Best Design for Airplane Structures

Hello tcorn137

Are you building to a commercially available set of plans? If so, advise please.

Please also advise the plane size = weight, length, wingspan, motor size in HP, expected ceiling, max speed, and number of occupants (max).

That will give prospective repliers a better understanding of what you are building.

You ask <"....Which is better for stress and fatigue purposes, a single thicker piece of material or several thin pieces bonded or riveted together?....">

At present nobody could inform you.

Both metal and plastic parts are liable to fracture failures, often with little warning.

It is sometimes not realized that aircraft metal sheathing is NOT Aluminium, but very high-strength Aluminium alloy, especially made to flex, whilst resisting both stress fractures and corrosion.

Laminated and glued structures may be made immensely strong, but failure of such has occurred, and in a plane that is generally fatal.

Rivets are good, but do present all those localised stress points, along with their holes, (punched holes cause more stress fractures than drilled holes with slightly countersunk on both sides of the sheet metal), and of course they would need to be of certified airframe type, and placed/riveted with the correct tools, generally pneumatic. (You normally need 2 persons to complete a proper riveted joint, and both persons may need to have personal Certification to do that work).

Wood is a natural product, as such liable to stress fractures without warning.

For that reason Wood parts are often laminated, glued with proper airframe or propeller adhesive, then certified part by part ready for installation.

Once all parts are fully certified, and each has received a certificate from the airframe inspector, the plane may be assembled, at which time a further complete inspection is required, to ensure the airworthiness of the craft, and the proper Airworthiness Certificates issued.

Your intended engine must also be Certified as suitable for your aircraft, then shall require regular recertification at a fixed number of hour intervals. All bolts must be aircraft grade also.

You haven't listed your Location/Country, (You may do that in your Profile, and it will then auto-update through all your Posts), I have assumed you are in the US, but if you are elsewhere, it would be useful to advise both Location/Country, as requirements vary widely for Location/Country.

Reply here please, with

Kind Regards....

In general, the answer to your question lies in the structure of the material you are using. Wood, for example, is very anisotropic and has dramatic differences in strength and stiffness depending on the direction. A thermoplastic such as polycarbonate, on the other hand, is very isotropic; direction does not matter.

So, as a rule of thumb, when you have an isotropic material (i.e., plastic or aluminum) lamination is of no use (and even has problems as noted below). If you have an anisotropic material (wood), lamination in which the grain direction is alternated can give good strength and stiffness in multiple directions.

Even for wood, lamination will be harmful when not needed. For example, an axe handle is properly made of ash or hickory and has the grain parallel to the long axis of the handle, with any irregularities along the sides. Since an axe is only stressed along one plane, this provides great strength in that plane. Were you to cut the handle into slices and laminate it, you might get more strength in the other planes, but you would weaken it in the one you use.

The other problem with lamination is that adequate bonding is very hard to do. In the case of glue, there are ones available that are quite strong, but they must be used over the entire area of the lamination. Rivets and screws are not as good as the metal. Welds may or may not be as strong as the metal being welded.

Now, there are structures that benefit from flexing, but generally in those instances, you don't bond, but allow slippage along some surface (think of leaf springs).

Hope this helps.

Thank You.

It is leaf springs I was thinking about. There would be a slight amount of slippage between the pieces. In a high flex and fatigue prone application, this might work better. I am working with sheets of plastic material that is available at a low cost. I can choose to use thin sheets and group them together or use a thicker sheet on its own. The thickness of either case would be the same. These parts would be subjected to a constant slight amount of repetitive bending as the plane is flying. I am making an original design so there are no real plans to work from. My question is more of comparing the structural integrity of a single piece of material to using multiple pieces that are joined together. With multiple pieces, each layer can flex a little better than the thicker piece. Cracks forming in one layer will not travel to the others. There would be a slight amount of slippage between layers that may be beneficial. On the other hand, there are extra stresses placed on using screws to hold the structure together. Drilling holes cause micro fractures that may cause problems. But again, going back to the leaf spring idea. I trying to gain some insight as to which approach may be superior. Should I consider multiple layered sheets or a thicker single solid one. This same concept should apply to many aspects of engineering design. In making bridges and other high fatigue applications, is there a preferred way to hand repetitive bending and stress. Thank You for your help.

