Hi there all

Here I come with another question which I'd like the answer of, and I hope it's not a silly question.[8D]

Is there any disadvantage of using not correct size prop which is recommended by any engine manufacturer? for instance a .46 2C engine required 11x6 prop, what if we use 10x5 in it, and a .61 2C size engine required 11x7 and we use 10x6? what would be the disadvantage or difference? would it not give enough power? or would it make engine hot despite running rich? and smaller size prop then recommended prop would damage the engine if used.

In addition to it another question is why Bench Props for breaking in are different in sizes for any engine? why after broken in one has to change the prop size from the one which was used in break-in?


Thanks to all in advance

Mody

lets see poor overall performance ,less thrust,less speed,possible engine damage from over revving dur to undersize props,

Prop sizes are chosen to keep the engine within a specific rpm range. 2 stroke engines develop their power on the high end, so you want your engine to spin at close to a specific rpm.

As a general rule you can change prop sizes by adjusting your pitch based on diameter. For instance, if the manufacturer recommends an 11x7, you might be able to go to a 10x8 or a 12x6. If you go up in diameter by one, you go down in pitch by one. If you go down in diameter by one, you can go up in pitch by one. This doesn't always work, but is usually a good indicator.

I have also heard that you can go by multiples. If the manufacturer recommends an 11x5, you can multiple 11 by 5 and get 55. Then you can divide 55 by whatever diameter prop you would like to use to get an idea of what kind of pitch you can get away with.

Diameter equates to thrust, and pitch equates to speed. The higher the diameter, the more thrust, the higher the pitch, the more speed.

Speed planes generally have pitches from 7+ while most others do not go beyond 6. 3D planes often have large diameters and low pitches, down in the 3-4 range often. 3D doesn't want speed, in fact, most of their planes cannot handle a lot of speed.




There are different props for breaking in for a few reasons. During break in period you are trying to reach a specific set of conditions for that period. The prop choice is to help the engine maintain a certain rpm to aid in the break in.

Whenever you use a prop that's too small for the engine, it will not put sufficient load on the thing. The engine will give you fits because it'll be overrevving. The carb is sized to provide appropriate mixture for a range of rpm, and the engine will be trying to operate outside that range. The consequences are greater than just a hard to handle engine. It'll be doing to itself all the bad things engines do when they're over revving.

All that said, the recommended prop sizes are very often a lot wider range props than you probably think. Look up the prop sizes the mfg of your engine suggests before you decide your 10x6 is too small. You might be wrong. The range of props for any engine is very often quite wide. A lot of the engine experts at our favorite flying fields often have only one favorite prop. And some of them often suggest that only one prop is "proper" for certain engines.

And that said, the size prop that actually suits the use might actually be a 10x6 or one outside the usually recommended range. The airplane often has more to do with the proper prop than not.

The idea behind using a size smaller prop during bench break in is to let the engine rev to its normal rpm in spite of a super rich fuel/air mixture. Not everyone breaks in engines this way and indeed the sky does not seem to fall when broken in otherwise.

If using the "wrong" prop size or break in procedure did cause sure fire falling of skys, then it wouldn't be so controversial.

The recommended prop sizes for various engine sizes are usually the ones that get the most performance out of the engines. Because modern engines often are tuned for lower rpm use than some of the older versions, the recommended prop sizes have crept up recently.

There is a fairly well known (and sometimes not very well known) prop sizing theory.

You square the diameter and multiply that by the pitch. That gives you the "power factor" of that prop.

10x6 = 600
11x5 = 605
9x8 = 648
10x7 = 700
11x6 = 726
9x9 = 729
10x8 = 800
etc

First thing you do is try a prop in the recommended range and see how it pulls the airplane.
If the engine is screaming, but not pulling well, try a prop that has a power rating that greater than the screaming, underachiever. etc etc

Actually, a number of engine mfg's now recommend that you break the engine in with the prop you intend to fly. OS does that today with a bunch of their engines.

Beyond the fact that mfg's say it works, is the fact that it works and has worked forever. There are a lot of "everybody knows" stuff that isn't exactly what everybody knows.

