I was wondering what is the difference between prop sizes, say a 10 x 6 and a 11 x 5. Does this make them faster, slower, give the more torque..stuff like that.
I put a 10 x 6 on my Trainer. It has a TT .46 Pro engine. My instructer said i Should replace it with an 11 x 6 or something close to that. I did not ask why and now I am wondering why.
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stonedeaf

The second number refers to the pitch. That's how far the propeller would move forward with one revolution if it was 100% efficient (no slippage). The first number, or course is the diameter.

So, a 10X6 moves 6" forward in one complete revoultion while the 11x5 only moves 5". Obviously, you would think, the 10x6 would make the plane faster. But, as is often the case in the world, it's not always that easy. Propellers are maybe 35% efficient, the engine won't load up above a certain RPM regardless of the prop and will have a "sweet spot" at which diameter and pitch are optimum. Your trainer has a lot of drag, so a longer prop probably does have the advantage of adding torque at the mid & high RPM ranges where the added speed potential of the 10x6 would be lost in slippage and drag.

To learn all the flies in the ointment read here: [link=http://en.wikipedia.org/wiki/Propeller]propeller efficiency[/link]

MOST 46 engines perform better with an 11" prop for SPORT FLYING. The larger diameter tends to "load" the engine more than a 10" prop.

So basically, the diameter is to add/reduce thrust and the pitch to add/reduce speed?
How do you determine how much thrust you need? depending on drag characteristics only?

Hey Hi there!

In reply to fadi....Charlie P. has explained the best way possible of what is happening there.....Torque is more effective when you have to fly heavy planes; scale planes and generally when these planes have limitations of high speed stalls. Torque or "thrust " as you say gives stabillity to your flow over your control surfaces.Of course this is also counting on what kind of flight you are into. If you have a sleek pattern or a pylon racer of course you are in different type of flying and you ask for more speed so you move into bigger pitch.
Cheers
Johnnie

PS Credit goes to Charlie P. this time....al the way!!!!

I see ok thanks!

Choosing a propeller is really more about testing than about theory.
There really isn't anything like a formula for thrust that works worth spit for our models and engines. You determine most everything about what prop you need by sticking one on your engine and doing two things.

You run it up on the ground and tach it. If it's turning too fast, no value flying it. You decide about the "too fast" by looking at the mfg's specs on the engine. You want to prop the engine to turn any prop on the ground just under the recommended optimum rpm.

Then you fly it and see how it flies on your chosen airframe. Your model loads the engine however it's going to load it. The engine mfg's got no idea what their engines can be hung on. They do know a way to help you adapt their engine to any airplane. They give you an optimum rpm.

Given the optimum rpm and those two tests, you can have a chance to match the engine to the airplane. And you'll have a chance to adapt the airplane to perform more like you want.

So you can test it two ways. But what do you look for in those two tests.

The first test is easy. The test prop should turn at or slightly under the engine mfg's optimum rpm. You're going to need a tach for this until you've got enough experience to tell rpm for your engine by ear. You can actually do that with 2cycle engines. If you're asking such a basic question about props, chances are good you need a tach. If you're using a 4cycle even experienced flyers use a tach.

The flight test is more complex.

When you have the needle set on your engine so it's running just slightly rich from optimum, you fly the airplane.

First thing to look at......... take off performance. A couple of things.........
Don't SLAM-IT-AND-PRAY. Line the airplane up on the runway and advance the throttle steadily. It ought to take you 4 or 5 seconds to get to FLAT OUT. The airplane should start to accelerate right away and the acceleration ought to increase with the throttle. That's pretty intuitive, but remember, if you've got the wrong prop for the airplane, it might not. Did the airplane accelerate with the throttle? Keep it in mind but look at how it climbs out. And how the engine comes up to top speed.

With the airplane at altitude, check out the throttle response. How does the airplane slow down and speed up in response to throttle position? Now, hit wide open throttle and see how it climbs straight up. Once up there, chop to idle and see how it dives at idle.

Now throttle back to landing speed. That should be about three-four clicks from throttle STICK (not trim, stick) all the way closed. After the model settles into that speed, how does that speed feel to you? Fast enough to be safe for landing? Too slow for safe landing?

So land it at whatever throttle gives a safe landing speed. And think all of it over.

Wow that's gonna be tough!!
What's the optimal rpm for my OS 46AX then? 16000rpm? I've never reached that number... i definately missed something here

I usually prop my ball bearing 46's (like the OS 46 AX) for 12,500-13,000 rpm on the ground (thats usually an 11x5, 11x6, or a 12x4). When the prop unloads in a full throttle dive, it will approach the manufacturers rated rpm of 16,000. Do not prop it for that many rpms on the ground. There are those that like to prop for even lower RPMs to reduce prop noise.

