BobbyRichardson

On an aerobatic airplane with a CG too forward will it pitch to the belly or canopy. and which will it pitch to with an aft CG?

mesae

I assume you are asking about knife-edge flight. There are other factors beside CG position that affect pitching with yaw, but CG does have an effect.

Considering only CG, a well forward CG will tend to cause canopy-ward pitching in knife-edge. This is because up-trim is necessary to counter the forward CG in upright flight. This up-trim is still present (unless commanded out) when the airplane rolls on it's side and yaws to maintain altitude. Now that gravity is not countering the up-trim, the airplane will tend to pitch canopy-ward.

The opposite may be true with an aft CG, but there are other more important factors that cause pitching in knife-edge when the CG is well aft. P-Factor is an important one of these, but not always the strongest.

Say you built a perfectly symmetrical airplane (thrust offset zero, wing and horizontal tail on thrust line, fin and rudder vertically symmetrical and centered on thrust line), and with a single propeller rotating in the conventional direction (almost like many precision aerobatic airplanes). The static margin is near zero and pitch stability is nearly neutral, so that little or no trim change is needed to maintain altitude upright or inverted. The rudder and elevator movement geometry and torque are exactly symmetric. If you roll this airplane 90° left from upright level flight and apply right rudder to maintain altitude, the airplane will pitch slightly canopy-ward if no elevator correction is applied. If you roll it to the right 90° and apply left rudder, it will pitch belly-ward. This is due to P-Factor. It's one of the same factors that cause an airplane to yaw left in a climb unless rudder is used to correct it.

This force is strong enough that it shows up on many well-trimmed aerobatic airplanes that are not set up precisely as described above. In fact, I have long considered this behavior a sign that my own plane(s) are set up and trimmed properly, because it means the airframe is neutral enough that the relatively weak (but significant and unavoidable) P-Factor is the only thing left causing knife-edge pitching. There is no other asymmetry on a well-built and set-up airplane that can cause it to pitch "up" with right yaw knife-edge, and "down" with left yaw knife-edge. Then I can simply apply a small amount of up or down elevator depending on which direction I'm yawing. Or, you can try to mix it out.

Caps and other low-wing airplanes often pitch belly-ward whether in right or left rudder knife-edge, due to airframe factors. This bell-pitching often becomes stronger at higher yaw angles, but may be nearly unnoticeable at high speed and low yaw angles. But every such airplane will pitch belly-ward more strongly when applying left rudder in knife-edge flight, because the airframe-induced belly-ward pitching moment is working with P-Factor in this direction, and against P-Factor when in right rudder knife-edge flight.

If you were not asking about knife-edge flight, then forget this whole post .

BobbyRichardson

I was asking about knife edge flight and thanks for your help. The planes starts to pitch towards the canopy during the first 30% degrees of roll before rudder is introduced to hold knife edge. after the ruder enters the scenario i have the pitching to the belly mixed out and will track straight with pure rudder input. Would the initial slight pitch (enough to track off line) be P factor. I added some weight to the nose 1 1/2 oz on a 90 size plane and it helped the pitching??

mesae

"The plane starts to pitch toward the canopy during the first 30° of roll before rudder is introduced...". This might be the result of an unconscious movement of the elevator as you are moving the ailerons. It's common. I catch myself doing it sometimes. It's possible to develop habits like this that you do consistently without realizing it. Or you might need to roll in faster? If you bank slowly to 30° or so, the airplane will naturally start to turn a bit. P-Factor is not a likely cause of the deviation you describe, since that only becomes significant when the propeller disk is not normal to the slipstream.

You don't have rudder slaved to aileron, do you? That might do it too. Check all your mixing on the ground to make sure it's doing what it's supposed to.

Which pitching did the nose weight help, the initial upward pitching, or the belly pitching that you have mixed out?

What kind of plane is it?

BobbyRichardson

It is a funtana 100. the initial upward pitching.

mesae

Anyone else with some Funtana time see this behavior?