Transverse flow effect

September 17, 2012 by Tim McAdams

When a helicopter starts to move forward from a hover another aerodynamic condition (in addition to effective translational lift that was discussed previously) that occurs is transverse flow effect. This condition involves a differential airflow between the front and rear parts of the rotor system.

Moving forward from a hover, with no wind, the edge of the rotor system over the nose moves into clean air while the rear portion moves into air that has already been accelerated downward. This causes the angle-of-attack of the blades passing over the nose to increase, producing more lift. Because of gyroscopic precession, the maximum reaction occurs on the left side of the helicopter causing the rotor disc to tilt to the right. To continue moving straight the pilot must compensate with left cyclic.

Transverse flow effect can be recognized by an increased vibration of the helicopter at airspeeds around 12 to 15 knots and can be produced by forward flight or from the wind while in a hover. This vibration happens at an airspeed slightly lower than effective translational lift (ETL). The vibration happens close to the same airspeed as ETL because that’s when the greatest lift differential exists between the front and rear portions of the rotor system. As such, some pilots confuse the vibration felt by transverse flow effect with passing through ETL.

9 Responses to “Transverse flow effect”

  1. mike vogel Says:

    thanks for some rotery wing info remember, the world is not all fixed wing …………………….

  2. Alan Stewart Says:

    Moving forward from a hover, with no wind, the edge of the rotor system over the nose moves into clean air while the rear portion moves into air that has already been accelerated downward. This causes the angle-of-attack of the blades passing over the nose to increase, producing more lift.
    Seems wrong. Wouldn’t the blades passing through the rear portion, where the air has been accelerated downward, need the greater angle of attack?

  3. Tr Says:

    Alan,

    Yes, the rear portion does NEED more angle of attack, but since it doesn’t get said increased angle of attack (it is actually reduced angle of attack due to the downwash), the resultant increased lift of the nose of the helicopter (and the end result of increased left side lift because of GP) occurs, causing the rotor disk to tilt to the right.

  4. Jean-Gabriel Says:

    “Houston, we have a problem !” :)

    Even if I totally agree with this phenomenon, I still can’t explain what’s happening when I fly the R22 model.
    One year ago, I exchanged very interesting e-mails with Ron Newman, the author of the excellent “Helicopters will take you anywhere” about this transverse flow effect and what I could feel and read about the R22 on this case. Here is an excerpt of what I wrote to him at this time :

    “….And when I consult the R22 ‘s POH, page 7-5, systems description, control trim and friction, I can read :

    ” The lateral cyclic is equipped with an on-off trim spring to cancel the left stick force which occurs during high speed flight.”

    There is for sure a left stick force increasing, but, as for me, it seems to go against the transverse flow effect theory with a counterclockwise rotor type (viewed from above) ?

    Furthermore, when I read the Special Investigation Report of the NTSB about the “Robinson Helicopter Company R22 Loss of main rotor control accident” (NTSB/SIR-96/03)(see attached file), you can read about the FAA’s review on flight testings of R22 and R44 models, page 23, second paragraph :

    ” the tests showed that, during cruise, the cyclic is forward and to the right of neutral, while the rotor plane is tilted aft and to the left, about 5° “.

    Statements which whom I agree too, speaking only about the cyclic position, but…….why ?

    Ron Newman told me that he couldn’t explain this statement and that he would ask for clarification with Tim Tucker of Robinson’s factory.

    Unfortunately, I don’t have the answer yet.

    Maybe you’ll have the solution ?

  5. Dave Says:

    Jean-Gabriel –
    The stick force which I think you are referring to is actually a result of the intentional tilt of the rotor mast to the left as a design feature to offset translating tendency. It is my understanding that Robinson incorporated a left mast tilt so that in hovering flight, where tail rotor thrust and therefore translating tendency to drift to the right is at it’s highest, would be offset. But as the helicopter accelerates, tail rotor thrust is reduced, the fuselage provides more slip-streaming, and so translating tendency diminishes; but the mast tilt to the left does not change. The result is a “pull” to the left as speed increases, which must be countered by the pilot applying/holding more right cyclic. When the “trim” knob is pulled up, the spring relieves some of this right pressure by offsetting the cyclic to the left.

    And remember, when pulling up the cyclic trim knob, be sure to reach around the left side of the cyclic, thereby reducing the likelihood of accidentally pulling up the mixture knob which is to the right of the trim knob.

    The transverse flow effect “roll” to the left is short lived and predominatly only encountered during the initial acceleration phase as described in the original article.

  6. Jean-Gabriel Says:

    Dave.

    Many thanks for your clear and detailed reply.
    At least I have my answer to this left stick force that I had to counteract using the trim knob.
    May I add that I’m a bit surprised by the fact that, as you mentioned, the tranverse fllow effect be “short lived”.
    Even tough I totally agree that the air acceleration difference between the front and the rear part of the rotor disc is at its most value during ETL, shouldn’t we still “feel” the right roll tendency at greater airspeed due to the rotor disc tilt to the front during the cruise ?
    I remember reading about this subject in the “Cyclic and Collective” Mr Shawn Coyle ‘s book citing that you could feel this tranverse flow roll during a zero ground speed hover with a side wind for example. So I imagined that , with a moderate wind, you could be above ETL and still have to counteract this roll….
    Anyway, many thanks again for your for your answer, it was a subject that I had difficulties for explaining to my students.
    ps : please forgive me for my “frenchy” english style…..and mistakes…

  7. Scott Wright Says:

    I AM NOT A HELICOPTER PILOT, but maybe this explains why I can never get airborne in Microsofts Simulator 2010. The moment I lift off I go out of control, unless I can somehow accelerate very quickly.
    This is just for humor, but I am curious if this is due to the realness of the program itself?

  8. Annie Says:

    Dave-the rotor mast is not tilted to the left on an r22, but rather the combination of controls rigged to the right and pilot input is what compensates for translating tendency. When the pilot picks up with the stick in neutral, he is actually (and unknowingly) pulling it to the left. I believe the left pressure felt is partly caused by pulling against the cyclic springs that are trying to pull back to the right. I also recently read in Professional Helicopter Pilot Studies that transverse flow can happen across the whole flight envelope, which is why the r22 has a trim knob to offset the lateral cyclic load. I think Jean is right in that at high speeds, your rotor disc is tilted in much the same way it is when taking off, which is associated with transverse flow. It seems to me that the tilt causes a horizontal airflow across the front of the disc and a more perpendicular flow across the rear, which would increase induced flow/drag which woulda be felt 90 degrees later by a right rolling tendency. So, in my opinion, I think it is a combo of the cyclic springs and transverse flow that cause that left pulling arm fatigue in forward flight…

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