Power Kite Forum

Different bottom ends ?

Bladerunner - 28-6-2007 at 05:12 PM

I suggested to someone that a 10m fixed bridle and a 10 m depower kite SHOULD have almost the same power when the depower is fully sheeted out ? That the difference starts as you sheet in and change the angle + shape.

In the field however it seems the the 9m fixed bridle will fly and produce power in a lower wind than even say an Ozone Access 10m. The bottom end for my 10m Flysurfer is almost the top end for a 9 fixed bridle. Yes, it's closed cell but once it's filled it doesn't get better.


Why is it that a fixed bridle will fly so early when a similar kite with depower losses that bottom end ? Why does the closed cell reduce that bottom end even farther ????

Pablo - 28-6-2007 at 08:05 PM

It's easies to think of the De-power system as a governor or dimmer switch, although it's great for reducing power, it always reduces power. I took my 9m Buster and tried an experiment on it. I fitted a USD depower system on it to see what would happen. So instead of comparing different kites to each other, we're comparing the exact same foil with diff control systems on it. Turns out I needed at least another 5mph of wind to get the kite to fly right, Almost another 10 before I was getting the same bottomless power feeling.

Makes sense as last time I put the 9m Buster head to head against a 14m Frenzy in 8-9mph winds, the Buster outpulled the Frenzy. Give it another 5-10mph winds and I'd take the Frenzy any day.

Bladerunner - 29-6-2007 at 08:15 AM

K,

I'll put my theory out there. In my pea brain, what I see is this. When you have a depower kite flying fully powered even though the canopy is as big a projected area as the equal sized Fixed bridle kite, the angle of attack is strong on the back lines so it fly's slow and doesn't generate enough power ( = speed ) in the power zone. As you pull in on the trim ( front lines ) the angle improves for speed in the window but you are starting to shape shift and now have a smaller projected area in your canopy ??????

Am I way off base on that???

Rye - 29-6-2007 at 09:38 AM

Don't forget the added weight of the lines/gear to make it de-power, these seem to make a difference in lower wind.

acampbell - 29-6-2007 at 02:45 PM

I think Rye has most of it. The complexity adds weight.

I can coax my de-powers off the ground in light winds by trimming the front lines all the way in and then even pulling more. But I don't think that changes shape so much. Rather it gets it flying faster with the lower AoA. but then you do not have a de-power; instead when you pull on the bar to" power up", it stalls and kills the kite. All bass-ackwards (like a fixed bridle) unless you can get in the bug or on a board and get it moving- then it behaves "right" for a de-power.

awindofchange - 29-6-2007 at 06:11 PM

One of the main differences is the actual shape of the air foil from one to the other. Depowerable kite sails are designed with the depower in mind, that is the sail is shaped in such a way that the wing will dump power when needed. Fixed bridle kites have a set foil shape that is designed for maximum power. That shape is held solid by a network of bridles that keeps the kite at maximum power through the entire wind range of the kite. On depowerable kites the bridles are designed to let parts of the sail change for the depower. Because the sail changes shape during flight on a depowerable kite, the wing tips have to be constructed in a special way to give the kite stability and keep it inflated. The wing tips become more like rudders instead being used for power production. On a fixed bridle kite the entire kite can be designed with maximum power output with stability built into the shape of the wing so you can use the entire surface of the kite instead of relying mainly on the wing tips for stability and the center for power.

The excessive bridling and extra depower lines will add some weight and drag to the kites, but this is pretty minimal to the actual power output of the kite. Most of the power differences is in the physical design of the kites themselves.

As for comparison, I was buggying the other day with a 10 meter Frenzy and was powered up pretty nicely. I then changed over to a 5 meter Haka and was powered up about the same but was able to go much faster in speed. The Haka rips! :)

acampbell - 30-6-2007 at 05:10 AM

I think the question was about the very low end of the wind range where the heavier de-power kite can't get out of its own way to exploit those properties and behave as you described. The lighter fixed bridle of the same power range in the same light wind will launch smartly and fly as it was designed.
Sure in their own elements (a bit more wind for the de-power), they are different beasts.

