NAVWEPS oo-80~-80 
APPLICATION OF AERODYNAMICS 
TO SPECIFIC PROBLEMS OF FLYl,NG 
If the rotor could operate within a wide 
range of rotor speed, the efficiency and per- 
formance could be improved. 
With the previous relationships established 
for the rotor in hovering flight, the effect of 
forward flight br rotor translation can be con- 
sidered. With forward flight, a third velocity 
component, that tif the forward velocity of the 
helicopter, must be considered in determining 
the relative wind acting on each rotor blade 
element. Since the entire rotor moves with the 
helicopter, the velocity of air passing over 
each of the elements on the advancfng blade is 
increased by the forward speed of the heli- 
copter and the velocity of the air passing over 
each element of the retreating blade is de- 
creased by the same amount. This is shown in 
figure 6.16. 
If the blade angles of attack on both advanc- 
ing and retreating blades remained the same 
as in hovering flight, the higher velocity on 
the advancing blade would cause a dissym- 
metry of lift and the helicopter would tend to 
roll to the left. It was this effect that created 
great difficu!ty during many early helicopter 
and autogiro projects. Juan De La Cierva was 
the first to realize what caused this effect and 
he solved the problem by mounting his auto- 
giro blades individually on flapping hinges, 
thus allowing a flapping action to automati- 
cally correct the dissymmetry of lift that re- 
sulted from forward flight. This is the method 
still used in an articulated rotor system today. 
The see-saw, or semi-rigid, rotor corrects the 
lift dissymmetry by rocking the entire hub and 
blades about a gimbal joint, By rocking the 
entire rotor system forward, the angle of at- 
tack on the advancing blade is reduced and 
the angle of attack on the retreating blade is 
increased. The rigid rotor must produce cyclic 
variation of the blade pitch mechanically as 
the blade rotates to eliminate the lift dissym- 
metry. Irrespective of the method used to 
correct the dissymmetry of lift, identical aero- 
dynamic characteristics result. Thus, what is 
said about rotor aerodynamics is equally valid 
for all types of rotor systems. 
By analyzing the velocity components acting 
on the rotor blade sections from the blade 
root to the tip on both advancing and retreat- 
ing blades, a large variation of blade section 
angle of attack is found. Figure 6.16 illus- 
trates a typical variation of the local blade 
angle of attack for various spanwise positions 
along the advancing and retreating blades of 
a rotor at high forward speed. There is a 
region of positive angles of attack resulting 
in positive lift over the entire advancing blade. 
Immediately next to the hub of the retreating 
blade there is an area of reversed flow where 
the velocity due to the forward motion of the 
helicopter is greater than the rearward velocity 
due to the blade rotation. The next area is a 
negative stall region where, although the 
flow is in the proper direction relative the 
blade, the angle of attack exceeds that for 
negative stall. Progressing out the retreating 
blade, the blade angle of attack becomes less 
negative, resulting in an area of negative lift. 
Then the blade angle becomes positive again, 
resulting in a positive lift region. The blade 
angle continues to increase, until near the tip 
of the retreating blade the positive stall angle 
of attack is exceeded, resulting in stalling of 
the tip section. This wide variation in blade 
section angles of attack results in a large 
variation in blade section lift and drag coefK- 
cients. The overall lift force on the left and 
right sides of the rotor disc are equalized by 
cyclically varying the blade pitch as explained 
previously, but the drag variation is not 
eliminated. This drag variation causes a 
shaking force on the rotor system and con- 
tributes to the vibration of the helicopter. 
RETREATING BLADE STALL. Retreat-