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Author Topic: air and water load equivalents  (Read 1109 times)

tobyker

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air and water load equivalents
« on: October 12, 2009, 02:58:42 PM »

Since most brushless motors are sold for use in model flying machines, they all come with a propellor recommendation. Does anybody know of any work done to equate airscrew and water screw loads - ie (FOR EXAMPLE ONLY) that a motor recommended for a 9" x 6" airscrew would be appropriate for a 30mm two-bladed waterscrew?
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Martin [Admin]

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Re: air and water load equivalents
« Reply #1 on: October 12, 2009, 04:04:48 PM »


That's a damn good question tobyker!   :-))
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GG

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Re: air and water load equivalents
« Reply #2 on: October 12, 2009, 04:36:26 PM »

Tried it once.  Fitted a 8x4 (inch) airscrew into a 540 motor and trimmed it down until the current reading was the same as a P35 waterscrew would produce with the same motor/battery combination.  I seem to remember that airscrew ended up about half its original diameter.
Lots of "ifs and buts" in such a method.  For instance do water and airscrews unload by the same amount when the model is running at full speed?  The airflow conditions must be standardised too, changing how the air is drawn into the propeller and then blown out could affect the motor speed.
From experience with RC aircraft, a SPEED 400 motor using a 6x3 airscrew would draw 8-10 Amps with the model held stationary.  Using the same motors and battery packs in a boat and I'd be reluctant to load the motor with more than half this current with the model held stationary in the water.  I know that in aircraft the motor can benefit from some cooling air flow (but they still get hot!) but I suspect the motors greatly unload (i.e. speed up) when at normal flying speed.
Even if an exact airscrew/waterscrew match cannot be made, a good enough approximation might be possible.
Glynn
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dodgy geezer

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Re: air and water load equivalents
« Reply #3 on: October 12, 2009, 05:35:12 PM »

Quote
Lots of "ifs and buts" in such a method.  For instance do water and airscrews unload by the same amount when the model is running at full speed?  The airflow conditions must be standardised too, changing how the air is drawn into the propeller and then blown out could affect the motor speed.
GG

The impression I get from my limited testing is that airscrews do not unload as much when flying as a boat does when running. For boats, as well, there is the hull form to consider. Some hulls plane, some are displacement and slow.

All aircraft move through the air at a reasonable speed - about 30-60mph. Some go faster. Boats may also do 30-60mph, or they may do 2 mph. The unloading ratios here are very great, and the propeller requirements vastly different. So I think it will be hard to produce one table comparing air and water screw requirements...
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