Tom Cornell

The flexural strength of one thick piece of material is greater than that of several unbonded sheets having the same overall thickness. Consider a sheet of thickness t and width b. The plastic modulus, Z is b*t²/4 and flexural failure occurs when M = Z*f_y (where f_y = yield stress of the material).

The flexural strength of one sheet of thick material compared to n sheets of 1/n thickness is n. Two sheets half as thick have only half the strength. Three sheets of one third the thickness are only one third as strong assuming each sheet is free to slide relative to its neighbor.

If the sheets are thoroughly bonded together, they act as one and would have the same flexural strength as a single piece having the same total thickness.

Is it passenger plane? in that case you are suposed to be qualified Aeronautical Engieer otherwise only you can fly the plane yourself.

"Is it passenger plane? in that case you are supposed to be qualified Aeronautical Engineer otherwise only you can fly the plane yourself."

That is simply not true. I was a Technical Counselor and did pre-cover inspection of many home built aircraft in the 70's and 80's, working closely with FAA engineering offices in the process. No where in the FAR's is there any statement mandating an engineering degree as a prerequisite for designing, building or flying these aircraft.

L. J.

I'd definitely consider both options but the followings need also be considered:

• How much flexing each components will be subject to?
• How good a quality of the material is you are going to use?

What about aircraft grade aluminium, can you use that or is it too heavy?

As far as flexing is concerned, I do not think a forcibly bound (laminated) metal bits will necessarily be your best choice over a one solid piece, providing you sized it properly! If they are forced to stay together (by either through riveting or screwing) it gives you hardly any better flexing than the single piece. When it starts to give away it will not be necessarily easier to detect any defects either compared to a single (heavier-gauge) piece. In a situation like this, I suppose, weight reduction would also be a considerable part and force you to make a choice.

So, using the lightest and strongest, yet flexible, material is obvious, but so is reliability!

Meaning, you'd better do more research because this blog can often give you no more than vague ideas, which sometimes can lead you to good answers but sometimes does not.

Since you're set on laminations, you might want to look into composites and/or aluminum honeycomb. There are some folks in the Columbia River Gorge and in California who do this sort of thing very well. You just have to Google for the skunkworks.

I agree with TVP45. It seems Carbon Fiber works very well in aircraft. You say this is your own design, and I'm curious. Are you a pilot?

A modern aircraft make extensive use of Carbon/Kevlar/Glass composites, Titanium both pure and 6Al4V alloy, 2024, 2014 Cu Al 7075 Al Zi, 6061 Al Mg grades of Aluminum alloys, of course, all these qualified under relevant AMS specifications. The most important criteria are design optimization to derive the maximum benefit using the unique characteristics of each material.

To design and manufacture even a very simple aircraft, require extraordinary technical skill and knowledge of materials and manufacturing processes, thorough knowledge of analytical tools like FEM, not to speak of huge capital requirements.

For instance:

• Most of the composite are thermosetting resin matrix prepreg materials, requiring storage at minus 80 deg C for a shelf life of about 6 months. Toolings are technologically intensive. Autoclaves require for the manufacturing are very expensive. Clean rooms are required for processing these materials.
• Titanium is another wonder material. Very good strength to weight ratio. Super plasticity is natural property. Extremely complex shapes are possible due to its super plasticity. Diffusion bonding is eminently possible; range of welding technologies can be employed. Most of these processes are done at elevated temperature, possible only in vacuum or in neutral atmosphere due to extreme affinity of Titanium to combine with Oxygen and form destructive oxide. To make use of the dream properties of this metal air craft manufacturers have gone to great lengths to create muti million ultra sophisticated facilities.
• Aluminum based fabrication is relatively inexpensive. Still require state of the art manufacturing facilities back up by precision welding, heat treatment & finishing process facilities approved by respective statutory aero nautical agencies.
• All materials on an aircraft, wood including, need to be covered under aeronautical standard (Aeronautical Material Specification "AMS" in US)

All said and done manufacturing an aircraft or helicopter how ever simple, require thorough knowledge, lot of resources not the least, a very brave heart! It cannot be done based on few books. The technology requires dedicated team of highly knowledgeable engineers backed up by very skilled team of manufacturing technicians.

Best of luck!

I don't think he wants it highly technical. What's more there are aircraft-kits available to build your own.

