Aircraft Composite Teardrop

[angib]

KC is right - chopping out one flange of the floor beams in the centre is not one bit the same as putting a hole in the shear web. If you intend to laminate the metal tongue in place, so that both top and bottom flanges are continuous and the shear webs are laminated onto the tongue, then I agree that the tongue tube will provide as much strength to the floor beams as is needed.

On basic structural principles, shear webs are there to connect the top and bottom flanges and to carry the shear force, hence its name. They are not an addition to the top and bottom flanges - without the shear web connecting the top and bottom flange, there is no beam and no strength.

[mach4]

Based on the feedback here, the design is evolving. Here is the current thinking regarding the tongue. I've got the front wall and the front bulkhead providing support for the floor up front, as well as the tongue box. If I shorten the tongue just a bit and tie it into a 2x3 foam/fiberglass box structure using a milled hardpoint and a pin to attach, I can eliminate the spar cap/shear web in that area and I should then have adequate strength for the floor and the tongue. A single spar cap/shear web and the rear bulkhead provide the rest of the floor's structural design. I've added an additional one in the very back, which might be replaced by an 8" rear bulkhead rather than have the hatch go all the way to the floor as currently designed.

Note the detail for the tongue hard point. Using a pin to attach the tongue means I can remove the pin and slide out the tongue for storage.

[DMcCam]

Howdy Mach4,

One good thing about the Atma Travelear design is it tows great, at least ours does behind a Honda Element. Of course she's a lot heavier than what you're proposing and your TV will be lower so that will change the aerodynamics too. Looks to be a great project to follow.

Thank you for the link too good Sir.

All the Best,

Dave

[RogHodge]

Oh yes, this looks good.

Beautiful design, to be sure! Sourcing the foam was a pita down here. It never gets cold enough to need the 2"
Lowes had 2'x8' 1" sheets

Finding the balance between starting too early, and analysis paralysis plagues me regularly. I would advise starting sooner than later, "take my advice, I'm not using it"

I'm concerned about the point that the tongue enters the shell, looks fragile

[pchast]

A glued in Hard Point of hard wood to spread the compression
forces in the forward bulkhead would likely suffice. (imho)
With, perhaps, a Sleeve below to align things... like your
attachment turned 90 deg.

[mach4]

I'm concerned about the point that the tongue enters the shell, looks fragile

I think there is far more strength there than what appears. For example, here is the nose wheel support structure on the Long EZ, a fiberglass/epoxy/foam homebuilt aircraft with a gross take off weight of 1325 pounds (710# empty). There is additional strength that comes from the skin of the nose cone for sure, but this is the primary structure. (Shown in yellow in the first image and as built in the second)





Granted I'm not using the same high-density foam, but I'm using thicker foam to make up some of the difference. I'm targeting 350# empty, so max weight on the TD will never be in excess of 500 pounds, which means that there will be a tongue weight of about 50 pounds. Which means (assuming a tongue extension into the floor of the trailer approximately equal to the front extension) a 50 pound force acting downward at the hardpoint under static conditions. I'm not going off road and I'm not using brakes, so the forces should never be terribly excessive.

And I'm considering a "nose cone" extension to the tongue "just in case".



The problem is that all the structural design is TLAR (That Looks About Right), rather than any kind of hard calculation.

[mezmo]

Check out this Build Journal thread, it is quite similar to
what you are considering:

viewtopic.php?f=50&t=44293

Cheers,
Norm/mezmo

[KCStudly]

Thanks for the link Norm, that's the one I was thinking of when I wrote,

There was another formed composite lightweight not too long ago, but I think that also used a triangular frame (IIRC) .

And he did use the triangular style frame. (At least some of my memory is intact!)

[aggie79]

The problem is that all the structural design is TLAR (That Looks About Right), rather than any kind of hard calculation.

Although the teardrop was never built AFAIK, this poster of this "build thread" did very sophisticated calculations and analysis for a lightweight composite teardrop: http://www.tnttt.com/viewtopic.php?f=21&t=28315&start=60&hilit=finite. Even though I only understood maybe 10% of the conversation, it was an interesting thread.

[RAYVILLIAN]

Just a thought on the tongue why not run it back and clamp or bolt it to the center of the axle. that way where ever the axle goes the trailer on top of it will follow. You might add a bit more weight but I would think that it would solve any shear problem. by fastening it to the body it would stay square.

Gary

[dmdc411]

I think building composite panels is the best way to go. In sense that's what we're doing anyway. I've worked in aviation a good part of my life, been exposed to the experimental side too. I took what I've learned and figured out to build my teardrop. I found that even building as I did, it's too heavy. I used the stressed panel technique everywhere I could. I'll be below 900lbs. Next time I'll go with no more than 1/4" floor skins and 1/8" wall skins. Roof, 1/8" might be too light, but with aluminum skin s it maybe fine. Your on the right track, and not that much more work.

[mach4]

Check out this Build Journal thread, it is quite similar to
what you are considering:
viewtopic.php?f=50&t=44293

I did a lot of searching in working on the design phase of this project and somehow missed that one. A great read and a nice project. Thanks for the pointer.