Hi everybody,

Thanks for all the responses. The reason I ask is I have two 55ax(s), one in an ultra sport 40, the other in a MKI P-51 PTS from Hangar 9. Both planes are swinging APC 11x5s. The performance of the Ultrasport and its 55ax is stellar. It is fast and has good vertical performance (but probably not unlimited, unless it comes to a crawl and goes vertical slowly inch by inch). The P-51's performance is not so stellar. It is not very fast and vertical is not really there. Now, I know that this is comparing apples to oranges. I usually tach my 55s around 11.5k rpms on the ground. I do not believe in *****g out the engines, thus shortening their lives. The p51 has about about 170 flights on it and it still has incredible compression. I did not tach the Ultrasport 55ax but I believe it is higher, maybe 12.5k. One thing that I notice is that on the p51, the wing, where the exhaust hits it, looks like a BP oil rig sits there. It is covering in oil. I am thinking that I should probably slowly lean out the engine until I get a more reasonable amount (read lesser) of oil. Therefore, I wonder what I should be taching the 55s at. I typically have a rule of thumb for props and engine sizes: 10" for .40s, 11" for .50s, 12" for .60s, 13" for .70s and 14" for .90s. Of course your mileage may vary on type of plane and type of flying. Also, OS seems to be a little enthusiastic when it comes to propping their AX line of engines. The 55ax will perform like a dog with a 12x6 or 13x5. I have tried them and seen another use a 12x6 on a Wild Hare with so-so performance. But I have used the 55ax and 12.25x3.75 APC in a Dazzler and it was a great performer with unlimited vertical (and we are talking about fast verticals). Kind of a pain in the butt to land with that heavy engine on the front. As far as in wear, I can only comment that my two 55ax(s) seem like new, even with 170 flights and 80 flights on them. But I do take very good care of the engines and do not max peak them for that last ounce of performance. I also add castor oil to my fuel (run about 20-22% oil synth/castor oil) and the bearing still look shiny and smooth (I have heard that OS bearings are total crap and do not last long, especially from one guy in our club, but his OS two strokes make my 2-strokes sound like 4-strokes if you know what I mean!)

Happy flying and soft landings!

SER00