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Do propellers really "unload" in flight?

Old 09-20-2008, 08:32 PM
  #51  
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Originally Posted by creyes123 View Post
....
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!...</EDITORIAL>

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Problem is the "blade angle" of a prop changes from root to tip. Because the tip travels much faster than the root section it needs a much shallower blade angle. If the blade angle divided by the distance from the shaft center is a constant then the blade has constant pitch root to tip. But constant pitch props are not necessarily the best so prop manufactures are free to vary the pitch along the blade any way they like.

Last edited by bz1mcr; 09-20-2008 at 09:33 PM. Reason: remove referance to angle of attack. It may have been wrong.
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Old 09-20-2008, 08:44 PM
  #52  
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It's standard practice to measure the propeller blade angle (or pitch) at a specific position, such as 70% or 75% of the radius.

You are getting your terminology mixed up. I think you meant to say that since the propeller tip travels much faster, it needs a lower blade angle so that it has the same angle of attack as the rest of the blade.

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Originally Posted by bz1mcr View Post
Problem is the "blade angle" of a prop changes from root to tip. Because the tip travels much faster than the root section it needs a much shallower angle of attack. If the blade angle divided by the distance from the shaft center is a constant then the blade has constant pitch root to tip. But constant pitch props are not necessarily most desireable so prop manufactures are free to vary the pitch along the blade any way they like.
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Old 09-20-2008, 09:31 PM
  #53  
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Seems we agree the blade angle is shallower near the tip.
"Standard Practices" in my field were normally documented by the Society of Automotive Engineers. Is the propeller "standard practice" documented some place?

I don't know enough about air propeller design to know if the angle of attack is normally lower at the tip or not. I would have thought so, but I do not know. I will revise the earlier post to eliminate the referance to angle of attack.
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Old 09-20-2008, 10:08 PM
  #54  
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Good question. There can only be one answer - the FAA. They regulate everything related to full-size aircraft in the United States. A quick search in the FAA website revealed this:

<style type="text/css"> </style>
Pitch. The propeller blade angle, measured in a manner and at a radius declared by the manufacturer and specified in the appropriate propeller manual.
So it is up to the propeller manufacturer to say at what radius they are measuring the blade angle. Probably explains why I've come across different values, though 75% radius is by far the most commonly used.

In theory, you want each propeller blade station to have the same AOA. But since the airfoil varies along the length of the blade, I can see why a manufacturer might not want to do exactly that.

I can think of one exception: high speed propellers. Mach effects can be very significant, so it is to their advantage to have a lower AOA near the tip.

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Originally Posted by bz1mcr View Post
Seems we agree the blade angle is shallower near the tip.
"Standard Practices" in my field were normally documented by the Society of Automotive Engineers. Is the propeller "standard practice" documented some place?

I don't know enough about air propeller design to know if the angle of attack is normally lower at the tip or not. I would have thought so, but I do not know. I will revise the earlier post to eliminate the referance to angle of attack.
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Old 09-20-2008, 10:52 PM
  #55  
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Default Interesting discussion....some Medusa Oracle data

I actually got to this thread while doing a search on Howard Chevalier's propeller book so as to be able to reply to a query on RCG. BUT....

It just so happens that I've recently done some logging with my Medusa logger and so have access to the graphing ability of PowerProView which makes it easier to see some of this stuff than the EagleTree graphs do (due to the way the axes are set up/defined).

Anyway, this is about the first two minutes of a log taken on my Mountain Models EZ Scout earlier this week. I'd been taking data on a recent trip to New Mexico so I could compare the same power system at different altitudes - but that's for another discussion.

The EZ Scout, like most of my planes, is one of those "it unloads a little" types - not really slippery but not so draggy as to not unload at all.

In the graph the first spike in RPM/power (and commensurate sag in voltage) is a full-throttle run up on the ground before flight. You can see a number of places on the graph where the in-flight RPM is higher (red) while at the same time the voltage (blue) is lower than that initial run-up.

Oh - since I'm sure you'll ask: the power system in the EZ Scout as shown in this graph is a Suppo A2208-17 motor turning a GWS 9x5 DD prop. The ESC (not "escape"!!) is a Jeti Eco-12 and the battery is a Fusion Packs 3s 1200. I normally get between 20 and 25 minutes on one in this plane, flown the way I fly it.

I could create a few more graphs from other flights from this plane (four total) and I also have Oracle data for my LT-25 and a beta ARC 36 mm motor so I have some data there.

