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Old 09-22-2008, 07:48 PM
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bz1mcr's Avatar
Join Date: Aug 2005
Location: Troy, MI
Posts: 136

Charles, its not at all that I dont believe you!

I said earlier that the graphs posted so far - including yours - clearly show unloading is indeed going on. They also show that the load on the motor increases/decreases with climbs and dives.

Your graphs supported the others that have been posted as far as that goes.

I dont think there is any question left on that issue - if there ever was.
So, the question asked in the first post is answered with data--props do unload with increased flight speed.

Maybe you missed it or I didnt word it very well but thats not really my question and never was actually.

My question still remains unanswered - how much of the "power unloading" between a static run up and inflight running is due to battery voltage drop and how much is due to actual prop unloading?................For example:

Lets say your static full throttle power level is 300 watts average.

You take off climb to altitude and go full throttle in level flight and your NEW full power level is 250 watts.

That seems to indicate an "unloading" resulting in a 50 watt drop in power.

My question is - how much of that 50 watt drop is due to the prop unloading and how much is due to the battery voltage dropping?
This is not a porp question but a battery question. Before getting into the details I would speculate that at normal conditions the battery sag will be a very minor (if any) contributer. To get an absolute answer you would need very accurate voltage measurments. But you can get a pretty good idea by just doing several static run up tests. I would you suggest you hold WOT for about 15 sec and average the result. Lets say you start with a fresh charged battery and run the first test and get 300W. Then you wait some time (maybe a minute) and do it again. You may get 295, 300 or even 305W. (It depends on the details of the battery and it's temperature.) You sould wait again and run more of these tests to get a feel for how fast the battery sags. I would run at least enough cycles so the power is definately dropping on each cycle. Now start over with a fresh charge on the same battery. Run 1 or 2 static tests to compare to the earlier results, if they are about the same, do the next run up in flight. Wait the same time (1 minute) then launch and go to WOT as soon as you dare and hold 15 sec. Now compare the power to the earlier static run up on the same cycle. My prediction is you will see a far greater drop in power than in the series of static tests. That would indicate the majority of the power drop is from unloading rather than battery sag.

The second question is: How does that vary from one setup to another?
I think we know the battery sag will be greatest on setups where the battery current is closest to its maximum output. Battery sag is also largest in fresh of the charger batteries during the first 20 seconds of heavy load.

Prop unloading will be greatest on fast flying models.

So in a draggy underpowered planes with a battery that gives low flight times the battery sag would contribute a higher percentage of the in flight power drop. I doubt in anything that would fly for more than 4 min it would ever approach 50%. That means the prop unloading would still dominate. (You might get over 50% loss due to sag if you can test the first few seconds of operation in a fresh off the charger battery.)

In a slick pylon or hotliner that will fly fast for over 8 min. The prop unloading is probably responsible for well over 90% of the in flight power drop off.

The third question is: Are there cases when the static power level is LOWER than the inflight power? The corolary being - can a prop load UP in flight, relative to the static condition.
The only time in flight power is higher than static (prop loads up) is if the prop is stalled in the static condition. That is said to happen with props where the pitch approaches or exceeds the diameter. As I said in an earlier post, the guys trying to fly at max speed tell me that if a motor is going to smoke it will be just after launch. That is when these stalled props are getting a bite (becoming unstalled). As the porp gets a bite the motor slows down while the plane accelerates! That means the inflight power can exceed the static but only for an instant. Then it immediately starts to unload. To get a recording of it you need a very fast plane with a pilot comfortable launching at WOT.

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