El Cheapo semi flat plate section

[Tom Harper]

My design goal is a minimal cost aerial photography platform. One approach to that goal is a wing built with 3/16" artist foam board. The stuff does not bend at all so I have used a straight line approximation of a common NACA camber line. The LE is a parabola and the TE is tapered. After assembly the thick spar is sanded to make a smooth transition at the max thickness point. This is not too far from a Jedelsky section.

I made a chuck glider to try it out and then a .15 powered model. Both fly fine.

I am interested in comments from others who have tried similar approaches. Is there any data on this kind of section?

[Sparky Paul]

Some time ago I bought a 20x30 sheet of foam board to see how large a plane I could get from that single sheet.
It sat around for awhile.. then I in December I picked up an E-Flite "Tribute" 3D aerobat, which was about the same size, made from Depron™ with CF spars for reinforcement.
I added all the equipment; motor, battery, servos, receiver to the foam board, and the same stuff to the "Tribute".. The "Tribute" weighs 14 oz ready to fly, the foam board thing 22 oz.
I didn't fly the foam board.. It would be a brick relative to the "Tribute".
You might look at Depron™ use in the 3D aerobat world for your plane.
It is a lot lighter than foam board.

[BMatthews]

Anything will fly fine if it's light enough. However when you start putting a camera into the model that's when it'll get heavier and problems could occur.

Sure it'll fly but you will probably find that it flies better with a proper airfoil. The sharp leading edge and flat area leading to the spar is going to have a higher drag separation bubble on it virtually all the time. That's going to slow the model down a lot which may not bother you since it's a camera plane. however it means you'll probably need more power. Here again that's not really a problem unless it means you need to go out and buy a new engine.

The stall charactaristics in the landing approach may suffer. A fair test for this is to add some ballast weight to the 15 powered model to bring it up to about a 3.5 lb weight and fly it like that. Don't do it all at once. Build up the weight in 4 to 6 oz steps and try a flight or two at each step to be sure it remains flyable. If you can hit 3.5 lbs and it's still fairly nice to fly then you've got a winner. Probably not the most efficient model but if it gets the job done then it's a success. Our models aren't flying cross country nor are they burning so many hundred lbs of fuel an hour so we can withstand a few poor efficiency factors without loosing any sleep over it.

[Tom Harper]

Bruce & Sparky,

It weighs 43 ounces with 3 sq ft. That's 14 oz/sq ft. It is not an aggressive flyer but it is not a slug either. I was going to shelve it after this weekend but I think I'll do what you suggest. How much weight it will handle looks like a good test.

I'm sure a built up wing or folded depron section would be superior. The question is whether this section is adequate. And, whether it is easier and cheaper to build than the alternatives. It might be worth the effort to perform the weight test and then substitute a conventional section using depron or fan fold.

[BMatthews]

It's certainly easier but you've got a lot of compromises there. First off the spar system in the 15 test model is fine for the present weight and quite possibly even up to the 3 1/2 pounds I suggested as a maximum (try some sharp pullups from dives and steep banked turns at full throttle to test). But the spar is working strictkly in a bending mode which means it's highly stressed when laid flat to fit the airflow. A larger and heavier model is going to stress such a spar design to the limit and depending on how much payload you want to carry quite possibly let you down.

A built up wing is going to offer a lot more options and strength. If you want to stick to foam board what about a flat bottom folded wing? With foam board you can force it into a curved shape by cutting slits through the one layer of cardboard on the inside of the desired curve. Space the cuts about 1/2 to 3/8 apart and then force one side down into the foam near the cut so it retains a tiny bit of bend at the cut so it overlaps the other edge when pushed together as in when beding the foamboard. Now when you curl the foamboard with the slits inside the curve the board should form a nice reasonably smooth curl. You can use this to form a nice flat bottom airfoil with a single piece of foamboard that starts at the trailing edge folds over at the leading edge and then curves nicely into a smoothly graduated upper curve and back to the trailing edge. Near the leading edge the slits need to be spaced at 1/4 inch for the first 15% of the chord, then at 3/8 up to the spar at 30%, 1/2 inch to about 60% and then 3/4 from 60% to the trailing edge for a 10 inch'ish chord. A few ribs and a spar of vertical sectioned spruce will provide the skin spacing and give you a much more strong wing to boot. The stall charactaristics of such a wing should be much better than the inverted V shape.

