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. Using Q allows you to eliminate speed, density, and temperature variables from your calculation. That is fine for structural loads and normal speeds and altitudes. The space shuttle sees very little Q on early re-entry, but I don't think you want to hang a radiator out there!

Q 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 based on To which is the stagnation temperature that you get if you decelerate a molecule adiabatically to rest from free stream velocity. Stagnation temp is what you are after, not pressure. Q is stagnation pressure, but it tells you nothing about temperature or density. It is only a pressure which could be at any arbitrary speed, density, or temperature combination. High altitude: Very high speed, high temp, and low density give the same Q as Low altitude: Low speed, low temp, high density. We don't have to worry so much with compressibility and stagnation temp at our speeds, but we still need TAS to calculate inlet efficiencies and sizes. We cannot just throw out speed, temperature and density since they are intimately related to what we need to calculate.

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. The inlet is an inlet and nothing more. 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 accelerate 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 radiator argument comes in. Theoretically if you slow the air way down and recover all the dynamic pressure and pass the air ever so slowly and delicately through a Mac truck sized radiator that is 1/4 inch thick you will get very little pressure drop (low turbulence) in the cooler and all the stagnation pressure will be available (plus a little from heat addition) to squirt out through the exhaust nozzle. Your airplane will also look like the Goodyear blimp to accommodate the radiator and diffuser. In the real world there must be a compromise.

We are not traveling at jet speeds or even P51 mustang speeds, and this jet effect is not as pronounced. Everybody who is trying to use this and exhaust augmentation to gain thrust is wasting their time. We do not have enough heat being rejected or enough dynamic pressure to make any appreciable thrust. It is doubtful that even the P51 made any thrust at cruise. Maybe at max manifold pressure and war emergency power at top speed it made a little at some altitudes.

Attached is a graph of what would happen if you did this perfectly and used all the waste heat of a 13b including the exhaust heat. Not too encouraging. The weight and size of the hardware to even get close to this would more than offset the miniscule gains.

Does this mean that all use of an augmenter is daft? No. If you use it to cool in climb and on the ground so that you can get away with a smaller cooling system, then that is OK, just don't think you are going to make a wankle-jet out of the thing.

I think I sent a copy accidentally before I was done. Sorry, stupid mouse slipped off the save and hit the send.

Tried to make it a little clearer than mud, not sure I succeeded. Hard to cram thermo one, two, and propulsion into a few email paragraphs.

Monty