Retreating blade stall is a hazardous and damaging flight condition in helicopters and other rotary wing aircraft, where the rotor blade on the retreating side of the rotor disc in forward flight and therefore with the smaller resultant relative wind exceeds the critical angle of attack. Retreating blade stall is one of the primary limiting factors in a helicopter's airspeed, and the reason even the fastest helicopters can only fly slightly faster than 200 knots (about 370 km/h) though various changes can be made to conventional helicopters to try to overcome this limit such as streamlining, lifting surfaces and secondary forward propulsion.
Aerodynamics of Retreating Blade Stall
In forward flight, the relative airflow through the main rotor disc is different on the advancing and retreating side. The relative airflow over the advancing side is higher due to the forward speed of the helicopter, while the relative airflow on the retreating side is lower. The rotor blades incorporate the ability to flap and this allows the advancing blade to flap up due to the increasing relative airflow across the blade as the blade moves forward from the 6 o’clock position. However, the upwards movement of the blade reduces the angle of attack giving constant thrust from the blade. As the blade reaches the 12 o’clock position, the blade is at its highest point. As it continues to rotate it experiences progressively less relative airflow and flaps down. This increases the angle of attack, again balancing the thrust. The blade ends the rotation at the 6 o’clock position at its lowest point. The blade will experience the lowest relative airflow at the point on the retreating side perpendicular to the airflow. With low airflow and high angle of attack, this area (at the 9 o’clock position on counter-clockwise rotors) will stall first as aircraft speed increases.
Due to differences in blade tolerances the stall may occur on one blade before the others giving an increase in 1R (once per rotation) vibration and a visible track split. As other blades stall a nR vibration will build (where n is the number of rotor blades). This vibration can be very damaging as it can impose high torsional loads and set off other harmonic vibrations in dynamic components.
Retreating blade stall is a major factor in limiting a helicopter’s top forward speed (Vne). High weight, low rotor r.p.m., high density altitude, turbulence and/or steep, abrupt turns are all conducive to retreating blade stall at high forward airspeeds as they increase the blade pitch to generate more thrust and hence increase the angle of attack. In particular, as altitude is increased, higher blade angles are required to maintain lift at a given airspeed. Thus, retreating blade stall is encountered at a lower forward airspeed at altitude. Most manufacturers publish charts and graphs showing a Vne decrease with altitude.
Retreating Blade Stall
Recognising Retreating Blade Stall
- First indication of impending stall is roughness and a low frequency vibration in the main rotor. This might be in one blade of the disc at first due to differences in design tolerances of each blade.
- If allowed to progress, the retreating blade side of the rotor will develop a stalled condition causing the nose of the helicopter to pitch up and roll into the retreating blade. The amount of pitch to roll is dependent on several elements including the type of rotor and density altitude.
- Reducing collective pitch. This reduces the angle of attack in the plane of rotation and should be done first.
- Gently Applying aft cyclic to slow the aircraft. Application of aft cyclic without a reduction in collective pitch will only worsen the stall. An attempt to roll away from the stall can also make the situation worse as it places even higher demand on the stalled blade.
Pilots should maintain an awareness of Vne taking into account the expected altitudes in flight, potential turbulence, and the weight of the aircraft.
Warning: Sustained flight in retreating blade stall conditions may cause reduction in the fatigue life of components through vibrations.
Developed in the 1980’s, the BERP (British Experimental Rotor Programme) rotor blade design has, by design, altered the area of effect of vortices and delayed the onset of retreating blade stall, allowing greater Vne and other performance improvements.
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