One of two problems. First, the fuel pickup (clunk) is stuck in the front of the tank and when the nose of the plane it up it can't pickup fuel any longer. Second, you have the motor running too lean. When you raise the nose it leans out even further and dies. Try richening up the engine a little bit. A couple of click rich. You should be able to see just a little bit of smoke in the exhaust when you have it rich enough.

Hope this helps

Ken