Propwash Rotation and Percentage parameters in TMD config files

  • I am trying to fine tune propwash / slipstream effects on prop aircraft, and I decided to edit these parameters in the tail surfaces and fuselage, but I would like to better understand what the rotation really means - plane and direction.

    For instance, by increasing Propwash-Rotation in the vertical fin from 0.05 default to 0.25 I get a noticeable yaw when power is applied, so this rotation was "around" the "z" ( yaw ) axis, which was what I was expecting for a CW rotating prop in the C172.

    Then I also applied that to the left horizontal fin / stabilizer, and it appeared to "twist" clockwise around the "x" ( roll ) axis, but it wasn't very evident in my tests inflight...

    I see that I can separately tune this parameters between the port and starboard sides, but I would really like to know more about the effects of these parameters, and their sign conventions too - I left them positive in all my tests.

    The available parameters, in "aerowing" sections are:

    - PropwashPercentage
    - PropwashRotation
    - PropwashOmega

    I see the Omega is there to harden / soften the propwash effect over an aerowing, while the other two scale the "twisting" effects it can have.

    Thx for any hints

    1983 ......……....2012.…………….......2018...

    fs1 ===>     MS FLIGHT ===>   WT 1.83

    Start ... Hope ... Realization ...

  • Hi jcomm,

    From my experience the propwash rotation value is a parameter for the amount of rotational wash at that surface. It describes how much of the prop wash is normal to the surface. A positive direction would be the natural spin direction of the wash as far as I know. I haven't experimented with props that rotate the wrong way that much... Maybe a negative value could be usefull there?

    As far as I know if you increase the propwash rotation on the vertical stabilizer you get the yawing effect. If you add it to the wings and horizontal stabilizer you just get roll. And on top of that there are the forces created by the prop itself (torque, thrust, side-forces, angle to the aircraft's x-axis described as matrix with X0, Y0, Z0). From experience I found that you can counter the torque of the prop with the propwash rotation parameter on the wings and horizontal tail.

    The propwash percentage, at least at the moment, is a value to adjust the overall effect of the wash on that wing. If a surface is entirely in the wash you could go up to 1.0 (100%), if the prop wash isn't hitting the surface at all then 0.0 would be the way to go. Most surfaces are just partially hit, then I'd estimate the propwash percentage as area of the wing hit by wash divided by the total surface. For a single engine aircraft that's like 15% (0.15) on the main wings, 80% on the tail or so. For twin engines it's probably 30% on the wings and depending on the tail configuration you may have 50% +/-...


  • Thank you Jan for your valuable answer.

    It meets my experience with the tuning I've been trying with the default C172, and whose results I am so far enjoying - obtaining higher yaw moment due to propwash, without too much roll.

    Will keep trying further adjustments.

    1983 ......……....2012.…………….......2018...

    fs1 ===>     MS FLIGHT ===>   WT 1.83

    Start ... Hope ... Realization ...