5 min read
The effect of altitude and temperature on kite performance
As a kiteboarder unfurling and inflating a kite for my first snowkiting experience, I wondered: Is there anything materially different about kite flying dynamics in cold weather or high altitude?
Lift is proportional to the mass of air displaced by a wing. (Though kiters don’t use the term, an inflatable power kite is a type of wing.) In less dense air, less mass is displaced by the flying wing/kite. So, in order to maintain lift as air density decreases, airspeed must increase. Therefore, at high altitude (where air density is lower), we need higher windspeeds (and/or we must move the kite faster to increase airspeed) in order to fly the same kite we would use at sea level.
Using the Kite Lift Equilibrium Calculator in the article “Helium-filled kites”, you can see how changing the altitude assumptions affects requisite windspeed. (e.g., Pre-filled assumptions reflecting sea level conditions could be changed to reflect a 10,000ft altitude situation: atmospheric pressure = 10.2 psi; ambient temperature = 30 degrees; air density = .001756 slugs/cu ft.) Note also that lowering the gauge pressure inside the kite (to lower the mass of the inflated kite, and thus lower the windspeed required to maintain lift) has a trivially small effect. Moreover, underinflating the kite will create significant drag and decrease performance further.
Now, snowkiting at high elevations usually involves substantially colder temperatures than a typical sea-level kiting experience. We might thus at first imagine that the net effect of high altitude (less dense air) and low temperature (more dense air) would perhaps be negligible. However, the increase in air density due to lower temperature is trumped by the decrease in air density due to higher altitude. So, when snowkiting, we are almost invariably dealing with significantly less dense air than when waterkiting, and accordingly we need higher airspeed to maintain lift.
In practice, if there is, say, 15mph wind on a high-altitude ridge-top, we expect it to be harder to fly the 12-meter kite one might fly in that same windspeed at sea level. At this minimum critical windspeed, we notice the need to move the kite around more than expected, in order to increase airspeed to generate lift and avoid a stall (or that we need to sit tight and wait for a higher windspeed). Happily, due to being on land instead of in water, we can increase airspeed not only by sine-ing the kite more aggressively, but in this case also by running backwards into the wind (adding a non-zero groundspeed to the headwind speed). At higher windspeeds where we no longer risk a stall, at altitude we still experience less lift – and thus less power – than we are used to from a particular size kite at sea level.
In light of kite wing performance decrease at altitude, one might logically wonder why we fly jet airplanes at such high altitudes. If airspeed must increase with altitude to maintain lift, and going faster of course requires more fuel expenditure, then high-altitude flying is more costly and presumably less preferable. However, less dense air also results in less drag. So, even though the plane must fly at a higher velocity, it requires proportionally less fuel to propel the plane at that velocity. In terms of fuel quantity, the benefit of less drag trumps the cost of higher altitude (most notably for long flights, due to the large fixed fuel cost of climbing to a high altitude). (Note: We also fly jet planes at high altitudes in order to avoid storms.) In contrast, with respect to inflatable power kite wings, we don’t experience a noticeable difference in drag due to air density.
Pilots are well-versed in the “density-altitude” issue, as they routinely consider altitude and air temperature when determining required velocity and engine power, and anticipating fuel consumption. A dramatic example of proportionality of wing lift to air density is available in the successful 2010 assassination mission against Osama bin Laden. The Blackhawk helicopter that crashed was evidently attempting a very high-power move: entering a hover from a high speed, and maintaining the hover above ground effect. (Hovering close to the ground gives the aircraft a buoyancy assist from the downdraft bouncing back up off the ground; hovering higher up misses out on that ground effect). The air was, reportedly, a little hotter than expected on mission day, and thus there was less power available to the Blackhawk than expected, and it abruptly lost lift. Available power may have also been diminished by a vortex from the target site’s high compound wall (turbulent airflow around a wing reduces lift). Even such textbook physical phenomenon can be overlooked by us ever-fallible humans in a high-adrenaline, highly-complex situation.
In the case of kiting, available power is similarly a complex and condition-dependent issue. Power is proportional to air density, to kite area, and to the cube of velocity. Given an understanding of the density-altitude effect, which decreases both lift and power for snowkiters, we might logically select a bigger kite — a wing with a larger surface area — in order have the same available power we are used to at sea level. If one typically uses an 8m kite in 22mph winds on the ocean, should one perhaps opt for the 10m in similar wind up on the snow-covered ridge?
Consider what happens once we are no longer just standing still and flying the kite, but having the kite pull us. There is much less friction on snow (unless it’s heavy, deep powder) than in water. The magnitude of this ground friction issue trumps that of the density-altitude issue. In order to race around on skis/snowboard across this high-altitude snowfield as fast as we would be able to in the ocean, we don’t need as much power. So, we might be able to get away with a smaller kite than otherwise expected, given the windspeed. Kiting on low-friction snow is equivalent to lowering one’s mass, in that you just don’t need as much kite power.
If we’re snowkiting up steep inclines, we try to generate sufficient power to overcome gravity by aggressively increasing airspeed with multiple kiteloops. Often, this is considered a non-beginner move, precisely because increasing kite velocity by looping as a cubic effect on power — this can be unexpected and unmanageable for a novice. When multiple kite loops still don’t generate enough power to do the work of pulling a kiter up a mountain, we’re back to the idea of selecting a larger surface area wing.
So, the practical answer to question I initially posed about kite dynamics in snow is this:
1. Flying a kite while standing still at high altitude will require more kite motion than an experienced ocean kiter would expect, given the windspeed.
2. But, kiting around on the snow with that kite will provide more groundspeed than expected.