Power Kite Forum

lift vs pull - do the kite specs tell the story?

DonaldLL - 30-4-2009 at 04:11 PM

I know y'all will get a chuckle out of this... I am new to the forum and new to power/traction kites. How new? Well so new that I don't even have a kite yet but I have already purchased 3 of them! Two of three, HQ Beamer IV 4m and Flexi Sting II 2.4m, will arrive tomorrow! Also bought a used 1.2m Sting and it's on the way too.

I am reading and learning alot but I wanted to confirm or ask:

Talking only about foils here.... can I tell by the specs on a kite if it is a lifting vs a pulling kite? Assuming the same size kite and just focusing on A/R.

From what I read I see that there is a correllation between greater A/R = greated lift but is that a given or just probable. I mean if a kite has an A/R = 3 will it always pull more and lift less than a kite with A/R = 4?

If someone tells me "My kite has an A/R = 4" is it correct for me to say "Mine has an A/R of 4.5 so it is a bit more lifty than yours" or do I need other factors to really know this?



Thanks

acampbell - 30-4-2009 at 05:04 PM

Excellent question Donald.

It's probably fair to generalize a bit and say that higher A/R will produce more acceleration and fast power build up but there is more to it. We left out L/D or lift / drag ratio. Using the word "lift" here can confuse things, so lets call a higher L/D kite a more efficient kite and not necessarily more lifty.

For more pull out in front of a buggy or board, a kite wants to be efficient enough to fly as close to the edge of the window as possible. It gets this not by grunty lift overhead but by low drag. Higher A/R can make a kite more efficient, and race kites have high A/R, but so do some lifty kites like the Blade, so there has to be more to it.

Reducing drag goes a long way to increasing L/D. One way is a thinner profile (rib or airfoil section). That gives up grunt lift for speed. We can also reduce drag by using thinner bridles and fewer of them. To use fewer bridles we we give up external support to hold the shape of the kite, so designers use internal support such as diagonal ribs (D Ribs) to support the kite where the bridles were taken away.

Take a look at a Flexifoil Blade, the king of lift. It is high A/R and uses D-ribs to reduce drag, so it is an efficient kite in a way, but it has a very deep profile / rib section. That produces the lift overhead and helps stability, but at a cost of speed.

Take a look at the buggy engines that broke records at NABX this year and they too have reduced bridles and high A/R's but very thin, fast profiles that take skill to keep flying properly.

Others can amplify or correct but this is a start.

Your kites arrive tomorrow. Have fun!!!!!!!!!!
And be safe.

heliboy50 - 30-4-2009 at 05:40 PM

Welcome to the forum! Like acampbell said- the AR doesn't tell the whole story. Narrow chord, more and smaller cells will be a faster kite good for pull and speed but not lift. Deeper chord width will be lifty. Case in point- my 2.5m profoil is wider wingtip to wingtip and has more and smaller cells than my friends 3.5m imp but the imp has a much wider chord (leading edge to trailing edge.) More, larger cells means more stable. The profoil is way faster than the imp but the imp will almost never luff and handles gusts. The racier profile kites are less forgiving- they want to be kept moving and when they luff, they will usually re-inflate going the opposite direction. When you look at the specs of the kite, look at the span, chord, cell count, and the AR. Hope this helps.

Bladerunner - 30-4-2009 at 05:45 PM

Wow,

That is pretty advanced thinking for a newbie!:eureka:

As you suspect there is more than just A/R at play. Generally a higher aspect kite will fly faster. That extra speed helps with the lift. The shape and " Angle Of Attack " are also factors.

Kites are usually measured flat. Their " projected area " can be very different depending on how it is brideled / shaped. So 2 kites of the same size can actually be very different in projected area. How the AOA is set in a fixed bridle also depends on the bridle. A depower kite alters the AOA and Projected area.

Often lower aspect kites have the other characteristics that lend to pull over lift in the design.

