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Do propellers really "unload" in flight?

Old 09-22-2008, 06:29 AM
  #76  
Larry3215
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Originally Posted by BEC View Post
Larry, your setup is different than mine. And if I understand what you were going to test correctly, you'd reach the zero thrust point dk_aero spoke of much sooner (low-pitched prop).

I'm sure a dyno and a wind tunnel would be a more precise way to measure this and it would allow controlling the other variable Larry's worried about - battery voltage sag - but a bunch more of us have loggers than we have wind tunnels and dynos....

(snip).)
Yeah, a dyno and wind tunnel would be great, but as Bernard says... we will have to get by with what we have

Yeah, my plane is a good bit different - which is a good thing. The plane Im using for the first test is very draggy and relatively slow and under powered.

I want to see how much things vary - if any - from one setup type to another.

My flights today were of no value. Somehow I had messed up the calibration settings on my Eagle Tree by a huge factor. It was reading 2.1 amps max current on a setup that pulls closer to 15 amps peak. Voltages were off as well but not as much. On top of that I didnt have it set to record RPM

So.. I'll have to wait for another day to re-fly the routine.




In looking at the raw data another bit of a problem occurs to me - thats how to decide which exact data points to compare.

For example - on the initial static run up should we use the first highest peak value or the lowest value it drops down to or the average or pick a point 3 seconds or 10 seconds into the runup?

Then later, during the flight, the same questions apply - where on the curve during the run do you pull a data point to compare?

Im going to approach it this way - unless someone can suggest a better method.

Im going to do the static run for aprox 5 seconds.

Then, during the flight, each subsequent full power run will also be aprox 5 seconds long.

Then I'll take data values from a point about 3 seconds into each run to do the calculations.

Oh - as Dr Kiwi mentioned and I as I said before - Im flying a specific flight patern that will hopefully make it easier to see and calculate the differences and reduce any variations due to climb/dive and throttle changes.

1)static run up on the ground right befor takeoff
2)level flight full power run starting from as low a speed as possible
3)level flight full power run as in #2, then pulling to vertical
4)level flight run as in #2 going into a full power dive
5)land and do another static run up.

All of those will be run at full throttle all the way thru with zero throttle points thrown in between so its easier to see on the graphs.
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Old 09-22-2008, 06:42 AM
  #77  
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Originally Posted by BEC View Post
I suppose one way to get closer to the answer to question 1 is to fly a low powered setup with a BIG battery (so that voltage is "stiff" and sags little) though of course the added weight will work against that objective. I don't see how you can completely eliminate the variable of power to the prop when the load is - by definition - not constant in this situation regardless of what means is used to twist the prop. That is, unless we go back to some huge overpowering means and the wind tunnel/dyno combo mentioned before.

Thats why I wanted the raw data files - so I could "see" even very small voltage drops. It takes very little voltage drop to make a significant power drop when power changes as the cube of the change in voltage.

The answer to two is probably as varied as power system/plane/prop/altitude/flight profile combinations.

I suspect thats true - but it would be nice to see if there is a trend we can spot - or if the general beliefs are correct

The answer to three, as I understand it, can be "yes" in the case of high speed aircraft with deeply pitched props that are partially stalled until the airplane gets moving. I imaging Schneider Cup racers from between the wars and early WWII types like the Hurricane Mk. I that had fixed pitch props would be examples.

Id like to see if there is a case with our models where that actually occurs. So far there is no sign of it on any of the graphs we have seen so far. Trams hi speed setup didnt show any of that behaviour.

I have one setup that may have a chance of showing it - a warmliner turning a 14x12 prop at around 550 watts or so. Its a relatively slick airframe as well. I wont be able to try it for a while - it needs some repairs.

Larry
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Old 09-22-2008, 07:24 AM
  #78  
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Originally Posted by Larry3215 View Post

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?

The second question is: How does that vary from one setup to another?

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.
I haven't done or read up on this type of testing, but here's my thoughts (for what it's worth)...

