Why is it so easy to go 3 times the wind speed with my 19m and so hard to go 3 times the wind once it has picked up? :duh:
nate76 - 15-9-2015 at 04:37 AM
Great question! Top speed is basically defined as the equilibrium point of a bunch of different forces: lift forces, drag forces, friction forces,
etc. You have one source of power (lift), but multiple sources competing for this power (drag, rolling resistance, frictions, etc). The quick/top
level answer is that most drag forces typically increase with the square of the speed. So for example if you are going 3x as fast, your drag hasn't
tripled but has increased 9x (3^2). So even though higher winds give you more power, your power requirements are rising much more quickly and your
speed tops out sooner.
One rule of thumb is that the power required for a given speed is a cubic relationship, meaning that to double your speed would require 8x the power
(2^3). One way of thinking of power is as a Force x Velocity. Since drag forces have a (Velocity^2) term in them, you can see how the power drain
from some drag forces is rising as (Velocity^2)x(Velocity) or Velocity^3
Sorry if that's a little wordy - Feyd says it much more succinctly. Some might find it interesting though. Feyd - 15-9-2015 at 05:04 AM
Annnnnnd I'm asleep. J/k.
Actually I would like to see a graph illustrating what you just described.
This has been discussed many times over the years. And with the advancements in kite design things we thought were impossible are now becoming
routine. Its becoming very easy to go 3x+ on a kite now. Especially in light winds. I couldn't believe how fast the Chrono would get me going in
10-12kt.
Kites like the early Speeds and the Chrono or Sonic make me faster in lighter winds. But not faster in general.
I guess in simple terms its just diminishing returns as wind speed increases.
BeamerBob - 15-9-2015 at 05:06 AM
I think the larger kites have more working surface area related to the turbulent edges of the kite. The larger the kite, the higher the wind speed
multiple. Bladerunner Your Silver Arrow was able to get me to almost 32 mph and I had loads of upper range left. Lots of sliding coming out of
turns, but I could've held a line with quite a bit more wind. It seems crazy to imagine 35-40 mph with a 19m Flysurfer!nate76 - 15-9-2015 at 05:13 AM
Ya, sorry Chris. When I get a chance, I'll have to look at some of my old books and see if I can't find a good graph. Interestingly, some of the
best charts on these sort of things that I've seen came out of the 30's and 40's when guys were trying to push speed limits on prop-driven aircraft.
You summed it up much more succinctly: its just diminishing returns as wind speed (or more importantly your speed) increases. BeamerBob - 15-9-2015 at 06:35 AM
I've noticed that the wind and effort to get over 60 is way higher than getting into the upper 50s. Like the parasitic forces are all coming together
above 57 on Ivanpah with me in my buggy. Feyd - 15-9-2015 at 07:48 AM
Yup. 0-50 for me, easy. 60mph takes a bit more wind but still not too, too hard. In 25 kts we can rip 60+ all day. 70+ on the other hand 35kts
gusting to 40+.
I agree with the drag becoming an issue. Something as simple as tucking down a few more inches has a considerable influence on my speeds. I keep
threatening to break out the GS suit but I'm honestly too damn lazy and it gets too damn cold. :P
I keep threatening to break out the GS suit but I'm honestly too damn lazy and it gets too damn cold. :P
Bah..... no one cares about the GS suit. Come the winter, this is what everyone want footage of underneath the kite.
Plus it's probably warmer.....
ATB,
SamFeyd - 15-9-2015 at 01:56 PM
The horns add drag and I'd be okay with that.:D TEDWESLEY - 18-9-2015 at 05:19 PM
Nate has it down I'd say. As the speed is increased all of the parasitic forces are increasing at an exponential rate. Don't forget the good old
apparent wind either, the wind the kite sees is much higher and thus more drag than might be intuitive. Our buggies and bodies are not
very aerodynamic since most of us do not enter the realm where aero is the limiting factor over say comfort and fear. A fully faired human
or buggy would go faster but it might not be comfortable or practical for anything but an outright speed attempt.
