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Do propellers really "unload" in flight?

Old 09-18-2008, 08:11 PM
  #26  
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Originally Posted by bz1mcr View Post
I know of a plane that was radar measured in level flight at a speed of 107 MPH, significantly above it's static pitch speed. This started a lot of controversy and resulted in many repeat measurments of both the static rpm and measured flight speed. The data repeated over and over. One parameter that could not be measured precisely is the pitch of the prop. Some props like the GWS 7X3.5 are known to not be what they are labled. The prop used on this plane was an Aeronaut 6X5 and as far as we could tell it was about a 5" pitch, but a difference of 10-20% would not be easy to measure.
the controversy ended when doppler sound measurements confirmed the radar speed measurements and provided an in flight RPM measurement that was in fact significantly higher than the static RPM.

Bottom line: The measured speed was well above the static pitch speed, but less than the in flight pitch speed.
This is a good example then of a prop unloading. If the in flight rpm is higher then there is definately unloading going on.

My question is still how much of that unloading is due to voltage drop and how much is aerodynamic?

Originally Posted by Old Fart View Post
Here's two graphs from my Brio - no "throttle monitoring, mut I can tell you the first drop is from unloading - I don't chop the throttle back that quick.
Yes, the power is dropping in your graph - but again how much of that drop is due to the battery voltage dropping? We cant tell because the voltage graph is too small to read.

It does look to have some serious voltage drop though. The average voltage is 10.78 and the minimum is 9.7. Thats almost a full volt difference. That would represent a drop in power of close to 33%. Thats a big drop in power. That 33% drop in power works out to about 150 watts on that frist run. The total power drop was only 166 watts.

My question is - the voltage drop looks to account for the vast majority of that power drop - not the prop unloading.

Still - there is not enough data to be sure what the relative percentage is because we cant read the exact voltages from that graph - rpm data woudl sure be nice as well.

Originally Posted by bz1mcr View Post
Larry,
I think you are making it more complicated than necessary.

If the RPM in flight ever exceeds the static WOT RPM Then it must be because the prop unloaded. (regardless of throttle position or aircraft attitude).

You correctly point out that it is possible for in flight Voltage to be greater than static voltage because the batteries performance can improve early on as they warm up. As long as your battery is above 70 degees F during the static run, I am pretty certian that you can forget about it. But to see for sure you would need to monitor V and current. If at any current level the V is higher in flight it than it was at the same current during the static run it has to be because of the temperature effect.

Don
Yeah I have to agree on that - but thats part of my point as well - we dont have all the data we need. In these cases we dont have any rpm data so far.

All we have is power to go on and thats not enough.

I think its clear that the there is some sort of unloading going on - Im still curious as to how much is caused by exactly what
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Old 09-18-2008, 08:14 PM
  #27  
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I should be more clear here I guess - in my mind as well.

I think from Dons example its clear the prop was turning at a higher rpm. So there was definately some unloading going on - or was there? Did the battery voltage increase from internal heating or was it all aerodynamic?

In the other cases so far all we have is the power reading to go on and thats not enough.

One of the commonly held beliefs is that the power drop is caused by unloading. Thsts more my disagreement than that the rpm is increasing.

But we still need all the data at the same time to tell for sure.
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Old 09-18-2008, 11:27 PM
  #28  
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I agree you really need RPM to see if it is unloading for sure. AND, without RPM you need very precise voltage and current, not just power.
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Old 09-19-2008, 12:45 AM
  #29  
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Hello,

Interesting discussion for sure. If battery performance improvement as it warms up is a concern, the static test could be performed again right after the high speed pass. Does the rpm go back down by the same amount or not quite? that could give a very rough indication of the effect a warm battery has during the testing.
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Old 09-19-2008, 01:03 AM
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I would say yes the prop unloads in flight, HP is HP, if its electric or gas, I do know that I have to set my nitro eng, 2 clicks rich, before a flight, because the prop will unload in flight and the eng. will lean out, if i dont do that, the eng will become to lean and get hot, I am talking about HP here electric or gas, its all HP, Just my 2 cents worth, Take care, Chellie
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Old 09-19-2008, 02:37 AM
  #31  
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Yes, but with a glow motor how much of "leaner running" once your flight begins
is due to increased airspeed and forced draft of air into the motor... rather than unloading

