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Learning how to build Lotus Seven replicas...together!
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 Post subject: widebody
PostPosted: Thu May 05, 2011 12:34 pm 
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As probably thousands of people have done before, I've been thinking about a "wide body" variant. Make the chassis wider ahead of the rear wheels and you can use a narrower track or get more cockpit room. Aerodynamics could be improved a bit if you were lucky, etc.

I'm thinking mid-engined here, for what it's worth.

The middle and back part of a widebody is easy enough - you pull the chassis out to the outside of the tires instead of the inside, put a small flat filler panel between the back of the cockpit and the tire, then another behind it, or wrap a flat section around back from tire to tire, just something to close off the back of the car below the beltline.

On the deck, just bend a flat piece of sheet metal across the top like Kurt's Midlana, except all the way across. No compound curves anywhere, and it looks pretty good on the Midlana; it ought to look about as good done wider. Basically, just toss the Midlana's fenders and pull the back out wider to cover the back of the car.

That gives you a full-width body from the scuttle to the taillights, in one fell swoop. And all single curvature, nothing unusual for a Locost builder.

Up front, of course, is where it gets rocky. I was looking at one of my old picture books the other day; it was about the old CanAm racing series. Some of those cars had front body sections that just sat on top of the frame, no more than a foot tall, sort of like C4 Corvette clamshell hood.

My sketching skills stink, so imagine something that looks sort of like this in the front, with flat Locost sides and a single-curvature Midlana-style tail:

Attachment:
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The problem is that even at Locost scale, that would be a really big fiberglass panel, and the bigger and flatter they are, the more likely they are to show the slightest warp, sag, or imperfection. Bodywork isn't one of my competencies, and since I hate it with a passion, it's not likely to become one, not at that level, anyway. I could make a mold off a buck and make a panel from a mold, but making a buck isn't nearly as easy as it looks. I know this from trying to make simple things like hood scoops.

In the old days you'd make a buck from stringers, lath, and chicken wire, then work in plaster or clay. In the not-so-old days you'd draw it on the computer, use the plotter to print patterns to cut foam slices, glue them together, and then use body filler or plaster. In modern times, you'd build a big CNC router or a CandyFab, and sanding and finishing would be minimal.

I admit the big CNC router idea is appealing; I could borrow the controller and steppers from my milling machine, and I probably have enough "stuff" laying around the shop to do the rest.

But then one of the Voices suggested something else... back in the *really* old days, automobile builders borrowed some techniques from the fledgling aircraft industry. Rather than beating a metal panel to fit, they'd make some wooden bulkheads, pull some thin strips between them... and cover the whole thing with cloth. Which would work great as long as you were working with a single curvature, anyway.

Hmm. Most of the stuff the Voices come up with has at least one major flaw. Now I'll have to start haunting some of the aircraft sites to see how that sort of thing is done.

Yes, you'd have bumps and ridges showing across the support structure. Yes, it would make painting "interesting." Yes, it would probably move with the wind, and I expect the tautness would vary with humidity and temperature. And it probably wouldn't last forever, either. And with my luck, it probably wouldn't work with the kind of curvature I'd want. But I'm keeping the idea in mind...

One downside that does come to mind is that with a normal height front tire, bump clearance, plus curvature over the fender, the hood would be quite high, in Locost terms, as well as going all the way across. It would probably feel a lot different to drive than a cycle fender car.

edit: I found this thread, from 2008, discussing basically the same thing. "What goes around, comes around!"
viewtopic.php?f=53&t=3583


Last edited by TRX on Tue Jun 21, 2011 6:00 pm, edited 1 time in total.

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 Post subject: Re: widebody
PostPosted: Thu May 05, 2011 1:10 pm 
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I used some "kerf cut" (for lack of a better term though I'm sure it has a real name) sheet a while back on a project, it might work for you.
Basically it was just really thin board with slots cut 1/2-2/3 the way through one side so it could bend easily and be a solid surface substrate.

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 Post subject: Re: widebody
PostPosted: Thu May 05, 2011 2:34 pm 
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'Tailwind' can probably give us more advice, but if you get up close to a steel tube and fabric airplane (my memories of an Aeronca are 40 years old) you will discover that the fabric is drum tight and stable - it never 'flaps'. It is sewn around the structure, shrunk (either with heat or 'dope' as I recall) and then painted.

