Different Example
Carl:
I don't have a whiz-wheel with me, and I don't doubt that it says what you say...but I think there is a way to adjust for non-standard temperature on the EA-6B. I'll have to wait until I can get my hands on one and figure it out.
In the jet I currently fly (T-38), our standard cruise-mach is 0.78 at FL 280. On a standard day, this will result in an IAS of 320 knots.
In the winter, it happens at about 313 kts. In the summer, about 327 kts. The T-38C is a glass cockpit; the Mach-number is determined by current true airspeed, which uses an input from a Total Air-Temperature probe.
When I did functional check flights in the F-4, we did them at FL400. We accelerated to 500 kts indicated, where the intake-ramps started to program. There was a table in the flight manual that predicted the resulting Mach number where this occured. Since it was always at FL400, there was only one variable in the charts: OAT.
The lowest I ever saw was Mach 1.7; the highest was 1.8; same 500 IAS.
The F-4 had a mechanical-looking airspeed and Mach gauge, but it, too, had a temperature input from a TAT, and the Mach-scale moved independently of the IAS ring.
Rickair:
I was pretty sure of this earlier, but looked it up to make sure: temperature is really the only variable; pressure and density are negligible (I had thought the same as you until mid-career, then learned this; emphasis added by me).
Wikipedia Excerpt:
....This equates to 1,236 kilometers per hour (768 mph) or about one mile in five seconds. This figure for air (or any given gas) increases with gas temperature (equations are given below), but is nearly independent of pressure or density for a given gas. For different gases, the speed of sound is dependent on the mean molecular weight of the gas, and to a lesser extent upon the ways in which the molecules of the gas can store heat energy from compression.....
Last edited by UAL T38 Phlyer; 06-15-2009 at 07:38 PM.