I just finished building my Alpha 40 trainer, with a Magnum .52 for the power and now its time to set the CG and break in the motor before its first flight. My question is when setting the CG do I check with the tank full, empty or 50% ? The directions that came with the plane list the target CG point on the wings but did not give any suggestions about fuel level. The tank on the plane is mounted front of the wings, directly behind the firewall so Im sure the fuel level will affect the CG, or am I over thinking this? My flying partner who will be the person to do the maiden flight is out of town and I cant ask him but would love to have the plane setup when he returns to test fly this weekend. I got my first fly time on one of his trainers a few weeks ago, all I can say is there is alot more to it than i though and who ever designed the buddy box idea was a great person LOL

Empty Tank

Yep, always find your CG with the tank empty. Good luck

Only when the tank is forward of the CG. If the tank is to the rear of the CGthen you balance it with the tank full.

Ken

good tip, thanks RCKen

Simple answers - what is the reasoning behind empty if in front of CG and full if in back of CG?

You want to balance the aircraft in the configuration that gives you the balance point as far aft as possible. That way, you'll never have balance point move any farther aft during a flight.

Bax - Seems that you are erring on the side of a nose-heavy plane during flight. What is BAD about a CG that is too far back, especially for a beginner?

When a plane is slightly nose heavy, it will become more stable and less aerobatic. The more nose heavy it gets, the more noticeable this becomes. It will also become difficult to slow down for landing - but it will still fly and be controllable.

As you move the CG back, the plane will become less stable and more aerobatic, but the problem here is that once you reach a certain point, it becomes so unstable that it is difficult or even impossible to control.

Remember, the tail feathers are there for a reason - to keep the rear of the plane in the rear!

Think of a dart. It has a weight on the front and feathers on the back. The broad, light surface of the feathers will catch air to keep the point facing the front. Even if you hold a dart by its point and throw it tail-first, it will flip around so the heavy part goes forward.

Now, remove the heavy point from the dart and make the feathers out of lead. When you throw it, the heavy tail will lead the way.

It's the same thing with an airplane. If the tail gets too heavy, the tail will want to swing out in front - Not a good thing!

So it is alway beneficial to err on the nose-heavy side. As the old expression goes, "A nose-heavy plane flies poorly, but a tail-heavy plane only flies once"

Thanks Mike - great explanation ...

Thanks for all the great info. If things go as planed I will have the CG checked tonight, and a few tanks of fuel in the new motor on the test stand tommarow and maby maiden flight this weekend