Ben Lanterman
Posts: 1286
Joined: 10/27/2002
From: St. Charles, MO, USA
Status: offline
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I was out flying today.
Since retirement, and since my wife loves me, that happens more often now than when I worked. Since 5 bypass surgery and a valve replacement it is important that I always have someone with me. She is by far the prettiest of the folks that I know. Off the track though.
I flew the Cap and a QuickFly III (KwikFli, QickFly, QuickFli, etc). Most of the aero moments in pitch, roll, and yaw are close enough to each other that they don't really matter. The horizontal tail height relative to the wing is within a few percent of wing mac of each other.
The main difference is that at least 35-40 percent of the rudder area on the Cap is below the location of the horizontal tail, along with a lot of fuselage area while on the QuickFly maybe 5-10 percent of the rudder area is below the horizontal tail along with less fuselage area.
The Quick fly has very little pitch response when in level flight and the rudder is deflected. In knife edge it seems to need a little up elevator but not much. Without rudder the deviation toward the belly of the airplane is small.
The Cap has a lot of pitch response nose down when the rudder is deflected. In knife edge it seems to need a lot of up elevator, a lot. Without rudder the deviation toward the belly of the airplane is a lot - big.
The conclusion is that the pitch response as a result of rudder deflection is a function of where the horizontal tail is located vertically along the vertical tail and rudder.
It has to be a function of pressure differential across the horizontal tail - after all the way all aerodynamic surfaces work (including the control surfaces) is by pressure differential. If you look at the two configurations from the rear and put in pressures before and after rudder deflection (just relative pressures will do) it works out fairly well.
Since in level flight the horizontal tail on these kinds of force and moment setups has an upload on the horizontal tail that is proportional to the wing lift (both are always the same angle of attack modified by downwash) you would think that an airplane that is rolled 90 degrees should not pitch at all. No downwash, wing and tail at the same angle of attack - the resultant is probably zero.
The airplane that pitches the least in knife edge is the one that is the most pure aerodynamically in all axes.
Granted the QuickFly rudder location with respect to the airplane roll axis will give a roll opposite the direction of deflection but this is probably less objectional than the rudder-pitch coupling.
One conclusion is that the old Quick Fly is better than the Cap in this respect. Something that on first glance I wouldn't have believed.
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Ben Lanterman
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rudder-pitch coupling - flight test - thoughts - 
