Hi.
I am wondering if you could tell me a little bit about how to choose propellers for my glow motors.

For example if I want speed.
As far as I understand, then I would want a small propeller as possible. But what about the pitch?

If I want a good propeller for 3d, low or high pitch?

Hope you can help me with this.

Thank you.

First of all, think about the numbers when you are looking at props. For example: 12-5. This is a 12 inch diameter prop with a pitch of 5 inches. The diameter is self explanatory. The pitch means that, in a perfect world, one complete rotation of the prop will move 5 inches of air through it, or it will move 5 inches forward (if turning in the right direction).

Now consider the transmission in your car. The lower the pitch, the less air it will grab or move, but it will have more power to pull something through the air because it will rotate faster, similar to lower gears on your car. If you want to climb a steep hill, you put your car into a lower gear. The motor turns at a higher RPM for a given speed, thus more torque, more energy passed on to the wheels through the lower gear, but the tires will turn slower than if you were in a higher gear.

Same with the prop. SO, if you want 3D, you will want a lot of power to hang the plane on the prop, so a lower pitch would be necessary. If you want a higher top end speed, then a higher pitch is what's needed.

This is a very basic description of what happens. There are limitations, of course, like maximum RPM capabilities of the particular engine, and of course the capability of that engine to provide the necessary power to do what you want it to do. Of course, increase displacement (increase engine size) and you get more power, but that is at a sacrafice to a lower RPM range. However, you can get more power at lower RPM's so you can prop it accordingly.

Someone already did most of the work in prop selection. There are charts that show ranges for props vs. engines, that will range from 3D performance to speed performance.

But, always remember one big point. Noise does not equate to power. So, if it makes a lot of noise, rotates at 16,000 RPM, the only thing you are doing is annoying people around you and destroying that engine. In other words, prop according to specs for the engine and don't exceed practical... PRACTICAL rpm ranges for your engine. Those numbers, such as 3.1hp at 17,000 RPM are nothing but numbers that mean absolutely nothing because you will never run the engine at those numbers.


Check this link by Top Flite. It provides a typical prop chart for various size engines.

http://www.top-flite.com/accys/topq5000a.html

CGr.

I run motors at thier peak HP rpm all the time


Don't let that confuse the issue though, CGR gave you a great explanation.

The manual for your engine will tell you which props are appropriate for it. Choose the bigger ones with less pitch for aerobatics and the smaller ones with more pitch for speed. If you're not sure, start in the middle and see how it flies, then change according to what you want the plane to do differently.

How do props relate when you drop the diameter.

on my TTPro .60 I'm running 12 x 6.

on the top flight prop chart, below the 12 x 6 comes the 11 x 10.

If I switched to an 11 x 10 I am now traveling 10" per revolution ( theoreticly (sic)) But I dropped an inch in diameter so how does this 11 x 10 relate to to 12 x 6 ????

Is there an overlap in the performance of the low pitch 12" and the high pitch 11" ?

Steve

The 11x10 (in general) will have a faster top end straight and level speed, accelerate slower, and be less appropriate for hovering and pulling through long uplines.

You're trying to maintain a similar load on the engine to keep it in its power band. So if you increase the diameter you want to lower the pitch unless you're trying to change your top end RPM for some reason.

Wow, thank you guys!
Means a lot to me.

ok,and what about swapping propeller's size? I mean, a guy from my club says that it would be the same if you use a 11x 6 or a 10 x 7 (example) but I think it wouldn't, what you guys say?

Chocoroll,

It may be the same load for the engine, but the model will be pulled faster thru the air in horizontal flight.
However, for vertical flight, the 10 x 7 will have less muscle to take your model as high as the 11 x 6 can.

Check this article:

http://findarticles.com/p/articles/m...g=content;col1

Regards!

OK I think the proper question would be comparing 12 x 6 to an 11 x 6, Each would move the plane 6" on one rotation.
What does the extra inch do for me? ( decrease my RPMs? )

Steve

Hi!
Of course not! They are not going to perform the same.
The only way you know what propeller to choose is by testing! By experience you will know what is best.

But there are some general rules. Rules that are unfortunately fairly blunt!
Large diameter and low pitch means lot's of low speed performance.
Small diameter and a little higher pitch means more speed.

Fort instance in international pylon racing where we use 6,5cc two stroke engines (F3D) and the models are 180cm in span and the speed is around 375km/h, we use 7x7,5" carbon props.

When using the same size engine however in a high winged trainer airplane like the Kyosho "Calmato (with a sport 6,5cc engine) flying at at sea level, the plane performs best using a 11x6 prop.
The same 6,5cc sport engine in a low winged 3D model used for hovering, then a 12x4 or 13,25x3,75 propeller gives best performance.

Steve,

I agree with Jaka.

Each propeller is to a glow engine, as each gear is to the engine of your car.

There is a range of rpms, at which that glow engine is happy and it can handle that propeller efficiently.
That range is between rpms for max torque and rpms for max power output.
Rpms for max torque are always slower than rpms for max power output.

For level flight, the engine-prop can be swinging at max rpms (rpms for max power output), but, if you start a stepped or vertical climb, the AOA of the blades change and they demand more torque from the engine shaft.
Automatically, the engine looks for more torque by reducing the rpms to the max torque rpms (which improves the breathing of the engine).

If you want to fly your model efficiently beyond that range for that particular prop-engine combination, you need to modify that combo.
It is easier to switch props than engines; hence, you determine the pitch x rpm that will produce the horizontal flying speed that you expect.
Then, the new diameter is determined by the muscle of your engine (torque) at those rpms.

The max power given by an engine is constant, and it is used to move a volume of air at certain velocity.
Diameter and rpms of each prop determine the volume of air (volume changes with the square of the diameter).
Pitch and rpms or each prop determine the velocity of the air (velocity changes linearly with the pitch).

