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Old Nov 09, 2004, 10:53 AM
Styrofoam Bird
Plainsboro, New Jersey, United States
Joined Sep 2004
100 Posts
Will the GWS 4-blade prop be OK for this setup ?

I am going to build another P-51, and was wondering if the 4-blade prop will be ok with this setup I have... ie: Will I burn up the engine or anything ?

(I believe the GWS 4-blade prop measurements are 254mm x 203mm or 10in x 7.9in)

Himax 2025-4200
Cobri 20
Tanic 3s 1550

Though I have been flying for a while now, I still don't know the "rules" regarding prop size, and have always gone with what was recommended by the manufacturer.

Any advice will be greatly appreciated.

Thanks, Brian
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Last edited by Warbird Brian; Nov 09, 2004 at 12:17 PM.
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Old Nov 09, 2004, 12:25 PM
Registered User
Birmingham Intl, Great Britain (UK)
Joined Jul 2004
969 Posts
Using a 6.6:1 gearbox, Motocalc estimates 15.5A so you should be within the 10-12C rating. Static thrust is estimated at 893g so you'll be well overpowered!
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Old Nov 09, 2004, 12:32 PM
Styrofoam Bird
Plainsboro, New Jersey, United States
Joined Sep 2004
100 Posts
Thank you Steve.

Is Motocalc something I can buy and download ? I'd like to figure out these things for myself if I could

Thanks again, Brian
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Old Nov 10, 2004, 03:06 AM
Registered User
Birmingham Intl, Great Britain (UK)
Joined Jul 2004
969 Posts
Brian, try MotoCalc. I'm a newcomer to electric flight, about 8 months now, and found it a pretty steep learning curve. After other recommendations I got MotoCalc which enables you to run various scenarios through to try to pick a motor/propellor/speed controller/battery setup. Its certainly no replacement for experience, speaking to some long term electric flyers they can quote a prop for a given motor/battery combination without ever actually having measured volts/amps.

I've also bought an electric flight meter which reads motor volts and amps up to 100A which can confirm setups. I used MotoCalc to find a better setup for a Mosquito I have. I was using direct drive 480BB motors with 6x4 props which was pulling over 30A but thrust was disappointing. I ran scenarios through and added 3.33:1 gearboxes plus 10x8 Graupner Cam folding props. For a 2Kg model it now flys well with a lower current. I've used it to predict thrust & current draw for an HVP Beaufighter I've just finished. MotoCalc got the current draw & prop rpm spot on.
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