i've given up on the Tower .75 (read my thread if you like)........................i should have bought the OS.75AX to start with.
i need an engine that isn't a "dead stick queen"................and the OS is it.
question is, through the search i see people using all sorts of props for this engine.
Bax says that OS recommends prop sizes based on noise rules in Japan.
the list shows 14x6 thru 15x7............but i'm thinking around 13x6.
i will NOT use APC for reasons i've witnessed, so my choices will lie in the wood prop selections.

thx
-mike

....throwing in the towel already ??? [X(]

yep..............although, i have an email into the nice lady i deal with at Hobbico...............let's say i will give anything a chance..................after that it's the 'ol paperwieght box..

Please send it to me, I need a another engine.

G'day Mike,
I ran my 75AX in on an APC 13x7, in a 60 size Diabolo, it spins it like a toothpick, now I'm running an APC 14x8, the engine handles it easily, & flys the plane beautifully, remember it's ported for larger props.
Depending on what plane you put it on, it would easily swing up to a 15x7.

15x7 on a 75 is got to be a joke. A fast prop on a 75 would be a MAS 12x7 and 13x6 is a 91 size. I would try the 13x6.........Sometimes OS way over prop recomends and the Japan noise rule would explain this.

G'day Mate,
Or is it possible that, to you, unless it does a zillion revs, it can't be running right.

Allan, did you ever tach it with those props?

I have a bunch of props here to try on mine. APC 13x6, 13x7, 13x8, 14x4W, 14x5N & 14x6 plus Bolly's in 13.5x6 & 13.5x8.

I'm sure i'll find the best match out of that lot

I'm using a MA 14-6 on mine and is pulling my CG Skylark 70 around with authority! I believe I've richen it to 8900 rpm's. Topped out at 10,400 w/ MA 14-6! Idled about 2900....

Jerry

G'day Mate,
Yeah I did tach it when I put the 14x8 on it, I think it was around 8,600, but I really can't remember, & I don't care anyway, it flew the Diabolo beautifully, but so did the 13x7.
I will be flying it this weekend, I flew my Katana last weekend, & I'm sure the 75AX will get better with some more flight time.

Tailskid I see a problem if you don't-- first 10400 to 8900 says your richened by 1500 RPM and not the norm 300-500 (running to hot otherwise) and 2900 RPM is way to high to brag about.

How is the spool up and throttle response with a 14x6?

Those numbers are the same as (if not slightly behind) most 4-stroke .91's which have the torque to deliver decent throttle response. I'm worried that 14x6 is over propping the 75AX.

G'day Mate,
A 14x6 is not overpropping the 75AX, remember some fellas think that engines must be revving at very high revs to run properly, but not the 75AX, it is designed to run slower with larger props, like a 15x6 or 14x6, 7 or 8.

OK MATE On the OS web site the OS75 is 2.4 HP at 15,000 RPM and your going to prop for 9,000 and under. OS states the break in prop for a .61 is 11x7 and a .75 does 15x6. FS 70 is 11x9 for break in and the .75 does 15x6 (remember the 4 strokes swing a much bigger prop ) and to top it off an FS 120 break in prop is 14x9. This will be linked to the likes of OS and the .91-2 stroke big prop recommendations. The fix to OS 91 problems is 2 fold. Redisgned HS needles and going down to a smaller prop than recommended.
I don't believe 4 stroke will swing much bigger props. Prove it to yourself. Look up size for size and the 4 stroke will probably go up 1 pitch. Remember size for size not a .46 2 stroke to a .82 saito 4 stroke. 91 to 91 120 to 120

Need to know what size and type plane for an answer to be of any value. Are you going to 3d or what?

yes that would be heplful eh?
the plane is a GP Big Stik.........1,000sq in wing...............right now with the TH75 mounted, the plane wieghs 6.25lbs.

here is a review that is posted using the OS.75 AX

http://www.rcuniverse.com/magazine/a...article_id=915

I have installed this engine in a Sundowner 50 size. I was thinking about trying an APC 12x12. Any thoughts on this? I have never set up a speed plane like this before. I have run a 14x6 and a 13x8, the engine has performed flawlessly. I have about a gallon through the motor now.

12-12 on a tiny .75? My tuned FX 91 gets 10000 rpm even on APC 12.5-12. Don`t expect your .75 to swing that prop much over 8K.
I really think it will struggle and run HOT

12x12 on a 75, regardless of ports sizes or magic is too much load.. A 11x9 or 11x10 would do the speed trick..

Well witha 12/12 at least we know he wants- speed. I have put together a few wm 40 size P-51s with 61fx's in them and they seemed to have run best on 11/7 or 8 apc. I would stay in the 11's and start with an 11/8 or 9 sport prop. The average hobbystore does not stock usually what you need and may need to go on line.

Don't forget your at 2000ft elevation. You have around a 6% overall power loss and nothing you can do about it. Experiment in the 11's.


http://www.apcprop.com/

....I use an 11-7 1/2 on my Tower .75, and a 12-7 0n my ASP .91

....a 12x12....no way. []

FBD.

That's where I'd start too - right in that zone with the 11 inchers and get a few props either side as well (maybe up to 12x7 or 12x8) just in case. Fly the model with all props and report back with your findings.

An APC 12x12 just seems like too much prop. It's unlikely to do more than early nines at best with that prop, which, even though the engine is targetted at a lower rpm band, seems just too low an rpm level for a good sport two stroke.

But you never know until you try them, right?

Thing is pick one, whatever one you want and stay with it for several flights. Then when you change, the differene will be seen immediately-whether good or bad. You won't have to guess. If it were me considering what you have, I'd start with an 11/8 for a while and see how it goes.

.....GP Big Stick....wood prop.....75 engine ?

Try this....[sm=thumbup.gif]

FBD.