What is retreating blade stall?

A retreating blade stall is when the retreating blade flaps down so significantly that the blade reaches its critical angle of attack and stalls.

The retreating blade does not produce as much lift as the advancing blade.  To equalize the lift across the rotor disc, the retreating blade must increase its angle of attack (AOA) through flapping.  However, there is a limit to how much the AOA can increase before the critical angle is reached.  Once reached, the blade will stall.  Retreating blade stall is a significant factor in determining a helicopter’s VNE speed.  Although the blade is at its highest AOA when directly to the left (270 degrees), because of gyroscopic precession, the stall is felt as a tail down/nose up movement.  In addition, the pilot will likely feel low frequency vibrations.

To recover from a retreating blade stall, the pilot should lower collective to reduce the AOA.  After lowering collective, aft cyclic can be used to slow the helicopter’s forward speed.  Using aft cyclic without first lowering collective will increase the problem as aft cyclic produces a flare effect and increases the AOA.  The flare reduces the induced flow as some of the air is received from under the disc.  When induced flow is reduced, the AOA is increased as the resultant relative wind is influenced more by the forward movement and rotation of the blades, if other factors remain the same.  Forward cyclic as an initial response also increases the problem.  With forward cyclic, changes in blade angles are most significant on the retreating blade. The retreating blade angle is increased due to forward cyclic, further aggravating the problem.  Because of the stalled condition, the blade flapping does not occur to equalize lift.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 11-11
Principles of Helicopter Flight, 2nd Edition, pg. 137
FM 3-04.203-2007 Fundamentals of Flight pg. 1-66

Other Helicopter Flight Conditions

What is a dynamic rollover?

A dynamic rollover is a catastrophic event where rotor thrust pulls the helicopter sideways around a pivot point, such as from catching a skid on an object.

There are three conditions needed for a dynamic rollover: lift, pivot point, and sideways movement.  The cyclic has only so much ability to tilt, usually 5-8 degrees.  Should the helicopter landing gear/skid become caught on an object, that skid may become a pivot point for the helicopter to tip or roll over.  Once airborne, the pendular action of a helicopter makes it quite easy for the angle to exceed the cyclic’s compensation capability.

Past this point, the rotor thrust compounds the problem as the thrust basically pulls the helicopter around the pivot point.  The risk of a dynamic rollover is one of the reasons a pilot must be extremely vigilant whenever there is any sideways movement of the helicopter.  Obstacles such as taxi lights, aircraft tugs, and signs can pose significant problems at airports.  When performing off airport landings, having a landing gear stuck under a tree or hooked on a foreign object is a serious concern.  A pilot should ensure the aircraft is free of any obstacles before ascending.

Slope operations are another area of flight where the possibility of a dynamic rollover exists.  When ascending or moving laterally, the upslope skid is a potential pivot point.  If too much collective is applied before obtaining a level flight attitude, a dynamic rollover may occur.  Should the collective be lowered too aggressively, the helicopter motion may initiate a roll towards the downslope skid.  Due to translating tendency, the conditions for a dynamic rollover are increased with a right skid/landing gear touching.  In addition, a crosswind from the left, left yaw inputs, and a right center of gravity also increase the potential for a dynamic rollover.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 11-12
Principles of Helicopter Flight, 2nd Edition, pg. 165
FM 3-04.203-2007 Fundamentals of Flight pg. 1-63

Other Helicopter Flight Conditions

What is a vortex ring state?

A vortex ring state is when the helicopter’s downwash recirculates into the induced flow and the helicopter descends while under power.

helicopter vortex ring state

A vortex ring state is a very dangerous situation but can be avoided.  The condition occurs when the vortices from the blade tips recirculate into the induced flow of the rotor.  For this to occur, there are several conditions that must be present:

1) The helicopter must be under power, generating lift
2) The helicopter must be descending at least 300 feet per minute
3) The helicopter must be below effective translational lift (ETL)

If these conditions are present, a vortex ring state could develop.  As such, a pilot should be very careful to avoid these conditions.  For example, when conducting a steep approach to a confined area, ensure not to descend more rapidly than 300 feet per minute.

A vortex ring state is very dangerous as the descent rate that can approach 6,000 feet per minute.  Because the aircraft is descending, many pilots want to increase collective to stop the decent, but this only increases the problem as the increased collective increases the induced flow.  The correct response is to lower collective and move the helicopter out of the downwash.  If altitude permits, an autorotation can be conducted.  Without power, the vortex ring state cannot occur.*

Some may refer to a vortex ring state as settling with power.  However, the two should not be considered synonymous.  Although the helicopter descends (or settles) while under power when in a vortex ring state, settling with power can occur under other circumstances.  Anytime the power demands for the flight conditions exceed the power available, the helicopter will descend, resulting in settling with power.  For clarity, it is best to use the term vortex ring state when referring to the aerodynamic phenomenon where the vortices are recirculated into the rotor system.  When people refer to a vortex ring state, they generally are referring to the main rotor system.  However, a vortex ring state can also occur with the tail rotor, potentially leading to loss of tail rotor effectiveness.

* The Vuichard Recovery technique is a new method for recovery from settling with power the requires increasing collective, left pedal, and right cyclic.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 11-9
Principles of Helicopter Flight, 2nd Edition, pg. 157
FM 3-04.203-2007 Fundamentals of Flight pg. 1-61

Other Helicopter Flight Conditions

How does the pilot handle low rotor RPM?

Lower the collective to reduce drag and increase throttle for more power, if available.  If in forward flight, gently apply aft cyclic.

