repeat an initial heading if it's part of my IFR clearance?

Yes, the FAA can and will take action in that scenario. I know because it happened to me :-)

A couple of years ago I was departing IFR from KPDK and my clearance included a right turn after departure. It was a busy and very hot day and I had to sit and hold for release (in a C182 in 100F heat...), so when Tower offered to get me out if I could depart immediately, I jumped at it. Tower cleared me for immediate takeoff without repeating the right turn instruction and I forgot about it in the rush.

I climbed out on runway heading and I started thinking that Tower was waiting a long time to turn me over to Departure. When they did, Departure told me to turn right immediately but I had to say "unable" because I now had obstacles to my right. I was vectored back on course and Departure asked me twice what the Tower had cleared me for, so it was obvious that something had gone wrong although they didn't give me a "possible pilot deviation" warning. Fortunately I was never close to busting the ATL Bravo.

I got home, listened to LiveATC, realized that I'd forgotten the right turn instruction, filed an ASRS report and forgot about it. 89 days later the FAA called to investigate (they have 90 days to act) because ATL Approach had reported it as a "course clearance deviation". Long story short, after a couple of calls and emails they dropped the whole thing without any record because they said it was a trivial incident and I'd shown a positive and "compliant" attitude with them. Although I have to imagine that if I'd busted the ATL Bravo or caused a loss of separation, the outcome might well have been different.

I don't know whether Tower is required to repeat the initial heading instruction (it may be in the ATC Orders), but my experience certainly shows that you can't rely on it. I commented in my ASRS report that if Tower had repeated the instruction it could have helped to prevent the whole issue.


Do any regulatory bodies require ISO 9001 QMS certification from airlines or service providers?

No, there are no civil aviation authorities that require QMS implementation.

ICAO documentation (Document 9859) discusses the integration of other systems, such as quality and environmental management systems. There are advantages of integrations, such as facilitating data management and reducing redundancies.

For example, a QMS has many of the same documentation requirements as an SMS, so an integrated system is logical. However, not all operator benefit from an QMS, especially the smaller ones.


How can I estimate wind speed and direction in VFR?

I used to do this when I was flying in the bush on floats, where there is very little time for planning and you are in a flying environment where you are living by your wits (which is what made it such great fun).
The only thing that really works to any (limited) degree, just by observation in the moment, is to look for bodies of water and observe the telltale signs of the surface winds.  Wave or wavelet size, streamer markings, lee side shoreline wind shadows. Or better, smoke columns from fires.
The wind typically veers (rotates clockwise) with altitude, so if you see a lake or someone burning something that suggests the wind is blowing on it from the west, with signs that suggest it's blowing at, say, 10kts, you can expect it to be more northwesterly the higher you go, and faster.  So if you guess the wind is 10kts and 270 at the surface, you can expect it to be maybe 300-ish and 20-30 kts at, say, 4000 ft.
Those are just signs that can help you make a better wild guess.  In the absence of any ground signs, the only other thing you can do is hold a constant heading for a couple of minutes on a timed run between two points, and note the difference between heading and track, and difference between ground speed and TAS.  Ground speed same as TAS, wind is at 90 degrees to your course (or velocity is zero), less than TAS it's forward, more it's behind. Ground speed less than TAS and significant crab angle to the left, the wind is quartering left/front.  Lots of crab and minor GS penalty, closer to 90 deg off your left, less crab and more GS penalty, closer to straight ahead.
With a bit of practice you should be able to make an educated guess on direction within maybe +/- 30 degrees and velocity +/- 15 kts.  With everybody using moving maps these days, these are skills that are becoming long lost.


Which fonts were used in old (1930s) aircraft cockpit displays and instruments?

I've seen a few times that even hand-written markings were used. This suggests that there hardly was any consistency.

  Photo from Darwin Aviation Museum:


Should you blow through the final approach course if the ATC forgot about you?

Yes, blow right through it.  You should never deviate from a clearance to "take a beating" later simply because you think you know better than the controller.  If the controller is busy it is for a reason, and it is very possible they are extending you on purpose for spacing.  You could very well conflict with traffic even more by taking your own turn.  It is far better to let them fix the overshoot they caused than to battle the FAA over a flight violation.

If you genuinely think the controller may have forgotten about you, you should continue to attempt contact, while pressing IDENT with each transmission.  If it is so busy that you can't get a word in edgewise, press IDENT anyway.  I have done this and received an immediate turn without even having to transmit on the radio.  It is designed to get their attention, just don't overuse it.

The only exceptions where you should deviate from an assigned vector are if you are lost comm or have an emergency.  In either case make sure you are squawking appropriately.

