Will there ever be a depowerable race kite?

[Simon Bailey]

The "de-power" on my combats is not based on AoA changes, but rather an increase in camber, which as you know increases lift and is a more suitable method than AoA at low speeds.

 

The next idea is to try and add vortex generators to the top skin to increase the stall speed, allowing a greater camber and therefore increased low speed performance.

 

The Combat has no issues at higher speeds (when the apparent wind kicks in), but really suffers at low speed, so my research is in trying to increase the low speed performance.

[sw67]

I can see a 7m speed being very good in winds were most others are on 2-3m fixed bridles but i am not sure about light winds

 

My speed just keeps going faster and faster until i run out of space or bottle

 

Stephen

[Simon Bailey]

At the worlds, there was a pilot that flew a FS (I think it was a speed), that was a nightmare to pass (big bulky ar$ed floaty thing), but once past, it was easy to leave him.

[sw67]

I have seen that movie on the flysurfer website - it was a 12 or 14m speed 2 prototype if i remember and it looked as if it was up against fixed bridles of a similar size

 

I think a 7m would give a 2-3m fixed bridle a good run for its money

 

Stephen

[spiky_dave]

The "de-power" on my combats is not based on AoA changes, but rather an increase in camber, which as you know increases lift and is a more suitable method than AoA at low speeds.

 

The next idea is to try and add vortex generators to the top skin to increase the stall speed, allowing a greater camber and therefore increased low speed performance.

 

The Combat has no issues at higher speeds (when the apparent wind kicks in), but really suffers at low speed, so my research is in trying to increase the low speed performance.

 

Yeah - thats sort of what i mean - i doubt people will abandon a fixed angle of attack, because of its advantages, but if 'de-power' can be acheived other ways people will use it.

 

How are you going to make the vortex generators - they are supposed to be perpendicular right? I guess you could make half a cylinder, then sew it so the opening is hemispherical, and the sides form a triangle to the back, with the rear sewn together. Kind of like a half a windsock with the TE sew flat vertically.

[Simon Bailey]

Dave, Vortex Generators are normally simple triangular protrusions on the upper skin, that penetrate 80% into the boundry layer, so with this in mind I had it in mind to sew in some mylar re-inforced additions, in the cell joins.

[spiky_dave]

Dave, Vortex Generators are normally simple triangular protrusions on the upper skin, that penetrate 80% into the boundry layer, so with this in mind I had it in mind to sew in some mylar re-inforced additions, in the cell joins.

 

Yeah, i know, i just have an aversion to solid bits in foils . In a bodging style, you could probably fashion some simple vortex generator from bits of those shatterproof rulers.

 

Oh, i have seen some vortex generators that actually aren't really vortex generators - they actually funnel air downwards, i'm not sure if it's from outside of the boundary layer into it, or just from the top of the boundary layer deeper into it.

[stumcgoo]

Would opening vents along the trailing edge depower the kite? Kind of like the flaps you get on some kites to get rid of sand etc.

[Simon Bailey]

Stu, we tried this with a kite modified with TE dirtouts...all that happens is the cell pressure drops and the kite looses it's profile shape.

 

Flysurfer use Jet Flaps as a boundary layer energiser...not as good as vortex generators, but prob the only workable solution on a flexible wing.

 

I have a prototype de-power on my 9M Buster that instead of changing the AoA or camber or any other normal method of de-power, I change the projected area whilst in flight. From rough calculations, the kites projected size ranges from 6.5^m to 9^m. This is combined with a new steering control method (bridling) and a robust safety system.

 

This system allows me to set the AoA and Camber at the optimum for the wing, with de-power being affected by simple projected size.

[8015]

All U-turns will take SPS/UDS/Wingwarp etc. to convert them to depower, as will centuries and anything bridled by rows.

 

I have experimented with UDSII on my 9.7 nitro, and for a while I really liked it, because it gave a smooth power up. I ditched it in the end though because the friction in all the pulleys meant that it was slower turning (like you pull in and then wait for the lines to work their way through the pulleys and change the profile of the kite). It takes some setting up to find the limits of travel for a race kite - I only tried it on the 9.7 because being big it will use a bigger range than a smaller kite, therefore it is easier to set the stops and to feel the different speed/power as you move the handles.

