How does aerodynamic cooling work?

[Raskolnikov]

How does aerodynamic cooling work?

I came across this question on an interview gouge for a regional airline.

I've searched several FAA books and the words "aerodynamic cooling" came up only one time, in Aviation Weather, ch 10. I couldn't find it an any other FAA source.

From Aviation Weather, ch 10: "Aerodynamic cooling can lower temperature of an airfoil to 0° C even though the ambient temperature is a few degrees warmer."

Can anyone explain how aerodynamic cooling works?

A solid surface can lose heat through radiation, conduction (convection with air), or evaporation. A gas can also lose heat adiabatically.

So is the air losing heat adiabatically, then taking heat from the aircraft skin through convection?

Is it the reduced pressure on the upper surface of the wing? If that's it, can aerodynamic cooling affect areas of the aircraft skin not on the airfoil?

Anybody have a good reference for this?

I've come across a few internet articles and blogs that attribute aerodynamic cooling to a "wind chill effect" from increased airflow around the aircraft. But I thought wind chill was just an effect that makes us feel colder, and didn't actually lower skin temperature. In fact, this should increase temperature slightly due to friction. It's why the space shuttle needed a heat shield, not a wind chill effect shield.

 

[Itchy]

Hmmmm. IIRC, PV=NRT? Solve for T? Cooling across the top of the wing as pressure reduces. DO NOT TELL THE CONTRAIL FOLKS however.

 

[Raskolnikov]

Of course. Thanks! Sometimes the answer is simpler than you think.

I knew there was something fishy about the "wind chill effect" answer I keep coming across.

 

[Minuteman]

The main method of "aerodynamic cooling" is convection (a combination of conducting heat to the fluid around that solid surface, and advection; the carrying-away of that heated fluid).

Anytime there's a temperature difference between two materials that are in contact, there will be heat transfer. The important point being the difference in temperature.

A hot cylinder head has cool air flowing past it and heat transfers from the cylinder to the air. Hopefully the amount of heat carried-away by the airflow is at least equal to the amount of heat addition (from the waste heat of combustion and friction) to maintain a reasonable cylinder temperature.

The main things that will affect the ability to transfer heat via convection are:

• Surface Area - more is better. That's why air-cooled engines have fins to increase the surface area exposed to the airflow.
• Temperature Difference - more is better. Heat conducts at a rate proportional to the difference in temperatures. Blowing hot air at a fixed speed on an engine will cool it less that cool air because the rate of heat transfer is lower.
• Thermal conductivity - more is better. This is the materials' ability to transfer heat for a given temperature difference. Typically good materials are good electrical conductors, like metal (this is also why is difficult to build an aircraft that is stealthy to both electromagnetic and thermal sensors).

(It's been too long since school to remember how "adiabatic" comes into play … I kind-of think it doesn't because the free airstream has effectively infinite mass?)

 

[Raskolnikov]

Fantastic response. Thank you!

I was thinking the air cooled adiabatically due to the drop in pressure. Isn't it correct to say the rising air in a thunderstorm cools adiabatically as it expands?

I'm a pilot, not an expert in whatever it is we're talking about here.

 

[Mike Massimini]

Look up adiabatic and you'll see it means "no heat transfer". The cooling of the air "packet" with altitude is due to a pressure drop (and expansion), not due to the packet losing heat to the surroundings.

Air flowing over, say, a cylinder fin is NOT an adiabatic process, There's hopefully a lot of heat transfer going on, mostly convective.

Obviously this is an idealized process. There's always some transfer, but in thunderstorm dynamics (huge masses of air being uplifted) it's relatively small, at least in the developing stages. Or so I'm told ...

 

[Mike Massimini]

Hmmmm. IIRC, PV=NRT? Solve for T? Cooling across the top of the wing as pressure reduces. DO NOT TELL THE CONTRAIL FOLKS however.

Errr, who is a "contrail folk"?

 

[Raskolnikov]

Errr, who is a "contrail folk"?

They are the anti-chemtrail folks.

 

[Mike Massimini]

Aha. Perhaps one will weigh in, and we can have some fun.

 

[Houston]

This is from AC 91-51A:


One of the hazards to flight is aircraft icing. Pilots should be aware of the conditions conducive to icing, the types of icing, the effects of icing on aircraft control and performance, and the use and limitations of aircraft deice and anti-ice equipment.

a. It is important that a pilot understand the conditions which are conducive to icing. An understanding of these conditions allows the pilot to evaluate the available weather data and make an educated decision as to whether an intended flight should be made. One of the best sources of available weather data is pilot reports. The Federal Aviation Administration (FAA) encourages all pilots to report their flight conditions when warranted.

(1) For ice to form, there must be moisture present in the air and the air must be cooled to a temperature of 0° C (32° F) or less. Aerodynamic cooling can lower the temperature of an airfoil to 0° C even though the ambient temperature is a few degrees warmer. However, when the temperature reaches -40° C (-40° F) or less, it is generally too cold for ice to form. Ice is identified as clear, rime, or mixed. Rime ice forms if the droplets are small and freeze immediately when contacting the aircraft surface. This type of ice usually forms on areas such as the leading edges of wings or struts. It has a somewhat rough looking appearance and is a milky white color. Clear ice is usually formed from larger water droplets or freezing rain that can spread over a surface. This is the most dangerous type of ice since it is clear, hard to see, and can change the shape of the airfoil. Mixed ice is a mixture of clear ice and rime ice. It has the bad characteristics of both types and can form rapidly. Ice particles become imbedded in clear ice, building a very rough accumulation.