Prop Size and what it all means - RC Groups
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Feb 03, 2003, 03:15 PM
Basil's Avatar

Prop Size and what it all means

I searched but found no easy def. of what the prop size means... 9x7, 10x4.7 , etc..

What excatly does this mean and how does this affect the performance on a plane and batty/motor life


I looked at the E-Zone FAQ and nothing is meniton....
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Feb 03, 2003, 03:32 PM
Motor Maniac
Here's the quick, and I hope accurate, answer. The first number is the diameter of the prop. The larger the number the larger the prop. The size may be in inches or in metric (cm usually), so this can be confusing. The second number is the pitch of the prop, essentially the angle of the twist in the prop, also measured in inches/cm. The higher this number the bigger the bite the prop takes out of the air.

A larger prop, either in diameter or pitch, or both, puts more load on the motor/gearbox, but is also more efficient in general. Smaller props are generally used for speed, larger props for higher thrust at lower speed.

Again, this is just a quick and simplistic explanation. There's a lot more to it than that, such as the current draw based on the motor, gearbox, and battery.
Feb 03, 2003, 03:37 PM
Basil's Avatar
Thats basiclly what I needed to know...

FYI for all those as lost as me..

So a 10.47 willl be slower and able to "carry" a larger load and a 9.7 will be faster but will not be able to "carry" an as heavy load.
Feb 03, 2003, 08:19 PM
Motor Maniac
A lot depends on the motor/gearbox/battery used. I tried switching between 10x4.7 and 9x7 props on my Gym-E this weekend and didn't really notice a difference, but the wind had picked up and it wasn't easy to compare fairly. The Gym-E is a draggy plane, so that could be a factor as well. Some of the people I respect the most on Ezone just experiment until they find the combo that works for them instead of using Motocalc or Pcalc to figure out the best combo.
Feb 04, 2003, 09:24 AM
God is good
Viper Pilot's Avatar
The pitch size, I've been told, roughly approximates the forward motion of the plane with 1 revolution. That is, a 7" pitch will move the plane 7" forward for each turn of the prop. This may be too simplistic, or even untrue.

Any response to this??

Feb 04, 2003, 09:51 AM
Registered User
Andy W's Avatar
That's the idea!
Feb 04, 2003, 09:52 AM
Registered User
columbiarcdude's Avatar
VP ... you are correct. BTW ... a prop like a 10x4-7 means 10 inches in diameter and 4 inches of pitch at the hub and 7 inches of pitch at the tip.

Props are interesting things. When I was involved in AMA pylon racing, I always tested large batches of props. You ended up with practice props and race props. What I mean is some props, with the same diameter and pitch, made by the same manufacturer performed MUCH better than others. Looking and measuring and weighing would show that the props were as identical as they could be ... just one or two worked better. I still have no idea why this happened.

Feb 04, 2003, 03:28 PM
Live to ride... and fly!
Tres Wright's Avatar
Oh man, the prop game. Unfortunately there is no magic machine that tells us what prop is most appropriate. I think I would be first to buy one of those if it became available I've found that Motocalc gives only a general idea of what happens when you swap props on a given setup. I usually go from there to static bench tests with a Whattmeter to see how different props affect the motor and battery pack. Then I flight test the 3-5 props that look the best. The one that works best is almost never the one I expected!

Also, you might switch props on a plane depending on what flying you're planning on. You might fly a Tiny outdoors with a 9x7 for example, but then switch to a 10x4.7 to fly indoors as the larger, lower pitch prop will fly it much slower with more thrust allowing walking speed flight at high angle-of-attack attitudes.

All props are not created equal either. A GWS 9x7 flies different than an APC 9x7SF. And an APC 9x7SF is different than an APC 9x7E!

I know that's all a little off topic since the question was more specific, but just thought I'd toss my 2 cents in
Feb 04, 2003, 07:29 PM
Balsa Builder
Paul Susbauer's Avatar
Originally posted by Tres Wright
....I know that's all a little off topic since the question was more specific, but just thought I'd toss my 2 cents in
Tres, its still on topic, besides your 2 cents is always welcomed.

Feb 05, 2003, 09:54 AM
Registered User
Patrick Plawner's Avatar
It also took me sometimes to get this well.

LEt me share with you my finding (I hope accurate)
Let's take a prop type 12x8

- Diameter 12 inch
- Pitch 8, meaning progress 8 inches per rotation

First number will give you more thrust
Second number will give you more speed

Then, you will have to balance what is most important to the type of plane you want to fly. Thrust, speed or both.
In all cases, both should be above the limit tolarated from the plane.

Meaning even if you have a thrust, able to carry double the weight of your plane, if your speed is too slow, you'll get in big trouble to maintain the plane.

Power will be given to Thrust, Speed and heat dissipated with some configuration making too much heat.

Next is the gearbox.


- Motors are most efficient at high speed
- Propellers are most efficient at low speed

---> Gearbox

Even Spitfire, during WWII had a gearbox, for the same exact reason.

Gearbox with enable you to have larger prop, more thrust.

That's what I learned, in short. There are rules of thumbs to balance the numbers for props, but the best is to use a "MotoCalc" type of software and play with the different options/setups.

Feb 05, 2003, 08:15 PM
Pedal Power!
lakedude's Avatar
Minor clarification.
Pitch 8, meaning progress 8 inches per rotation
This is only in an ideal case as Andy said. What ideal means is that the prop in this case would travel 8 inches per rotation if air was a solid and the prop was a screw. In pratice there is a lot of slip cause air is not solid. in fact on takoff just before the plane moves the slip is 100% I don't know the ratio of travel once the plane is in the air but you ain't never gonna see the whole 8 inches. (Edited by Paul)
Last edited by Paul Susbauer; Feb 06, 2003 at 11:26 AM.
Feb 08, 2003, 01:32 PM
Registered User
The one thing that I don't think was mentioned here is the speed required for the wing to provide adequate lift. I'll try to put it into the basic terms as I understand them. The two extreme cases would be a helicopter and an unpowered glider. The helicopter gets its lift exclusively from the prop, and the glider gets its lift exclusively from air flowing over and under the wing.

A powered, winged aircraft gets some of its lift from the prop and some from the wings. As the nose is pulled up into a high angle of attack, the prop provides more and more lift. If it has enough power to reach hover, it effectively becomes a helicopter with no lift coming from the wings. In straight and level flight, most of the lift comes from the wings. Each wing design requires a certain air speed in order to provide the lift required to keep the aircraft in the air, so the prop must have enough pitch to provide that speed.

Bottom line is that the prop needs to be matched to the aircraft. A large diameter prop with little pitch will be great for high angle of attack flying, where the prop provides most of the lift. But it may not have enough pitch to provide enough forward speed in straight and level flight to generate adequate lift from the wings. A small diameter prop with lots of pitch will be capable of high forward speeds in a low-drag airframe. But if it has too little thrust, it would not be able to overcome the aerodynamic drag of an aircraft designed primarily for slow flight.

As you look at the various types and sizes of props recommended by manufacturers for different types of aircraft, the pattern will soon become obvious.