[Tom Harper]

Bruce,

Thanks for the comments.

The design problem here is for a minimal cost, minimal effort airplane. I want to photograph areas that are covered with volcanic rock and/or creosote. Built up wings usually get shredded. And airplanes don't last long. I started this design project when I got tired of rebuilding my Telemaster.

You are correct about the spar. It needs to be continuous through the fuselage and the center 18 inches should be plywood. That is a weakness of this model - the wing has folded once. I have now reinforced it externally. The triangular cross section does give it stiffness. However this is not an aerobatic aircraft. It has to get a camera aloft, cruise in a stable manner and get the camera back on the ground in one piece. It would be nice to get more than one flight out of the model without repair but that is not necessary.

Some efficient airfoils are flat from the peak camber to the TE. The Gossamer Condor was one. Benedek and Jedelsky offered others. I have grooved the leading portion of some foam wings but I am not sure that the resulting series of flat segments is any better than a single straight, flat run from LE to max camber. That, in fact, is the question being addressed here. If I can't economically provide a smooth, continuous parabolic surface wouldn't I be better off with a clean straight line approximation. It seems that two segments and a single, smooth transition would be better than a number of short breaks.

I priced 3mm Depron and it looks like a wing would cost close to $15 . The present wing is under a dollar. I am sure that a full envelope or a continuous parabola would offer less drag. The question is - how much? An indication might be found by determining the glide angle of the chuck glider then replacing the wing with a curved plate of the same camber and measuring the glide angle again.

I may have time to try that tomorrow.

[Sparky Paul]

Here's a real ambitious fan-folded plane.. 24 foot wing..
Good images of the construction technique.
http://www.rcgroups.com/forums/showthread.php?t=307945

[adamdb]

Tom,
Years ago there was a foamboard kit for 1/2A glow engines called the Folding Flyer. I had one with a Norvel .061 and it flew fine at 5000 feet in altitude. I agree that it is probably much "draggy" than a normal airfoil, but if an .061 will fly it at 5000 feet, it isn't too bad.

The FF used strapping tape wrapped around the wing from tip to tip to help the stiffness. I usually put a strip at the leading edge, one at the trailing edge, and one at the bend location. I used 3/4" tape for these models, you could probably use 2" tape. The FF did not use a spar at all, so with the spar and the strapping tape I am certain this would stand up for your purposes.

I constructed several models out of foamboard with a lot of success. Then I discovered corrugated plastic and didn't look back. You might check on the SPAD (Simple Plastic Airplane Designs) forum on this website and on www.spadworld.net. I know there have been other efforts to build a camera plane with SPAD techniques. A SPAD might also be durable enough that you don't have to rebuild so often. The construction cost would also be similar to foamboard.

Good luck,
Adam

[BMatthews]

Good point about the area you fly being rough.

The faceted slit form approach is not as good as a proper airfoil but it's WAY better than the inverted single surface V you're using now. More importantly the folded over flat bottom form provides you with the room to fit a proper spar. Apparently my first thought that the spar would be a problem is correct so you may want to consider the full form just for that reason.

The Gossamer Albatross uses a form with a flat, or was it actually concave(?), rear section but the forward section was carefully designed to flow the air onto that section. Just because one airfoil uses a feature doesn't mean that feature is desireable in isolation.

[BMatthews]

Just a thought, if you're just looking for a big, slow, easy to fly and easy to fix camera model what about the big pink styrofoam delta wing by Crazy Herb? Cheap to build and if a chunk gets taken out by a branch you just scab in a block of foam and carve to shape with a sharp bread knife....

Lots of inspiration at http://www.crazyherb.com/ .

[Tom Harper]

Agreed, the Gossamer Condor used a curved LE. I'm not convinced that the straight line approximation is any worse that a FF with span wise stringers on the LE. Anyway that is what I am investigating.

The big delta works great. It is the ideal solution for a large AP model. I made mine with half inch foam board, parabolic LE and an angled dihedral break that provides wash out. This one was 7ft span. I'm just starting another one now (3rd). Notice that he uses a triangular section with a flat over the spar. There are a couple of commercial designs that also use triangular sections. These are straight line approximations of the envelope.

