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Oct 11, 2019, 09:02 AM
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Bounce 60" Plank


After three editions of the 1.8 and 2 meter Max Planks and the 1.8m PsiClone, it occurred to me that I was missing a lot of impromptu flying sessions on our local hillside, the one with the landing zone made of steep slopes, gnarly trees and immovable boulders. This area needs a shorter wing span and lots of bounce built into the fuselage and wing leading edge. So... the 60" Bounce Plank.

The second edition of the 72" Max had an experimental covering of hemp fabric that worked, well, okay. Not great, but it warranted another try, so I ordered another bunch of hemp fabric. It turned out to be burlap... I must have hit the wrong Order Now button. It's an open weave, which meant lots of epoxy would be needed to fill between the strands. And maybe a dimpled surface that would show through the mylars from the vacuum bagging. But that fabric was intriguing: it is REALLY tough, and the strands were stuck together so the fabric wouldn't unravel when cut. And cutting it was intriguing too; I ended up using metal shears instead of fabric scissors. It is TOUGH!

Then an idea occurred to solve the burlap's problems, hopefully without too much weight: add Q Cell filler to the epoxy for the spaces between the threads, and instead of mylars, use a sheet of aluminum roof flashing to keep the surface smooth in the vacuum bag, and finally, use a layer of 3 mil mylar on top of the fabric as a release layer between the aluminum and the epoxy. It makes a slick surface, adding a bit of tear resistance at the same time.

I wanted to try Peter Wick's new airfoils, the ones he developed for the 3 meter slope plank. The lift/drag curves are impressive for the Reynolds numbers range the Max will fly in: 100,000 to 250,000. But those airfoils are THIN! The first attempt had far too much variance in thickness, so I redesigned the indexing on the hotwire templates, and it came out just right. I wanted to use EPP for the whole wing, but EPP just doesn't cut well in thin sections, so blue foam went onto the aft half of the cores. It saves on the more expensive EPP at the same time.

Balsa could have been used for the elevons, but after looking at the lift/drag curves for the airfoils, it was clear that the performance would fall off if the trailing edge geometry was not accurate. Even a 1 mm thickness creates some squirrelly wiggles in the lift/drag curves. Planing and sanding six feet of balsa did not appeal to me, so the cores had to be cut accurately. The photo with the light behind the core trailing edges shows the results of accurate registration of the hotwire templates.

One core had some ugly corrugations in the blue foam on the top surface at the tip. I have no idea how they got there, but decided not to cut another set, just to see if the aluminum sheet will keep the surface true under vacuum. We shall see.

The cores and bottom skin went into the vacuum bag last night, using very low vacuum to avoid distorting the surface over the burlap. When I pulled them out this morning and uncovered them (it's always like Christmas morning as a kid!) the core surface was excellent! Not as good as a mold, but very nice for some sporty flying over rough terrain. The metal shears and a sharp knife were needed to cut the epoxied hemp around the edges... reassuring toughness!

I will calculate the skin weight per square foot later on... breakfast calls.

Core prep today for servos and spar, then into the bag again. It's 18 degrees outside, so it may be better to build an airplane than fly one.
Last edited by EdSoars; Oct 11, 2019 at 09:08 AM. Reason: wanted to submit as Build Thread, not Discussion
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Oct 12, 2019, 01:25 AM
Very nice and pretty innovative! I think you are heading in the right direction with the "blue foam". I have been using very light-weight EXP-foam (the cheap, light-weight home-depot stuff) for complete wings with ample reinforcement and it lasted at the slopes... even at 4Bft without a problem. So, you line up well I think.
Oct 12, 2019, 08:18 AM
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Thanks Eric! I also use the cheapest foam I can find at the local building supply. The blue Dow foam isn't the cheapest, only left-overs in my garage. I've been bagging under very low vacuum, and Bounce isn't a stressed-skin structure, so I don't need high-compression foam.

The main problem with styrofoam is skin separation. After trying pecked holes to increase the bond, but with little improvement, I thought to try a full- EPP wing, but those skinny airfoils prohibit using EPP further aft than about 70% back from the leading edge.

Yesterday's challenge was the spar. I was planning to use a vertical CF ribbon, 2 x 12 mm, but again, that airfoil is too thin. The spar may have to be laid-up CF tow in a channel melted into the foam.

I'm hoping for a light wing loading, under 10 oz/sq ft. Eight oz/sq ft would be even better, but that means keeping the weight to about 21 oz. The reason for a light wing loading is WINTER. If the Bounce needs a 10 knot breeze to stay up, I will freeze solid.
Oct 13, 2019, 07:28 AM
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I found some lightweight, skinny, tapered CF tubes in the corner of the shop. Installed with epoxy and Q cells they add 3 oz to the wing. The top surface will add another 7 oz, so the finished wing will weigh 18 oz. The wing area is 2.7 sq ft, so if the wing loading is to be less than 10 oz/ sq ft, the fuselage and 2-servo RC system will have to weigh less than 9 oz total. Should be doable.

