My next project
 

OK guys- here's the deal: I'm working on a BIG slow plane. I've been fiddling with it off and on now since about November or so, and I'm ironing out the last details now. I'm going to use it primarily as sort of a giant scale slow flyer, but I also want to use it as a cargo lift and a glider tow, but mainly just BIG and SLOW. Specs below. My question: what angle of incidence should I use on the wing? Gut instinct says 0 degrees, but I want to get some oppinions from the pros.

Name: eh, I haven't given it that much thought. Desn't really matter too much:)
Construction: Balsa, Ply, CF, Foam.

WING
Span: 144"
Chord: 24"
Aspect ratio: 6
No taper, diehedral, or sweep
Airfoil: Clark Y
Ribs, foam with balsa cap
18" center section permanantly mounted to fuselage, 8" above thrust center line.
Two outer halves joined on aluminum tube

FUSELAGE
Profile, foam core sheeted with balsa and ply, CF rods to reinforce.

TAIL SURFACES
Foam and balsa , removable for transport.

Thrust line and elevator at 0 degrees unless otherwise sugested.

Thanks guys in advance.
-Steve

RE: My next project
 

I would align the bottom of the wing at zero degrees to the long axis. This will give the wing a bit of positive incidence and in my experience with planes of that type, it should be about right. I would also: Keep the stab at zero, make the engine thrust adjustable or just set it at 3 degrees down or so (since the wing is 8" above the thrust line), and 2 degrees or so right thrust.

RE: My next project
 

Good for you Lomcevak! Airplanes you build fly better, and airplane you design always fly best!

Mesae is right regarding the wing incidence. It's largely a guessing game anyhow, but I've had the best results with the wing set slightly positive as he states.

If you set it at 0-0, some airplanes fly tail low which just looks weird. A little positive on the wing, and everything gets back in line where it ought to be.

After some test flights, you could tweak the incidence a bit with shims.

Enjoy the project! Be sure and post some pics of your handiwork.

RE: My next project
 

I'd make the center section wider, and the outer panels plugin with about 3 degrees of dihedral. Big and slow needs a little help from the airplane itself.
And for real slow, an NACA 6409. Tons of undercamber, lots of drag.

RE: My next project
 

I am using Profili, so I can print out airfoils galore and use the true chord line for setting the incidence angles exactly. I love that program. I understand what you mean about flying tail low- I've seen that lots of times, just never really thought about why it happened.
TP: I considered using an undercambered airfoil, but after some consideration I decided to go with the Clark Y because it can be built on a flat surface without a jig, and I didn't want to mess with building a massive jig for 6' wing panels just for this one plane, but I may play with undercambered wings on later versions if all goes well with this one. I am hoping it wil come out very light. Keeping weight and wing loading down is the key to flying SLOW and taking off in its own length. I'll attach a wing loading chart below. One of my goals with the design was to make it easy enough for anyone with a kit under their belt to consruct, and I have found in the past that the simplest answer is usually the best answer. I did not design any dihedral in the wing because as the horizontal vectors increase, the vertical lift vectors decrease. I did design it with a very high wing so that gravity would help with roll stability. This plane will have split flaps and big barn door ailerons.

24 square feet of wing area goes a long way!

Weight in lbs Wing loading in oz/square foot

10 6.6
12 8
15 10
18 12
20 13.3
24 16
28 18.6
32 21.5

No way this plane will come out at 32 Lbs, just put that in there for reference. I am really shooting for 20 lbs or less. I don't know. I plan to pull it around with a spare O.S. 1.60 FX I have, so lighter would be great. These are some really intrigueing numbers.
-Steve

RE: My next project
 

'Duck, towing gliders often needs a modicum of speed. Not a lot but more than a pure slow flyer would provide. Don't spare the power. But add plain flaps for drag and a bit of extra lift for slow flying.

For a true slow flyer build light, use an airfoil with at least 5 to 6% camber and at this size stick with something around 10 to 14% thick. The 6409 is nice but the 6412 at this sort of chord would likely work a touch better. And then there's my own favourite the Goettingen 501 as used on my 84 inch Flamingo old timer. I've often been amazed at how slow it can fly yet how well it can penetrate for the sort of polywog shape it has.

But if you're going to tow gliders, especially the faster flying jumbo scale types, then stick with the ClarkY and add the flaps for quick descents.