Brad

If the model didn't accelerate on takeoff worth spit, the engine needs more. More what? We'll get to that.
If the model didn't climb worth spit, it needs more.
If the model was too fast at idle (throttle (not trim) closed) to land, it needs less.
If you're just beginning this hobby, this is enough to deal with.

When the model "needs more", that could be more thrust or more fan area or more pitch. Most times, what you need is more thrust and you get that a couple of ways. On takeoff or climb, there are two ways to get more. Very often, putting a larger diameter prop on the engine will get more. It gives you more fan area, and that's very often the key to get more thrust. If the engine can turn the prop that is. That's where you go back to your ground/tach test. Don't assume that if you go up in diameter that you'll have to also change the pitch too. That's not always the case. So slap a longer prop on and run the testing from the start. From the start. Run the engine to set the needle and then tach the sucker. The engine mfg will have given a range of props to try. If your prop is in or near that range, it's worth testing. Get near the optimum rpm. If the new prop won't do it, try another one.

If the engine needs more and you longer prop doesn't give it, you might need a prop that long but with a little less pitch. Sometimes, less gives more. Less pitch might give more pull.

If the test prop wouldn't fly your airplane at idle slowly enough for a safe landing you need less pitch. If you simply slap the same diameter prop on for the next test and it's only got less pitch, chances are good the engine will be at risk sometime down the road. It will be fairly easy to set it to run at too fast an rpm. When you need less speed, it's a good idea to put a less pitch prop on, but one with more diameter. Hey, you'll be getting more pull for free.

There are some problems that go with certain props. Don't sweat the reasons behind the problems but they're nice to know. You'll understand more about them later.

Props with the least pitch for a given diameter are usually less efficient props. They're also a bit more difficult for a beginner to set the needle for. But they often give the least range of flight performance differences. Sometimes they make a good handling engine become touchy.

Props with "fast pitches" are often a bit risky for a beginner to set the needle for. Plus, they often make the engine/airplane a bit difficult to land. They often won't let the airplane slow down. Sometimes they make a good handling engine prone to lean runs. Lean runs kill engines.

Lots of airplanes that would benefit greatly from a larger diameter prop don't have the ground clearance to run the sucker. ARF ARF ARGGGGhhhh.....

I don't have the manual with me, but there is an easy way to get an idea what to use. Your needle setting. Yeah, I know....

Tune the needle from obviously rich into 2cycle. Turn the needle slowly. You'll know when you've hit the peak rpm when you pass it. Yeah, I know.... As soon as the engine sags or drops rpm as your closing the needle, back out a couple of clicks. The engine picks back up and you got the baseline rpm.

Do that with the most popular size prop for that engine size and you're good to go.

BTW, props are made in a number of pitches. The pitches we have available to us are perfectly adequate. And the pitch range is usually perfect. And the differences between pitches is fine enough but not too fine nor too coarse. Change the pitch one inch and the difference is noticable but not too great.

On the other hand, change to a different diameter prop and you got a major change. The one inch diameter difference isn't fine enough. But it's all we got.

So figure that you can play with a lot of different pitches, but you'll probably only have a couple of diameters you can try.

So, would an 11 x 5 prop be good for my Avistar trainer that has a TT .46 Pro?

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StoneDeaf

Hi!
If you fly at sea level ...a 11x5 APC or 11x6 APC would be the prop of choice...not a 10x6.

Ah. There's the rub. A propeller generates more thrust at the tips, and the longer props have more tip (relatively speaking). But if your're cutting grass as you try to rotate onto the step for takeoff you've lost any advantage.

I run 11X6 on my TT Pro-46. Nice engine. I have an assortment of props I carry in my field box (all threaded on a 6" bolt) so I can swap out props and see what works on an airframe. I have been surprised at times. As a rule of thumb: if you move an inch in diameter move an inch the other way in pitch (i.e. 10X6 => 11X5.). I have a model with a ST .51 that is a dog with a 10x8 but has lots of vertical and slows well with an 11X7, and the 10X8 is not noticably faster. You just have to play around.

Thanks Charlie P.
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StoneDeaf

In reply to stonedeaf
Darock and Charlie P are right.

Also kindly I would like to add that what Charlie P knows but forgot to mention; if you run 3 blade props you have to move 2 inch in diameter/pitch and if you run 4blade then 3inch in diam/pitch.

Hey fellas stay well
Cheers
Johnnie