F1A Thermal Trim

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    6.. As a final aside, would anyone like to comment on thermal hunting trim?
    I take conventional trim to be about 2mm wash-in on the inside wing and
    trimmed near the stall to tighten up in lift. I am confused by reports
    of Wakefield fliers flying with the outside wing washed in (or inside
    washed out) and (as I understand it) the fin lifting away from the turn
    to resist spiraling in if lift is strong. I don’t really understand how
    this would prevent a spiral dive once the model speeded up, say in a
    dive after a stall. In a normal glide the outside wing would be dragging
    against the rudder turn – but if it speeded up the outside wing would
    start lifting adding to the rudder turn – with no way to recover from
    there as far as I can see? Do any glider flyers use this trim, is it
    really superior and how does it work in reality?

    Neil Murray posted by Jim Parker


    The “F1B trim” was used on his gliders by a well known double world champion, Rudolf Lindner… And do not say he built old-timer like models, please. The difference between the two wing warpings for thermal hunting was explained by Arthur Schaeffler in his great series on A1 models, 1963. Positive warp on the inside wing increases the drag in a thermal turbulence, but requires a small SSM – small static stability margin, you write “near the stall” – in order that the increased drag is lasting a bit longer. With an opposite warp (F1B, as you say) the model must have a greater SSM, in order to avoid diving long after a perturbation. The tighter turn is started by the increased model velocity in vortices. Lindner used to release his model in a thermal with too much nose-up, so that the model stalled and next acquired the desired speed.

    Returning to the current F1B practice : these models have a very giant SSM… no danger for a dive. On the other hand the long tail prevents from quick yawing motions, what is rather bad for thermal spiralling, you will agree. So the rolling action of the wing warp is welcome as soon the model undergoes a longitudinal acceleration.

    Concerning the opposite fin section, again remember that the SSM is very huge, and dive is not concerned. His action above all concerns the flight in a downer : here the model is supposed to glide straight ahead out of the bad field, and an inverted section is supposed to help.

    I’m only a rubber flyer… sorry for that. And for my D/Ting English .JW

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