Aircraft Composite Teardrop

[mach4]

Greetings. I'm well along into the planning stages for a tear drop build. Haven't pulled the trigger on ordering materials and getting started, but I thought I'd share my design and construction thinking with the brain trust here on the forums.

My basic idea is to build a 5x10 teardrop using the general profile of the ATMA Travelear.

The thing that makes my project unique is that I'm planning on using homebuilt aircraft composite construction technology involving moldless fiberglass/foam/epoxy. I've pushed the numbers and I think I can build mine at about 300-350# empty. I'm not planning on using any wood in the project, except for some small structural "pads" for the mounting of the axles...and I'm not going to be using a frame either. The tongue is part of the floor structure and the axles attach directly to the floor (see image below).

A typical 4x8 sheet of 3/4 plywood weighs 75-80 lbs. A comparable composite structure of 1" styrofoam covered in 2ply BID (bidirectional fiberglass) on each side, will weigh 16 lbs. My cabinets and bulkheads will also be composite so there is significant weigh savings there as well. My tongue will be aluminum estimated to weigh 7-8# plus whatever the receiver weighs.

This kind of construction is extremely strong as evidenced by this image of bags of shot piled on a Long EZ canard.



The upper structure is a no-brainer as foamies constructed out of foam, canvas and paint are quite common and seem to survive nicely. Since I'm not going with a frame, but rather a monocoque design, the floor structure needs to be pretty stout. The plan here is to use 2" foam covered in 2ply BID with the equivalent of a shear web constructed of BID laid up at a 45 and spar caps constructed from UNI (unidirectional fiberglass) on 2' centers. This will provide the primary structural rigidity from edge to edge while the walls and bulkheads will provide additional structural rigidity and strength to the sides. Walls, roof, doors and hatch are 1" foam with 2ply BID per side. Structural bulkheads are 1" foam and non structural bulkheads are 1/2" foam, or possibly 3/8" nomex honeycomb.

The tongue storage box is an integral structure to the main trailer structure.

I plan on using torsion axles, half or full depending on factors yet to be determined. I plan on 13" alloy wheels and fiberglass fenders.

I don't planning on pulling 6g's and if something breaks it won't be at 10,000 ft so I think my design is sound and adequate for the intended use. If I do come in at 350# empty and run at 450-500# or so, that gives me a tongue weight of about 50#.

Some may say a trailer can be too light. That has yet to be proven, but if it does prove to be a problem, I'm prepared to add ballast tanks that can be filled during high wind situations. In fact, if you notice the area under the lower shelf in the front, this could be the front ballast tank easily and and a second one at the galley bulkhead would get me additional weight.

The intended tow vehicle is my 380SL Mercedes in which I installed a diesel engine. I get 33mpg on the open road, so by keeping the wind resistance and weight down, I hope to keep the tow penalty to a minimum. Plus the light weight means there will be plenty of excess capacity in the hitch.

Here are some current design images.

[noseoil]

Best of luck on the project. As far as "too light" is concerned, it is possible to end up with a sail and not a trailer. Think about passing or being passed by a semi at 75-80 mph and the wind load it creates on the sides of a vehicle. Nothing wrong with light weight, but there does need to be some stability and a low center of gravity to make it pull well.

I worked in the aircraft industry and we used exotics for cabinetry & interior panels (honeycomb, nomex, balsa-core, etc). It can be strong & light, no doubt about that, but there are limits to the "light" part of the equation. I'm sure others with more experience with foamies can shed a bit more light on the subject. Welcome & nice design!

[mach4]

Best of luck on the project. As far as "too light" is concerned, it is possible to end up with a sail and not a trailer. Think about passing or being passed by a semi at 75-80 mph and the wind load it creates on the sides of a vehicle. Nothing wrong with light weight, but there does need to be some stability and a low center of gravity to make it pull well.

I worked in the aircraft industry and we used exotics for cabinetry & interior panels (honeycomb, nomex, balsa-core, etc). It can be strong & light, no doubt about that, but there are limits to the "light" part of the equation. I'm sure others with more experience with foamies can shed a bit more light on the subject. Welcome & nice design!

I don't know what the stability will be, but a quick back-of-the-napkin calc shows that the sail-area to weight ratio is essentially identical to an unloaded semi-trailer at 20:1. Unladen tractor-trailers traverse the interstates on a regular basis, mostly without incident. A tractor trailer will have a higher CG and the effects of wind disturbance from an 18 wheeler will tend to be above the level of the TD.

Ultimately the effects come down to coefficient of lift and drag which are impossible to know.

[tony.latham]

I've seen several of Rutan's homebuilts in the air and on the ground and am surprised that this construction method hasn't come up before. Perhaps I've missed it.

Wind is a consideration, here's a link on a lightweight teardrop that was blown over: viewtopic.php?f=55&t=44431&start=120

Press on and please maintain a build log on here.

Tony

[pchast]

Then again you can easily add 1-200 lbs in your tongue box and galley.
That would likely be ballast enough and a low CG.

[warnmar10]

Following.

[mach4]

Wind is just another weather phenomena to deal with. I wouldn't drive into a hurricane area; I wouldn't fly into a thunderstorm; I wouldn't drive into a blizzard or severe icing conditions and so forth. Yea, it limits travel a bit but I'm knowingly entering into uncharted territory with a micro-light project and understand the risks (and rewards, too).

For those that are concerned about winds, here is a pretty amazing site that takes all the surface wind data and presents it in a easy to understand visualization - Wind Map

[lrrowe]

Very interesting.