Surely, there still must be available good basic aircraft grade materials available which should enable him to build one.

If he's to use such sophisticated materials and the process along with it, you suggest, then no one would have ever built one out of his own effort.

Since the original question is about "building a plane with various plastic and aluminum pieces that are available" my answer is quite relevant. The question was not about assembly of air craft from kits.

As far as aviation technology is concerned there is nothing like not highly technical. The ready to assemble kits manufacturers would have complied with the mandatory flight worthy requirements.

Air worthy materials is certainly available. It is only beginning. Highly regulated mandatory procedure laid to covert them to air worthy components is the issue.

Incidentally it is these concerted technical requirements that has made aircrafts highly dependable and safe.

I agree with you it may be possible to assemble from knocked down kits, merely by strictly following to the letter the instruction of the module supplier, but than it cannot really be termed as own effort to design an aircraft

This discussion has gone way off course. I am getting my private pilots license. I have been interested in flying for years. I am quite aware of the many fine pre-made kits that are available. I also do not have the kind of money needed to even consider one. I do have available to me a selection of various metal and plastic materials that I might be able to design with. I have a tech degree in electronics and a reasonable level of engineering skill. I also have a library of airplane design materials. I do not wish to enter a discussion about all sorts of outside issues. My question is a basic engineering one. I have been reading these discussion groups for a while. It seems like a person can not ask a simple question with out a bunch of jokes and snide comments from the others. In the end, everything is being said EXCEPT addressing the original question. I have seen this happen in 9 out of 10 discussions I read.

This is a basic question of engineering. I wish to design parts for a project that may be subject to fatigue from constant bending and stress. I am thinking that several thinner layers of material screwed together at several points may be superior to a solid piece of the same thickness. I am trying to gain some advice from the groups. Each layer can actually bend easier and may not crack as easily. The strength comes from using multiple pieces. If a thick piece forms a crack, it will travel through the thickness leading to a failure. With thinner pieces, this can not happen. Cracks in one layer will not cross to the others. A single piece may have a slightly higher stiffness. I am thinking there might be a safety issue here. I am asking for the experience of others. I have seen building designs where sections are layered. I have seen a 'sandwich' type of construction for composites and such. The choice is between thick solid beams and supports or ones using multiple pieces bonded together. Are there differences that I should be aware of.

Thank You,

Tom Cornell

Simple, straightforward answer to your original question is impossible. The best direct answer is yes/no/maybe. Out of thousands of parts on a typical airplane each must be considered individually on its own merits.

Are you talking about wing spars? longerons? formers? bulkheads? doublers? triplers? control fixtures? firewalls? engine mounts?

Each has its pluses and minuses regarding material and method of manufacture. You really need to be more specific.

PS - I would daily in aircraft modification and every bit of my work is thoroughly reviewed by several levels of authority for completeness, accuracy and engineering feasibility even after 30 years around aviation.

Sorry, I should have been a little more specific in my last. In this wing structure you describe, is it a traditional wing? ie, with spar, ribs, longerons, intercostals, control surfaces, fuel tanks, etc?

Or are you alluding to a single piece solid structure (possibly laminated)?

How big is it? Is it a NACA standard airfoil? What kind of loading do you expect? What provisions do you need for control equipment? Fuel delivery? Electrical (for required lighting)?

Tom. I happen to have several sets of plans for asst aircraft design. The particular design that appeals to me the most is the Amphibious design by George Perrera of Sacramento Cal.. The Osprey Two. He has also designed the GP-4, a very fast air craft design. His plans call for construction from wood, foam and fiberglass, with some metalic hardware. I am not aware of any aircraft design that is constructed from only one material. If you are planing to construct a simple design, the Pietenpole is a good place to start. There are a number of good designs out there to choose from. What you choose should reflect on your area of expert experience. Remember this, however. When airborn, there is no place to pull over for repairs. As you seem to be leaning toward metal laminates, I suggest that you consider that "It is impossible to inspect the interior laminates for fatigue"

TMF

I agree with Hooker. I have 20 years of aircraft repair under my belt and I don't care if you think some of the answer are snide. The biggest question that has not been answered "is your aircraft a remote control". If it is then have a great time if not you should build anything for the purpose of carrying a human into the air. Your are too stubborn to take advice and out of your element.

