RUBBER BAND POWERED AIRPLANE HELP REQUIRED.
PS THIS IS MY FIRST MODEL AND I AM PLANNING TO MAKE AN INSTRUCTABLE.

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Build FULLY then ensure it balances left and right and the point of balance front to back should be about 1/3 of the distance back from the front edge of the wing. this is with ALL the plane assembled prop rubber and all.
This set up should glide Ok when pushed off in a slightly downward direction although it may need a bit of speed depending on it's weight/wing area.
If this works OK then you can put a few turns on the rubber motor and see what happens. You may need to block up the rear edge of the wind under power to keep the nose down.
It's good to try a proven design first so you can see what does work. A great proven design is the Squirrel rubber band plane. Make your own designs after that. Unless of course you are determined to come up with something new and have a lot of time to do this.
Pitch stability is one thing your plane needs. For that you need a wing and a horizontal stabilizer (back wing if you are making a traditional airplane). The wing should have a positive angle of attack compared to the back wing. The balance of the airplane should be somewhere on the wing.
Roll stability is another thing it needs. There are a few things that can work for this. Dihedral which is having he wings sloped up this works in conjunction with a fin at the back of the plane to create yaw roll coupling which results in roll stability.. Back sweep can also contribute to roll stability. Winglets can do this as well (like in the Squirrel design).
You also need yaw stability. A rear fin works for this.
This is all for a traditional tractor type airplane. It's different if it is a canard style or a flying wing style. Flying wings use reflex to get pitch stability and are much more sensitive to adjustment. I don't know as much about canards.
If you need roll stability, add dihedral deflection where necessary. If you need to prevent yawing motions, add and increase the size of the vertical stabilizer and/or the wingtips.
Generally, model biplanes are easier to make if they use annular wings like my YD142-1 prototype, seen below.
All of these planes are very comparable to the prototype above, and all have instructables already posted. I have found them very good testbeds for many configuration experiments.
THE BIPLANE I CREATED IS MORE STUBBY AND LOOKS LIKE A BIPLANE AND IS MADE OF CARD. MAYBE THAT IS THE REASON IT DOES NOT FLY. PERHAPS ILL SEND YOU A PICTURE. UR ANSWER IS REALLY GOOD. THANX FOR BOTHERING.
COMING TO THE TUMBLING MY PLANE SPINS FORWARD AND GLIDES VERY SLIGHTLY JUST BEFORE PLOUGHING INTO THE GROUND.
Aerodynamically, I really never design planes that aren't "clean". I generally have as much of the plane made sleek as possible. When drag is unnecessary or not desired, it should be fixed. That should increase range and potential speed.
As for stability, if you are describing correctly "spins", then your airplane seems to be stalling very quickly. If your plane is to have or has an engine, then it is likely pulling into a flat spin. To cure for this problem, increase the size of the vertical stabilizer, and perhaps even trim the plane if you need to.
Are you an aerodynamics genius or do you have plans ?
Is the band the engine ?
You are vague on lots of points.
A
http://www.rubber-power.com/make-it.htm !!