(Edited to add schematic)

Yes, it will decrease your RPM by putting a greater load on the engine.

Tinkering is the only way to find the right prop for your airplane, engine, and style of flying combination.

The transmission gear analogy doesn't seem to be getting through. I'll try the literal approach.

First, deal with pitch. As stated above, pitch moves your plane forward. The more pitch you have, the more forward you go with each turn. If you engine can turn 13k rpm, then raising the pitch increases your top speed.

However, more pitch means the engine must work harder. Therefore, we must lower the diameter of the prop in order to keep turning that 13k rpm. When we do that, we lose some pulling power because the prop is now not moving as much air per turn.

"But," you say, "I want more acceleration and the ability to go vertical better." So we go to a bigger diameter prop to move more air which helps it to hold its position when there is lots of drag or when the engine is having to do the work to fight gravity. So, again, we overload the engine by increasing diameter and so we must give up some pitch to keep our rpms up.

So prop selection is finding the sweet spot between acceleration and vertical performance vs. top speed. In flight, the bigger props really do act like low gear in a car because you get instant acceleration and instant deceleration, but can't go fast. And the smaller ones with more pitch are the opposite. To find the right one, start in the middle and then decide what, if anything you would like to change.

Basically what to do is to buy a bunch of props of different brands around the size, both larger smaller in diameter and pitch. Then fly them and see which one works for your particular airplane, engine and flying style. Keep the rest as one of them may well be the best for your next airplane/engine combo. I've done this and been surprised at the results sometimes.

For those playing the "home game"...

Don't try the "swapping props" experiment with electric motors!

Or your electric may end up trailing a cloud of smoke!

you guys are way overproping your engines
a .46 will never turn a 11x10, 12x6 at any reasonable rpm. try a 9x8-9 (speed) 10x7-8 (fast sports flying) , 11x5-6 (sports flying), 12x4 12.25x3.75 (3d)
a few things are wrong with the above
a larger dia prop with a low pitch (12x4) DOES grab more air than a 10x7 for example, but it wont move the air it grabbed as fast this gives you lower stall speeds and more vertical.
a 10x7 would give you more speed, less vertical and a higher stall speed
what does all that mean, well it really depends on what the plane was designed to do. most sports planes like a 11x5 or 10x7 with a .46 sized engine.
what plane are you going to use? if its a trianer i recommend a apc 11x5, its plenty fast at full throttle but will slow the plane down at idle.

Hi!
A two stroke .46 engine can cope with a lot of prop sizes.
For use in a high winged trainer airplane like a Kyosho "Calmato" a 11x6 or 12x4 is the best prop to use.
The same. 46 engine used in a low winged Q-500 type airplane (130cm in span weighting 1600-1800g)...then use a 9x7, 10x6 or 10x7 prop.
11x10 is way way to much prop for a .46 engine!

Best sport props on the market are RAM, APC and the new Graupner "Cam-prop" or "Sonic" series (not the old blunt tipped grey one).

I'll go back to what I've said before, not only in this thread. Generally, a prop of "typically normal size" like a 10-7 for a 46 size trainer engine, is a pretty good bet. However, more often than not, a pilot can have a small collection of props of various sizes and swap them out to get the best fit for the flying style, airplane configuration including weight, and the variables that may or may not be assumed in most of our aircraft.

Still, the manufacturers of, say Hobbico NexSTAR, put a prop on the plane based on what is most likely to fly well. And for a trainer, it probably won't make much difference until the student pilot gets more proficient.... he or she will never know the difference.

CGr.

what about prop blade profile.. squared tip vs Scimitar like if i had a gms 47 and I put a 11 x 7 on it.. what would be different if i had a 11 x 7 scimitar vs a 11 x 7 blunt.. if any?

Now you just opened up a can of worms!!!

From personal experience a friend of mine tried to fly his Sig LT40 trainer with a 40 turning a 10X8 Top Flite wood prop, this is a very high load prop meaning it moves a lot of air at the same rpm as compared to another brand /style at the same rpm.
The LT40 wouldnt get out of its own way.
He dropped down to a 10X6 and the plane woke up.
Testing will tell. If the engine cant turn the prop fast enough to get up to the power range it will fall flat on its face quickly. 2 strokes are very "rpm dependent" in making good power.

sorry but Im clueless to this part... my instructor said to use only scimitar... but I like the looks of a wood blunt on my cub.... also ive heard to only use 2 blades on single engines... but doesnt a corsair look better with a 4 blade???

Hehe now you're getting into the weeds! But if you want to go there......

For many uses it may not matter. If you're flying precision aerobatics, 3D you can get into some pretty serious tuning with prop choice.

Some props like APC, will out perform the same size wooden Zinger prop because the airfoils are more efficent or smoother. You might be able to get the same RPM on a 10x7 APC as a 10X6 cheapie wood prop (for example) or more rpm just by sticking with same size.

So to say you "have" to fly a Scimitar prop is hogwash. Though for some uses they may be a better choice. Personally I hate the things, they are too heavy and don't spin up fast enough for me, I use Xoar or Vess props. But the style of flying I do demands better response to throttle than I can get with a Top Flite prop.

3 or 4 blade props load the engine more so if you have a 10x6 2 blade you'd probably have to step down to 9x6 3 blade. A 4 blade would be worse. Now if you stuck that on the front of a Corsair as you used for your example, the prop tips would barely clear the cowl unless you have an oversize engine on it.

If you look at the big time scale guys they have pretty multi blade props for static judging and 2 blade props for the flight rounds. (yes not all of them but a high percentage)

So basically it really boils down to personal preference on how you want your plane to perform.. and the only way to figure that out is to experiment with different prop/engine combos... sound right?