RE: My next project
 

the structure needed to make a plane that large -- will be your biggest challange.
As far as airfoils - well -- your choice -for what you are shooting for -is fine.
keeping it from twisting or breaking and still being really light is the real task.
On really big models (I have a 127" span 2600 sq in aerobatic turkey) we found that the "surprise" in the whole effort -is the power needed to pull it along - same thing full scale guys found out when the early, really big planes were designed in the 1930's
The wing will lift the weight but the power needed to move it along goes up at an alarming rate.
So pick an airfoil which has the best strength -unfortunately - the thicker the better for this .
We did some big biplanes which had to have 10% foils to make the thing responsive for aerobatics .
the structure was stressed skin-plus top n bottom spruce full span surface spars . the wings flex easily but don't twist readily. the tubes you plan on using to hook the panels together -- really add weight . -once the structure at the joints is all added in.
If you can afford it - one large full span composite I beam spar would be a good possibility-OR a full span carbon fibre tube which has a diameter close to the proposed wing thickness.
If you can get anywhre near 20 lbs - no aids such as flaps will be needed -

RE: My next project
 

This is all true and I'm not trying to get you to go one way or the other, but if you think you might benefit at all from the added roll stability dihedral gives you, consider that 10 degrees of dihedral per panel (which is a lot) only decreases the effective span for each panel by 1.52%. 5 degrees per panel only costs you 0.38%. You can gain significant additional roll stability and lose almost nothing by using a few degrees of dihedral. If it were my project, the tiny amount of lift loss by itselft would not be a deterrent to a few degrees of dihedral. You will of course have some dihedral effect by mounting the wing high, but this effect will be relatively weak, considering that your 12 foot wing will only be 8" above the thrust line, and presumably close to the same distance, or maybe less, above the CofG.

Having written all that, I'm sure it will fly just fine with no dihedral; you just might find yourself using a bit more aileron in turbulence.

The only reason I mention this at all, is that I have considerable experience flying (similarly configured) 200 lb, 18 ft and 600 lb, 27 ft, high, flat bottomed wing UAVs that were built with zero dihedral (for simplicity and efficiency), and I wished they had been built with some dihedral because I had to fly them for long periods in turbulence, and the additional drag from the additional aileron inputs easily ate up any tiny efficiency gain they had from zero dihedral, not to to mention it got a little annoying at times. Even though they had high wings (there was no "cabin" so the wings were mounted on top of the box fuselage), their roll stability was so weak that it was essentially ineffective. In short, they would have been more pleasant to fly with some dihedral.

Just curious: Why split flaps? Do you plan to use them as brakes to get down faster for the next tow? If so, that makes good sense.

RE: My next project
 

Those UAVs were far heavier (wing loading) than the project mentioned by L Duck.
That makes a fair bit of difference in how they fly --
One of my long time friends, David Stuart ,flies these UAVs as part of his living . Dave and I have had many long conversations about these things .Some - not to flattering about the designs used.
It seems the original design requirements often do not match the later task requirements
That is however, SNAFU for many government designs .
Having designed a fair amount of industrial equipment -mostly for clothing manufacturers and later on a number of competition models -- the real problem is each case -- is really nailing down the task -FIRST.
One patented concept I developed for Levi Strauss -in the 60's was a case in point-
The competition was not willing to come up with a precision heat and control setup which was a must for the fabrics being processed - We were because the task showed nothing available could do the job..

Unless the task is thoroughly researched - all the rest is just wasted effort.
A classic case was the Ford EDSEL.
Ford asked select goups what they wanted in a car --they should have asked what they thought their neighbors wanted in a new car.
Really nailing down criteria -and sticking with it - can do much of the design work for you.

RE: My next project
 

Actaully, yes- sort of. I know split flaps are not what you want to use for adding lift, but drag. I have designed this plane not to need flaps for takoff and landing because of the afore mentioned wing loading, but I was thinking that it would be interesting to see what the added drag did to the decent angles. When I started the design, I was drawing with the following factors in mind: 1) I wanted a slow flying plane, sort of like a old timer. I was picturing one I saw at a field I used to fly at- the guy was an engineer for Lockheed, and had built an electric powered plane so slow he could fly it backward in a light breeze. This was back when electric planes were a rarity, and NiCds was top of the line:) 2) I was looking really hard at the Sig Kadet, but it really didn't do justice for the old timer flair I was thinking about. Nothing on the market really tickled my fancy, so why not roll it myself? 3) Big planes are cool, so why not make it BIG. I had an extra .91 four stroke that needed a home, so let's see how big we can get for that power. No- wait- just for grins and giggles, how big could I get if I dropped a 1.60 in it? Na that's too big, right? Why? My hero, Howard Hughes, never thought the H-4 was too big, did he? Well, the cool factor just increased exponentially. 4) Make it simple. The best answer is usually the simplest. Ie: profile fuselage, airfoil that can be built flat on a building board. 5) Cut cost as much as possible, let's use some foam in this thing! 6) Cool factor can't hurt. Bush planes are cool, it's GOT to have flaps.

