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Old Aug 30, 2002, 01:27 PM
Canadian E-Tailer
Slow Riser's Avatar
Calgary
Joined Apr 2002
343 Posts
Wind-milling Prop or Stopped Prop?

I have a wing with a pusher. I'm using a 3 x 2 propeller and when I'm in a glide mode, I turn the throttle off to do some soaring. Unfortunately there are no 3 x 2 folding props.

The question I have, is what creates less drag? To let the prop wind-mill or if I use an ESC with a break, a stopped prop?
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Old Aug 30, 2002, 01:49 PM
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Kings Park, New York, USA
Joined Sep 2001
813 Posts
Prop Drag

Hey Slow Riser

Stopped prop is better than a windmilling one.

Just so happens there is a Propellor MFG. right in Calgary, that has the potential to make a terrific folder to take place of a 3x2.



YOU!!!!!!!


Get hold of a 4.7x2.4 Graupner Cam folder and cut down diameter, and width and thickness to suit. Take a static reading on what you have and match.

Would help a bit if you could make or get hold of a pitch guage if you are going to go to a 2 instead of a 2.4. Eyeball of the old prop or new will work.

Try it you will like it.

If you want more climb as compared to now go larger diameter and less pitch. If you want faster go less diameter and more pitch.

If you need to decrease amps, narrowing prop and thinning will do the trick, at the expense of less thrust. Don't know your model, but if it does not need gobs and gobs of thrust because it is a drag monster, this works well.

You can make your own custom 2 or 3 or even 1 blade hub for 4.7x2.4 blades. 1 bladers are hard to get balanced, dynamic and thrust. That said easy to mount balamce weight to a folder hub, there is hardware there already!

You will probably need a brake on ESC, if not you can try putting a small rubberband or spring to help close the blades.

I am a bit lazy to take a look, but I have some 4.2x2 APC props, meant for a Pee Wee Cox IC, You could probably cut 1 up and make your own folder out of it. i kinda think you are not making much HP. They are really light and really easy to file and cut. Cheap too.

Good Luck
ron Parigoris
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Old Sep 08, 2002, 08:30 AM
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Canada, QC, Gatineau
Joined Jan 2001
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A windmilling prop creates drag equivalent to the disk area projected by the prop. However, if you use an ESC with a brake, you will be reducing the drag quite a bit. On a stopped GWS 3" pusher prop, I would think the drag would be minimal, especially if it in line with the wing or pylon.
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Old Sep 08, 2002, 01:56 PM
Canadian E-Tailer
Slow Riser's Avatar
Calgary
Joined Apr 2002
343 Posts
Thanks for the responses.

Ron,
Thanks for the suggestion. I did get a 4.7 x 2.4 but felt that this is too big for the GWS 50XC to swing even if cut down.

The good thing is, although my ESC does not have a brake, is the prop stops windmilling during gliding. While not as effective as a folding prop, it is much better than just windmilling.

Based on Pierre's comments, why aren't all ESC's braked? Why would you want to buy a no-brake ESC?

Hank
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Old Sep 08, 2002, 02:14 PM
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United States, VA, Warrenton
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A windmilling prop can help slow a sport plane down as it is coming in for a landing. I know that with my E3D, I would like the added drag of the windmilling prop, but I can't disable my ESC's brake.

A brake can also put a lot of stress on gears. In other threads, I have read of people stripping their Great Planes GD-600 gearbox when the brake kicked in on their ESC.

The best ESC is one that allows you to disable the brake if you don't want it.

-Dan
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Old Sep 08, 2002, 03:43 PM
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United States, CT, Enfield
Joined Nov 2000
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Quote:
Originally posted by Ninjak2k
A windmilling prop can help slow a sport plane down as it is coming in for a landing. I know that with my E3D, I would like the added drag of the windmilling prop, but I can't disable my ESC's brake.
-Dan
Just apply 1 or 2 clicks of throttle to tick the prop over slowly if you have a brake. I use this trick a lot when landing my LT-25. It will float forever if you let it, and letting the prop turn over slowly helps to slow it down and lose altitude.
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Old Sep 08, 2002, 07:47 PM
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Richmond, VA
Joined Apr 2002
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OK I know nothing about aerodynamics but it just seems logical that a stopped prop would be more resistance than a freewheeling one. The fact that the wind turns the prop tells me it is less resistance to do so.