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Build Log: viewtopic.php?f=35&t=11601


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 Post subject: Re: widebody
PostPosted: Thu May 05, 2011 4:47 pm 
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Quote:
I used some "kerf cut" (for lack of a better term though I'm sure it has a real name)


Heck, that's what I have always called it...

Quote:
the fabric is drum tight and stable - it never 'flaps'. It is sewn around the structure, shrunk (either with heat or 'dope' as I recall) and then painted.


I built some model airplanes as a kid, back when you had to cut out all the little bits from balsa and then cover it with "Silkspan" paper that shrunk when you wet it.

I'm sure the bulkhead-and-stringer underpinnings would be clearly visible, but considering how many people want their "carbon fiber" weave exposed with clear resin, I expect nobody could make too much fun of it. Those ricer "carbon fiber" parts always seemed like the automotive equivalent of yanking your pants halfway down your thighs and walking around in Scooby Doo boxer shorts anyway.

One interesting thing I've turned up so far is that the fabric weighs almost nothing - 3.5 ounces per square yard was one specification. Of course, the weight of the supporting framework would have to be added to that.


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 Post subject: Re: widebody
PostPosted: Fri May 06, 2011 7:55 am 
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I thought there would be more out on the net about fabric covering aircraft, but my google-fu must have been weak last night. I did find that some planes have the fabric shaped and sewn and pulled over the airframe like giant socks, they're called "envelopes" in airplane-speak.

I also found that the fabric is attached by glue, by sewing, or by lacing. Just wrapping a hood covering around and doing up the laces on the back sounds gonzo to me.

Mostly, what I'm interested is how much double curvature can be accommodated by the fabric. I expect I'll find it eventually. Apparently wood-and-fabric isn't very popular among builders, so there's a lot less information out there than there is for metal or composites.

Somewhere in another room I have a rigging manual for a Curtiss JNE, but I can't get to it while I'm still on crutches.


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 Post subject: Re: widebody
PostPosted: Fri May 06, 2011 10:24 am 
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TRX,

Think 'convertable top'. An aircraft fabric skin is similar, except that the fabric has been stabilized by shrinking and painting. You create compound shapes with a series of facets that don't have to be 'developable' because the shrinking process allows the fabric to adapt to facets that are neither cylindrical nor conic surfaces. If you wanted a compound curvature you could approximate it in tube and fabric as a series of radial segments, in the same way that school globes are covered with a paper map of the world. This picture of the DH88 Comet shows how it could be done, albeit for a more gently curved surface. There are many other examples.


Attachments:
800px-DH_88_Comet_G-ACSS_Farnborough_10_09_88R.jpg
800px-DH_88_Comet_G-ACSS_Farnborough_10_09_88R.jpg [ 64.63 KiB | Viewed 2346 times ]

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Build Log: viewtopic.php?f=35&t=11601
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 Post subject: Re: widebody
PostPosted: Fri May 06, 2011 10:57 am 
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Talk to Jack McCornack. He's an aviation guy.


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 Post subject: Re: widebody
PostPosted: Fri May 06, 2011 11:08 am 
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Gotta strap on my hiking boots for this one, its a loooong walk down memory lane to get there. ..

Maybeck HS was building a HUGE pyramid kite, able to lift several full grown humans off the deck.
It took 6 bodies to keep it under control in a relatively light breeze :shock: 8' tall as I recollect, bolt together sections of multiple triangles for transport.
Used cloth and "dope" to make the panels. ..
As I recall, the cloth was stapled onto the birch framework, pulled taut and wrinkle free as you went along with the stapler, then painted with airplane dope, it was a long time ago and my memory is, well. ..
As it was described to me (the way I remember it anyways) the "dope" cooked the cloth fibers a little and pulled the cloth tighter during the drying process.
What I definitely remember is it made a very nice smooth surface, rigid but not structurally rigid, like a single layer of fiberglass wiggles when you flex it but with less strength.
Convex curves were easy but the attempts to make concave curves failed. ....... probably more due to being a bunch of inexperienced students in Berkeley in the 70's than anything else. Might be an interesting method to make a mold. ..
Did I mention that airplane dope is really pungent? :roll:
Might also be an easy way to make the skin then backfill it with a chop gun for strength.

