Fauvel AV36 glider 1:33 by Philippe Rennesson

[Gil]

Leif,

No, you're not mistaken. The Carbon Dragon was designed specifically to test "Dynamic Soaring" capabilities. Dynamic soaring is the hot topic in the ultralight sailplane category and is just beginning to be picked up by the mainstream soaring community. It's a welcome technique in the never ending quest to stay aloft without an engine (the ultimate green machine).

Best regards,
+Gil

[Leif Ohlsson]

And dynamic soaring is... let me try to remember; it's what the seagulls and albatrosses do. You dive with the wind at your back, right - 'cause then you'll enter the wind gradient close to the surface with extra relative speed and get free lift (less tailwind closer to the surface means more airspeed as if out of nothing). You convert that lift into free extra speed, and then make a sharp turn into the wind and use it to get free lift.

Repeat, and if your name is Jonathan Seagull, you'll stay aloft and might even go forward, against the wind, without a single flap of your wings, even though there are no thermals. Right?- Or was it the other way around? Figuring out these things is getting harder and harder.

I suppose glider pilots are not using the stronger wind gradient close to the ground. But any wind gradient ought to do, even if it is just a difference in wind direction at different altitudes.

Funny thing is, I once made a calculation for a couple of kites, hooked up together so that one would fly some hundred yards or so lower than the other (with a bit of common ballast at the bottom of the V-shaped line, to stabilize the whole thing). My guess was that such a contraption, if it was light-weight enough, might well stay aloft for ever, since there is always a wind gradient around (difference in wind strength or direction at different altitudes). The larger the difference in line length, the greater the chance to stay aloft, since then you are more likely to capture a sufficent amount of gradient.

The idea came to me as I once lost a kite, which carried a smaller kite some hundred yards lower on the line (tied there just for fun, no plan or anything). What struck me, as they sailed away over the sea, was that the pair of kites didn't descend, but seemed to continue climbing (or at least maintain their height) until I lost sight of them. The only explanation I could find was that the lower kite acted as a brake, thus providing enough line tension to keep the top kite barely expanded but still stable and providing lift.

This could only be if there was a difference (gradient) in wind strength at the respective heights of the two kites. So I sat down and worked it out geometrically on paper. Turned out to be some pretty interesting diagrams, not least when I realized that it is not even necessary to have different wind speed, different wind directions at different altitudes is enough to provide lift. And there's always both different wind speed & direction, which doubles your chances of creating lift.

Lines could actually be extremely light-weight, much lighter than flying an ordinary kite, since once aloft the line tension would be just the difference caused by the wind gradient between the respective altitudes of the two kites. The only problem I saw when contemplating an experiment, was the start, since until the pair of kites could be released there would be full tension on the lines. Otherwise common sewing thread joining a couple of light-weight delta kites would suffice, I thought.

If I were a Jules Vernes, I could easily extrapolate that into a full-scale, man-carrying contraption, which sailed the skies of the world for ever, navigating by entering wind streams of suitable direction. Lift (and thus altitude) could be regulated by shortening or lengthening the difference in line length. Just turn on a little winch, making the line lengths more even, and you'll go down. Increase the difference in line length, and you'll go up again. Pretty idea for a 19th century novel, right? (But how would good old Jules have contemplated landing, and taking off again, I wonder? On sea, most likely.)

Anyway, that would be the same basic principle as dynamic soaring. Which seems like a wonderful adventure - learning how to keep aloft from the seagulls.

Gradients of all kinds are wondrous things. That's the principle behind heat pumps, for example. Net energy as if out of nothing.

Leif

[Gil]

Leif,

Yep, that's it. Following YouTube shows RC guys with nearly supersonic sailplane models. The first up meets with a ground of explosion. Really don't know how they can control the planes at those speeds?

YouTube - More Dynamic Soaring

This one clocked at 357 mph!

YouTube - 100" Kinetic Sailplane 357mph Dynamic Soaring World Record


+Gil

[Leif Ohlsson]

I would seriously appreciate some feedback on my choice of wood textures. Below you will see the present state of my trial sheet for the Fauvel in complete wood texture, inside and out.

I have tried to go for beechwood for formers, mahoganny for the instrument panel, some undifferentiad wood for load-carrying parts, and aircraft birch ply for the exterior.

I would have liked the exterior ply to be somewhat lighter, but haven't found any. In some of the detailed views you will see my attempt to get at the feature which fascinated me most, namely the differing grain directions in some parts of the fin.

The fabric is too coarse at present. I will have to come up with something better for that.

What I am hoping for is to generate some enthusiasm for the possibilities of modeling true wood & fabric structures in paper. To me, they are very, very beautiful.

Leif

[shrike]

The differing grain patterns on the fit are perfect for the (possibly mahogany) plywood skin of the structure.
If built in the conventional manner, the rudder however should be individual members of spruce (very pale) with the grain running along the piece. At each joint there should be a gusset of plywood reinforcing the joint. Often this would be mahogany plywood of a contrasting colour.

I should mention that my own airplane was built using douglas fir for structure with baltic birch gussets. The wood took on an orangey shade when varnished with pale yellow plywood

An illustration will follow

[Leif Ohlsson]

Thanks, Shrike! Points on the rudder well taken. I was a bit lazy there. Good opportunity to change the fabric structure as well. Will remake completely. Looking forward very much to photos of your aircraft!

Leif

[shrike]

Quick illustration of a typical gusseted joint

The fuselage structure (part 23) would also be made in this fashion, although the reinforcement around the wing would probably be a very large plywood plate

[B-Manic]

The re-colour looks like it is coming along great Lief. Shrike's suggestions match what I have seen in other models. The wood grains themselves are nice and show up well even at the reduced scale.

[Leif Ohlsson]

I can see the point about part 23 now. C.f. close-up below.

A lot of work. But I asked for it. Thanks.

While we're at it - do you see any chance of the pattern used for part 23 now (the golden yellow) being used as pattern for exterior plywood covering? It's just that it is so beautiful.

Or would the pattern now used on the instrument panel be better?

Or should I use the present pattern (sort of greyish brown)? These things better be decided now, since it is a fair amount of work to change patterns (until someone comes up with some more or less automatic procedure).

Leif

[shrike]

The golden yellow pattern on 23 looks great for birch -use it for the plywood covering. The greyish brown looks like mahogany plywood with a little age to it. For the most part they can be used interchangeably, the mahogany is cheaper but heavier and takes more varnish to finish(even heavier), so birch would be the better choice for the large areas of skin. (How is that for rationalizing an artistic decision?<G>)

I'd use the grey/brown for gussets to add some contrast and visual appeal. If you are going to do lot of work getting the wood right, make sure it can be seen!