The fin is the critical weight component now, so it may be depron over a CF spar.

This is a different animal from my normal go-for-speed, 11 to 15 oz/sq ft target wing loading, where weight doesn't matter nearly as much as landing survival. Fun!
Oct 13, 2019, 09:50 AM
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Plans!
Oct 16, 2019, 10:45 AM
I am sure you have a big advantage with having some decent slopes in the region where you live. That means that a little bit of weight here and there should be no problem. In The Netherlands most of the time one needs a floater-type-wing. As far as I can see from the drawings this can be the wing to take with you everytime you are close to a slope :-).
Oct 16, 2019, 10:50 AM
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I am close to a slope everywhere I go! My choices are light slope breezes, moderate slope winds and strong slope winds! So 8 to 15 oz / square foot wing loadings. I have discovered that a lightly built wing with lots of ballast doesnt survive many landings, so I build for the conditions.
Oct 18, 2019, 04:02 PM
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Elevon detail; 1/8" CF rod inserted at elevon leading edges to stiffen in torsion and transfer servo movement spanwise. Gap on top is only 2-3 mm, or less than 1/8" when not deflected.
Oct 22, 2019, 08:30 AM
That’s a nice detail flight performance will benefit from especially in moderate to strong wind-conditions.
Oct 22, 2019, 09:33 AM
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Got the wing top surface bagged, came to 22 oz with servos. 3 mil mylar "release film" works well. All up weight should be under 30 oz, so 9 oz/ sq ft...light!
Oct 22, 2019, 11:06 AM
27gr/dm2 .... perfectly fine IMHO
Oct 22, 2019, 11:16 AM
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Looks great! I'm very interested in how the new airfoils fly.
Oct 22, 2019, 01:14 PM
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9 oz/ft2 does indeed fly nicely in most conditions...most.
But 14 oz/ft2 flies in surprisingly modest winds, and the energy going into big aerobatics, even limited to elevon-only, is SO much fun!
Having only started into the heavier ships this summer, I'm learning some basics. One is that a clean, low-drag ship with tight control surfaces is important. Planks fly at low lift coefficients, and can stay up in light air as long as they can be moved around the sky without losing too much altitude. In other words, high Lift / Drag is more important than low sink rate... on a clean ship. And of course, I'm talking about flying slopes with lots of interesting topography to explore. Flat fields are something else.
Oct 22, 2019, 06:26 PM
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Gbreeze, I'm expecting noticeably better speed than the good old standby PW 51. I've built three wings now with Peter's Rennbrett series, omitting the root section. I use only the Strak 1 and Strak 3 sections. They are impressive! Thinner and faster than the PW 51, though not as good inverted. But I don't spend much time in the air upside-down!

I used Nur.exe to see what difference there would be in the cruising speed of the Bounce planform at 30 ounces, or 833 grams, with the Rennbrett to the 3m sections. The Rennbrett wing came to 29 mi/hr, and the 3M wing came to 33.2 mi/hr. That's a 14% increase.

But the flying is the proof. I'm hoping for some nice afternoons in another two weeks.
Oct 28, 2019, 07:30 PM
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Nice afternoons, HAH! We got a foot of snow on the ground last night! But it quenched the Decker fire that's been burning on the outskirts of town for the last two months, and that's a relief.

The top surface went on without a wrinkle-- in the skin. I did manage to allow some epoxy + Q-Cell glop to get into onw of the servo output arm pockets, and that took some micro-excavation to clear out. But the good news is that the hemp burlap-plus-glop created a VERY stiff, tough skin.

Well, swimming in the River is no longer very much fun, so there will be more progress soon.

ed


The 3 mil mylar provides a shiny, tough skin, but I have to wonder if it adds much protection. I'll try a sample wing section with the shiny side toward the epoxy to see how it acts as a release film.

It may be months before a crash test, er, flight test can happen. In the meantime, there may be a solution to one of my pet peeves at hand. I like nylon bolts for the wing hold-down: one #10 just in front of the spar, which is very close to the balance point, and one #8 about 25% in front of the trailing edge. But it gets frustrating at the flying site to pick out the nylon bolt stubs in the T-nuts in the wing saddle.

The solution is to bolt the wing to a "false" saddle that is in turn bolted through to the bottom of the fuselage. Steel bolts can serve for this, since the nylon bolts provide the stress relief. If bolt stubs in the false saddle don't come out easily, just unbolt the F.S. and unscrew the suckers.If I haven't left enough bolt protruding under the F.S., it's my own fault.

As soon as the fuselage looks less like a junkyard, I'll get some pics out.


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