1. Prop unloading vs. battery voltage drop. Need to isolate the two. I think this can be done via static testing. Preferably with a low pitch prop so that static thrust should be fairly constant (i.e., prop is not near stalled condition). Precise measurement of speed, voltage and current... Problem may occur since inadequate cooling will cause an increase in current (increased motor coil resistance). You may be able to correlate voltage drop with current draw over time, thus isolating voltage drop.

2. I would think that battery, motor and prop effiency would affect each setup. Correlations are only possible among one variable.

3. As BEC said, one condition where static power level would be lower is in the case of high pitch props which are stalled at static incoming flow.
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Old 09-22-2008, 03:31 PM
  #79  
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Originally Posted by BEC View Post
mred - your racer case is where we should see more unloading once the airplane is flying well. That it's glow powered magnifies the effect assuming you set the needle a bit rich before takeoff so that when the unloading happened the mixture leaned out to "just right". Electric motors ARE different - their RPM/torque curves are very different from an IC engine.

That may be true to some extent with the engine speeding up because of the mixture changing to some extent after take off, but most of the increase is due to the prop unloading. I have heard the difference in sound from a prop standing still and one that is increasing in rpm while taking off and flying. It's not just Racers that do it either. My pattern planes do the same thing and they are not running high pitch props.

Now voltage sag is nice to think about, but it would be just backwards would it not? After the prop unloads, the voltage would increase, not decrease. The load on the battery is less after the prop unloads, so should not the voltage go up and not down like under load? As soon as I get my Eagle Tree I'll run some tests and see just what is going on, but until then I have nothing to prove my point that you can look at. Later.
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Old 09-22-2008, 04:36 PM
  #80  
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What Larry's concerned about is that even though the prop unloads to some extent, as energy is drawn from the battery its voltage goes down. You can see this in some of the graphs already posted as a general downward slope to the voltage plot even though there are local peaks and valleys due to changing throttle settings/load.

And I just realized I've contradicted myself about fast planes with deeply pitched props. In one post I suggested that's where we'd see the most unloading and in another that that might be the case where the in-flight load is higher than static. I suppose I'd better make up my mind, no? Actually I think that which situation one sees depends on the power, prop and flight condition. I can conceive of a case where both were true on the same airplane - a slightly stalled prop statically loads up as the plane accelerates and the prop is no longer stalled, then unloads again as the plane continues to accelerate.

That, however, is all outside the realm of the sorts of planes and flying where I've done any onboard logging.
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Old 09-22-2008, 05:41 PM
  #81  
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I agree that the battery voltage is going to drop as the battery is used up, but that is not the point. The point I was trying to make is the prop unloading and that is not going to change with the battery voltage going down as it is used up. The prop is going to unload and then stay that way. Any farther drop is going to be from the battery voltage dropping, not the prop unloading. Once the prop is loaded for flight, it stays that way. It's not going to change unless you start pulling the nose up or putting it down. Fly straight and level and the prop loading stays the same, so only the first part of the chart should be relevant, not the later part where the voltage is dropping.
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Old 09-22-2008, 08:00 PM
  #82  
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More specifically - how much of the power drop in flight is due to voltage sag and how much is due to the prop unloading.
Post #72

Voltage 15.78 to 15.30 for a 0.48V or 3.1% difference

Amps. 24.65 to 29.52 for 4.87 or 16.5% differenct.

Sorry I fall so see where the question has not been answered.

Charles
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Old 09-22-2008, 08:48 PM
  #83  
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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.

Don
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Old 09-22-2008, 09:09 PM
  #84  
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Adding to the brew...

I went back and changed the new chart to display propeller blade angle of attack and power required by airspeed. Attached is a screen shot of a generic 12x12 gas propeller at 15k RPM.

The initial dip in the power required occurs at about 15 degrees AOA, which is the stalling angle of the propeller blades. According to the aerodynamics books, this is as expected.

Maximum power required occurs at about 5 degrees AOA. This is close to the best L/D of the propeller's airfoil, which I found counterintuitive. I looked but couldn't find an easy explanation for this (though it matches what the books say). The interactions that these variables have with the propeller blade rotational airspeed and Reynolds number are definitely not obvious.

