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Old Sep 24, 2007, 06:20 AM
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Question
What do the prop numbers mean?

I'll see a propeller and then 9x4.7 or 8x3.8 etc.
What does it all mean?
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Old Sep 24, 2007, 06:42 AM
The Revegetator
Chris F's Avatar
Melbourne Australia
Joined Jun 2006
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9X4.7 would be a prop with a diameter of 9 inches and a pitch (angle of prop blades) of 4.7.

There is more info on props and pitch speed in this guide

http://www.rcgroups.com/forums/showthread.php?t=739069
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Old Sep 24, 2007, 06:46 AM
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East Anglia, UK
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Quote:
Originally Posted by The_RattleSnake
I'll see a propeller and then 9x4.7 or 8x3.8 etc.
What does it all mean?
Sadly less than it should.

9x4.7 means its roughly 9" diameter and VERY roughly 4.7" pitch..the amount in theory it would travel forward every revolution in a totally incompressible liquid

In practice props vary widely, even if they have the same numbers stamped on them.

- the marked pitch may be completely illusory.
- they may not have the same pitch along the blade
- the blade sections can vary from prop to prop
- The blade areas do differ widely and from make to make.
- And over the actual blade span.
- and it says nothng about how many blades there ARE.

So although the marked stuff is a start at specifying the prop, in the end everyone scratching for top performance buys half a dozen, and tries them all to get the one that feels best.
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Old Sep 24, 2007, 07:07 AM
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Joined Jul 2007
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Thanks for that link...


So if I have understood correctly, for example, an indoor 3d foam plane would suit a bigger
prop with less pitch than a smaller one with more pitch?
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Old Sep 24, 2007, 07:09 AM
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Joined Sep 2002
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Quote:
Originally Posted by The_RattleSnake
Thanks for that link...


So if I have understood correctly, for example, an indoor 3d foam plane would suit a bigger prop with less pitch than a smaller one with more pitch?
Definitely.

On the other hand, in wind you want more speed to beat upwind, and keep the model above stall in turbulence, and the ability to climb steeply is actually less of an issue, so you want it the other way around!
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Old Sep 24, 2007, 10:27 AM
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Dr Kiwi's Avatar
Chattanooga, Tennessee, United States
Joined May 2003
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Quote:
Originally Posted by vintage1
Sadly less than it should.

9x4.7 means its roughly 9" diameter and VERY roughly 4.7" pitch..the amount in theory it would travel forward every revolution in a totally incompressible liquid

In practice props vary widely, even if they have the same numbers stamped on them.

- the marked pitch may be completely illusory.
- they may not have the same pitch along the blade
- the blade sections can vary from prop to prop
- The blade areas do differ widely and from make to make.
- And over the actual blade span.
- and it says nothng about how many blades there ARE.

So although the marked stuff is a start at specifying the prop, in the end everyone scratching for top performance buys half a dozen, and tries them all to get the one that feels best.
As usual Vintage is on the ball:

Take a look at this collection of 8x4 props (a couple of other close enough ones in there too).

On the same motor at the same voltage, each would draw a different current, and get you different thrust at different rpm - and at a different [hypothetical, and/or actual, if one could determine it] pitch speed.
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Old Sep 25, 2007, 05:58 AM
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I guess that they are cheap so the best thing to do is just buy heaps of different
ones until I find something that works well and learn from that.

I didn't think the same size props could be so different.
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Old Sep 25, 2007, 06:10 AM
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Quote:
Originally Posted by The_RattleSnake
I guess that they are cheap so the best thing to do is just buy heaps of different
ones until I find something that works well and learn from that.

I didn't think the same size props could be so different.
None of us did..till we tried Master Airscrew
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Old Sep 25, 2007, 08:16 AM
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Chattanooga, Tennessee, United States
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Hi Vintage, as you know I've found the MAS Electric Only series to be hopelessy inefficient in converting watts into thrust, but I believe I have actually found a half-way decent MAS - admittedly a very small sample but I just tried the 7x3 and 8x4 MAS G/F Series props (using the same Justgofly 450TH 12-magnet (1700Kv) motor). They take a lot of balancing, but after that they do run very smoothly and generate decent thrust - the 7x3 G/F drew more than a 7x3.5 GWS HD, but also produced more at only marginally lower "efficiency" (774g @ 4.3g/W v. 707g @ 4.71g/W). Pitch speed is moot since the GWS HD might only be 7x2.5.

Comparison of the 8x4 G/F (837g @ 4.76g/W) versus 8x4 GWS HD (852g @ 5.01g/W) and 8x4 Graupner Slim Prop (844g @ 4.85g/W) surprised me in how similar they all were.

Cheers, Phil
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