## Power Relative to Windspeed and Kite Size [Reference]

forum for kitesurfers

edt
Very Frequent Poster
Posts: 5011
Joined: Fri Jul 16, 2010 6:27 am
Kiting since: 2010
Local Beach: ford lake Michigan
Gear: slingshot liquid force wainman pansh naish north cwb burton ronix rpm ozone

### Re: Power Relative to Windspeed and Kite Size [Reference]

taiguy wrote:
UKSurf wrote:Why does a 10m kite have 4x the power of a 5m in your first chart? I would think it would have 2x approx. also the power seems to be increasing with the power of 4 not power of 2 for wind speed. We need Bill Hanson to confirm these figures before they are used to purchase a quiver
I haven't seen any information that would corroborate edt's claim.

Power increases with respect with the square of the area and cube of the windspeed. But please have Bill come and give us all a short course

taiguy think about it for a second, it is ridiculous to go with cubed.

If you go with v^3, then if you fly a 12 meter kite in 20 knots, in 30 knots you need a 3.5 meter kite.

If you go with v^2, then if you fly a 12 meter kite in 20 knots, in 30 knots you need a 5 meter kite.

You can't just look at wikipedia without noting that when they give these figures of v^3 is it not for a kite it is for a turbine. The turbine spins faster the higher the wind. This means it extracts one additional factor of v. If instead of a turbine you are measuring the amount of force pushed back against a car it's 1/2 p v^2 Cd A go look it up, that's because unlike a wind turbine the car doesn't spin around in circles it just sits there in the air.

Anyway, just crank out a few numbers and example kite sizes it's pretty easy to see right away v^3 makes absolutely no sense.

Also kite size and rider weight scales linearly. Only wind speed scales as square for figuring kite size.

So a 20 meter kite pulls twice as hard as a 10 meter kite, and a 200 pound rider needs twice the kite size as a 100 pound rider.

SSK
Frequent Poster
Posts: 284
Joined: Thu Aug 30, 2012 2:04 am
Kiting since: 2008
Local Beach: Salvo
Favorite Beaches: OBX
Style: Surf
Gear: Cabrihna Drifters, Surfboards with 3 Fins
Brand Affiliation: None

### Re: Power Relative to Windspeed and Kite Size [Reference]

the force due to lift = 1/2 * rho * V^2 * Coefficient_Of_Lift * A

Double the area double the force
Double the wind speed and square the force.

SSK
Frequent Poster
Posts: 284
Joined: Thu Aug 30, 2012 2:04 am
Kiting since: 2008
Local Beach: Salvo
Favorite Beaches: OBX
Style: Surf
Gear: Cabrihna Drifters, Surfboards with 3 Fins
Brand Affiliation: None

### Re: Power Relative to Windspeed and Kite Size [Reference]

A - being the area.

UKSurf
Frequent Poster
Posts: 327
Joined: Wed Feb 22, 2012 9:09 pm
Kiting since: 1999
Local Beach: South Coast of UK
Gear: Slingshot Rally
Brand Affiliation: None

### Re: Power Relative to Windspeed and Kite Size [Reference]

taiguy wrote:
UKSurf wrote:Why does a 10m kite have 4x the power of a 5m in your first chart? I would think it would have 2x approx. also the power seems to be increasing with the power of 4 not power of 2 for wind speed. We need Bill Hanson to confirm these figures before they are used to purchase a quiver
I haven't seen any information that would corroborate edt's claim.

Power increases with respect with the square of the area and cube of the windspeed. But please have Bill come and give us all a short course
Well I think you can use common sense atleast as far as the area and power goes. look at the first line of your chart. You are saying that a 17m has 11.56x the power of a 5m. So what you are saying is that a 17m kite would have more power than 11 5m kites flown like flexifoil stackers (with at total area of 55m). Did you use this spreadsheet model to work out the low end on the Epic Infinity for Dmitri btw

taiguy
Rare Poster
Posts: 45
Joined: Mon Jun 23, 2008 2:13 am
Kiting since: 0
Brand Affiliation: None

### Re: Power Relative to Windspeed and Kite Size [Reference]

edt wrote:
taiguy wrote:
UKSurf wrote:Why does a 10m kite have 4x the power of a 5m in your first chart? I would think it would have 2x approx. also the power seems to be increasing with the power of 4 not power of 2 for wind speed. We need Bill Hanson to confirm these figures before they are used to purchase a quiver
I haven't seen any information that would corroborate edt's claim.

Power increases with respect with the square of the area and cube of the windspeed. But please have Bill come and give us all a short course

taiguy think about it for a second, it is ridiculous to go with cubed.

If you go with v^3, then if you fly a 12 meter kite in 20 knots, in 30 knots you need a 3.5 meter kite.