Yes, my Oliver Tiger 2.5cc diesel needs to be a bit rich to start with, but, as you say, it leans out....... but, is that unloading or is it something else?
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Old 09-19-2008, 02:57 AM
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Originally Posted by Dr Kiwi View Post
Yes, but with a glow motor how much of "leaner running" once your flight begins
is due to increased airspeed and forced draft of air into the motor... rather than unloading

Yes, my Oliver Tiger 2.5cc diesel needs to be a bit rich to start with, but, as you say, it leans out....... but, is that unloading or is it something else?

Thats a Good point Dr Kiwi
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Old 09-19-2008, 03:36 AM
  #33  
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I found my phase reading rpm sensor for my Eagle Tree so I'll be able to get some pretty good RPM numbers along with voltage etc on my flights.

The major difference between my flights this weekend and Trams is the prop ratios. Ill be running much slower setups with much flatter props.

The conventional belief is that they behave quite differently than the square or hi pitched props.

I also have a Hot Liner I might be able to fit the Eagle Tree into that runs an almost square prop - 14x12. Its on the bench for some repairs though.
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Old 09-19-2008, 03:47 AM
  #34  
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Originally Posted by drh View Post
Hello,

Interesting discussion for sure. If battery performance improvement as it warms up is a concern, the static test could be performed again right after the high speed pass. Does the rpm go back down by the same amount or not quite? that could give a very rough indication of the effect a warm battery has during the testing.
Not a bad idea.

What I had planned to do was note the voltage and rpm data at several points on the flight and compare the rpm/volt numbers.

If the prop is unloading then the rpm per volt should increase. That should actually be a better number to look at than just the rpm I think.

The greater the unloading the closer the prop will come to the theoretical max rpm/volt numbers dictated by the kV of the motor.
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Old 09-19-2008, 02:52 PM
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The greater the unloading the closer the prop will come to the theoretical max rpm/volt numbers dictated by the kV of the motor.

Exactly!
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Old 09-19-2008, 04:22 PM
  #36  
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Originally Posted by Dr Kiwi View Post
The greater the unloading the closer the prop will come to the theoretical max rpm/volt numbers dictated by the kV of the motor.

Exactly!
I agree with your statement....If the aircraft is a low drag....sleek model this "unloading" efect will be more dramatic since the aircraft speed will nearly aproach the prop blast speed. With bulky....slow flyers....trainers...etc., as you can see the "unloading" is less dramatic.....in fact the motor will labor to pull a high-drag....bulky aircraft through the air. These are the aircraft that would yield better performance and longer flight times with a lower pitch prop.

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Old 09-19-2008, 05:07 PM
  #37  
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Originally Posted by Flash1940 View Post
I agree with your statement....If the aircraft is a low drag....sleek model this "unloading" efect will be more dramatic since the aircraft speed will nearly aproach the prop blast speed. With bulky....slow flyers....trainers...etc., as you can see the "unloading" is less dramatic.....in fact the motor will labor to pull a high-drag....bulky aircraft through the air. These are the aircraft that would yield better performance and longer flight times with a lower pitch prop.

Flash

Thats what I suspect as well - but I dont think anyone has shown that conclusively - yet

Thats also why I think it important to do these tests on different models with different pitch ranges and different planes.

I suspect you're correct and we may well find some large differences in the results.
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Old 09-19-2008, 06:32 PM
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Larry-

I've performed the test you laid out..

1)Plug in the pack and do a full power static run up for 5-10 seconds then chop the throttle.
Can be seen in the graph from around .25 to .60 minutes is the static run.. I did 20 seconds worth..


