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Geodetic Fuselage Flop

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StarJar

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Here's another geodetic scale fuselage. The problem is when I went to put on the second layer of lattices, the bends were too great.
This will work eventually. Just a few problems to work out.

IMG_20161111_202240_965.jpg
 
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FritzW

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Looks good. Why did the first layer work but the not the second? Wouldn't they be the same just going the opposite direction?
 

rtfm

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Hi,
Hey - don't give up. I built a complete airframe like this, but out of foam planks. I wouldn't have done a second layer, except that the foam I had on hand was too thin (only 5mm) So I overlay the second layer going the opposite direction. It worked like a charm.

I'm not sure i still have the photos.

I'll post this now, go look for the photos, and update this message if I find them

Regards,
Duncan
[EDIT] I found them...
Templates RHS.jpgPlug strip 1.JPGplug strip 3.JPG
 

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choppergirl

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StarJar

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Looks good. Why did the first layer work but the not the second? Wouldn't they be the same just going the opposite direction?
That's a good observation.
The reason is, as best as I can tell, is that the first direction was aided by the shape going up, and back, at the nose.
The second layer had to go down and back which, from the top of the nose is a tighter bend.

Thankfully the nose does not have to be that steep, since lowering the height of the second and third bulkheads will let the canopy come down for better visibility.

So the sticks were trying to tell me something, lol.

That's how you go from dummy to pro in a hurry, lol.

To answer Choppergirl's question, they're store bought...Revell balsa wood. Which reminds me, I need to order another pack of 30, lol.
 

FritzW

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...a couple of questions that might not make any sense.

1) Are the problems with the 16th? balsa sticks going to scale to the full size sticks?

2) Do the sticks (first and second layer) have to intersect at a bulkhead? Couldn't the second layer sticks just find a natural curve they're happy with and intersect the first layer wherever it happens?

3) Would steam and/or ammonia bending solve the problem?

4) What's this airplane going to look like? ...the cockpit "pod" looks very interesting.
 

StarJar

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...a couple of questions that might not make any sense.

1) Are the problems with the 16th? balsa sticks going to scale to the full size sticks?

2) Do the sticks (first and second layer) have to intersect at a bulkhead? Couldn't the second layer sticks just find a natural curve they're happy with and intersect the first layer wherever it happens?

3) Would steam and/or ammonia bending solve the problem?

4) What's this airplane going to look like? ...the cockpit "pod" looks very interesting.
OK since you numbered them, I can't dodge any questions, lol.

1. I tried a full scale fuselage a couple years ago and learned two important things. First, rectangular crossection (lattices) don't like to twist (talking about wringing them like a mop when they are pointed straight) but square crossections will (twist). They get twisted a lot, so I went to square. Also square has more buckling resistance for a given quantity of material (weight).
So this was the first step in negotiating the nessary bends.
It appears the breaking point is very similar to full scale, maybe small scale being a little more fragile/brittle. My full size was 1:1 RF-4 fuselage with tighter bends than this design. The balsa sticks are exact scale here by the way.

2. This brings up another lesson from the RF-4; between bulkheads the diagonal will go a little off course skin-wise as they go between bulkheads. They con be concave or convex so their outer edge cannot be use for lateral stringer mounting.
If the lateral stringers are at the bulkheads and there is no lattice trying to 'push them out' anywhere along the line, everything will be fine when covered with Dacron.
This also guarantees the lattices will meet between each bay, to resist shifting or buckling.(They can be pulled together and glued even if one or both gets a little concave or convex). Also they have a little room to get convex, because the space of the lateral stringer is between them and the fabric.)

3. A spray bottle with water helped a lot on the RF-4 experiment. The bends will all be less than on the RF-4.

4.This computer image shows it pretty much how I expect it to be. Very similar the one you and I corresponded about, about a year ago Fritz..I'm still wrestling with it, lol.

IMG_20161107_012332_269.jpg....but with an engine on the front.

I think I might have missed a point. Get back to me with any more questions, okay Fritz?
 
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Aerowerx

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Is your fuselage flat-wrapable? That might make a difference in how the strips curve.

I am interested in this because I have been thinking along the same lines (pun not intended).
 

Tiger Tim

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Have you tried wrapping without notches in the formers? It's possible the members would rather lay just to one side or the other of where you're trying to force them.
 

StarJar

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To Aerowerks: I'm not sure what flat wrappable means. Obviously not by a sheet. By a rectangular strip?---- no it would have twists in it like a mop, in addition to flat curves.

To Tiger Tim; I understand what your saying. The strips can swing 15 degree or so either way from the notch if they had to (from a perfectly aligned notch). The excessive bending is a combination of fuselage roundness and bad notch path. I think I solved both of those for the next attempt.

Thanks both.
 

Aerowerx

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A flat wrappable fuselage is designed in sections. Each section is curved in only one direction and straight in the other. The curves are always circle sections, elliptical, parabolic, or some similar shape called "conic sections". Each section can then be covered with a flat sheet.

You can still get some fancy looking curves by making the sections small enough, and they can be either concave or convex so long as they curve in only one direction---no compound curves. The problem is making sure (from what I have read---haven't actually tried this yet) there is a smooth bump-free transition from one section to the next. Dan Raymer's simplified aircraft design book has a brief introduction to it.

Most wood boat hulls are flat wrapped.

I was just comparing the two pictures you posted above. It appears that the scale model one has some concave areas. Is that correct? It might be better to have it entirely convex like the foam covered one.
 

StarJar

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Oh I see..I think. Duncan had an interesting approach for a composite fuselage.

No there are no concave areas but there are areas that have much greater curves than others.
The cockpit is almost 33" wide to accommodate the bending. I knew that dimension was important because of my RF-4 attempt.

Everything would have been fine here, except for the area between the firewall and windshield. Some sticks there had to go too far up and then back down. I had it too high for good visibility anyway. For those lessons I'll gladly eat this first model.
 

plncraze

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Thank you for sharing these experiences. These examples seem very different from the geodetics in Sport Aviation years ago which recommended rectangular section cross sections for the strips and where the strips went around the fuselage a few times.
 

FritzW

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...Very similar the one you and I corresponded about, about a year ago
I thought it looked familiar. I'm glad your still working on it, geodetic construction is fascinating stuff.

I hate to mention the "FG" word, and I know it's been discussed before, but is there anything in the composite world that would work for what your doing?

Tow.jpg d26.jpg ...interesting fiberglass/CF tow structures
 
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