## [ksurf] Re: Kite size / weight ratio

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• Dave - excellent theory but you speak of windspeed at the kite - When I (and most others) speak of windspeed this is measured at the beach at ground level -
Message 1 of 23 , Oct 1 6:34 AM
Dave - excellent theory but you speak of windspeed at the kite - When I
(and most others) speak of windspeed this is measured at the beach at
ground level - Which is the only practical place to measure it !!

then you go out and the wind is probably 6 knots at altitude out on the
water but who cares ! as far as we know its still 3 knots on the beach -
and those windsurfers will be very impressed.

I am not saying your theory is wrong - lets not go there - I am saying that
when we kitesurfers speak of low wind planing limit - its most probably the
wind on the beach at head level... depending on the beach the wind "at the
kite" (as you said) can be anywhere to up to 3 times as much ! (2 knots vs
6 knots)
• Dave, Reality is normally more complex than hypothesis and scientists never assume anything CORRECT until they actually experiment and experience it. Are
Message 2 of 23 , Oct 1 6:38 AM
Dave,

Reality is normally more complex than "hypothesis" and scientists never
assume anything CORRECT until they actually experiment and experience
it. Are you near Silicon Valley? The next time I am in town, I can
give you a call and we can try this together. I will bring my XXXL. Is
there any shop in town that we can borrow an FOne230?

The one who loose will pay the beer.

Cheers :-)

Hung.

