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Folding Props and Spinners dimensions

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Bruce Austin05/01/2016 18:14:46
242 forum posts
1 photos

Happy new year for 2016.

Hey, nice electric Guys, can you please tell me if there is some sort of dimension standard for our folding prop accessories???

I fly electric gliders and due to "wear and tear" it is time to purchase some bits and pieces. This is proving to be a wee bit of a puzzle.

Spinner diameter seems logical enough. BUT then there is the Yoke!!! or is it the Hub!!! I am used to 8mm prop root width, (10x6 prop) but it seems that this might be smaller for say a 7.5x4 Prop

We want the folding blades to fold flat against the fuselage, but the hinge pin centres need to be greater than the diameter of the spinner.

e.g. spinner base diam 38.2mm with integral hub pin centres 45mm having a prop root width of 8mm, and prop adaptor hole 8mm. Scanning the various online suppliers catalogues gives no clue about how to mix and max the different items other than the shaft diameter for the motor.

I have just put together a wing with a motor option for its nose.

As an experiment I used an HK Wingnetic prop/ spinner assembly.

Spinner base 29.75mm with hub pin centres 19.25mm, having a prop root width of 6mm. This hub slides directly onto the 3mm motor shaft . Note that in this case the prop pin centres are inside the spinner diameter

So back to the question, Is there a Standard for ease of mix and match???

Go With the Flow

Bruce

PatMc05/01/2016 19:20:48
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4199 forum posts
521 photos

There is no standard unless you use the prop maker's yoke/hub.

For instance, I have Graupner 10 x 6 blades on a MPX Easyglider spinner/yoke assy. Due to the hinge pin centres the blades extend to 11" - that means the prop is now an 11 x 6.6. Similarly on an OD E-glider I use an ali spinner/yoke which extends 9x5 blades to give a 9.75 x 5.4 prop.

Bruce Austin07/01/2016 08:51:31
242 forum posts
1 photos

Hey there PatMc thank you for that prompt and encouraging response.

I had thought it was just me having difficulty in understanding the details.

Thanks for your own examples and experiences. That has helped me understand what is going on.

Go With the Flow

Bruce

Dickw07/01/2016 09:54:46
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454 forum posts
75 photos

Just to add to what PatMc said, it is also possible to buy yokes that have a slight twist which has the effect of increasing the pitch - see here **LINK**

I believe Aeronaut blade sizes are quoted assuming a 40mm dia centre piece (pin to pin).

Some more useful stuff here **LINK**

Dick

Bruce Austin07/01/2016 10:31:01
242 forum posts
1 photos

Hi Dick

Thank you for the links.

I am flying Phoenix 2000 and 1600 foamies, plus, just setting up a Zupair Zulu Wing.

The information on the links was quite fascinating, hmmmm and quite a lot of ££££££s too.

It all adds to the store of knowledge in this fascinating hobby of ours.

Thanks for the help.

Go With the Flow

Bruce

Dickw07/01/2016 11:08:19
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454 forum posts
75 photos

If you think thgat is a lot of ££££££s you should see some of the really specialist stuff - **LINK** £90+ for a prop and hub surprise.

Not suggesting you need anything like that (yet devil) but fascinating reading as you say.

Dick

Glasshopper09/01/2016 10:16:13
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102 forum posts
 

"There is no standard unless you use the prop maker's yoke/hub.

For instance, I have Graupner 10 x 6 blades on a MPX Easyglider spinner/yoke assy. Due to the hinge pin centres the blades extend to 11" - that means the prop is now an 11 x 6.6. Similarly on an OD E-glider I use an ali spinner/yoke which extends 9x5 blades to give a 9.75 x 5.4 prop"

PatMcc's posting above on 5th Jan made me wonder about the effect the size of the hub has on the efficiency of the prop.Obviously a longer hub will increase the diameter of the prop but does it change the dynamics?

Dickw09/01/2016 10:54:11
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454 forum posts
75 photos

I am no aerodynamcist, so the following is from practical observations.

Most props don't seem to be a constant pitch along the length of the blades, as I assume the designers try to make them effective over a range of speeds, and I believe many producers measure the "nominal" pitch at about 75% of diameter. So the answer to your question is almost certainly YES it will change the dynamics BUT probably not enough for you to notice a change in efficiency unless you use a drastically different length of yoke. In my experience the small changes in pitch and diameter just act like a different prop size on the correct hub.

Hubs that are twisted to add pitch work best with blades designed to work with those hubs unless very small angles of twist are used. It is easy to change the pitch by a large percentage with just a few degrees of change in angle, and this can make for a noticeably inefficient prop (5 degrees of twist adds about 5 inches of pitch to some of my props).

Dick

Glasshopper09/01/2016 11:50:33
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102 forum posts

Thanks for your reply Dick. I have always imagined that the least effective area of a prop is around the centre. I suppose a simple test would be to fix the same set of blades to several different hubs and measure the rpm & watts....if I had the inclination!.RayMac

Dickw09/01/2016 12:24:37
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454 forum posts
75 photos

Simple tests like those are more than adequate for our basic needs, and I have more inclination to do that than fly in this weather smiley.

If anyone wanted to take such analysis a bit further then the Martin Hepperle site has a lot of useful information on props including a prop design applet "Java prop" - just look down the menu under "Other Topics" and "Java Applets". **LINK**

For the really dedcated theorist the "Selig" site is good for a few wet days reading **LINK** but most of it is beyond my understanding.

Dick

Geoff Sleath09/01/2016 13:09:46
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3356 forum posts
272 photos

I think the main reason for twisted or offset hubs is to allow the folded blade to lie closer the fuselage to reduce drag. Like these at Hyperflight. Changing pitch is a side issue, I think.

Geoff

Dickw09/01/2016 15:07:29
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454 forum posts
75 photos

Agreed, but some people do not realise the effect of hub twist on pitch and and can get themswelves into trouble. If you add 5 degees of pitch due to twist you really need to consider if you also should use a lower pitch prop as well to counteract the effect of that increased pitch.

Dick

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