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Dec 22, 2010, 10:21 AM
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Thank you for a generous response!I am studying it carefully as I lack the theoretical background, that you have.I have tried a lot of wings(see my collection on " show your wings thread"), some quite successful. In this project I was following my nose( as usual!)and it will serve as a model for more, and bigger.Have you found any other way around a reflexed profile?It seems to me that that is not the ideal for a glider wing. I am going to call it the Arc for simplicity.
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Dec 22, 2010, 02:36 PM
I like your Arc. I'm going to make a Sling Wing version of that planform, maybe today.

My hang glider(s) have cambered airfoils without reflex, except a couple which have only a mild amount of reflex in the root area of the wing.
My high-performance ride is a Seedwings Sensor. It has a variable geometry system, where it can be flown with varying amounts of tension on the sail. In the loose configuration it has more twist, or washout if you prefer, and also the flaps are lowered in this configuration. When pulled tight, the VG system flattens the sail, reducing twist, and brings the flaps up to a slightly reflexed position. This little 136 sq. ft. glider can slow way down for tight radius turns that allow efficient thermalling in weak conditions, and she also can "scootate bovinely" when conditions get strong. It has a delta shaped tail fin, which it needs due to having only minimal sweep angle for a HG.

So how do HG's get away with having cambered, non-reflexed airfoils? They use what I call "Planform Reflex", that is, when you combine sweep-back with twist you can get a glider with a positive (nose-up) pitching moment.

To better appreciate this, draw a top view picture of a Piper Cub. Right beside it, draw a swept-aft flying wing. Note that where the wingtips of the FW are is where the stabilizer of the Cub is, and the nose of the FW is where the wing is on the Cub.

Now draw a side view of both aircraft, one above the other. Draw a straight line under each, this is your datum, or reference line.

Draw the Cub's wing so that the leading edge is slightly higher than the trailing edge, in reference to the datum, and now draw the stabilizer with the leading edge slightly lower than the trailing edge. This angle, relative to the datum, is called the angle of incidence. If the wing has 2 degrees of positive incidence, and the stab has 2 degrees negative incidence, you have an airplane with 4 degrees of Decalage.

When you draw the FW side view, have the root of the wing at a positive incidence angle, and the tip airfoil at a negative angle. Now you see that the FW is basically just like the Cub, except we've eliminated the fuselage and vertical fin. The sweep angle places the tips back where a stab would be, and places the root where a straight wing would be. Also, the sweep provides directional stability, which is the job performed by the Cub's vertical tail.

So if you have plenty of sweep, like your Arc has, and enough twist,or washout, you will have plenty of positive pitch stability. You should note that if you have zero twist in the main body of the wing, but have upwardly angled elevons in the outboard area of the wing, this is somewhat the same as having twist, but not entirely. It will provide the nose-up positive pitching moment, but won't provide resistance to tip stalling. For that, you need either geometric washout, where the tip airfoil is twisted to have less angle of incidence that the root area does, or you can have aerodynamic washout, which means having an airfoil section (shape) at the tip area with a higher stalling angle of attack than the root area airfoil has. The idea here is to have the root area stall before the tip area does, and there's forty ways of getting there. You pays your money and you takes your choice.

So back to reflex. Cambered airfoils have a negative, or nose-down, pitching moment. But if you've got enough planform reflex, which is sweep combined with twist, you can get away with using non-reflexed airfoils. My hang gliders do just fine using this arrangement. But one thing must be kept always in mind, and that is that all aircraft, any aircraft design, is nothing more than a big list of compromises flying through the air. The only difference between any two designs is the priority assigned to the compromises. Piper Cubs don't look like jet fighters, and vice-versa.

I mentioned earlier the difference in "design philosophy" between a high aspect ratio flying wing intended for up-right thermal soaring, and a high speed, lower aspect ratio aerobatic FW. Here's where you decide what you want to do with this thing. As I said, you pays your money and you takes your choice.

My HG has variable geometry. I can add twist and camber (flaps) and carve really tight, very slow turns for squeezing everything I can get out of a small, weak thermal. There's been lots of times where I've watched my friends give up and go to the LZ while I just sat on top of the only active thermal on the mountain, me and my little Sensor with flaps.

So let's say I'm racing cross-country. When I top out of the thermal it's time to haul ass down the ridge. At this time I don't want all that twist, I'm hauling butt at a low angle of attack, I want span efficiency and reduced trim drag. So I tighten the VG, getting rid of the extra twist that served me well for low speed circling but now works against me. And of course I can dump those flaps, reflexing that part of the wing to make up for the reduced pitch stability that resulted from getting rid of the twist.

I'm telling you this to illustrate the decisions you'll be making as you design different flying wings for different flight profiles. For RC, I'd go with having three different designs with me at all times. One for low winds and weak thermals, a middle of the road design, and one that can penetrate higher winds and provide an adrenaline jolt.

