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 Post subject: Re: High Jumps kitePosted: Thu May 17, 2012 8:59 am
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Joined: Mon Jun 28, 2010 6:58 pm
Posts: 322
& 1 more question

back to the topic - is a Torch able to boost (jump high and hang time)?

I don´t see a C-shape as the best design for either height or hang time.

Bellow the lift distribution over the span: the final lift is just the sum of all those small vectors

on a C-shape just the mid third is really contributing with a vertical component, near the tips the force is more horizontal than vertical.

If we think in a flatter platform the force over all the span is poiting in a vertical direction

Thus my conclusiong that a flatter kite would be able to generate more CL & will have a better L/D (for the same airspeed, area, etc) than a C-shape

Again - am i missing something?

Attachment:

lift distribution over span.JPG [ 45.44 KIB | Viewed 2461 times ]

note
in reality my feeling is that a C-shape will turn quickly & somehow balance things in terms of Clmax. but from a theoritically stand-point, with enough strenght, both would turn at the same rate

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 Post subject: Re: High Jumps kitePosted: Thu May 17, 2012 9:54 am
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Joined: Wed Aug 10, 2005 9:49 am
Posts: 221
Gentlemen,

I have all the answers to these questions, but sadly I c.b.a. to put the time into addressing them buried on page 7 of a derailed thread that will be lost in the ether like many before it.

Some people here like JS are very close to the truth, while I am afraid others have just enough understanding to get very muddled up.

Instead here are some things to think of before getting into too much detail about angles of attack etc.

1. state your frames of reference: the water/ground or the airmass is one choice. the other is whether you are talking about the kite, the rider, or the kite and rider as a unit. essentially we have a selection of six baselines, most of which have appeared through this thread out of context with each other.....

2a. It can be very helpful to consider how this thing plays out with a simpler unit than a kite/rider combo, starting with infinite l/d (or zero drag assumptions), and then gradually adding in the complexity as you build up your understanding.

2b. total lift is the product or airspeed and Cl. with airspeed being much more important. never forget this as you think on.

e.g. imaging a glider with no drag flying along horizontally at some notional speed(40m/s is a good one) which then pulls up. it can pull up steeply or gently. assuming we require it to still have enough speed left to glide at 1g just above stall speed(20m/s) the ultimate height it would reach(60m gain in this example) is not affected by the gradient of the climb in this case. for a steeper climb, a period of higher Cl is used initially to change the trajectory from flat to upward, but once we are on the upward trajectory of our choosing, the Cl can fall back to a lower value as we continue on upwards.

now a long gentle climb bleeds more energy over the time it takes to get up there than a shorter climb, but the initial pull up into the shorter climb bleeds more energy than the gentle pull up. without knowing the wing loadings and polars one cannot specifically answer which comes out better, but in the case of low l/d units like a kite+rider, typically a steeper pull up is better for us in most cases. intuitively all this agrees with practice.

3. considering kite+rider. in a glide at 1g, the rider contributes a lot of drag to the sum. probably halving your effective kite l/d from maybe 7 to 3.5. In dynamic maneuvers like take/off or kiteloops, the kite is operating at a higher airspeed than the rider(delivering higher "G" to accelerate us in the direction of our choosing), so the main energy loss to drag is coming from the kite. effectively we can operate at a higher unit l/d that is closer to the kite l/d when we are using it for accelerations. This explains more intuitevly why it is best to boost aggressively and lose less total energy in a steep pull up and rapid climb, than in a gentle pull up and slow climb- climbs and glides are effectively done a a lower total kite+rider l/d than sendings/loopings!

4.conveniently for us kiters(though not glider pilots) because rider drag is so significant on the system, when gliding and climbing, our total l/d is maxed when we operate the kite at Cl max, rather than at it's max l/d.

5. here is a bit of a kicker hehe, (3.)and (4.) don't matter as much as you might think, there is quite a wide range of AoA and Cl/Cd that we can utilise and still get good jumps

6. as we near apex, only in the steepest climbs is it possible to get near max acheivalble energy conversion(after losses) from speed into height. i.e. going ballistic and getting between 0 and 1g at the top. In most cases we top out when we drop to the airspeed that gives us total lift=total weight. i.e. if we sheet in for max Cl while going faster than the glide which gives us min sink, we go up until we lose speed enough that we are now gliding at min sink.

7. to glide furthest(downwind) as a kite+rider, we choose min sink, not best l/d. this is because are groundspeed is largely made up of the speed of the airmass downwind, through which we are gliding.

8. small kites mean higher wingloadings in the glide so we come down faster, but if we helicopter the kite above us, it can operate at higher airspeed than our rider-through-the-air glidespeed, so gives more total lift , thus allowing a slower sink rate than if we just glide statically.

