Gliders have long, narrow wings to get the best possible lift:drag ratio at moderate speeds, so they can go a long way from a given starting elevation. Kites have short, wide wings to get maximum lift at low wind speeds, but don't care very much about their lift:drag ratio.
You can convert a glider into a kite by filling in the angles between the wings and fuselage with sails. You can then take off like a kite in a moderate breeze, using a rope to stop you simply blowing away downwind (the tangent of the angle of the kite string is your lift:drag ratio). Climb like this until you reach the height you want to start gliding from, then roll up your sails onto booms hidden in your fuselage, disengage your kite string, and off you go.
A remote control mechanism that disengages your kite string at the ground would be good, if you want to take the string with you. And a winch to reel it into your glider of course.-- Cosh i Pi, Apr 17 2007 kite takeoff hang gliding kite_20takeoff_20hang_20glidingLike this? [ldischler, Apr 18 2007] [+] but a question: why are the requirements of a glider different from those of a kite? Surely the kite also would prefer to have a high lift:drag ratio to climb as high (ie, as steep) as possible? In which case, why aren't kites designed like gliders, and why couldn't you launch a conventional glider this way?-- MaxwellBuchanan, Apr 17 2007 Just as with a kite, I think you'll need a bit of initial velocity, e.g. towing behind a car. Actually, are normal gliders ever launched by towing from a car?-- xaviergisz, Apr 17 2007 //are normal gliders ever launched by towing from a car?// Yes, although it's not very common. I had a quick Google, but I couldn't find any decent articles/pics/vids.-- neutrinos_shadow, Apr 18 2007 when you say climb...
most kite launches I've seen have entailed a fair amount of running and huffing and puffing.-- po, Apr 18 2007 [maxwell] Kite requirements are different because the glider is using its height above ground as a potential energy source, whereas the kite is using the wind as a power source. As long as the wind blows, the kite can stay up; the glider has to land once its potential energy is used up. You can launch a conventional glider this way - as long as the wind speed is greater, at ground level, than the stall speed of the glider. Which is a tad unusual, but not unheard of.
[po] Huffing and puffing to get a kite airborne is a sign of a bad kite, an incompetent operator, or an attempt to loft a kite on an almost completely still day (or possibly someone flying a stunt kite, I don't know much about them). I've watched experts loft kites (in India, where it's still a common pastime) in the gentlest of breezes, while standing absolutely still. If the string is attached to the right point on the kite, and you hold the kite at the correct angle above your head, it'll just sail away, no effort at all. The important feature of a kite, for it to do this, is that its stall speed must be less than the wind speed (at head height - wind velocity increases with height). This requires a large wing area in relation to the weight of the kite, that's all. This corresponds to a poor lift:drag ratio, however - which means that the string makes a large angle to the vertical, which isn't a big deal.
The glider wants a high lift:drag ratio to get as far as possible using a given initial potential energy, but doesn't need a very low stall speed. It wants to fly reasonably fast, so as to be able to make progress upwind. There are compromises: at higher air speeds, its lift:drag ratio might be worse, but it gets further upwind because it's going faster relative to the ground. Going downwind, it cares more about its sink rate, which is a function of both lift:drag ratio and air speed.
[BrauBeaton] Weight isn't the enemy of a glider. Weight greater than the design weight is an enemy, but weight equal to the design weight isn't merely not an enemy, it's better than having less than your design weight. Such a glider would be designed with the weight of the sails, winch, booms, and release mechanism as part of it. It would need a correspondingly larger wingspan, and would be more expensive - but the performance would be roughly the same.-- Cosh i Pi, Apr 18 2007 I really must get out and see some expert kite flying sometime. thanks!-- po, Apr 18 2007 I was thinking to accommodate the rope and winch in the glider, but either is possible. With a sufficiently powerful, high speed winch at the ground end you might manage to avoid the cable snaking about all over the place - or you could attach a small parachute to it.
The big advantage of keeping the rope and winch on board is that you can land and take off anywhere, you're not dependent on any facilities other than a flat enough field to land in, and a friendly tree or somesuch to attach the remote release to.
(You'd probably want to use pretty nice cable. 3000m of 4 tonne breaking strain Kevlar line weighs 20kg and occupies 17 litres of space. E&OE.)-- Cosh i Pi, Apr 18 2007 You'll need one hell of a wind.
You'll risk a bad stall immediately upon releasing the tether, when the wind begins pulling the glider backward.
You'll need a stunning amount of open land to pay out your tether - and rewind it safely - without it endangering people on the ground.
An expert lofting a kite in India is entirely different from an excellent glider pilot trying to get the wind to pick up his airplane, no matter how kitelike it has been modified to become.
Unfortunate gusts and slacks of wind while the plane is still low means lots of expensive carbon fiber repair.-- elhigh, Apr 18 2007 I read about kite fighting in India as a kid, and introduced the sport in my neighborhood. Of course, I didn't tell the other kids about razor blades on kites until I had demonstrated it against theirs. Ah, carefree youth...-- normzone, Apr 18 2007 //You'll need one hell of a wind.//
No, that's the whole point. In "one hell of a wind" you can get an ordinary glider up with just a rope and no sails at all: if the wind speed at ground level is more than the stall speed, up she goes.