2)Take off and climb to altitude at a lower throttle setting and get lined up for a speed run.
2.75min indicates the actual launch.. Everything between is a botched launch, I was trying to keep the throttle low and the VecJet needs some push on the launch..

3)allow the plane to slow down as much as possible without stalling then briefly chop the throttle then go imediately to a full power straight and level run for say 10-20 seconds. At the end chop the throttle again briefly.
3.75min is the chop then back to full.. Followed by a level run at 3.75 to 4.0min..

4)get lined up again for a speed run then chop the throttle and go back to full. Fly level for 5 seconds, then pull to a full power vertical for as long as possible. Chop the throttle again.
This is exhibited at 4.25 to 4.75.. Looks like we may have reversed the time of the runs in part 3 and part four..

This is the vertical climb, as exhibited by the drop in speed.. This thing gets high, quick.. Also gets very small..


5) stay up hi as and in fact climb as far as you dare - then chop throttle again and go back full. Now start a full speed level run for say 5 seconds and then go into a full power dive and hold it as long as you can then chop throttle.
[/quote]

Throttle is off from 4.75 until ~4.85 and then the full throttle, vertical dive is commenced.. Recording a 136mph dive..

From just shy of 5.5 to the end of the recording is me just out and playing around with the plane.. Had juice in the batts, not gonna waste a flight..

Here is the graph..

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Old 09-19-2008, 08:44 PM
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Interesting thread.

Airplane propellers have been studied for over 100 years now. Propeller theory is now quite extensive and sophisticated. The little propellers behave just like the big ones, too. There's no shortage of books out there that cover their behavior in detail. Let me know if you want a recommendation on a full-size airplane propeller book.

There's a couple of books out there that deal specifically with model airplane propellers:

Prop Talk
by Donald Brooks

Model Airplane Propellers: Selection and Performance

by Howard Chevalier

They are both good books.
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Old 09-19-2008, 10:19 PM
  #40  
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Originally Posted by Tram View Post
Larry-

I've performed the test you laid out..


Here is the graph..

[/quote]

Excelent!

Can you attach the data file here or send it to me again? Id like to be able to zoom in and get exact numbers at specific times.

Thanks!
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Old 09-19-2008, 10:40 PM
  #41  
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Originally Posted by Larry3215 View Post
Can you attach the data file here or send it to me again? Id like to be able to zoom in and get exact numbers at specific times.
I'll e-mail it.. It won't let me upload it..
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Old 09-19-2008, 10:40 PM
  #42  
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Originally Posted by creyes123 View Post
Interesting thread.

Airplane propellers have been studied for over 100 years now. Propeller theory is now quite extensive and sophisticated. The little propellers behave just like the big ones, too. There's no shortage of books out there that cover their behavior in detail. Let me know if you want a recommendation on a full-size airplane propeller book.

There's a couple of books out there that deal specifically with model airplane propellers:

Prop Talk
by Donald Brooks

Model Airplane Propellers: Selection and Performance

by Howard Chevalier

They are both good books.
If you have copies of both you might see if there is any reference to our question.

I used to have a copy of Howards book and I dont recall anything specific to this question. IIRC it came out 9 or 10 years ago? Well before there were hi powered electrics or lipos on the market.

IIRC, wasn't it Howard that came up with the formulas that most of the prop thrust calculators use? I tink he was the one who posted an artical in MA way back with the formula. There was some controversy because he claimed the pitch had no bearing on thrust figures and left it out of the formula comletely.

Im specifically and mostly concerned with whats causing the power drop off in the air compared to static - is it the unloading or is it mostly battery voltage drop - or some combination.

Secondly - are there some prop diameter/pitch ratios that act differently than others?
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Old 09-20-2008, 12:05 AM
  #43  
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This a great Thread Larry I gave you a 5 Star Rating
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Old 09-20-2008, 01:15 AM
  #44  
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Howard Chevalier had it right in his book. Propeller thrust formulas are not supposed to have a separate propeller pitch term. You see, the thrust coefficient parameter is supposed to take into account the pitch. Howard's book clearly explains this. In an attempt to simplify the computations for model airplanes, Boucher and others broke out the pitch angle out of the propeller thrust coefficient. I hadn't heard of this controversy before, but it sure sounds like the other folks didn't do their homework.