Dave Culp wrote:
>
> >I don't think I could ever get planing in 3-5 knots on the board I have
> >now... I need too much speed to plane with it. I think even with a kite
> >that I could barely handle I couldn't get going fast enough to plane unless
> >there was more wind higher up. An 8ft or bigger board is on my shopping list!
> >
> >Mark Frasier
>
> I know that Hung is keen to plane under very light conditions. However, consider the following: I have yet to see a vessel capable of both a) carrying an adult human being, and b) planing--fully planning--at a craft speed below about 8 knots. Hung is a small, strong guy, by all evidence, which certainly enhances his chances of planing at very low craft speeds, but I'd still bet a goodly sum even he cannot maintain a full plane (no help from flying the kite overhead--I'll explain why in a moment), below 8 kts craft speed (I keep using the term "craft" to remind us all that boards, skis and boats aren't really any different, physically).
>
> In order to sail any craft at 8 knots in 3-4 knots true wind, obviously, one needs to sail at 2--2.6 times windspeed. In order to achieve this, first, the apparent course will be well to windward, while the true course will be to leeward. Putting numbers to it (assuming, for the moment, a Vb/Vt ratio of 2:1, ie, 8 knots craft speed, in 4 knots true wind), the true course will need to be about 120 degrees off the wind. Vector diagrams will show that apparent wind will be just under 7 kts, apparently from 30 degrees off the bow of the craft.
>
> Kitesailors will recognize this to be almost impossible--I'm talking about beating to within 30 degrees of your apparent wind! The kite must be right down low, pulling for all it's worth. The board will need to be sailed dead flat, with all sideforce coming off the fin. Edging the board will cause too much drag, and this sailing angle will become actually impossible, not simply "almost."
>
> Taking the apparent wind angle (30 degrees) apart into kite drag angle, plus board drag angle, I'm presuming a kite angle of 15 degrees (perhaps) and a board angle similar. This means a kite capable of a L/D of 4, which sounds OK, until we note that this includes *all* parasitic windage--line, bar, sailor, everything above the water's surface. this indicates a very large, powerful kite, short lines, and *very* skillful flying.
>
> Taking the craft, in order to achieve the same 15 degrees of drag angle, of course, we need the same 4:1 L/D. Note that this refers to * net horizontal* L/D. In other words, the board must generate 4 lbs of sideforce for *every* pound of drag it produces. A planing hull, even a very efficient one, has a vertical L/D on the order of 3:1 or a bit more--to lift Hung's 125 lbs, plus the board's 20 or so (including bar, gear and water--everything's wet, no?) onto a plane, he'll create between 35 and 50 lbs of residual drag, before creating an ounce of sideforce. A very efficient fin might have an L/D near 7; so to get his total net horizontal L/D up to 4:1, he'll need that fin to create something like 300 lbs of sideforce, yielding 43 lbs of induced drag. Added to the 35 lbs of vertical, residual drag, this yields a total board drag near 77 lbs, approaching the needed 4:1 overall L/D.
>
> In order to create 300 lbs of sideforce, of course he'll need to create and maintain 300 lbs of tension in his kite lines (Actually, a bit more due to angle, and yes, it is possible for a 125 lb man to maintain 300 lbs of line tension, for several minutes at least). For a kite to generate 300 lbs of pull in a 7 kt apparent wind, assuming a lift coefficient of 1.5, which is very large, Hung will need a kite 1500 sf in size (140 sq meters), which is not possible. Presuming he could fly the kite in a zig-zag at twice the apparent windspeed, he could use a 35 sq meter kite (about 425 sq ft). I am not at all sure this is possible, since the kite will lag back as it is zig-zagged, and overall L/D will suffer--very likely below that critical 4:1, or 15 degrees drag angle.
>
> Both board and kite drag angles are very likely to be higher than this--Hung will only be able to sail within perhaps 40 degrees of apparent wind--and boatspeed/windspeed ratio will drop. He'll either not be able to plane, or will wait for more wind.
>
> Going through the above exercise demonstrates why very efficient sailcraft (on water), at low wind and craft speeds, tend to be dispacement hulls, not planing hulls. A small displacement catamaran, moving through the water at 8 kts might only have a total hull drag of 4-5 lbs, compared to Hung's board's residual drag of 35-50 lbs. It is thus far easier to achieve the overall 4:1 or better needed to get the hull's drag angle to 15 degrees. Similarly, wingsails on rigid masts, or even soft sails hung from rigid masts are capable of higher L/D's than any current kite, again making it more likely such a rig will achieve the small drag angle needed for such high boatspeed/windspeed ratios.
>
> To my knowledge, no windsurfer has sailed as fast as twice his windspeed, except under suspicious conditions (short runs, where inertia has an effect; un-measured gusts, and the like), nor any kite craft over 1.5 times its true wind speed *at the kite.* We are certainly pushing these limits, and will exceed them fairly soon, but Hung's desired 2.0-2.6 times true windspeed isn't very likely any time soon.
>
> Dave Culp
>
> --
> Hi! I'm a shareware signature! Send \$5 if you use me, send \$10 for manual!
> Dave Culp Speedsailing dave@... http://www.dcss.org/speedsl
> Kite powered boats, high speed sailing, proas and more. Check it out!
>
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• ... That brings up a question I ve been meaning to ask - is the wind at 30 ft to ground wind ratio bigger in lower or higher ground winds? Or about the same?
Message 3 of 23 , Oct 1 7:28 AM
>Dave - excellent theory but you speak of windspeed at the kite - When I
>(and most others) speak of windspeed this is measured at the beach at
>ground level - Which is the only practical place to measure it !!
>
>then you go out and the wind is probably 6 knots at altitude out on the
>water but who cares ! as far as we know its still 3 knots on the beach -
>and those windsurfers will be very impressed.
>
>I am not saying your theory is wrong - lets not go there - I am saying that
>when we kitesurfers speak of low wind planing limit - its most probably the
>wind on the beach at head level... depending on the beach the wind "at the
>kite" (as you said) can be anywhere to up to 3 times as much ! (2 knots vs
>6 knots)

That brings up a question I've been meaning to ask - is the "wind at 30 ft
to ground wind ratio" bigger in lower or higher ground winds? Or about the
same?

Mark Frasier
• ... Or not enough board volume. Why the name scalloping (edging)? ... I have not done any research nor experiment on this matter (I did try a couple boards,
Message 4 of 23 , Oct 1 7:55 AM
Mark Frasier wrote:
>
> >There is always more wind higher up (using 45 m lines, I felt it all the
> >times) but I found them not very useful, especially when you have to
> >turn the kite upward after diving it down. This will make you fall if
> >you don't have enough wind below. One trick I always do in this
> >situation is to turn the board windward (to add more apparent wind) when
> >I start to turn the kite upward.
>
> Buggiers call that "scalloping". I can't seem to make that work ont he board
> - it seems to sink too fast. I think a lot of the trouble might be my lack
> of board handling skill.
>

Or not enough board volume.

Why the name "scalloping" (edging)?

> >A bigger board may help too. I have yet figured out the relationship
> >between board size and rider weight. There's definitely a correlation
> >between board volume and rider weight, but I am not so sure about the
> >correlation between board length, width and rider weight?
> >
> >Hung.
>
> Hmm...so do you think I should look for a board with a higher volume instead
> of a larger planing surface?

I have not done any research nor experiment on this matter (I did try a
couple boards, but not enough). You may want to do some research and
experiment and let us know. Just on the "surface", I guess board volume
could be proportional to rider weight. Larger planning surface could
help but I have no idea how much for the length and how much for the
width.

Hung.
• ... The tracks that are left in the sand look like the edge of a scallop shell... sort of like a bunch of arcs end to end. Makes for slow progress but it s
Message 5 of 23 , Oct 1 8:04 AM
>Mark Frasier wrote:
>>
>> >There is always more wind higher up (using 45 m lines, I felt it all the
>> >times) but I found them not very useful, especially when you have to
>> >turn the kite upward after diving it down. This will make you fall if
>> >you don't have enough wind below. One trick I always do in this
>> >situation is to turn the board windward (to add more apparent wind) when
>> >I start to turn the kite upward.
>>
>> Buggiers call that "scalloping". I can't seem to make that work ont he board
>> - it seems to sink too fast. I think a lot of the trouble might be my lack
>> of board handling skill.
>
>Why the name "scalloping" (edging)?

The tracks that are left in the sand look like the edge of a scallop
shell... sort of like a bunch of arcs end to end. Makes for slow progress
but it's better than walking :)

Mark Frasier
• Hung s right! My kitesurfing skill is only slightly greater than 0, yet I managed to plane and get about quite nicely with an XXXL in about 8-10 mph. I weigh
Message 6 of 23 , Oct 1 3:03 PM
Hung's right! My kitesurfing skill is only slightly greater than 0, yet I
managed to plane and get about quite nicely with an XXXL in about 8-10 mph.
I weigh about 190 lbs. The necessary skill may be with the kite more than
the windsurfing.
Dave
----- Original Message -----
From: Hung Vu <hungvu@...>
To: <kitesurf@egroups.com>
Sent: Thursday, September 30, 1999 10:11 PM
Subject: [ksurf] Kite size / weight ratio

