I will be running a Magnum 91 2 stroke on a Plane I just put together; the Engine Manual calls for 13X6 to 14X8 size Props; before I fly it, I am trying different size and style props on the test stand; so far the best performance was Master Air Screw S-2 14X6 at 12,000 rpm and Bolly Clubman 12.5X8 with the same rpm; what performance can I expect on a APC 14X5 Narrow Pattern Prop and a APC 14X4 Wide Fun Fly Prop; the reason I ask, I haven't used these 2 APC Props and are still in the unopened packages, I am happy with the other Props and I'd rather just return the 2 APC Props if their not going to any better.

Thanks, Doug.

What are you looking for in performance, and what are you measuring?

APC props tend to use a little more energy, but return more than that in performance. The Wide blade will spin slower, put more load on the engine, and give you a lot of instantaneous thrust, or torque. The Narrow will give more revs.

Just measuring the RPM doesn't tell you much about performance, but it does let you know if a prop is too much or too little for the motor.

If you're happy with the props you have, that's all that matters. Totally up to you if you want to experiment; you will never know what you like until you try it.

In full support or gaRCfield here it is impossible to select any given prop based simply on max static RPM. What you need is a PropWall for every displacement class and engine type you fly.

My suggestion is go ahead and open those props and have them ready when flying commences along with a reasonable selection.

If You are new to that displacement class having primarily always done other sizes or types its time to start on that 90-120 class Prop wall

John

Hi!
First of all forget about what rpm you get! It isn't rpm that is meaning full, it's what prop that works best in the air that counts!
APC is superior to MA at least when you compare MA white-blade -tiped -props, but that doesn't mean APC is superior to all other brands.
Graupner "Cam-prop" series or Graupners still newer G-Sonic props (resemble APC) are good too, as are the British RAM series of props.
MA Scimitar series are rather good too! But avoid the MA white tipped props, noisy and no good performancevice!

Wow!
I'm loving that prop wall!
Guess I'm just jealous!

I'm with John, you can't have too many props. I don't put too many on the wall, because I don't have that much wall space. But I have maybe 5 or 6 hundred props (maybe more) in boxes, drawers, and also a wall. Most are Rev-Up or APC.

I have to disagree about the statements that static RPM is unimportant. Nothing could be further from the truth. You need to operate the engine at the rpm where it makes it power. So find out where it is rated to make it maximum power and target a ground RPM several thousand down from that spec figure. This allows the engine to unload in flight, operate at it's peak and make horsepower. Then you find the selection of props that puts you at that ground RPM +/- a thousand by changing diameter or pitch.

last spring I had a plane and engine combination , that I felt that I had to get the most pull out of the engine.

I put a fish scale on the tale of the plane and measured the pull of each prop gave. It worked great, found the prop that gave max pull.

I used a second safety line to make sure it did not take off, be careful

I can't say it was accurate , but it was a good and easy way to select a prop.

sticks

Someone at the field I fly at made up a very cool thrust gauge sort of like you just mentioned. We tried several different planes with different engines and props then recorded the findings. It was placed in the club web site. Thrust on the ground tells you a bunch more then RPM but in flight prop testing is the only way to find the prop that will fly the plane the way the PILOT wants it to fly. RPM means nothing to me at all. I like a slower flying plane with a lot of thrust for vertical so I tend to use longeer props with smaller pitch. That's just me, another pilot may want the same plane to fly differently.
I have never had a wall of props like John. I have had around 30 props in the sizes I know work on the engines I have on hand and use. Most of them from 13 to 20 inches. The smaller props I have had I have given away to guys at the field. I was going to load up on more props at the club swap meet but the guy with the van full of props and stuff didn't show up this year. Swap meets are a good place to find props for very cheap. I think it was John that reminded me of that?

Please elaborate on how they "use more energy." Thank you.

Kurt

APC props need a little more power to turn the same RPM as a same size Zinger on the same engine but because they are designed better will actually give better performance, pull and speed.

So they're more efficient.

Kurt

I have heard that APC's are efficient and if they are I wonder if we can or do use a little less throttle . If so we may use the same amount of fuel or even less?

As to the thrust measurement and the fish scale, I don't think its the last word on selecting a prop. I think it just give a good measurement on pulling power and a place to start.

It did help me get an idea as to the engine and prop I had being able to fly a heavy plane. I used the prop that pulled the most and it did very well, even with our field being small for big planes ( corn did not help ) I was able to pull it off with room to spare.

sticks

Doug,

Here's my take on props for a .91.

I tried the 13x8, 14x7, 14x6 and 15x6, all APC. The plane that helped me decide was an old Cermark Sukhoi, used for aerobatics, no hovering.

I found a happy medium with the 14x6. It provides a little braking, to slow the plane for landing, better than the 13X8 or 14x7. The 15x6 was also good, but I found the braking to be excessive.

For me, the 13x8 and 14x7 were faster, but I didn't need fast. I needed straight, strong uplines. The 6 pitch props seemed to pull the plane straight up a little better than the others.

If you're looking to hover, you'll probably want a 4 or 5 pitch prop. For racing, one of the higher pitch props might be better.

Just my 2ΒΆ worth,
Dave Olson