None of that stuff is as important as cool graphics for float though !!!! ;)

+ Like Angus said !

krumly - 30-4-2009 at 06:44 PM

Like Angus said, thinner foil profiles generally have less drag than thicker profiles, but that isn't as big a factor as other things like camber (convex curvature of the meanline of the foil). Increased camber generally results in raising the entire lift curve and increasig the max L/D.

Thicker foils with a less sharp leading edge generally have a more gentle stall. The airflow doesn't break away from the upper surface so quickly and radically as with a flatter, sharper nosed profile. That makes them less 'nervous.'

Reflex foils are those with some upturn to the trailing edge - a sort of convex followed by concave meanline. These have 'autostable' properties, much like an elevator set to keep the nose up on a traditional airplane configuration. If the nose drops too far, or luffs, as when the kite overfies the window, a nose-up moment is automatically applied.

Conversely, a gust does not make them nose up and increase angle of attack even more as it does with most cambered airfoils. Gusts don't exacerbate the problem of increased AOA when you least want it. This foil shape does have lower lift than a cambered foil, so you need more area for the same lift, or you have to have a less draggy profile and fly the kite faster to get the same line force.

Physics gripe: we use the term 'power' very indicriminately when we're talking kites. Power is force x distance/time. Flying static, the kite may be generating power (it pulls iwth a certain force while flying across the window at a certain speed), but it is not transferring any power to you nor is it doing any work on you.

krumly

DonaldLL - 30-4-2009 at 06:44 PM

These are truely some great answers. This is what I was looking for... thank you. Eventhough I am a 'technical' guy and educated in the sciences, I wanted explanations without equations and responses from people with a passion.

Bladerunner now that you mention it, I guess the question is advanced for a newbie. That's an interesting point. It is a technical question but then while I am reading these fantastic responses I started to wonder why did I even ask this question? I mean it's a good question but what's in it for me? Then I realized the answer is.... you know ever since I was a child (decades ago) I have always been fascinated by anything that flies. My grandfather used to make small, delicate single liners for me out of paper and boxwood I think; i don't know I was just a child. We would fly those kites. He would also drive us out to the local small airport and we'd park and watch the prop planes land and take off. It was like therapy before I even knew how to spell therapy. As far back as I can remember I was awestruck by the fact that something could fly - inanimate or otherwise.

So now here I am on the day before I will have my 2 new kites and I feel this connection that I forgot about. It's a feeling that goes waaaayyy back but now suddenly, it is very present. So when I ask this rather advanced/technical question I realize it is asking about the A/R and the L/D ratio but all of a sudden it hit me..., it is about trying to connect with the beauty of how things fly. Compared to those paper and wood kites back then, these kites today are a marvel of materials and design engineering but at the root of it, it is still the heart of the child and the ability to appreciate the beauty of the concept of flight.

I think you guys can see how excited I am about this. Thank you for your warm welcome and your willingness to share your passion for your avocation.

Donald

macboy - 30-4-2009 at 09:32 PM

Welcome Donald! Thanks for a GREAT question! I learned a pantload from it. I'd like to drop in 2¢ if I may.

I was chatting to my dad about the airfoil shape of the kite's ribs and how that makes kiting more than just "catching wind in a tarp" and turns it into a dynamic flying wing capable of generating lift/pull by moving through the air. So Dad, who comes from a sailing background starts explaining apparent wind as he understands it to me and how the sail on a yacht has the same airfoil characteristic...only the sail doesn't have a bottom surface, only a top skin (as it were). My brain immediately took that thought and combined it with a love I too have of flying things - especially RC planes and realized the there are simple trainers that have concave airfoil shapes (effectively no bottom surface) and they fly because they create lift. I learned this early on in my kiting life - that there is more to it than just catching the wind - when I felt the smooth surge of power come on through my handles on the Brooza. I'll never forget that first real experience of what apparent wind feels like. (That is, if apparent wind is what I was feeling ; )

It's a fascinating thing, these planes/sails/kites. Quite neat to harness the science through four unbelievably thin set of "strings".

