GWS Spitfire, twichy in flight, why the big tail?

[Spiteful]

The electric GWS Spitfire is very twichy in flight, and one has to be very conservative with the control throws, especially for the elevator. At the same time the plane is very hard to balance due to the short nose.

Why has the empennage been enlarged so much when the plane already is over sensitive to controll input?
The real spitfire was also very sensitive to elevator input, so wouldn't a smaller (more scale) and better looking elevator make it more sedate and controllable in addition to removing tail weight which would make it easier to balance? Not having to add nose weight would make it lighter too.

Any thoughts?

For those who don't know the model:
Length 720 mm (28.4 in)
Wing Span 876 mm (34.5 in)
Wing Area 13.8 dm² (213.9 sq.in)
Flying Weight 390 g (13.8 oz) (usually more)
Wing Loading 28.3g/dm² (9.5 oz/sq.ft)

GWS Spitfire page (http://www.gws.com.tw/english/product/airfly/spitfire.htm)

[Ollie]

The symptoms you describe are of a CG which is a bit too far aft. Making the horizontal tail smaller will move the neutral point farther forward and make an even more forward CG necessary to reduce control sensitivity. Your best bet is to find a way to move the battery a bit farther forward so the CG is about 1/4 inch more forward than at present. You can also reduce the elevator throw with dual rates or by moving the linkage farther out at the elevator control horn or farther in at the elevator servo end.

[Spiteful]

Thank you for your answer.
I've already done what you're suggesting and it flies fine like that. My question had more to do with the aerodynamics of it all.

Why is it that a smaller tail moves the CG forward? Does the tail contribute to the total lift, or is it that the plane kinda leans backwards onto the tail, making it possible to move the CG further back?

I know that most small scale models have very large tails. I thought it had to do with reynolds numbers or something like that. I'm not sure I'm getting this.

I also know that the torque from bigger props creates a need for a bigger vertical tail to counteract the increase in yaw, so wouldn't a smaller prop with more pitch make it possible to have a smaller vert. tail?

Just wondering.

Thanks!

[Ollie]

The plane has a neutral point which is the point through which all the resulatnt aerodynamic forces work. The aerodynamic center of the wing is at about 25% of the mean aerodynamic chord of the wing. The horizontal tail has an aerodynamic center at 25%of its mean aerodynamic chord. The tail moment arm length is the distance between the wing and tail aerodynamic centers. The neutral point (aerodynamic center of the whole plane) is located at a point where its distance to the wing aerodynamic center times the wing area is equal to the distance from the neutral point to the tail aerodynamic center times the tail area. The neutral point gets its name from the fact that when the CG is located at the neutral point, the plane will be neutrally stable and go where it is pointed without any tendency to correct. When the CG is ahead of the neutral point, the plane will be stable and correct from any minor disturbances like gusts or abrupt control return to neutral. The distance that the CG is ahead of the neutral point is called the static margin. The bigger the static margin the stronger the correction from a disturbance and the more control deflection it takes to change the attitude of the plane. The location of the CG can be used to increase or decrease the static margin and adjust the stability and control response to the desires of the pilot. Increasing the tail area or moment arm length moves the neutral point aft and increases the static margin. Decreasing the tail area or moment arm length moves the neutral point forward and decreases static margin.

Calculating the location of the neutral point is complicated by the fact that the turbulence behind the wing and around the fuselage slow the air flow over the tail, rendering its area less effective. Accurately estimnating that reduction in effective area is quite difficult in most cases. However, the neutral point can be located quite accurately by moving the CG aft in small increments until neutral stability is obtained.

Warning! If the CG is behind the neutral point, the plane will be unstable and the flight path will diverge. If the CG is much behind the neutral point the plane will become uncontrollable and diverge faster than the pilot can correct.

[Spiteful]

Now THAT's an answer. Thank you.

[Ollie]

I forgot to mention damping. When a stable plane corrects from a gust or abrupt control input, it can overshoot the correction and go into a damped oscillation. If it does not overshoot it is said to be critically damped if it takes a long time to complete the correction and over damped if it completes the correction quickly. The degree of damping increases as the square of the tail moment arm length. Long tail moment arms result in "groovy" handling planes.

If the oscillation drives the plane into a nonlinear region of lift vs. angle of attack (stall) the oscillation can change from a sinusoidal oscillation to a relaxation oscillation where the energy input exceeds the energy dissipation by damping and the amplitude of the oscillation grows.