Morning all!

A long time lurker here finally breaking his silence, and choosing the topic that has been driving my crazy since I started thinking about building a UAV - Airframes.

I have a decent idea of what I want as a design for my UAV (twin boom, high wing etc), but every time I sit down to start working out specific designs, I keep heading down the same rat-hole with the payload. My original thought was to ensure that the design is modular, allowing the removal/addition of the nosecone to allow for the eventual use of an FPV pan/tilt camera arrangement, but early on this is not necessary, and I'm only planning on a simple, cheap camcorder-type arrangement.

What need advice on is whether to hold off on the construction until I can assemble the payload, then finalise the design of the airframe to allow easy access to the various components, or to just build the airframe to the specs I have now, then try and then try and fit the [modified] payload inside. Of course, this would risk having a shortfall in space, or a change in CG from where I want it to be...

Anyway, what have other people done when building their own airframe from scratch? Ideas are really welcome as it is really playing with my sanity now.

Oh, and if I get some decent designs up for what I have planned for the design, I'll get them posted up.

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  • Developer
    This is a very good method for RC aircraft design, I made my first glider design back in 1979, I used may of the ideas discussed here, but i also designed in crash resistant methods, that shed mass in rough to full on head in crash. I still fly this plane, I made the nose a bit too short, but helps when wings shed off in crash, it has a 3 meter wingspan, weighs 7 pounds bungee launched to 300' I use a EP 305 airfoil, it will penetrate a 25 mph head wind and floats well too. Have fun !
  • i'm in the same sort of situation , DYING to make a damn aircraft but I want it to be really optimised so I guess I should wait till i can get an precise weight estimate.
  • A couple of other things. With a pusher design, your motor or engine will be behind the wing CG, and you will need a longer or heavier nose to balance than if you have a puller design with the motor/engine out front.

    If you don't have an airfoil picked out, you can buy all different size foam wing cores from FlyingFoam on the web. You can size your wing to lift the payload that you have chosen. Put a fiberglass tube in the wing for strength, and sheet it with 1/32 balsa. Kite suppliers like GoodWinds have all sorts of fiberglass stuff. You'll get a light, big wing!

    Make sure that you pick out a motor/prop combination that will fly the total weight of the plane. The motor (and batteries, if electric) has got to be included in the mock up balances.

    Stephen from Tucson
  • Your question is excellent. What I've discovered from the standpoint of someone who has designed a few custom electrics, and made a lot of mistakes....
    1. lay out the components on a table, and try different configurations
    2. estimate total weight and pick wing area so that the wing will be able to lift everything (lbs/square foot of wing)
    3. pick a tail boom long enough to stabilize the wingspan, and big enough tail feathers
    4. plan a LONG light nose, decently shaped to reduce drag, with a LOT of hollow space inside, and many hatches
    5. ** allow the wing to be attached anywhere along a flat surface on the top of the fuselage: this gives you a number of CG choices that will work
    6. put heavier stuff near the proposed wing CG, lighter stuff further away (increases maneuverability)
    7. *** do mock up balance tests: hang gear from a stick, as if it were the fuselage
    If you can balance a mock up of the body with the equipment and the tail, and the CG is somewhere on the fuselage where you could attach a wing, you have a winning possibility. If you're ballpark close, then use the space in the long nose to move weight back and forth until you get a perfect balance.
    8. Think about the design, even if it balances acceptably. Could you change gear, and rebalance easily? Can you replace servos? If you crash, are heavy items going to destroy expensive items (cameras, gyros, speed controllers, Rx?). Make sure that your control surfaces are big enough to easily orient the plane. They can be overbig, and you can dial down your radio movement as needed.
    9. As you build the plane, do repeated CG mock ups. Especially after you get the fuselage built and the tail put on, but before you start to install any gear in the fuselage. Make sure that you balance as you go.
    10. with twin tail booms for a pusher, make sure that they are far enough apart to fit a propeller that will fly the plane!
    11. buy 30 feet of servo wire, and put servos where they are needed and balance, not where their standard wires will reach

    By the way, this is a challenging problem to solve. Most ARFs designed for nitro motors have VERY short noses, due the heavy metal engine. When you modify them for electric motors, the nose often has to be lengthened from 3-8".Most RC designs are not designed with a lot of nose room for gear, or easy access to the nose. Feel free to depart from the standard ARF designs!
    Stephen from Tucson
  • Moderator
    What Mike said
  • For small systems, Mission, then Payload, then Platform(aircraft). The other way around is a lot more difficult and expensive.
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