Wind Speed, Power & Lift - How does it all work out?

[4x4xFinch]

This is really a follow on from Andy S post about Aspect Ratio.

(More sums than I'd normally care to read, but good info :-0)

 

http://www.flexifoil.com/community/forums/showthread.php?s=&threadid=7953

 

So having found out lots of stuff about how power is generated etc, this brings me onto the question of how does the power generated by given kite translate into lift?

 

Rather than being an academic question, I have a very pressing need to know - at what windspeed will my Blade III 4m start to generate enough power to lift me?

 

Ok, so there will be complications about some of the following and these may or may not be relevant but I just started to think about the whole process from a laymans point of view - I know nothing about aerodynamics or kite design:

 

>Actual wind speed - how hard the wind is blowing,

 

>Apparent wind speed - if kite standing still then presumably, this is the same as the actual wind speed, but if the kite is moving anywhere on the wind window arc, there must be a change to the apparent wind speed.

 

>Angle of attack - at launch, the leading edge is near vertical but at the zenith it is near parallel to the gound. Thus there must be some form of rate of change to the power generated in any given direction as the kite moves vertically - at very least, on the ground most of the pull would be horizontally and at the top most of the pull (lift) will be vertical. Presumably the angle of attack to the wind also has a relationship to the power generated as well as the direction the power is pulling.

 

>Weight of the flyer - lighter flyer must be lifted with less power, (presumably by a lower apparent wind speed) than a heavier person.

 

I'm sure there must be loads more variables that I'm not aware of, but for now ,those are enough.

 

Presumably trying to work out the power from any other position other than parked at the Zenith involves loads more variables than I care to think about and at least in a fixed position some form of comparison between kite sizes and models could be made?

 

So, I'm 77kg (just over 12 stone), and flying on 25m lines (not sure that makes any difference). At what wind speed will my 4m Blade lift me when at the Zenith?

And what's the forumla to work it out?

 

Sleep well,

 

Simon

[Andy S]

Finch...

 

Try the search function.... or alternatively follow

 

http://www.flexifoil.com/community/forums/showthread.php?s=&postid=48420#post48420

 

Unfortunately, there are some variables that we don't know the answer to to precisely calc. it (like the lift coeffficient of the Blade), but you'll see the maths is just about the same as the aspect ratio

 

 

QED

 

Andy

[burgy]

Originally posted by 4x4xFinch

>Angle of attack - at launch, the leading edge is near vertical but at the zenith it is near parallel to the gound.

 

on take off the kites not generating litf due to its section, its basicly the wind hitting the underside of the kite & getting pushed in to the air.

 

once the kites moving the apparent wind at the kite builds & the kite starts to act like a wing.

 

this is one of the reasons why the bigger kites can be a bugger to launch in very light winds. once its moving & generating lift you have no problems but getting to that point can be a problem.

[Bob Monkhouse]

Gordon Bennett geezer !

 

Just fly the blinkin thing dude.

All this talk of maths makes my brain hurt.

 

Cheers Bob.

 

Don't forget to consider the relative humidity in your calcs though

[mgs]

Gosh ....now thats a question.Car'nt answer all of that but from a buggying point of view this might be of some interest..

 

One day at the beach the wind speed was about 10mph flying a 5.5M Mosquito Pro in a Libre Full Race Parakart I managed to achieve a maximum speed of 29.5mph....There is an equation that will explain all this....Windspeed...size of sale etc ..With the right size kite it is possible to travel 3 x the apparent wind speed..with a bigger kite I would have have just been overpowered and dragged sideways and forward speed would have been reduced...

[Andy S]

Originally posted by burgy

on take off the kites not generating litf due to its section, its basicly the wind hitting the underside of the kite & getting pushed in to the air.

 

Don't think that is right (for a foil - I think it is for a NPW ). To launch the frenzy, you just bring the angle of attack into the wind slightly and it takes off. If you think about it, that is what you are doing launching any kite on pulling the handles towardsyou - generating apparent wind. It will be more difficult to launch in low wind as there will be less apparent wind across the kite.

 

Andy

[Andy S]

Originally posted by Bob Monkhouse

Don't forget to consider the relative humidity in your calcs though

 

That's in the maths - it's the air density

 

Andy

[Bob Monkhouse]

I was referring to your sweaty sister dude

[lazydog]

apparent wind results in apparent lift, ie so long as you keep believing in it your safe ... well it's worked for me so far.

[burgy]

Originally posted by Andy S

Don't think that is right (for a foil - I think it is for a NPW ). To launch the frenzy, you just bring the angle of attack into the wind slightly and it takes off.

 

I understand what your saying but unless the kites moving forwards theres very little air flow over the wing.esp at that ridiculosly high initaial AOA.

