Autorotation

What is a vortex ring state?

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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

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

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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?

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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

What is the height-velocity diagram?

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The height-velocity diagram is provided by the aircraft manufacturer and indicates speeds that should be avoided as a safe autorotation might not be possible.

diagram showing the different regions on a helicopter hight-velocity diagramShould there be an engine-failure or other issue that requires an autorotation, operating within the shaded areas of the height-velocity diagram reduces the chances of a successful autorotation.  As the aircraft changes from powered flight to autorotative flight, there may not be enough airflow to provide adequate rotor RPM to arrest the descent if the aircraft is not high-enough or does not have enough forward speed.

Conversely, if the aircraft has excessive speed, but not much height, the pilot may not have enough reaction time to slow the aircraft to a safe landing speed.  Or if the pilot attempts to do so, the flare may be so excessive that there is not enough clearance for the tailboom and it may strike the ground.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 11-8
Principles of Helicopter Flight, 2nd Edition, pg. 153
Principles of Helicopter Flight, 2nd Edition, pg. 28
FM 3-04.203-2007 Fundamentals of Flight pg. 1-28

Other Helicopter Performance Topics

How do changes in center of gravity (CG) affect helicopter flight characteristics?

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The aircraft manufacturer sets the helicopter’s center of gravity limits.  If operating outside of these limits, the helicopter may be uncontrollable in certain situations.

The effects of CG are a reason to perform a weight/balance calculation before each flight.  When the CG is forward of limits, there may not be enough aft cyclic travel to stop the helicopter, in particular during the flare at the end of an autorotation.  With a forward CG, the helicopter will hover nose low in no-wind conditions.  This condition may not be as noticeable in windy conditions.

When the CG is aft of limits, excessive forward cyclic will be required for forward flight.  Should a gust of wind occur, there may not be enough cyclic travel to adequately control the helicopter in forward flight.

Lateral CG limits are also established to ensure that cyclic travel is available to adequately control the aircraft during all phases of light.  Smaller training helicopters may have certain limits, such as solo flight from the right seat, primarily due to the fuel tank locations.  Depending where the fuel tanks are located, the CG will often move forward and to the opposite side as fuel is burned off during the flight, especially in small training helicopters.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 6-2
Principles of Helicopter Flight, 2nd Edition, pg. 270

Other Helicopter Performance Topics