 

I have also used SPS on my 4.5m Butane. My thinking was that SPS has less pulleys, so will react more quickly. Unfortunately I never really got round to tuning it properly before I needed to switch back to fixed bridles for racing - and I haven't had time to go back to it. The Butane is a good one to play with because it is really stable at low angles of attack, but believe me if you set the stop too far it will become a bag of washing like any other race kite flown past the edge!

 

As for pulley handles, I just found they added extra resistance and made the big kites harder to fly and slower to turn.

 

Depower race kites, maybe but they won't give significantly better wind range and won't come from the manufacturers, they will be home grown tweaks from people like Simon and I seeking to make the existing kites more efficient, although to be honest I don't really have time to work on it any more.

 

Didn't the flysurfer guy at the worlds get dubbed "Demolition man" by one of the English team who got repeatedly wiped out by him?

 

Jim

[philpig]

maybe this why im at the back of the pack but si you lost me about 6 posts ago and you look like you know bugger all when your at hoy ,smart arse

[DaveN]

So given the popularity of the access there is obviously a market for depowerable non-lifty kites. So if Arcs are quicker and more responsive than foils, could there be a non-lifty Arc - or is there something in the design that makes them lift?

[Ad Branger]

Hi Simon,

 

I like the way you are writing about your research and development experiences with your kites. After reading the story about the brakebridles I was sometimes surprised about the (expected) results. I know that you have a good knowledge of aerodynamics, so I was wondering if you had have the same thoughts as I have.

 

 

Brake bridle of the Combats

You changed the brake bridle on the Combats from the TE (the E-row) to the D-row. When you’re applying the brakes you’re now pulling the D-lines instead of the E-lines. You wrote that you’re cambering up by doing so. If you’re really add more camber by doing this, the foilkite will turn to the opposite site of what you want (you pull left > add more camber left > increase the lift on the left site > kite will go right). But that’s not happening.

 

In my opinion the profile will change into a reflex profile. Because it’s not a rigid profile the more you pull on the D-lines (your new “brakelines”) the more negative camber the profile will have. Changing the camber near the trailing edge have a proportionately influence on the pitching moment. If you add some negative camber to the TE-part of the profile you can easily change the negative aerodynamic pitching moment in a positive pitching moment.

 

As you know a reflex profile has a lot of advantages. The most important one is stability, but a negative camber at the TE has also a lot of properties which are not in advance of efficiency (L/D ratio). Think about more drag, lower maximum liftcoefficient (less lift at the same AoA), more separation of the airflow at the lowerskin at the TE (increase of drag), etc. All those properties are the reason that the brake lines attached to the D-row on your Combats are working fine as brake lines. When you’re pulling on these brakelines the drag will increase and the lift will decrease.

 

 

Performance at low speed

Low speed and using the brakes is a normal combination off course. In my opinion forming a reflex profile will happen in a situation where the brakes from the E-row are changed to the D-row. So in a turn, when you’re using the brakes, you add some negative camber to the TE.

 

As mentioned above a negative camber will give a lower maximum liftcoefficient. A lower maximum lift coefficient will also result in a higher stall speed (the foilkite will stall earlier). So when you’re applying the “new” brakelines there is a new (extra) risk of stalling. In my opinion you must stay away from the possibility of forming a reflex profile when you’re after better performance at a lower speed for a racekite.

 

 

Extra power while using conventional brakelines

Another thing you will miss when you don’t have the brakelines at the normal location at the TE is the extra power when you’re applying the brakes a little bit. Pulling “normal” brakelines will both increase the camber and the drag a little bit. The first (small) part of the brake-action will give more extra lift than extra drag. That’s why the power is increasing in this first part. When you’re pulling more on the brakes the drag will increase more than the lift. Than you’re braking. When you’re pulling the D-row instead of the E-row you miss this little extra power.

 

 

Conclusion

The change of the brakelines will definitely work as brakelines, but will not be in your advantage when you’re thinking about drag, liftcoefficient, efficiency, separation and performance at low speed. Although it’s very important to keep the TE in shape with enough brakelines, I think it’s more efficient to think about reducing the drag of the brakes by making the lines as light and thin as you can.

 

 

 

Cheers, Ad