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Old 02-24-2013, 09:51 PM   #1
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Default Prop unloads

As the prop unloads in the air does it draw more power or less? My motor is right at it max rating on the ground 55a and a little over 815 watts, (834w), so when it unloads in the air am I going to toast it? or am I safe?
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Old 02-24-2013, 10:03 PM   #2
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They do generally unload (meaning draw less) in flight. How much is unknown unless you have a good method of measuring in-flight.

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Old 02-24-2013, 10:05 PM   #3
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Mike is absolutely correct. However ... remember that you're right on the edge even so.

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Old 02-24-2013, 10:23 PM   #4
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Most motors have some 'fat' built into their ratings so providing you have decent cooling I'd not worry if you are on the stated limit. Also bear in mind that you probably measured watts and amps with a fresh battery, after a few seconds the power drops off as the battery loses it's initial surge of power, then it unloads again in flight.

Some of my motors are running well over their factory stated maximum power when measured static but I've never yet burned a motor and with good cooling they don't even get unduly hot. Efficiency does go down the pan a bit when you work the motor very hard but unless you are running flat out all the time that's not really an issue.

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Old 02-24-2013, 11:44 PM   #5
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Fly it for a bit, land it, see if the motor is cool enough to touch and the batteries aren't swollen.

I prefer my motor to be close to ambient temps after a wot hard run, but the plane moving through the air gets much more cooling then on the ground, so don't do a long run on the ground.
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Old 02-25-2013, 12:26 AM   #6
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Great, thanks guys.
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Old 02-25-2013, 01:25 AM   #7
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Originally Posted by soarrich View Post
As the prop unloads in the air does it draw more power or less? My motor is right at it max rating on the ground 55a and a little over 815 watts, (834w), so when it unloads in the air am I going to toast it? or am I safe?
Thanks
What motor is this? Are you saying 55A is the distributor's rated max. (typically optimistic) or is it a "real" number?
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Old 02-25-2013, 02:07 AM   #8
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http://hobbyking.com/hobbyking/store...dProduct=16231

The numbers I posted, 55amps, 834 watts where off my Watts meter with a 11x7 MasterAirscrew. I pulled 45amps with a 11x6 MA, and 43amps with Zinger 10x6 woody. I think the watts where in the low 700's. I didn't get rpm readings as I was under florescent lights in the garage.
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Old 02-25-2013, 02:27 AM   #9
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First time I tried doing an rpm check, I couldn't figure out why my motors all ran the same speed with different props, till my dad came in and shut off the lights. I felt dumb.
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Old 02-25-2013, 02:35 AM   #10
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Originally Posted by soarrich View Post
As the prop unloads in the air does it draw more power or less? My motor is right at it max rating on the ground 55a and a little over 815 watts, (834w), so when it unloads in the air am I going to toast it? or am I safe?
Thanks
Generally, these motors "Unload" once they are in the air. How much depends on a lot of factors, the main one being the propeller blade pitch, and the flying speed of your model.

I've got several Castle Creations ICE ESC's that record flight data that can be downloaded after a flight. On my Giant Scale Big Stick model, with a Hacker A50-16M motor running a 19X12 prop, that prop is slightly stalled at take off. (Yeah, you can hear it) The ICE download shows that the motor current actually increases by about 10% during the take off. But, once the model is at flight speed at full power, that same current drops off to about 70% of maximum or so.

It's interesting, backing off on the throttle on these electric models can result in your motor current dropping to 50% or more, even at perhaps 70% of full throttle. That giant scale Big Stick model will actually fly quite well at a motor power input of about 500 Watts. It pulls near 3000 Watts at full power, and that motor will haul that 16 pound model straight up, out of sight.

It's interesting, with the same throttle setting, the current pulled by the motor can vary widely, depending if your model is climbing, or descending. Even a decent angle of perhaps 30 degrees, can cut the motor current in half, or more.

What to watch for, is the current pulled while climbing straight up. Or worse, the current pulled when you're doing 3D and making a helicopter out your model. Maximum current, and zero forward speed is worst case for motor/ESC/battery cooling.

