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Folding Pusher Prop - Help please

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John Cole22/01/2013 17:17:17
615 forum posts
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I am building a 1-kilo model that will need a folding pusher prop. to go on a 5 mm shaft 300 watt motor, probably an 11 * 6 conventional prop equivalent. I know nothing about folding props, but almost all the ones I see are used in tractor mode.

Do I need to buy something special, or will pretty much any folder do?

Do I need to leave the spinner-part in place, or remove it?

Simon Chaddock22/01/2013 17:50:33
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A folding pusher is a bit of a problem.

If you simply reverse the blades on a normal folding prop hub you are likely to find the blade root off set will cause it to foul the spinner back plate or the front of the motor when the blades try to extend.

This sort of hub might have enough clearance to work as a pusher.

The other thing you have to watch for is the blade being able to fold too far so when it starts up again it tries to extend the wrong way! Obviously in a tractor layout the fuselage prevents this but if full power is applied with only one blade extended it is likely to rip out the motor!

The aerodynamics of most pusher designs do not warrant the benefit of a folding prop hence they are rarely used.

I would be interested to see a picture of the plane.

Wingman22/01/2013 18:10:08
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Seems a bit unbalanced to me - a 300 watt motor (presumably max?) with a 5mm shaft(??) with an 11x6 prop means low revs which is not brilliant for a folder prop. What is the plane?

John Cole22/01/2013 19:21:15
615 forum posts
24 photos

Wingman: O/D canard. Yes, low kV motor.

Simon: thanks. Good point about reverse-opening. I think I would need to put some kind of stop in to prevent that. It's for prop-protection not aero. With this size of prop I was concerned that a standard O-ring based propsaver could drive the prop into the aileron of the canard's wing, damaging it.

ken anderson.22/01/2013 20:12:24
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john-i'm going to fit one to my pushycat.........and intend to use normal blades and spinner.....i dont see any problems......with fouling etc..the spinner will stop the blades going too far back...and the motor will do the rest.........i'll be using a set up as in simon's link...if i do it before you -i'll report back...mod4.jpg

 

ken anderson....ne...1. pushing the frontiers dept...

Edited By ken anderson. on 22/01/2013 20:41:59

PatMc22/01/2013 20:37:39
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Instead of a spinner with backplate you could use a prop hub yoke mounted on a collet (not sure if this one is 8mm shaft) with these prop blades . I've found these prop blades to be as good as Graupner & Aeronaut at half the price.

Alternatively you could simply cut away clearance for the extended prop in the backplate of the one Simon linked.

BTW an 11x6 on 3s at 300W will be turning at about 8500rpm.

GONZO22/01/2013 22:40:41
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1371 forum posts
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Stealthwing covered

Found this on another forum to show whats required (this is on a flying wing). Note the ally stops, front and rear, installed on the prop yoke. These prevent the prop from going to far forward and possibly striking the wing, or when folded going off centre and shaking everything to pieces when restarting the motor after a temporary shut down in flight.

HTH

Edited By GONZO on 22/01/2013 22:41:39

Frank Skilbeck22/01/2013 22:54:50
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Posted by Simon Chaddock on 22/01/2013 17:50:33:

The aerodynamics of most pusher designs do not warrant the benefit of a folding prop hence they are rarely used.

Why do the aerodynamics of a pusher not warrant a folding prop, I would have thought the drag from the prop was the same whether it's a tractor or a pusher.

I have a MPX Easystar 2 which comes with a pusher prop, which isn't really need for landing protection as prop is above the wing.

PatMc22/01/2013 23:36:13
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Gonzo, I think you'll find that the ally stops aren't necessary. They're not needed with tractor props, I can see no reason why they should be with pusher.

Tim Mackey23/01/2013 00:18:27
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20920 forum posts
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On a review model I did some time back ( Borjet Maja ) I used a conventional folding prop on the pusher layout, with no modifications - worked a treat.