Using it as a glider tow was an after thought. My dad has always wanted to build a glider, but we don't have any good slopes, so towing it is probably the best bet.
Thanks for all the good ideas guys.

-Steve

RE: My next project
 

True, but they were not terribly heavy for their sizes, and dihedral effect is dihedral effect, regardless of size. It's more a question of proportion and geometry than size that determines roll stability. As I mentioned, just a consideration--wasn't really trying to persuade.

Comparing wing area loadings between aircraft of different sizes is completely meaningless. A meaningful comparison of flight performance between similar airplanes of different sizes requires exponents at the least.

Here is a comparison using area loading between: a 1/6 scale Cub at about 5 ft^2 wing area and weighing 7 lbs - 1.4 lb/ft^2.

and

a full-scale Piper Cub at 179 ft^2 and 1100 lb: 6.14 lb/ft^2.

To get close to the right “scale” weight, you can use the cube root of the wing loading of the prototype times the wing area of the model: (1100/179)^(1/3) = 1.83. Then: 1.83 * 5 = 9.16 lb, which is heavy for a 1/6 scale Cub, but not surprising, since we are trying to get the “scale” weight.

For the “scale” hp, we can get in the ballpark by using the linear scale as an exponent: 65^(1/6) = 2.01 hp, which is probably about what you would need on our 9 lb Cub for scale performance. Most models Cubs are kites anyway.



Now a 33% scale model of a 340 hp, 1600 lb Edge, with 106.2 ft^2 wing area and a 25.17 ft wing span:

(1600/106.2)^(1/3) = 2.47. 2.47 * 11.8 (wing area of the 1/3 scale model) = 29.14 lb. Heavy for prime 3D performance, but this is for approximately “scale” performance.

For the hp: 340^(1/3) = 6.98 hp. A little doggy for a 33% Edge, but again, probably about “scale” climb performance, and if we didn’t need to accelerate straight up from a dead stop on a hot day, (like the full-scale one can’t), it would be adequate. Considering that the Edge's max sustained climb rate is 3700 ft/min (42 mph, or about 20% of it's cruise speed), and a typical model Edge climbs at probably 80% or 90% of it's cruise speed, these predictions pass the "reasonable" test.

Going the other way, if the full-scale Edge was to have performance similar to our model with say, 14 hp at 25 lb, it would have to weigh 1009 lb, and have over 2700 hp! I don't know if this relationship holds up to such extremes when horsepower is converted to thrust.

We could also have used the formulas for power required, but since we are doing ratios instead, we can skip all that.

I know this can be improved upon, and it doesn’t do anything to predict scale speed, if there is such a thing that can be meaningfully expressed. It deals primarily with scaling down of power and weight for scale climb performance, and scale inertial properties vs. lift. This method doesn’t get into aerodynamic theory; it’s basically just curve fitting. It does provide a somewhat meaningful rough first approximation comparison between different size aircraft, and shows the folly of comparing wing area loadings between different size models, or between models and their prototypes.

RE: My next project
 

yeh - scale comparisons - not much real use for developing a good model tho - as you noted -
The 20 lbs end weight for 24 sq ft of wing -- seemed a bit optomistic for a wing with any ability to handle more than a few G's.
I built some minimum structures long ago - just to see -- and lost a few models due to folded panels -- found out real fast that structural requirements really ramp up quickly as size increases.
far more critical than the airfoils for our models .

RE: My next project
 

Dick makes a accurate point about stiffness. My 82" A-10 Bluefoam can qualify as a glider.
6 sq. ft plus 1.5 sq. ft. from the tail area when correctly trimmed.
I made a " pirated " performance wing by copying a Lanier U-2 foam wing. It is a thin taper wing. It also flexes so much staticly that I put a layer of Poly & Ester on the under side of the center section. Still flexes at 1# per sq. ft loading.

I then made a blue foam A-10 scale wing. Looks like a runaway drunk made the under camber too thick.

NO flex at all. With NO spars. I can HARD hand toss it into a fast, steep, flat, flared out glide.

This plane needs a powerfull toss to go to a landing 30 ' away.

Thin wing would curl up and lose lifting area as dihedral got worse.

Go thick and strong.

Pieces are larger and easier to find.

:D

RE: My next project
 

Duck, if you make the model light enough that it flies at the speed you want and chose an airfoil that works nicely at that speed then I fully agree with your use of split flaps for drag only. They still add lift but likely nowhere near what a standard flap would provide.

