ETL, Translational Lift and Transverse Flow Effect

Effective Translational Lift, Translational Lift and Transverse Flow Effect

Effective Translational Lift vs Translational Lift

Translational lift - Translational lift is sometimes thought to mean only effective translational lift (aka ETL), which is incorrect. Translational lift occurs when any amount of clean air passes through the rotor system. Even 1-knot of air introduced into the rotor system from either wind or forward speed provides translational lift which improves the efficiency of the main rotor to some degree. Translational lift increases continuously as airspeed increases, however the negative effects of induced drag prevent any further benefits of translational lift above about 45-knots of airspeed. This is to say that increasing the airspeed increases translational lift and thereby improves the flight characteristics of a helicopter through about 45-knots of airspeed. Note that this airspeed may vary slightly from one helicopter make/model to another.

Effective translational lift - is translational lift advanced to the point where all air flowing through the rotor system of a helicopter is fresh or undisturbed air. That is air that has not already passed through the rotor system while the helicopter speed is slow or when it is in a hover. Effective translational lift can be recognized by the sudden tendency of the helicopter to climb as it passes through about 16-20 knots of airspeed As stated above, this airspeed will vary slightly from one helicopter make/model to another. Some pilots also refer to the result of effective translational lift as blowback. Passing through effective translational lift while on approach can be identified by the sudden tendency of the helicopter to sink when an increase of collective pitch becomes necessary to maintain a continuous rate of descent.

While in a stationary hover in any wind there will be translational lift and if the wind is strong enough, you can even have effective translational lift while in a stationary hover. This wind can be blowing from any direction creating enough airflow through the main rotor to maintain ETL, or gusts of wind causing the helicopter to pass in and out of ETL. Airflow inducing translational lift does not have to be from the front of the helicopter as often thought and/or taught.

An increase in translational lift will reduce the amount of power required to sustain flight in a given profile, therefore as translational lift varies by the wind while hovering, it will be necessary to work the collective pitch to maintain a given hover height. The collective workload will obviously be relative to the variance of translational lift. This will also be true during takeoff or landing when the winds are gusty.

Transverse flow effect

While in a hover the rotor disk must be tilted into the wind to prevent a drift with the wind similar to the way it is tilted forward for departure from a hover. Transverse flow effect will be noticed by a vibration of the main rotor caused by a lift imbalance between opposite sides of the rotor disk at a relative wind or airspeed. This vibration occurs because the low part of the disk has not yet passed into effective translational lift while the high part of the disk has. This imbalance of lift is caused by the flow of air through the rotor while the rotor is tilted into the wind or airflow during takeoff. The air on the low side of the rotor disk is recirculated to a varying degree due to ground effect which causes a higher angle of attack, while the air on the high side of the disk passes through and is accelerated clear of the disk with a resulting reduced angle of attack. This vibration and imbalance of lift occurs at about 12 to 15-knots of airspeed and can be induced by forward flight or from the wind while in a hover.

Transverse flow effect can occur from any direction of flight, or any wind direction while in a hover just as translational lift can also occur from any direction. However due to the fact that the aircraft is generally faced into the wind most text on these phenomenon usually refer to forward flight only. As a result, figures depict transverse flow effect and translational lift only from the front and likewise the references are made to the fore and aft portion of the disk (see figure 17). The airspeed of occurrence is relative to the helicopter being flown but is always slightly less than the airspeed at which effective translational lift occurs.

Many pilots confuse the vibration of transverse flow effect with effective translational lift and as a result, these pilots will state that they are passing through effective translational lift when they feel this vibration on approach which is incorrect. The fact is that the rotor vibration caused by transverse flow effect occurs at an airspeed slightly slower than effective translational lift so any pilot using this vibration as a sign that they are passing through effective translational lift while on approach has actually passed through ETL before the vibration was felt.

It is important that pilot's understand the difference in these two conditions to prevent the possibility of entering into settling-with-power inadvertently while using the wrong factor to determine the passing of effective translational lift. It is far better to use a safe minimum airspeed of say 30-knots to stay above ETL until the rate of descent is safely less than 300-feet per minute. END Jump to Top