Translational lift

September 5, 2012 by Tim McAdams

A hovering helicopter can require a lot of power. However, as it moves forward the horizontal flow of air across the rotor system improves the efficiency by changing the induced flow, and therefore the relative wind, which increases the blades’ angle of attack. This added efficiency is called translational lift. The forward motion also causes other aerodynamic issues with the rotor system, like dissymmetry of lift and transverse flow effect (a later discussion).

Wind can also create translational lift. Trying to hover at a constant altitude in gusty winds requires the pilot to constantly add or reduce power to compensate. Gusty winds can affect the tail rotor and power changes require pedal input as well. Holding a precise hover in these conditions is challenging.

With no wind, translational lift starts with any amount of airspeed and continues to develop as the helicopter’s speed increases. However, somewhere around 50 knots (it varies between different helicopters) induced drag increases to the point where it overtakes the gain in efficiency from translational lift.

Effective translational lift (commonly referred to as ETL) is a term used to describe the airspeed at which the entire rotor system realizes the benefit of the horizontal air flow. This happens when the helicopter’s rotor disc moves completely out of its own downwash and into undisturbed air. Depending on the helicopter this occurs between 12 and 18 knots of airspeed. The pilot will recognize effective translational lift on departure when the helicopter begins to have a noticeable tendency to climb and on approach when the helicopter starts to sink as the airspeed drops below ETL.

5 Responses to “Translational lift”

  1. Jon Farhat Says:

    Tim

    Thank you for this feature you write. I with the AOPA had more rotorcraft specific information and your articles are great. I’d love to see an article or hear your thoughts on perfecting skills like landing on carts or pinnacles etc. Your basic are always great!
    j.

  2. Rick Longlott Says:

    YES, I agree with Jon and may I add slope landing operations to it???

    RL

  3. Jean-Gabriel Larroche Says:

    Hello Tim.

    I just wanted to thank you too for your articles.

    I found these to be very insteresting and I wish we had the same quality in France.

    Each time It’s a real pleasure to read them and to revise or to learn something new about rotorcraft theory.

    It’s remind me of your work as a contributor in Rotor and Wing Mag years ago.

    All the best,

    Jean-Gabriel (part time helicopter CFI in France)

  4. Ernest MacQuarrie Says:

    Your comments about requiring a lot of power in a hover should have described the difference between IGE and OGE. In ground effect rquires about the same power as when in translational lift. Out of ground effect is the condition that requires a lot of power

  5. Ron Resch Says:

    I to always look forward to hover power,I always look forward to the articles every month.The only complaint I have is that I wish AOPA had more on rotorcraft.Perhaps we need a section for rotorcaft of our own.For instance I did a cross country from San Diego to Myrtle Beach in an Enstrom 280C, and I would enjoy hearing about other pilots experiences

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