Apothekary

Greetings,

I am a student at McMurry University in Abilene, TX and I am currently doing my senior project as a physics major. I have already been approved to do a UAV design which is essentially just a RC airplane design. I hoped this forum could help me pick good materials for this project plane. It's primary purpose is to be a stable, relatively easy to fly aircraft with high tolerance to crashes. It also needs to be able to take a small camera and perhaps other instruments into the air.

With that said, I already have played with some RC airplanes from nitroplanes.com, and I have a good transmitter and receiver. I also Have some decent batteries, servos, and a few different choices for electric motors. I'm not really interested in internal combustion motors for this project, I'm really wanting to use all electric.

My primary objective is to design the aircraft. I'm thinking of basing my design on the A-10 Warthog, except I will move the airfoils up on the fuselage for more stability. I will need to make a mold of this particular aircraft, then use the mold to make my aircraft. Once that is done, I will make my changes that I want for the design.

My main question is, what would be the best choice for the material? Has anyone done anything like this on these forums?

Thanks, Aaron Ward

DMcQuinn

My Feedback: (13)

You can use fiberglass cloth and epoxy resin to make both the mold and the final part.

What is your likely all-up weight, ready to fly? A fairly small electric plane can be built at 10 ounces with depron foam, for example, but this will not involve molds and fabricated parts. A larger plane - like 2-3 pounds will be more in-line with fiberglass technioques.

Is the mold-making a required part of the project?

BMatthews

I hate to say this but if you are putting the cart before the horse. The A10 was designed to fullfill a mission based on a lot of factors. The weight of the engines and the main payload (the very heavy anti tank gun and weighty armor tub around the pilot) played a major part in the design of the basic layout. The designers looked at their payloads and other factors, such as battle damage, and placed the components, wings, engines and tail all according to decisions on how best to balance them in the final design. In your case you need to do the same thing for your effort and not just take a layout that is based around a lot of factors that are not pertinent to you. Espeically since the A10 has a lot of things about it that make for a poor model design. Think about pitch stability and the factors that go into making a model pitch stable. THink about where you intend to mount the motor, camera and other parts of the payload into the model and how the location of these items will affect the final balance point of your design. Think about the flying speed you expect to attain and where and how you'll take off and land. Use all these factors to decide on the aircraft's design look instead of picking something that you think looks cool. Because as it sits now that sounds like what you have done.

The same considerations come into play when you design the structure of the model. You need to pick a style of building that results in meeting the target weight, one that can withstand the expected rigors of flying and reasonable ground handling. Forget about making it "crash resistant" other than making it a design that doesn't get easily caught in grass and such things. Although a decently hard nose isn't a bad option. But you can do that through the use of a strong design based on triangles that resist the forces and direction that will occur in a typical nose in catch that best use minimal amounts of materials in ways that put them most directly under tension and compression instead of shear.

And also you will want to work with materials that you're most familiar with. For many of us this is balsa, spruce and aircrafy plywood. In your case you need to figure out which you understand the best and let us know if you expect to get advice that will prove useful. "Composites" sounds fancy but it's just another material. If you don't fully understand how to use it to achieve a light but strong structure then it'll either come in too weak or too heavy. Each choice of material carries with it the need to understand it's strengths and weaknesses if you want to get the most out of it while keeping the overall weight of the model to a minimum.

If all this sounds complex it's because it is. You're embarking on a project that requires a combination of engineering disciplines. Aerodynamics, structural and electronics. Each of these three is a complex topic all it's own. Many would consider trying to tackle all three at the same time in a short time frame a bit much.

One way to short cut these would be to figure out what you need to carry aloft and then just buy a suitable model to carry it. That way the aerodynamics work is minimized to that needed to actually select a suitable model. It'll leave you more time to work on the other factors that turn a basic model into a UAV.

Things you'll need to look at are payload volume, weight of the model and suitability for carrying the payload intended and likely a self stabilizing design that requires less control to remain in level flight. All of this begins to sound like a Graupner Telemaster or maybe a polyhedral glider with a fat fuselage substituted for the slender stick they usually use. If you go with something like that you only need to design a good fuselage. For the wings and tail from the glider just be sure to maintain the same tail length so the pitch stability is still as per the original design.

diggs_74

You took the words right out of my mouth when you mentioned the Telemaster.. We converted one to a "UAV" in about 30 minutes at the field.. It was harder to learn to fly using the camera/screen than it was to set up the UAV part.. I think we put about 3-4 flights on it that day with many taking turns flying..

Apothekary

Thanks for the replies

Matthews,

You are right, the A10 isn't the best choice for stability, however, I though if I made some adjustments to the fuselage and wing location, perhaps I could increase stability laterally and longitudinally. I know what you mean though, starting with a more stable design to begin with is probably a better idea with the limited time that I have. With that said, I will look into your suggestions.

As far as materials go, I am pretty limited on funding ($200), so I initially was looking into balsa wood and/or foam. Recently though, I found some carbon fiber molding kits and the carbon fiber kits such as this:

[link=http://compare.ebay.com/like/190326162997?ltyp=AllFixedPriceItemTypes&var=sbar&rvr_id=171133540147&crlp=1_263602_304662&UA=WXS%3F&GUID=6023c64212c0a0e2041213b2fe887b42&itemid=190326162997&ff4=263602_304662]Carbon Fiber Kit[/link]

The molding kits sell for around $50. By the way, all $200 can go to materials, because I plan to use servos, motors and batteries from previous models I have used in the past. I also have a good transmitter and receiver.

