http://www.tvbf.org/archives/velocity/msg02818.html
The pressure we have to work with is
limited to the dynamic head. And if
air isn't treated right in the
ducting it will form back eddies and pressure
waves, and find lots of ways
to give you less flow than you calculate from
your intake area. Core
thickness can be traded for x-sectional area only if
the ducting is
designed to get the air slowed and through it.
http://www.vansaircraft.com/pdf/hp_limts.pdf
Because the airspeed indicator is The Gauge
That
Lies. Despite its name, an airspeed
indicator does not
measure speed. It measures "q" – dynamic
pressure
caused by packing air molecules into a
tube.
http://duxford.iwm.org.uk/upload/pdf/Instrumentation.pdf
Airspeed
The airspeed is directly related to dynamic
pressure. To find out what the Dynamic
Pressure is, Static Pressure (the pressure
of the air surrounding the aircraft) is
subtracted from the Total Pressure, which is
the force of the air impacting with the
aircraft (this is measured using a pitot
tube which protrudes from the aircraft to meet
the oncoming airflow directly).
So it would seem to me for
apples and apples you would want to compare your cooling at different
altitudes at the same dynamic pressure (or IAS) in order to isolate the
effects of ambient temperature on cooling. But, then I've been wrong
before {:>)
Ed A
Ed,
Q is indeed Q at any altitude and speed, which
makes it very handy for structural calculations. It involves density, which
involves Temp and pressure. So using Q alows you to eleminate these variables
from your calculation. That is fine for structural loads.
It is not fine for thermodynamic calculations.
What is important here is how fast you are actually traveling through the
fluid. In the jet world all calculations are bassed on To which is the
stagnation temperature that you get if you decellerate a molecule adiabaticly
to rest from free stream velocity. We don't have to worry so much with
compressibility and stagnation temp at our speeds, but we still have to have
the TAS to calculate inlet efficiencies and sizes. We cannot just throw out
density (pressure and temperature) since they are intimitely related to what
we need to do here.
TAS is free stream or Vo. What goes through
your inlet gives you Vi/Vo or Vinlet/Vfree stream. typically .6-1 depending on
what you are doing on the other side of the inlet.
If the inlet is perfectly sized to the task,
the capture area will ingest the proper amount of mass flow at Vo (TAS) for
CpDt to carry away our cooling load WITHOUT
DIFFUSION. Now that you have the air in the airplane you use a
diffuser to slow it down and raise the pressure in a controlled manner to the
point that it will cancel the pressure drop in your heat exchanger (or better
yet overcome your exit nozzle). As the air flows through it picks up heat and
expands a little. If your cooler is the proper size and relatively
efficient your pressure on the back side of the cooler will still be greater
than ambient and you can accellerate the cooling air in a
nozzle to Po (ambient pressure) If you did good you will get close to Vo.
If you did real good you will get Vo+ a little. This is where the thin vs
thick argument comes in. Theoretically if you slow the air way down and
recover all the dynamic pressure and pass it ever so slowly through a Mac
truck sized radiator that is 1/4 inch thick you will get very
little pressure drop in the cooler and all the stagnation
pressure will all be available (plus a little from heat
addition) to squirt out through the exhaust nozzle.