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Feb 02, 2019, 08:08 AM
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EdSoars's Avatar
The search for an earth-friendly building material continues.

I bought a kilo of unprocessed hemp bark fibers, thinking this super-strong, LONG fibers would be perfect for laying up a spar. They would be, BUT photo 1 shows what they look like as taken from the brown cardboard box they came in: a CURLY, TANGLED MESS.

So, I cut the spar slot, photo 2, weighted the foam cores in the beds, and wetted the fibers with epoxy and stuffing them into the slot. No good: the fibers would NOT lie flat.

On to plan B, 3rd photo: a built-up spar mold. Predictably, it behaved no differently than the slot in the wing cores. There was no way to hold the fibers in the mold. They are free spirits.

Onward to Plan C: bark fibers in the form of hemp twine, photo 4. The twine is as curly as the bark, so it wouldn't lie flat in the mold. At this point I almost abandoned the hemp spar idea, but gave it one last test: I bundled about a dozen pieces of twine together, wetted them out with epoxy, and wrapped them with fishing twine. When the epoxy set up, it was STRONG. Very slightly resilient, but stiff and lightweight.

But how to make the twine lie flat in the molds?

Plan C1: Photos 5 and 6 show the solution... I hope. The idea was to stretch the string across my bench, like a harp, then wet the strings with epoxy to (still hopefully) keep them straight when they are placed in the spar mold.
Photo 5: 12 screws in a 1 x 2 x 7" block to hold the string about 3mm apart, with mylar underneath to keep the strings from becoming one with the bench top.

Will they stay straight after the epoxy sets? Stay tuned.
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Feb 02, 2019, 11:26 AM
Everything's A Composite
Knoll53's Avatar
The box of hemp fibers looks quiet "raw". Maybe a combing process could work. Like combing wool to make yarn. the process I use with carbon tow is to stretch is out over mylar, wet it out, then comb it and stretch it while it is wet and sticky. It works pretty well to tame the loose tow, but your similar method didn't get the job done..................If I'm following correctly.

You might consider going big. Like a 1x4 mold. Stretch and pack the fibers in and press with epoxy, cure, then run it through a tablesaw for spar caps. Getting straight fibers is the goal for strength for both tension and compression.

Also, polyester resin might be a good choice............for economy.

Feb 02, 2019, 02:07 PM
You know nothing....
Stuart A's Avatar
Thread OP
Might be worth trying-soak a length of cord in water,see if it will stretch out some of the kinks.
Feb 02, 2019, 07:55 PM
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EdSoars's Avatar
Stuart, wetting in water and stretching would work, but it doesn't seem to be necessary.

Kent, yes, it is spades. Combing would work for sure, but it would be a lot of work. The main difference in hemp and CF is that the CF fibers are virtually endless, but the hemp fibers are from 15 feet to 15 inches long. What a mess.

But the good news is that I just pulled the hardened, STRAIGHT twine off the harp and it stayed straight enough to lay down in the spar slot in the wing. No need to cut it apart.

The bad news is that it is not straight fiber, for sure. It is twisted into twine, and the 3mm twine leaves a lot of space in the spar slot that must be filled with epoxy. I haven't yet used polyester resin but will look into it. That slot accepted a lot of resin for sure.

But, although this spar will not be anywhere near the maximum strength to weight for hemp, it may be strong enough and light enough for my flying purposes, for which a wing loading of 10 to 16 lbs/sq ft. is perfect, and I will be using multiple layers of hemp fabric to distribute skin strength where it's needed: the center and the leading edges.

One improvement may be to use 2mm or 1mm twine to increase fiber density.
Feb 03, 2019, 11:23 AM
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I like the hemp idea ... but perhaps consider some other materials, in more "spar friendly" formats, as alternates?

Oak planks

The bowyer guys use those materials from (according to their posts) ... Home Depot.

To manage weight, you'd probably want to cut spar caps and use balsa as the core/web.

On a similar note, I'd think straight-grained hardwood veneer would also work nicely for spar cap laminations.

Anyhow, the hemp experiments are certainly interesting.

Feb 04, 2019, 04:00 PM
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Jebera's Avatar
Hemp is cool, but it likes resin just like crazy...
Feb 24, 2019, 06:21 PM
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EdSoars's Avatar
Finally some progress.

Yes, hemp is VERY absorbent. I wonder what that does to the strength/weight? The real potential of the hemp fiber may not be realizable until I can figure a way to wet out the processed roving and compress it into a mold.

And there are definitely some better spar materials. I like the idea of bamboo. The biggest challenge is to be able to shape the caps and webs to the spar slot in the wings. I don't have a table saw to cut those little tapered oak strips!

I do know a fly-rod builder though. And a guitar maker. There may be some synergy here !

Anyway, this will not be a high-performance DS wing for sure. More like some snappy rock-n-rollin' on the front side with the ability to survive rocky landings. The bottom skin seems very tough. It sands a bit like kevlar; a little fuzzy on the edges. But the fibers are friendly, not like FG or CF at all.

