I have a scratchbuilt Curtiss SBC4 biplane which has retracts which fold up into the fuse. When the LG is down there is a hole or opening which goes thru the fuse; completely thru from side to side. The model flies just fine with the landing gear down. When the gear is retracted it still flys great with positive control; It's quite a gentle plane to handle. Now the funny part, with the gear up it swims like a fish, almost like I am swinging the rudder from side to side when in straight and level flight. It seems to be worse when flying downwind but I'm not sure. It has a very large cowl and the prop, a 3 blade which only extends 2 inches beyond the cowl but has adequate pull on a 90 FS. It does not swim when the gear is down????

Would appreciate any thoughts