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Old 01-09-2006, 01:12 AM   #1
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Default Some confusion on prop size

Ok I know a 10x4.7 means 10 inch diameter and it will theoretically pull itself 4.7 inches in one rotation, but how does diameter factor in? Do you lose power and speed if you go to a 9x4.7? Does somebody have a link to a prop primer somewhere?
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Old 01-09-2006, 10:06 AM   #2
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The simplest way to think of it is that diameter controls the amount of thrust you have, i.e. how it accelerates and how much weight it can pull, whereas pitch (the 4.7) basically controls the maximum speed you can fly at.

But ubfortunately it's not really that simple because it also depends on how fast the prop is turning. Going down in prop size from 10 to 9 may lose you some thrust so the plane won't accelerate as well. Or the motor may be happier with the smaller prop, turn it faster and the whole thing will fly better.

One thing some people find tricky to understand when first learning is that the motor, gears, prop and battery together form a complete power system. Changing any one thing affects all the others and the overall results are often not easy to predict.

Steve
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Old 01-09-2006, 11:37 AM   #3
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Thanks Steve that helps some.
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Old 01-09-2006, 06:19 PM   #4
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One thing some people find tricky to understand when first learning is that the motor, gears, prop and battery together form a complete power system. Changing any one thing affects all the others and the overall results are often not easy to predict.
I agree with your first sentence completely. Just add "airframe" to the list, and I couldn't have said it better myself.

I agree with your second sentence somewhat... but, I note that designers of (real) airplanes, boats, submarines, and other propeller-driven craft make these kinds of predictions all the time. If the manufacturers of the various props, motors, and batteries would publish a few performance curves for their products, you and I could do it too. It ain't rocket science (*)... it's just basic engineering.

I find the lack of meaninful design information to the most frustrating thing about this hobby (in my short time in it so far). Almost all design and configuration work seems to be based on hearsay and folklore. Why can't we see a few speed-vs-torque curves?

Eddy

(*) At the level we need to do it for a hobby, it ain't rocket science. If you're optimizing a Mach 2 fighter, or a commercial transport that has to compete and make money... now that's rocket science.
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Old 01-09-2006, 06:57 PM   #5
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I thought it was the other way around. Diamater=speed, Pitch=thrust.

Man, you guys are right...some basic power curves by the manufacturers of the planes using various popular motors and props would be great info to have.

[QUOTE=slipstick;35640]The simplest way to think of it is that diameter controls the amount of thrust you have, i.e. how it accelerates and how much weight it can pull, whereas pitch (the 4.7) basically controls the maximum speed you can fly at.
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Old 01-09-2006, 07:05 PM   #6
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Originally Posted by Steve View Post
I thought it was the other way around. Diamater=speed, Pitch=thrust.
Just think of the prop as a screw going thru the air. The more pitch, the faster it trys to "screw" itself thru the air.
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Old 01-09-2006, 09:52 PM   #7
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Thanks for the replies guys, Rugar I have that part down it's just the daimeter thing that throws me, say I have a 10x4.7 on a given motor and switch to a 9x4.7 without changing anything else will I slow down?
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Old 01-09-2006, 11:37 PM   #8
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Originally Posted by fabricator View Post
Thanks for the replies guys, Rugar I have that part down it's just the daimeter thing that throws me, say I have a 10x4.7 on a given motor and switch to a 9x4.7 without changing anything else will I slow down?
Good question . I think you may actually speed up. I'm thinking you would gain more motor rpm's with a smaller diameter prop which in turn would make the model fly faster. BUT, you would also loose some thrust with the smaller diameter prop and loose some power. Hopefully someone has a good answer to this question for both of us.
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Old 01-09-2006, 11:49 PM   #9
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Originally Posted by Rugar View Post
Good question . I think you may actually speed up. I'm thinking you would gain more motor rpm's with a smaller diameter prop which in turn would make the model fly faster. BUT, you would also loose some thrust with the smaller diameter prop and loose some power. Hopefully someone has a good answer to this question for both of us.
That is exactly the way I see it too, thanks rugar, lets see what develops.
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Old 01-10-2006, 10:46 AM   #10
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Yep that's how it works but you do have to look at both aspects.

If your 9x4.7 allows the motor to run faster you will have better theoretical top speed. But your plane still has the same weight and drag and you need thrust to overcome them and accelerate up to that speed. The reduced thrust from the smaller prop may stop you ever getting to the theoretical top speed in level flight. It's not easy to tell without knowing ALL the parameters of the motor/gearbox and particularly the plane. The effect on a draggy WW1 biplane will be very different to that on a really sleek monoplane racer.

