First post, a question on plastics

Hi everyone. Hope I’m not too dumb by posting this - I did a brief search but couldn’t find anything quite specific enough. I’m not an ID student by trade, I do 3D/Graphic design, but I have a large interest in the field, so…yea, here goes :slight_smile:

Anyway, I have an aircraft design project where I have a high strength carbon fibre tube frame, covered with (maybe) plastic. One of the top three design goals is extreme ease of construction - the frame itself goes together like a plumbing set (tubes into PVC joints, joints covered with carbon fibre). Now, I want to cover the airframe with flat plates of plastic, for a faceted design. Other than wind resistance up to about 200 mph, the panels will not support structural loads.

So my issue is that I need an easily cutable, paintable (if not immediately opaque, then must be able to be turned opaque via painting), high strength plastic. It needs to be low cost (as much as feasible), come in sheets, and be resistant to avgas, oil, and possibly battery acid. UV resistance is also a huge plus. Also should be able to be secured to the Carbon Fibre skeleton via either epoxy or bolts (preferably the former).

So far it’s a toss up between ABS, PVC, and PC plastics. This is the first time I’ve actually examined plastics as an alternative skin covering, so I’m still a bit lost, but as far as I can tell, PC is the strongest, followed by ABS then PVC? I don’t know if PC is paintable though, and doesn’t it have problems with battery acid/gasolene?

Cheers everyone, thanks for any help you can provide. Maybe I’ll run the gauntlet and post design pictures later :smiley:

You’re correct that PC and ABS offer some of the best strength among the commonly available polymers, but you might want to reconsider using a plastic at all for the application you’re describing. You might notice that plastics are almost never used for external covering on aircraft, and this is primarily because they don’t do all that well under constant exposure to sunlight – the UV tends to break them down, leading to creep and brittleness after a few months. Painting can slow this process down, but not completely eliminate it.

PC and ABS are both take paint quite well; ABS is probably the better of the two as it is inherently opaque, cheaper, and has better chemical resistance. Painting a plastic for external use on an aircraft is asking for trouble though, IMHO – it’ll start scratching off in no time.

This is an interesting question. I wonder if using a stretched fabric “skin” like an ultralight that is serviceable would be a better solution? This company makes all kinds of crazy fabrics:

I would agree with hitch, that if you must use a rigid plastic, abs is probably best for your application.

airframe panel skins only see shear load so can be surprisingly thin.

Bill Lishman, of the “Fly Away Home” movie fame - training geese to fly, is famous for his ultralight aircraft: aluminum tube structure skinned with transparent plastic film, but, I never found out what plastic film it was. Very thin film.

If you are going for a faceted look however, the frame will need to express facets through the thin film skin. Actual flat panels will be a nightmare to seal, poor seals between skin panels will cause excessive drag, gross instability as well as failure mid flight. Bill Lishmans plastic film skinned ultralight was sealed with aluminum tape.

I can’t offer any help on the material question, but are you sure you want faceted aerodynamic surfaces? It’s possible to make a flat surfaced plane fly (F-117), but it’s certainly not how you would do it given a choice. And using carbon fiber tubes is OK, except for the part where you’re putting them into PVC (!?) connectors. The joints are the point of highest stress on a tubeframe structure, and if PVC is good enough, then you probably don’t need CFRP everywhere else.

Obviously I don’t know anything about your project, but traditional aircraft construction (spars, bulkheads, stringers, external skin, etc.) is probably easier to assemble than a tubeframe, is more efficient (lighter), distributes loads better, and is much more amenable to creating aerodynamic surfaces. In a really low speed aircraft (like an ultralight), you can get away with a lot of sins, but something capable of 170 knots is a real airplane.

Also, drag and lift forces at that speed are considerable. 16 times higher than they are at 50mph in fact. A Cessna 182 has over 200 horsepower and a well optimized shape, and it tops out at around 140 kts. Your faceted shape (and heavy airframe) is going to need a lot of power to go 30 kts faster.

Is this for fun, or something you’re planning to actually fly?

Er, sorry for the triple post, but can someone help me out? My previous post didn’t show up, so I hit reply again and now…well now I’m making a third post :stuck_out_tongue:
EDIT: Apparantly they’re not showing up because they have a word marked as spam in them (which one?). I’m editing my last post into this one;

Thanks for the help all.
pier, I’ve looked at cloth/plastic covering, and will continue to do so. There are other difficulties with those methods though, that I have yet to determine if they’ll be actually “easier”.

