[FF-2] Retreating Blade Stall

Retreating Blade Stall. The speed of the airflow over the retreating blade (the blade moving away from the direction of flight) decreases as forward speed increases. The retreating blade, however, must produce an amount of lift equal to that of the advancing blade. Therefore, as the speed of the retreating blade decreases with forward 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 (exceed the critical angle-ofattack) at some high forward speed.

As forward speed increases, the no-lift areas move left of center, 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 the 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. In the negative lift area, blade flapping and rotational velocity are sufficient to produce a positive angle-of-attack, but not to a degree that produces appreciable lift.

Upon entry into blade stall, the first effect is generally a noticeable 4-per-rev vibration. This is
followed by a left rolling tendency and a tendency for the nose to pitch up. If the cyclic stick is held forward and collective pitch is increased (or not reduced) this condition becomes aggravated. The vibration greatly increases, and control will be lost.

In operations at high forward airspeeds, the following conditions are most likely to produce blade stall:
(1) High blade loading (high gross weight).
(2) High density altitude.
(3) Steep or abrupt turns.
(4) Turbulent air.

Warnings of approaching retreating blade stall are:
(1)Increase in 4-per-rev vibration level.
(2)Pitch-up of the nose.
(3)Tendency for the helicopter to roll left.

You need to take corrective action when blade stall is likely. Exercise extreme caution when
maneuvering. An abrupt maneuver such as a steep turn or pull-up may result in dangerously severe blade stall. Aircraft control and structural limitations of the helicopter would be threatened. Blade stall normally occurs at high airspeeds. Low rotor RPM can contribute to blade stall; therefore, increasing rotor RPM is a corrective action. This, however, is normally not a likely concern in the H-60 provided 100% Nr is maintained.

To prevent blade stall, the pilot should fly slower than normal when:
(1) Density altitude is high.
(2) Operating near maximum gross weight.
(3) Flying high-drag configurations (ESSS).
(4)The air is turbulent.

When blade stall is suspected, corrective action includes:
(1) Reducing power.
(2) Reducing airspeed.
(3) Reducing G-loading during maneuvering.
(4) Checking pedal trim.

In severe blade stall, the pilot loses control. The only corrective action, then, is to accomplish the procedures indicated previously, to shorten the duration of the stall and regain control.