BeamerBob - 1-5-2009 at 05:41 AM

I love this technical stuff and even though I think I've asked the same question before, I really enjoy reading again what the masters say about it.

Donald, you are in big trouble already. I bought one kite and realized in time I would probably need 3 kites to cover most of the wind range I would fly in. You started out at 3 and you haven't had the issue of not being able to go to sleep yet from all the endorphins your system will be full of after your first good session and all the good ones after that. Just realize, we don't have a 12 step program but the 12 kite program seems to work well, or at least be a good starting point.

acampbell - 1-5-2009 at 05:42 AM

A great demo of apparent wind can come with flying large, efficient foils. On some low wind days I will break out my Peter Lynn Reactor 10.8m. I have a 6" length of yarn hanging from a small mast strapped to my headstock on the buggy, and when then end moves about two- three inches from hanging straight down - about 2-3 mph - I know I can launch., I also keep a few segments of PVC pipe to form a 4' mast stuck in the sand with some light, bright streamers hanging from it as a true wind indicator. When it flutters I know I can launch.

The trick is to launch in that tiny puff an then keep the kite moving enough to get the buggy moving from a stand-still. Then I feel a small surge of acceleration and a burst of wind in my face and the GPS is clicking off 15+ mph and the little yarn tell-tale on the buggy comes alive. But I look at my streamers sticking out of the ground, the flags on nearby flagpoles and the few tree-tops and realize that there really is no local wind to speak of. I am creating my own.

At that moment, life is so good I have to marvel...

B-Roc - 1-5-2009 at 05:51 AM

Side note - Angus mentions D-ribs. I've been kiting for years now and know I have some kites with D-ribs but can someone explain for me what makes a rib a D-rib?

bobalooie57 - 1-5-2009 at 05:55 AM

A O X O M O X O A... Hey Don, D'ja fly your kite yet??? Bob

acampbell - 1-5-2009 at 05:56 AM

"D" for diagonal. when you take away a bridle point from the bottom of a cell corner, it would deform from a square shape to a parallelogram. A D-rib spans that diagonal space to help keep the cell's shape. I will try to find a picture....

acampbell - 1-5-2009 at 06:04 AM

Here, look at this picture of a Blade leading edge and note the white D-ribs inside the foil. They support the shape of the foil where the bridle points are not present to do so, such as at the location of the "x" in "Flexifoil"...

Blade 013.jpg - 43kB

PHREERIDER - 1-5-2009 at 06:29 AM

great forethought for flying. i did the same in the beginning.

the dynamic system of kite propulsion no matter the surface is a great feeling, with alot of feedback that is learned not though knowledge but experience and time in use.

the pendulum equation between you and the kite and resultant power/ movement with the kite stable in the air is where you are going with the kiting experience.

as the system moves, the kite and you, the wind going around the kite gradually builds providing more system wind (apparent to you ad the kite, hence the name)

the better it slips through the air (an increasing AR =less total drag =faster in the system=more power(accel and rate are built into the kite)

ok super geek what does this mean.

HI AR 7 =fast! through the air, stall prone, slower turning (not much surface for brake area)

lower AR = handling! turns quicker at slow speeds USER FRIENDLY and can teach you more with less hassle

PHREERIDER - 1-5-2009 at 08:03 AM

failed to mention the obvious wind dependent element

more is more across the board ..lift, pull whatever

remember power and speed can be in coincidence but always

and as a newbie when starting to fly and ride and move around ...think balance and timing

balance between rider and rig is where it all comes together using heels and hips(bending yours knees)

NOT toes and hands


the rest is a matter of time , may it be fill with fun

DonaldLL - 1-5-2009 at 08:15 AM

bobalooie57 - none of the kites have arrived yet. I work from home so I keep checking to see if the delivery made it without my hearing it or just to see if there is a truck coming down the street. I am pacing like an expectant father!

BeamerBob - Hello my name is Donald and I am addicted. I first admitted my addiction when I bought 3 kites in 2 days and waited outside for the deliveries.