[rowerwet]

the foam "tear" that got blown over had a very high profile and was rather boxy. yours should be fine with a low profile like you are showing, and a wide axle.
The Rutan designs fold the nose gear and sit with the nose resting on a small bumper on the ground to reduce the tendancy to fly away, Just like Rutan's cool airplanes, a foamie that light may need to be tied down. the foamie that got blown on not quite over was traveling on a day with wind warnings, a much better idea would have been to hunker down and even invest in some ground anchors (they screw or drive into the ground and have a anchoring rated in pounds)
The standing joke in the foamie forum is you need 300 lbs of beer to ballast the tear.

[GerryS]

Cool windmap! Just got added to my favorite.

[noseoil]

One more for wind.
http://earth.nullschool.net/

[mgb4tim]

What glass cloth are you going to use?

[angib]

I think that is only a monocoque in the sense that it doesn't have much of a metal frame - but it is still just a body-on-frame design made mostly from composite. It seems a real waste to use composites to imitate a metal frame - Rutan certainly wouldn't do anything so wasteful. All the floor reinforcement is only needed since a crude single metal tongue has been used. If the tongue was replaced with an A-frame that picked up reinforced points in the front panel and the sidewalls, the floor reinforcement wouldn't be needed.

A more immediate concern is that the metal tongue appears to be cutting those floor beams in half. Putting the tongue under the floor beams, which can then be continuous from side to side, would be much better.

Better still would be to modify the tongue box so that it replaces the tongue as a true monocoque. Here is a design I did for an all-plywood tear like this:

[mach4]

One more for wind.
http://earth.nullschool.net/

Nice find. I like it.

What glass cloth are you going to use?

I'm just planning on using standard bi-directional 6oz eglass. I'll likely use some uni-directional cloth for my "spar caps".

I think that is only a monocoque in the sense that it doesn't have much of a metal frame - but it is still just a body-on-frame design made mostly from composite. It seems a real waste to use composites to imitate a metal frame - Rutan certainly wouldn't do anything so wasteful. All the floor reinforcement is only needed since a crude single metal tongue has been used. If the tongue was replaced with an A-frame that picked up reinforced points in the front panel and the sidewalls, the floor reinforcement wouldn't be needed.

A more immediate concern is that the metal tongue appears to be cutting those floor beams in half. Putting the tongue under the floor beams, which can then be continuous from side to side, would be much better.

Better still would be to modify the tongue box so that it replaces the tongue as a true monocoque. Here is a design I did for an all-plywood tear like this:

My understanding is that monocoque means that all structural loads are taken by the skin alone, so maybe my working design would be more of a semi-monocoque structure (where the load is also carried by longerons and stringers, etc.) particularly if my floor reinforcements qualify as stringers. I don't see where you are seeing this as a "body-on-frame" design unless any structure supported with an axle is automatically this by definition.

You suggest that "the floor reinforcement is only needed since a crude single metal tongue has been used". I don't have the floor reinforcements in place for that reason, but rather to reinforce the 5' span between the walls. I agree that the reinforcements are likely not needed, but since I'm not a structural engineer, it's just as easy to put them in by way of over-designing and over-building - better safe than sorry, as it were... so as not to be broken in half on the side of the road.

As far as the tongue is concerned, the bulk of the load is being carried by the "spar caps" (sorry if there is a better non-aeronautical term I should be using). The "shear web" is used to support the spar caps. The bottom one carries the bulk of the load in tension while the top one is in compression (and of course the shear web is in shear). Composites do far better in tension than compression, so to keep the spar caps from buckling under load when supported only by polystyrene foam, the shear web is used to stiffen and delay this from happening. Under most of the span, the shear web is a "c" structure that bonds to the spar caps. Where the tongue intersects this, the tongue acts as a shear web preventing buckling of the spar caps at that point. Would it be better to have the tongue not intersect the shear web? Of course. It appears that it's not the end of the world to have an interruption in the shear web as Rutan's Long Ez central spar actually has a hole right in the middle of one of the shear webs.



I like your thinking on the monocoque tongue, but I'm concerned that the tongue box structure might have a tendency to tear away from the shell of the trailer. In my design I at least support a part of the length of the tongue with the tongue box, shortening the effective length of the cantilever.

I'm not terribly concerned about the strength - especially since there are tons of "foamies" safely traversing the highways with nothing but canvas and paint holding them together. E-glass has a tensile strength of 22,000psi, probably an order of magnitude greater than canvas.

Thanks for the thoughts...it pays to have critical analysis and other sets of eyes at this stage of the design process.

[KCStudly]

There is a huge difference between poking a hole thru the middle of the web on a spar or beam, than interrupting the fibers of the flange. The outer fibers are more highly stressed so you do not want to interrupt those.

What Andrew's design does is move the high stress point at the intersection of the tongue and cabin/tongue box back into the cabin structure gradually, using the radius shape and tapers. You are correct to be concerned about shifting the point of failure to the front of the TB or the front wall of the cabin thru the TB.

Don't rely on the successful results of previous foamies absolutely. I can't recall any that use the foam structure as a mounting point for the tongue. There is one wooden example where the tongue and axle attach separately to blocking, but most foamies attach on top of a std trailer ladder frame (yes they do stiffen their trailer frames up considerably, but the tongue to frame joint is already handled in steel.

Have you seen the triangular shaped frames of Alaska Teardrop or Andrew's ultra light chassis?

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