I also have a bunch of EagleTree data - you MAY be able to see the same effects in some of the graphs posted in my SA SHAFT review on RCG (http://www.rcgroups.com/forums/showthread.php?t=740836) which includes some with one of the CSRC motors (for you, Jeff ) but the voltage is hard to tease out because of the axis scale thing. Someday EagleTree needs to allow us to set up multiple axes like Medusa does.
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Old 09-21-2008, 01:31 AM
  #56  
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Excellent Bernard!

That RPM data very clearly shows that the prop - in your particular case - is unloading.

Im still curious about the power drop numbers.

If you get a chance, could you send me the raw data file? I'll PM you my e-mail.

I agree - I like the way Medusa handles the graphing much better than Eagle Tree. I wish I hadn't lost my Oracle!
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Old 09-21-2008, 01:41 AM
  #57  
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What kind of file extension does the Eagle Tree create???
If it's some sort of text file....maybe Notepad would read it....then you could import it into Excel....then you could make any graph you wish.
Does The Eagle Tree allow for selection of high resolution.....or a high number of acquisitions/sec. If so....set it as high as you can.....it will likely run out of memory....but the data would be superb!
With all these prop calculations and postulates I haven't seen where anyone has mentioned tip velocity (pardon if I missed it). That's the limit of RPM. Depends on the prop diameter....but the limit is 1100 ft/sec. at sea level. I still don't see how you can calculate or predict a theoretical thrust without involving pitch. Air density and all that stuff is part of the calculation. The more advanced calculations use Reynold's numbers. In my next post I'll link you to Don Stackhouse's site. They're doing a build of the Chrysalis. He talks about props there.....I think he would love to get in on this thread.

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Old 09-21-2008, 01:52 AM
  #58  
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Here's the link I promised.... http://tinyurl.com/3zykv8 .....Hope you find it interesting.

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Old 09-21-2008, 06:19 AM
  #59  
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Larry,

I can send more than one file. One from my trip to Santa Fe has the 9x7.5 prop on the same airplane (but flying at 7000+ feet MSL on an 85 degree afternoon). I'll just have to do it from the other machine where the files are (one of the few reasons I have to have a Pee Cee in the house that's not furnished by my employer).

Flash,

The EagleTree logger lets you go up to 10 samples per second. I usually run mine at eight (no, I'm not sure why). Both EagleTree and Medusa let you export files as CSV so one could play in Excel if one was inclined to do so.

I would LOVE to see Don Stackhouse weigh in on this thread....does he post here at all? He's occasionally posted on RCG, but it's been awhile.
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Old 09-21-2008, 06:21 AM
  #60  
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Great!

I sent you an e-mail with my address - I'll PM it as well.

Thanks!
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Old 09-21-2008, 06:56 AM
  #61  
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The files are in the process of being sent right now (of course this post slows it down a bit....) Thunderbird says 49% so far.

Oooops - it timed out. I may have to do it in two notes (four files).

Added: looks like it went on the second try. 9335 Kb.

Last edited by BEC; 09-21-2008 at 07:20 AM.
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Old 09-21-2008, 07:00 AM
  #62  
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Excelent! I'll have something to play with if it rains all day tomorrow
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Old 09-21-2008, 07:57 AM
  #63  
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Yup, got 4 files.

Thanks!
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Old 09-21-2008, 05:06 PM
  #64  
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Default Great Planes Elector Stick scorpion 3020-12 Eagle Tree V3

Flew GP Electro Stick ,Scorpion 3020-12,APC 11X7E, OUTRAGE XP25 4S 3700.

Very short blip at beginning where I went barely past 50% Throttle for takeoff. Came back to 50% flew around level for a bit then did some loops,climbs and dives without touching the throttle. Next I bumped throttle up tp approx. 65% and did the same thing.

Added second attachment of complete flight.


Charles
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Old 09-21-2008, 08:12 PM
  #65  
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Originally Posted by Tram View Post
I have to put myself firmly into the "things unload in the air" group of myth believers..
Quick! Someone call Mythbusters and suggest this myth to them!!

Of course, then there would be the inevitable Mythbusters-thread-war like we had with the "plane on a treadmill" myth. I don't know if stevecooper's heart can handle another discussion like that one.
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Old 09-21-2008, 11:09 PM
  #66  
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Originally Posted by BEC View Post
The files are in the process of being sent right now (of course this post slows it down a bit....) Thunderbird says 49% so far.

