First wattmeter results and subsequent changes

[Peter Beeney]

Hi Mitchell,

 

I think I would go along with Matt too, I would have a little play around with it just to find out exactly how it all works.

 

One way of doing that might be like this.  Using the nominal voltage of 14.8 for a 4 cell lipo to make a datum line to start from the unloaded speed of the motor is 11,840rpm. I consider for my purposes I want the motor to run with a prop that lets it run in the most efficient way possible, that would be about 20% down from the unloaded speed, so that’s a rather precise 9,472rpm. Your current? (present) set up at a 7 minute run suggests the the average current flow throughout the flight was approx, 37 - 38 amps. All these figures are very general fag packet figures by the way. If by changing prop sizes I could arrive at an average flight current flow of around say 18 -19 amps but with the same flight performance as the 6 - 7 minute flight then that would extend my flying time to something like 12 - 14 minutes. I know that as the prop size gets smaller the motor speeds up and the current flow reduces proportionally so I’d start with a small version and try various sizes ether way until I found the best all-round answer. I think it’s possible that a smaller but faster turning prop may provide almost as much tug as a larger but slower turning one in some cases at least.

 

This is a very brief idea, perhaps a little bit different too and may want some explanations but they are all there. Two small points, your model’s wing loading appears to be 20 oz to the square foot, that’ll do nicely for me and if the 8 inch pitch prop starts turning at 9,472rpm that will give the model a theoretical forward velocity of 70mph……….

 

 Enjoy it all…

 

PB

[Matt Carlton]

I would be tempted to go with a pretty low pitch, maybe a 12x5 or so, even a 13x5, as you really don't need a high pitch speed. 

The beauty of electric is that you can underprop a motor and unlike an IC engine, you don't end up with ridiculous RPM so you can effectively fit a smaller motor by simply under propping and being kinder on the components. 

I think you'll find the model much more controlled with a lower pitch prop and everything will be much cooler.