9. wind gradients and gusts encountered while airborne effectively give us transient increases in airspeed(energy) and because this is a dominant input, can have dramatic effects on total achievable height.

10. given known conditions and known kite polars( and known rider drag) you can use all the available info to predict how high a rider with perfect technique can go in a given situation.

11. i haven't mentioned technique, but things like carving into the apparent wind vector on take off to modify the rider through airmass momentum(granting more airspeed) have large effects due the geometric impact of speed. likewise the faster you get your kite from sailing position, to upward acceleration, the less rider momentum is lost, and every little bit counts. If rider a loses 10% momentum(relative to airmass) in the sending phase but gets it back from the carve upwind and rider B doesn't carve and loses 20% momentum while sending, rider A will go 56% higher, all other things equal!

12. small fast turning kites therefore often more than make up for their lack of total lift compared to big kites. within reason. apostage stamp will never have enough area to give you even 1.1G of lift to get airborne in anything less than a nuclear blast, but then you have other problems...

13.C-kites arguably turn faster and get turning faster than Bows, so the whole sending process can be done with less rider momentum loss, so maybe they start the jump with more energy. Bows have higher acheivable Cl so can increase the lift by a greater factor than C's and arguably have a fractionally greater l/d when static so they should do better when airborne and turn inputs are low. It all kind of balnaces out, though small flatter kites have it best theoretically(they don't pivot as much proportionately per span than a large bow does for a arc of a given radius, so incur less turning penalty compared to C than a large bow does)
Modern hybrids seem to have the balance pretty right

14. following on from 13, C-kites probably have a better l/d in the turn than a flat kite, so are more suited to looping.
A flat kite truns mostly from drag increas on one side. a c kite turns by the inside "rudder" adopting a high AoA and pulling against the outside "rudder", the outside "rudder" equalizes the pull at it's lower AoA by going faster round the outside[think of it like it is a mini-kite in some kind of lateral transient powerzone, trying to fly forward to the edge of a window that keeps moving if you will] there is still assymetric drag involved, but less than a flat kite, which is the main point of contention.

i used to race sailplanes and do aerobatics in them. this energy conversion stuff is bread and butter. if you think boosting is fun, you should try doing a pull up from a Vne beat-up. altitude goes from 3ft to 7-800ft as airspeed bleeds from 150kts down to 40kts. you can do it at 2g or you can do it at 4g. Or entering a thermal at 100kts and doing a full pull-up combined with the rising airmass, you peak out at zero G, stick hard over boots of rudder but featherlight touch, carefully nursing it round into circling at 1.5g without stalling or initiating a spin, looking back down at the guy 1000ft behind an below chasing you into the same thermal....it's like boosting, just 5 times better!

monster post eh? and i said i wouldn't

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 Post subject: Re: High Jumps kitePosted: Thu May 17, 2012 10:25 am
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Joined: Fri Jan 09, 2009 4:49 am
Posts: 2775
Location: Japan
Thanks for posting!
This - In most cases we top out when we drop to the airspeed that gives us total lift=total weight. - is confusing though ...

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 Post subject: Re: High Jumps kitePosted: Thu May 17, 2012 12:15 pm
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Joined: Mon Jun 28, 2010 6:58 pm
Posts: 322
wow! thanks that you c.b.a.

Did´t realize 4. Is this true both while airborne and sailing?
The reason i´m asking is because when sailing 1 way for the kite to move forward in the window is to release a bit the pressure on the bar. I always though that what we were doing was reducing the AoA to move from the kite Clmax to L/Dmax (less lift but also less drag => thus the kite advances in the window).

But then this seem to somehow contradic what you said on 4. => I am now in www.confused.com

Back to the original topic & from 13. should we conclude that the best design for a heigh jumping kite is:
1. for high winds/small kites: hybrid that tend towards a flat platform
2. for low winds/bigger kites: a hybrid that tend towards a C-shape
(directional stand-point)
?

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 Post subject: Re: High Jumps kitePosted: Thu May 17, 2012 3:43 pm
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Joined: Fri Jul 16, 2010 6:27 am
Posts: 1330
Location: Ford Lake, Michigan
respect Rabidric, you know what you're talking about.

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 Post subject: Re: High Jumps kitePosted: Thu May 17, 2012 4:17 pm
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Joined: Wed Aug 10, 2005 9:49 am
Posts: 221
apologies for all the typos in that monster post above...

Kamikuza wrote:
Thanks for posting!
This - In most cases we top out when we drop to the airspeed that gives us total lift=total weight. - is confusing though ...

assuming you are trimmed at a fixed aoa or Cl this is true.

e.g.
1.start with you and the kite are travelling at 20m/s through the air horizontally with a Cl of say 0.25 and at 1g.

2.now you sheet in to an AoA that gives Cl of say 0.5. At this point in time kite is now making twice the lift compared to your weight. you will accelerate upwards at initially 1g, feeling 2g(gravity is the other 1g). but we will turn airspeed into height, and so lose airspeed.

3. as we climb we lose airspeed, so we lose the excess lift and acceleration. at some point we end up with the same total lift from Cl=0.5 as we had from Cl=0.25 , except now we are higher and are travelling 29.3% slower(14.1m/s)...not 50% slower, because lift is proportional to the square of windspeed

4. how much higher? about 10m minus the losses to drag, so maybe 6-7m higher. (kite l/d of say 5:1 costs 2m and rider drag another 1-2m)

That was the tame scenario

now imagine you raised Cl from 0.25 to 2 at the start. kite lift just went up 8 fold for a fraction of a second, so you will go up at 7g, feeling 8g. in this case we do almost all our vector adjustment in a fraction of a second, the kite won't be adding anything else in the climb, we are ballistic. In this case we top out a 8m, feeling 0g. we got 1meter higher than the slow climb. we still lost 20% of the available energy to kite drag, but rider drag is much much less relevant this time as the kite forces have quadrupled compared to the previous example.

this goes to show that it isn't worth worrying too much about squeezing out the little bit extra heigh from a ballistic trajectory following a violent takoff. the gentler ride will get us nearly the same height, but won't break our back or kite lines heheh.

in practice when jumping we don't just raise Cl, we increase the kitespeed by sending it through the top of the powerzone, where it flies faster. if your kite is big enough and you are " fully powered up", then you don't need to sheet in at all, you just send the kite aggressively enough and all your excess total lift is generated by the extra kitespeed.
so in reality the kite will be travelling maybe 1.5-2 times the speed through the air that the rider is feeling. but since the rider is effectively the center of mass, once you are airborne all the kinetic energy remaining to be converted into height comes from the airspeed of the rider . if the kite travels faster, then the conversion is quicker, but the total limits on height are dependant on the energy the rider is carrying at takeoff. [ ok, ok, plus whatever extra stuff we pick up from gusts and wind gradient while airborne, lets just leave that alone though ok ]

the tradeoff with making the kite fly quicker, by sending it through the zone deeper, is that the vector of it's lift becomes more horizontal, so we will get more brutal takeoffs but won't go as high.

the sweetspot is if you can combine a bit of Cl increase(from sheeting in a bit of AoA) with a bit of kitespeed increase(from sending it back through the window) , at the appropriate angle( maybe 20-30 degrees back off vertical) that you get a decent initial boost to get you going up fast enough that you don't bleed all your energy to drag before you can convert it to height, but not so violent that your vector is too much back off vertical that you start losing out on potential height to the laws of trigonometry (35-45 degrees back off vertical or more).

the sharp ones amongst you may have spotted that if kitespeed through the window is best at max L:D then you might benefit from not sheeting all the way to Cl max until the actual moment your board leaves the water. In practice this is understood intuitevly as good technique. hopefully I have helped some of you understand the theory and made the two marry up.

edit: if max fun is what we want, well, then we might want to get the kite further back off the vertical by looping deep at takeoff. It will be more violent, so we will end up mostly ballistic(weightless the the top) but since the vector is lower back we will get a big yank downwind but won't go as high. this is like a unhooked kiteloop+railey. In kinetic terms it is slamming on the brakes/reverse thrust in style.

edit2:If we start with a max height boost but as we go up we modify the goal into a max yank, we basically end up with the hooked in mega loop scenario, so we get semi ballistic, we get a pretty big yank, and we get a pretty big height. a nice all round kinetic combo of upwards into backwards.

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 Post subject: Re: High Jumps kitePosted: Thu May 17, 2012 7:40 pm
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Joined: Mon Jun 28, 2010 6:58 pm
Posts: 322
i´ll still need to repeat the reading 2 or 3x times to get all the details but:

thank u!

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 Post subject: Re: High Jumps kitePosted: Thu May 17, 2012 9:20 pm
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Joined: Wed Aug 03, 2005 6:25 pm
Posts: 758
Location: Vancouver
Rabidric wrote:
I have all the answers to these questions...

Thank you for taking the time. Very well thought through, and explained. Many kiteboarders get tripped up by failing to understand and apply the simplest aerodynamic and other physical principles first, and then interpreting them in the context of kiting.