> Robert Pratt wrote:
> > These guys are experts, there is NO WAY!!! your going to be able to
plane in these conditions for a long time (years?)
> > when I first got my 8.5 Wipika I kept taking it out in winds <10mph, if
I was able to get it off the water the best I could manage was way downwind.
>
> It's more dependent on how much a person weight than skill. My weight
> has been "published" on this list a number of times since last year and
> the way to calculate the kite size based on another person's experience
> is pretty straight forward and has been also "published". Here it goes
> again: kite size is "approximately" proportional to weight. If a 50 kg
> person uses a 5.0 m2 kite, a 100 kg person should use 10.0 m2 kite. The
> kite quiver that I currently have to kitesurf from 5 knots (start to go
> upwind) to 30 knots (start to feel way overpowered). I uses lines
> ranging from 35 - 45 m. All the kites I have are very good so far and I
> am very happy with them; however, I am planning to sell them and change
> to a new quiver next year (just to have new experiences with other
> kites; so many kites, so little time!):
>
> Hung at 125 lb:
>
> XXXL 9.66 m2 foil 5-6 knots to 10 - 11 knots
> Wipika 8.5 m2 8 knots to 16 knots
> Wipika 5.0 m2 12 knots to 22 knots
> Wipika 3.5 m2 19 knots to 28 knots
>
> The wind was too strong (especially the gusts) today for my 3.5 m2
> Wipika. I spent a third or half of the time waiting on the beach for
> the wind to "calm" down a bit. Someone could have made a "fortune"
> selling me a smaller kite then.
>
> To achieve the same thing, someone 170 lb would need:
>
> 13 m2 foil
> 11 m2 Wipika type kite
> 6.8 m2 Wipika type kite
> 4.76 m2 Wipika type kite
>
> To achieve the same thing, someone 220 lb would need:
>
> 17 m2 foil
> 14.9 m2 Wipika type kite
> 8.8 m2 Wipika type kite
> 6.16 m2 Wipika type kite
>
> These numbers are pretty good given or taken a few .1 m2 or knots;
> however, they are still just an approximation. Have a good selection and
> have fun!
>
> Hung.
>
> ------------------------------------------------------------------------
> To unsubscribe, send a message to kitesurf-unsubscribe@...
>
>
>
• Message 7 of 23 , Oct 1 7:03 PM
• ... . huge clip full of things to think about.... . Keep the faith, Dave. I for one enjoy reading your posts immensely. I may not agree with everything you
Message 8 of 23 , Oct 1 9:14 PM
----------
>From: dave@... (Dave Culp)
>To: kitesurf@egroups.com
>Subject: [ksurf] Re: Kite size / weight ratio
.
huge clip full of things to think about....
.
Keep the faith, Dave. I for one enjoy reading your posts immensely. I may
not agree with everything you write, but it gets me thinking & learning!

Art
• I ve got another 8.5 Wipika for sale just came on this one. It is in good shape and is complete with everything you need. Kite, boom, 30meters line, quick
Message 9 of 23 , Oct 1 11:39 PM
I've got another 8.5 Wipika for sale just came on this one. It is in
good shape and is complete with everything you need. Kite, boom,
30meters line, quick release. I'm asking \$550 USD + your choice of
shipping, insurance etc.
This one has been tuned and flys great.
Finally got to try out some longish lines on my XXXL. I used about
200ft in about 8mph then the wind picked up to 12-13mph. I got my
world rocked when it picked up. Lots of fun. The kite flys much
better on longer lines in my opinion. Up to today I've only used about
90feet-100ft(off of my XL). On launch it dragged me about 60ft across
the sand. Good fun. Pretty good jumps waves were about 1-3feet.
About twisted my ankle off during a crash.
I will now recommend owners of a Quadrifoil XXXL to go with a long
line set. I'm going to cut mine down to about 150feet because 200 is a
bit to long.
Pray for wind and pray that it does not come up while your flying your
big guns.
• Hi, Dave, I am one of those lurkers and as an engineer I enjoy your posts, however, I do not feel I have much to add on top of your long experience and sound
Message 10 of 23 , Oct 2 4:21 AM
Hi, Dave, I am one of those lurkers and as an engineer I enjoy your
posts, however, I do not feel I have much to add on top of your long
experience and sound analysis.

I also mess around with radio control sailplanes and I have an
appreciation of the 3-D wind picture which most sailors don't. It is
amazing how many lulls on the water are actually the leward side and
bottom of a thermal. This is significant for kite surfers, can you use
the upward velocity?

FYO there are several small telemetry devices available to measure air
speed on RC aircraft. Should be light and small enough to put on a kite
to measure on kite wind velocity. I don't know about water proof though.
I will e-mail you specific vendors if you are interested.

