1. Retreating Blade Stall
Front Range Helicopters

2. Definition
The stalled condition of the retreating blade when it operates beyond its critical (stall) angle. The condition is generally (but not only) associated with high forward airspeed.

3. What Occurs as Forward Speed Increases
As the helicopters forward speed increases the advancing blade gains
the advantage of the aircrafts forward speed plus the speed of rotation.
In this example the forward speed of the blade in rotation is 400knots.
The forward airspeed is 20 knots. So on the advancing side, right side of the aircraft, the blade tip speed has now gained 20 knots and the tip speed is now traveling at 420 knots. This allows the advancing side to produce more lift.

4. What Occurs as Forward Speed Increases
Just the opposite is true on the retreating side, left side, of the aircraft.
The blade tip which was moving at 400 knots, now looses the 20 knots due to the forward airspeed. Blade tip speed is only 380 knots on the retreating side. This would result in less lift being created than on the advancing side.
This results in a condition referred to as Dissymmetry of Lift.

5. Angles of Attack in the Plane of Rotation in Forward Flight
Without some correction, dissymmetry of lift would cause the helicopter to roll to the left in forward flight, a problem encountered by the early designers of the helicopter.
To compensate for the increase in lift on the advancing side and the decrease in lift on the retreating side, the angle of attack has to change in the plane of rotation to make lift equal on each side of the helicopter. Angle of attack must be reduced on the right side and increased on the left side.
This is accomplished by Blade Flapping, as shown in the diagram to the left.
(Angle of Attack)

6. Blade Flapping
With the addition of the flapping hinge, the advancing blade flaps up, changing the angle of the relative wind and decreasing AoA and flaps down on the retreating side increasing the AoA

7. Compensating for Dissymmetry of Lift
The blades flap up on the advancing side and down on the retreating side changing the relative wind and angle of attack. Lift on each side of the rotor is now in balance. The approximate angles of attack throughout the plan of rotation are shown in figure
The degree of flapping will vary based the forward airspeed of the helicopter to maintain this balance………at least up to a point!
Angle of attack distribution in forward flight

8. Blade Flapping
Do the Main Rotor Blades “Flap” while in a stationary hover with calm wind?___________________________
Under what conditions would that be different?______________________________________
While in a stationary hover, would the blades always flap up on the left and flap down on the right?____________
Other than wind and forward airspeed, what else would have an impact the angle of attack throughout the plane of rotation?____________________________________

9. What makes the Blades Flap?
LIFT = ½pV S 2 C
(LIFT FORMULA) L
(Produced) C
L= Lift Coefficient as established by the design of the airfoil and it’s given angle of attack
p= Density of air the blade is moving thru
V = True Air Speed at which the airfoil (blade) fly's thru the air
S = Surface area of the airfoil (blade)
Given the lift formula above, why would the advancing blade flap up and the retreating blade flap down?? ____________________________________What component of the Lift formula changes in the plane of rotation?_________________

10. Retreating Blade Stall
The ability of the main rotor to flap up and down compensates for dissymmetry of lift, but as forward airspeed increases eventually there is a speed reached where the angle of attack on the retreating blade reaches its maximum lift coefficient and the blade is stalled. This stalled region begins at the blade tip and moves inward and is at it’s maximum at the 9 o’clock position in the plane of rotation.
Additionally when the retreating blade reaches these critical airspeeds, the inboard section of the retreating blade may actually experience a flow reversal where the airflow is moving across the trailing edge toward the leading edge of the blade…….oh my!!!!!!
Retreating Blade Stall

11. Another Look

12. What’s Happening on the Advancing Blade
To complicate matters the advancing blade tip speed, if forward speed is sufficient, is now approaching the speed of sound (trans-sonic). The blade designs on symmetrical rotor blades were not intended to go trans-sonic, as a result the blade will experience a sharp rise in drag associated with shock waves along the blade section, usually the tip, resulting in a totally disrupted airflow and pressure changes that can even cause structural failure of the blade.
VNE speed limits on helicopters are associated with both the limitations on the advancing blade AND the retreating blade, though the symptoms associated with the retreating blade will get the pilot’s attention first. .

13. Recognizing Retreating Blade Stall
After looking at the placard for the Schweizer 300C below, what conditions would lead to retreating blade stall?
________________________
__________________________

14. Conditions Leading to Retreating Blade Stall
High Forward Airspeed
Temp/Pressure Altitude Combinations (High Density Altitude)
Maneuvers involving high load factors such as steep turns and pulling out of a high speed dive
Use of excessive or abrupt control movements, particularly at high altitudes.
Flight in turbulence where excessive airspeed changes occur.
Any of the above maneuvers in combination with low rotor RPM

15. Recognizing Retreating Blade Stall
Now that we know airspeed, temperature, pressure altitude and weight of the aircraft are contributing factors for retreating blade stall, lets look at some symptoms the pilot can recognize at onset…..
First the pilot will experience a roughness and vibration in the main rotor. This has often been described as a “buzz” on initial onset.
If allowed to progress the retreating side will develop a stalled condition that will cause the nose to pitch up. This is due to gyroscopic precession, stall on the right side, reaction is 90 degrees later causing loss of lift over the tail and the nose to pitch up.
At this point the helicopter may start to roll, usually to the left but not always.

16. Recovery The recovery is accomplished by:
Reducing collective pitch. This reduces the angle of attack in the plain of rotation and should be done first.
Gently Applying aft cyclic to slow the aircraft. Application of aft cyclic with out a reduction in collective pitch will only worsen the stall.
Maintaining the main rotor RPM in the upper part of the green arc.

17. Conclusion
The best way to avoid Retreating Blade Stall is the pilots recognition of the conditions that may lead to the occurrence.
Have a clear concept of what the aircrafts VNE is for each day, in the existing conditions (including potential turbulence), expected altitudes and at the current aircraft weight.
Unless corrective action is taken by the pilot, retreating blade stall can led to the total loss of control of the aircraft.
Always THINK AHEAD of the AIRCRAFT!!