Oooops - it timed out. I may have to do it in two notes (four files).

Added: looks like it went on the second try. 9335 Kb.
Originally Posted by everydayflyer View Post
Flew GP Electro Stick ,Scorpion 3020-12,APC 11X7E, OUTRAGE XP25 4S 3700.

Very short blip at beginning where I went barely past 50% Throttle for takeoff. Came back to 50% flew around level for a bit then did some loops,climbs and dives without touching the throttle. Next I bumped throttle up tp approx. 65% and did the same thing.

Added second attachment of complete flight.


Charles
Originally Posted by gzsfrk View Post
Quick! Someone call Mythbusters and suggest this myth to them!!

Of course, then there would be the inevitable Mythbusters-thread-war like we had with the "plane on a treadmill" myth. I don't know if stevecooper's heart can handle another discussion like that one.
I dont think we need the Myth Busters to generate a controvercy in most on-line forums

Thanks to everyone who has posted files/graphs so far.

Its clear from the data that the props DO indeed unload in flight.

The inflight max RPMs are higher than the static and the peak power levels do go down.

However, its also clear that under certain conditions, inflight loads can increase.

There are places in Bernards and Charles and Trams graphs where the power level is INCREASING while the battery voltage is dropping. Its obvious that the load is increasing at some points and decreasing at others.

Whats much more dificult to tell from any of the data so far, is how much of the drop in power is due to the prop unloading and how much is due to the battery voltage dropping.

I can tell that climbs and dives are occuring and that they do indeed effect the load on the motor - but Im not always sure that full or constant throttle was being maintained - or if the flight was level or not etc.

Tram hasnt sent me his graph yet, but it looks like I may get to do a flight in a few minutes. I can log all the relevant data and I will know how I was flying at any gievm point so I can - maybe - better determine how its working on my setup.

I'll be flying a very slow floater with a relatively small power system thats designed for 3D type slow flying. In other words - the system is pushing the limits at full power and is designed to be flown at 1/4 - 1/2 stick most of the flight with only short power bursts.

Thats how many systems are set up so it should be of some interest.
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Old 09-22-2008, 12:25 AM
  #67  
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The flights for which I sent you data were seldom at full power for long and almost never in level cruising-type flight. Places where the power is higher than static are probably vertical climbs. There will also be, on occasion, power spikes due to prop strikes on a touch and go that didn't go smoothly.

I also tend to pull the power all the way off on the back side of loops and such rather than just reduce it. You can see that in the series of regular-appearing peaks in the graph I posted. That's a series of big loops. In two of them I consciously tried not to go all the way off the throttle and you can see that in that the power level doesn't drop all the way off.

It is obvious to me that the props do unload in flight but nowhere near the degree that some postulate. I should look closely at that Santa Fe 9x7.5 file myself with that in mind as there I should see more unloading in relatively horizontal flight and perhaps heavier loading in vertical climbs. I know that that was enough added pitch that even at that altitude there was more load on the motor/battery than with the 9x5 here at sea level-ish altitudes. I wish there was a 9x6 or 9x6.5 in that prop series (GWS DD).

As an aside on that graph - and probably the data you got - the Medusa phase sensor doesn't apparently report RPM when the throttle is "closed" all the way. The brake is not enabled and I never fly the airplane slowly enough for the prop to stop windmilling altogether, so it IS turning on the backsides of those loops for example, even if the RPM graph falls to zero along with the power level.
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Old 09-22-2008, 12:25 AM
  #68  
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Since I am an incredibly incompetent pilot I can't offer to test fly any of these "scenarios" but I wonder whether a series of absolutely simple flights, of level circuits at various, (constant as possible) throttle levels [1/4 (if it will fly level at that?), 1/2, 3/4, full] would tell us something???

One with a sleek, fast plane, with a small high pitched prop... one with a slow, draggy, plane, with a large low-pitched prop.
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Old 09-22-2008, 01:05 AM
  #69  
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Props do unload in flight - the amount of "unloading" is dependent on on the incoming velocity flow -







Tthrust[N]
Dpropeller diameter[m]
vvelocity of incoming flow[m/s]
additional velocity, acceleration by propeller[m/s]
density of fluid[kg/m]
(air: = 1.225 kg/m)




For a typical, fixed pitch propeller, the largest induced velocity occurs under static conditions, where the efficiency is small. It decreases with increasing flight speed, until it reaches zero: no thrust is generated.