Bladerunner - 3-7-2007 at 06:49 PM

Thanks for the insight ! The difference in the shape / cosntruction of the kite needing to be different was the missing piece of the puzzle. I agree that the extra bridle + pulleys must add a bit of weight as well. That alone just didn't seem to be the major factor.
From the bit of experience so far if I HAVE to launch with the added pull on the trim lines the kite won't fly well enough to entertain at all anyway.

DenisLaMenace - 3-7-2007 at 07:37 PM

As for the closed cell kite such as Flysurfer, the power is also reduced a bit since the air traveling through the cells is reduced compared to an open kite.

More air = more apparent wind = more power.

Pablo - 3-7-2007 at 08:10 PM

It's got to be in the actual depower setup. I've taken a fixed bridal kite(9m Buster I) and converted it with a UDS system to a proper depowerable foil. So the foil is exactly the same foil being flown first fixed bridal, then depowerable. Normally the kite fly's well in 4-6mph winds and pulls great by 10mph, by 15mph I'm getting overpowered. With the USD fitted the kite would barely stay in the air at 10mph winds, by 15 it was just starting to fly properly. The Pegas Wing Warp system works a little better, keeps more of the bottom end, but you lose a lot of control on the depowerability. After a couple weeks, I got so fed up of the low wind performance that I swapped it back to a fixed power bridal.

So it's either got to be the added weight of the depower setup, more drag, or the difference in control. I think it may have something do do with the fact that fixed bridals can be turned on the front lines by warping the foil, while most depowerable foils turn by applying brake to one side of the wing. Basically like trying to turn a fixed power foil by only using the brakes in low winds.

You really need enough airflow over the depowerable kite to allow you to hit the brakes on one side without collapsing the foil.

krumly - 13-7-2007 at 10:10 PM

Man , I got in late on this one - just back from a Yellowstone family roadtrip w/o kites!

I'm with Pablo. I've mostly used a 3.5 m Concept Air freestyle for my depower rig experiments, so all tests are on the same chassis. It's set up with separate secondaries for A's, B's, and C's like the PKD's. I've also used UDS II and Wing Warp/Frenzy pulley rigs, and noticed pretty much the same thing as Pablo (Smooth power progression with UDS II, but less bottom end; more bang on/off depower with Wing Warp, but better bottom end).

But here's the deal: The depower rigs work much better with handles than on a bar with this kite, 'cuz it seems to be pretty balanced between steering off the fronts as well as the brakes. The steering is way more sluggish on a depower bar, even in a blow. I'd estimate 2-3 mph better low end with any of the depower rigs on handles vs. a bar with this kite.

In really light winds, the weight and aero drag of all the additional lines, pulleys, stoppers, and straps really affects the kite. Not surprising, as lift is proportional to apparent wind velocity ^2.

Kites with reflexed foils usually need quite a bit more surface than a positively cambered foil of the same aspect ratio to develop the same pull at the same apparent windspeed. The price you pay for pitch stability is usually a 15 - 25% decrease in lift over a "standard" positively cambered foil section at the same angle of attack.

I differ with Pablo right now in that with the gusty stuff that is the norm here in MN, I'm totally re-rigging my old Kitesurferr XXL to a depower setup. I measured the bridles and bottom shape of the foil and laid it all out in CAD to get separate secondaries for A, B, C, and D's. I'm tuning it to fly well as a fixed bridle again at 6 degrees design AOA before I put the UDS II on. I'm having problems with getting the wingtips right, but I'm close.

It's such a light, valved foil (.5 oz Icarex polyester) it works great in low winds, once it's inflated. Closed cell does not equate with bad low end. At 127 lbs, I've skiied behind it in winds 5-7 mph when tube kiters were just sitting on the ice in lawn chairs with parked kites. It worked well as a fixed bridle on a crossover, but I really feel safer having either brakes on handles or a depower bar. I have my fingers crossed.

BTW, at the junction of Hiway 14 and 14A in the Wyoming Bighorns, near "Burgess Junction." National Forest land , open range, bald, rolling plateaus at around 9000 ft. I'm going back this winter. Also, Yellowstone Lake - sweet. Gotta see if they allow snowkiting. The park was packed over the 4'th, but the lake was empty, and it offers awesome access to the backcountry at the south end of the park.


krumly

Pablo - 15-7-2007 at 10:41 PM

I can see why you'd like the depower in those wind conditions, here in the summer months our wind speed is usually in the 8-12mph range max, some days more like 5-10. That couple mph range that you lose by going depower was often enough to make the difference between getting moving or not. I hadn't even thought of running a depower setup on handles though, should have tried it. I may still give it a go on a smaller kite that would be flown in stronger winds.