The photo shows the top surface almost ready to skin with hemp fabric. Servos are HS-85MG.
Apr 16, 2019, 06:42 PM
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EdSoars's Avatar
After a long winter away from Colorado's best slope site, FB Mike and I got out for a Sunday afternoon of soaring in glorious conditions. Wind was directly onto the slopes at 15-25 mph, with big thermals flying past us. I got to compare the old stretched Tiger Moth to last year's Max Plank. After getting reacquainted with the air above the slope with the Moth, I launched the Max and flew until I was brain-tired. I'm not sure of the relative wing loadings of these two planks, but the Max at 12.5 oz/sq ft was definitely faster than the Moth, with better penetration and glide. Some of the difference is due to the many fuzzy repairs on the old Moth, but the PW Amokka airfoils on Max seem to be a little more efficient than the old standby PW51 on the Moth.

The site comprises two lava-flow ledges, the lower about 15 feet of vertical cliff above a 100 foot slope, and the upper about 500 feet downwind of the lower cliff, with 70 feet of cliff above another 180 feet of slope. Flying back to the upper slope takes good eyesight and a ship with a fast, flat glide to punch back upwind to get out front. I have to brag that Max performed very well admirably.

And I apologize for not having a video camera. Someday soon!
Maybe with a tall blonde videographer...
Apr 17, 2019, 02:55 AM
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Stuart A's Avatar
Thread OP
Nice one!If you find a tall blonde(I presume of the fair sex)videographer a selfie of same will be required.
Apr 17, 2019, 10:21 AM
Everything's A Composite
Knoll53's Avatar
My Plank 101 is extra fuzzy now......................time for a new plane! Maybe one that can be sanded and refinished.

Would like to learn more about the new airfoils.

After shooting AND posting over 600 videos, I am beginning to just fly without shooting video. Maybe the video Gods are transferring from me to you.........time will tell.

Beat this landing with your new fancy schmancy airfoil.
Los Banos - 12 mph (0 min 17 sec)
Apr 19, 2019, 07:40 AM
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EdSoars's Avatar
Stuart, If the blonde shows up, she will be required to supply a selfie!

Airfoils: Peter Wick designed them for the Amokka, which is a CNC-molded wing aimed at DS'ing. They are a series of four, named PW Rennbrett Wurzel (Root), PW Rennbrett 1, -2, and 3 (tip), and are best used at the Amokka's chords and spacing along the 1 meter wing panel.

I am too lazy to cut four cores per panel, so used the PW RB -1 and -3.

The PW-51 is a wonderful all-around airfoil, and tapered balsa TE stock fits the shape very well with a minimum of sanding. The hemp-based Max Plank uses the PW-98, also suited to a balsa TE, but with a higher maximum lift coefficient than the PW-51 at the cost of some inverted performance.

The Rennbrett series really need to follow the coordinates, with a thin, reflexed TE to get the most out of them, and since they are aimed at DS, they like a goodly wing loading to get the Reynolds numbers up. And I gotta say, they do work well... at least the two I used on Max Plank! I really do need to build another version at 14-16 oz/sq ft. ... or more!

Ask the video gods to include a good headband camera and time to edit in that transfer!

Yes, that was a sweet landing, but I notice a lot of fluffy grass underfoot. If I could find a place to land on grass, I'd try to emulate your sterling example, but our rock-strewn landing zones would likely result in damage. I also noticed that the aircraft that started your video lost its fuselage and tail before the landing. That makes the landing REALLY impressive.
Apr 19, 2019, 06:56 PM
Everything's A Composite
Knoll53's Avatar
I have gotten in the habit of doing all of the video editting the same day as I fly, you know, while the interest is high. My videos are simple, mostly just chopping up clips and rearranging. Now that I am using a Gopro at 2K and 4K, I find 1080p just laughably blurry. Consider a camera with and external mic jack, which allows a live commentary. I have found this really useful for taking notes in the field...........if you're into that sort of thing.

Hobby King probably has something great.
Apr 20, 2019, 07:49 AM
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EdSoars's Avatar
Thanks Kent!

Now that spring has sprung, irrigation ditches need cleaning, and the field needs cultivating, and irrigation lines need cleaning out and setting up, and seeds need planting, and et cetera ad infinitum, my sailplaning time will be a bit limited. Why are days limited to 24 hours? Chores should let up in June.

I can bolt my old Canon G12 on top of a far older aluminum hard hat and call it good for now... when I next get to the slopes.
May 19, 2019, 07:55 PM
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EdSoars's Avatar
The Hemp Max Plank is close to launch. O boy, do I love having only 2 functions to program! Not to mention only two sets of linkages, control arms, etc, to match up. Compared to recently programming a full house electric 3m thermal ship, this is almost boring. Almost.

But then, the Hemp Max will absolutely SUCK at flat field hand launches. Horses for courses. Nice to live in a place that DEMANDS a large selection of site-specific aircraft, all within minutes of the shop.
May 21, 2019, 06:47 PM
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EdSoars's Avatar
Edging closer to launch. ..

Final configuration:
Span 72 in
Taper 9 to 5.63 in (the Golden Ratio!)
Area 522 sq in
Aspect Ratio 10:1
Weight 51 oz
Loading 14 oz/sq ft
Airfoil PW 98

Structure: hemp fiber spar, epp foam cores, wing skins: hemp fabric w/ 7 mil mylar.

The vacuum bagging got away from me. I should have used thicker mylars, so the skins look like they were cured in a hailstorm. Not pretty, but the launch will decide the issue: pretty is as pretty flies.

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