hey guys....still plugging along with my build of DE F95, and think that I want more trim. Experimental Amateur Built means just THAT....and "that being said", I am asking some input to my considered sketch of this modification. If you see some glaringly stupid things here, please advise. This mod would increase surface area from about 65 sq in, to about 125 sq in. Comment and help please. Thanks....bob
Unfortunately here I really don't have a pat answer since the size and shape of a trim tab will depend partly on the application and partly on the designer's preferences. Basically, a trim tab is nothing more than a plain flap applied to another surface. As a matter of fact, all the loads on the trim tab are calcualted in exactly the same way that you would calculate the loads on an elevator or aileron. If we use just good ol' proportionality as applied to flap design, the chord of a trim tab should be about fifteen to twenty five percent of the control surface's chord length. For an ultralight, where the low Reynold's Number and gap leakages tend to reduce the effectiveness of any deflected surface, I would tend to look at about the twenty five percent range. You of course don't want to make it too big though since then you decrease the effectiveness of the elevator itself. If you have a flying tail, you'll want to stay toward the low end of that range. Span-wise, you'll want a good aspect ratio for good effectiveness. Again depending on a designer's preferences and the airplane's requirements, elevator trim tabs seem to extend anywhere from half the surfaces' span to their full span. The bigger the tab, the less motion it will need and thus the less drag it will create. But, a too large a tab will make the trim such that you may not have enough finnesse to trim to a specific condition (you'll tend to over-trim). In that case you'll need a long lever arm to reduce the amount of deflection per movement of the trim servo, or just a smaller tab. In short, cook-book, rule of thumb and eyeball methods will get you close but the final size will, as George said, require a bit of trial and error.
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