Sorry for yet another answer that you wont like.

Mike

I absolutely agree with you.

That is why I said at the end of my first comment to you - you'd better do a research than follow this blog - that can sometimes help you but more often it won't!

This is a said fact and you hardly, if ever, get a sensible answer to your questions and problems.

As far as my qualifications are concerned they're along the same line as yours. And I myself used to work quite a bit with metals including aircraft grade aluminium, not to making plane components, but to build enclosures for our electronics circuitries we used to developed for customers for their mobile applications. My boss-man used to say, 'it is a better choice than going for a cheaper grade that may or may not be able to handle vibrations so well'.

I am however, not familiar with plastics and other composite materials which on the other hand I do believe can be laminated and used more successfully to absorb vibrations than metal could (Forget the bridge concept here!).

You will need to do some homework on this, but:

Look up in any engineering desk reference the Youngs Modulus and Shear Modulus of the materials you wish to use. Then you'll need to calculate the required thicknesses to acheive at least 2X your worst case scenario in each. There may also be a reference to laminated thicknesses.

It is true that laminating will offer resistence to material failure, but in many cases the bonding between layers can be your first point of failure due to non-uniform flex. A "leaf spring" design may not have the same structural strength because multiple layers acting individually lack the synergy of laminate multi-directional loading.

Finally, once you have your required thickness worked put, will your materials be light enough to fly? (It sounds odd, but heavier aircraft can actually take more stress than lighter ones).

You are placing yourself in a bit of a disadvantage in that your are trying to design an airplane around available materials you have sitting around rather than choosing materials best suited to your design based on their properties - which is the tried and true method.

Check out Phoenix Composites at Falcon Field, Mesa, AZ (airport ID is FFZ). They assist homebuilders and chances are - whatever your idea - they've seen it.

Also, Bellanca aircraft: They use wood laminate for the wing structures and may have the strongest wing in the industry. Also, older Bellancas often sell for less than the cost of a homebuilt.

Caveat: Plasitcs and composite airframe materials have a limited life after which the entire airframe is junk.

If you build and keep it in the experimental catagory, there is pretty much no limit on what parts, accessories, engine types etc that you can use.

And if you build it in a hurry, it will probably kill you in a hurry.

Good luck.

Craig

To just answer your question - go with single thickness for plastic and aluminium lamination for wood is recommended. In this application the lamination would increase the work while the bonding or riveting would increase the uncertainty of the result.

There is basically nothing to be gained as per FVP45's comments.

Due to the high cost of destructive testing for amateur airplanes there is a benefit to keeping it simple and going with what is proven/known to work. While you may not be able to afford the kit now - you can afford plans. You need them. This project will take quite a bit of time. In time you will have more money.

If you want to build an experimental project that will provide years of fun in relative safety build a sail boat. Frankly your questions and comments clearly indicate you do not have the correct temperament or level of expertise to safely build an airplane.

The great difficulty with asking others' their opinion is - you may get them.

Best wishes,

Go investigate the Experimental Aircraft Association, EAA. You will find many many experienced members who can advise you. They have books on acceptable materials and practices and have designated members who can check for airworthiness. The odds are that if you say you want to build an airplane out of old beer cans and recycled take-out boxes, there is a member who has tired it and can tell you about his experiences. It's always best to learn from failure, other people's.

Do not be discouraged by professionals. The Wright Brothers would never have earned an airworthiness certificate, but they flew, anyway. When it comes to strange structures and materials, go look up the Horton Brothers, or the Goodyear inflatable airplane. Almost anything can be made to fly, if you are determined. Of course, it may not fly well. (Consider the V-1 "buzz bomb", which had a solid, one piece wooden wing, just a plank, and could be built for about 200 man-hours, yet was as fast as the early jet fighters. Unfortunately, it never made a landing you could walk away from)

What you say is true, with a caveat. I used to know an aerospace engineer who built all her airplanes out of old junk she salvaged here and there. I was pretty impressed until one night over a few beers when she began talking about how to crash without being seriously hurt - seems she went down fairly regularly. By the way, she went on to become a regular pilot and gave up crashing.

tcorn137 , I have built many parts for many aircraft over the course of the last 35 years. In my opinion, the material that offers the greatest strength and utility is "S" grade fiberglass in a matrix of appropriate 2 part epoxy.

