Porpoising . . . - Page 2 - RCCanada - Canada Radio Controlled Hobby Forum
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Old 12-06-2012, 01:11 PM   #11
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I am: Kevin C.
Join Date: Jan 2007
Location: Courtenay,BC

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Re: Porpoising . . .

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I'm borderline old-codger myself, but also happen to have a formal education in aerodynamics as well as a lot of chuck glider and RC experience. There are a lot of myths about airplane stability and control in modelling circles.

CG position relative the aircraft neutral point is what sets the aircraft stability. Wing incidence angle is for setting the fuselage angle in flight, at cruise usually. Stab incidence angle is only for adjusting the trim speed, and has nothing to do with aircraft stability.

For example, take a glider with a full flying stabilizer. Moving the elevator control effectively changes the tail incidence. Moving the stabilizer position only changes the trim AoA of the wing, it does not effect the aircraft stability. If the CG is ahead of the aircraft neutral point, the aircraft stays pitch stable regardless of the stab incidence. The trim AoA can be adjusted from slow upright flight, to fast, to straight down, to slow inverted flight through changing the stab incidence. The aircraft stability stays the same through the whole range, ignoring secondary effects like CG and wing heights, etc., for the moment.

The same thing occurs with a stab with an elevator. The zero lift angle of the stab changes with elevator deflections, the same as with a FF stab. The stab load stays pretty constant throughout the AoA range.

The elevator most definitely gets more sensitive as the CG moves aft, and unless the throw is reduced the model will seem difficult to fly at aft CG positions. If the throw is set properly, aft CG positions are easily flyable, and are in a lot of ways easier to fly. Most sailplanes and pattern models are flown with near neutral CG positions.

Biplanes can have several other effects that could cause pitch issues, but getting the CG in the right place is critical to the pitch behaviour. This is a very good CG position calculator that handles biplanes:


If you have a model you like the pitch response of, finding what it's static margin is, and applying the same static margin to a new airplane will result in a similar pitch feel. That can take a lot of the guess work out of setting up a new airplane.


Last edited by kcaldwel; 12-06-2012 at 02:06 PM.
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