Normally, the power listed for a specific 2C engine is at a specific RPM. Most flyers don't approach that RPM because it is way to high and sustained operation at that RPM may eventually cause problems. Also, the noise level at that RPM is sometimes not tolerated at certain clubs. Remember that noise does not necessarilly equate to power.

For instance: The OS 45 AX engine shows 1.65ps at 16000 RPM. Operating it at RPM would probably drive every one crazy and would use a small prop. Normally you may operate that engine at a peak RPM of about 12000. This would probably work out well using the props listed above.

I normally use a 15-10 prop on my OS 1.20 AX. That gets me a peak RPM of about 9800. I chipped that prop and the only one I had on hand that was balanced was a 15-6. It tached at about 11000 RPM and was very noisy. The prop tip speed was making tremendous noise and the prop was not that efficient. The original prop would have been much better.

I'm sure lots of people would shoot for the max RPM they can get, but that normally equates to short engine life. You are better off sticking to what is recommended and working around that number.

DS.

Thank you very much for your replies,

I understood it very much, but there are 2 points which I need the clarification, pitch is used for speed, higher the pitch higher the speed, and higher the diameter higher the thrust, thrust is the force which pushes in the backward position, so, it can be said that if the thrust is higher speed won't be much, or what exactly the difference is between thrust and speed in technical terms.

Mody

Think of a gear in a car (a real car). Lower gears the motor runs at a higher RPM but goes slower, and has much more pulling power. Shift to a higher gear and the car goes faster but the engine RPM is lower and the power is less. You accellerate faster in first gear than you will in, say third gear, but you go faster in third gear than in first.

Same thing with props. Lower pitch means more pulling power but less speed and higher RPM. Higher pitch means faster speed at lower RPM and thus less power. The engine has only so much power to turn a prop and adding a bigger ie. higher diameter higher pitch puts more demand on the engine and it only has so much power do it will not turn as fast as with a smaller prop.

Decide what you want out of the engine prop combination and go from there. If you want good vertical performance, go with a lower pitch for more pulling power, but beware of the RPM and noise level.

DS.

Thanks DS

It pretty much clarified me. Im using .61 HB's 2C german engine in my easy sport locally made plane, as engine gave me hard time, and I had received so many great advices how to tune the engine up and run it well, and hopefully this sunday, Im gonna put in the feedback how well it flew. At the moment Im using 11x7.7 prop of Thunder Tiger's Cyclone, but I read in prop chart that 11x7 is the right prop for it. I wanna do maneuvers on it and ofcourse I love vertical pull, so which is the best prop for it?

Thanks

If I were in your shoes, I would try to use a 12x6. The 12x6 is in the middle. To fly slower a 13x5 would work too. Nothing wrong with your propellor, but it will be on the fast side, and may not slow down easily on landing. Propellors need to be matched to the airframe's needs AND be within the recommended range of your engine. For your Easy Sport and a .60 engine, a 12x6 prop is a good starting point.

Here's some info on HB Engines http://www.mecoa.com/hb/index.htm

Happy Flying!!

Another thing to consider is that the higher prop efficiency of a larger slower turning prop offsets the engine power loss of the lower rpm. This is why four strokes and electrics perform so much better than their horsepower numbers would indicate. The optimum prop is actually a tradeoff between one that's small enough to let the engine perform and yet big enough to let the prop perform.
Put a gear reduction unit on a .46 two stroke and then you could use its 1.6 horsepower at 16000 rpm to turn a 16x10 at probably 8000 rpm for enough thrust to take an 8 pound plane verticle to the moon.

Many times, too, the manufacturer's recommended propeller is one that gives you the best combination of "reasonable" power output with good noise reduction. Usually, though, you can get more power and performance if you turn the engine faster with a different propeller. The result will be better aircraft performance.

For example, some .61-size engines recommend 12-inch propellers with 7 or 8 pitch. The engine's running in the low 11,000's or high 10,000's. Re-prop the engine to turn in the mid-12,000's, and the airplane may just start to "come alive". It all depends upon the engine/airplane combination, and whether the engine will actually turn that RPM and still give good throttle response.

Thank you very much all for your precious advices, as warhawk advised, that 12x6 is good choice, so, Im gonna try to use that size prop as well, I hope it would give me good vertical too. Thanks a lot all of you once again, I really learned a lot from it.

Mody