After the observations of big kite and little kite on Silecroft beach I resolved to make some measurements. Tonight I got a spring balance to measure the tension in the string. The 1N reading was probably a bit low, with the spring balance being upside down. I felt the tension in the string and then felt the newtonmeter - felt more like 2 Newtons in truth. I estimate the angle of the string as 45 degrees to the horizontal. The kite itself leans forwards - the string is not perpendicular to the kite at the top so I've assumed it was at an angle of 40 degrees above the horizontal. I measured the kite - 69cm by 59cm so area was 0.41 square metres. I assumed the density of air as 1 kg per cubic metre. I calculated the force of the air on the kite by assuming a component of the wind normal to the kite and that the air stops dead when it hits the kite. I then used Newton's Second Law as rate of change of momentum of the normal flow of air hitting the kite. My method is shown below.
I put in the numbers and got 3.6 metres per second as the wind speed. That's 13 km per hour or about 8 mph. That doesn't sound too bad. The wind speed was too low to fly the bigger kite. Will repeat with a stronger wind and try to measure the angles more accurately. Thanks to Adam for holding the kite!