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Old 09-22-2008, 09:22 PM
  #85  
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Originally Posted by everydayflyer View Post
Post #72

Voltage 15.78 to 15.30 for a 0.48V or 3.1% difference

Amps. 24.65 to 29.52 for 4.87 or 16.5% differenct.

Sorry I fall so see where the question has not been answered.

Charles
Sorry Charles, obviously Im not doing a very good job of getting my idea across - but no, it doesnt answer my question at all. There is still too much missing data

For starters, that 3.1% drop in voltage would have accounted for a 9.6% drop in total power all on its own - but I have no way to reference it to the current or rpm at exact same time and no reference back to a static value. So - no way to tell what % of the over all power drop was due to voltage drop and what % due to unloading and what % was due to your loops, dives etc.

Im at work and have no time to go into details now, so more later....
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Old 09-22-2008, 11:04 PM
  #86  
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Larry3215
I've had that same exact question on my two models, powered by Hacker A50-12S and A50-16S with 6S2P A123 cells. Over the past year or two, I've put together a Microchip (Picchip) on board metering system that records total flying time, total AmpHours for the flying season, as well as the AmpHours and Amperes for the current flight. Recently added, it records the current pulled by the motor every 5 seconds for play back without requiring a PC computer. (I wrote the RCReport article last year on an "RCHourmeter")

The results were somewhat surprising. The motor current does drop off somewhat after the model reaches flying speed, but what was not expected was how much this same current varied, depending on whether the model is flying level, or climbing or diving. And it did not take much of a dive to cut the motor current by 50% or more.

The A123 cells in my models have each passed 10 hours of actual flying time, pulling 57 Amps out of the 6S2P cells. Total AmpHours to date is 175 Ampere Hours on each battery pack. The A123 cells have not lost ANY RPM on the motor during this flying season.

Also note, the Hacker A50-12S with 14X10 APC prop turns 7950 RPM fresh off the charger, and 7840 RPM after flying 60% of the battery capacity.
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Old 09-23-2008, 02:24 AM
  #87  
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And it did not take much of a dive to cut the motor current by 50% or more.
Which is why in the first attachment the time for the second min. of flight at approx. 65% Throttle shows between 400 watts and 750 watts at a fixed / constant throttle position.


http://www.wattflyer.com/forums/show...7&postcount=64


In short long high power climbs eat battery capacity big time.

Guess I am not smart enough to explain it but I know I can fly 3D, low , slow and close in for a whole lot longer than sports / pattern style flying with same aircraft.



The A123 cells have not lost ANY RPM on the motor during this flying season.
One of the great benefits of the A123 cells is that while capacity drops slowly the IR stays the same and the only change is a slight reduction in flight times if you fly them till dry. If you never use more than 2000 mAh from them then flight # 500 is the same as flight #1.

I do have A123s with over 300 flights 6 min. average flight times and one right at 500 flights with 8 min avergae flight times .

I charged my two 125Ah deep cycles that I use as a power source at field Fri, morning. I flew Fri. evening,Sat. and this morning. In the past I determined that it required approx. .66Ah to recharge each A123 cell on average. This afternoon it took 60Ah to recharge my deep cycles.
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Old 09-23-2008, 05:21 AM
  #88  
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Well, Im ticked off - at myself! I had a loooooooong answer to everyones posts since yesterday and then lost it!

grrrrrrr!

Well, Im out of time now so I'll give you all the short version - I dont really disagree with anything anyone has said - for the most part

Everyone has made comments and observations that make sense.

However - I still want to run some numbers. I need time to go through Bernards data files and collect some relavent data points and do the math. Then I want to do a few flights on my systems and see what I come up with when the throttle positions and flight profile are more of a known factor.

I still believe that battery sag is going to be significant - the question remains how significant and how does it vary.

I can test setups with marginal and much larger battery capacities on the same plane so that will help in that reguard.

Tram - if you sent me that data file of you flying that particular profile - I never did get it.

As I said before - this isnt a question that will change the world or even make much - if any - impact on how anyone sizes thier systems - Im just curious.

