A tendency for the retreating blade to stall in forward flight is inherent in all present day helicopters and is a major factor in limiting their forward speed. Just as the stall of an airplane wing limits the low speed possibilities of the airplane, the stall of a rotor blade limits the high speed potential of a helicopter. The airspeed of the retreating blade (the blade moving away from the direction of flight) slows down as forward speed increases. The retreating blade must, however, produce an amount of lift equal to that of the advancing blade. Therefore, as the airspeed of the retreating blade decreases with forward aircraft speed, the blade angle of attack must be increased to equalize lift throughout the rotor disk area. As this angle increase is continued, the blade will stall at some high forward speed.

As forward airspeed increases, the "no lift" areas move left of centre, covering more of the retreating blade sectors:

This requires more lift at the outer retreating blade portions to compensate for the loss of lift of the inboard retreating sections. In the area of reversed flow, the rotational velocity of this blade section is slower than the aircraft airspeed; therefore, the air flows from the trailing to leading edge of the airfoil. In the negative stall area, the rotational velocity of the airfoil is faster than the aircraft airspeed, therefore air flows from leading to trailing edge of the blade. However due to the relative arm and induced flow, blade flapping is not sufficient to produce a positive angle of attack. Blade flapping and rotational velocity in the negative lift area are sufficient to produce a positive angle of attack, but not to a degree that produces appreciable lift.

This figure shows a rotor disk that has reached a stall condition on the retreating side:

It is assumed that the stall angle of attack for this rotor system is 14 degrees. Distribution of angle of attack along the blade is shown at eight positions in the rotor disk. Although the blades are twisted and have less pitch at the tip than at the root, angle of attack is higher at the tip because of induced airflow.

Upon entry into blade stall, the first effect is generally a noticeable vibration of the helicopter. This is followed by a rolling tendency and a tendency for the nose to pitch up. The tendency to pitch up may be relatively insignificant for helicopters with semirigid rotor systems due to pendular action. If the cyclic stick is held forward and collective pitch is not reduced or is increased, this condition becomes aggravated; the vibration greatly increases, and control may be lost. By being familiar with the conditions which lead to blade stall, the pilot should realize when his is flying under such circumstances and should take corrective action.

The major warnings of approaching retreating blade stall conditions are:

• Abnormal vibration
• Pitchup of the nose
• Tendency for the helicopter to roll in the direction of the stalled side.

When operating at high forward airspeeds, the following conditions are most likely to produce blade stall:

• High blade loading (high gross weight)
• Low rotor RPM
• High density altitude
• Steep or abrupt turns
• Turbulent air

When flight conditions are such that blade stall is likely, extreme caution should be exercised when manoeuvring. An abrupt manoeuvre such as a steep turn or pullup may result in dangerously severe blade stall. Aircraft control and structural limitations of the helicopter would be threatened.

Blade stall normally occurs when airspeed is high. To prevent blade stall, the pilot must fly slower than normal when:

• The density altitude is much higher than standard
• Carrying maximum weight loads
• Flying high drag configurations such as floats, external stores, weapons, speakers, floodlights, sling loads, etc.
• The air is turbulent

When the pilot suspects blade stall, he can possibly prevent it from occurring by sequentially:

• Reducing power (collective pitch)
• Reducing airspeed
• Reducing "G" loads during manoeuvring
• Increasing RPM to upper allowable limit
• Checking pedal trim

In severe blade stall, the pilot loses control. The helicopter will pitch up violently and roll to the left. The only corrective action then is to accomplish procedures as indicated previously to shorten the duration of the stall and regain control.