CG question from a novice

[rosco]

I am guessing that a normal CG on a traditional type of trainer would be close to or on the main spar of the main wing...
If so, what would be the result of using weights to move the CG toward the rear of the plane, and the result of moving the CG to the front of the plane?
Is there a simple or general answer tho this question...
cheers
rosco

[Sparky Paul]

Tail heavy planes may fly once.
Nose heavy planes will fly like logs.
Tail heavy makes the plane more responsive. Too much so, and it rekits itself.
Go aft cautiously.
If the plane flies well, there's no need to go forward.

[rosco]

When you say 'tail heavy' are you implying that the CG is at the main spar and weight is added to the rear? Which is essentially saying that the CG has been moved to the rear, because the ballance point would then be more towards the rear...
Appologies for the confusion.

[Sparky Paul]

That's correct. "Tail Heavy" means the c.g. is aft of the recommended location, however it may be defined.
"at the spar" is common, but a more correct definition relates the c.g. position to the wing chord... 25% say, or 3-1/8" aft of the leading edge... depending on the plane's geometry since some planes don't have spars. :)

[rosco]

Nice one!
Thanks for that info
cheers
rosco

[BMatthews]

Rosco, high performance glider pilots and some aerobatic power pilots use something called the dive test to help set the CG back as far as they dare.

For a power trainer it involves setting the trims so the model flies hands off in level flight at about 1/4 throttle. Not 1/4 stick but about 1/4 the RPM sound up from idle. Trim for level and then push forward into a 45 degree dive and release the stick and watch the pull up. Most trainers will rotate to a nose up attitude quite quickly. A high performance glider or aerobatic model won't pull up at all but won't tuck into a steeper dive either.

To ease toward this "high performance" setting you need to move the CG back a little, reset the trim for level flight as per above and try another dive. Keep moving the CG back in 2 to 3% chord steps with elevator re-trimming and dive testing until the model pulls up but does so with a loooooooong and gradual pullout. If it's gradual enough you'll need to pull it out yourself before it lawn darts itself. Be particulary careful about watching for any sign of the model trying to tuck into a steeper dive. Some of the flat bottom trainer airfoils won't show this until the speed builds to a high value. If you see signs of this tucking then move the CG back foward to the last spot you don't see that happen.

The model will drastically change how it flies and reacts to stuff at this setting. For starters it'll be far more sensitive to the elevator inputs and you may need to reduce the throw. Beyond that it depends on the model.

If you're doing this to learn then learn away. But be prepared to need to "fly" the plane more as a lot of the autopilot like effect of a trainer will be gone and it's now looking to you and your skills for survival. In reality the best spot will be where you find the model suits your skill level. As you progress you'll likely find you can shift the CG back a little at a time until the model suits how you fly.

At the end of this there's something called the Neutral Point. You'll be getting close to this if you do the above dive testing but don't go past it or the model will start acting like you're trying to balance a broom handle on end in your palm.

[rosco]

Thankyou also for your input. I will have a play with some small bolts and double sided tape. hehe (this plane is a bit of a 'war horse'!)

[B717 Furloughee]

All airplanes have a CG fore and aft range within which flying will be safe and controllable. The forward range of CG will lead to stable characteristics at the expense of manueverability. Conversely, the aft range of CG yields higher manueverability at the expense of stability. Flying outside the recommended CG envelope is risky.

There are advantages/disadvantages to both extremes. Generally, a forward CG is better for training while an aft CG is used once the pilot is experienced.

BTW- Shifting battery/components fore and aft (if possible) to adjust CG is preferable to adding weight to the aircraft.