So far Im finding the discussion very interesting and informative
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Old 09-24-2008, 04:54 AM
  #89  
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I still do not understand this thing everyone has about battery sag. What in the world has battery sag got to do with prop unloading in the air. ( NOTHING ) Battery sag is not an issue for prop unloading and never has been. The prop either unloads or not - period. Battery sag is an issue of the condition of the battery and a whole bunch of other things, but that does not make a prop load or unload. It makes the motor run faster or slower, but that has nothing to do with prop loading, ever. Put a plane in a steep dive at WOT and tell me that battery sag has something to do with the motor screaming it's heart out. If you are worried about battery sag, then put a 4000mah battery in a 200W powered plane like I did and tell me battery sag is causing the prop to load up. I could hear the prop speed up as I took off and when I put it in a dive. That is prop unloading. The battery is big enough that battery sag does not come into it. Yes the voltage drops as I use up the battery, but what has that got to do with prop loading???
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Old 09-24-2008, 05:41 AM
  #90  
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Ed,

I think the connection is that guys were trying to determine how much of the observed drop in in-flight amp draw was simply due to battery sag (voltage) and how much was actually due to "prop unloading".

Cheers, Phil
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Old 09-24-2008, 07:31 AM
  #91  
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Default Beating the dead horse...

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.
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Old 09-24-2008, 08:25 AM
  #92  
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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...


GRU
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Old 09-24-2008, 08:14 PM
  #93  
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Originally Posted by Dr Kiwi View Post
Ed,

I think the connection is that guys were trying to determine how much of the observed drop in in-flight amp draw was simply due to battery sag (voltage) and how much was actually due to "prop unloading".

Cheers, Phil
I know the battery has an effect on motor speed and it always will, but it can be taken into account if you have a big enough battery with a good charge on it so that the load on the battery is not so high. Yes you will still have changes in battery voltage, but they will be much smaller and can be put on a chart to compare the change of voltage against prop unloading. The motor itself will cause a change of voltage with different loads. That's why I was talking about an IC engine instead of a battery powered one. It's much easier to tell if the prop is unloading with an engine then a motor, unless you can build a constant current, constant voltage power supply that can maintain voltage and current regardless of the load the motor puts on it. The problem most people have with voltage drop is because they have a battery that puts out 50 amps, but they are loading it to 40 amps. Try a 50 amp battery and put a 20 amp draw on it and the voltage sag is MUCH less.

I know the engine will speed up because of the fuel mixture changing during flight, but no where near 10,000rpm. Static rpm while adjusting the fuel flow goes to 23,500rpm and then decreases, so you back up the mixture to 23.000rpm. Now, on take off the motor starts to unload and speeds up, partly do to the mixture changing, but mostly do to the prop unloading. If the best you can get on the ground is 23,500rpm and it changes to 30,000rpm in the air, then that is not mixture change causing the rpm to increase that much, but prop unloading.

The point that I was trying to make is, prop loading or unloading has nothing to do with voltage sag, motor load does. If the chart has amps and voltage recorder on it and the rpm can be logged, then you have a way to measure the overall effect of prop unloading. You must chart rpm, voltage, and current to be able to tell if the prop is unloading. If the load on the motor decreases, then the amps will drop and the voltage will increase. This cannot be done very accurately with a heave load on the battery, you need a lite load, or the power supply I was talking about.

If a wind tunnel is used, then you must know the air speed of the plane and be able to control the air flow in the wind tunnel to the point that you can match that. Then you can use a GOOD power supply and run your test.

Now I will admit that I am new to electric planes and I have a lot to learn, but I have been flying IC models and full size planes since the 50's and I understand general flying and prop unloading. I know a prop changes load during and after take off. The only problem with proving it is using a proper power supply with the motor such that the battery is taken out of the circut altogether. As long as the battery is able to change voltage because of load, you will never prove prop unloading to the point of setteling the issue. The IC engine comes closer to that then a battery ever could.