It is possible to calculate the result for a specific airframe, motor etc. But props are very cheap. It's much easier just to buy a couple and see what the effect is .

Steve
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Old 01-10-2006, 11:31 AM   #11
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Thanks Steve.
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Old 01-10-2006, 11:48 AM   #12
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So many Steve's.....

I like the screw analogy. I helps me visualize the effect. It all does come down to "swapping props" though doesn't it?

Swap and screw, swap and screw.

Are we flying yet?
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Old 01-10-2006, 02:38 PM   #13
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That's why I use a o-ring prop saver. Real easy at the park to prop "up" or "down" for best results.
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Old 01-10-2006, 04:55 PM   #14
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Default Play with This!

Hey guys... see your debate here. Interesting.

I went thru the same learning curve. There is a great tool HERE to help you understand prop sizing and load on the system.

Here's a few guidelines to help put it all together.
Watts = Amps x Volts
Output Watts (power of thrust) = Motor Watts x Motor/Prop Efficiency
~(typically 75-85%)
Select SF Props to run 6000-9000 rpm
~(BELOW 250 Watts)
Select E-Series Sport props to run 10,000-12,000 rpm
~(Over 250 Watts)
Pitch has a linear impact on thrust/load, Diameter is to the 4th power!
~Change in Diameter will have a far greater impact than a pitch change.
~With all hardware same, Larger prop will turn slower and draw more current and produce more thrust
To really see the impact of change, use a Wattmeter!

TMan
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Old 01-10-2006, 10:26 PM   #15
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Originally Posted by TManiaci View Post
Hey guys... see your debate here. Interesting.

I went thru the same learning curve. There is a great tool HERE to help you understand prop sizing and load on the system.

Here's a few guidelines to help put it all together.
Watts = Amps x Volts
Output Watts (power of thrust) = Motor Watts x Motor/Prop Efficiency
~(typically 75-85%)
Select SF Props to run 6000-9000 rpm
~(BELOW 250 Watts)
Select E-Series Sport props to run 10,000-12,000 rpm
~(Over 250 Watts)
Pitch has a linear impact on thrust/load, Diameter is to the 4th power!
~Change in Diameter will have a far greater impact than a pitch change.
~With all hardware same, Larger prop will turn slower and draw more current and produce more thrust
To really see the impact of change, use a Wattmeter!
That's what I'm talkin bout, that link is now in my favorites Thanks TMan
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Old 01-10-2006, 10:59 PM   #16
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Originally Posted by fabricator View Post
That's what I'm talkin bout, that link is now in my favorites Thanks TMan
You like that one... here are some more resources that I find very useful.

Thrust Calculator

another Thrust Calculator

Aircraft CG and Balance Calculator

Listing of all APC Propellers Made

Servo Wiring Diagrams

Lipoly Battery Technical Information

TMan
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Old 01-10-2006, 11:07 PM   #17
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Nice list T-Man. I like that APC Prop link.
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Old 01-10-2006, 11:50 PM   #18
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Originally Posted by TManiaci View Post
Good stuff thanks TMan
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Old 03-30-2007, 01:21 AM   #19
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Maybe I missed something here,but, I did not read anywhere that motors have a 'sweet spot' to which one must find. This is where any combination allows the motor to run at FULL efficiency with ease. Going in either direction is just an experiment and a risk. Once you find that perfect combination(prop,motor, amps,gears,etc) then your motors will perform perfectly in every which way possible. Motors will have less stress and so will the batteries,etc. A smaller prop is generally a better prop compared to oversize. Most times> BIGGER is not better in overall life of your electrical components. You can go with a smaller prop with more pitch(equals less weight also) to achieve what a larger prop with less pitch will do. In some cases you can achieve more with a smaller prop. Pitch is as important as prop size. Most motor specs will have the proper prop info included, that is best used for that particular motor,but , adding another battery and going a prop size bigger/pitch, MAY mathematically come close. This could be an endless BUT very interesting discussion,but, I hope the info I tried to pass forward was useful and fairly accurate. If not, I WANT to be corrected. Thanx, Scott.
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Old 03-30-2007, 08:20 PM   #20
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Scott, you are correct with everything you said. I doubt anyone would argue with you.