Anyway this experiment is an attempt to get something a bit handier than a 7ft Delta.

[Tom Harper]

Adam,

SPAD is a possibility. But, like a lot of the suggestions I get, I haven't had time to try them all. I gotta fly this one first.

The straight line approximation of the camber line does offer some advantages if the drag cost is not too high. So, I plan to run this investigation a bit further and then use it or move on to something better.

[Tom Harper]

Bruce,

I was out taking GPS readings at the target site so I snapped this shot of the 'field'. It's not the worst - it's just tough to avoid the creosote.

[BMatthews]

I see what you mean. What about a SPAD wing? The corrugated plastic is quite tough and slitting a few lines will still let it bend into a close approximation of a Clark Y so you can bury a decent spar. That stuff is tough as nails and adds it's own strength to boot.

[Tom Harper]

Bruce,

I need to take another look at SPAD. I think I initially rejected it because of weight and possibly cost. But, maybe it's time to reconsider.

[BMatthews]

Cost is pretty cheap at about $10 for a 4x8 sheet even if you DO have to fold it. And it's a lot tougher than the cardboard clad foam.

[adamdb]

Tom,
When you consider fuel proofing for the foamboard, I don't think a SPAD solution will be significantly heavier. An 8'x4' sheet of 4mm coro costs me around $9 at the local Regal Plastics so the price is right also. I usually use an RNAF (search the above forums) style wing which is a combined 2mm/4mm hybrid wing in my combat planes and it is plenty tough while being a little lighter than the all 4mm option. You can also build a fuselage to whatever size you need using the 4mm (again do a search).

I do like the idea of the foamy delta. Seriously cheap, fast building, and lots of wing area to carry a camera.

Adam

[Tom Harper]

Adam,

The white foam board has a plastic coating that is resistant to oil. I just wipe it off or use some household cleaner.

Will definitely look into SPAD.

Thanks

[Tom Harper]

Spent a couple of hours this morning working with this design. The problem so far is not lift. A question on this design is whether elevons will provide sufficient lateral control. The answer is a resounding no. In perfect trim the elevons will turn it but if there is any yaw error they do not have enough leverage to provide correction.

There are a number of solutions. Could use an inverted V configuration on the stab. This is the best but it is not feasible structurally. A V tail has cross control issues. A single aileron is attractive but I don't want to add drag to the wing. I think the best is a large rudder in the prop wash. It's easy to do and the model may ROG with it. We'll see.

[BMatthews]

Tom, there's nothing wrong with rudder and elevator. Just be sure you use enough dihedral to work with the rudder so you get the proper roll response. But with rudder comes the advantage of a longer fuselage so the rudder has more leverage to work with.

If you frequent those rough sites the biggest requirement is a model that is not damaged easily by bush strikes. I can't help but keep coming back to the Delta planform as a good compromise for that. The one piece structure resists tearing off bits, no fuselage to bend and break and an angled leading edge to sort of "ease" or "wedge" its way through the bush. And then there's all the crud you fly in. No wonder you're using a high mounted pusher engine!

The inverted V is really a non starter for your case unless you mount it up on twin booms with a high apex joint.

Nope, I think that if you don't want to use a delta then your best bet is closer to what you're using now but perhaps with a bit longer fuselage and either a mid mounted stab and fin off the low boom or a standard V tail. In fact a mid mounted tail with 3 tailbooms would be superb for you. One low like you have now and two off the wing's center section to support the tips of the stabilizer so weeds whacking the stab won't flex the single support may offer a tougher more survivable airframe. Something to think about at least...

With the ultra close coupling of the wing and stab you're actually not far off a plank style flying wing. Or perhaps you have more in common with a Dellane planform considering your low mounted and very large stab... or rear wing in the Delanne option. The problem with the current planform is that the stab is very susceptable to bush strikes and damage.

You mentioned that you didn't care for the delta option due to it's size. But what you had there was HUGE. Granted a delta requires more area than a conventional model but without doing the research I'm guessing that it's only about 1 1/2 to 2 times as much to make up for the lower aspect ratio. Deltas often have symetrical airfoils as well but if you only plan on using it for load carrying upright then there's no reason you can't use a neutral or positive pitching airfoil like those used on plank style flying wings. The MH110 being one example of such an airfoil. How big would a delta wing be if it was about 175% of the wing area that you intend your current design to be?