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Old 02-25-2013, 04:31 AM   #11
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DennyV
Thanks that's good info. No 3d here, it's going in a doped Kaos 40, little tiny ailerons. I haven't flown a Kaos in 30 years so this is going to be fun. We used to fly Kaos 40's with a 10x6 so this may be over-powering it. After the maiden I may switch to the 10x6 to see if it flies the way I remember, and get the bonus of longer flights too.
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Old 02-25-2013, 05:48 AM   #12
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Originally Posted by soarrich View Post
DennyV
Thanks that's good info. No 3d here, it's going in a doped Kaos 40, little tiny ailerons. I haven't flown a Kaos in 30 years so this is going to be fun. We used to fly Kaos 40's with a 10x6 so this may be over-powering it. After the maiden I may switch to the 10x6 to see if it flies the way I remember, and get the bonus of longer flights too.
Yeah
If your power system is running perhaps 130-150 watts per pound of airplane, that model is going to be a very good flyer, with an appropriate motor/ESC/Battery and matching prop.

It's been my experience that a well designed electric conversion of a glow type model can equal or outperform that same model with glow power. Of course the glow engine can fly longer on a tank of fuel, but you get that back when cleaning the glow engine powered model before putting it back in your vehicle!

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Old 02-25-2013, 05:51 AM   #13
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Originally Posted by hayofstacks View Post
First time I tried doing an rpm check, I couldn't figure out why my motors all ran the same speed with different props, till my dad came in and shut off the lights. I felt dumb.

LOLOL
Yeah been there done that. I've been using a high powered 3 watt LED flashlite running on two "AA" batteries. That thing pulls some two Amps when running with a pair of AA Nih cells. Works very well for a light source on a tach.

But, now with my models that are running the Castle Creations ICE series of ESC's, the ICE data recording feature records all of the important stuff during a flight, allowing it to be downloaded to graphs after landing.

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Old 02-25-2013, 01:17 PM   #14
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Originally Posted by soarrich View Post
http://hobbyking.com/hobbyking/store...dProduct=16231

The numbers I posted, 55amps, 834 watts where off my Watts meter with a 11x7 MasterAirscrew. I pulled 45amps with a 11x6 MA, and 43amps with Zinger 10x6 woody. I think the watts where in the low 700's. I didn't get rpm readings as I was under florescent lights in the garage.
Couple of things to help you out. First HK VASTLY overrates their motors (for the most part). A good rule of thumb is to take the weight of the motor in grams and times that by 3. that gives you a good safe margin for operation.

So that 171g motor is really good for about 513watts. The good news is that motor is still enough for the Kaos if it is 5lbs or so ready to fly.

I would use a 11x7 or 11x8 or so prop on 4s. A 10x10 might be worth a try too.

Second - get rid of your old glow props by MAS and Zinger. Sadly they are eclipsed by MUCH more efficient offerings. So try the APCe propellers. You will get more thrust for less energy.

Mike
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Old 02-26-2013, 02:36 AM   #15
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A few years back I did some testing on "unloading" Admittedly, it was a small sampling but the results were interesting - to me at least

On the few motors/props/models I tested, virtually all of the "unloading" was due to the battery voltage drop during the flight. The largest voltage drop is during the initial few seconds of the flight, so most any setup will show this type of "unloading".

However, if you made allowances for the voltage drop, there wasnt all that much drop in power once in the air. In fact, on the models I tested, the power used in a full throttle climb in the air was virtually the same as the static power on the ground and in some cases it was higher.

When I substituted over sized/over rated battery packs that DIDNT drop much voltage under the applied loads - I saw very little "unloading" unless you were in a dive. In fact, in some cases the load when climbing was higher than static.

So, at least with the limited number of setups I tested, props do not really "unload" much, if at all, in the air.

BUT - the power draw does drop as you are flying - most of the time.

So, the myth that props "unload" in the air is BUSTED - but it really doesnt matter because you ARE drawing less power as the flight progresses. Its just not for the reason thats commonly put forward

What does that all mean to most flyers? It means your setup probably draws less power in the air than on the ground - probably

On the other hand, if you have a largish pack thats more than capable of handling the full throttle loads AND if your pushing your motor and/or esc, then you could still be in trouble.

I think I need a signature.
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Old 02-26-2013, 02:54 AM   #16
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Yep suspect that is right - the pretty quick decrease in voltage is likely what most see as unloading.

Good point.

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Old 02-26-2013, 05:45 AM   #17
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Originally Posted by Larry3215 View Post
A few years back I did some testing on "unloading" Admittedly, it was a small sampling but the results were interesting - to me at least

On the few motors/props/models I tested, virtually all of the "unloading" was due to the battery voltage drop during the flight. The largest voltage drop is during the initial few seconds of the flight, so most any setup will show this type of "unloading".

However, if you made allowances for the voltage drop, there wasnt all that much drop in power once in the air. In fact, on the models I tested, the power used in a full throttle climb in the air was virtually the same as the static power on the ground and in some cases it was higher.

When I substituted over sized/over rated battery packs that DIDNT drop much voltage under the applied loads - I saw very little "unloading" unless you were in a dive. In fact, in some cases the load when climbing was higher than static.

So, at least with the limited number of setups I tested, props do not really "unload" much, if at all, in the air.

BUT - the power draw does drop as you are flying - most of the time.

So, the myth that props "unload" in the air is BUSTED - but it really doesnt matter because you ARE drawing less power as the flight progresses. Its just not for the reason thats commonly put forward

What does that all mean to most flyers? It means your setup probably draws less power in the air than on the ground - probably

On the other hand, if you have a largish pack thats more than capable of handling the full throttle loads AND if your pushing your motor and/or esc, then you could still be in trouble.
H'mmm
Doesn't agree with what I've found on my various models equipped with the Castle Creations ICE series of ESC's.

Methinks a lot of the "unloading" effect can be wiped out if you have a high pitch high RPM small diameter prop, as compared to the flying speed of your model.

My Giant Big Stick model as an example uses a 19X12 APC-E prop. Castle Creations shows this prop turns at about 6800 RPM on the ground, and increases to about 7300 RPM in flight under a gentle decent, all at full power.

The voltage of the 12S2P A123 battery pack is pretty flat during the flight, so the battery voltage does not enter into the results.

The CC ESC graphs for that Big Stick shows that the current hits 68 Amps at full power during take off, and holds at about 60 Amps on a vertical climb out. The same current pulled by the motor drops to about 37 Amps on a gentle descent again at full power.

I've found similar results in my other models that are also equipped with the CC ICE series of ESC's.

It's quite interesting, the motors with the very high efficiencies, such as the Hackers, show a very substantial drop in current when the RPM is allowed to increase slightly due to prop unloading. This is not the case with some of those motors that have lousy efficiency.

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Old 02-26-2013, 06:30 AM   #18
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Originally Posted by kyleservicetech View Post
H'mmm
Doesn't agree with what I've found on my various models equipped with the Castle Creations ICE series of ESC's.
Nor mine. When i put the data logger on my models the power draw shows clear rises and falls inversely proportional to how fast the plane flies. For instance if I pull out from a hover at WOT (effectively the plane is static) the are very much higher than if the model is in fast forward flight.

This agrees with theory because the faster inflow into the prop disk effectivly reduces pitch, which reduces load on the prop. The likes of Motocalc predict this 'unloading' with increased airspeed.
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Old 02-26-2013, 07:49 AM   #19
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Steve and Denny - dont forget that the power varies with the CUBE of the RPM and RPM is directly proportional to voltage. That means that even very small changes in voltage result in significant changes in power.

Secondly - sure there is less load when in a descent - gravity is helping you out. The motor doesnt have to do as much work - and thats one of the problems I had doing the testing. See the next note....

