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Old Dec 13, 2014, 12:09 AM
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PW51 plank build

Needing a new plank, and feeling inspired by EdSoars fantastic TigerMoth build, I decided to try a similar project with slightly different materials. Since I've got access to good LZ's, and wanted to have good light wind performance, I'm building a 48" PW 51 plank tightly based on the Moth, but with white foam cores & balsa skins. The planform is close to the Moth, but with a little more chord at the root. Here's the basic idea:
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Old Dec 13, 2014, 12:17 AM
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The wings get skinned in two stages. This makes it easier to build in the joiner hardware, and add ribbon spars to stiffen the ailerons. Cores were CNC cut by Eureka Aircraft, balsa is 1/16" contest grade from National Balsa. This is the first time I've used contest grade balsa for anything, and it's amazing how light & consistent the sheeting is; each 3"x48" sheet weighed between 9 & 11 grams. Cores were sheeted with polyurethane glue, no water (ambient humidity is fine), and kept under 5" vacuum for 6 hours.
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Old Dec 13, 2014, 12:26 AM
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Once the skins are on, it's time to mark out the ailerons, joiner block, and wing hold downs. Since none of this will be visible once the top skin's on, measuring everything accurately first is helpful.

The joiner's a 9/32 brass tube (1/4" ID) with a ply sub rib. The hold down areas get marked out and then diced before sheeting; this makes it easier to clear them out with tweezers later.
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Last edited by c.walker; Dec 13, 2014 at 12:37 AM.
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Old Dec 13, 2014, 01:04 PM
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Wow, great idea and concept. For light-winds, but probably able to fly through big air too. Why did you choose the 51? 75 seems fine for light winds also.
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Old Dec 13, 2014, 02:01 PM
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PW 51 would seem to be a better choice for higher wing loadings, but you are building light.
I think it will rip nonetheless!

Good build, thanks for sharing,

Andy
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Old Dec 13, 2014, 02:12 PM
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I played around with the PW98, PW51, and blended variants; I went with the PW51 since it's got a very neutral pitching moment and better inverted performance. The goal here is light air aerobatics with good energy retention.
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Old Dec 13, 2014, 04:34 PM
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Quote:
Originally Posted by c.walker View Post
I played around with the PW98, PW51, and blended variants; I went with the PW51 since it's got a very neutral pitching moment and better inverted performance. The goal here is light air aerobatics with good energy retention.
I use pw51 on my home brew flying wings and find it very good indeed
This is my home brew "Hawk"1200mm epp flying wing at the beach dunes in 16knots 8m/s wind speed
Cliff
rc hawk flying wing (1 min 46 sec)
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Old Dec 13, 2014, 06:03 PM
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Old Dec 13, 2014, 11:33 PM
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Quote:
Originally Posted by 16141 View Post
I use pw51 on my home brew flying wings and find it very good indeed
This is my home brew "Hawk"1200mm epp flying wing at the beach dunes in 16knots 8m/s wind speed
Cliff
https://www.youtube.com/watch?v=jLOV...yer_detailpage
That's some lovely flying, great energy management. Exactly what I'm going for, albeit with a bit more lift. What's the total weight?
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Old Dec 14, 2014, 12:23 AM
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Applying the bottom skin first in a separate process is a good idea. As you say, it allows spars and joiners to be placed and sanded down to the top shape before adding the top skin.
I will copy that someday.
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Old Dec 14, 2014, 04:16 PM
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With the wings out of the bag, it's time to fill in the hold down blocks and add the root rib. I'd diced the foam in the hold down blocks with a razor before putting on the top skin, so it came out fairly cleanly with tweezers. The root rib is 1/16" ply, epoxied on. I'd glued in the joiner tubes so that about 1/16" stuck out past the core, giving the rib a good mechanical link to the tube.
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Old Dec 14, 2014, 06:06 PM
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Weight tracking, or everything adds up:

I wasn't sure how light I could build this. I originally wanted the completed plane to be under 16 oz. That's probably not going to happen. While the base components are quite light, here's a few numbers showing the progression:

Blank core: 0.7 oz
Wing skin: 0.5 oz
Wing with one skin bonded, joiner block glued in: 2.1 oz
Same with with top skin bonded: 3.0 oz => glue layer is 0.4 oz, joiner block is 0.5 oz glued in

Wing with hold down blocks filled in, and root rib glued on: 4.5 oz
With shaped tip: 4.9 oz

No single step adds that much weight, but they do add up. Also, it looks like if I had built this as a one piece wing, the total weight for the wing would be about 6 oz instead of 10 oz. That's significant for a 48" wing; with a nominal target weight of 16 oz, a 4 oz difference is 25% of the total. That means you'll need 11% more lift to stay up (though it'll also fly 11% faster ).
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Old Dec 14, 2014, 06:56 PM
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I like the idea of doing the top and bottom skins separately. Will try copying this. How do you keep the skins aligned, do you just use tape?

Mike
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Old Dec 14, 2014, 07:12 PM
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Here's the alignment process:
1) oversize the skins by 1/8" in each dimension
2) line up the front edge of the core so that the skin just barely protrudes past the foam
3) roll the core back carefully, press down evenly all over

The polyurethane is quite "tacky", so at this point, the skin won't shift out of alignment. You don't need tape. Throw in the bag & cure.

When it's out of the bag, line up a straightedge on the foam TE, and trim off the excess skin at the back. When you repeat the process, this will be your alignment guide to trim off the excess skin from the other side.

This is slower than doing both at the same time, but much less stressful, and you're more likely to get good results. Picked up this trick from the instruction manual for an old CR Aircraft Turbo.
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