• ... Absolutely! Oh, kites don t have use for upward motion, per se. What we *do* have use for, just like a racing sailplane, is being able to dive the kite,
Message 11 of 23 , Oct 2 5:22 PM
>Hi, Dave, I am one of those lurkers and as an engineer I enjoy your
>posts, however, I do not feel I have much to add on top of your long
>experience and sound analysis.
>
>I also mess around with radio control sailplanes and I have an
>appreciation of the 3-D wind picture which most sailors don't. It is
>amazing how many lulls on the water are actually the leward side and
>bottom of a thermal. This is significant for kite surfers, can you use
>the upward velocity?

Absolutely! Oh, kites don't have use for upward motion, per se. What
we *do* have use for, just like a racing sailplane, is being able to
"dive" the kite, down into it's apparent wind, without actually
losing altitude. This is a major plus, if and when possible. I'd bet
most kiteflyers cannot tell the difference between a small thermal
and an ordinary gust. The difference is great, of course, in that
thermals can be predicted from visual clues (clouds), while gusts
normally cannot.

>
>FYO there are several small telemetry devices available to measure air
>speed on RC aircraft. Should be light and small enough to put on a kite
>to measure on kite wind velocity. I don't know about water proof though.
>I will e-mail you specific vendors if you are interested.

Please do. Perhaps off-list, if you will, so as not to offend anyone
wiht the extra bandwidth: dave@... Thanks!

Dave Culp
• Hi Gorge , Dave and others! Sorry for this misunderstanding, I like too the group the expertise from Dave and the others. I don t wont to break down the group.
Message 12 of 23 , Oct 3 2:12 AM
Hi Gorge , Dave and others!
Sorry for this misunderstanding, I like too the group the expertise from
Dave and the others. I don't wont to break down the group.
From my past I have also many experiences to build and fly my tail less
gliders, so aerodynamic terms are not new for me. For example on the
races we was able to start with the winch the glider, using the special
tracking path, push the glider up to 200km/h with the max. winch speed
of 16-20m/s. The physic is the same, winch is the wind speed and the
glider on the line is kite.
Dave can we set up the table of relations from the weight of the kite
and the surface area. With other words, how much wind we need to keep
the kite on the top of the window position.

touch and go
Primoz

Web page of SEAJUMP group,
http://www.olimp.si/seajump
• This kind of technical discussion is what the foildesign
Message 13 of 23 , Oct 3 7:15 AM
This kind of technical discussion is what the >foildesign< group is all
about. Why not post details there so all the technoweenies can benefit?
It's Foildesign@onelist.com . Seems like that group would love all this
stuff.
Dave

----- Original Message -----
From: Dave Culp <dave@...>
To: <kitesurf@egroups.com>
Sent: Saturday, October 02, 1999 8:22 PM
Subject: [ksurf] Re: Kite size / weight ratio

> >Hi, Dave, I am one of those lurkers and as an engineer I enjoy your
> >posts, however, I do not feel I have much to add on top of your long
> >experience and sound analysis.
> >
> >I also mess around with radio control sailplanes and I have an
> >appreciation of the 3-D wind picture which most sailors don't. It is
> >amazing how many lulls on the water are actually the leward side and
> >bottom of a thermal. This is significant for kite surfers, can you use
> >the upward velocity?
>
> Absolutely! Oh, kites don't have use for upward motion, per se. What
> we *do* have use for, just like a racing sailplane, is being able to
> "dive" the kite, down into it's apparent wind, without actually
> losing altitude. This is a major plus, if and when possible. I'd bet
> most kiteflyers cannot tell the difference between a small thermal
> and an ordinary gust. The difference is great, of course, in that
> thermals can be predicted from visual clues (clouds), while gusts
> normally cannot.
>
> >
> >FYO there are several small telemetry devices available to measure air
> >speed on RC aircraft. Should be light and small enough to put on a kite
> >to measure on kite wind velocity. I don't know about water proof though.
> >I will e-mail you specific vendors if you are interested.
>
> Please do. Perhaps off-list, if you will, so as not to offend anyone
> wiht the extra bandwidth: dave@... Thanks!
>
> Dave Culp
>
>
> ------------------------------------------------------------------------
> MyPoints-Free Rewards When You're Online.