I'm trying to balance my trainer and I'm told that the cg should be 1/3 the way back. How full should my tank be?

Thanks, Dave Trimmer

Dave,
The fuel tank should be completely empty when you balance your plane.

Ken

Dave,

Which trainer. 1/3rd the way back is a decent start but it's better if you know exactly where. If you don't someone else might

never balance a plane with any fuel in it.

1/3 back makes perfect sence. take the length from the leading edge of the wing to the trailing edge of the wing and divide by three. set it 1/3 back from the leading edge of the wing, 9 times out of 10 this is the rear edge of the spare

Most trainers I've seen balance on the spar.

enter the measurements and this site calculates your cg for you

http://sky.prohosting.com/air2/cg_calc.htm

Ive always balanced front spar bottom of the wing, slightly nose tipping. With an empty fuel tank.

depends on the trainer, I've seen ones where the CG is almost 35% of the root cord, others were 18% - 25% ... well, wait a minute now ... Were you just told this? Or is this what the manual calls for?
I had a Hobbico trainer and the cg on that was 2 1/2 inches back on a 10 inch wing cord ( 25% )... which, after the second tree it hit, I found out is exactly where the spar is. There is a trainer ... a tail dragger ... can't remember the name of it, but the guy was talking about the CG being 4 inches from the LE and had a 12.5 inch root cord. Well, just find out for sure, because there's a big difference between landing and crashing with style.

Every plane I've assembled to date gave exact measurements for the CG range from the leading edge of the wing, not fractions or percentages.

With your plane--yes, you balance the plane empty of fuel. This represents the aft most position of the CG. So, when you fill it up with fuel, the CG moves forward--which is manageable. However, on pusher type planes--and in any case where the fuel tank is located behind the CG, the plane is usually balanced full of fuel--again, representing the aft most point of CG.

To reitereate--with your trainer--empty of fuel, and on the spar would be a great spot to start--especially with such vague instructions!

Try reading the manual? They usually tell you where it is.

Maybe next time I shouldn't by it used from the flea maket.

Dave does the plane have any identification ?
Lots of times a plane will have its name right on the wing.
If no marking to ID the beast if you post a picture here someone might know what it is.

I am not trying to be trite or crass, but I think it would be extremely important that you do a little studying up on the aerodynamics of airplanes. Believe me I truly understand the urge to get out there and fly, but understanding the basics will make you a safe flyer, as well as a happier one too. It can be a pain, but knowledge is power. Good luck and welcome to the hobby.

For a trainer -- tank empty.

25% aft of the leading would be safe. It might not be optimal for that model, but it will be safe.

About the only time you would balance with fuel in the tank is if the tank is AFT of the CG.

Hi Dave, I agree with the responses so far, & my recomendation would be, get an instructor or experience pilot to check out your plane, he will check directions of flying surfaces, balance engine tuning & so on, & to fly it & trim it for you. better than doing it yourself & decking your new bird.
Cheers, & good flying.
Allan