However, the conditions and flight style changes many of those "rules". The bigger prop is better, if you want to do 3D acrobatics, when the larger prop blast is necessary to provide control at high alpha and static flight attitudes. Also, the spec's on motors are always about peak running points. They are generally not the ideal when you start talking about accelleration or throttle response. These "uncharted" needs push you into conditions the manufacturers don't address, and quietly don't support. It usually means the hardware selection is skewed and tuned to provide a better platform for a given flying style.

Same applies to hotliners and go-fast stuff. Over-amping and over-proping is standard procedure to get the most out of the lightest possible setups.

It's not wrong, just different...

TMan
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Old 03-30-2007, 09:35 PM   #21
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Originally Posted by ElectricFlyGuy View Post
Maybe I missed something here,but, I did not read anywhere that motors have a 'sweet spot' to which one must find. This is where any combination allows the motor to run at FULL efficiency with ease.
You're right in essence but if you take the time to check the current and speed for maximum efficiency for a few of the motors we use you'll soon realise that they are remarkably low compared with what everyone runs in the real world. Hardly anyone buys a motor capable of 400W and then runs it at <100W just for efficiency.

Where you are absolutely correct is that it's the total combination of motor/gearing/propeller and battery that must work well together. It follows that changing any one of those things may have all sorts of effects on performance that are not immediately obvious. For example putting on a smaller prop will certainly reduce current, increase run time, reduce strain on the motor/battery etc. .....but it may completely ruin the flight performance of the plane .

Steve
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Old 03-30-2007, 10:05 PM   #22
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Originally Posted by fabricator View Post
Ok I know a 10x4.7 means 10 inch diameter and it will theoretically pull itself 4.7 inches in one rotation, but how does diameter factor in? Do you lose power and speed if you go to a 9x4.7? Does somebody have a link to a prop primer somewhere?
Working out the Prop Load Factor helps to visualize the change.

PLF = diameter^3 x pitch

10x4.7 = 4700
9 x 4.7 = 3426

The smaller prop takes 73% of the energy to turn as the 10x4.7; at the same watts, it may turn fast enough to give you the same thust due to its higher rpms.

Go here for some real world data - same motor, different props:

http://www.ampaviators.com/index.php...d=42&Itemid=27

The GWS 1180SF and the HobbyZone 1080 deliver about the same thrust at the same power levels, with the HZ turning faster rpms, so going down from 11x4.7 to 10x8 had essentially no thrust change, although the level flight speed might be a tad lower with the 11x4.7.

In your example, with the same prop pitch but one inch diameter lower, if the thrust is equal, I believe the speed will be the same also.

My site for electric flying articles:

http://www.ampaviators.com
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Old 03-31-2007, 09:41 AM   #23
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Am I right in thinking that the biggest problem with selecting props is that different brands perform differently? what I am saying is quite often people run, lets say a 5x4 gws xx prop then the change to a graupner xx 5x4 prop and find a massive difference in performance. Its confusing enough with pitch and diameter but it gets very confusing with all the different brands and types of prop performing differently, for example you get slowfly props, slim props, quiet props all sorts and they all come from different manufactures as well They should be standardised any two props of the same pitch and dia. should give the same results, I assume the all have different profiles like different wing sections?
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Old 03-31-2007, 01:28 PM   #24
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Difference - yes. Massive? I tend to doubt it.

As to prop types - sure there are differences - that to me is a great benefit as you can match a prop better to your requirements.

There is no way there will be standardization as you suggest - then why have more than one company making props?

Props are wings and yes - they have different profiles depending on what works for a given manufacturer considering their capabilities, marketing strategy, etc. That's the beauty of it - lots of choices for tailoring to specific needs.

My site for electric flying articles:

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Old 03-31-2007, 03:59 PM   #25
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Originally Posted by CitaFly View Post
Difference - yes. Massive? I tend to doubt it.

As to prop types - sure there are differences - that to me is a great benefit as you can match a prop better to your requirements.

There is no way there will be standardization as you suggest - then why have more than one company making props?

Props are wings and yes - they have different profiles depending on what works for a given manufacturer considering their capabilities, marketing strategy, etc. That's the beauty of it - lots of choices for tailoring to specific needs.
Yes it has its benefits but its plain annoying as well, someone says use a 7x4 prop so I get an APC and the performanace is not waht I expect so someone says get a graupner 7x4 instead, surely you can see how this can be annoying?. I know props are cheap but we don't all have enough money to go out and by 10,000 different props to try. For the inexperienced it makes choosing a power package a minefield.
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