(From part of an email from Gene)
Me: Another thing I was thinking about is that I read large cells have a higher air pressure, which is better (is this true?) now, to me putting DRibs in splits the volume of the cell in two, do you cut nice big holes in the DRib?

Gene Matocha [gmatocha@yuroka.com]: This is one of those common misconceptions that has some basis in truth. Smaller cells are not really lower pressure. If, however, you have a large number of small cells with NO cross ventilation, the kite will be more prone to collapse. The reason is not lower pressure. It's, I believe (read, this is a theory), due to the bulge of the skin between cells. The bulge of the skin acts like a "buffer" that allows changes in pressure to have less of an impact on the structural integrity of the cell. DRibs, however, don't reduce the volume of the cell simply because they do not extend completely to the ends of the cell...they don't "enclose" any volume of air. If you make your DRibs extend 100% of chord, that would be a different matter of course.

Me: Would increasing the number of cells, but cut big holes in the odd numbered ones, and little holes (normal cross ventilation) in the even numbered ribs. Would this give the 'large' cells some form of support?

Gene: You are on the right track here. You can get away with smaller cells if you have adequate cross ventilation. There's no advantage to different size holes in different ribs, however. A rib is a rib, regardless of it's relationship to a DRib.
A simple tip for making the cross ventilation holes. Get a cheap pencil-type soldering iron and remove the small tip. Use the iron to melt holes (they are about the size of a penny) in a grid pattern. I usually make a grid 5 x 15 holes on the larger ribs...less as the ribs get smaller of course. It works well because the iron seals the edges of the hole and makes its own little plastic reinforcing ring so you loose very little strength. It's fast and you don't need to be super precise, just consistent.

Andrew Beattie [http://www.tug.com] [note: TE = Trailing Edge, i.e. the back of the kite]:
Or use a gauze rib TE.
Or use square cut TE (leaving a semi-circular vent)
Note that the the TE is the place where cross-venting should be because that inflates first.

Andy Beverage, 50mm dacron:
50mm dacron is the black edging on the edge of the vent holes. The Gamma is designed not to need mesh over the vent holes in the front.
I slit the 50mm into 2 x 25mm and folded a piece of that in half and sewed it along the front edge of each rib, along the leading edge of the bottom skin and along the leading edge of the top skin, to give strength all the way around 'the opening'.

Cross vents and valves:
Cross vents are the holes that are cut into each rib to allow the air to flow between the cells. It is only the rib on each side that goes from open cell to closed cell that needs to have any form of flap on it....I made mine too tight and had to unpick them to help the kite tips inflate correctly!

How to sew ribs:
Sew the bottom panels together, not forgetting to include the vent pieces that mean the tips will end up being closed, and not forgetting the bridle loops. Then sew the ribs to the bottom skin, then sew the top skin together then sew the top skin to the ribs...then sew up the trailing edge, bridle and fly......

Materials for foil:
Carrington - cheep, absorbs water, is floppy.

Daniel Gagnon [dag@bbsi.net] replies to my email:
Why do the Gamma wingtips have no profile? [see foilmaker .kte file of the Gamma to view this]:
Reducing the drag

Why is there a canopy curve when I read a flat canopy is best?
Canopy curve among other details, enhance the turning rate or a foil.

If I tweak the aspect ratio to greater than 4.34 will that make it quicker to turn? (I'd like this :-)
Greater then 4.34 some luffing behaviors If present at 4.34 will be enhanced.

How do I calculate the amount of material I'll need to build it? Is it 3* the area (15-16 sq m?)
Its actually rather 3.8 but it all depend of the number of ribs used.

> Thanks for yet another FM feature set. But I have to admit that I don't have
> a clue what skin tension exactly does.....:-)
>
> Could you explain this enhanced feature?

I'm not entirely sure what it actually does either, but this is what I
_think_ it will do :-

Putting a taper in at the TE should reduce 'coalsacking' and hence reduce
drag. There have been posts in Foildesign in the past about this, and I've
used the suggested values as the defaults in FM. Search for 'TE pinches' in
the archives for info.

Of more interest to me though is tensioning of the LE. I think that putting
a taper into the skin at the LE will tighten the skin & hence improve the
airflow at the point where it matters most. There may well be an anti-banana
effect as well. I suspect that Peter Lynn's NGen used a very similar
principle, though of course the panels were cut spanwise instead of
chordwise.

On the downside, you can't increase tension in one part of the cell without
decreasing it elsewhere. The uptapered sections will therefore be a bit
slacker than they otherwise would be. This will mean a bigger difference in
profile depth between the rib & the center of the cell, but I'm not sure
what effect this will have on performance.

I haven't tested in anger yet by actually building something - whose going
to be guinea-pig????

Peter.


Well, here's the results of what I've tried.
Note that all these require mesh on the inlet hole, to stop the valve
blowing back out.

A simple flap, about 1 1/2 times as wide as the inlet hole, sewn at both ends
but open on the other two sides.  Simple, effective, slows inflation by
an enormous amount.  Useful for internal valving due to its low weight and
simplicity.

Triangular inlets, with square panels sewn on two edges (as per the Blue
Spot, scribbly drawings somewhere on my website
http://www.geocities.com/Pipeline/Ridge/2349).  Slightly more complex, first
launch out of the bag is a bastard due to the fact that they tend to get folded
when the kite's scrunched up.  After that, they're fine.  They still slow
inflation quite a bit.

Tube valves.  Simply a tube of material running into the kite.  Optimum length
of tube looks to be about 1 1/2 - 1 3/4 the width of the inlet hole.  More would
seem to be overkill. It's possible to tack these onto one surface of the kite,
or make them 3-sided with a rib/surface as the fourth side to reduce weight.
Inflation speed is roughly the same as a 'normal' kite, although first flight
out of the bag can suffer the same problems described above.