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Old 09-25-2008, 06:23 PM
Look out for that tree!!!
Larry3215's Avatar
Join Date: Mar 2006
Location: Gig Harbor, Wa USA
Posts: 7,061

Originally Posted by mred View Post
Well I guess in a round about way, that was the point I was trying to make. The flight MUST have a given flight profile to do any real testing and make it meaningful. Straight and level at WOT from the slowest speed you can maintain or better yet, a standing start to level flight at WOT, but the battery MUST be big enough to maintain a constant draw without a heavy load being placed on it to keep the voltage as steady as possible and the amp draw steady. I know the amps are going to go down when the motor load decreases, but given that, the battery voltage should also go up with a liter load. Using a graph on a stunt flight is almost meaningless because the load is constantly changing as the attitude of the plane changes.

I am not trying to put anyone down, but if you are going to run a test, then it must be a test and nothing else period. Anything thrown in can change the results to the point of being meaningless. Changing throttle position is not going to help either. The standard test for anything has as few extra data points as possible so as to get some meaningful data.

I worked in PMEL for quite some time, and when we calibrate something, we input a constant for the test and nothing else, or the test is of no use. The same applies to this unloading point. To do a proper test, you MUST do a proper test or the results mean nothing. THAT is the point I was trying to make and if it didn't come out clear before, I'm sorry.

Yup - thats pretty much the plan. The one difference is that I WANT to see the battery sag thats happening during the flights.

We dont fly planes with infinity large batteries so the voltage drop does play some part in the power drop - I want to see how large that roll is.

With the on board loggers we can see what the battery voltage is doing pretty accurately. Down to better than .1 volts easily at any point in the flight.

If we take Trams static run the power starts out at 318 watts and 10.74 volts.

Then 5 seconds into that static run the battery has dropped to 10.39 volts.

If the theory is correct, that 3.2% drop in voltage will result in a roughly 10% drop in power. That means the power should have dropped by 31.8 watts to 286 watts. The power actually dropped to 275 watts or about 13.5%

After looking at a number of static runs on the data files, the theory is a pretty close match to the recorded data. In practice, the power tends to drop off a little more than the thoery predicts. Probably due to motor heating.

I just took a quick look at Jeffs data file for the run where he did some level flight speed runs.

As I said his static run up folows the thoery very well with the power dropping off a bit more than the theory preditcs as the voltage drops.

On his first level speed run the total power drop after 5 seconds is aprox 27 watts from 290 watts down to 263 watts.

Of that 27 watt drop aprox 9 watts or about 1/3 is due to voltage sag from the packs. That doesnt take into account the motor heating.

So the power drop in flight on Trams speed plane is looks to be about 40/60 battery sag/motor heating vrs prop unoading.

The total power drop in level flight is about 9.3%

About 3%-4% is due to battery sag/motor heating and 6% is left over for "prop unloading".

I need to go through the file more carefully but thats the rough values for the first part of the data. I still need to look at the climbs and dives and later level flights to see if the data is consistant.
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