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Old Oct 11, 2012, 01:00 PM
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Joined Jan 2007
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The CG position (static margin) affects pitch stability. It determines the strength of the airplane's pitch response to return to trim speed if the airplane is flying faster or slower than trim speed.

If you have the glider that is trimmed for a "normal" glide speed in speed mode, if the CG is forward it will pitch up on launch; if the CG is near the neutral point it will not pitch up on launch. The CG position (stability) determines how much the glider pitches up to return to trim speed.

If you trim for launch speed in speed mode (high speed), then with the CG forward the glider will not pitch up on launch, but it will try to go launch speed all the time in speed mode.

If the CG is near the neutral point, you can basically set the flight speed by the airplane attitude, and the airplane does not change it's pitch angle to return to a trim speed.

You can use these characteristics to adjust the pitch up on launch. If you have the CG slightly ahead of the neutral point (perhaps a 5% static margin), and trim it for a reasonable glide speed in speed mode, the glider will pitch up at the higher launch speed. The pitch up will die off as the speed bleeds off during the launch. If you get too much pitch up, move the CG slightly aft and re-trim your your speed mode. This will give less pitch up during launch. Do the opposite if you have too much pitch-up during launch: move the CG aft, and re-trim your speed mode.

Unfortunately, adjusting the CG position will affect your trim speeds in all your other modes as well, and the pitch rate in response to elevator; More forward CG require more elevator deflection, aft CG position requires less.

Another drawback of the method of trimming the launch pitch-up are that it will pitch up more as the wind speed increases, which is the opposite of what you really want. You would have to adjust the CG position and your trims for all your modes as the winds speed changes.

When the CG gets near the neutral point, many other small factors effect the stability margin. such as dihedral angle, wing height above the CG, tail position relative to the wing wake, etc. This can result in different response to speed changes with different airplanes with small static margins.

Kevin
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