Of course if you can make it light enough then almost any airfoil will be fine.

I also second the idea of adding 2 or 3 degrees of dihedral to this design. It'll help the stability and also prevent the "tired wing syndrome" that seems to go hand in hand when you don't have any dihedral at all.

RE: My next project
 

I have to admit, part of the reason I didn't draw in any dihedral is because I feared designing a weak point in the wing. It is easier to join two wings together at 0 degrees than to join them with any dihedral, when using the wing tube. This is a limit of my ingenuity. I could see how to draw up a .40 sized wing with diehedral, and even how to make it removable, but I just couldn't work it out in my mind how to do it at this scale and make it light enough and yet strong enough. That's what I get for taking Psychology classes instead of Engineering.:eek:

Edit: By the way, I have drawn up an "I Beam" type construction, I was thinking it might save some weight over the D-Tube type. I have also been browsing the giants section, and found a couple of build threads for the giant Telemaster, and it's suprisingly similar to my design, not by any intention on my part. Major difference is that I am using a wider chord and therefore a slightly lower aspect ratio, but the same span, and very similar other deminsions. And of course, my Profile fuselage and parasol wing rather than a very trainer-esque fuse.

RE: My next project
 

I did toy with the idea of building a polyhedral wing, that I could see working at this scale with a little added weight, but I was concerned about not having ailerons with this big of a plane, and I wanted flaps.

RE: My next project
 

I had not noticed until you mentioned it again about the profile fuselage. Frankly if you're trying to keep it light I'd dump the profile idea and go for a built up fuselage similar to an old timer or a scaled up rubber model with longerons and diagonal truss uprights. By placing the wood out on the corners of a box like this and usign diagonal truss uprights you can actually build a far lighter AND STIFFER fuselage with less material and at lighter weight. It'll even be stronger since the wood is spaced out further apart so any bending loads will not put as much stress into the longerons as it would a pair of closer spaced sides. I'd use 1/4 sq spruce for the longerons and 1/4 sq balsa for the truss uprights. Leave it rectangular or add a few stringers along the sides and top and bottom so it looks like it has more shape to it. If you're after light this will definetly help. In fact why not go for a semi scale Taylorcraft look with a rather large side by side inspired cross section? IT would look dynamic add to the stiffness by spacing the longerons that much further from each other and generally make the model that much more dynamic. Not to mention all the room inside the cabin for any sort of camer work that you can imagine.

If you're using 2 panels plugged together with a tube or flat joiner then it's actually almost as easy to make the wings with dihedral as not. A couple of small templates and stagger the outer slip fit tube in the ribs to allow for the angle and yer done. Just block in the dihedral before buttering up the slip tube in the ribs and between the spars with a filler mix of epoxy and microballoons or chopped glass. If you use the I beam style alter the first couple of rib bays to a box with caps with carbon or kevlar tow wrapping at the center line ribs and out at the ends of where the blade joiner goes. These are the two critical load spots for a blade joiner. Then make up a joiner from aircraft aluminium (something like 6061-T6. Use a T4 or T6 to ensure the highest yeild point) with the dihedral in the angle and the root ribs angled slightly to match. Simple and no need to even jig it up since the dihedral comes from the joiner and not how a slip tube is included in the wing roots.

Don't pass up on another big spot to save some weight. Using music wire for landing gears is very typical. But the wire size and lengths used on a model of this size can introduce a LOT of weight. Instead I'd suggest using carbon fiber arrow shafting for the leg struts with some form of coupler or joiners at the ends. If the diagonal trusses are lined up to help take the shock loads at the strut attachment points then you can similarly reduce the amount of heavy mounting material at the landing gear attach points by quite a lot. You'd still need some but it can be cut back a bunch since the truss work will pass the shock loads around and into more of the structure.

And finally, for now at least :D, since you're using two peice wings you may want to consider not worrying so much about the joiner and shoot for the wings attaching via simple mounts on the fuselage sides and use functional wing struts to take the actual flight loads. In such a case the wing panels would mount to the sides of the fuselage that would have an airfoil shaped upper cabin area with a strong compressive box beam that runs side to side with some sort of bracket or light joiner rod to take the rather small lift loads and temporarily hold the wings up while you attach the strut(s). Here again you can make the struts from carbon fiber arrow shafts with a balsa fairing glued to the rear side and nicely made ends bonded in and pinned. Overall this would allow you to not only make the center section joiner structure lighter but to allso make the wing spars a lot lighter as well since the struts are holding up most of the load and the wing portions carrying the cantilevered portions are a lot smaller. Doing it this way even a 144 inch span model could get away with something like 1/2 x 1/8 spruce caps with proper webbing to form a nicely supported I beam. I would then cap the spruce with carbon fiber tapes of about 7 to 10 thou thick for insurance. Light and strong by using the material to it's best advantages.

