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.