Fiber orientation can be aligned to be coaxial with load paths thus insuring maximum strength with a minimum of weight, an important consideration for any application but especially so in aircraft.

Burt Rutan said it best when he said that had airplane manufacturers known as much in the 50's about composites as they did aluminum, they never would have chosen aluminum for aircraft.

Avoid carbon fiber. The benefits of using that material in a small prototype aircraft are not sufficient to justify the costs.

You can make virtually any shape you want with glass and thus optimize the aerodynamics of the design, something not possible with more conventional materials.

L. J.

Two issues not covered tcorn137 are galling and foreign materials.

To flex they must rub something. Aluminum and (most) plastics both gall.

Also unless you seal the edges with something very elastic foreign matter will get between them causing all types of problems.

You might test alternating layers of plastic and aluminum fully laminated. I don't know if it is possible to vacuum bag using gorilla super glue as your binder.

More information is needed to give more pertinent feedback.

As the information stands no you can not do better with laminates vs solid unless you bond with something that is stronger than the sheet stock matrix.

Brad

A few rules of thumb in homebuilding and design:

- Don't put an untried engine in an untried airframe. If the airframe design is your own, use a certificated engine.

- From Aircraft Spruce and Specialty, I think you can probably still get Rutan's manual for building in the moldless composite fashion. Don't swallow everything in it, but use it a a good guide. (It makes a fairly heavy, but moderately easy to build strucure.) If you decide to build in composites, get several books on using composites in aircraft structures -- there is a world of difference between designing and building for marine purposes vs aircraft purposes.

- Join the EAA, and meet the people building locally.

- There are books of not too much more than 100 pages that describe the design and building process surprisingly completely. One is "Light Airplane Design" by Pazmany. Another is "Elements of sport Plane Design for the Homebuilder" P.E. Bird.

- Don't laminate aluminum alloys with themselves or anything else. Don't laminate any anisotropic material.

- Laminate wood only if you know what you are doing.

- Composites for aircraft should have designed-in fiber orientations -- otherwise they are too heavy and weak.

- If you build a design that has not been built before in composites, read many books on composite construction.

- Your test pilot should have a minimum of 1000 hours, and should be experienced in test flying homebuilt aircraft.

- Highly-schooled aeronautical engineers have carefully designed and built planes that have gone on to kill test pilots. Burt Rutan was involved in the design of the BD5, which went on to kill many people. The Beach Bonanza earned its reputation as "the forked tail doctor killer".

- If you build in wood (a pleasant experience as compared to building in composites) use only aircraft grade spruce where required by the plans.

- Use only conventional materials: wood as used in well-built wooden aircraft, composites as used in well-built composite aircraft, aluminum alloy as used in well-built metal aircraft.

- Laminating a thicker piece of aluminum out of thinner sheets is a very poor idea, unless you have loads of equipment for fatigue and stress testing. The last thing in the world you want is a completely untested design using completely untested material configurations. Typically, the bonding agent has far different properties than the aluminum.

- For a first effort, always build a proven design using proven materials.

- If you have not already built a museum quality wooden boat or the like, I'd do that first for practice. The standards of design and workmanship for anything that flies must be far higher than for things that float or run along roads.

- You've been reading the wrong books if "the engineering and design material have not made it clear if one structure excels over the other." Stop in at a local university and talk with people in the aeronautical engineering department -- they can recommend some texts.

I have a choice between using single thick pieces of plastic or multiple thin pieces for the same thickness. Ordinarily, sheets of plastic have no place in a wing structure.

Also, a crack forming in any one layer will not continue to the others. This is incorrect. It might or might not -- it depends upon the nature of the materials and stresses.

Have fun, and realize people get killed doing this stuff.

Hello,

Good and enlightening discussion.

My question is how can possibility one building air craft as bobby using glass fiber etc, as suggested here, without having to depend on the following state of technologies suggested in these columns? I know it is impossible here in India, but if possible in US or Europe it should be interesting to get educated.

• Air worthy advanced composite structure is high capital intensive technology
• Auto clave for post curing is a must. Very expensive to buy operate and maintain.
• Cryogenic storage facilities to store prepregs.
• C Scanners for quality assurance of structural integrity.
• Tooling for advanced composite manufacture is technologically intensive requiring great expertise.
• Clean room with humidity and temperature requirements for fabrication.