Ed
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Old 09-25-2008, 06:02 AM
  #94  
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Originally Posted by Dr Kiwi View Post
Ed,

I think the connection is that guys were trying to determine how much of the observed drop in in-flight amp draw was simply due to battery sag (voltage) and how much was actually due to "prop unloading".

Cheers, Phil
Exactly

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 (snip)

All variables that affect the independent (measured) variable must be controlled for accurate measurement (characterization) of the independent variable.
All good points, but Id still like to get as close as possible with the tools we have. I think we can at least get in the ball park with the on board logger data.

Your absolutely correct on needing to limit the "unknowns" as much as possible. We obviously cant control or know every detail perfectly - but I think we can get 'close enough' to at least get a general idea of the relationship between how prop unloading and battery sag effect the power drop in flight.

Originally Posted by mred View Post
I know the battery has an effect on motor speed and it always will, but it can be taken into account if you have a big enough battery with a good charge on it so that the load on the battery is not so high. (snip)

The point that I was trying to make is, prop loading or unloading has nothing to do with voltage sag, motor load does. If the chart has amps and voltage recorder on it and the rpm can be logged, then you have a way to measure the overall effect of prop unloading. You must chart rpm, voltage, and current to be able to tell if the prop is unloading. If the load on the motor decreases, then the amps will drop and the voltage will increase. This cannot be done very accurately with a heave load on the battery, you need a lite load, or the power supply I was talking about.

(snip)


Ed
Hi Ed,

I see your point and I dont disagree with it for the most part. Your comments Ive highlited above are almost exactly how I am proceeding.

Im not saying props dont unload in flight. I want to know how much of the observed power drop in flight is due to battery sag and how much is the result of prop unloading.

Untill today I didnt have any data that would really even start to answer that question. All the previous graphs and data were missing too many details.

Tram just sent me his data file where he flew the patern we discussed previously and recorded voltage, amps, watts and speed. The one data element thats still missing is RPM. In this case I at least know what the plane was doing and what the throttle setting was at each moment along the graph.

I'll have to wait untill I can fly my own planes to get that RPM data as well.

As soon as I look at the file I'll post some results.
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Old 09-25-2008, 07:40 AM
  #95  
BEC
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Larry,

I have a couple of EagleTree files that have GPS speed as well as RPM logged (from the SHAFT review on RCG). Interested in those? No throttle setting logged, though.

Motor winding and ESC temps are also logged, along with Volts and Amps of course.
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Old 09-25-2008, 09:01 AM
  #96  
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Originally Posted by BEC View Post
Larry,

I have a couple of EagleTree files that have GPS speed as well as RPM logged (from the SHAFT review on RCG). Interested in those? No throttle setting logged, though.

Motor winding and ESC temps are also logged, along with Volts and Amps of course.
Thanks Bernard, but unless I can tell if your climbing/diveing and throttle settings it doesnt do a lot of good. I havent been able to figure out how to reference back to static values without that data as well.

I think the ideal data file will include volts, amps, rpm and possibly speed.

Then the plane needs to fly a specific flight profile so we can tell whats happening when.

In particuler I think a straight and level full throttle run starting from a low speed to max speed will be the most relevant and revealing as far as pure unloading in reference to a static reading. We also need the static full power data.

Climbs and dives will be interesting all on thier own but I dont think they help with the basic question that much. Id still like to see what % of change they cause just for fun.

I probably wont have time and good weather to do any logged runs untill this weekend some time.

Jeff sent me the wrong file so we are still waiting on that

Last edited by Larry3215; 09-25-2008 at 09:16 AM.
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Old 09-25-2008, 04:12 PM
  #97  
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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.

Ed
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Old 09-25-2008, 04:47 PM
  #98  
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Ed,

That's what I was saying when I suggested a series of smooth level circuits at "slowest possible", "moderate" and "continuous WOT" - with V, I and rpm all recorded.

Cheers, Phil
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Old 09-25-2008, 05:59 PM
  #99  
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I was agreeing with you, but it does not always come out right. Something about my thinking and my fingers doing their own thing.
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Old 09-25-2008, 07:23 PM
  #100  
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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.

Ed
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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