OS 91FX vs 75AX
 

I am not sure I see a lot of difference in these engines but I am probably missing something. It appears the 91 is lighter with a muffler and more powerful. Is that correct?

I am looking for an engine for a UCD46 and I am definitley into hovering.

RE: OS 91FX vs 75AX
 

The O.S. Max .91 FX has more torque and can throw a larger propeller. Raw horsepower numbers are not what you need to look at. The .75 AX weight 14.29 ounces without muffler, the .91 FX weighs 19.42 without muffler.

We'd not recommend either engine for use on the Great Planes U-Can-Do 3D. The maximum two-stroke engine recommended for it is a .50-size. The model was not designed for larger. If you do go larger than the recommendations, you do know that the warranty on the model would be voided.

RE: OS 91FX vs 75AX
 

Thanks Bax,

I had an OS 50 SX-H on my UCD and it could hover it but no pull out.. This summer I wrecked the UCD and put the heli engine on my OMP profile Edge which had an OS-46. WOW - what a great combo!

According to Tower Hobbies and the OS website the OS-75 weighs 20.39 oz w/o muffler - so I am not sure where you get 14.29. I hope your number is correct! Can you verify that?

RE: OS 91FX vs 75AX
 

On the weight, we accidentally got the one for the .55 AX! Our mistake! For planes like the UCD, matching the propeller to the airplane/engine combination is essential. Many flyers use a 12 x 3.5 propeller or so for at .46-.50 size engine. That gives them the high RPM they need, along with good vertical performance.