Speed of Heat

"David CL Francis" wrote in message
...
On Tue, 15 Mar 2005 at 22:10:56 in message
R5QZd.71896$Tt.15712@fed1read05, BTIZ wrote:
All of the others talk of temperature rise do to compressibility based on
mach number, but no one really addressed the speed accounting for friction
of air molecules on the sheet metal which warms the aircraft. War story
time, low level over the plains of eastern Montana, near Conrad and Havre
Bomb Plots (Radar Bomb Scoring sites) and the outside air temp was
about -15F, however, we were moving along at about .88mach at 500ft AGL,
and
the skin temperature was about 100F, not a worry about accumulating icing.

Some one else referred to "Speed of heat" as being Mach 1, because most
aircraft need after-burner or "heat" (reheat) as the Brits would say.. to
make Mach1. BTDT

The stagnation temperature I recall is roughly given by the following:

Ts/T1 = (1 + (M^2)/5)

Where T1 is the absolute air temperature and Ts is the stagnation
temperature. So if the outside air temperature is -10C and the Mach number
is 0.5 then T1 = 263

Ts = 263*(1 + 0.25/5) = 263 *1.05 = +3.15 degrees

That is the _maximum_ temperature rise at the stagnation point. Elsewhere
it will be lower. The heat transfer through the ice and aluminium and
their thermal capacity will affect how long it takes.

The fact that it depends on the square of the mach number brings the
effect down markedly at lower speeds.
--
David CL Francis

I have flight manuals that give a correction for airspeed and altitude and
the numbers are much higher than 3C even though the speeds are below M0.5.
In the MU-2, the temp sensor is in the tail section where the cross section
of the fusilage is actually decreasing.. Is there both a compression and a
friction heating of surfaces due to airflow?

Mike
MU-2

The stagnation temperature I recall is roughly given by the following:

Ts/T1 = (1 + (M^2)/5)

"Mike Rapoport" wrote in message
link.net...

I have flight manuals that give a correction for airspeed and altitude and
the numbers are much higher than 3C even though the speeds are below M0.5.
In the MU-2, the temp sensor is in the tail section where the cross
section of the fusilage is actually decreasing..

I think the problem is only with the numbers that got plugged in. At M =
0.5, you get
Ts/T1 = 1.05. In other words a 5% temp rise. 5% of about 273 K is about 13
K,
i.e. 13 degC.

Is there both a compression and a friction heating of surfaces due to
airflow?

It's the same phenomenon. The "friction" occurs because the air is brought
to stagnation in the boundary layer.

Julian Scarfe

On Fri, 18 Mar 2005 at 23:27:50 in message
.net, Mike Rapoport
wrote:

I have flight manuals that give a correction for airspeed and altitude and
the numbers are much higher than 3C even though the speeds are below M0.5.
In the MU-2, the temp sensor is in the tail section where the cross section
of the fusilage is actually decreasing.. Is there both a compression and a
friction heating of surfaces due to airflow?

Of course; but I don't think the driving temperature can get higher than
the stagnation temperature. Let's try it for Concorde:

At 55,000 ft the outside standard air temperature is 217 deg K

So Ts = To(1 + (2.05^2)/5) = 408 == 126 C

That is very close to the value quoted in Brian Trubshaw's book about
Concorde. The chart on page 34 shows the nose temperature at 127 C, the
leading edge of the wing at 105 C, pilots cockpit area skin temperature
at 97C decreasing towards the rear fuselage at 91 C. Mind the fuel in
the wings kept down their temperature from rising too high!

It does not show the fins or air intakes but I doubt they were much
higher than the forward fuselage section just behind the nose at 100 C.
--
David CL Francis