Most of the propeller formulas that I use in my calculator were already old fifty years ago. They are similar to what Howard uses, but I didn't get them from his book. They predate Chevalier's book by a couple of years, at least.

I just added a Thrust/Power Required vs Airspeed graph to my calculator (released about half an hour ago). I compared APC Sport 10x6 and 10x9 propellers. Screen shot attached. Note: RPM is being held constant at 6,300. The darker lines correspond to the 10x9 prop.

Both propellers actually unload right after the start of the take-off roll, the 10x9 propeller much more. But then it sure looks like power required is increasing, doesn't it? As a sanity check, I cracked open the big heavy aerodynamics books. Nothing wrong with my calculator.

There's more I can say about this, but I have a dinner date. Later!

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Originally Posted by Larry3215 View Post
If you have copies of both you might see if there is any reference to our question.

I used to have a copy of Howards book and I dont recall anything specific to this question. IIRC it came out 9 or 10 years ago? Well before there were hi powered electrics or lipos on the market.

IIRC, wasn't it Howard that came up with the formulas that most of the prop thrust calculators use? I tink he was the one who posted an artical in MA way back with the formula. There was some controversy because he claimed the pitch had no bearing on thrust figures and left it out of the formula comletely.

Im specifically and mostly concerned with whats causing the power drop off in the air compared to static - is it the unloading or is it mostly battery voltage drop - or some combination.

Secondly - are there some prop diameter/pitch ratios that act differently than others?
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Old 09-20-2008, 01:30 AM
  #45  
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Sure some props unload and as stated it deopends on the motor,prop, aircraft and flying style.

I can hear the prop loading ./ unloading on mt GP Electro Stik. Scorpion 3020-12, 5S A123 or 4S LiPoly,APC 11X7E prop. At approx. 60% throttle 600 watts there is a noticeable audioable difference between climbing,diving or level flight.

I have flown other models (mostly 3D type) at a constant throttle for 30 sec. at a time and recorded the flights. You can see amps. increasing and RPMs decreasing in a climb aand the opposite in a dive.


Charles
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Old 09-20-2008, 07:08 AM
  #46  
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Originally Posted by creyes123 View Post
Howard Chevalier had it right in his book. Propeller thrust formulas are not supposed to have a separate propeller pitch term. You see, the thrust coefficient parameter is supposed to take into account the pitch. Howard's book clearly explains this. In an attempt to simplify the computations for model airplanes, Boucher and others broke out the pitch angle out of the propeller thrust coefficient. I hadn't heard of this controversy before, but it sure sounds like the other folks didn't do their homework.

Most of the propeller formulas that I use in my calculator were already old fifty years ago. They are similar to what Howard uses, but I didn't get them from his book. They predate Chevalier's book by a couple of years, at least.

I just added a Thrust/Power Required vs Airspeed graph to my calculator (released about half an hour ago). I compared APC Sport 10x6 and 10x9 propellers. Screen shot attached. Note: RPM is being held constant at 6,300. The darker lines correspond to the 10x9 prop.

Both propellers actually unload right after the start of the take-off roll, the 10x9 propeller much more. But then it sure looks like power required is increasing, doesn't it? As a sanity check, I cracked open the big heavy aerodynamics books. Nothing wrong with my calculator.

There's more I can say about this, but I have a dinner date. Later!

Carlos Reyes
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Before I forget - great job on your web site! Im impressed!

The key points in the debate over pitch values effecting thrust centered on these points:

1) A prop with zero pitch will produce zero thrust. A prop with infinite pitch will produce zero thrust.
2) Measured static thrust values DO increase (and then decrease) as pitch increases.

