Kiteline strenght equation

[Vojta]

Edit: it's quite accurate I have 2 kg reel which somewhat levitates, meaning sometimes the pull is enough and sometimes is not. And I am flying at lower speeds than 10 m/s. 

(Ignore: I just got confused why I have 91 kg line when 4 is enough  pardon confusion 

I am not sure where could be a mistake but 4 kg strength kite line seems too little!!! And in my opinion the simplifications I made increases the result.)

[Edmond Dragut]

for a single line kite i think you calculation is a bit off,  is no way at the surface you present in to a 24 m/s wind the pressure to be just 4 kg. I may agree with that 4 kg just if the wings are having possibility to flex back allowing the wind to pass without to encounter opposition (something like an geometrical adjustment wing on planes) and may be also correct for launch and cruse position but you may count the hole process in this. at one point your kite will pass troth the power of the wind window trying to stay on the wind direction when the wind is shifting. Math and chemistry are give you the absolute numbers because are suppose to by on ideal conditions  

 My opinion thou 

[frob]

I've studied a bit of the math behind it, it is quite complex. The general equations are accurate enough but for situations rarely found in the wild. 

 

For the general drag equation you are using, I immediately see two issues. 

You are using the angle of the kite, which isn't part of the equation. Instead you need to use the projected surface area at the given angle between the kite and the flow. While the sine of the angle is part of computing the projection, it by itself is not the projected area. While it might be similar, the wind hitting dead on a flat kite does get close to zero surface area, and hitting full on gives close to 1x the fabric surface, those are not the projected surface areas. 

The second is the drag coefficient you are using. The values are always experimentally found and are unique to each wing and a range of values. The coefficient is usually not a single number but a table of numbers for many angles because of varying turbulence, laminar flow changes, cavitation changes, and more. At the kite's strongest angle it should be around 1.8 or so, but maybe closer to 2 for a strong pulling design, or lower near 1.5 or less for weaker pulling designs. 

[Vojta]

On 5/14/2020 at 8:12 AM, Edmond Dragut said:

24 m/s wind the pressure to be just 4 kg

I used 10 m/s wind.

On 5/15/2020 at 8:25 PM, frob said:

You are using the angle of the kite, which isn't part of the equation. Instead you need to use the projected surface area at the given angle between the kite and the flow

I am using sin() to do that. Bcs sine(0) =0, sin(90) = 1. I think it's correct bcs if you are calculating for a rectangle A=a*b, then multiplying by sin gets the job done. And if rectangle approximation is not enough. I am not able to complete these calculations.

On 5/15/2020 at 8:25 PM, frob said:

At the kite's strongest angle it should be around 1.8 or so

 I used the coefficient out of table on wiki, which was 1.28 for flat plate perpendicular to flow. 1.28 is one of the largest numbers there above that is only Empire State Building 1.5 and Eiffel Tower 2. So I doubt 1.8 is correct, it's too much. Increasing it from 1.2 -> 1.8 will mean 4 kg -> 6 kg. Next question is the role of a tail.

 

I still think it gives good prediction because to lift up my 2 kg reel I need 7 m/s wind for this kite. And as I said when I am kiting the reel is jumping up-down. 7 m/s sounds like real conditions (nearby meteo station measures 5 m/s at h = 3 m height and I kite at h = 50 m*). Still, I am surprised why I am recommended 90 kg line.

Thank you for contributing!

*just so its above trees

[Edmond Dragut]

probably nobody overthink to much like you do and just by experience with all kind of kites shapes and models generally recommend 90 Kg lines. For sure also is a safety issue here, better to fly the kite with a ticker line than to lose it in the first gust. Yon may agree the wind speed is different at 50 m then at 100 m.  Also is the kite line production on the play witch is having less weight options then the fishing line and the main focus is on anti friction and elongation more then in sock strength .

Again is mi opinion not a scientific fact 

[davide63]

NASA have a few pages of examples you can work through.

https://www.grc.nasa.gov/www/k-12/airplane/shortk.html