I would consider a vector towards terrain a pending emergency if not turned within some appropriate period of time.  When I was instructing in the Navy, during simulator rides I would force a missed approach at McChord AFB, give a climb to 5000' and a left turn towards Mt Rainier.  I would then fail the crew's radios.  It was always a good lesson in terrain awareness, especially for young aviators from flat states.


How do you maximize an electric engine's thrust?

Generally, if you want to optimize for area/diameter (without the complexity and weight of the many/multi stage option mentioned in comments to the question), you'll want to use a ducted fan (roughly similar to the fan on a turbofan).  Not only do you avoid tip losses due to the end plate effect of the duct (if the clearance isn't excessive), you can run the fan at arbitrarily high speed, even with supersonic tips, without the extreme drag and noise production of an open propeller in such a regime.

Look at the design of EDF (Electric Ducted Fan) engines used to simulate jets in small radio control models; some of these can product thrust comparable to fans driven by high performance methanol/nitromethane glow engines (15-25 kW/L displacement, with 10 cc, 1.5 kW engines driving fans 15 cm diameter), with 10-20 minute flight duration on a charge with light enough batteries to fly well.  Scaling these up to 500 mm diameter will generally require shortening the blade chord relative to diameter, and you'll need to optimize pitch for the combination of rotation speed and torque available.


Why are movable rocket thrusters not used in airplanes?

Any time the question is "why don't we have...", the answer is almost certainly in the trade-off between weight, maintenance cost, fuel cost, & utility. If it weighs a lot, needs maintenance, burns fuel, and will only be useful once a decade, then it just is not productive.

Putting rocket thrusters on a plane will be heavy, meaning fewer passengers, or less fuel or cargo on board.   All that extra weight has to be flown around, requiring fuel.  They would need regular maintenance checks, which is another cost.   And they would only be useful very, very infrequently (on most planes... never).

So, safety issues completely aside*, it is just impossible to justify adding new equipment for a once-in-a-lifetime scenario, which can be better managed with good maintenance, good training, and good planning.

This same line of reasoning goes for "why don't more airplanes have parachutes", "why don't planes have anti-missile systems", "why don't planes have airbags", and many other random items.

*Safety issues are a big deal with rockets.  Rockets are a controlled explosion.  What happens if that explosion is not controlled?  What happens if the rockets are triggered by accident, or blow up all at once?  Even if rockets passed all the tests of utility vs. costs, they would still increase risks to the plane, rather than decreasing it.


Birds have integrated active control systems that rely on several sensors to adjust the control surfaces to stabilize the flight. Since it is very difficult (perhaps impossible) to study the aerodynamic properties of the bird independently from its control system it is hard to make any claims on the static or dynamic stability of a bird.

What can be said is that from a control system point of view, most birds are observable, controllable and stabilizable in flight. This does not apply to the class of flightless birds.

If you are interested in the similarities in aerodynamics between insects, birds and aircraft I recommend getting a copy of "The Simple Science of Flight; from insects to jumbo jets" by Henk Tennekes, an  Emeritus Professor of Aerospace Engineering at Pennsylvania State University.

After sitting in the garden on a sunny afternoon, observing various bird species and tinkering a bit more on the subject I actually think that many birds are unstable. Especially the smaller species that fly in the garden through bushes and trees, but also those that hunt for flying insects, are capable of extreme rapid changes in direction of flight; they are very agile.

Stability and extreme manoeuvrability are difficult to combine. In fact modern jet fights are designed to be unstable to improve their manoeuvrability. They then need a computerized control system to stabilize them.

An online search showed that some research has been done on the field of flight stability of birds Petter Krus of the Linköping University in Sweden has published an article on Natural methods for Flight Stability in Birds.

Abstract:

  There is a stark contrast between the behaviour of flying man-made
  artifacts and the effortless elegance of bird flight. In this paper,
  mechanisms are proposed that seems to be at least related to the way
  birds fly when they are gliding (the no propulsion case). It is
  recognized that biological structures and actuator systems (muscles)
  are characterized by a great deal of compliance that produce a
  completely different dynamic behaviour of flight. In this paper it is
  demonstrated how this can be applied on a geometrically unstable
  bird-like configuration. The result is that geometrically unstable
  configurations can be stabilised without the need of rate gyros and/or
  accelerometers. Although the bird might have a fast acting control
  system for flight control, it is unlikely that it is fast enough for
  active stabilisation of the short period oscillation, and this is
  unnecessary with the model described here, which only requires
  stabilisation of the long period oscillation.

In his introduction he also briefly refers to the work of Tennekes, which does not include much about the stability of birds. Also in the introduction he observes that birds have a cambered wing and must have an aft centre of gravity. This combination makes the bird unstable (if it were a rigid body).