Ben Lanterman
Posts: 1286
Joined: 10/27/2002
From: St. Charles, MO, USA
Status: offline
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Don't Panic Dick. However Dave Brown is wrong! He thinks the airplane flies with a small down load, not true. He should stick to flying.
Start with a very nice layout to start with, Pick one, the Hydeaway or something similiar. No funny configuratons allowed. Nice round fuselage so that doesn't effect the results. If it is being used competitively then in general the tail configuration has been worked out to be reasonably nice regardless. Now take that and start fine trimming it.
Remember what you are seeing in level flight is a wing and horizontal at the same angle of attack, as Flexible Flyer noted, with a set deflection in the elevator trim (that does vary with CG location for a constant weight) that changes the upload on the horizontal tail from a big upload (with no elevator deflection) to a smaller upload (with a little up elevator deflection) to maintain that angle of attack.
Roll it 90 deg and the wing is still forced to the same angle of attack that was seen in upright flight. The gravity isn't there when rolled, BUT, the pitch moments are still balanced about the CG and the angle of attack and 1 g of lift (wing lift and tail lift) that it produced still is. With no rudder deflection at 90 deg of roll it responds with a linear pull to the canopy. Assume with rudder deflection you get a nose down pitch. The nose down pitch overpowers the linear pull to canopy and the airplane continues to pitch nose down.
So it is heading to the belly. You would have us move the CG forward to correct it. With the more forward CG you need a different elevator setting in level flight - a little more up elevator which reduces the up load at the tail. The tail forces a slightly higher wing angle of attack which is subsequently shared by the horizontal tail. The results will still give 1g total in trimmed level flight - the wing and tail moments are balanced. Note that he horizontal tail is at the higher angle of attack too but the elevator trim is greater than the horizontal angle of attack change causing the lower up load.
So the wing lifts more, the tail less but it is still in level trimmed flight with a forward CG. Roll it to 90 deg and hit the rudder. Almost the same thing as happened before with the more aft CG. Note that you are trimmed in level flight with both CG's and both have the same moment response whether upright or rolled 90 degrees. Both CG locations will cause the same linear airplane response - mainly an initial linear pull to the canopy. But from flight testing we find that the nose down pitch has gone away, how?
There is a tendency to say the tail lift is lower so it will stop the pitch, not true, remember that the tail pitch (moment) is balanced by the wing lift (moment) in both CG locations and roll angles.
If not the change in horizontal tail load with trim then what has changed? We can assume the flow field variation about the horizontal tail with rudder deflection is essentially the same in either case since we are at the higher (but small difference) angle of attack. It must be that the rudder deflection is changing the pressures around the horizontal tail in such a manner that the higher elevator deflection configuration responds with less nose down.
But the pitch effect is not cured due to a change in basic elevator trim and the loading that comes with the CG change. The moments are balanced at all CG locations if the airplane is in level flight.
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Ben Lanterman
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What causes pitch with rudder? - 
RE: What causes pitch with rudder? - 