I am under the impression that use of advanced composites and honey comb bonded airworthy structures are possible only by big corporate sector capable if huge investment. Is the suggestions are about wet lay method which has very limited secondary applications in aero plane parts fabrications?

It should be interesting to know.

Great questions.

Air worthy advanced composite structure is high capital intensive technology

Many composite homebuilt aircraft are not truly advanced composites. -- although some are. The Vari EZ, a Burt Rutan design, has been built in the thousands (I'd guess) and is composite, but not what I would call advanced composite. The wings are formed by hot wire cutting Styrofoam to the wing profile, and then very carefully fairing the shape, followed by filling the surface pores with microbubbles in epoxy. This is very carefully squeegeed on to avoid adding too much weight. Then several glass plies are laid up by hand, with fiber orientation according to plan. The layup is carefully hand squeegeed to remove excess resin, bubbles, etc. It is room temperature cured, and then the surface finish is made smooth by filling the cloth grain with more microbubbles in epoxy. One could easily sand through any high spots, so surface fairness prior to filling in the cloth weave is very important. After all the finishing steps, the whole assembly is typically post cured in a jury rigged oven. (For the resins used, something like 90-100 C is adequate, and the structure must be supported to avoid sags.

The resulting structure is heavy because the core fills the entire wing. However, these aircraft perform extremely well, (much better than the typical production plane) partly because of surface finish much smoother than a typical small metal aircraft, but mainly because the basic design is very efficient.

When I built this,

I needed a much lighter wing structure (than could be produced by the Rutan method), so built foam-cored vacuum-bagged flat panels on a plate glass table, which provided a very smooth surface finish. Then I used these panels as if they were plywood. Using a several-step, tedious process, I was able to avoid loading up the surface of the foam with excess resin. Using a similar process, I could have substituted honeycomb for the foam, but the cell sizes for the light weights I needed were too large to avoid inter-cell dimpling under compressive load. The entire wing, which was about 6 meters long, 1.8 meters front to back and .3 meter thick, weighed 27kg. The main structural tension and compression members (of the spar) were carbon fiber pultrusion, which were easy to work with, exceptionally strong and stiff, and not too expensive.

Some homebuilders work with prepreg, using vacuum bagging for consolidation and an oven for curing, but that is very much the exception.

With very careful, and rather tedious work, it is possible to make wet layups that are lighter and stronger than the alternatives generally used (wood, wood and fabric, metal). Making use of things like pultrusions selectively can also keep weight down. On the other hand, sloppy or careless work, or even good workmanship but using the standard Rutan moldless composite construction technique can yield a heavy structure. Make the workmanship sloppy enough, and the craft won't even be airworthy.

The most popular homebuilt planes here are built from kits, in which many of the pieces are made in more typical aircraft fashion (autoclaved, honeycomb core, carbon fiber). The builder is then mainly gluing together pieces and doing mechanical work, interior work, etc.

There are a great many homebuilt aircraft flying of more conventional construction (metal, all wood, wood and fabric, tube and fabric, etc.) Most of the well-built ones out-perform production planes. An FAA inspector is always involved, and the aircraft is inspected at several points in its construction.

Hello Blink

Just to advise your graphic

It does not actually show as a picture, so we are left wondering.

Kind Regards....

How bizarre.

I simply used a cut and paste, which I have generally done all along. Perhaps my graphics have not typically shown up on remote computers, and show up to me because they can be found on my machine. I've assumed that anything that ended up as visible in the editor window was uploaded with the post. In any case, this is the pic:

Thanks. I'll have to look into other posts in which I've added pics.

Hello again, Blink

Picture of Windrocket now visible in your latest post, thanks.

What is maximum speed, and was that on fresh water or salt water?

Kind Regards....

TEECORN (137) from your original posting here on CR 4 I have determined that you hardly know a hang glider from a parachute and even less about materials that are suitable for air craft construction. If you really want to fly something, I suggest that you begin with a hover craft. There are some really nice designs available, and at least one even flies with wings just like a real air craft. And there are easier ways to bring a terminal end to your life with less work involved. However, for the benefit of your family, I suggest that you consult your insurance carrier. You may learn something about what is hidden between the several plies of paper, discuised as fine print.

TMF