The use of the D tube style sheeting is still worth using since this is as much for torsional stiffness as it is for covering support.

RE: My next project
 

I don't see what the problem is putting dihedral in. All you have to do is put the wing tube fitting in the tip part at an angle rather than flat. Instead of an I beam, I would build a box spar with about the same amount of material, and taper the amount of material from the center section out. I would build up the ribs with the curved top laminated. I'd end up spending many happy hours over complicating things. ;)

RE: My next project
 

Thanks for all the advice again guys. While I am thinking that what I have might fly, and well, the basic idea has great potential, so I am thinking that a harsh revision may be in store. Before I start rolling again, 1)what do you think about a polyhedral wing in comparison to dihedral? 2)What do you think of a plane this big without ailerons? 3)How hard do you think it would be to build an undercambered wing on a building board without a jig, or should I bite the bullet and just build a jig? I've never used a wing jig before, so I don't know much about them. And while I'm taking suggestions, 4)what do you think about rib construction: Balsa sheet, Built up balsa rib (which I've never done before, and would really like to se some pics and oppinions), or foam ribs? I know, lots of questions, lots of possibilities, and thanks again.


Edit: This is Steve, AKA Lomcevak Duck. I have gone home for the Easter weekend, and didn't think to change screen names before I posted:eek:

RE: My next project
 

1)polyhedral sound like a good idea, I'd washout the wing tips too
2)A plane that big without ailerons, not to good, weight shoud not be all that much of an issue, for what you gain in control. How about spoilerons???
I would even add flaps. lol
3)just suport the spars like you would for symetrical wing. Design your spars so that they hold your ribs in position. The truss idea sound like a real good way to start!
4)Built up balsa rib, lots of work, but would be lighter and stronger than balsa sheet.
Someone I know has a built up rib for a full size Jenney hanging in their kitchen. you would be suprised at how strong and light it is.
I think foam would flex too much to be useful on something that size.
Carbon fiber on the other had is one of the best tips you have gotten so far, you would be amaside at how much you can stiffen a wing with it.
Check out:
http://www.airfieldmodels.com
He has a lot of good design and building tips!
Best of luck have fun with it!
Paul

RE: My next project
 

Bruce is right about the fuse....
Stick built is stronger and lighter!

RE: My next project
 

Polyhedral is fine. I'd want ailerons for sure, even if they were only on the inboard section, which wouldn't give them ideal leverage, but you could make them flaperons and save the weight of the extra servos for separate flaps. I'd stick with the Clark Y idea for simplicity - you can use wing area for more lift if you need it, instead of undercamber. Built-up ribs would be great, but a whole lotta work. Foam is decent at resisting compression, but lousy at buckling and bending, so I'd be careful about foam ribs, but they might be OK if well capped and thick enough.

I don't know how light they would be--you would have to use a lightweight adhesive, but I wonder how well a foam rib/balsa sandwich (and capped on the edges in the open bays) would work as a rib. That would take full advantage of foam's compression resistance, and be simpler to build than built-up ribs. You could still put lightening holes in them. 1/32" balsa on each side should add tremendous strength to the ribs, and they would be very stiff because of the thickness of the foam (1/2"? 3/8"?).

RE: My next project
 

I had planned on using 1/2"- 1" foam ribs capped with 1/16" balsa , and of course the 1" in the critcal areas, and light ply reinforcement drawn in around the wing tube. I think I figured out the dihedral thing last night while I was trying to go to sleep. I think of these things at the weirdest times of the day... Well, maybe all day, but:D

Well, I'm off to work where I'll actually spend most of the day thinking about this design. I'd tell you what my weekend job is, but you'd all give me heck:)

RE: My next project
 

Male stripper?

RE: My next project
 

I read only the posts through DBMathews first...

I didn't see a mention of the size of the plane. I think you're building way too big of a model for the engine you're using and that's before you add a glider to tow.

I think about 1600 inches of wing should be good and a final weight of no more than 15 lbs (and hopefully closer to 12).

The Eindecker I finished last fall as a G-26 and weighs a little over 18 pounds. It flies but it does not have an excess of power.

Your design sounds like too much for the available power, but then I've never been anywhere near a model tug so take it for what it's worth.

Now if you want to build a motor glider, your numbers are better, but taper the wings.

- Paul