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Old 09-24-2008, 07:25 AM
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Join Date: Jul 2008
Location: Pflugerville, TX
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Originally Posted by dk_aero View Post
I don't think the inflight testing will produce any useful quantitative data. For instance prop unloading (power decrease) vs. airspeed for a particular prop or percentage of power drop from prop unloading vs. voltage drop.

That being said, we might get closer by -
1. Testing the battery statically. With a variable load, you can characterize the battery in terms of current draw vs. voltage drop, voltage drop over time at constant current, etc.

2. Once the battery is characterized, you might be able to characterize prop unloading using inflight tests. Highly doubtful since you have no control over incoming flow - example: flying the plane at a "constant speed"... what are the winds aloft? How do you fly at a constant airspeed when you're on the ground? Propeller thrust (power) is determined by incoming flow vs. discharge flow.

Again, the only method I can come up with to accurately measure power drop due to prop unloading vs. voltage drop is a wind tunnel utilizing a motoring dynamometer. Accurate measurement and control of incoming air flow, propeller speed and torque will enable characterization of the propeller (power drop vs incoming flow). Next, replace the motoring dynamometer with our battery/motor/esc combination. Now, any changes in power drop from the baseline prop characterization would be due to our combination.

All variables that affect the independent (measured) variable must be controlled for accurate measurement (characterization) of the independent variable.
Give me a shop fan, 10,00o drinking straws, four sheets of plexiglass and a case of beer and I'll settle this argument once and for all!

All kidding aside - I've been lurking on this thread for ahwile... I'm tempted to shoot off an email to those guys working on full-size electric aircraft - one of their EE's is sure to know the answer to this question, as it is likely central to their drive to squeeze every drop of flight time out of the power train...

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