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  1. #1

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    CG <-> Down thrust relation?

    Hello Don,
    first of all, as usual, thank you for taking your time to respond (and what replies!) to us pattern guys in this GREAT forum! I'm very grateful to you.

    These days, while I'm in the process of setting up a new plane, I'm thinking about down thrust and CG. Since now, I never thought there was a relation between the two, but after some thinking... I think they are related: am I wrong?

    Here is my way of thinking: one of the common ways of assessing if CG is correct is the 45 degree climb followed by a half roll. Regarding down thrust, one of the test that can be done is to pull in a vertical climb and see if the plane follow the line. Ok (if I'm mis-understood something, please correct me).

    Now: if we have a wrong down thrust setup (for example, we have too much down thrust), also the CG test will be affected, since in the 45 degrees line we can see that the airplane follows the line but it does so beacuse the engine is "pulling up" the airplane: we can conclude that CG is right even if it is too far.
    Now let's assume we have the wrong CG (i.e., too far), so even the down thrust test can be falsed, since with the CG in a forward position we have more up trim, with the result that we can conclude we have the wrong down thrust...

    What do you think about that? In which order do you set up CG and down thrust?

    Thank you very much and sorry for the long post...

  2. #2
    Don Szczur's Avatar
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    RE: CG <-> Down thrust relation?

    I set the CG as far forward as I can. Basically pull to 45 degrees half roll and see what happens. If it noses over move the CG back, if it climbs to the belly add nose weight. I like mine so it will, ever so slightly, nose drop.

    THrust line is simple. Get the wings level and pull vertical. See what the nose does, add some rudder trim, but as it slows down you'll see if it pitches to the belly or canopy. Add up or down thrust as appropriate so its hands off on the vertical upline.

    Thats it!

    See you,
    Don


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