Almost everything you wrote is right on the mark, however there's at least one relatively minor point worth clarifying:
Rabidric wrote:
8. small kites mean higher wingloadings in the glide so we come down faster, but if we helicopter the kite above us, it can operate at higher airspeed than our rider-through-the-air glidespeed, so gives more total lift , thus allowing a slower sink rate than if we just glide statically.

When we helicopter an otherwise well trimmed kite directly overhead, we don't increase it's airspeed. (If anything, the induced 'warping' of the kite [to turn it], and the varying AOA across its span [due to turn-induced varying airspeed] likely reduce net lift, L/D and airspeed, but increase sink rate [relative to the kite, not necessarily to the surface of the earth].)

As I supect you recognize, all you can do to increase a given well-trimmed kite's airspeed while airborne is a) adjust your bar outward, effectively reducing AOA (before and after the kite/rider-system transitions to a new steady state), or b) increase line tension by directing the kite so you swing (rotate) about some vertical and/or horizontal axis (eg jumping and/or looping) to induce centripetal acceleration and reactive centrifugal force.

Tight 'helicopter' loops overhead don't significantly achieve this acceleration/ force/ lift/ airspeed increase. Big loops do though, because the rider swings through an arc, causing centripetal/ centrifugal force to increase his effective weight beyond the force of gravity alone (1g), resulting in additional tension in the lines and therefore increasing kite airspeed.

Considering that line tension is almost equal to lift (neglecting drag component vector), wide kiteloops can increase lift (perpendicular to airflow relative to the kite), but they'll unfortunately also increase sink rate (relative to the kite). However, because part of the kite's sink rate has a horizontal component relative to earth, the kite/rider's vertical sink is less than the kite's 'aerodynamic' sink. That said, I don't think looping significantly reduces the rider's sink rate relative to earth, although I don't have time to go through that quantitatively.

Related topic: Of course, we haven't taken into consideration any vertical ambient air currents. But if we do, 'helicopter' looping can be a valuable tool, because it allows the rider to stay in a rising air mass, in particular a region of rotor turbulence. I wrote an article about that years ago as well. Here it is:
http://kiteboardbc.com/phpBB3/viewtopic.php?f=1&t=57

And here's one I wrote about gusts, force and acceleration. Some of it is relevant to the concepts at hand:
http://kiteboardbc.com/phpBB3/viewtopic.php?f=1&t=268

Thanks again for valuable contribution,
James

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 Post subject: Re: High Jumps kitePosted: Fri May 18, 2012 8:38 am
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Joined: Wed Aug 10, 2005 9:49 am
Posts: 221
Hi James

well i didn't specify how tight a helicopter loop but of course if the inner tip is near the center of the rotation, then the looping is going to be worthless!

whilst it is theoretically difficult to "prove" wider helicopter looping works I take comfort from the field data(admittedly not scientific)that in practice I am prettysure that I get longer "float" by looping small kites above my head in some fashion after a very big jump which reduces my airspeed within the airmass to lower than my glidespeed under a stable 7m or so of fabric. The kite is definitely moving through the air faster than I am, just like an auto-gyro's main blades pass through the air on a faster circuitous path than the actual underslung machine does. The key to it all is that I can operate the kite at a Cl of best L:D instead of min sink, and lose less energy overall. the kitespeed compensates for reduced Cl but the achieved kite l:d is better than in a stable min sink glide scenario.

probably not much in it though, and most of the real benefit is that I keep the lift vector more squarely overhead(or "counter-orbiting my opposite phase rotating mass vector" ,ahem), more consistently, for longer, and maintain a steady kitespeed; compared to back and forth overhead sining with large amounts of time spent hairpin turning the kite at either side inefficiently, or just hanging underneath and having the kite outfly to one side and drop me. (also, stable overhead static min sink glides are hard to setup after many climbs with all the vectors out of sync and alignment, but easier to setup if we just ski off a hill....)

happy landings

there were other factual errors that i glossed over because I was trying to illustrate in exxagerated terms so that it the reader could form a picture, e.g.the second example in my second post, the 7g @ 40knots is never gonna happen because 40knots does not contain the energy to supply more than a few G of acceleration, and that those G is assume an instantaneous application, which is also impossible, as the moment g>1 the mass starts moving upward and the ability to apply further G increase is reduced immediately etc etc etc.

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 Post subject: Re: High Jumps kitePosted: Fri May 18, 2012 9:23 am
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Joined: Wed Aug 03, 2005 6:25 pm
Posts: 758
Location: Vancouver
Rabidric wrote:
Hi James

...there were other factual errors that i glossed over because I was trying to illustrate in exxagerated terms so that it the reader could form a picture.

Likewise. To explain these concepts without inviting excessive confusion, I find it best to leave out many details and assumptions which would require pages to expand upon, but don't actually affect the conclusions.

Cheers,
James

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