If you go with v^2, then if you fly a 12 meter kite in 20 knots, in 30 knots you need a 5 meter kite.

You can't just look at wikipedia without noting that when they give these figures of v^3 is it not for a kite it is for a turbine. The turbine spins faster the higher the wind. This means it extracts one additional factor of v. If instead of a turbine you are measuring the amount of force pushed back against a car it's 1/2 p v^2 Cd A go look it up, that's because unlike a wind turbine the car doesn't spin around in circles it just sits there in the air.

Anyway, just crank out a few numbers and example kite sizes it's pretty easy to see right away v^3 makes absolutely no sense.

Also kite size and rider weight scales linearly. Only wind speed scales as square for figuring kite size.

So a 20 meter kite pulls twice as hard as a 10 meter kite, and a 200 pound rider needs twice the kite size as a 100 pound rider.
Going with a v^3 model, and assuming 12m in 20 knots, an increase to 30 knots puts me on something just slightly smaller than a 7m. Where did you get 3.5m from?

edt
Very Frequent Poster
Posts: 5011
Joined: Fri Jul 16, 2010 6:27 am
Kiting since: 2010
Local Beach: ford lake Michigan
Gear: slingshot liquid force wainman pansh naish north cwb burton ronix rpm ozone

### Re: Power Relative to Windspeed and Kite Size [Reference]

taiguy wrote:
Going with a v^3 model, and assuming 12m in 20 knots, an increase to 30 knots puts me on something just slightly smaller than a 7m. Where did you get 3.5m from?
(20^3/30^3 ) * 12

how'd you do your math? don't see where you can get 7m. (20^x/30^x) * 12 = 7

x log 20 - x log 30 + log 12 = log 7, x = (log 7 - log 12) / (log20 - log30) = 1.33

To get 7 meters you would have to be using force is proportional to v ^ 1.33
Last edited by edt on Fri Dec 20, 2013 11:30 pm, edited 1 time in total.

taiguy
Rare Poster
Posts: 45
Joined: Mon Jun 23, 2008 2:13 am
Kiting since: 0
Brand Affiliation: None

### Re: Power Relative to Windspeed and Kite Size [Reference]

I'm not multiplying by a factor of 12.

Square of the area and cube of the speed were things beaten into me back when I started in 08 and I took it for granted then. I recall reading one of Bill's posts about the subject last year and went to go dig it up for you.

viewtopic.php?f=1&t=2372383&p=751233#p751233

He states available power increases with the cube of the velocity as well...

taiguy
Rare Poster
Posts: 45
Joined: Mon Jun 23, 2008 2:13 am
Kiting since: 0
Brand Affiliation: None

### Re: Power Relative to Windspeed and Kite Size [Reference]

I also see you participated in that discussion as well

The cubed relationship seems to match my own experience /shrug.

edt
Very Frequent Poster
Posts: 5011
Joined: Fri Jul 16, 2010 6:27 am
Kiting since: 2010
Local Beach: ford lake Michigan
Gear: slingshot liquid force wainman pansh naish north cwb burton ronix rpm ozone

### Re: Power Relative to Windspeed and Kite Size [Reference]

taiguy wrote:I'm not multiplying by a factor of 12.

Square of the area and cube of the speed were things beaten into me back when I started in 08 and I took it for granted then. I recall reading one of Bill's posts about the subject last year and went to go dig it up for you.

viewtopic.php?f=1&t=2372383&p=751233#p751233

He states available power increases with the cube of the velocity as well...
ok now when you talk about "power" "force" "energy" all these are precise physical terms with precise mathematical definitions.

Power is not "power" power is defined as the amount of energy consumed PER UNIT OF TIME.

But we don't feel time! We feel an instantaneous force of the kite pulling us. We don't integrate the force of the kite. Therefore if the "power" is proportional to v^3 then you take the derivative of that which is proportional to v^2.

Needless to say it can get confusing because as kiters we don't normally deal with precise units of physics, like impulse, energy, power, force. To a kiter all of those 4 terms, impulse energy power force are all equal, it's how hard the kite pulls you.

But they all mean something different.

Anyway, in terms of kite size it scales as to

v^2
linear with weight
linear with kite size

SSK
Frequent Poster
Posts: 284
Joined: Thu Aug 30, 2012 2:04 am
Kiting since: 2008
Local Beach: Salvo
Favorite Beaches: OBX
Style: Surf
Gear: Cabrihna Drifters, Surfboards with 3 Fins
Brand Affiliation: None

### Re: Power Relative to Windspeed and Kite Size [Reference]

Even if you are talking power (not sure how you feel power I understand physically measuring a force), it is still linear to the area. There is no squared term for the area.
1/2*rho*A*C*V^3.