Hm appears to possibly be a Jr flitestreak or similar. For this to work your engine needs to point straight forward, and the wing as well as the stabilizer incidence needs to be zero. 0-0-0 means the wing, stab and engine are all pointing in the same direction.

Also measure carefully to make sure the left wing is not longer than the right wing (this is common with some controlline airplanes but it won,t work in RC)

Ballance the airplane no further aft than in this case about 20% MAC. Thats one fifth of the wings average width from the leading edge. Yes that is further forward than much of RC practice but will be neccessary for this airplane to acheve pitch stability. It also is further back than what would be normal for this airplane as a controlline ship.

The ailerons are to small in cord near the tips to be very effective at any but the highest speeds so they will be ineffective at low speeds adding to the flight difficulties. A control set up like this with no rudder and just elevator, ailerons and throttle can work well but for the already mentioned smallish ailerons.

Part of the control senario i.e. Elevator, Aileron and Throttle is also limited by the engine. The OS Pet .099 with the early Bottle insert carburator, this from about 1959 or 60 is essentially a two speed carburator. The barrel in the bottle has two holes (high speed and idle) and there is no mid range bleed or needle making anything other than full throttle or idle a non issue.

What prop are you using?

John