My feeble attempt at an analogy. If you hold a fan out a car window while moving it will turn trying to match the wind speed. It won't of course but it lets air pass around the blades. Now stop that fan blade and hold it out the car window again. Won't the blades that are now not turning create more force on the blades pushing it back harder?

Just seems logical to me.
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Old Sep 08, 2002, 08:11 PM
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Parkflyer,

I'm not quite sure how to explain it, but a free spinning propeller will have much more drag than one that is stopped.

A propeller that is still is like taping a popcicle stick to your plane. A spinning prop is like taping a disc to your plane. The air is causing it to spin, but it's bleeding energy (speed) from your plane in the process.

Oh, just thought of a good example. You know those plastic blade-stick toys that you spin up in your hands and let go? A very simple helicopter. Why do they float on the way down like that? Because the air is causing them to spin. If they stopped spinning, it would drop like a rock.

Is that more clear?

-Dan
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Old Sep 08, 2002, 10:12 PM
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Cambridge, MA USA
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Quote:

I'm not quite sure how to explain it, but a free spinning propeller will have much more drag than one that is stopped.
Not true in general. If the prop has a large Pitch/Diameter ratio and very little bearing or motor friction, it will cause less drag if it can freewheel.

Non-folding rubber power props fall in this category. They have very large P/D and no motor to turn over, and are almost always set up for freewheeling.
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Old Sep 09, 2002, 12:40 AM
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Colonial Heights, Virginia, USA
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But a prop (free-wheeling or not) is Not a disc...it's just two fairly narrow blades that just looks like disc when spinning... air just goes around & thru the open space even when turning...... kw
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Old Sep 09, 2002, 03:50 AM
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It takes quite a lot of energy to get a motor and prop to turn. If it didn't we wouldn't need to carry those heavy batteries around.

When it's not the battery and motor doing the turning where exactly do you think the energy comes from ?

Steve
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Old Sep 09, 2002, 08:24 AM
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Yes. To expand on what Steve said, the prop isn't truly freewheeling. It is attached to the motor and must overcome its resistance to turning.

-Dan
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Old Sep 09, 2002, 09:28 AM
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Guys,
I don't know how to explain this in engineering terms, or technical terms, but I'll try. In my experience flying real, full scale aircraft (commercial pilot). When you lose an engine on a twin engine aircraft, you have a handy little feature called "feathering" the prop. That is turning the pitch of the blades into the wind so that the prop will not turn. Without being able to do that the drag from the windmilling prop is so great, that the plane will not maintain altitude, and is a bear to maintain control. I would think that this would apply to rc aircraft also, because a plane is a plane, they all basically fly alike no matter if it has a 30" wing span, or 30 feet. Aerodynamic principals are the same, the biggest difference is that rc models have a lot better power to weight ratio than full size aircraft. Anyway, a windmilling prop is drag. A prop that is stopped still creates a small amount of drag, but no more than the reciever antenna you have dangling out into the wind. That's my 2 cents worth.

Phil
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Old Sep 09, 2002, 09:14 PM
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Richmond, VA
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Quote:
When it's not the battery and motor doing the turning where exactly do you think the energy comes from?
The energy is coming from the wind that is pushing the prop. The same wind that pushes a stalled prop.

Granted, there is certainly some resistance to turning the prop with the motor attached. But it is mostly inertia, after it is turning it takes very little to maintain the spin. Similar to an automoble that is on the highway. It took a hundred horses to get up to 60 mph but it only takes 10 horses to maintain that speed.

Remember we're not using the battery to turn the prop slowly. We're turning them very fast to push against the air at sufficient force to overcome gravity. That's why we need those heavy batteries.

Not trying to be argumentative, it's just my anal analytical mind wants to understand how things work. I want to hear something like; the freewheeling prop creates vortices that cause a differential pressure across the prop creating an increase in drag.

Sounds pretty good anyway.

Grant
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Old Sep 09, 2002, 10:41 PM
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Grant,

I'm going to search on the next and find the scientific answer for you, but for now, you'll just have to accept the fact that a freewheeling prop does have more drag. It takes more than just a little bit of force to spin the motor, btw.

-Dan

Scientific answer on next page.
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