What is effective translational lift?

Effective translational lift (ETL) is when the lift generation from the rotor disc is more efficient due to increased aircraft speed or wind.effective translational lift or ETL

When at a hover in calm, no-wind conditions, the induced flow is a significant factor affecting the resultant relative wind.  As a result, the blade angles are significant and it takes more power for flight.  As the aircraft increases speed, approximately 16-24 knots indicated airspeed, the impact of induced flow is reduced.  Because of forward movement or wind, there is undisturbed air* meeting the front of the rotor disc.  As speed increases, the portion of the disc receiving undisturbed air increases.  As a result, the inflow angle is decreasing as more air is received horizontally versus vertically.  As such, a lower angle of attack will produce the same lift, resulting in less power needed for flight.

The tail rotor also becomes more efficient with an increase in forward speed.  There are two factors involved.  First, like the main rotor, the tail rotor becomes more efficient when it operates in undisturbed air.  Second, the forward movement of the aircraft reduces the amount of anti-torque thrust needed as the horizontal stabilizer or similar component, becomes more effective.  As the need for anti-torque lessens, there is more power available for the main rotor.

It is a common mistake for pilots to refer to flying through ETL.  An aircraft achieves ETL.  Once in ETL, the aircraft is receiving the benefit of transitional lift until the relative wind is changed so the ETL is no longer achieved, such as by slowing down, flying downwind or a change in wind velocity.  In general, the increase in horizontal speed or wind, the more efficient the rotor system.

* Some people will make reference to clean/dirty air instead of undisturbed/disturbed air.  The use of the term clean air and dirty air should be avoided as the level of particulate matter in the air is not a factor, it is the turbulence of the air that is relevant.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-20
Principles of Helicopter Flight, 2nd Edition, pg. 64, 99
FM 3-04.203-2007 Fundamentals of Flight pg. 1-41, 1-43

Other Helicopter Flight Conditions

What is translating tendency?

Translating tendency is the movement of the helicopter to the right due to the combination of main rotor torque and tail-rotor anti-torque.helicopter translating tendency

If uncompensated, power applied to the main rotor would turn or yaw the helicopter to the right.  To prevent turning, anti-torque is applied via left pedal.  As anti-torque is applied, the tail rotor produces thrust which pushes the tail to the right, stopping the potential turn.  There are now two forces moving the helicopter to the right, and one force to the left.  The forces to the right are greater and the helicopter tends to drift to the right, assuming no pilot inputs.  Translating tendency is often called rotor drift.

Some helicopter designs compensate for translating tendency by various methods, such as tilting the mast or designing a bias into the cyclic.  Most training helicopters, such as the Robinson R22/R44 and the Sikorsky 300CB, do not have any of these compensating characteristics.  In these helicopters, the pilot must counteract the translating tendency, or rotor drift, through pilot inputs to tilt the rotor disc to the left.

Not all helicopters experience translating tendency, specifically dual rotor helicopters such as the CH47.  In these helicopters, the torque produced by one rotor is counteracted by the other, which rotates in the opposite direction.  As such, forces are equal in each direction.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-14
Principles of Helicopter Flight, 2nd Edition, pg. 70
FM 3-04.203-2007 Fundamentals of Flight pg. 1-36

Other Helicopter Flight Conditions

What is transverse flow?

Transverse flow is the decreased lift at the rear of the rotor disc due to an increase in induced flow as the disc moves through the air, producing a roll to the right.helicopter transverse flow

Transverse flow occurs as a result of forward flight or a significant wind.  As the helicopter moves forward, the airflow at the front of the disc has not started its downward flow.  At the back of the disc, the induced flow or downwash is more significant, which reduces the angle of attack on the blades.  As a result, the front of the rotor disc is more efficient and produces more lift.  The rear of the disc wants to descend, but because of gyroscopic precession, the result is that the helicopter wants to roll to the right.  The transverse flow effect is also referred to as an Inflow Roll.

The transverse flow effect is felt as a vibration when just below effective translational lift (ETL) on takeoff and after losing ETL on landing.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-22
Principles of Helicopter Flight, 2nd Edition, pg. 101
FM 3-04.203-2007 Fundamentals of Flight pg. 1-42

Other Helicopter Flight Conditions

What is blow-back (flap-back)?

Blow-back is the rearward tilt of the rotor disc during the transition to forward flight.

Blow-back is due to the combination of flapping and the transverse flow affect.  In forward flight, the advancing blade produces more lift than the retreating blade.  Flapping is used to correct for the dissymmetry of lift across the rotor disc.  As the advancing blade flaps up, it causes the front of the helicopter to rise.  In addition, the induced flow will be greater at the rear of the rotor disc during forward flight, increasing the lift on the front of the disc, thus increasing blow-back.  As forward movement occurs, the pilot will have to continually increase forward cyclic to compensate for flap-back/blow-back.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-19
Principles of Helicopter Flight, 2nd Edition, pg. 94
FM 3-04.203-2007 Fundamentals of Flight pg. 1-40

Other Helicopter Flight Conditions

What is reverse flow?

At high airspeed, the airflow from the helicopter’s forward movement may exceed the rotational speed of the retreating blade.helicopter rotor blade reverse flow

The rotor blade is slower at the root than at the tip.  As the helicopter’s speed increases, the forward speed may exceed the rotor speed near the root.  In this portion of the disc, the airflow will be reversed.

Although the blade is not stalled, it is not generating lift.  As a result, the other sections of the blade need to provide more lift to equalize the lift on the rotor disc.  The angle of attack (AOA) must be increased on the retreating blade, but there is only so much increase in AOA before a blade will stall.

Reference(s):

Principles of Helicopter Flight, 2nd Edition, pg. 98

Other Helicopter Flight Conditions

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 ground resonance?

Ground resonance is a vibration that can be destructive to a helicopter.

Ground resonance is primarily a problem in a fully-articulated rotor system.  In these systems, ground contact can jolt the aircraft and produce a shock wave that can resonate in the aircraft.  This resonance can then destroy the aircraft in a short amount of time.  Basically, the helicopter will shake itself apart.  Ground resonance is the reason that many helicopter’s landing gear has some sort of damping feature, such as shocks or OLEO struts.  Many helicopters also have damping features within the rotor system as well, such as the 300CB.  Should ground resonance be suspected and rotor RPM allows, the pilot should immediately lift the helicopter off the ground, stopping the resonance.  Recovery from ground resonance is one reason to keep rotor RPM at flight speed until the collective is completely down.  If unable to lift off, an option is to immediately lower collective and shut down the rotor.  Wheeled type helicopters are at increased risk as the wheels do not flex horizontally.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 11-11
Principles of Helicopter Flight, 2nd Edition, pg. 162
FM 3-04.203-2007 Fundamentals of Flight pg. 1-68

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

What is loss of tail rotor effectiveness (LTE)?

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