User Guide

3: GROUND SCHOOL
3.11
The other blade moves from the rear of the aircraft toward the front (from the 6
o’clock position to 3 o’clock). It is the advancing blade because it is traveling
“forward,” or in the same direction as the helicopter’s motion.
The advancing blade is moving in the same direction as the helicopter (forward),
so the speed of the air passing over the blade is the sum of these two speeds
(150 + 400 = 550 mph). The retreating blade is moving in the opposite direction
of the helicopter, so the speed of the air passing over it is the difference
between the speed of the advancing helicopter and the speed of the retreating
blade (400 - 150 = 250 mph).
Now, return to a basic principle of lift — more lift is created at higher airspeeds.
The speed of the advancing blade is 550 mph, while the speed of the retreating
blade is 250 mph. The advancing blade creates more lift, because it’s traveling
faster. This additional lift only occurs on the side where the blades are advanc-
ing. Thus, helicopters in the U.S. tend to veer up to the left. This phenomenon is
called
asymmetry of lift .
550130
= 550
Retreating
blade
Advancing
blade
150
400
150
400
650
250
150
400
= 250