Engine overheat and altitude

[Tyro48]

Dale/Doug:

    I was wondering if the FM in Aces High models the adiabatic temp change over altitude and the relation that would have on how fast the engine temperature overheat would drop off as altitude is acquired. Seems to me the time it takes for the engine to return to a normal temperature is the same at sea level as it is at 25k, at an altitude of 25k the outside air temp must be somewhere on the order of -30 deg below zero and therefore the eng should cool much faster, agree or disagree?  

[funkedup]

Density

[moose]

uh...

::falls asleep as his desk as funked begins the lesson::

[Am0n]

Some one give this man a beer.

[pbirmingham]

Originally posted by funkedup:
Density

Funked isn't calling you dense ... he's pointing out that though the air up there is very cold, there isn't very much of it.  Thus its heat capacity is very low -- each cubic meter of air carries away less heat than it would were it as dense as at sea level.  Thus, engines do not necessarily cool off much faster.

[Swoop]

But it was very nicely put, all the same.  

 

[fdiron]

I disagree with the density theory. I think an engine should return to its normal operating tempature much sooner at 30k than at sea level.  The air at 30k is still breathable by humans who have acclimated themselves.  Early B17s had trouble with their machine guns freezing shut from the extreme cold.

[Eaglecz]

yes air is very cold up there, but dense is lower. But it should could way better anyway.

btw.: what pilots in LA7 at 20k+ breathe ?

[Eaglecz]

Originally posted by fdiron:
The air at 30k is still breathable by humans who have acclimated themselves.

wow did you ever try to make alt change from 5km to sea level ? It is possible to acclimatize. But from 10Km down to sea lvl in 1 hour ? .... WOW

if is it sou easy why TA152 had pressure cockpit ?

[Toad]

Originally posted by fdiron:
The air at 30k is still breathable by humans who have acclimated themselves.

Oh, it's is most certainly breatheable... you just pass out REAL fast.  

 

Time Of Useful Consciousness (in seconds) without supplemental oxygen

Altitude (feet)

40,000  15 sec
35,000  20 sec
30,000  30 sec
28,000  60 sec
26,000   2 minutes
24,000   3
22,000   6 minutes                  
20,000  10 minutes
15.000  Indefinite

Source: "Physiologically Tolerable Decompression Profiles for Supersonic Transport Type Certification," Office of Aviation Medicine Report AM' 70-12, S. R. Mohler, M.D., Washington, D.C.; Federal Aviation Administration, July 1970.


HIGH ALTITUDE BRIEFING

[Tilt]

Originally posted by Tyro48:
Dale/Doug:

    agree or disagree?  

Only at the same IAS because only then could you argue that the same mass of air is flowing thru your cooling interface. (with a lower temperature)

Typically we fly higher up at lower IAS (cos we dont cruise anywhere!!) so the lower density reduces the thermal capacity per volume unit.

DRY Air at sea level has a thermal capacity of just over 1 kJ/(kg.'C)and it does not reduce by much as it gets colder.

So its a function of delta T (between heat exchanger surface and air temperature) and the kg per unit period of time passing over/thru the heat exchanger.

There is also a conductivity issue at lower pressures  but as cooling is largely "forced convection" we need only interest ourselves in the boundary layer at the interface.

Tilt

[Wlfgng]

lol moose.. you (and I) (and Runny )  knew this would get detailed (tedious)  

[ 12-21-2001: Message edited by: Wlfgng ]

[pbirmingham]

Originally posted by Wlfgng:
lol moose.. you (and I) knew this would get detailed (tedious)  

<tosses hair>
Science is hard!
 

[fdiron]

My point about the air being breathable at 30k was to stress that the air is not as thin as you might think.  There have been people who have climbed Mount Everest without oxygen tanks.  In fact, I watched an IMAX documentary about a guy who climbed to the peak of everest without oxygen tanks.  It takes at least 2 weeks to get acclimated.

[AKIron]

Take it to extreme, out of our atmosphere. Is heat unable to radiate in outer space? Personally, haven't been there, can't speak from experience.