Scott; we’re doing faceted construction because we postulated that it would be easier for us or another home builder to simply cut out the desired shape and put it on the fuselage, no bending or jigs required - the only large “specialized” tool/knowledge required for construction would be a jigsaw and the knowledge of how to measure a line and cut it. It’s not the most aerodynamic solution, but it is the easiest to build (we hope).

As for the PVC connectors - you must have missed the part where I said that they were covered with carbon fibre for their real strength. No, we’re not relying on PVC plastic to hold the whole thing together - they’re simply there to make the tube connections easy and to provide shape for the carbon fibre covering. I would have to see, though, if the strength would be the same if only the outside was covered with CF and not the inside - I think it will be (basing this conclusion off of personal experience with another project and from research), but at the same time I look at sandwich construction and wonder about that second layer on the other side of the foam that’s used in that process.
Yet we’ll probably end up doing a lot of custom joint angles for our specific purpose - so we may just end up doing custom CF parts and not have to worry about the PVC. :slight_smile:

While you’re right that traditional construction is more efficient, I wouldn’t say it’s easier than tab A slot B construction. Cessna’s in particular, as they are all metal and would require specialized tools, knowledge, and a lot of time to construct. I threw out 200 mph ( I actually used mph instead of knots - that’d be only about 170 knots Smile ) as a top line - we may very well end up being less. Not discounting your point, it IS a good one, but we will be obviously be examining whether the airframe can handle the stress.

Covering PVC in carbon and relying on the carbon for strength just means you’re carrying your tooling around with you. And no, it won’t be as strong. You’re right to wonder about the sandwich panel example- a one sided sandwich doesn’t work very well.

“Only” 170kts? Using the Cessna example above, increasing its top speed from 140kts to 170kts would require increasing the horsepower from 225 to 400. Drag power increases with the cube of velocity. And your faceted body is going to be very draggy.

And honestly, these questions are best answered by engineers rather than art school designers. I hope you have some on board (engineers I mean). I’ve designed airplanes (worked on the Adam A500), and it is Hard. By all means post pictures, and don’t forget to pay your life insurance and liability premiums. :slight_smile: (Better yet, let someone else take that first flight.)

Thanks for the help Scott :slight_smile: I didn’t mean “only” as in 170 knts is “slow,”, I meant it comparatively to 200 knts. On the airspeed issue, our top speed (after just now remembering to look at the LSA certificate requirements) is maxed out at 120 knots, so…while still fast, it’s relatively slow.

As to the PVC conundrum, like I said, we’ll probably end up doing a lot of custom angles, which means custom parts, which means we’ll probably end up forming it ourselves and be able to make it a CF sandwich.

We do have engineers, although their areas of expertise are…not in this field. One worked several years at Boeing doing CAD design, and the other works for the Air Force doing missiles, I believe. However they are both quite busy, so most of the time I end up having to do the research and whatnot myself, which leads to many questions on internet forums and late nights browsing Google search results. Once we (I) do another redesign, I’ll put some pictures up if I can. But I’d take the first flight myself - if anyone should get hurt because of my mistakes, it should be me, non? :wink: Plus it helps being the only pilot out of the group…

And you did the Adam A500? I just Googled that, gorgeous aircraft! What exactly did you do with it?

Oh, and on the topic; I know there are UV resistant paints for materials such as zylon, which are also UV sensitive - how long could I expect UV-paint coated plastic to last? And are there any other alternatives? Cloth covering is relatively difficult, and I don’t think wrapping it in seran wrap will work as well :wink: Metal requires specialized machining, and I would like to avoid wood. Carbon fibre requires layups, and even though it could work, it’s probably over engineered for this task, and honestly with a frame/truss construction it’s not the best way to use the material. I went with plastics because they’re relatively cheap, easy to cut into shapes, and easy to glue/work with. But if it really does degrade so rapidly, I guess that option is out.

do the whole plane with carbon fibre. That way your parts all heat up and cool off at the same rate. Your tooling is also carbon fibre for the same reason.

The cost of the raw material is rapidly going down because of Boeing who is the largest manufacturer using it… and all the defense contractors. Hell even armord trucks get carbon fiber interiors. All the drones use it. Its relativly cheap to do one-offs and not terrible to make your own tooling. Its easy to finish and strong light and …

Its not too clear but it is cheap when you consider the material strength to weight ratio.

Paint is heavy so be careful what you ask for. Try to die the resin. Maybe with some experiments you could obtain an opaque fiber structure?

with a sister company to design engine I worked with carbon fiber to aid in the development of an aircraft for the military over the past several years. it is about to fly too and at that point I can talk about it more.