Static vs. Flying amp draw

[pallen_2006]

Hello,
Does anyone know a rule of thumb for how much amp draw decreases in electric models after takeoff? Let's assume a mid pitch prop so the prop is reasonably efficient at low speed (or static) and in flight speed.

Let's say, a motor is pulling 50amps at WOT static. Once at top speed in straight and level, still at WOT, how much would one expect the amps to drop? 10%? 20%?

Any ideas?

Thanks

[ADB2]

Hi,
A quick search in the Power Systems forum for "prop unload" will turn up all the information you could possibly need, and more! :)
Just a warning, most answers will be of the kind "it depends", because... it depends! ;)

[slipstick]

Fortunately people have recently been using onboard dataloggers and publishing the results. So they have exact results, at least for their specific configurations. Again Power Systems is the place to look.

There are also some useful results over on Wattflyer http://www.wattflyer.com/forums/showthread.php?t=39096

Steve

[pallen_2006]

Thanks for the replies. I will check out power systems as suggested.

[Pat Daily]

The simple answer appears to be about 15% decrease as the prop unloads.

[vintage1]

VERY roughly, it will unload about half as much as the ratio between diameter and pitch.

e.g.. an 8x4 might unload 25%, an 8x6 16% an 8x8 won't unload at all, and an 8x16 will draw 50% more in flight as static..;)

[JetPlaneFlyer]

VERY roughly, it will unload about half as much as the ratio between diameter and pitch.

e.g.. an 8x4 might unload 25%, an 8x6 16% an 8x8 won't unload at all, and an 8x16 will draw 50% more in flight as static..;)

Whats the basis for this theory Vintage?

My take on it:
As the airspeed increases then the effective angle of attack of the prop blade will decrease which will tend to produce less drag for all props. So (assuming RPM ramains constant) the load should get lower as the airspeed increases, regardless of pitch or diameter. The only time this would not be true is if the pitch was zero or when the airspeed exceeded the pitch speed (at which point you would start the generate reverse thrust and the load would increase again)

I'd actually propose that the reverse of your theory could be true in that the course pitch prop will potentially unload more with increasing airspeed because the zero airspeed drag of the course pitch prop is greater (as its has a higher blade AoA)

[Work in Progress]

Motocalc agrees with Vinto, but I don't know the theories from which the modelling is derived.

It seems intuitive that anything that gets very close to its theoretical pitch speed is going to have unloaded a fair bit, and that for most models there is a dive angle that unloads the motor completely. In real life model flight, with our (in comparison to full size) draggy airframes and sticky air, a model with a very coarse pitch prop is far less likely to reach a high proportion of its pitch speed in level flight than a model with a fine pitch prop.