 

It is in effect stalled & the point of seperations way forwards. from my understanding of this the kite cant be generating lift like a tradtional wing at this point. the kite is redirecting the air which IMO is what give us that first lift. but as to whats actualy happening???

 

If you think about it, that is what you are doing launching any kite on pulling the handles towardsyou - generating apparent wind. It will be more difficult to launch in low wind as there will be less apparent wind across the kite.

 

Andy

 

think about the direction the kites accelerating when you give that first tug. your pulling in the opposite direction to the wind so all your really doing in increasing the true wind strenght. I guess its similar to when you hold a foil stalled in the window. while its stationary the only thing holding it up is parasitic drag. untill the kites moving forwards again its not producing lift in the tradtional air flow over the wing stylie.

 

to be honest andy non of my ideas have an impirical evidence to back them up & speaking to the designers no ones 100% sure whats happening at the point of take off. Its not something thats an issue for soft sails like paragliders & its certainly nothing a fixed wing plane ever expreances.

 

I guess we need someone to build a windtunnel we can fly our kites in & a massive reaserch budget

 

 

burgy

[Andy S]

Originally posted by burgy

I understand what your saying but unless the kites moving forwards theres very little air flow over the wing.esp at that ridiculosly high initaial AOA.

 

Hmmm.. I've always thought that by pulling the kite towards you you do two things:

 

1) Ram air inside it

2) Pull the leading edge forward just enough to create some vertical lift over the front of the kite - not a lot, but enough

 

It is in effect stalled & the point of seperations way forwards. from my understanding of this the kite cant be generating lift like a tradtional wing at this point. the kite is redirecting the air which IMO is what give us that first lift. but as to whats actualy happening???

 

I guess by ramming air into the foil, and deflecting the air down (which you'd do if you pulled the kite forward) would be an explanation... I guess your explanation of reverse launching must be the same - you bring the trailing edge far enough forward to deflect air down to get the kite to rise backwards...

 

How does the Frenzy launch then - it definitely seems to fly away rather than being pushed into the air...

 

I guess we need someone to build a windtunnel we can fly our kites in & a massive reaserch budget

 

 

burgy

 

Until that time I'll have a closer look and think a bit more next time I fly a traditional 4-liner (nearly always fly a Frenzy now )

 

Andy

[Simon Bailey]

Remember that there are two lift forces acting......bernoulli lift and plate lift....

 

you can reverse a kite using plate lift....and you'll find that most of the time, when launching, that the foil is using plate lift, until the aerofoil has gained it's shape and airspeed, for bernoulli lift to start acting.

[Simon Bailey]

bugger....caught out by the elusive "old thread reviver"!!!

[Cornish Blade]

Another one rises its head again lol

[Exhale]

All of you theories are great sounding. Simon Bailey is the guy you want to listen to. The bernoulli effect is what gives you lift. In short, There is a greater surface area over the top of the kite than the bottom. Therefore the same volume of air occupies less space under the kite as opposed to the top when moving, giving you a greater partial pressure underneath the kite. With that in mind, the method of increasing partial pressure under the kite is to move it faster through the air. That is why a 5 knot wind will only push a sailboat at a speed of 5 knots, and push a kite 10 knots. Secondly, dont confuse the kite lift with a person being lifted. Anytime the kite is pulling, it is generating lift. This does not mean you will come off of the ground. In order for you to come off of the ground you need to create lift in a vector perpendicular to the gravity. This is where angle of attack comes into play. The angle of attack is what causes foward movement of the kite, not lift. The best way for the person flying to get off of the ground is to drop the kite down the side of the window. Then very quickly aim the kite towards the centerof the window (you will be pulled very quickly). Before the kite hits the center of the window, re-aim the kite towards your zenith and fly straight up with arms outstretched. You should now be flying. This method will provide the most amount of lift capable for the kite you are flying. The reason is that the kite is already in motion before it starts the ascend to zenith, utilizing a greater percentage of the window. Happy flying, and watch out for the landings, they are the hard part.

[Pablo]

Great technique for jumping, only thing I do differently is use the brakes to loop the kite say to the right, keep the brake on till the kite's all the way around, then head down the left side of the window, when your past say 10:30 then loop the kite to the left full power, if done right you'll end up with the kite powered up about 10ft off the ground on the left of the window heading right, before you hit the centre of the window point the kite to zenith. It's about the most powerd up I can get the kite without doing a pendulum jump.

[Andy S]

God... That Andy S bloke should just chill a bit, forget the theory and just fly

[Old Tom]

Subscribe to the Lanchester theory of flight (say the one about 1890) and why kites (especially power foils) fly as they do at the airspeeds they do becomes staggeringly clear.

 

Bernoulli never gives a decent result, plate lift isn't bad and sailing theory just doesn't do it.

[archkiter]

one more thing to add to the equation - the weight of the kite.