Retreating Blade Stall Flashcards
What is retreating bladestall
When at a point the retreating blade can no longer compensate for the dissymmetry in lift and begins to stall starting at the tip.
When can an aerofoil enter retreating blade stall? (3)
- airspeed
- AoA too great
- Blade loading too great for given airspeed
Contributing factors to retr. Blade stall? 7
Airspeed Collective angle
RRPM low Load factors (handling rough/aggressive, gusts, turbulence)
High AUM Fwd cyclic position
High DA
Symptoms of retreating blade stall?
Cyclic vibration
Airframe vibration
Kicks in controls
Uncontrolled pitch nose up
Usually a roll to the retreating side
Recovery from retreating blade stall?
- Lower Collective lever ( reduces AoA)
- Aft cyclic reducing airspeed ( if done first inflow angle is reduced increasing AoA, if pushed fwd AoA increases further. Both aggravate the stall)
- Increase Throttle (Nr)
- Unload the disc, ensure stable flight
Which 2 things eliminate the dissymmetry of lift and how?
Cyclic and Flapping hinges work together to eliminate the dissymmetry of lift.
Cyclic by feathering the blades. Fwd cyclic the advancing side pitch is reduced lowering angle of attack, retreating side pitch increases, increasing AoA.
This is due to increased airflow over adv side vs retreating side. Airspeed has a significant effect on lift produced
What does retreating blade stall limit the most in terms of helicopter performance?
Forward speed
Airflow Reversal may occur….
At the root end of the retreating blade. It has the lowest rotational velocity and is also one of the limits to fwd speed.
3 main features which define maximum speed of a helicopter are?
Retreating blade stall, compressibility limit of advancing blade, design limits of cyclic control.
