there is no unbalance of pitching, yawing, or 
rolling moment. If the airplane traverses a 
sharp wind shear equivalent to a horizontal 
gust, the resulting change in airspeed will 
disturb such an equilibrium. For example, if 
the airplane encounrers a sharp horizontal 
gust which reduces the airspeed 20percent,the 
new airspeed (80 percent of the original value) 
produces lift and drag at the same angle of 
attack which are 64 percent of the original 
value. The change in these aerodynamic forces 
would cause the airplane to accelerate in the 
direction of resultant unbalance of force. 
That is, the airplane would accelerate down 
and forward until a new equilibrium is 
achieved. In addition, there would be a 
change in pitching moment which would 
produce a response of the airplane in pitch. 
The response of the airplane to a horizontal 
gust will differ according to the gust gradient 
and airplane characccristics. Gmcrally, if the 
airplane encounters a sharp wind shear which 
reduces the airspeed, the airplane tends to sink 
and incur a loss of altitude ‘before equilibrium 
conditions are achieved. Similarly, if the 
airplane encounters a sharp wind shear which 
increases the airspeed, the airplane tends to 
float and incur a gain of altitude before equilib- 
rium conditions are achieved. 
Significant vertical and horizontal gusts may 
be due to the terrain or atmospheric conditions. 
The proximity’of an unstable front or thunder- 
storm activity’in the vicinity of the airfield is 
likely to create significant wind shear and gust 
activity at low altitude. During gusty condi- 
tions every effort must be made for precise con- 
trol of airspeed and flight path and any changes 
due to gusts must be corrected by proper con- 
trol action. Under extreme gusts conditions, 
it may be advisable to utilize approach, land- 
ing, and takeoff speeds slightly greater than 
normal to provide margin for adequate control. 
POWER-OFF GLIDE PERFORMANCE 
The gliding performance of an airplane is of 
special interest for the single-engine airplane 
NAVWEPS O&ROT-80 
APPLICATION OF AERODYNAMICS 
TO SPECIFIC PROBLEMS OF FLYING 
in the case of powerplant failure or malfunc- 
tion. When a powerplant failure or malfunc- 
tion occurs, it is usually of interest to obtain a 
gliding flight path which results in the mini- 
mum glide angle. The minimum glide angle 
will produce the greatest proportion of glide 
distance to altitude loss and will result in 
maximum glide range or minimum expendi- 
ture of altitude for a specific glide distance. 
GLIDE ANGLE AND LIFT-DRAG RATIO. 
In the study of climb performance, the forces 
acting on the airplane in a steady climb (or 
glide) produce the following relationship: 
where 
Y=: angle of climb, degrees 
T-thrust, lbs. 
D-drag, lbs. 
W=: lbs. 
In the case of power-off glide performance, the 
thrust, T, is zero and the relationship reduces 
to: 
D sin y= -- W 
By this relationship it is evident that the mini- 
mum angle of glide-or minimum negative 
climb angle-is obtained at the aerodynamic 
conditions which incur the minimum total 
drag. Since the airplane lift is essentially equal 
to the weight, the minimum angle of glide 
will be obtained when the airplane is operated 
at maximum lift-drag ratio, (L/D)ma,. When 
the angle of glide is relatively small, the ratio 
of glide distance to glide altitude is numeri- 
cally equal to the airplane lift-drag ratio. 
glide ratio= glide distance, ft. 
glide altitude, ft. 
glide ratio = (L/D) 
Figure 6.6 illustrates the forces acting on the 
airplane in a power-off glide. The equilibrium 
of the steady glide is obtained when the sum- 
mation of forces in the vertical and horizontal 
directions is equal to zeta. 
369