G`Day
Am buying the Boomerang 60 and want to fit the OS 75AX motor, have done some research on the web and it states that the prop for this motor is in the range of 14x6, 14x8 or 15x6.

I`m a bit concerned as I dont know if I will have enough ground clearance between prop and ground?

Have read and article on the OS 75AXand they fitted this motor to a 60 size planeand had little clearance using the 14x 6 prop!

Can I use a smaller prop on this motor (13 ish) Maybe even 12 ish, and still get peak performance?

Regards
dutchy50

Smaller prop with more pitch gives you more speed. Bigger prop with less pitch "bites" a bigger hole in the air and gives you more thrust. You will end up buying 3-4 props and fly them all to see which one you like best. I just tested props today and found "the one" that works best. Scoot

13x6 or 12x8 is what i would try

Yep, I've flown a 13x6 on the engine and it works fine.

I'm using 12x8 in 10 lb. plane and have run 14x6 with 4 star 60

I'm using a 14x6 and 14x8 on my OS 95AX and it only turns a little over 9k RPM. I'd guess a 14" prop might be a little large for the 75AX.

The OS .75 AX instructions call for a 13x8, 13x10, 14x6, 14x8, or 15x7 size props. My .75 AX turns a MA 14x6 prop at 10,000 rpm.

Bruce

Actually dutchy50, there are two cases here

1) Try and get a 14x6 or 14x7 (depending on what is available) and check ground clearence. If you feel you wont hit at landing or you fly off tarmac, you should use these props. They will offer lower top speed and greater accelration

2) If the 14xX prop is too large, try to get a 13x8 (The manual suggests that too as an option). Basically, as a thumb-rule, if you reduce dia by 1" you should increase pitch by 1. Be aware that the smaller prop will give you more speed so you will have to manage throttle at landing and fly in a more pattern than 3D sort of way (smooth accelration and deceleration)

Like everyone says, you will need to buy two or three props before you settle on what you are comfortable with. There is no harm in taking abigger prop. Keep it stock so you can use it on another model later. If you are concerned with durability or ruggedness get Master Air Screw. Their softer GRP material is more forgiving to ground strikes and mishandling and performs on par with APC atlease based on my experience.

I think 12x8 is too small for this engine, though others have been using it. I felt that 12x8 was more suitable for my 55AX. No bad feelings intended.

Personally, I tried 13x8, 14x6 and 14x4 on that engine. On my current FS- 91SII, I have put a 15x4 on the same airplane. The manual suggests upto 15x6 so ideal hovering prop would be 16x4 but the model is a UCD clone from Phoenix (called Topstar 3D) and that is the biggest size prop it can take. I have kept a 14x8 (as recommended by the manual) as spare in case the performance on the 15x6 is not good.

Ameyam

The main point is to get the best performance that you can while watching engine RPM and your good old ground clearance. If you can get something to work for you at a lower RPM, then you are better off using that prop. You will increase the life of your engine and keep the noise down at the same time.

I would call that a good trade-off. Good performance with lower RPM and low noise.

I have the OS .75 AX on a Goldberg Skylark 70 and I am using a 14-6 prop and getting outstanding performance with reasonable noise levels. In fact, our club has been doing a noise check on our models and my Skylark 70 - OS 75 AX - 14-5 prop are below the maximum noise limits that are recommended by the AMA.

CGr.

<div>Regardless to what the manufacturer says is the prop size, if your plane will not stay stationary at idle, then the prop is to big or the pitch is to great, just a newbies opinion.</div><div></div><div>TB</div>

Well, if you are on pavement, it's pretty hard to meet that "requrement". On grass, it's not a problem because most will not taxi on their own without adding throttle.

And, of course, in either case, it depends on how high your idle is.

CGr.

You can also switch to a three blade prop to help with clearance problems.

When stepping up to a three blade prop, reduce the diameter by 1" to compensate.

e.g. if the manual calls for a 14x6, you could substitute a 13x6 three blade prop and achieve almost the same performance.

G'day
I have a Boomerang 60 and at first was fitted with a 13x10 but now have a 13x6 prop and runs like a dream. The recommended props for the 75 is 13x6-13x10, 14x6/7.
I hope this helps.

Regards
Albi-Wan

It must be a really powerful engine to run such big props. Just kidding, because OS calls out props that tend to overload the engine.
Whether that is a marketing ploy or a noise abatement strategy is unclear. But I doubt the laws of Physics has changed that much since the last time I ran a .75 sized engine. On a draggy biplane, I ran a 13-5 prop which would peak at around 12,000 on the ground. A 13-6 (depending on maker) is likely quite good too. If a faster design, then a 12-7 would be a good starting point or perhaps 12-8.

Now what nobody will tell you, but here is the straight dope. As you increase the load on an engine with an over-sized prop, your needle valve (high speed) get more sensitive to it's setting. Thus it becomes much harder to get a good setting that will overlook slight problems with tank position and changes in the conditions from the morning to afternoon. You also get a lot less power out of your engine, since the power is a combination of the rpm and the torque. Generally an engine will unload somewhat in the air, so you always prop 2 or 3 thousand below the point of peak power. For your AX 75, that is 2.37 hp at 15,000 rpm. So you don't want to peak on the ground above 12,000 or so. OS lists the practical rpm up to 16,000 so you would be fine at 12K.

Or you can load the engine with their recommended props and suffer with touchy needle valve and low power. If you need more power, well they make a 95 too.

G'day HighPlains
My 13x6 prop is the minimum and it has a 14x 7 at the top end of propellor choice, as stated in the thread and also most of the members of Central Coast RCA Club use the same prop on their .75 size engines. With living on the east coast of Aus, we get hot humid days all year round. My Plane flies well, my other Boomerang has an ASP .61 with a 12x6 prop on it and flies reasonably well. Thanks for your input and will check it out.

Yours Sincerely

Albi-Wan

The O.S. Max .75AX will run well on 12 x 7 or 8 propellers. O.S. suggests such large props as they do because of the severe noise restrictions in Japan, their largest market. The .75AX will run well when turning near 12K RPM or even a bit over. They have peak Hp coming at 15K RPM, which means the peak torque will likely be near 12K RPM or so. If you run the engine there, it will run very well, but a lot louder than if you're running near 10K RPM, where their suggested props will have you running.

Performance and handling will be much better at the higher RPM levels.

Yep, my .75ax ran well on an 11x9APC in an 8lb Curare (old school pattern ship) and pulled ok as well with an APC11x10. 5% nitro, 22%oil

I currently run a 12x8 APC cut down to 11.5x8-ish. I'm ground limited to 12" diameter max and know I giving away heaps of static thrust but the engine's happy and the plane likes it. If you're stuck with ground clearance and noise issues then more blades would be a good way to go rather than adding pitch for a trainer OR just as good is to learn early on what a throttle is for and reduce it a bit after takeoff rather than fly around flat out.

Cheers

That's like saying if your bank account is negative, you need a better bank.

Kurt