Send me a PM and I'll mail you some stuff to make Sling Wings with. You'll be pleasantly surprised to find how much you can learn from simple free-flight models. They'll give you a firm grounding in stability and control, and because they're cheap quick and easy you can explore a wide range of configurations. It'll save you time and money when you go to design and build your RC models.
Last edited by dayhead; Dec 22, 2010 at 02:42 PM.
Dec 22, 2010, 03:29 PM
Registered User
miniphase's Avatar
nice distillation of information dayhead, please share the details of some of your rc models with us
Dec 23, 2010, 07:41 AM
Registered User
Thanks again Dayhead. As miniphase says this is very clearly explained. I will do your diagram and pin it to my wall.I cant resist adding 2 more photos- took a drive up Doi Inthanon,(highest place in Thailand)A beautiful ,forested mountain range.Just happened to have my Arc in the back of the car!Found a likely spot beside the road, with highland tropical forest below and wind in the right direction.Feeling sufficiently confident of the plane I launched it into the enormous space, and soon into a strong thermal- a "neckbreaker". More followed, and landed in a user- friendly bush.
I have other questions, about size and weight, for another thread perhaps.
Dec 23, 2010, 07:44 AM
Registered User
I would also like to see your planes! What is a PM? Sorry!
Dec 23, 2010, 11:01 AM
internet gadfly
nmasters's Avatar
I don't have time to write today (holiday preparations ya know) some of the comments in these links may be useful:

Dec 23, 2010, 06:10 PM
Originally Posted by miniphase View Post
nice distillation of information dayhead, please share the details of some of your rc models with us
At one time I was very actively building and flying flying wings (FW's). I haven't built any RC versions for several years, I think the last one was before I got a computer or digital camera.

But the ones I built were very much like the ones that others have made and posted pics and articles about.

When I was doing it I concentrated my efforts on wings with no vertical surfaces, simply because I enjoyed the challenge. It was rather silly, in a way, because such wings are hard to see and stay oriented on. When doing aerobatics I could be inverted and not know it sometimes!

My best one was one of the last that I built. It had an 8' span, and system that allowed easy adjustment of the sweep and dihedral angles before flight.
For would-be FW designers I highly recommend incorporating as much adjustability as possible. Once you've arrived at a configuration that pleases you, you can then make a one piece glider, which will be somewhat lighter and more durable.

In a nutshell, I ended up with pretty much constant chord planforms, with sweep angles anywhere from a low of about 10 or 12 degrees and as much as about 25.

I prefer having a flap that can be lowered to configure the glider for low speed flight. One thing I never liked was raising the elevons to pitch the nose up. Just when you're asking for more lift, you're damaging the airfoil's ability to create more lift. So as a compromise I made most of mine where when you pull back on the stick, the flap would go down, and when it hit a stop, then the elevons would start to come up. I had to do all this mechanically 'cause I've never had a fancy radio, mostly just the cheap Hitec ones.

My 8 footer had a tray that allowed the battery pack to move fore'n'aft. It was operated by the throttle stick, once banked into a thermal I would move the CG back to trim. Hang gliding has taught me that although using weight shift for roll control is a bit laughable, using it for pitch control is the ideal way of controlling pitch on a flying wing, although I should mention that the type of FW I'm referring to is a cambered and fairly highly twisted glider intended for thermalling, and relaxed, sitting-in a-chair-with-a beer type flying.It may not work well with the flying wings usually discussed here, but it's worth looking into.

Also, my wings all had anhedral. I must admit I made the walk of shame more than once while experimenting with this. I generally used foam wings with stifferers or sheeting. I often cut lightening holes in the wing, with the holes getting larger going outboard, this reduced the amount of balancing weight considerably.

I did a couple of experiments using very thin airfoils, with the battery mounted about a chord length below the wing on a pylon. Pendulum weight seemed to allow good pitch qualities with less twist required. A lot of the stuff I did because I wanted to learn more about my hang gliders.

I'm not very active in RC anymore. I have a Flyzone Playmate airplane that I really love, it has excellent aerodynamics and can be flown practically anywhere. The only mod I've done is moving the battery back behind the landing gear, which made it more efficient. It thermals very well, I'd like to have a folding prop for it. It's only drawback is short motor life, around 3 hours I'd guess. But the motors are cheap, so what the heck. The airplane fits up inside the double surface of my HG sail, so I have something to play with when I land. When the airframe gets beat up I'm going to make a "trike" style powered HG with it's guts. I plan to make it so it folds up and fits in a mailing tube.

I'm glad I've been able to help a little. NMasters seems to have a good grasp of the subject, he seems like an engineer. I'm just a self-taught experimenter, learning all I know from some reading but a whole lot of trial and error, mostly error probably.
Foufly, a PM is a private message you can send me. Click on my name and surf around a bit, you'll find where you can send me your address. Your flying site looks pretty. Maybe someday I'll come over and bring my HG and we can go fly.
Dec 23, 2010, 06:43 PM
Foufly, there is another thread in this forum that I just saw, and you would do well to connect with this guy. He's taking a mathematical, scientific approach to the problem.

Even if all you do is make something that looks like his pictures, you'll be ahead. He's using a variable CG, which I think is the key to expanding the performance envelope of a FW.

I get the impression, based on your Arc design, that you prefer the artistic approach, as opposed to the engineering way. Nothing wrong with that, it may actually be more entertaining that way. I like to think that my efforts were more art than science, but I used some science to help reduce the failure and frustration rate.

But Keep On Truckin'. The neat thing about flying wing design is that you'll never get bored, because you'll never feel that you've mastered it.

After doing a lot of flying wing stuff, you'll really learn to appreciate the value of a tail
Dec 24, 2010, 04:39 AM
Registered User
miniphase's Avatar
thanks for sharing dayhead, so much of what you say strikes home with me...great posts
it's been a while since this section of the groups has had any decent wing stuff in it

hope that foufly doesn't feel too badly 'hijacked'
Dec 24, 2010, 04:42 AM
the answer 42 is
Well in fact I am trained as architecht and I learned that one need to combine both science and design to reach maximu performance. I designed and build many FW in the past, just from the gut and they fly well, but also I want to understand how this things work and bring them to the next level.

Dec 24, 2010, 11:38 AM


Hang Gliding is the ultimate, for me and a few other poor souls that made the mistake of trying it, and are now hopelessly addicted for life.

But while I love it, I also resent that it sucks up my psychic energy to the point that I can't seem to work up the ambition I once had for RC and free-flight models. But if the ambition returns, here's where I would go with flying wing design:

I would try to emulate the variable geometry of the HG. Variable airfoil shape, from having a drooping LE and a trailing edge that can be either cambered down or reflexed up. Variable washout, and variable CG. Maybe even have the CG move laterally, as well as longitudinally. The idea is simply to be able to configure the glider so that it is optimised for any given flight condition, such as slow and stable for thermalling, streamlined for high speed running, and quick maneuvering on the slope. I think that using a semi-flexible airframe using EPP foam, and some geared down servos or even muscle wire, a very interesting aircraft could be built. The low profile and light weight and high energy density of LiPo batteries will make heretofore impractical ideas a possible reality. Oh, to be young and full of piss and vinegar again!

Maybe even go with varying amounts of wing sweep angles, like an F-111 or F-14. This feature would be great on the slope, get her up high and swing the wings to a pointy shape for high speed dives, just like a Falcon or Hawk does. I believe this wouldn't be all that hard to do, because I did make some catapult launch F-14 and F-111 free-flight gliders that had wings that swept aft or forward using only aerodynamic forces. The mechanics are very simple and I believe could be adapted to a FW design.

It's all good, whether you're doing 3D in a gym or braving the elements on an alpine slope. Or just cruising a micro in the driveway or releasing a stick and tissue rubber powered job in a pasture. It's all good, it keeps us young, and stimulates the brain. Viva the model airplane!
Dec 25, 2010, 12:17 AM
Ochroma Pyramidale Tekton
Fly Wheel's Avatar
Wow! this is easilly the most educational FW thread I have ever read! Maybe it should be stolen, filled with all sorts of "wing"-odynamics and made a sticky. (or at least used as inspiration for one).
Dec 25, 2010, 07:10 AM
Registered User
I am going to stretch the strand a bit further ,with another (dubious) wing I have started.This will also have the option of a non- moving horizontal stabilizer with a large moving fin on a fairly long carbon tube (4 mm).The aim is a compromise (what else?) trying to achieve the flat turn my "mercury" 3D does easily.I have been close to this with a large, light, crescent wing (photo). Since parasitic and skin drag are apparently less important than profile drag at low Re ,I think a fairly big thin rudder at some distance from the aerodynamic centre may be worth a try ,keeping elevons as well.
When I have the chance I go flying full size gliders,I have to keep the red thread upright on the windscreen in front of me to avoid side- slip and loss of altitude,so this model may be a wild goose chase! However I am in the lucky position of being able to bombard a problem with models!
I have adapted my flying to these light soaring planes: I trim them for a flat straight glide,use up- elevator in the thermals, and mix in down in down elevator with the throttle. To get to the next thermal if I am too low, or in sink, or down-wind, I use the motor.I have usually A 19 gm motor and a 360mah 2S battery and seldom exhaust it,so the drag cant be excessive.
Dec 25, 2010, 10:58 PM
I like that Crescent look, kinda alien like, futuristic science fiction styling.

If you're flying with that much up-elevator, you're paying too much in the form of trim drag. I don't know where your CG is, but if it's right at or behind the trailing edge there at the wing root, try adding a flap there. Just tape on a sheet foam flap of about 6 inch span, and bend it down a little at a time and test fly. There's a possibility you can get rid of some of that up elevator, which looks to me to be so much that you're paying a huge drag penalty to trim that airplane.

I do like the Crescent look. I think if I were to try it I'd tone it down a little, so that the tips aren't going straight back, but swept. I think it would be more efficient.

Some years ago I designed a free-flight model for a company trying to market a full scale airplane they called "Geo Bat". It was a circle, or ring shaped wing. Anyway the front half of it looked a lot like your airplane does.

Good luck and Merry Christmas!
Dec 25, 2010, 11:43 PM
Registered User
Merry Christmas to you! No it doesn't fly with anything like that up elevator.I will have to be more careful when taking pictures!I have a version ( I think )of your geo bat, thanks.

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