speed-heat-friction

[roux_de_secours]

I was wondering about what is the speed at which air (friction) start heating you up.

Here's an exemple. you've got a car, a very good one (with no roof), and the air is at 5 celcius. and you're at sea level.

If you go at a low speed, the cold air will cool you up. At high speed, the friction heats you. so, at which speed the air will stop cooling you, and its friction will heat you up?

[Dodgey]

While friction does cause some heating, the majority of the heating due to traveling very fast through the air is due to compression. As the air is compressed it heates up and causes things such as re-entry plasma trails. Another small thing is that you have to define at what pressure, unless you were talking about calibrated airspeed. With regards to your question I have no idea what so ever.

[R0cketC0der]

As already said, most of the heat comes from compression. Compression happens when you travel supersonic, so heating starts happening then.

[PakledHostage]

Compression happens when you travel supersonic, so heating starts happening then.

As a rule of thumb, air can be considered compressible above Mach 0.3. Compressibility effects such as heating are already noticeable on subsonic airliners. Readouts of "Total Air Temperature" (TAT) and "Static Air Temperature" (SAT) are available to the pilots as these values are important to engine performance. The two values typically differ by many degrees.

[Ralathon]

It also depends on the angle of attack of your test surface. Compression happens in front of your craft and air moves fastest along the edges. So in general the front will get much hotter than your sides.

[roux_de_secours]

that answers most of the question : )

while we're talking about that. about compression and reentry, how does it ionise the air (or what's in it)? is it only heat due to compression? what happens on the molecular/atom level?

[Dodgey]

It is because when matter is heated up it allows the electrons to move between atoms with much more ease. When it gets to the point that electrons can flow freely that is where it becomes plasma. Ionisation is where atoms contain more or less electrons than normal.