The bad stall as you release the tether is avoided by starting to fly as you furl your sails, BEFORE releasing the tether.
The tether never touches the ground anywhere more than a few feet from where it's initially attached. (Unfortunate incidents excepted, of course. But they can happen with conventional techniques, too.)
//An expert lofting a kite in India is entirely different from an excellent glider pilot trying to get the wind to pick up his airplane, no matter how kitelike it has been modified to become.// Now there I have to agree with you. You do want a good amount of open land the first few times you try it. Whether you still would once you'd had a bit of practice is a question that could only be answered with experience.
//Unfortunate gusts and slacks of wind while the plane is still low means lots of expensive carbon fiber repair.// Possibly. You could say the same thing about landing a glider, yet people seem to manage. Mostly. Leaves blowing around in the wind don't crash to the ground very hard when the wind drops. 200kg of glider with 100m² of sail can't fall very fast.-- Cosh i Pi, Apr 18 2007 A really good glider is called a sailplane.
Weight isn't always bad in a sailplane. Some actually carry water ballast to let them fly faster and punch through to good lift--it's complicated, but true.
Kites can be built like gliders, but they tend to climb up overhead and fall on you. A bit of drag, like a small parachute, actually helps.
Winch and automobile tow launch of gliders has been done for years, but not as much lately.
An aircraft on a tow-rope is legally a kite. I've been in a gyrocopter trainer behind a pickup truck. Wheeee.
The proposed idea is really for variable-lift wings, although a novel method.
A kite-launch would require that the wind be so strong that landing would be quite risky. Variable lift wings would allow a safer landing, but only safer, not easy.
A strong wind would blow away all thermal lift, so gliding would be kind of pointless. Unless there was slope-soaring landscape nearby, which would allow slope launching, likely.
Most people can't fly kites for shit. It's both an art and a science, and a lot of fun. It's not for just for kids.-- baconbrain, Apr 18 2007 [baconbrain] Thanks for that! 8~)
//Kites can be built like gliders, but they tend to climb up overhead and fall on you.// Indeed. They're also harder to launch, because of their higher stall speed - you either need more wind, or you have to run to get them up into the stronger wind higher up, higher than you can reach.
//...blow away all thermal lift, so gliding would be kind of pointless.// Indeed - unless the point is simply to get from A to B - relaunching yourself kite fashion every few tens of miles... okay, tedious...
The question I don't think we've really got an answer to is how light a wind a glider with such sails could take off in - I think everyone is overestimating the amount of wind you'd need. You might need a somewhat longer fuselage than is ideal on a glider, to get the sails big enough, but even a light breeze produces quite decent size forces on big sails, and gliders don't weigh much. Your lift:drag ratio might only be 1:1 or worse, but for a kite you don't care: it's only stall speed that matters.-- Cosh i Pi, Apr 18 2007 [ldischler] (link) Like that? Yes, quite like that - very closely related idea.-- Cosh i Pi, Apr 19 2007 I'm interested in where the sails attach to the wings, and how these fold up without fouling the aerodynamics of your wings. Do the sails detach from the wings and fold into the fuselage? Can we have an illustration?-- TheLightsAreOnBut, Apr 19 2007 I've not got round to doing illustrations yet, but I think I can describe the idea I have at the moment (obviously it would be subject to development if this were ever actually tried).
The sails would run out along the wing attached to a series of sliders running inside a track, pulled by a thin rope (probably Kevlar or some such) with a pulley inside the wing tip. When the sail was rolled up around the boom in the fuselage, there'd be a flexible plastic lip that closed the slot in the track, and made it as smooth as possible.
The sail, being triangular, would make a thicker roll at the root of the wing than at the tail - just as well, because you want the fuselage skinny and light at the tail end. There'd be a smooth door to cover each rolled up sail.-- Cosh i Pi, Apr 19 2007 I stand behind my original statement. A quick peek at some specs for Schleicher sailplanes - among the best in the world for competition purposes - indicates a stated "minimum speed" of 75kmh, about 45 mph. On the ground, that's one hell of a wind. I'm guessing that the stall speed can't be much below that. I don't know about where you live, but 45mph winds around here are not associated with sailplane flying weather.
Also, those same specs indicate a full-up empty sailplane masses 400kg. That 100m of sail is looking like less and less, esp. when you add mass of pilot, passenger, ballast, etc. Max. takeoff weight is listed as ~750kg. It'll fall pretty fast, I think.
Now, increasing the wing chord with a deployable kite will reduce the stall speed radically while increasing the effective lift and increasing the maximum allowable angle of attack. To what degree I don't know, it depends on the native wing and what you're doing with the kite.
I'm not saying it couldn't be made to work. I'm just saying I'm not daredevil enough to rely on the capricious wind getting me to a safe altitude for a successful sailplane launch.-- elhigh, May 25 2007 750kg takeoff weight is a pretty hefty glider - but then, that's a two seater. What's the wingspan, and wing root to tail distance? I'd imagine it's big enough to have rather more than 100m² of sail.
Obviously you can't take off in less than stall speed wind without the sails - that's why you've got the sails.
A glider without its sails extended would fall pretty much like a stone initially - but with its sails extended it would fall more like a leaf.
All that said, I'll agree with your last paragraph. I'm not that daredevil, either.-- Cosh i Pi, May 25 2007 random, halfbakery