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Old Jul 08, 2012, 03:59 PM
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Quote:
Originally Posted by skitchen8 View Post
Do you have a source on that? It doesn't make sense to me that moving would create more drag. Stopped it seems that air would build at the front of the prop generating more drag whereas moving it still allows air to flow toward the rear of the plane. I also think a moving prop would be more efficient as a stopped prop will create turbulence flowing over and under the fuselage where a moving prop will lessen this turbulence.
A windmilling prop has more drag than a stopped prop... just like an autorotating helicopter falls slower than one with the blades stopped.
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Old Jul 08, 2012, 04:56 PM
UMs & parkflyers... for now.
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Here's one thesis on the subject.

http://www.peter2000.co.uk/aviation/misc/prop.pdf

From its conclusion:

Quote:
From the experimental results reported here we can conclude what we could have figured out with a little thought: drag force increases with length and wind velocity, and decreases with pitch. What is less clear is how the drag force increases and decreases with these variables. Does it increase linearly or quadradically with length and wind velocity? When considering the pitch, does the windmilling drag force also follow a cosine-squared curve? More accurate data are needed to determine the characteristics of the crossover point. Does it depend on wind velocity? Arguments both for and against rely on data that could be drastically changed if just a couple of data points were moved. Further work in this topic should begin either with an increase in the range of the variables, or increasing the precision of the data. Improving either one of these will help answer all of the questions posed above.

The main goal of this investigation was simply to determine whether a stationary or a windmilling propeller has more drag. The answer is complicatedly simple: it depends. It is clear that it depends on the pitch and length of the propeller, and it is probably independent of the wind velocity. A crossover point was discovered where the drag forces for the windmilling and stationary states were the same. This crossover point is also dependent on the pitch, the length, and probably independent of the wind velocity
And a 1922 NACA Technical Note on the subject of Propeller Energy Losses:
http://naca.central.cranfield.ac.uk/...naca-tn-91.pdf

Bottom line is that windmilling props are draggy. It's why I add a touch of power to overcome it... unless I want to come down faster.
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Old Jul 08, 2012, 05:00 PM
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If the drag of the blades were the only component of autorotation you could weld a large disk the size of the rotors and drop a heli from the sky and it would float down gently. The reason autorotation happens is because of the lift created by spinning rotor blades that is only created with forward motion. If you kill the engine on a heli and let it drop straight down you'll fall pretty much as fast as you would with no blades at all so that's not really a valid comparison. Add to that the fact that an airplane prop has very little in common with a helicopter rotor (the shape is totally different for example) and what you're saying doesn't prove anything. I'm not saying you're wrong just that I'd like to see some proof because what I understand about physics would say that a spinning prop creates less drag than a stopped prop. Easy enough to test really, anyone have a wind tunnel? Lol
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Old Jul 08, 2012, 05:03 PM
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Quote:
Originally Posted by davidterrell80 View Post
Here's one thesis on the subject.

http://www.peter2000.co.uk/aviation/misc/prop.pdf

From its conclusion:
Thank you, that clears it up a bit and makes sense. I suppose we were all right
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Old Jul 08, 2012, 05:22 PM
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Originally Posted by skitchen8 View Post
If the drag of the blades were the only component of autorotation you could weld a large disk the size of the rotors and drop a heli from the sky and it would float down gently. The reason autorotation happens is because of the lift created by spinning rotor blades that is only created with forward motion. If you kill the engine on a heli and let it drop straight down you'll fall pretty much as fast as you would with no blades at all so that's not really a valid comparison. Add to that the fact that an airplane prop has very little in common with a helicopter rotor (the shape is totally different for example) and what you're saying doesn't prove anything. I'm not saying you're wrong just that I'd like to see some proof because what I understand about physics would say that a spinning prop creates less drag than a stopped prop. Easy enough to test really, anyone have a wind tunnel? Lol
You need to look up the dynamics of autorotation, because you don't seem to understand it.

An autorotating helicopter doesn't accelerate @ 10M/S^2 and it doesn't have the terminal velocity even 1/2 as high as a rotorless helicopter, even with zero "forward velocity" and even with negative AOA of the rotor blades (the same as a prop plane would have with forward motion) and yes I have an Aeronautical Engineering degree and yes I have 20+ years working for major aerospace companies.
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Old Jul 08, 2012, 05:43 PM
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Originally Posted by davidterrell80 View Post
Here's one thesis on the subject.

http://www.peter2000.co.uk/aviation/misc/prop.pdf
This thesis leaves a lot to be desired.

He testing the drag on an 8 pitch prop with a MAX Speed of about @ 11 mph. His "wind tunnel" sounds like a box fan
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Old Jul 08, 2012, 05:58 PM
UMs & parkflyers... for now.
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Originally Posted by UNGN View Post
This thesis leaves a lot to be desired.

He testing the drag on an 8 pitch prop with a MAX Speed of about @ 11 mph. His "wind tunnel" sounds like a box fan
Sort of like the things we fly....

BTW, I used to live in Keller and still own a house there. The war keeps me away.
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Old Jul 08, 2012, 06:10 PM
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The only information I can find compares a feathered prop to a windmilling prop, and that much is clear. What I'm not finding is a comparison between windmilling and stopped when a prop is not feathered.
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Old Jul 08, 2012, 06:47 PM
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Originally Posted by davidterrell80 View Post
Sort of like the things we fly....
If some one is using an 8 pitch prop to go 11 mph they are doing it wrong.

His data is pretty clear that a 3 pitch prop @ 11 mph causes significantly more drag windmilling than stationary, so its pretty much cased closed for a micro.
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Old Jul 08, 2012, 06:53 PM
Scout CX | mCX2 | mSR | 120SR
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I feel like I'm back in my guitar forum.








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Old Jul 09, 2012, 09:25 PM
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So bottom line, A stationary prop creates less drag than a windmilling prop, right?
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Old Jul 09, 2012, 09:30 PM
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So bottom line, A stationary prop creates less drag than a windmilling prop, right?
Yes. Unless you are using an 8 pitch prop on a Champ, apparently.
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Old Jul 09, 2012, 09:33 PM
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Thanks. I wanted to make sure I understood the consept correctly.
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Old Jul 09, 2012, 09:49 PM
Scout CX | mCX2 | mSR | 120SR
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Canada, AB, Edmonton
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I have been working with a buddy on an LED nav light and strobe project for my Champ... he wants the same specific strobe pattern I want in my tail in his wing tips with his nav lights. I was too busy flying all weekend to breadboard prototype the whole thing but he was working on figuring out the resistor and capacitor values to set the timer chips to do what I showed him I wanted... based on YouTube videos of real planes like this one... you'll need to click the link in the player to take you to YouTube to watch it at the proper higher framerate to see what the strobe is actually doing, it won't look right in this embedded player...

STOL CH 750 with super-bright Zenair AeroLED 3-position wing tip nav / strobe / position LED lights. (1 min 16 sec)



I'm also working on 2 sets of Champ floats... one from scratch using the plans available here at RCG and one from modified Carbon Cub floats (size reduced and hollowed out). I'll use the same styrene I used for my wing struts to make the mounting strut system look more scale than just having the piano wire

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Old Jul 10, 2012, 08:48 PM
I just need one more plane..
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United States, VA, Fairfax
Joined Jun 2012
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The cowl broke off my champ so I Modded it with stale french bread from an old hoagie I had laying around. I had to tape a piece of salami to the tail to balance it out but man that thing flies great now.

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