Retreating blade stall
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Retreating blade stall is a hazardous flight condition in helicopters and other rotary wing aircraft, where the rotor blade rotating away from the direction of flight stalls. The stall is due to low relative airspeed and/or excessive angle of attack (or AOA).
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[edit] Advancing vs. retreating blades
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| retreating blade side | advancing blade side |
Since lift in a rotary-wing aircraft is generated by the movement of the airfoil blade rather than passive air movement over a fixed wing, balancing lift is important to the aircraft's stability. When stationary in X-axis movement, this isn't a problem; the blades of the rotor, regardless of their position in rotation, are moving at the same speed relative to the aircraft. Non-zero X-axis movement, such as forward flight, creates an inherent instability: as the rotor turns, those blades moving in the same direction as the aircraft have a higher relative speed than those moving in the opposite direction. The blade that is moving in the same direction as the craft is called the advancing blade, while the blade moving in the opposite direction is called the retreating blade.
One of the inviolable truths of aeronautics states that as airspeed above an airfoil increases, lift increases, and vice versa. Consequently, the advancing blade will experience a stronger lift effect than normal, and the retreating blade will experience a weaker effect. A fuller treatment is provided in dissymmetry of lift.
[edit] Compensation
Most helicopter designs compensate for this by incorporating a certain degree of "flap" in the blades. Rather than being rigid, the rotor blades are built to have a certain degree of flex. As such, the blade twists up during its advance, creating a smaller AOA and lower lift. When the blade retreats, the blade twists down, increasing the AOA and generating more lift.
Another system to compensate for the advancing/retreating blade effect is to automatically adjust the blade AOA depending upon its position in rotation. This system works well with blades that are more rigid and, thus, require more accurate positioning.
[edit] Failure
These compensations can only do so much, and it is possible for a rotary-wing aircraft to move so quickly that the retreating blade no longer moves fast enough relative to the aircraft to provide lift. This situation is called retreating blade stall. All airfoils have a stall speed, defined as the minimum speed at which the airfoil must move through the air to generate lift. Below this speed, slow-moving turbulent air replaces the fast-moving slip air going over the airfoil, disrupting the Bernoulli effect that generates lift. When a fixed-wing aircraft drops below its stall speed, the entire aircraft loses lift and enters a condition called a stall. The usual results of a fixed-wing stall are a sharp drop in aircraft attitude and a dive. Stalls in fixed-wing aircraft are often a recoverable event for an experienced pilot. In a retreating-blade stall, however, only half of the lift surface experiences a stall. The advancing blade continues to generate lift, but the retreating blade enters a stall condition. The resultant flight condition is extremely unstable.
[edit] Flight performance during a retreating blade stall
The aircraft reaction during a retreating blade stall event involves a rapid, violent upward pitch in the aircraft's attitude. Simultaneously, the aircraft will begin to roll towards the retreating blade as that side of the aircraft begins to "fall." Once the aircraft begins to both pitch and roll at high flight speed, control is rapidly lost. The aircraft can quickly invert, and continued progression of the pitch effect can leave the aircraft falling backwards and upside down. The uneven effect of the main rotor, coupled with the violent attitude changes of the aircraft, make recovery impossible in most cases. Generally, aircraft that survive a retreating blade stall event are commanded by an exceptional pilot and are at sufficient altitude that an attempt to stabilize the aircraft can be made before impact.
[edit] Causes of retreating blade stall
Retreating blade stall is more likely to occur when the following conditions exist at high forward airspeed:
- High gross weight
- Low rotor RPM relative to vehicular airspeed
- High density altitude
- Steep or abrupt turns, particularly those that place the retreating blade side on the concave face of the turn.
- Turbulent ambient air
[edit] References
Basic Helicopter Handbook, US Department of Transportation, Federal Aviation Administration
