NAVWEPS OD-80140 
APPLICATION OF AERODYNAMICS 
TO SPECIFIC PROBLEMS OF FLYING 
blade must operate at a greater angle of attack. 
If the blade pitch is increased or the forward 
speed increased the stalled portion of the rotor 
disc becomes larger with the stall progressing 
in toward the hub from the tip of the retreating 
blade. When approximately 15 percent of 
the rotor disc is stalled, control of the heli- 
copter will be impossible. Flight tests have 
determined that control becomes marginal and 
the stall is considered severe when the outer 
one-quarter of the retreating blade is stalled. 
Retreating blade stall can be recognized by 
rotor toughness, erratic stick forces, a vibration 
and stick shake with a frequency determined 
by the number of blades and the rotor speed. 
Each of the blades of a three-bladed rotor will 
stall as it passes through the stall region and 
create a vibration with three beats pet rotor 
revolution. Other evidence of retreating blade 
stall is partial or complete loss of control or a 
pitch-up tendency which can be uncontrollable 
if the stall is severe. 
Conditions favorable for the occurrence of 
retreating blade stall are those conditions that 
result in high retreating blade angles of attack. 
Each of the following conditions results in a 
higher angle of attack on the retreating blade 
and may contribute to retreating blade stall: 
1. High airspeed 
2. Low totor RPM-operation at low 
rotor RPM necessitates the use of 
higher blade pitch to get a given 
thrust from the rotor, thus a higher 
angle of attack 
3. High gross weight 
4. High density altitude 
5. Accelerated flight, high load factor 
6. Flight through turbulent air or gusts- 
sharp updrafts result in temporary 
increase in blade angle of attack 
7. Excessive ot abrupt control deflections 
during maneuvers 
Recovery from a stalled condition can be 
effected only by decreasing the blade angle of 
attack below the stall angle. This can be 
accomplished by one or a combination of the 
following items depending on severity of the 
stall: 
1. Decrease collective pitch 
2. Decrease airspeed 
3. Increase rotor RPM 
4. Decrease severity of accelerated ma- 
newer or control deflection 
If the stall is severe enough to result in pitch-up, 
forward cyclic to attempt to control pitch-up 
is ineffective and may aggravate the stall since 
forward cyclic results in an increase in blade 
angle of attack on the retreating blade. The 
helicopter will automatically recover from 
a severe stall since the airspeed is decreased in 
the nose high attitude but recovery can be 
assisted by gradual reduction in collective 
pitch, increasing RPM, and leveling the heli- 
copter with pedal and cyclic stick. 
From the previous discussion, it is apparent 
that there is some degree of retreating blade 
stall even at moderate airspeeds. However, 
the helicopter is able to perform satisfactorily 
until a sufficiently large area of the rotor disc 
is stalled. Adequate warning of the impend- 
ing stall is present when the stall condition 
is approached slowly. There is inadequate 
warni,ng of the stall only when the blade pitch 
or blade angle of attack is increased rapidly. 
Therefore, unintentional severe stall is most 
likely to occur during abrupt control motions 
or rapid accelerated maneuvers. 
COMPRESSIBILITY EFFECTS. The highest 
relative velocities occur at the tip of the ad- 
vancing blade since the speed of the helicopter 
is added to the speed due to rotation at this 
point. When the Mach number of the tip 
section of the advancing blade exceeds the 
critical Mach number for the rotor blade sec- 
tion, compressibility effects result. The criti- 
cal Mach number is reduced by thick, highly 
cambered airfoils and critical Mach number 
decreases with increased lift coefficient. Most 
helicopter blades have symmetrical sections 
and therefore have relatively high critical 
Mach numbers at low lift coefhcients. Since