Thirdly - its a real bear trying to do this kind of testing because its very difficult to know if your actually flying straight and level or in a slight descent or in a climb or going through some turbulence etc. Then, how do you go back to your graph - maybe hours later - and be certain you are looking at the section of data that corresponds exactly to that particular flight mode.

Then throw in things like motor heating - which lowers efficiency and battery heating - which, up to a point, increases output voltage all on its own.

I tried analyzing many flights with EagleTree, Oracle and ICE data logs.

Its not at all easy to correlate - with certainty - what the model was doing during any particular moment on a graph other than the first few seconds and the last few seconds of any flight.

I tried doing the flights in 10 second intervals: ten second initial climb out, then 10 seconds of level flight, then 10 seconds of climb, then 10 seconds of descent, etc. Try doing that type of flight, at full throttle every second, and maintain perfect up lines, down lines and perfect straight and level and perfectly level turns. Even top pattern flyers have some trouble doing that sort of flying

If you do that one time, it looks like maybe you might have some data worth looking at - until you try repeating the experiment and get very different readings. Maybe the next pack was warmer or newer (lower Ir) or older, the motor was hotter, the air warmed up, the wind came up from a different direction, you turned later, climbed at a steaper or shallower angle, etc etc.

There are too many factors influencing the load on the motor at any given moment of a typical flight and I didnt have a wind tunnel handy

I finally settled on a much simpler test. Start with a fresh pack and fly it around for 1 minute or so to take the fresh charge off and get it down to a flatter place on the voltage curve. Then do a 10 second full throttle run up static on the runway, release the plane and maintain full throttle while doing a climb out at the shallowest angle possible and level out as low over the runway as you can. Then hold the plane there as it accelerates.

I figured that would give me some reasonably consistent and repeatable data to look at - and it seemed to work ok.

You would expect the power usage to drop as the model accelerates down the runway - and it does - as you've both noted it should.

However, how much of that power drop is due to the battery voltage dropping as the pack drains and how much is due to aerodynamic unloading of the prop?

In order to determine that, I took a close look at the voltage drop while the model was static and again at different points while it was accelerating.

The measured voltage drop will tell you how much the RPM would have dropped which in turn, will tell you how much the power would have to drop as a result.

Then you can compare that power drop to the actual measured drop in power as the model accelerates.

Any drop in power that was greater than the expected power drop due to the voltage decreasing as the battery drained would then have to be due to aerodynamic unloading.

Note that Im ignoring changes that could be caused by motor/battery heating, motor efficiency curve, esc issues etc etc.

All I can say is that my test data indicated that most of the drop in power could be accounted for by the voltage drop alone. I forget the exact numbers but IIRC it was 80% or more.

Of course, I had no good way to make allowances for pack heating, motor heating, bumpy flying, motor efficiency changes with rpm. etc etc.

You also have to assume that the formula are accurate at our scales and with the particular motor/prop/model combinations I was using. Also, I only did this with two or maybe three different models at the most and over a dozen or so flights. Thats not a huge base to draw conclusions from.

Also, all of the models I used would have had props with about a 2/1 diameter/pitch ratio and setups propped to load the motors and packs pretty hard at full throttle. They were also fairly draggy airframes.

Still, those are the results I came up with

I think I need a signature.
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Old 02-27-2013, 12:41 AM   #20
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Originally Posted by Larry3215 View Post
Also, all of the models I used would have had props with about a 2/1 diameter/pitch ratio and setups propped to load the motors and packs pretty hard at full throttle. They were also fairly draggy airframes.

Still, those are the results I came up with
Yeah, that same power relationship to the RPM^3 power also applies to the drag of the model vs airspeed. And, methinks a draggy airframe would even things out a little bit.

The models I used were fairly fast models, with high pitch, comparatively low RPM on the propellers used. My tests were to run the motor at full throttle for one minute, and after reaching about 200 feet or so, try to hold that same altitude, even during the required turns at each end of the field. Doing the same tests with a motor with poor efficiency, using a small diameter prop turning at real high RPM's would likely have very different results. As you've indicated, the LiPo batteries also have a notable effect on everything. My A123 cells are pretty much flat in their discharge curve, varying only a few percent in voltage from 90% to 20% charge or so.

