Penut size rubber powered wing - Page 3 - RC Groups
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Dec 29, 2012, 01:07 PM
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Gotcha. Thanks for the info on the other, previous, wing.

Am looking forward to flight reports on your new wing ... when you complete it!!
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Jan 03, 2013, 01:43 PM
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Stuart A's Avatar
Looking good!I'm very tempted to have a go at one of these( once you perfect it
Jan 05, 2013, 02:59 PM
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slebetman's Avatar
Lots of build progress today (and last night) so I'm going to split this into two posts.

First the top covering. I covered the top "wet" (actually, just a bit moistened) while the wing is still attached to its jig. I decided to split the covering into several parts. This allows me to do both the left and right side simultaneously. Once both dried I covered the center.

I realize now that I should have put stringers in parallel to the spar along the leading edge of the wing. The extreme curve of the airfoil causes the tissue to sag a little. I'll add it to the final plans.

Once the top is covered I left the wing to fully dry for another 12 hours. Only then do I cut out the wing from the jig.
Jan 05, 2013, 03:12 PM
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slebetman's Avatar
The bottom of the wing I covered dry to prevent warping. Like the top, it is covered in panels. Once covered I mist the whole bottom with water to shrink and tighten the covering a bit. I did this several times misting very lightly to prevent overdoing it and warping the wing.

While waiting for the glue to dry for each panel I also built the fuse.

Now all that's left is the nose block, tail block and the prop.

Glide tests are encouraging
Jan 05, 2013, 07:00 PM
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Stuart A's Avatar
Excellent!I can see I'll have to build one of these.
I think the top stringers are a good idea(Norms post If you made a few well placed holes in the ribs(maybe main spar?)you wouldn't be any heavier.
Jan 05, 2013, 11:07 PM
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nmasters's Avatar
Beautiful, delicate, little airplane
Jan 06, 2013, 10:20 AM
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slebetman's Avatar
Stuart, if you decide to build this then please post your plane here. Or at least link to your build thread. I would love to see another one this built.

I've finished making the nose block, installed the peg and made the tail block and prop. So now it's basically completed.

The prop is made from a plastic cup with a balsa stick hub. I whittled and sanded as much balsa from the hub as I dare to reduce the weight of the prop. Prop angle at the root is 45 degrees but the prop template has a 20 degree tilt so there is some washout at the tips. The prop is a little bit less than 5 inches.

The nose and tail blocks are solid balsa. I'm not using a prop bearing this time. Instead, I simply punched a hole in the tail block with the prop shaft and wiggled it a bit until it's loose and then hardened the entry and exit holes with a bit of CA. This may turn out to be a bad idea but it works so far.

The prop shaft is a length of no.2 guitar string.

Did several short test flights in my garden and found the 5 degree down thrust in the plan to be way too much. Kept pushing the nose down into a dive. Which was annoying because I've already made a hole in the tail block with the 5 degree down thrust and the prop shaft was already glued onto the prop. So I had to cut the prop shaft and make a new one. Made another hole in the tail block, this time with zero down thrust. Wound up to around 200 winds and let go and it flew perfectly - no stalls, no nose dip. It has a tendency to circle to the left so I'll try trimming it for left circles first.

The hole in the tail block is much too wide now due to my attempts to reduce the down thrust. But since it flies well I'll use this for now.
Jan 06, 2013, 10:39 AM
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Very pretty ....!

Jan 06, 2013, 11:11 AM
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slebetman's Avatar
Forgot to mention. Weight (minus rubber) is 5.6g
Jan 06, 2013, 12:45 PM
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Stuart A's Avatar
Marvellous stuff!I think the c/a hole for the prop is good,especially if it's running without any down thrust.
I'm thinking 1mm depron would be ok for the ribs(easier for my old hands to cut accurately at this scale)
With the twist,fuselage and pusher prop it might be worthwhile trying without the tips?
Regards Stuart
Jan 07, 2013, 08:32 PM
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slebetman's Avatar
Go for it Stuart, I'd love to see this in depron.

I didn't have problems cutting out the balsa. I tacked on the templates with a light coating of spray mount. The paper protects the balsa from splitting when cutting and stops the blade from wanting to follow the wood grain. Just don't overdo the spray mount or it would be almost impossible to remove the templates without destroying the ribs.
Jan 07, 2013, 09:04 PM
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slebetman's Avatar
Noticed a weird behavior last night. At higher speeds, either from throwing hard or from using thicker rubber, it dips its nose and dives straight to the ground. My guess is that the airfoil changes pitching moment at higher speed. So I'll need to make some changes to improve its behavior.

But at the right speed it flies great. I'll try flying it at the mall this weekend and see if I can trim it as is. This probably explains why it doesn't need down thrust - it's built in to the airfoil.

I have several options to fix this:

- Use a thinner airfoil
- Use turbulators
- Use a symmetrical airfoil
- Use a proper flying wing airfoil with low pitching moment
- Use symmetrical or inverted airfoil on the wing tips
- Increase twist

Feel free to suggest other ideas
Jan 09, 2013, 01:32 AM
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slebetman's Avatar
I decided to rebuild the prop and tail block. I realized that the rather loose fit of the prop shaft would make trimming up or down thrust impossible. I'm taking this opportunity to document how the prop is built.

This time around I decided to build a counter-clockwise/reversed prop. The reason is because I found winding the regular prop awkward because in pusher configuration, a regular prop needs to be wound counter clockwise. A reversed prop is wound in the regular clockwise fashion.

The prop is cut from a plastic cup based on a hand drawn template which is drawn on another cup. Drawing templates on a cup makes it easier to align the washout angle properly and consistently.

The hub is simply a 2 cm length of 5mm square balsa stick. Since the root of the prop is 45 degrees it's easy to cut the slot for the prop blades by simply cutting from corner to corner. Then I whittled and sanded away most of the non-structural bits of the hub to reduce weight. The end result looks a bit like a tiny motorboat prop.

After dry fitting the prop and I'm satisfied that the alignment is correct I mark the position of the prop on the hub with a pen. I mark on only one prop. Then I overlap the props and trace over the marking on the other prop. This ensures that both prop are symmetrical when installed. You'll still need to eyeball it and check for crookedness but at least the markings make a good starting point.

The root of the prop blades are sanded to improve adhesion to the glue. In fact, I also lightly sanded the rest of the blade to make it slightly opaque and remove any remaining sharpie outline.

The prop blades are installed with a small drop of CA on both sides.
Jan 09, 2013, 02:19 AM
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Stuart A's Avatar
Neat.Just put one inside the other and mark over.Locating plastic cups without any "ribbing" could be a problem,I'll have to see what I can find,something I could tinker with in a quiet moment.

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