Paul-Douglas Rivers asked:
I recently started commuting by bicycle, but now that it's becoming winter, I've been wondering more about windchill. I understand much of it involves the moisture in the air, but how come commercial airplanes land from flights with cold icy surfaces, while some supersonic jets and reentering spacecraft land with a hot surface because of atmospheric friction?
At what point is the threshold?
Is there a room temperature flight velocity?
And how fast will I need to ride my bike for atmospheric friction to overcome windchill and keep me warmer?
We put this question to Kevin Knowles from Cranfield University and Holger Babinski from Cambridge University...
Kevin - I'm Kevin Knowles. I'm professor of aeromechanical systems for Cranfield University and I work at the Defence Academy of UK in Shrivenham, Oxfordshire. In essence, the answer to the question is that aircraft wings are cold because they fly high in the atmosphere, the region known as the troposphere, and atmospheric temperatures fall with altitude in the lower atmosphere. So an aircraft flying at about 10 kilometres which is typical above the ground, a day where it’s only 15 degrees Celsius at sea level, will be experiencing temperatures of about minus 50 Celsius outside. If however, you move fast enough in excess of the speed of sound then there's a significant kinetic heating effect and that's what spacecrafts experience. For example, if an aircraft were again flying at 10 kilometres altitude, but now at nearly at 7 times the speed of sound, then the temperature that it would feel due to kinetic heating, the highest temperature would be about 10 times the ambient temperature, but measured in degrees Kelvin. Now 10 kilometres altitude, the temperature in Kelvin is 223, so Mach 7 spacecrafts would experience 10 times that, so its heat temperature would be 2,200 Kelvin or about 2,000 degrees centigrade. At 10 kilometres, those two effects balance out if you're flying at about Mach 1.2, so at that speed, the kinetic heating is just enough to bring the peak skin temperature up to sea level temperature.
Diana - So it’s not so much the wind-chill effect as the fact that planes fly around in the colder part of the atmosphere. Travelling at Mach 2, Concorde used to experience surface temperatures over 100 degrees Celsius causing the fuse lodge to extend by as much as a foot, but what if you're a cyclist?
Holger - Hello. My name is Holger Babinski and I'm professor of aerodynamics in the engineering department of Cambridge University. How much friction you need to compensate for the fact that the air is cold around you, depends very much on the temperature difference between your body temperature and the air temperature. You asked how fast you would have to cycle, so I shall assume that you're cycling at sea level. There, the surrounding temperature on a cold day might be something like 17 degrees. And in order to generate enough friction to bring the air up to body temperature, you have to cycle at about 63% of the speed of sound. With the speed of sound at sea level, that is pretty much 480 miles per hour. So you have to be pretty fast to achieve that.
In this case, cooling comes by water evaporation and heating comes from friction of air moving over surfaces.
how does "wind chill factor" play into this?
For conductive heat loss, you likely have 2 (or more) equations.
The power required to move a vehicle thru the air varies as the cube of the speed so presumably the heating effect varies in the same manner at least at subsonic speeds. syhprum, Mon, 13th Jun 2011
The point at which frictional heating overcomes convection cooling on your skin is a velocity that will sand blast you away. willyp00, Fri, 4th Nov 2011
Re: my idea of covering a bike rider's skin with vaseline to block sweat evaporation in order to experience heating by air friction, I forgot that vaseline has some insulating properties which will cause a rise in skin temperature.
Interesting idea of adding a wind barrier (I think I'd just put on a windbreaker coat). Certainly if you are working hard, you will generate more body heat. When I had a bicycle commute over a big hill, just a windbreaker would be enough to keep me warm even on the coldest of days.
I just joined today ...... so don't throw rotten eggs if I make a fool of myself with my first contribution.
May I add an alternative to racing down the highway at 200km/h in a cabrio, naked and covered in vaseline, ie. Get a scuba tank(fulll) and attach a hose to the valve. Hold the hose opening close to your forearm, and open the valve slowly. Stop when the skin starts peeling off.
I though heat generated while experiencing drag came from the compression of air molecules in front of the moving body, not just friction between air molecules and skin molecules. hakonmarcus, Thu, 20th Dec 2012
ok but what if the effect desired isn't speed but certain amount of cooling lets say -100c and you had a skin of metamaterial on the bird. Trevor Wilson, Tue, 24th Sep 2013