Helicopter Performance

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 is loss of tail rotor effectiveness (LTE)?

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Loss of tail rotor effectiveness, commonly referred to as LTE, is when the ability to provide anti-torque thrust from the tail rotor is ineffective or highly unreliable.

Notwithstanding mechanical problems, there are several wind conditions that impact the tail rotor’s ability to provide anti-torque thrust: main rotor disc interference, tail rotor vortex ring state, and weathercock stability.  Main rotor disc interference may occur when there is a wind between 285-310-degrees.  From this angle, the main rotor vortices can be blown into the tail rotor, making the tail rotor operate in turbulent air.  A tail rotor ring state may occur when the wind is from 210-330 degree as there is the potential for the wind to blow tail rotor’s vortices into the tail rotor and the tail rotor can end up in a vortex ring state.  When the wind is from 120-240 degrees, the helicopter will want to weather vane into the wind, making it operate in extremely turbulent air.

Diagram showing wind conditions leading to loss of tail rotor effectiveness or LTEWhen there is pilot discretion to approach a landing site with left or a right cross wind, a right crosswind minimizes the likelihood of LTE.  Often, a pilot has the discretion on the approach to a landing site, so the wind should be considered, particularly on final where tail rotor use increases.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 11-17
Principles of Helicopter Flight, 2nd Edition, pg. 69

Other Helicopter Flight Conditions

Why are left pedal turns often considered safer?

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More power is required for left pedal turns.  If a left turn can be completed, there is enough power available to safely control the helicopter in the current flight conditions.

When wind is from the right, it is similar to having more induced flow, which decreases the angle of attack on the tail rotor.  With the decrease in the tail rotor blades angle of attack, more left pedal is needed to provide adequate anti-torque thrust, which uses more power.  As such, there may not be enough anti-torque thrust available to turn the aircraft to the left.  If this is the case, the flight should be aborted.  If a right pedal turn is conducted under the same conditions, the turn will likely not be able to be stopped.  A right pedal turn uses the torque from the main rotor so is can often be considered a turn with less application of anti-torque thrust.  As such, the right turn could easily become uncontrollable.  The problem would become worse as the right turn reached the conditions where LTE is more likely (tail wind or left crosswind).

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 9-4
Principles of Helicopter Flight, 2nd Edition, pg. 68

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

What is total drag?

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Total drag is the sum of parasitic drag, profile drag, and induced drag.diagram of total aerodynamic drag

Parasitic drag is caused by non-lifting portions of the aircraft, such as the rotor mast, landing gear, etc.  Parasitic drag is present anytime the aircraft is moving.  Parasitic drag increases significantly with airspeed.

Profile drag develops from frictional resistance as the rotor blades passing through the air.  Profile drag is comprised of form drag and skin friction.  Overall, profile drag increases moderately with increases in speed, but does not change significantly with changes in the airfoil’s angle of attack.  Form drag is the result of turbulent wake caused by separation of airflow from the surface of a structure.  A flat plat creates more form drag than a symmetrical airfoil (teardrop).

Diagram of various profile-form drag combinations

Skin friction is caused by surface texture.  The smoother something is the less skin friction.  Dirt, ice, and other items that impact the surface texture have a significant effect on friction drag.

Induced drag is a result of the production of lift.  Lift production generates downward velocities and vortices that increase induced drag.  As the aircraft increases forward speed, induced drag decreases.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-5
Principles of Helicopter Flight, 2nd Edition, pg. 28
FM 3-04.203-2007 Fundamentals of Flight pg. 1-28

Other Helicopter Performance Topics

What is the proper airspeed to fly for the best possible range?

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When flying for range, operate the helicopter at the speed for least amount of drag.Helicopter performance diagram showing the best possible range

To cover the most amount of distance, or range, with the current fuel load, the drag must be at its minimum.   This speed will be slightly higher than that used for endurance, which maximizes the time in the air.   A horsepower required curve can be used to identify this speed.  In a calm, no-wind scenario, a tangent is drawn from the zero airspeed to the bottom of the horsepower required curve.  Where the two intersect is the proper speed for range.  This speed will correspond with the lowest total drag as well.   If there is a wind, the tangent would be drawn from the headwind or tailwind component.

NOTE:  Although they are related, drag is calculated in relation to indicated air speed, whereas range is calculated in relation to true airspeed.  The two are shown together to demonstrate that minimum drag is not at the bottom of the horsepower required curve.

Reference(s):

Principles of Helicopter Flight, 2nd Edition, pg. 108

Other Helicopter Performance Topics

What is the proper airspeed to fly for endurance?

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When flying for endurance, operate the helicopter at the speed using the least amount of power.Diagram showing best possible endurance for helicopter performance

To stay in the air for the longest amount of time, power use must be at its minimum in order to conserve fuel.  When looking at the horsepower required curve, the best speed for endurance is the speed that corresponds with the bottom of the curve.  This speed does not correlate with the lowest drag.  The speed for endurance will be slightly less than that needed for range.

Reference(s):

Principles of Helicopter Flight, 2nd Edition, pg. 111

Other Helicopter Performance Topics

How does density altitude affect helicopter performance?

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A high-density altitude decreases helicopter performance.

Density altitude is pressure altitude corrected for non-standard temperature.  As such density altitude is a key factor in performance.  With a high-density altitude, it is if the aircraft is operating in an environment that would exist at a higher altitude in standard atmospheric conditions.  If the temperature is higher than standard, there will be less air pressure for that specific location.  With less air pressure, the density is less.  As a result, less lift will be produced as density is a key factor in the production of lift.  Conversely, lower-density altitude will increase performance.  If the temperature is lower than standard, the density altitude will be lower than the pressure altitude for that area.  If the air density is high (higher pressure), there will be a lower-density altitude.  If the air density low (lower pressure), the density altitude will be high.

Reference(s):

FAA AC-00-6B Aviation Weather pg. 5-16, 5-17
FAA-H-8083-21A – Helicopter Flying Handbook pg. 7-2
FAA-H-8083-25B Pilots Handbook of Aeronautical Knowledge pg. 8-7, 12-5
Principles of Helicopter Flight, 2nd Edition, pg. 248

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

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

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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