Hi Davide

Although I haven't yet bought an AX myself, in my training and frequent maiden/test flying role I get to setup and fly an larger number of .46AX's than if I actually owned several examples myself. They are the popular 'safe' buy amongst nOObs/studs as either their first or second RC engine. For basis of comparison, I also get to fly most of their common sport contemporary alternatives, many of which I won't deign to call "class competitors". I personally use a myriad of similar class .45s & .46's everyday, some in multiples of several thereof. eg: O.S.'s 46SF AB'C', the AX's immediate predecessor the O.S. 46FX ABN; ENYA's stunning 45CX AAC; and of course TT's legendary PRO 46 ABC; et al.

O.S.'s AX needs to be propped to operate to advantage itself of its peak torque and power curves. Otherwise, typical of a two stroke of its class, you're sacrificing a large potential power percentile. Next, due to the fact that our RC props are of fixed pitch, the prop chosen needs to also take into consideration (A) the airfame the engine is going to be married to, and (B) the intended flight envelope/task. The final choice of prop is a compromise based upon these three requirements. This still allows for a fairly wide range of props this engine can be successfully operated with. But IME, task and airframe (parasite drag) dependent the optimum choices for the AX would be a selection from one of the following.

Small or aerodynamically efficient (clean) where speed is desired = APC 10x6, Bolly 10½x6, or my favourite an APC 10x7 will all work very well without challenging tuning skills. If you desire to do aerobatics requiring sustained vertical using any of these, they will give excellent vertical, but you will need an understanding of energy managment and to fly the model accordingly. Research Bob Hoover for a demo of what I mean by the term energy management. On any of these sizes, the engine should rev hard and the model should go like stink. Omit the silicon exhaust deflecter and use 10% nitro in the AX for optimum sustained vertical performance of the prop under load.

Training, or general flying over a wide range of trainers and sport airframes. APC's 11x5 or 11x6, or a Bolly 11½x5 all work in the band, the 5" pitched ones offering superior acceleration and aerodynamic braking. All 3 are pretty much no brainers on the AX. For optimum performance, match to airframe.

For 3D fun flying, APC 12x4 or their 12¼x3¾ both work very well. Superb acceleration and deceleration, but anticipative throttle control is needed as they'll wind up fast, especially in downlines or a power on dive.

Although people will turn up with them from the LHS all the time, IME&O an 11x7 of any brand is too much prop for this engine. Sure the AX turns it, but..... you can achieve superior performance in almost any airframe with an alternative prop selection from one of the above mentioned range. Whilst I haven't actually tried a 12x5 on the AX, clearly anything 12" above that pitch would similiarly be working this engine outside its band, so if a 12x6 or > is what's desired, either go up in engine capacity or go four stroke where its torque and power curve will be more suited to turning that diameter prop in a usable RPM range. Other than large capacity 2 strokes timed for pattern, sport engines such as the AX are ported to develop their high relative HP for capacity in the upper ¼ end of their spec. RPM range. For the AX, that means from 12k upwards.

Edit:sp