There are a number of dangerous situations for helicopters and other aircraft that use rotors. One such situation is known as retreating blade stall. This is where the rotor blades traveling away from the direction of flight enter a stalled condition.
This is caused by a combination of fairly low airspeed, and/or the angle of attack being too large. Retreating blade stall is what limits the speed at which helicopters are able to fly, even the fastest helicopters can only fly at around 200 knots.
Rotor blades travel in a circular movement, and so some of them will be traveling in the direction of travel (advancing) and some will be moving away (retreating blade).
The majority of helicopter designers have overcome this problem somewhat by allowing a certain amount of flex in the blades.
The blades flex during the advance of the blades. This means that the angle of attack is much lower. The blades flex down when the blades retreat which increases the lift and angle of attack.
The cyclic pitch control can also be used to increase the pitch of the retreating blades and also decrease the pitch of the advancing blades. This make is possible to generate more lift and balance out the effects of advancing and retreating blades.
Failure
The compensations built into the modern helicopter designs they can only reduce the effects, not completely eradicate it. It’s possible for helicopters to move too fast, which means that the retreating blades do not provide enough lift. This results in retreating blade stall.
Performance
As a helicopter reaches the airspeed where retreating blade stall will occur, the helicopter will begin to shake and the noise will get louder. If the nose is pulled upwards the condition can be corrected as the helicopter will begin to slow down. If this condition is not corrected in time the helicopter is likely to roll over.
Causes
Retreating blade stall can occur much easier when one or more of the following conditions are met. The more conditions that are present the more likely that retreating blade stall will become a problem.
Things that can make retreating blade stall more of an issue include turbulent air, low speed RPM of the rotors, abrupt turning, or high airspeed.
Few of these factors are due to pilot error. It is particularly important that all helicopter pilots know how to correct this should it ever arise.
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