Low rotor RPM is a significant concern for pilots.  The slower blade speed reduces the amount of lift as velocity is a key factor in lift production.  If the pilot tries to retain the same altitude, additional lift will be needed.  Should the pilot attempt to raise the collective for more lift, the increased drag will further reduce the rotor speed.  As the rotor RPM decays, the centrifugal force reduces and results in increased conning of the blades.  The increased coning compounds the problem as the rotor thrust is directed inward, reducing its vertical lift capacity.  In addition, the size of the overall disc becomes smaller, further increasing the needed for lift is the altitude is to remain the same.

Lowering the collective reduces the blade angle, which reduces the angle of attack.  With a reduced angle of attack, the induced drag will be less, and the rotor RPM should increase, assuming the same power requirements.  In addition, aft cyclic will tilt the rotor disc aft which in turn lowers the inflow angle on the rotor, which reduces the angle of attack, and reduces the induced drag.  Due to the aft cyclic application, some of the airflow is now coming from underneath the disc, which reduces the induced flow, which in turn reduces the angle of attack, induced drag, etc.  In addition, the coning angle will increase with aft cyclic.  The increased coning angle reduces the size of the disc, and because of the Coriolis effect, the disc will increase rotation.  Should the rotor RPM be allowed to decay beyond the lower limits, the lack of adequate centrifugal force may allow the blades to collapse.  Remember, lower the collective and increase power to maintain rotor RPM within limits.  Do not let RPM decay.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-15, 11-15
Principles of Helicopter Flight, 2nd Edition, pg. 171

Other Helicopter Flight Conditions

What are the three primary regions of the disc in an autorotation?

The three primary regions of the disc in an autorotation are the stalled, driving, and driven.diagram of the three primary regions of the disc in an autorotation

The stalled area of the disc is the area closest to the hub.  Because the rotation of the disc is slow at this area, the angle of attack of the blade is beyond its critical angle.  As a result, it is stalled and not producing any lift.  The middle section of the blade is the driving region.  The thrust or lift from this section of the blade is slightly forward in the direction of rotation.  As a result, this lift provides the thrust needed to rotate the blades.  The driven area is the outside portion of the disc.  This area is producing a lot of drag.  As a result, its net contribution is not assisting in lift production and is being “driven” by the middle section of the disc.

Unless descending vertically in a no-wind condition, the stalled, driving, and driven regions will be a different size on the advancing and retreating side of the disc.  During autorotation, the pilot changes the size of these regions to control the speed of the rotor disc.  For example, raising the collective decreases the size of the driving region and increases the stalled and driven region.  As a result of the decreased driving region, the rotor RPM decreases.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-25
Principles of Helicopter Flight, 2nd Edition, pg. 144

Other Helicopter Flight Conditions

What is the purpose of the flare at the end of an autorotation?

The flare at the end of an autorotation is to arrest the decent.

When in an autorotation, the decent rate is significant, upwards of 1,500 feet per minute.  This rate of decent must be reduced before attempting a landing.  The flare reduces the decent rate.  As a side benefit, the flare also increases the rotor RPM and reduces forward speed.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 11-4

Other Helicopter Flight Conditions

Why does turning left require more power during forward flight?

Turning left requires more power to overcome the increased induced drag caused by the increase in angle of attack on the rear of the rotor disc.

To turn left, the AOA must increase at the rear of the disc. Due to precession, this change is felt on the right side of the disc and turns the helicopter left.  In forward flight, the front of the disc receives airflow more horizontally, while the rear of the disc has more induced flow.  As a result, the AOA changes to turn left requires a significant increase in the AOA.  This increase caused more induced drag on the rotor disc.   To maintain the same rotor speed, more power is required to overcome the increased drag.  The opposite is true when turning right.  A right turn changes the AOA on the front of the disc, which is operating with less induced flow.  Also, the reduction in the AOA on the rear of the disc reduces induced drag and less power is required.

The impact of this change is more significant at higher speeds and higher gross weight.  In high performance turbine helicopters, abrupt changes from right to left can place significant stress on the aircraft systems.

Like most items in a helicopter, there are many different forces working with and/or against each other.  In a turn, the increased g forces from the turn also speed up the disc as there is more weight.

Reference(s):

Fundamentals of Flight pg. 1-141

Other Helicopter Flight Conditions

How does a helicopter perform in-ground effect vs. out of ground effect?

The rotor disc becomes more efficient within close proximity of the ground, so it takes less power to provide the same amount lift.

The benefits of ground effect are received when the rotor disc is about ½ its diameter from the ground, usually about 3-4ft skid height.  The benefits of ground effect are the greatest over hard surfaces.Diagram of helicopter performance in ground effect and out of ground effect

When over grass or similar surfaces, more power will be required then when over a smooth hard surface. When over a hard surface, such as a taxiway, there is some pressure received which lowers the velocity of the induced flow which reduces the angle of attack. With a lower angle of attack, more power is available for lift.  In addition, the tip vortices are reduced, which lowers the induced drag, providing more power available for lift.  When in ground effect over grass or similar surfaces, the horizontal movement of the air along the ground is reduced, increasing the induced flow, and requiring more power.

Another factor with grass or non-hard surfaces, pilots tend to slightly increase their altitude.  Any increase in altitude, significantly lessens the benefit of ground effect.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-10
Principles of Helicopter Flight, 2nd Edition, pg. 62
FM 3-04.203-2007 Fundamentals of Flight pg. 1-34

Other Helicopter Performance Topics