Just got a 91FX and noted in the manual that 6 different props are suggested. Kewl that so many are recommended, but them suckers are fairly expensive. I don't feel like doing the old "try every prop possible and pick the best" dance.

Once upon a time, if you had a .35 engine, you couldn't go wrong with a 10x6 on it. Is there a prop size or two that're like that for a 91??

The manual shows a 15x8, 16x6, 16x7, 16x8, 17x6 and 14x7(3blade) as suggested for sport flying.

BTW, that 14x7(3blade) seems too small. I figure a 3blade might be in the cards since the 91 is going on a 60size airplane and those size airplanes were mostly designed for 13" props. I can't see swinging a 16" on a design that just clears a 13", and REALLY can't imagine a 17" on the same.

Darock

I use an APC 13X8 on this engine in a GP Shoestring with a Bisson Pitts style muffler. This setup works well for a fast plane, but might not be so good in a 3-D or slower scale model.

Sandy T

Thanks for the info Sandy T

I bet that engine of yours screams. That does seem like a very small prop for a 91. It's 2" smaller than the smallest OS recommends. It's sort of amazing what we can actually do, isn't it.

Darock,
I have two of these engines and I felt the same way when I read the instructions. I've run almost all of the props on the list including the 14-7 three blade.

For me, I felt the 14-6 was the best prop.
15-8 was OK for scale flight.
16- 6 flew suprisingly well but spool up was really slow.
The 14-7 had really slow spool up also.
I have not tried anything smaller than the 14-6 since I was so happy with the performance on my Kyosho CAP 232 60 size.
Tower/Bax et al suggest getting the engine to reach 11K+ and the 13-8 prop. I was getting 11,500 on PowerMaster 10-22 and the 14-6.
I know you are not too far from me so send me a PM if you want to barrow some props.

darock....

....that's one of the main reasons OS has had so many problems with the
.91 FX....most of those props are....

....way too big for a bored out 60 engine....

13 X 8 or 14 X 6 work well on this engine...

ditto. I use 13-8 or 14-6. rews about 11000 on those. Os recommends way to heavy props

You got that right Flyboy. Those props are way to big for even the old, big case OS90. Those are 120 size props. I use a 13/10 or 13/8 3 blade for pattern. A 15/4 for 3D

Denis

man, I'm amazed.....

It really sounds like OS screwed up with the recommendations in their manual. And I just ordered a couple of the middlesize props (their "middlesized") to try out.

Hey, it just dawned on me how awful this is!!! What can I possibly do with props that're for an engine size I don't have???? Awe, man, am I depressed!!!

NOW I GOTTA BUY AN ENGINE TO RUN THOSE PROPS!!!!

ain't that a shame [sm=wink_smile.gif]

And then.... I ain't got no airplanes that engine would go on....... Jeez, it never ends.......

I bet an OS 120AX would run em real nice or maybe a 120 Webra

I run a 14-8 on an OS 1.08 FSR, which is a powerful engine.

The thing is, if you over prop an engine, it is like a car that is driven
uphill forever, it will heat up, and never get a chance to cool down,
it's always under a load. Engines that are propped so the engine can
not "rev freely" are destined to give problems....where as a similar
engine that is running cool and free, will give better running results.

Add to the over propped engine...some hot running fuel, a Pitts
style muffler, a cowling.....and have it running up hill all the time....

....you have a recipe for an over heated, and sometimes damaged
engine, as well as dead sticks and crashed airplanes. []

FBD. [sm=teeth_smile.gif]

So OS is actually recommending up to 16-8??????? Well, I tached my 91 fx when it was bone stock and on a 16-6 wood prop (don`t remember the brand) it only managed 8000 rpm peak. It was slooow and spool up was non-existent, how bad wouldn`t it be on a 16-8? Maybe if you only fly vertically straight down[sm=bananahead.gif]

I think I know what it is...

It's either a vast conspiracy for O.S. to cook their engines so they can sell more of 'em (or parts )

Or...

They're in cahoots (sp?) with the airplane manufacturers...right darock? I mean now, you gotta go out and buy new airplanes just to go along with all the extra props, and the engines they will need to spin 'em...right?!

J/K

I just remembered that Tower's catalogs always have lots of information. I just went to the online one and the Tech Info page for the engine has a list of props and the rpms the engine gave with each.

My .91FX works best for me with a 14X6, on my old Cermark Sukhoi. The 13X8 works, but the idle is less reliable and the plane doesn't slow down very well. The 15X6 slows down almost too well, and the engine spools up a little slow. I like the 14X6 because it provides some braking, a reliable idle, and it spools up quick. Great combination.

Good luck,
Dave Olson

Please remember that O.S. is making recommendations based upon testing in Japan. In our discussions with them, they acknowlege that the engine will run quite nicely on the smaller props that allow it to turn between 11K and 12K RPM. They also mention that the noise level becomes unacceptable. Japan and Europe (and likely other places) have fairly strict noise restrictions, so the larger props help keep the noise down...a lot! Go with a lower-nitro fuel and the larger props, and you'll find the engine won't have an overheat difficulty.

Propping the engine closer to 10K will also make it less-suited to the 3D-type of flying that's so popular in the U.S.

Thanks Bax for that info.

It reminded me to not get fixated on the rpm range. We often get some number in our heads and wind up test flying only props that will turn those r's on the ground. And wind up missing out on sometimes finding better performance.