If I go with a carbon fiber design, I know this is going to mean a small model, even with more than one of these kits. I'm just trying to look at all of my options. Has anyone here done anything with carbon fiber? I have a friend in the Air Force who actually gave me a design that the Air Force is developing that is 100% carbon fiber. It's small and lite weight, with approx a 2ft wingspan and weighs less than 1lb.

My supervisor wants me to do something that involves making a mold and then using the mold to make the aircraft. So it's not so much about the design process as much as it is the building process and then data retrieval with the finished product. So the design phase needs to be very easy. However, I do want to show some of the design process on paper even if I don't do a design all my own (such as CG calculations, some of the integrals and derivatives that go with the various coefficients and what not). Most of the data will come from instruments that I am able to put on the aircraft (very small camera, thermometer, pressure, anything like that).

Speaking of payload, I don't think the aircraft will need to lift any more than 10oz. Most small cameras are well under 5oz, so really the weight of the aircraft plus power plant, and 5-10oz additional payload should be plenty. The question will be how to keep the aircraft stable with the additional payload.

Thanks for any and all advice.I really do appreciate it.

-Aaron

reaper31

My Feedback: (1)

i really dont think it would be nessesairy to build something like that out of carbon fiber. if you really have to mold it why not use fiberglass? that 31 dollar would get you about 48sq feet of cloth. btw that little tube of epeoxy in that kit wont get you anywhere. i say if you ahve to mold it keep it simple and use fiberglass not carbon fiber.
have a look at plasticareinc.com 4oz fiberglass is 9.38 per yard while CF is 49.95 per yard...just saying

DMcQuinn

My Feedback: (13)

I have made many parts with carbon fiber and have documented some techniques on my personal web site: http://webpages.charter.net/mcquinn/...s/projects.htm

I show how I made a CF propeller from a mold and also a CF landing gear. Both of these projects were to make rather small parts for a large airplane, but the techniques will be the same. The sailplane guys (I'm a powered plane guy) tend to do more light weight molded fuselages.

I made a CF spinner - about 4 inch diameter - using a female mold, but I don't have good pictures of that. I used two layers of CF cloth (about 6 oz cloth). for this one and it is very light.

DMcQuinn

My Feedback: (13)

Oh and by the way, I tend to agree with reaper. If you use fiberglass the cost will be a lot less and the resulting part will be almost identical in strength and weight. The techniques are the same, but fiberglass is easier to acquire and a little easier to work with. There is a certain bling to the look of a CF part, however.

Apothekary

Fiberglass would be almost as strong as carbon fiber? I don't really care about the look of the part, just that it is aerodynamically trustworthy. If fiberglass will provide a good structurally sound aircraft for less money I will definitely try to go that route.

How is fiberglass easier to work with?

BMatthews

With either fiberglass or carbon fiber the material needs to be used in a way that is appropriate to get the most from the material and uses the least of it. It's a delicate balancing act to achieve adequite strength with light weight. Carbon cloth in thin layers is very strong in tension but flexible when compression is trying to buckle the skin. So a supportive structure is needed to hold the skins in a way that allows the carbon to do it's job. Think corrugated cardboard as an example of this. On it's own it's just a bunch of easiy crumpled paper. But formed into two thin skins supported by the inner corrugations we get something that is very strong for it's weight. For your model this is the sort of thing you need to manage to do.

Generally by the time you get enough supportive structure to carry the skins adequitely the skin does't have much left to do. It's at times such as that where fiberglass will be "good enough". Especially if you're going to mold the whole model.

Also look into "honeycomb" panels or "foam core" as examples of double skinned panels. Now you can't MAKE a model from flat double skinned panels other than simple flat plate wing stuff but it serves as examples of now strong a thin core can make it possible to use very thin and LIGHT skins that would never be stiff enough on their own to produce strong light model panels.


Since you're stuck for time and that doing mold is part of the assignment you may want to opt for a simple flying wing design. Something with big surface area done with easily made molds. For the weight of payload you're considering I'd suggest a tapered flying wing of about 50 inch span with a root chord of about 16 inches and a tip chord of about 8 inches. The trailing edge can be straight or slightly swept back. The molds could be hot wired from blue or pink foam and the panels formed from two layers of 1 oz fiberglass cloth and epoxy with 1/8 thick blue foam cores between. Foam that thin will easily flex to match the shape of the airfoil with only a little pressure. And if you use a symetrical airfoil you can use set of molds to do symetrical top and bottom shells that then glue together to finish the model. The sort of way to do this to avoid complex and time consumning mold fabrication is that your wings would be laid up as a sandwhich as per;

Heavy mylar waxed with mold release wax as a surface finish mold skin
One oz cloth impregnated with resin.
blue or pink foam core sheet
One oz cloth impregnated with resin.
Heavy mylar waxed with mold release wax as a surface finish mold skin


This sandwich would be clamped between the foam beds cut to the shape of the wing and airfoil. Lots of weight in the form of stacks of magazines, sandbags or bricks would then ensure that the form is true. You'll want to stack a couple of hundred pounds onto the mold.

What comes out when the mylar finish skins are peeled away will be a light one piece top or bottom shell that is self supporting. All you'll need is a single spar that spaces the two shells apart so that they can't collapse in towards each other. I'm guessing that the two wing shells should end up no more heavy than about 8 oz. And likely less if you don't overdo the resin. It takes very little to actually fill the cloth weave. Especially when it's compacted under pressure as described.

Apothekary

Wow, I wish I had some of you guys around Abilene so I could use your expertise first hand.

Matthews,

You said you thought a 50in wingspan would be good for this? Do you think we could go a bit smaller than that? I'm not sure if my budget will support an aircraft that large. Also, how are you estimating these measurements?