So the theory didnt seem to match real world observations very well. Its obvious that pitch does effect thrust values.

I dont really want to get off into that debate again though
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Old 09-20-2008, 07:21 AM
  #47  
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Originally Posted by everydayflyer View Post
Sure some props unload and as stated it deopends on the motor,prop, aircraft and flying style.

I can hear the prop loading ./ unloading on mt GP Electro Stik. Scorpion 3020-12, 5S A123 or 4S LiPoly,APC 11X7E prop. At approx. 60% throttle 600 watts there is a noticeable audioable difference between climbing,diving or level flight.

I have flown other models (mostly 3D type) at a constant throttle for 30 sec. at a time and recorded the flights. You can see amps. increasing and RPMs decreasing in a climb aand the opposite in a dive.


Charles
Still have any of those old graphs or data files laying around Charles? Thats what Id like to see.

Im not really saying that props dont do some unloading - especially in a dive situation. Although Id still like to see some actual data

Im more focused on power side of it.

How many times have we heard some one say something like this:

"Im pulling 60 amps static and I know my motor, packs and esc are only rated for 45 amps - BUT it will unload in the air so I'll be fine."

Then they show an Eagle Tree graph where the power drops off dramatically during the first few seconds of the flight and say - "See - that proves it".

On the one hand - who really cares WHY the power drop occurs. If the power goes down to a safe enough level once he's in the air - then he IS fine.

Im still curious about the real why. I believe that power drop is largely due to voltage sag rather than unloading.

Dont know if I'll get to do my tests this weekend or not. Weather may not cooperate
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Old 09-20-2008, 10:02 AM
  #48  
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Can't wait to see those results.

I challenged the unloading idea a while back over at RCG. I found that with a power to weight ratio of 1:1 or greater, my results suggest that there is some unloading in flight vs. static tests. If I remember correctly, this was about 20%. In my case it was an Alfa P-47 with an Esskay 400XT...I'll have to see if I can dig up that data. I AM NOT SAYING THIS IS TRUE FOR ALL SET-UPS!

There are so many variables involved in trying to prove the theory of "prop unloading" including:
1) The airframe itself...a Mustang vs. a Christen Eagle vs. a fully rigged WWI Sopwith Camel
2) The prop...APC TE vs. APC SF vs.GWS SF...
3) The power to weight ratio
4) Batteries...yes they do sag...but they all do so differently...at different times
5) Atmospheric conditions....Heat, Humidity and Wind...all could have an affect on the results
6) I am sure that I could think of more, but I will stop now
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Old 09-20-2008, 05:26 PM
  #49  
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Originally Posted by Larry3215 View Post
Before I forget - great job on your web site! Im impressed!
Thanks. It's been a lot of work.

Chevalier's book talks about propeller unloading in section 4.5. He says that the propeller unloads when the airplane starts moving because the angle of attack (AOA) of the blades is decreased. The point that he makes is that when the propeller unloads, the RPM increases, which in turn increases the AOA and the power required. I don't disagree with his comments.

Propellers are rotating (twisted) wings. Instead of using propeller pitch, a much more meaningful measurement to talk about would be propeller blade angle. Too bad that's not the convention!

If you think of a propeller as a rotating airfoil and look at the AOA that it makes to the oncoming air, it suddenly becomes a lot easier to make sense of the behavior. I compute the AOA internally, but don't expose it in the interface.

<editorial>Editorial: I think propellers are absolutely fascinating. They're the simplest power system component - just a chunk of maple, in many cases. But they are the most complex to understand (for various reasons). Chances are that you are using a suboptimal propeller in your model. It is also very frustrating for me that so few manufacturers publish performance test data.
</editorial>

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Old 09-20-2008, 06:18 PM
  #50  
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I've had setups on the bench with lower power nimh batts, that would not even hit half throttle before going into LVC. In the air there was no problem running at full. I always assumed that was due to the effect of unloading
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Quick Reply: Do propellers really "unload" in flight?


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