Airframe design - Payload design first, or fit payload to airframe design?

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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For small systems, Mission, then Payload, then Platform(aircraft). The other way around is a lot more difficult and expensive.
What Mike said
Cheers guys. In that case, I'm going to shelve the plans I have now (except for when I get bored) and get to work assembling the payload. I think we have a decent idea of "Mission" already.
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
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
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.
shouldn't you choose your airfoil somewhere in there?

not trying to trip you up, just wondering were airfoil selection would go
Airfoils get very weird, very fast! The low Reynolds numbers of RC planes change some of the effects that are present in full size airfoils. For a sport plane, a simple flat bottom airfoil will work. A lot of the sport planes are about 12% thick. And a lot use an airfoil that is close to the NACA0012. Generic.

If you are really concerned with low drag, I've heard that an 8-9% airfoil is optimal for lift-drag ratio.

But an airfoil this this thin will fly very fast, and you will have to design the whole plane to fly much faster than most sailplanes. This means a different motor and prop combination for more airspeed.

Thicker than 12% will cause a lot of drag, although if it is semi-symmetrical, it will be forgiving as to angle of attack. Do you want a forgiving, trainer like behavior (but inefficient, requiring a lot of power to power it through the air)? Do you an efficient airfoil, but with a "Driving Miss Daisy" stable glide, with no tip stalling? If you want a fast, efficient airfoil, you're going to have to plan a heavier plane that penetrates the wind. These are very different styles of flying.

The reality is that the airfoil cross section is not as critical as in a full size plane. The surface area of the wing, and the aspect ratio and whether the tips are tapered, probably makes as much difference as the airfoil profile. Tapered wings try to emulate a low drag elliptical front edge, but they tip stall. Do you want your UAV tip stalling at glide speed? No taper, and a decently large aspect ratio will give you a lot of lift and stable wing tips.
I've heard it said that triangular wings (jets) pretty much offset any airfoil cross-section that is used.

Longer aspect ratio main wings (5-7) give better lift. But tail feathers are typically a lower aspect ratio (5).
I guess that the thinking is that the main wing should stall before the tail feathers. Typically, the tail feathers are non-lifting (symmetrical). A lifting tail means that the plane will have to be trimmed differently for different speeds (not impossible, just a complication). Y-tails are studly looking, but harder to trim. An SD8020 is symmetrical, and can be used for tail feathers.

So, I guess, more than the choice of the individual airfoil, is to design enough wing surface, choose a wing that is not too thick, avoid symmetrical airfoils (they have no neutral lift), avoid tapered wings and tip stalling, and think a bit about how fast you want your machine to fly. Build a few different main wings with different airfoils, and try them out. See which type of flight you like. And, of course, build a strong spar into the wing, so that your beautiful airfoil will not fold under stress. flyingfoam.com has about 15 standard airfoils. Research them, and try out a few. Don't believe everything you read.
Looks like the decision has been made for me!

One of my co-workers is an FPV flyer and has gifted me his old airframe. It's a lot like an easystar, but a little smaller and a fair bit more battered (mental note, take duct tape off the wings and replace with something better!). It's got enough space in the nose for the autopilot kit and radios etc and looks to be a stable design, so I may be able to have something ready before the new year.

Mind you, it may take longer if I need to do any modifications/repairs to the fuselage to repair previous damage.

Really good of him though. I must buy him a beer or two for that.
James, Hamish, all!

Seems we all have similar goals here.

im in the same boat, and probably at the same step as you guys.
I have been thinking about this for quite some time and lately have been thinking about putting the motor in the back (pusher) rather than the front. to counter this, the simplest thing to do is put the battery in front of the wings CG (in the nose essentially) to counter the weight and balance the plane.

another plus side to this is that if you crash, nose first (most of the time), your electronic components will be safer somewhere behind the wing CG and your battery should be dense enough not to get damaged.

Thoughts?
PS, also thinking about making wings out of balsa encasing very thin solar panels to increase flight time, wings will be wrapped in clear plastic wrap.
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 !
Yeah, I'm also going to put my power in the rear (lol) so i can put a quaint little camera up front.

i'm curious about solar panels, i'm certainly not putting any on my design just because of cost, but just how thin/light are they?

and to Mark below, I'm also planning on launching my plane of a bungee or just chucking it real hard discus style (if it can take it) but not beause it's a glider, because I'm hoping to make a blisteringly fast model which will result in a high stall speed :D

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