Without having much experience in the rc world, I had the ambition to design and build an own rc plane. The first plane I built was a 3 channel and looked about like this http://4.bp.blogspot.com/-SCkEBwUEDQ...0/IMG_2487.JPG but I made it from foam. I was able to fly it more or less successfully a few times, but it was way too tail heavy (the nose was pointing up in the air and I constantly had to push the elevator). But other than that it was pretty stable. Then I shifted the center of gravity to the front but it crashed right at the start. Now I am thinking about rebuilding it, but I am afraid that I'm gonna make it too nose heavy and it will crash again right at the start.

So my question is, how can you balance the plane before flying it? How can you figure out where exactly the center of gravity should be?

This online calculator will give you a good starting CG.

http://adamone.rchomepage.com/cg_calc.htm

Being tail-heavy doesn't make a plane fly with its nose pointing up. That's determined mostly by the angle of incidence of the wing.

I use the calculator Al posted for planes that were given to me over the years if they aren't constant cord. It works great. Then when I'm at the field I may make slight adjustments to get the CG exactly where I want it. All adjustments are very minor moves, big moves can jump up and bite you in the butt!!!

Looks great and a very easy one to balance at the 25% mac. All that means is balance at one quarter of the wings chord. That is where that airplane will fly best at.


Since that wing is straight chord (no tapers) you simply need to measure the distance from the front edge of the wing to back edge of the wing and divide that by four. And that sum is the distance you measure back from the front edge is where you want the airplane balance level at. Do not balance it a little nose down for mother and country, balance it level at that point!

Easy and you did not have to use a computer and makes so easy for your future airplanes that happen to be straight chord and you learned something. After all that's what designing you own airplane is all about

John

Thank you very much for all those hints! The length of the wing is 23cm, so the 25% will resuilt in about 5.75cm from the front edge. The calculator returend about 5cm, so I think I am gonna go with that=)

The calculator just gave you a safer place to start but after doing a lot of trim flights you may find the 25% rule of thumb is where you end up. I'm like John and on any constant cord plane I usually go right to the 25% mark but with swept wing or tapered wings I use the Calculator to start with. I was given a pattern plane and used the calculator and it was right where I liked it and never made a CG change. I tried a few different places but it ended up at the original spot.

Just curious: Where was the CG when you flew it before?

Well I just had to reconstruct it: It must have been around 10-12cm back from the front edge, and that obviously explains why it behaved the way it did=) When I built it I must have assumed that the cg just has to be in the middle of the main wing, as it is what supports the weight in the air. I am sorry that I cannot provide any images since my camera broke a few weeks ago, and I haven't bought a new one yet.

work on the basis of the wing spar is close to the cof g

The fact that the plane flys with the nose up does not necessarily mean that it is tail heavy. It more likely means that the alignment of the wing and stab are not properly set on the fuselage. If you changed how the wing mounts to the fuselage (in your case lower the trailing edge of the wing and change the elevator angle of attack the same amount) it will fly with the nose lower. With the exception of a change in total drag, the flight characteristics will remain the same as before the change, just flying with the fuselage more horizontal.