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