Forum: Technology

Hello I don't think this has been asked before, but I am just curious as to whether anyone has any idea what the current fighters skin temps would be at top speed?? I ask because I know the old SR-71s used to rate at around 522F at cruise. Just wondering how hot this would make the skin on a Viper, Eagle, Hornet, etc??

Is there a chart or scale showing how speed increases skin temps at all??

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Elite 1K Joined: Aug 19, 2004 - 09:19 AM Posts: 1092

There's a way to calculate it given some basic variables. I had to do such a calculation once for a viscous flow course. I don't remember it being to involved. I don't have the textbook on hand to quote the proper equations though. Sorry about that.

Good question, wish I had an answer. There has got to be a few hundred variables to get to the answer I'm sure, but I'm just an ol'crewdawg, what do I know?? Shrug

Salute!

Go find a good aero site and find "stagnation".

A great example is that you will not ice up if you are flying over 300 knots IAS or so. This is because the air will heat up as it slows (relative) from 300+ knots to zero knots at the leading edge of your wings.

ROSCOE!!!!!!!! We need ya!

I personally know of a troop who melted the paint on a VooDoo when doing high-q missile launches at low altitude. JR also can tell all about the same thing in the Zipper.

At high altitude and low air density, things change. The heating becomes less dependent upon "indicated airspeed" and starts to track mach number rather than the tiny molecules hitting the leading edges.

A friend that flew the SR-71 talked about seeing the RTV (used to seal fuel leaks and such) ionizing and trailing the bird as it zipped along at 3+ and 75,000 feet or higher. A roommate told me about the same thing when coming home in the shuttle.

So bottomline is it ain't easy to figure all this out unless you do serious homework, heh heh.

out,

F-16 max sustained airspeed is limited to M2.05, due to thermal friction weakening of the plexiglas canopy above that number. I don't know the temperature numbers but that is what I have been told by folks at LockMart and Texstar.

Assuming that the air is brought to rest adiabatically (thats thermo talk for no losses, i.e. reversible), the temperature of air brought to a complete stop (or, more accurately, air is taken from rest and accelerated to aircraft speeds by being trapped on the leading edges of the airplane), the temperature of that air mass is called the Total Temperature.

Total Temp = Ambient Temp * (1+M*M/5) [Temp is in absolute scale]

Assuming M=2.05 in the Stratosphere (T=216 deg Kelvin)

TT = 125 deg C or 257 deg F

Assume M=3.5 (SR-71 on a good day)

TT= 472 deg C or 881 deg F

(Quick math, won't guarantee accuracy, but they sound right)

Sounds about right for the Blackbird Smile

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The true speed limiter on that beast was the engine inlet temp which shouldn't exceed 427 deg C. I got that from Brian Schul (the author of Sled Driver) and it was the way he chose to price the very rare limited edition of his book.

Cruise speed was about Mach 3.2 with very rare excursions any faster but I seem to reacall hearing of 3.3 or maybe 3.4 in the past. I'll see if I can find some public info to back up the speed numbers Wink

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AFAIK in the case of the MiG-21 some elements of the airframe reach 110 deg Celsius if you take it to Mach 2.05. Regular aluminium receives plastic deformation (meaning it won't regain size and shape after cooling) beyond 140 deg C.

Forum Veteran Joined: Dec 15, 2006 - 12:28 AM Posts: 760

Aussie_Falcon_Freak wrote:

Surface heat from airspeed friction? Hello I don't think this has been asked before, but I am just curious as to whether anyone has any idea what the current fighters skin temps would be at top speed?? I ask because I know the old SR-71s used to rate at around 522F at cruise. Just wondering how hot this would make the skin on a Viper, Eagle, Hornet, etc??

Is there a chart or scale showing how speed increases skin temps at all??

Calling it friction is somewhat misleading. Thermodynamics has a compressibility element as well as friction.

You're looking for an easy answer there isn't one. Its engineering, it's thermodynamics. Like they say it's all a matter of the variables.

Put simply in another way, part of the heating is not just caused by friction; it is caused by air compression from the increase in pressure.

Aussie_Falcon_Freak wrote:

Hello I don't think this has been asked before, but I am just curious as to whether anyone has any idea what the current fighters skin temps would be at top speed?? I ask because I know the old SR-71s used to rate at around 522F at cruise. Just wondering how hot this would make the skin on a Viper, Eagle, Hornet, etc??

Is there a chart or scale showing how speed increases skin temps at all??

Rosco's numbers sound about right. On the Concorde the leading edge temperature at 212 degrees C was the cruise limiting parameter.

Here's a link to the structures section of a good NASA Report on the X-15, the entire report makes interesting reading.

http://www.hq.nasa.gov/office/pao/Histo ... uctur.html

The AIAA paper has some more technical info.