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Old Oct 17, 2008, 02:45 PM
Registered User
Philadelphia, PA
Joined Apr 2005
98 Posts
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How does prop size/pitch effect a plane?

Could someone please explain in simple English how the size and pitch of a prop makes a difference.
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Old Oct 17, 2008, 03:28 PM
Deletedfor proving Nauga wrong
Joined Mar 2005
15,855 Posts
In simple english?

Best I can quickly do for you:
(ask if you need more explaination)

Excess prop size + pitch will overload the motor/engine and substantially reduce performance. It can also lead to premature failure of the power system.

Prop selection is a trade-off. You select based on the aircraft's desired performance (after selecting the appropriate power system to make the propeller turn...)

For fast accelleration and steep climbs you want low pitch and large diameter. This is similar to low gear for an automotive transmission. Lots of pulling power, but low maximum speed.

For speed you use high pitch and low diameter. Again compared to a car transmission, this would be high gear. Good while at speed but if you are trying to tow a trailer uphill you'll be in trouble.

For sport flying we generally choose a medium pitch and diameter. For ".40" size glow planes the 10X6 is a commonly used propeller. Its not always the best choice though...

For example on the Four Star 40 when using a common .46 glow engine a better prop is the 11X4 or 11X5. The plane isn't going to go as fast with the 10X6 because the plane has a lot of drag (like pulling a trailer uphill) So the 10X6 is just beating the air into submission.

A good target is "Pitch speed" = 1.1 to 1.5 times "full throttle level flight speed"

Pitch speed is RPM/1048 * pitch

use 1000 to make the math easier... (and it accounts for the engine speeding up in flight vs ground tachometer readings)

So if the engine is spinning a 10X6 at 13,000 rpm, pitch speed is appx 13 X 6 = 78 mph.

The Four Star 40 typically has a max speed of appx 55 mph. 78/55 = 1.418... (high in the useable range...)

But spinning a 11X5 at 13500 would give more static thrust (better acceleration and climb) and 67.5 mph pitch speed. 67.5/55 = 1.227

For most purposes appropriate to the plane, the 11X5 will have the Four Star 40 perform better.

Lower pitch also means the plane will slow down more at low throttle settings. The Four Star has a reputation for "floating" past the runway when using a 10X6. Idle speed for most glow engines is high enough that the 10X6 can keep the plane flying at idle. the 11X5 will reduce the idle pitch speed to where the plane lands easilly.

*******************

Learning what prop is best for which plane and engine is a matter of experience and/or experimentation... I have a LOT of different props and I am often swapping props looking for which will give the best results for the way I want a model to perform.
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Old Oct 17, 2008, 09:21 PM
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Jacksonville Fla.
Joined Mar 2007
2,161 Posts
nice explaination...in simple terms a bigger diameter prop with a higher pitch is like driving up hill in high gear....your going to get to the point that you over load the engine...too small a prop and pitch and its like a C-130 at idle, the engine is turning at 100 per cent but the pitch of the prop is zero and no thrust is produced.....
between the the 2 explaintions I hope you get the info you're looking for......
Ron is a good teacher...whats with the Texas location?
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Old Oct 17, 2008, 09:43 PM
Deletedfor proving Nauga wrong
Joined Mar 2005
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Note that when dealing with "the need for speed"... you can go to a prop that has more pitch than diameter... but you have to be very careful when attempting to take off, climb or do hard maneuvers. Its all to easy to "stall" the prop blades resulting in severe loss of thrust and can easilly end up turning the model into toothpicks.

I've used up to a 8X9 (hand carved) on a .35 engine burning 60% nitro. Engines don't last long run that way... but the plane sure goes fast (If it was designed for speed...)
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Old Oct 17, 2008, 10:24 PM
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Wellington, NZ
Joined Aug 2006
837 Posts
Thanks, fh, I found your explanation useful.

Lindsay.
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