As we've all indicated, the prop unloading does exist, but even a slight increase in climb out, or descent can completely change the propeller loading. That is easy to note, as you can hear the motor RPM change with different pitch angles of your model. Even a 10% change in motor RPM on a high efficiency motor like my Hacker A60-16M motor will result in a very significant change in the motor amps and watts.

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Old 02-27-2013, 07:41 AM   #21
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Larry,

To be honest you dont even need a logger to identify unloading, you can very clearly hear it. It's quite easy to hear the increased RPM when the model is moving at speed compared to say, climbing steeply or accelerating from rest.

Assuming reasonably constant volts then Higher RPM on an e motor always means less amps (due to back EMF generated in windings). It's also easy to correlate the relationship between RPM and amps on a data log.
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Old 02-27-2013, 09:53 AM   #22
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Steve, I think you've misunderstood my point. Im not disputing anything you said. Im also not talking about what happens in a climb or dive.

When most people talk about "unloading in the air" they are generally comparing the full throttle static power draw vrs level flight full throttle power draw or as close as you can get to that.

Most people ask about unloading because they are over propped when static and want to be sure they will OK once they get into the air and try to do some full throttle speed runs.

Since you cant climb or dive for the whole flight, you have to assume level or some average in the range of level flight.

I wasnt questioning the existence of unloading in the air or that the load changes when you climb or dive. As you say, thats obvious.

I wanted to know how much of the unloading - in straight and level flight - was due purely to moving through the air and how much was due to voltage drop.

In the tests I did, the majority of the "unloading" seemed to be due to voltage drop.

I'll have to see if Ive still got any of the logger files. If so, I'll post a log or two and you can check my math. Its not unheard of for me to screw up

If your curious enough, try it yourself Id love to see what you come up with.

Hold a model at full throttle, static, on the runway or in your hand for a few seconds so you have a good full throttle static reference point on the log later. You wont have a straight line on the log. The voltage, rpm and current will all be dropping during your static run up.

Then release the model (I like to blip the throttle just before release so you can be sure when that point was exactly) and fly it at full throttle for several seconds - trying to keep it as straight and level as you can. That should give you some data points as the model accelerates and again once it's reached full speed.

Then look at the logs, run the numbers and see what you come up with

Unless your a much better pilot than me or have a very stable, trimmed out model to test with, your "straight and level" graph will be varying a lot. Check some hi's and lows or average it out and let me know.

I would expect to see a wide range of results depending on the drag characteristics of the model, the motor, esc and battery quality and condition and also how highly loaded the setup is and what prop your using and probably a host of other details.

I wasn't able to get anyone else to do the same tests before. No one seemed to care enough

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Old 02-27-2013, 10:10 AM   #23
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I forgot to list the biggest variable - where is your static reference point on the battery discharge curve compared to the in-flight point on the curve?

Most people do static run ups on fully charged packs and pick the number thats on the watt meter after about 10 seconds of running. Even waiting 30 seconds is going to give you a static pack voltage (and resulting power) thats well above the average voltage a minute or two latter in the flight.

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Old 02-27-2013, 10:11 AM   #24
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Steve - Out of curiosity, how much unloading would there be on a full scale prop? Say a Cessna or some other typical propped aircraft?

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Old 02-27-2013, 12:49 PM   #25
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Originally Posted by Larry3215 View Post
Steve - Out of curiosity, how much unloading would there be on a full scale prop? Say a Cessna or some other typical propped aircraft?
I'm not really sure. i.c engines have very different charecteristics to electric power. Generally on i.c. when holding open throttle the power of the engine increases as the RPM rises. This is opposite on electric motors where (input) watts reduces (eventually to zero) when the motor is allowed to rev out and is at a maximum when the motor is stopped dead.
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