USA_Eli_A - 1-5-2009 at 08:22 AM

lift happens when you make the kite fly faster than it's trim speed. Once you make your wing fly faster than trim the wing produces lift. Kites the have very low trim speeds are usually the' big ones' and the kites with high trim speeds are 'small ones'

All Kites 'pull' a little. But big kites are the ones usually used for lift.

Some kites are build for speed and usually don't produce much lift vs speed. Some kites are lifters so the more speed = more lift.

I GOTTA RUN. THE WIND IS CALLING ME.

bobalooie57 - 1-5-2009 at 08:22 AM

I know the feeling, it's like sitting here at my computer looking out the window at the treetops... b

B-Roc - 1-5-2009 at 09:08 AM

Quote:
Originally posted by acampbell
"D" for diagonal. when you take away a bridle point from the bottom of a cell corner, it would deform from a square shape to a parallelogram. A D-rib spans that diagonal space to help keep the cell's shape. I will try to find a picture....


Thanks Angus. That's what I thought they were but was never too sure and felt foolish asking after years of flying :puzzled:

DonaldLL - 1-5-2009 at 12:56 PM

Ok so this is torture? I have a box just delivered and I am on a conference call!!!!! I already tried once to mute myself out of the conversation and get to THE BOX and I got busted when they asked me a question and I had no idea what we were talking about LOL

Looking_Up - 1-5-2009 at 02:14 PM

:cool: flying on the apperant wind alone is the bomb

out yesterday trying to teach some young grasshopers about light wind and the ability to still fly (inland not very consitant wind) when we showed up it was perfect and everybody got into the air with realative ease but were not continuing to fly but they started to learn the more they watched that if you move the kite verry little if any wind is needed to keep it in the air so when the wind is light kite speed high in the window and small imputs from the brakes to turn and with a 4.5 Little Devill i was creating enough lift from the wing to do some parking lot surfing when the flag wasn't even wiggling

so practice in the light winds and you'll be flying when knowone else is.

Looking_Up - 1-5-2009 at 02:30 PM

as far as aspect ratio goes look at a glider compared to a cesna 150 the gliders wing is long and eliptical verry thin and creates more lift per squar ft than the cesna's wing and in the proper hands can stay in the air almost indefinatly where as the rectangular short fat wings of the cesna are good for heavier lifting using a faster airspeed w/an engine to sustain flight and if the engine gives you'd better be flying through that check list looking for the best landing spot.


don't know if i got that explained right trying to translate my aerodynamics classes for you

take the Ozone manta and the Acess both generate lots of lift (Pull on the Lines) the manta is a High A/R kite that has allot of float and is verry efficiant it flies in the upper part of the wind window and near the edge because it is faster (it also is not a good gusty condition kite) And the acess is a low A/R kite but good in its own performance for example it is slower it will sit a little of the edge of the window and generate more lateral pull it munches gust whith ease because a gust across the kite does not immediatly translate into +speed=more pull the lift from this type of kite is often refurred to as Grunt kindof like a disel trucks low end tourqe and the Manta would be the race car with handling on the edge


hope i did'nt confuse any one and that this was somewhat usefull:dunno:

Although i am taking aerodynamics classes it does not mean i can spell so take it easy on me

coreykite - 2-5-2009 at 10:45 AM

Hey Sailors,
This is great! I had to chime in.

When speaking of "lift" we should be careful.
In the back of most people's minds, "lift" is an UP force.
But we are talking airfoils here and words can be tricky.

The faster a foil moves through the fluid (wind) stream, the greater the pressure difference between the top and bottom surfaces.
The low pressure on top sucks the wing "up".
"Aerodynamic lift" is the direction away from the top of the wing.
We fly tethered aerodynes, so that aerodynamic lift we've created is also called "pull".

Just a moment of clarity amidst the dust storms of knowledge and opinion.

Besides, it usually isn't a kite thing anyway... It''s usually the flyer.
I can play Eric Clapton's guitar but completely different s**t comes out of me.


Safen Up! Buggy On!

"Often wrong... Never in doubt"

the coreylama