Friction & Heat??

[Peter T]

So we all know that the faster you travel through the air the friction between the moving object and the air causes an increase in temperature. So what given speed causes a temperature increase? Is it in anyway related to the objects shape or material it is made of?
I ask this question as when i travel on my Honda Fireblade i get colder the faster i travel. Why??

[Simpo Two]

Ah now that's a nice idea - we need to invent you a suit where the friction counters the windchill...!

Also, 'anti-windchill friction suits for Antarctic explorers' - I can feel my first million coming on...

[Nimby]

On a cruising conventional jet aircraft (say 550 mph) you get ice forming on the windows, but at Concorde's Mach 2 the skin got hot, so I'd guess somewhere between the two. Mind you that's at different altitudes and it's probably rather different at ground level.

I thought the heating of a re-entering spacecraft is due to compression of the air in front, not friction. Not sure about aircraft though.

[Pints]

Simpo Two said:

I can feel my first million coming on...

That'd be wind.

[Eric Mc]

Nimby said:

I thought the heating of a re-entering spacecraft is due to compression of the air in front, not friction. Not sure about aircraft though.

Both happen. The air ahead of the spacecraft will eb compressed into a bow shock. This bow shock actually acts as a thermal barrier and helps prevent the surface of the spacecraft getting even hotter. It's why re-entry vehicles tend to be what is known as "blunt body" shaped (like an Apollo Command Module) rather than needle nosed (like a V2). Even the Space Shuttle tried to emulate a blunt body as best it could.

Friction certainly does take place as some of the spacecraft body will be directly impinged by high speed airflow. The secret is to keep the main forward facing part of the structure protected by the bow shock.

[Eric Mc]

I've heard of Fish on a Bicycle - but not Fish on a Caterham. I'd love to have seen that.

[Dr Jekyll]

TheLastPost said:

There are two separate mechanisms at work, here.

You get cold because:

A) The frictional heating you get at the sort of speeds a Fireblade is capable of is negligible.

B) The airflow increases the evaporative cooling of the sweat your body produces. This is what weather forecasters and the like refer to as 'wind chill factor'

Isn't there also a bit of
C) The warm air heated by your body gets blown away.

contributing to wind chill

[Peter T]

Tthank You. That explains it perfectly.

[AJI]

TheLastPost said:

If you were entirely dry and produced no sweat, you wouldn't get colder, you'd get (maginally, and possibly immeasurably at those speeds) warmer.

I'm not sure this is correct.
What you are talking about is simple heat transfer. In this example the warm body is transfering heat to the outside air.

The clothes act as a barrier of sorts so that the air flow is on the clothes and not the skin, but the heat from the skin would transfer to the air between clothes and skin and then through the clothes in to the air flow outside.
If the 'barrier' of the clothes is not all that efficient and allows cooler air flow to come in then heat transfer between skin and cooler air would mean you would feel cooler quicker.

[wormburner]

AJI said:

TheLastPost said:

If you were entirely dry and produced no sweat, you wouldn't get colder, you'd get (maginally, and possibly immeasurably at those speeds) warmer.

I'm not sure this is correct.
What you are talking about is simple heat transfer. In this example the warm body is transfering heat to the outside air.

The clothes act as a barrier of sorts so that the air flow is on the clothes and not the skin, but the heat from the skin would transfer to the air between clothes and skin and then through the clothes in to the air flow outside.
If the 'barrier' of the clothes is not all that efficient and allows cooler air flow to come in then heat transfer between skin and cooler air would mean you would feel cooler quicker.

The body heats the air around it. If that air is constantly being replaced with fresh, unheated air, the body loses heat faster. That's windchill. Clothes diminish the effect by hindering the rate at which the wind can remove the heated air close to your body.

If one was to ride one's fireblade though air hotter than the body, one would get hotter than just standing still in a reverse windchill effect.

[Japsteeze]

Peter T said:

So we all know that the faster you travel through the air the friction between the moving object and the air causes an increase in temperature. So what given speed causes a temperature increase? Is it in anyway related to the objects shape or material it is made of?
I ask this question as when i travel on my Honda Fireblade i get colder the faster i travel. Why??

To start off, when you compress a gas to a fraction of its original volume, as you can probably notice when using a bike pump.

When you drive/fly at subsonic speeds (less than 760mph, although effects do become noticeable at 500mph+), any compression the air undergoes is really quite negligible in terms of heat buildup. The reason concorde heated up was because it traveled at Mach 2 (twice the speed of sound), causing the surface temperature at certain parts of the plane to reach 130degs celcius.

Below sub-sonic speeds, fluid flow (air flowing over you & your bike) greatly affects heat transfer rates. The air passing over your bike is at a lower temperature than you, so it removes heat energy from you.