Errors with FlexBuilder 3

Hi!

I have .fla called Helicopter and a movieclip in the library with a linkage of Fly. If I use the code-snippet (as a seperate class called Helicopter.as) below with Flash or Flashdeveloper it works fine. If i use FlexBuilder (Actionscript project)I get these errors. Does anyone know why?
Is there some settings I have to change in FlexBuilder?

Severity and Description Path Resource Location Creation Time Id
1046: Type was not found or was not a compile-time constant: Fly. Helicopter/src Helicopter.as line 7 1247730594859 770

Severity and Description Path Resource Location Creation Time Id
1180: Call to a possibly undefined method Fly. Helicopter/src Helicopter.as line 7 1247730594868 771

Severity and Description Path Resource Location Creation Time Id
1180: Call to a possibly undefined method Fly. Helicopter/src Helicopter.as line 7 1247730594869 772

//--------------------------------------------//
package {
import flash.display.Sprite;

public class Helicopter extends Sprite
{
    
    private var f:Fly = new Fly();
    
    public function Helicopter()
    {
        
        addChild(f);
        
    }
}

}