In my opinion the best way to measure the amount of "unloading" would be utilizing a wind tunnel and motoring dynamometer.

~ DK
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Old 09-22-2008, 01:36 AM
  #70  
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Default Prop Unloading

I know this has nothing to do with electric motors, but flying fuel engines, I can hear the engine unloading in the air and you can hear the rpm go up when the plane starts flying. The is real noticeable with a FAST engine and plane like a pylon recer that is turning 23,000 on the ground and unloads to around 30,000 in the air. I see no reason to think that electric motors would be any different. They also have constant speed props on some full size aircraft that I have flown and you can see the manifold pressure change as the prop tries to change rpm in flight. You can also hear the change in rpm in a big loop when a heavy load is put on an engine. I also hear the same thing with my electric models, but I have an Eagle Tree coming and can get a graph of the motor in flight if you would like to see one. I should be getting it some time this week and I can get the graph shortly after to settle this once and for all maybe.
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Old 09-22-2008, 02:07 AM
  #71  
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I have some results that appear at first glance to show a much more dramatic aerodynamic unloading in the air than Bernards data indicated.

However, I think I have a problem with my Eagle Tree readings so my data may be suspect.... more later after I look things over more closely.......
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Old 09-22-2008, 03:13 AM
  #72  
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I have some results that appear at first glance to show a much more dramatic aerodynamic unloading in the air than Bernards data indicated.

My first 1 min. section at 50% throttle show almost flat(constant) voltage line,and amps. between approx. 16 and 30 depending on if climping or diving. I guess I am dumb but to me that clearly shows major unloading.

Anyone else here drive a car, ride a bicycle? Sure it takes more energy to go uphill than downhill.

I guess I could hook up the motor tach sensor and the throttle position sensor and fly around at a fixed throttle position with logger verification but I figured a simple volts,amps. watts and my word as to throttle would be proof enough.

Bob Boucher stated when back when that a typical electric would unload approx. 10% in flight. He was speaking of sports setups as there was no such thing as 3D, Hotliners or pattern in thw world of electrics back then. He was also refering to setups using props with resonable pitch to dia. ratios as he has stated many times tha low pitch props are only good to stir paint with.


Fly a fairly powerful electric setup at say 70% throttle for three minutes in level flight. Chop the throttle and land as fast as possible and check motor temps. Now recharge the battery and run the smae setup, throttle for three minutes and see if you have a working motor left.

Someone mentioned that a plane will fly faster than calculated prop pitch speed. Yes indeed they will. It was proven about 35 or 40 years ago with RC Pytlon racers flying around a closed course. A couple of years ago when I first stsrted post logger files I figured that many of these old questions would be put to rest but I guess to many just choose not to believe their eyes or figure their fellow flyers are out to deceive them.

Here is a section of the 50% Throttle position of the flight with only volts and amps. displayed. If this does not show unloading clearly then I guess there is no way to show it.


Charles
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Old 09-22-2008, 03:22 AM
  #73  
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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....

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.

I was just out in the cul-de-sac with the Blu-Baby and I almost put my Oracle in it before I did so just to gather more data. I got chased inside by a big shower after about 15 minutes of flying up and down the street and shooting landings with the occasional loop or rudder roll thrown in.....maybe next time. And maybe I can log throttle position too...just need to figure out how to do it right with the Oracle. (I don't want to pull the EagleTree out of the Switchback Senior right now.)
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Old 09-22-2008, 04:27 AM
  #74  
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Originally Posted by everydayflyer View Post
My first 1 min. section at 50% (snip)
A couple of years ago when I first stsrted post logger files I figured that many of these old questions would be put to rest but I guess to many just choose not to believe their eyes or figure their fellow flyers are out to deceive them.

Here is a section of the 50% Throttle position of the flight with only volts and amps. displayed. If this does not show unloading clearly then I guess there is no way to show it.


Charles
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.

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?

Your graphs clearly show that loading and unloading occur during maneuvers - but that doesnt answer my question.

More specifically - how much of the power drop in flight is due to voltage sag and how much is due to the prop unloading.

I cant tell anything along those lines from your graphs.

Only portions of Bernard's data are usefull because of the unknowns in manuvering and throttle position.

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?

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.
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Old 09-22-2008, 05:18 AM
  #75  
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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.

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

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.
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