UltraLight Body Design

[schaney]

I have no personal experience with Dacron. Althought the site http://www.gaboats.com has given me some ideas. Oh ya, they use both Kevlar and Dacron.

At $7-10 a yd for 72" wide, 3.7 oz Dacron, price doesn't sound that bad.

[GPW]

Schaney , I'm working on the Vintage aircraft construction TD... Very similar to your TD... Thinking just a nice thin canvas covering (or similar) , possibly with a 2 0z. fiberglass cloth over that for little weight gain and maximum durability... Inexpensive and easy to patch , should the odd puncture occur ...
Thinking on an aluminum frame such as yours , a good contact cement should adhere the fabric easily to the frame... If kept in a garage , like old planes are kept in hangers, the covering should be good for many many years ...

I also had the thought of padded decorative panels , like a quilt (sorta) that could be Velcro'd to the frame for a simple light interior ... Sewn in pockets could hold small gear , personals , eyeglasses , etc.

[schaney]

GPW, My initial thought is a "floating" fabric cover, something like a waterproof urethane or PVC coated polyster. A fabric cover would make it easy to sew in windows and doors.

Then as noted some form of inner liner for heat retention and condensation control.

[GPW]

So like a big TD "cozy" ... That's a Cool idea !!! You doing all the sewing yourself or subbing it out ? I'm guessing you could use snaps to attach it .. That would be easy Maybe the same for the inner liner too eh ?

[schaney]

For prototypes I normally sew them myself, when it comes to products for sale, I farm it out to the experts.

For the interior wall I'm thinking velco or snaps. For the exterior I'm not sure yet.

[GPW]

Great you're able to do the prototyping fab. yourself ... You have a big machine or just thinner fabrics to "test" the shape ??

[schaney]

Yes being able to test out ideas yourself is very helpful. My current commerical mahine is good for 6-8 layers of canvas although it can have feed challenges. I plan on upgrading, looking for a industrial walking foot machine.

[GPW]

WOW!!!! That IS a Serious machine !!!! Great to have the right tools eh!!! Small wonder your work looks so Good !!!

[schaney]

Thanks ... Yes the right "tools" make all the difference.

[kennyrayandersen]

I’m not trying to be critical – just a couple of things to think about:

You originally ask about a fixed skin. Generally when we refer to a fixed skin (in engineer land) we mean that it is attached. That’s actually where some of the strength CAN come from as the ‘panel’ can react shear loads (though it’s often not designed that way). It’s not that cheap to build out of Dacron, or other aircraft type fabric. You could substitute a long-lasting latex paint for dope (Ragwing aviation does that on their planes) -- that would cut you finishing costs way down. How are you going to attach the skin, or are you just going to drape it on, dope it and then shrink it? If you actually look at the .025 aluminum (your panels are small) it would probably work, be quite light-weight, and far more durable than the fabric (not to mention it would look tremendously cool buffed out). Also, aluminum is not that expensive, especially if you are actually comparing it to aircraft-type covering (which is really quite expensive).

The load on a TV from a tear is aerodynamic drag and friction (tires, bearings etc.) The flat back is nearly the worst shape possible (a flat plate normal to the direction of airflow is the actual worst) – you’d be better off to run it backward. In real life it may not make that much difference though depending on your TV. If the TV is big and brick-like an aerodynamic shape would be not so much benefit; if you car is small and relatively aerodynamic there would be more impact. As stated by others unless the weight difference is significant (in the 300# range) there won’t be that much gas savings unless you are towing it someplace that has a lot of hills.

There are several ways to build really light-weight. Fiberglass, foam core and fiberglass is probably as light, and certainly more structural than the frame and skin you have laid out (Andrew’s Pico frame would be enough to support a composite body whereas with this concept you will very likely need to frame the entire thing – not that that is necessarily bad).

I think you have to look at what you goal is – to build light-weight? Are you trying to save some gas? Are you trying to find a ‘cheap’ way of building light? Do you want to build something that will last? What kind of trade-offs are you willing to make (cost, manufacturing, lifecycle etc). No matter what you do, it’s a compromise in some way or another.

Now, having said that, it’s not that it won’t work, it will, but it may not be particularly robust/weight compared to some other methods. Also, I really wouldn’t build this way unless I had a garage to store it in because I think the weather/sun is going to be hard on it (you could cover it though).

[GPW]

I think it's a "given" that a fabric covered TD would be best garaged when not used ... All the old fabric covered planes (mostly Stearman's and Cubs ) around here are hangered' , metal planes sit outside ...
But being so much lighter , should be easy to roll in and out of the garage , and could make a great office or getaway (doghouse ) whilst in the garage ...

[schaney]

kennyrayandersen, you misunderstood what I meant by "fixed body". My soft sided trailers normally use a folding tent unit, this design has a "fixed" body that doesn't fold closed.

I agree, you must understand your criteria and make the appropriate trade-offs to achieve your objectives. Being well versed in many building methods, IMHO this type of skin on frame design is a very cost effective way to achieve ultra lightweight bodies. Can you achieve a similarly sized composite structure of the same weight? Sure, but a what cost difference?

Yes I agree, the tall flat back isn't ideal from an aerodynamic standpoint. Also for me, storage out of the sun is just a given.

[GPW]

Schaney , I don't really see the tall squared off back as an issue really , especially with the nicely rounded front of the trailer as you've depicted in post #1... The most drag comes as the trailer enters the air stream , and that's a turbulent flow anyway since you're pulling it behind another vehicle ...not a problem unless you're going supersonic... and you can use little aerodynamic tricks to help the flow off the back ...
For what you're building , and what with your abilities to sew... I could see a form fitting tent-like covering that snaps on at the edges... Simple and replaceable if the need arises ... Just an idea !!!
Sure would be light too ...

[kennyrayandersen]

The most drag comes as the trailer enters the air stream , and that's a turbulent flow anyway since you're pulling it behind another vehicle ...not a problem unless you're going supersonic... a

This actually is incorrect. In the front the air piles up creating kind of an artificial boundary that is fairly aerodynamic (of course it is better if it is smooth, but that a different deal). The real problem is the aft end. NASA did some experiments with a doctored up van (AeroTruck E-38096) which was an investigation into truck aerodynamics. What they showed was that the front had little effect on the drag, but the back had a bunch. They were able to show that just putting an aerodynamic bobtail on the truck had a huge input on the drag (some people are now suggesting that big rigs be modified by putting bobtail doors on the back), while changes to the fron only have a minimal impact on overall aerodynamics.

The little vortex units can help a bit (I guessing that’s what you are referring to), but nothing like a full bobtail and in some cases not much at all. Also, I’m guessing the OP isn’t going to glue vortex generators all over the back of this thing, but that’s just a guess – cost issues as well as tearing up the fabric as the buzz in the wind). A teardrop seems like it would be quite similar, especially when towed by a smaller more aerodynamic vehicle. Having the flat back is exactly the worst configuration and the NASA data only looks at highway speeds – you really don’t need to be going that fast to have a big aerodynamic effect – it starts kicking in north of 40 mph.

If you TV is big and brick-like, as I stated earlier, it doesn’t make that much difference, but if you are driving a smaller, more modern and aerodynamic vehicle, it could easily mean a few mpg difference.

[GPW]

Can't argue with NASA huh ??? Still , better than towing a cube eh .. I guess If I were going for a pure aerodynamic shape that would be a concern ... many possibilities , but to trade a few mpg for practicality , I'd take lighter weight over aerodynamics ... Just me ..
Thinking Schaney's ultralight would be slick enough ... we drive slow ...