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 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 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