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Old Dec 09, 2012, 10:00 PM
davidterrell80 is offline
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UM-to-60-inch fixed wing
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United States, VA, Herndon
Joined Apr 2012
4,639 Posts
Just don't call him "Shirley."

In a sense, the short answer is "Yes." The tail force works to balance the "wing pitching moment". As the horizontal stabilizer is usually designed to produce negative lift, then the wing must fly at a slightly greater AOA to provide additional lift, so that the net aircraft lift balances weight.

I'm visualizing your aircraft in level, unaccelerated flight, with the long axis of the fuselage parallel to the direction of flight; for minimum drag. If the CG is behind the combined Mean Aerodynamic Chord you will need a down force generated by the Horizontal Stab.

But, surely, any change to speed, throttle, or attitude will change the direction and magnitude of the force needed.

If you hinged the stab at the elevator hinge and use a set-screw at the leading edge, you make a incidence changeable on the ground.

The real thing does it with stacked washers...

...this is the way it has always been done. Its' one of those places where the normal rules about washers doesn't apply.

The method for adjusting is usually to fly the airplane in the most common load configuration and trim for level flight. Have someone in a chase plane observe and photograph the tail. You then know to add or remove shims until the elevator and trim tab are in trail.

It would be interesting to poll flying airplane owners to find out what angle worked best on particular airplane/ engine/ pilot weight combinations.

With the Acro II, the tail incidence range seems to be from zero to 1.5 deg leading edge up. The heavy engine and prop combinations seem to favor close to zero, and the heavy pilots, like myself at 240 get close to 1.5 deg LE up.

I think the Skybolt has a longer arm distance for pitch force (This would mean the distance from the effective pitch center of the wing cell to the hinge line of the elevator) than the Acro II so less change would produce more affect.

To figure the amount of rise at a given point, such as at the "H" tube (front spar carry through) multiply the sine of the desired angle by the distance between the centerline of the stabilizer spars. In the case of the Acro II, to raise the incidence 1.5 deg multiply.026 (sine of 1.5 deg) times 19" (the spar centers) which is .497. Call that 1/2". (From the Acro Sport Newsletter-Rigging)

You could use 1/4" spacers and each spacer would be 3/4 of a degree.

It is beneficial to know this angle and record it in the airframe log book for future reference. Like where you are now. (John)

It would be in the builders best interest to set the incidence at the carry through spars before building the stabilizers, then build the stabilizers to fit the carry through's. This will prevent binding of the bolts in the "H" tube. Because the stabilizer swings in an arc when adjusted, you have a very small range of adjustment that doesn't bind the bolts.
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