Rye - 16-7-2007 at 04:14 PM

You have peaked my interest krumly. I never thought of trying a depower rign handles. Do you mean it will still operate as a depower? I have further questions if this is the case, I would like to hear more on this please.

speleopower - 16-7-2007 at 08:33 PM

I think a lot of the lower end pull is in the weight and drag of the lines, bridle and kite.
For example my Flexi Blade 10.5 will fly quite nicely in barely a breeze. I like to have long tails on my kites...it's silly but I like it. When I put a single tail on the 10.5 it has a hard time getting up with 2 tails it takes even more wind.

At the low end you need to minimize drag and weight. For example I fly my bigger foils with 300 pound mains and 50-80 pound brakes. If I put 150 pound brakes and 600 pound mains the kite has problems taking off and maintaining itself in the air. In really light air if I'm desperate I've even put 80 pound for mains and something like 20 for brakes. No jumping allowed with this but it helps.

The Quadrifoil XXXL is a super light weight kite and can fly in barely any wind at all. I have even flown it in almost no wind by running and have flown a complete circle around myself.

It's all weight and drag. Remember when you double the cross section of a circular object you quadruple the drag.

That's my story and I'm sticking to it whether it is 100% correct or not :puzzled:

Scott

krumly - 17-7-2007 at 02:03 PM

Yeah - I think you're right, speleo. Amazing what you'd think is an inconsequential amount of weight or line drag does to hamper you in low winds. And it's also useful to note both lift and drag increase as apparent wind velocity squared. But at a constant wind velocity, lift generally increases linearly with changes in AOA while drag increases parabolically(squared) with increased AOA. You can power up your depower kite for increased lift as it climbs and momentum keeps it going for abit, but you know it's gonna settle way back in the window if you leave it like that.

Rye - Hooking the front lines to the top of the handles and rears to the bottom has you powering up by pulling in the brakes, but let's you have the subtle control of handles. I prefer long handles to give me the throw I have on my bars - long enough to depower the kite so it's just about to luff ("brakes off"), then powered up enough to backstall and reverse the kite ("brakes full on"). Try it on a small kite first to be safe...

krumly

Bladerunner - 17-7-2007 at 06:25 PM

Quote:
Originally posted by speleopower
I like to have long tails on my kites...it's silly but I like it. When I put a single tail on the 10.5 it has a hard time getting up with 2 tails it takes even more wind.



By tails do you mean streamer type tails ?? I see how they would be a problem in low wind but what do they do for you in good winds ?? I mean besides look real pretty, I'm sure :wink2:
Don't they get messed up with the bridle ?

Dagon - 18-7-2007 at 12:30 AM

You guys got it all wrong. Depowers are slower because of the plutonium that is used to make the fabric and lines. not only does this add a lot of weight, but the intense radiation reflects off of the gogonosphere effectively creating an "antiwind" that can be adjusted as needed. This is why it is always important to wear a tinfoil hat when flying a depower.

Some people might argue that a reverse matter propulsion amplifier would do the same thing but I have tried this and it doesnt work.:P

Rye - 18-7-2007 at 06:11 AM

Yes, but with the reverse matter propulsion unit you could ditch the silly hat, right?

speleopower - 19-7-2007 at 05:49 PM

The gogonosphere is quite real.

The tails are just long streamers. They look cool. Once the kite is flying they don't seem to do much except make it harder to recover from a stall/deflation event.

I've not had them tangle in the bridle yet...I hope it doesn't happen. They really trail not directly behind the kite but fly slightly "downwind" and behind the kite making it unlikely to get tangled up.
I started by attaching 2 streamers to the wing tips on the trailing edge. But lately I've tried just one streamer attached at the trailing edge in the middle of the kite. The Flexi 10.5 meter has 2 small flaps of velcro to hold the bridles when lines aren't attached. This is where I've started attaching the streamer.
Before I get in the water and kitesurf with the streamers I'm going to keep flying on land with the mountain board. So far I've not had a problem while flying with streamers. But I want a few more sessions on land before I get in the water with a Blade.

Scott