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 Post subject: Re: widebody
PostPosted: Fri May 06, 2011 11:36 am 
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Modern airplane fabrics are very easy to do. Polyester fabric, sewing needle & polyester thread along with a clothes iron.

http://www.polyfiber.com/stits/index.htm


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 Post subject: Re: widebody
PostPosted: Fri May 06, 2011 12:16 pm 
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What Tyrod said. For big simple curve areas, use metal sheet, but aircraft cloth (polyester in these modern times) shrinks nicely to compound curves; the framework will be the guide. Sew yourself a big "sock" to fit fairly snugly, slip it on, glue the edges, shrink as desired with an iron, apply dope, IV resist, and paint. Just be careful you don't leave any serviceable items underneath because the only way to remove it is to tear it off and start over.

Or...you could just make fiberglass fenders and drape sheetmetal betwixt them, a la the concept on my avatar. A body doesn't really need many compound-curved pieces. As you've noted, everything from the scuttle back can be simple curve aluminum sheet, and for the front, the only truly necessary compound curves are the upper outside edges of the fenders...like the Bugatti Type 32 "Tank". Or it could be made much like the Corvette nose if you made fiberglass (or aluminum) parts at the seam lines and sheetmetal between them.

Here's a modernized Tank concept, left over from a year and a half ago. If I were really making one, I'd curve the side panels so the sides were a bit cylindrical (for stiffness, but it'd help the looks too) but the Type 32 had flat sides and that's what I was copying.


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JTank1.jpg
JTank1.jpg [ 19.75 KiB | Viewed 2332 times ]

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 Post subject: Re: widebody
PostPosted: Fri May 06, 2011 12:35 pm 
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I should have known you guys would side with the Voices...

If you haven't seen a C4 Corvette with the hood up, it's basically a shallow clamshell. The bumper cover and side filler panels are are bolted to the chassis and don't come up with the hood. The hood itself is a big shallow clamshell, not nearly as much curvature as the airplane in the picture Jack posted.

So the fabric does shrink? Like I said, I'm coming up with bupkis for useful fabric stuff.


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 Post subject: Re: widebody
PostPosted: Fri May 06, 2011 4:39 pm 
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JackMcCornack wrote:
Or...you could just make fiberglass fenders and drape sheetmetal betwixt them, a la the concept on my avatar.


I've been keeping an eye on that. It reminds me of some of the drawings I once saw for the Hirondelle, the car The Saint drove in the original books. Your design should take care of most of the objections to the clamshell fenders and look good too.


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 Post subject: Re: widebody
PostPosted: Fri May 06, 2011 5:21 pm 
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I can't stress enough the versatility of modern day aircraft fabrics. Using the recommended paint & painting procedures, visually you can't tell them from composites. On the one job I did, I could stick my finger like an inch into the finished wing fabric and a few minutes later you couldn't tell anything had happened there. The memory is fantastic. But the principle advantage is light weight. If I were doing an exo, I'd seriously consider fabric for body panels in lieu of fibreglass.

Plus, unlike doped cotton, polyester won't burn on it's own. Once the heat source is removed, it stops burning. The one hugh downside is polyester is it's UV unstable. Without sun protection polyester will degrade very rapidly. Ray Stits develped 2 methods of UV protection, one is the conventional painting products for polyester fabric which will give years of serviceability (see my earlier link). The other is a lightweight process developed for ultralight vehicles (I use the term vehicles because the Gov'ment considers ultralights vehicles intead of aircraft. That way they can circumnavigate aircraft rules). It's less durable but is light weight.


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 Post subject: Re: widebody
PostPosted: Fri May 06, 2011 6:51 pm 
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Tyrod wrote:
If I were doing an exo, I'd seriously consider fabric for body panels in lieu of fibreglass.


Six of the seven Voices agree!


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 Post subject: Re: widebody
PostPosted: Fri May 06, 2011 8:42 pm 
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I struck out on homebuilt aircraft and kayaks, but I'm finding a bunch of bulkhead and stringer stuff on model airplanes, model boats, and full-size boats.


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