C of G confusion

[Hangar Rat]

I am having trouble getting my Beaver to loop / roll (Yes, I know thats not a scale characteristic) The aircraft is very stable and the C of G is at the manufacturer recommended location. Should I move the C of G Forward or Back to get more sensitivity. Also, what are the results of going too far?

I'm supposed to know all of this but... :p

Thanks

Ratty

[steve lewin]

Moving CG backwards will make it more sensitive in pitch, i.e. to the elevator. It won't affect the roll characteristics very much. Too far back usually makes a plane so sensitive it's impossible to control. It's likely to porpoise all the way to the crash :(.

But if it won't loop OR roll it may just be short on power. Can you get it to loop or roll from a dive ?

Steve

[Ollie]

The factory recommended CG location is usually a good, safe place to start adjusting the aircraft. Every plane has an acceptable range od CG locations. This range is usually about 5% of the average wing chord in width.

The CG position determines stability and control response. As the CG of a stable plane is moved gradually aft:

1. The plane becomes less stable in pitch and yaw. That means that the plane has a weaker and slower return to the previous flight condition after a small gust or controls return to neutral.

2. The elevator and rudder control response increases.

3. The pitch trim becomes more nose-up and it may become necessary to apply some down elevator trim or reduce the decalage (raise the trailing edge of the wing or the leading edge of the horizontal tail) to retrim for level flight.

4. When the CG reaches the neutral point, there is no stability left and the plane goes where it is pointed. There is no tendency to return to normal flight after a gust or control input. This is where the control response and quickness is maximum.

5.When the CG is moved aft of the neutral point the plane becomes more and more unstable. The flight path becomes difficult and eventually impossible to control.

Because instability has such adverse consequences, the CG should be moved aft, only in very small increments, between test flights. When the plane becomes slightly difficult to control, move the CG forward a bit and leave it there.

The final CG location will depend on the experience, skill, eye sight, reflexes and concentration of the pilot as well as the purpose of the plane. Trainers need lots of stability. Aerobats need little or none.

[Viper Pilot]

Ollie,

Very complete explaination. Concise, and to the point.

Thank you for your input.

VP

[Hangar Rat]

Well, thanks Chaps - very helpful. Had the first flight of my Twin Star today :)) and it seems as though the C of G might be very slightly too far back but on the other hand, as it is a new aircraft to me, it might just be that old favourite 'Pilot (T)error'

As usual, the e-zone helps once again!

Cheers

Ratty

[vintage1]

hehe. Time to expo and dual rate the controls..