ORIGINAL: jetnuno
Still I am flying my F18 with unbalanced stabs and I have over 50 flights now.
I want to balance in the pressure center and not in the c.g. of the stab.
The PC, should be backwards of the rotating axis and the actual stab pivot of the F18 flying stab, is too much forward of both PC and rotating axis.
I have flown my F-86 with unbalanced stab for 400 flights and it is fine, but it would be better balanced! It would draw less current and put less stress and wear on the servo.
I think you mean the Aerodynamic Centre, the Centre of Pressure is a different thing, it is a combination of lift and pitching and it moves about a lot. The AC of a stab or a canard is the same as for a wing, roughly at 25% of the MAC. The exact point of the AC depends on the wing section being used, if you plot the AC position for a lot of sections you find the majority clustered close around 25%, a few out around 24 or 26% and a very few out near 23 and 27%. Thus for model flying purposes it works to use the approximation of 25%
A model may replicate the position of the pivot and balance of the full size but that may be totally unsuitable for a model! We have to remember that aircraft like the full size F-18 will be operating in the transonic and supersonic regions where airflows and centres of pressure change enormously compared to our strictly subsonic models. The full size will have some features designed to cope with the dramatic changes that we will not eno****er. It could well be optimised for cruising in the transonic region and therefore be utterly wrong shape for our subsonic flight. Also the full-size is designed by very clever professionals with super computers, ours are designed by amateurs using rule-of-thumb approximations, so the full-size can do things quite differently because they have calculated exactly what is happening while we are guessing. And finally the full size is destabilised and as a result is fly by wire, whereas although our model is the same shape our model is most definitely stable with amuch more forward CG and does not have the luxury of fly-by-wire.
So if we make a model of Spitfire which operates in the same subsonic region as us and has the same classic controls, we can make our model using the same rules. But when we model something that is supersonic and destabilised and fly-by-wire, but we still have to use the same Spitfire rules of subsonic, stable and classic control, we are going to see things that look bizarre to us and wonder if we should emulate the pivot point and balance of the full-size.
H