Ailerons do not add drag unless you use them and then they only add drag in proportion to how much they are used. It's a small price in any case. I'm not sure what you have against them other than the complexity of an extra surface to hinge. Don't bother with a single aileron. That is only a good option for a racing model that only has to turn one way and always is flying at very low lift coefficients. You're not in that mode with your camera model. Not that I have anything against lots of dihedral and rudder control but use it because it's more convienient if you must rather than because you think it has an advantage over ailerons. It doesn't. The control response from rudder control is quite a bit slower than from ailerons.

This sure is an interesting design exercise. I'm going to ponder this a bit more....

PS: How about some numbers to work with. Camera and control payload and coverage angles, fixed or pan and tilt internal to the model?

[Tom Harper]

Bruce,

Thanks for the discussion. My conclusion on Deltas is here:

http://www.rcgroups.com/forums/showthread.php?t=323919

I agree that the Delta is the best planform but the small ones just won't carry the load.

I've been using a left side only aileron - even on the Telemaster. These are not aerobatic aircraft. I did find that it worked better when I doubled the size of the 'flap' area.

Gotta go, more later.

[dtrip]

Use an Easy Star :P

[Dereck]

Tom
Have tinkered with foamboard some, and talked with others about it, a couple of years back.

What mihgt help you is that you can put a little curve into an airfoil, to end up with what could crudely be called a 'flat plate undercambered section'. to 'bend' the stuff, you make score lines in the underside, depressing but not quite cutting through the lower paper skin. This will allow for some bending, the closer the scroring, the sharper the curve, up to point, but enough to put in enough for a better airfoil than a flat plate. When happy, secure the curve by rubbing foam-safe glue - epoxy or wood glue - into the score-lines.

Might just get you the lift you need, also about as quickly repairable / replaceable as a flat plate

Controls - don't overlook rudder / ele. Set up right, it works fine for steering around type flight and is real simple to build / actuate

Good luck

Dereck

[Tom Harper]

Dereck,

Thanks for the comments.

I have installed a rudder in the prop wash. The weather has been too unsettled to try it out this week. All indications are that the rudder will be sufficient. Last week flew OK but it started a flat turn (either right or left - it's choice) and tightened until it spiraled down to the ground. The elevons would not correct the turn. It did pass the ruggedness test as it would land in the creosote bushes and sit there with the engine running waiting to be picked up.

This model is just a platform to test a number of features:

Flat plate approximation of airfoil camber line

'High' aspect ration wing constructed of foam board

Very short coupled stabilizer

Elevon for yaw and roll control

Of course any real engineer would only vary one parameter at a time, but foam is cheap and so far the effort is progressing reasonably well. The wing is strong enough when it is swell braced at the root. The short coupling with the huge stab also works very well. The elevon idea does not work at all.

The straight line approximation of a camber line carries the model but I have no way to make a qualitative judgement. What I may do is increase the wing loading, as Bruce suggested, and make max and min flying speed measurements. Then I will curve the leading of the existing wing so that the incidence is not disturbed and all dings, breaks, repairs etc. are retained. Then repeat the max/min measurements.

The other goal is ruggedness. I don't think this configuration is a strong as a Delta, but it appears to be adequate.

Thanks again for your comments.

[Tom Harper]

I added a rudder in the prop wash and left the other two in place. The triple fin looked cool. It also worked.

Flew the model last Sunday. The rudder gave good ground control even on the single wheel gear. The model accelerated nicely and took off in about three feet. The climb was strong and the control was more than adequate. The elevator throw was excessive making it touchy. It also still required a lot of up trim to compensate for the high thrust line.

During recovery from a shallow dive the left wing separated - my fault, the center structure was too weak.

Conclusions:

The configuration is stable and aerodynamically sound

The straight line approximation of a camber line worked fine. I do not have any qualitative data. However the model was fast and responsive. It did not display any sharp stall tendencies or other bad habits.

The weakness of the center section was due to cutouts for the tank and engine. Something that can be cured.

I will rebuild the fuselage sometime and fly it with different wing sections. But that will have to wait. For now I have enough information to move on to rev 2. Below is a photo of this model, without the center fin, and an early one of it's successor: