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By TjW
#402530
While it's structurally very simple, aerodynamically I don't think it is. The Pou du Ceil, or Flying Flea, has a similar tandem-wing configuration. Several of the pre-WW2 models dove in because the forward wing was used for pitch control.
Pulling out of a dive at high speed, the gap between the wings decreased, which increased the airflow over the rear wing, increasing the lift on the rear wing, which pitched the aircraft more nose down. The harder the pilot pulled back, the worse the pitch down got. Apparently this didn't happen at low speeds, because people were able to fly them.
Post WW2, the pitch problem was fixed by some changes in airfoils, and, I think, using a conventional elevator on the rear wing and having the gap between wings be fixed. Then several of those crashed due to Dutch Roll. There are people building and flying them today, but the configuration is not without its issues.

I'm not saying it isn't feasible at all, but I suspect that like many unexplored corners, Here There Be Dragons.
By Lazypilot
#402537
I've been having a lot of fun making and flying some "joined wing" free flight gliders using sheet foam.

The forewing is swept aft and the aft wing is straight, or forward or aft swept, depending on my mood while I cut it out of the sheet.

The first one I made from an existing simple delta glider. I just cut big holes in each wing that removed about a third of the foam, in the same shape as the delta. They remind me of the Platz glider.

The basic Platz concept is a good place to start. Imagine the pilot comfortably seated between two eels, the forewing (or "jib") having a modern planform and construction and mounted so that it is aerodynamically balanced, using a reflexed airfoil. Use anhedral in the aft-swept forewing, and some dihedral in the fwd swept aft wing.

I have a question for you educated guys:

Where is the line that determines if it's a tandem wing, a canard, or a conventional design?

I'm of the opinion that if both wings are the same size it's a tandem wing, if the forewing is smaller than the aftwing it's a canard, etc.

Anyway I'm enjoying trying different ways to get decalage, sometimes lowering the forewing's trailing edge or raising the aft wing trailing edge. Also, lowering the leading edge by adding camber, Does anyone know of efforts made to have leading edge control surfaces? I haven't seen any but the idea sounds interesting.
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By red
#402543
Lazypilot wrote:
Tue Mar 06, 2018 2:41 am
I have a question for you educated guys: Where is the line that determines if it's a tandem wing, a canard, or a conventional design? Does anyone know of efforts made to have leading edge control surfaces? I haven't seen any but the idea sounds interesting.
Lazypilot,

I ain't one of them edjumcated guys, but I read a lot . . . :mrgreen: A tandem wing splits the flying load between the front wing and rear wing. It can be pitch-positive if the front wing has a higher AoA. A canard has a small fore-wing that stalls before the main wing. The fore-wing does not carry much load, but when it stalls, it drops far enough to prevent the main wing from stalling. It's difficult to fly canard designs in thermals. "Conventional" to me means wings and tailplanes, although some may include the deltas.

At least one sailplane design uses a hinged leading edge (think of "slats" with no air outlet) as spoilers to reduce the glide ratio, but not they are not used separately as roll controls.
User avatar
By raquo
#402549
I saw this mentioned quite a bit, but I don't think I've seen a good explanation of why canards are bad in thermals. Could you elaborate on that please?
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By red
#402551
raquo wrote:
Tue Mar 06, 2018 3:38 pm
I saw this mentioned quite a bit, but I don't think I've seen a good explanation of why canards are bad in thermals. Could you elaborate on that please?
Raquo,

I have no direct experience there, but as I understand it, the fore-wing of the canard design is in air going up (or down) before the main wing, and the pitch controls are done by that fore-wing. Bottom line, the pilot is constantly making pitch control moves, maybe large pitch control moves, and can't just relax and let the canard design do what it wants. Since the nose responds to turbulence before the main wing, pilots reported "rough air" in a canard, when conventional glider pilots did not.
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By raquo
#402553
Hm that's weird, I'd think it would have an opposite, stabilizing effect, e.g. in case of flying into rising air the canard would stall first, and the glider would begin to nosedive just as it flies into rising air, preventing a nose pop and loss of airspeed that you typically get from that.

I wonder if anyone here has firsthand experience in this, as so few gliders had canards.
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By magentabluesky
#402555
Moving on . . .

Another design is the Marske Monarch

In its current form it is a little heavy for foot launch. The carbon fiber version is supposed to weight 155lbs. Using “Fledge” construction and reducing the span and square footage, there may be a possible foot launch-able Hang Glider with the Monarch plan form.
Attachments
Monarch.JPG
Monarch.JPG (14.54 KiB) Viewed 1299 times
By Lazypilot
#402559
raquo wrote:
Tue Mar 06, 2018 3:38 pm
I saw this mentioned quite a bit, but I don't think I've seen a good explanation of why canards are bad in thermals. Could you elaborate on that please?
It should be kept in mind that a properly designed and built canard aircraft flies with the canard having a higher wing loading than the main wing. Since it is more heavily loaded than the wing it flies with a slightly higher AoA than the wing does, and so the stall occurs first on the canard. In a 'normal' slow deceleration stall, the canard stalls and the nose drops.

This is considered a safer platform than the 'conventional' design that has the little wing in the back. They never gained much popularity outside of the homebuilt airplane arena. A design I always admired is the Burt Rutan designed "Defiant", a twin-engined canard having centerline thrust. Twins with their engines mounted out on the wings are horribly inefficient with one engine inoperative, what with the ailerons and rudder having to be deflected just to keep flying in a straight line. And if the pilot isn't right on top of things when the engine quits, especially when flying with full throttle and a low airspeed, well, the result is often ugly.

Rutan's Defiant is a push-pull design, if an engine conks out the pilot notices a drop off in performance, but no control problems. And on top of that, you can't stall it, and if you can't stall you can't spin.

I don't understand how a manufacturer can get product liability insurance for conventional twins. I imagine a courtroom scene where the lawyer for the plaintiffs (a woman and two kids crying) asks the designer "If you knew that designing your airplane like the Defiant would have saved those children's father, why didn't you do it?"

I've heard that the canard sailplane that Rutan designed was not all that efficient. It's difficult to get the main wing up to it's highest lift AoA because the canard stalls before it gets there.

I have built and played with some model gliders that use a "free floating" canard wing. It is in one way the opposite of a normal canard design, in that the canard wing NEVER STALLS. The canard has a reflexed pitch-stable airfoil, and is mounted in such a manner that it can rotate in pitch all the way around, 360 degrees. If I adjust the glider's Cg too far aft, or tail-heavy, the glider will nose up and slow down, then the MAIN WING stalls and the glider descends vertically, and I can easily see the canard pointing straight down, and it is still "flying" at it's trimmed AoA.

The canard is now "lifting" forward, and it pulls the glider forward and the main wing un-stalls, starts flying, and then the whole scene repeats.

It' looks funny when I toss the glider straight up, it stops and tail-slides, and I can watch the canard pivot nose down and point back at the wing. You should make one, they're fun to play with. They seem to "co-ordinate" the glider in pitch when circular airflow, or airflow curvature if you prefer, exists, such as in circling flight, because the free-floating canard always seeks it's trim AoA. When the Cg is corrected it flies like a regular glider.

I keep hoping that someday I'll find a practical use for the design, to justify it's existence. I came up with it one day when I was thinking those "what if" thoughts that seem to plague me most of the time. Maybe with pitch controlled by weight shift it would prove to have desired characteristics. If not, then I guess it's just a novelty.

Hey, I just realized that if the glider flew into a thermal the canard would adjust itself to the new relative airflow direction and....... hmmm, gonna have to Rc one and see what happens...Maybe the problem of getting the main wing up to max Cl would be solved by using weight-shift to "unload" the canard....Gonna have to experiment some more.
By Lazypilot
#402560
magentabluesky wrote:
Tue Mar 06, 2018 8:18 pm
Moving on . . .

Another design is the Marske Monarch

In its current form it is a little heavy for foot launch. The carbon fiber version is supposed to weight 155lbs. Using “Fledge” construction and reducing the span and square footage, there may be a possible foot launch-able Hang Glider with the Monarch plan form.
Your suggestion is right on the money there. One design I've been contemplating looks similar to the Monarch, but mine has a short (less than 30 feet) span, a large root chord with a bird-like tail, and gobs of wing area. But the wings would have more forward sweep, due to the reduced span and to get the Cg right. I won't need the extra forward sweep if I use a bowsprit cable that can also be used to mount a canard wing. I want to sit or recline comfortably with my head resting on a cushioned nose-plate. Excellent visibility, I could look over or under the wing to keep track of things while thermalling.

The short span/low aspect ratio is because I value maneuverability and light weight more than a high L/D. I see the PG's having as much or more fun than the Atos drivers, so taking a page from their playbook (something Hg should have done at least 20 years ago) I'm aiming for the lowest stall speed of any glider ever, no wind launches a walk in the park, and I won't be restricted to positive G only flight. That's right, Axial rolls, thermalling while inverted. Yee Haw!
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By magentabluesky
#402575
An early hang glider that I thought would be a good build is the SR-1 Hornet. The SR-1 Hornet started out as a hang glider and then the ultralight people naturally put an engine on it. The SR-1 Hornet is a Bi-plane design with three axis control. I believe the airfoil was simply a Clark-Y, but could be upgraded to the airfoil of your choice depending on if you want a climber, cruiser, or aerobatic machine. There are many possibilities, seated, prone, landing gear, and electric power assist.
Attachments
SR-1_isometric.JPG
SR-1_isometric.JPG (121.51 KiB) Viewed 1161 times
By Lazypilot
#402582
That type of design has way too many parts for my tastes. A Cantilever monoplane will weigh more, for a given strength, but I just don't wanna make all those parts and cables. Also I'm going short span with lots of area, so the low aspect ratio will drive me to make her as aerodynamically clean as I can possibly make her. I'm prone to setting up hurdles, but climbing over them keeps me in spiritual shape. :P

I want the parts count as low as possible. Parts have weight, parts cost $, and parts require labor to make. Remember, I'm Lazypilot.

If I thought it was possible to just carve the glider out of a big block of Styrofoam, and didn't have to break it down for transport, I'd do it. That's just how cheap and lazy I am. Not bragging, just admitting. :ahh:
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By magentabluesky
#403238
Here is another idea on making hang gliders more maneuverable.

Both 7075-T6 Aluminum and Carbon Fiber are very stiff materials whereas fiber glass has much more flexiblity. “Billow” shift could be made more flexible if the out board leading edges were fiberglass tubing.

Custom fiber glass lay ups could make the leading edge tubing flexible in one direction and stiff in another.
By Roadrunner71
#403239
I have wondered for a very long Time why Hang-Glider designers have never designed a FLEX - WING that was both a Topless design, and also a Bottomless Wing? I presume that the main objection to a Fully Cantilevered Flex wing would be weight. How about it? does anyone have the answer that I am seeking?


Think about it: A Flex - Wing Sans rigging that need to be moved through the Air.
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By raquo
#403241
If you need / want a fully cantilever beam, putting a flex wing on it would just reduce performance compared to using rigid ribs like an ATOS, without necessarily improving on weight. Not to mention that such a strong beam would not provide any flex needed for a flex wing to billow shift.
By Lazypilot
#403243
magentabluesky wrote:
Sat Apr 14, 2018 6:57 pm
Both 7075-T6 Aluminum and Carbon Fiber are very stiff materials whereas fiber glass has much more flexiblity. “Billow” shift could be made more flexible if the out board leading edges were fiberglass tubing.

Custom fiber glass lay ups could make the leading edge tubing flexible in one direction and stiff in another.
This post reminded me of when I got off on a forward swept wing kick for awhile, Most of the discussion I found was about the need for advanced materials tech, in the form of arranging the fibers to provide torsional rigidity. It seems that as the wing gets loaded up it wants to twist up at the L.E. tip, creating wash-in. The airplane being discussed is the X-29, a possible platform of a fighter jet. Just as flex wing Hg's get extra wash-out when loaded up and then experience "positive aero-elastic divergence" so does the X29. Wash-in combined with fwd sweep produces the same effect of positive aeroelastic divergence, which means that as you pull more G's the aircraft wants to nose up even more. It's divergent.

Some aero nuts believe that a fwd swept wing is more stall resistant, particularly in the area of tip stalling characteristics. They also claim higher maneuverability. I've long thought that an Rc model with the capability of sliding the wings along the fuselage and varying the sweep direction at the same time would perhaps either prove or disprove these claims. Swept aft wings experience a washin effect, so the amount of washout seen on Hg's by your eye is actually more than what the wing sees. If the wing were flat, with no twist built in, it would be a tip stalling b---- and maybe pitch divergent, negatively. On the other hand a fwd swept wing has wash out even if it isn't twisted.

So after reading about all this it occurred to me that using flex wing construction tech on a fwd swept wing might be advantageous. As G load increases the wing would wash out, just like they do when swept aft. But instead of being divergent, it would tend to be convergent: As G load builds and washout increases, the pilot would experience a nose down tendency. I wonder if that would feel any more scary than not being able to hold the basetube back and looping involuntarily, something I experienced with Hg's having large sweep angles (UP Comet was one, while pulling out of my third loop I lost it and she went over hard, bending a leading edge. I belly landed while holding my body as far over to the right as I could just to fly straight) :oops:

The FAA requires certified aircraft to have an increasing stick force per G experienced, the common method is to hang weights on the "down elevator" actuating cable or somewhere so that the G load makes it harder to pull up any harder, hopefully reducing the pilot's chances of overstressing the airframe.

A swept fwd Hg using the "sailboat" wing construction would require a pilot to experience ever greater bar pressures as G load increases, due to the increasing washout. The question is: Would that be good or bad? We would likely see instances of pilots flying into the ground because they couldn't push hard enough, but then the chances of over loading the structure when aerobatic would be reduced, possibly allowing a lighter airframe. You wouldn't need a spar good for 6 G's if you can't get the thing past 3 or 4, right?

Hg manufacturing results in many compromises being made in the interest of economy. Hand lay-up of composite fibers is labor intensive, but would allow for customization, maybe stronger in one direction than another.

I still wonder about the "joined wing" concept which is one way of reducing the airframe weight-to-span ratio. Nasa has or is still looking at this layout for designs that have to carry a lot of fuel so they can orbit an area for surveillance for long periods of time. The lower the airframe weight the more fuel capacity. This is what we want, more glider, less weight. Joining tandem mounted wings can provide torsional rigidity with less material, hence lighter.
Last edited by Lazypilot on Sun Apr 15, 2018 3:28 am, edited 1 time in total.
By Lazypilot
#403244
Roadrunner71 wrote:
Sat Apr 14, 2018 10:50 pm
I have wondered for a very long Time why Hang-Glider designers have never designed a FLEX - WING that was both a Topless design, and also a Bottomless Wing? I presume that the main objection to a Fully Cantilevered Flex wing would be weight. How about it? does anyone have the answer that I am seeking?
Think about it: A Flex - Wing Sans rigging that need to be moved through the Air.
A structure having enough stiffness and strength to go without wire or strut bracing might not be flexible enough for pure weight shift control of roll. I guess maybe someday an invention will come around that makes weight shift for roll control of a High Performance flex wing a practical thing. But I ain't holding my breath. There was a guy in Germany who pursued that idea for over 2000 flights and died. I concede that inadequate roll control may not have had anything to do with it. It will always impose limits on wingspan though, and that's where the best glide angles come from.

I'm a lonely voice in the wilderness, it seems that everyone is Ok with positive G only ultralight soaring. I flew that system for over 40 years and I'm just bored, that's all. I just want ridge soaring to be fun again, and for that I need more capability.
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By raquo
#403245
Lazypilot wrote:
Sun Apr 15, 2018 3:06 am
So after reading about all this it occurred to me that using flex wing construction tech on a fwd swept wing might be advantageous. As G load increases the wing would wash out, just like they do when swept aft. But instead of being divergent, it would tend to be convergent: As G load builds and washout increases, the pilot would experience a nose down tendency.
Interesting thought, that's the kind of stuff I come here for!

I've never heard of anyone trying forward-swept flex wings. It certainly sounds like a tricky balance that would take a lot of ironing out on a test rig. Which is not necessarily bad, it just means it'd be hard for garage designers to do this safely.

If this does work it seems like bowsprit would be a great technique for this design – it will have both a structural advantage, and allow the sail to shift easier (since there would be no crossbar in the way). Might be the hike-and-fly glider we've been waiting for! :)

Though, I wonder if weight shift roll control would even work with a forward swept wing. I guess it should, but just intuitively, kinda hard to imagine that it would be effective. Would probably need a vertical tail too for yaw stability.
By Lazypilot
#403246
A vertical tail would definitely be required. Aft swept wings are inherently yaw stable, although wide nose angles are less damped, The Aolus has a very wide nose angle, and so are most Seedwings Sensors.

Aft swept wings also have a form of built-in dihedral and fwd swept wings have anhedral.

I agree about the bowsprit having a structural advantage with fwd sweep, as opposed to swept aft which disadvantages the bowsprit. The bowsprit cable might be used to mount a fore-wing of some kind (jibsail?)

If you're so inclined try making some fwd swept gliders using foamboard. I think you'll like the ones using a bowsprit wire, after you get it flying as a flying wing then try adding the fore wing using a pitch stable section. I test fly the fore wing as a glider all by itself, and once it's trimmed properly I add it to the glider. I believe that a free-flying forewing could prove to be a good idea for ultralight gliders, especially so if the pilot can use it for roll control.

INMHO using purely weight shift for roll just won't cut the mustard on hi-perf gliders, it is adequate for slack sailed beginner kites though. Please keep in mind that I'm an eccentric crusty old curmudgeon with sometimes unpopular sentiments regarding aviation. That is why I refer to myself as an "aeronautical philosopher". For me, "airworthy glider" means fully controllable at all times, regardless of attitude, airspeed, or direction of flight. Some call that a tall order, I call it necessary for a healthy flying experience. :popcorn:

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