what does this necessarily promote? if I go from 13x6 to 12x6 will I achieve more speed?...

More information. Engine, plane.

I have an OS .61fx, and I had a 13x6 on it for 1 flight, then put my 12x6 back on. The engine was able to spin the 12x6 faster, created faster top speed. Both the 12x6 and the 13x6 are props recommended to use by OS.

cool, just what I needed to hear...

I run an 11X8 on my Magnum .61 it will turn it at 14000 rpm

14000 RPM? Ooooh kay..

Noise does not equate to power. Turning 14000 RPM means absolutely nothing if it does nothing for performance. You can put a 9-5 on an OS 46 and get 15000 rpm and the only thing you will succeed in doing is destroying that engine.

It has been said so many times here in the Beginners Forum that the RPM numbers quoted by manufacturers mean absolutely nothing to use in actual flight.

Prop these things so that you get good performance. It takes several trys to get the right engine, airframe, prop combination that suites your aircraft. Excessive RPM does nothing but chew up aluminum and internal parts of your engine.

Here is how it works: the prop has two numbers: Diameter and pitch.. 12-8 for instance. This is a 12 Inch diameter with an 8 inch pitch. The pitch number will tell you how far that prop would move in absolutely perfect circumstances, in one revolution. So, a 8 inch prop will move 8 inches per revolution. At 10,000 RPM, this is 80,000 inches per minute. Do the math. That is 6666.666 feet per minute or 111.11 feet per second. 88 feet per second is equal to 60 miles per hour, so 111 feet per second is probably somewhere around 75 miles per hour... in perfect conditions.

Increase the pitch and you will increase the speed, but at the cost of load to the engine, and lower RPM.. thus most likely you won't get better speed out of that prop. It's a big circle.

Keep the RPM of your standard off-the-shelf 40 - 60 size engines below 12,000 and you will be just fine.

CGr.

I am gonna run spare 12x6 prop today and see how it goes...

No, this is not true. And if it only ran 15000 on that size prop, it would be a surprise.

This was intended as a vague comparison of prop size versus RPM.. intended to demonstrate that the user should use the correct prop size for an engine. Excessive RPM is not going to gain a darned thing except make more noise and perhaps destroy the engine.

Did you ever try a 9-6 on an OS .46? If not, how do you know it won't reach 15,000 RPM? I under proped an OS 61 one time and the result was an RPM range that was way to high and enough noise to upset those on the flight line, let alone the neighbors.

The point is to prop them correctly to get the best advantage out of that engine/prop/airframe combination.

CGr.

so I used the 12x6, couldn't see much of a diff... wind was pumping... but I think it was better to keep up the RPMS... never sounded like it wanted to run out of them... with 13x6 it does pull dead on me...

Keep in mind that not all props are created equal. If you installed an APC a Master Airscrew and Top Flite you would get 3 different rpm readings and they would each fly different. I would try every one and see what I liked the best.

This everyone should agree is the goal.

I've run an 8-6 on a TT 40 for break-in. Also as small as a 10-6 cut down to 6" diameter on a 40 engine. 40 sized engines are pretty much bullet proof well past 20,000 rpm.

I said I'b be surprised if it didn't reach 15K


There is a specific rpm that every engine produces maximum power. It is quite often listed in advertising or a spec page. Generally with just a muffler, it is a fairly broad range of rpm, say + or - 500 to a 1000 rpm from the stated peak power rpm.

You do not want to prop the airplane so that it turns this rpm on the ground. This is because every engine and prop combination "unloads" in the air. The amount of unload depends upon a number of factors, including the timing of the crank and sleeve, compression ratio, and the drag of the airframe. For most sport engines and planes, you can expect about 10 to 15% increase in rpm over the peak reading on the ground. This is one of the reasons you have to set the needle valve a bit on the rich side, so the engine does not go lean as it unloads.

So if the engine has it's peak power at 15,000 rpm, you would want to hit close to that in the air for maximum performance (generally either speed or climb). Working that back by 15%, you might select a prop that turns 12, 750 when peaked, but one that is from 12K to 13.5K would fall within a range that would work. Buy multiple props and test fly.

As stated by Jeff, not all props are equal or even equivalent when they have the same diameter and pitch. Variations of blade width, airfoils, and the pitch curve make it difficult to compare. Even little things like the ridge of the mold separation line can affect the prop and it's performance. The shape of the nose of the airplane may need a completely different pitch curve to work well. Test test test.

For most sport models I generally use just enough pitch to knife edge well, and trade away more pitch in the prop for more diameter to improve the climb.

Don't any one forget that many off the shelf fuels are sold with the least amount of oil that the mfg thinks will do the job for the average flyer. If you want to experiment with smaller props and more rpm, be sure to use at least 20% oil. With ample oil, moderate load prop and a good needle setting the engine will run amazingly cool for ya.
For beginners wanting to try 8 or 9 inch props on a .40-.46 engine, you will need to try it with planes smaller than your typical .40 sized trainers to see positive results.

neglected to mention this is a FA65 four stroke... max is 11500... I am gonna try props to get 11 with the same setting as I have now... I use Morgans Fuel 15%... you guys know the oil content?

I believe Morgans is only 18% I personally use Sig Champion, it's 20% with a 50/50 blend.