Simon Chaddock23/01/2013 00:48:21
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5711 forum posts
3034 photos

I am sure Gonzo has the right solution.

Fold limit stops are essential to stop the blade folding even a short way past the motor shaft axis. If it does it will then not extend yet the other one definitely will with likely catastrophic results - I know I've done it.

This cannot happen with a folding tractor prop as the fuselage is in the way. wink 2

 

Frank

The question is how much would the glide performance of the Easy Star be improved with a folding prop?

In my own experience with foam pushers the benefit over a conventional 'braked' prop was only significant at quite high speed and this had to traded against the slightly lower 'power on' efficiency of the folder.

 

Edited By Simon Chaddock on 23/01/2013 00:49:14

Frank Skilbeck23/01/2013 13:22:10
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Posted by Simon Chaddock on 23/01/2013 00:48:21:

Frank

The question is how much would the glide performance of the Easy Star be improved with a folding prop?

In my own experience with foam pushers the benefit over a conventional 'braked' prop was only significant at quite high speed and this had to traded against the slightly lower 'power on' efficiency of the folder.

 

Edited By Simon Chaddock on 23/01/2013 00:49:14

Isn't that the same for a tractor prop also though ?

Edited By Frank Skilbeck on 23/01/2013 13:22:24

Erfolg23/01/2013 13:53:39
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11745 forum posts
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From what has been written, some have had issues with respect to folders as pushers. Simon being one, and myself is another, i do accept that there may not be issues with all folders.

The problem I have experienced is the over centre blade, not unfolding. As Simon has done, I used cocktail sticks to act as stops to prevent it occurring.

Contrary to others experiences, I have found significant improvements with a folder. In essence the same level of improvement that you obtain with a tractor folder.

I am not convinced that there is any loss of efficiency using a folder either. The central, hub area, contributes little, if anything to the thrust. Although I believe there is benefit from a smoothed form to the body that a spinner produces, reducing the drag (with a tractor mounting). Though not true with my pusher. I could have made an effort, though more interested in cooling and reducing propeller drag.

Simon Chaddock23/01/2013 14:11:00
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5711 forum posts
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Frank

Yes but on a high performance sail plane the airframe drag is as low as possible particularly at speed for good penetration so the drag from an unfolded prop is proportionally much more significant.

It is difficult to design a pusher layout that overall is as aerodynamically 'clean' as a nose mounted tractor configuration..

John Cole23/01/2013 14:36:47
615 forum posts
24 photos

Thank you all for your comments and suggestions.

Simon: what you say about cleanliness is true on the glide, but with power-on there's a significant improvement in efficiency in pusher mode, as the fuselage is not in the prop-wash.

ken anderson.23/01/2013 15:21:51
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8681 forum posts
807 photos

iv'e bought one of the 'rival' mags(RCM World) to get access to an article on foodmixers(autogryo's) .....also there is a couple of pages about prop's etc and there is a photo of a multiplex model with a pusher set up......as it comes out of the box.......also some canny figures about the stresses exerted on the prop blades as well......dont tell young asher's i've bought a different mag..... kulou

ken anderson...ne..1 ...pusher dept......

Erfolg23/01/2013 15:33:44
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11745 forum posts
1337 photos

Right at this moment the dark riders are making there way northwards. Repent whilst there is still time, cast the unmentionable artefact away. You know there is only one magazine which is forged in the breath of the dragon, each month. All others are forgeries, meant to tempt, but must be resisted.

ken anderson.23/01/2013 17:59:00
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8681 forum posts
807 photos

teeth 2 ........... no word yet from ashers tower's........ i may have got away with it...allthough he has a delete thread key......with him having super powers and all....and also he has BEB and chris....and not forgetting his other clone........ young/ish hargreaves......with a bit of luck-they may be snowed in.......glad to see your on my side/not erfolg .......

ken anderson.....ne...1..... wanted dept.....

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