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There are some designs which have no horizontal stab: flying wings
for example. There are also canard setups (the speed canard, for example). For a sailplane, I was thinking about how one might design away the typical T-tail stabilizer and elevator. First of all, how much dynamic stability does the horiz. stab contribute? If it were eliminated by design, would it be absolutely necessary to compensate by using a swept wing (either forward or backward)? When deflected, how much torque does an elevator provide? I'm considering these factors, because eliminating the elevator and stab would reduce drag. From there, one could potentially design a ducted surface, or use moveable weights in the tail to change C.G and therefore pitch. In the first case (ducting), there are commonly used NACA ducts (they look like little triangles on power planes) that are commonly used as air vents on power planes. They have the advantage of producing minimal drag when the vent is closed. On a glider, they could be used in the tail to direct airflow and produce pitching moments. There is a tail-rotor free turbine helicopter which uses ducted bleed-air, I believe, to control yaw this way. The other option, which is more elegant, is to use a moveable weight in the tail for pitch. Move the weight forward to pitch down, backward to pitch up. One difficulty is if the weight must be quite heavy, or the stick movement needed to move it is too heavy. I suppose this in some part is a function of the length of the tailboom. Another complication is that a regular elevator is more effective at high airspeed, and less effective at low airspeed (more deflection is required for the same torque). This isn't necessarily true with a weight-shift pitch control. Hmmm...anyone have data about forces provided by the elevator is flight? Drag caused by the elevator/ vert. stabilizer in level flight? How about torque produced by weight shift near the arm of the elevator? I suppose the best way to experiment with this is in a model glider first, then in a full scale glider with BOTH pitch systems (elev/stab, AND weight shift). Then finally with the elev/stab removed. |
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