Speed at FL65 question

So I'm flying in the Raptor F22 to PHNL and it struggles to go to mach 2.35 where I usually fly at FL50. It's like it's harder to speed up way up there. Both ground speed and mach. Why is that?

So I'm flying in the Raptor F22 to PHNL and it struggles to go to mach 2.35 where I usually fly at FL50. It's like it's harder to speed up way up there. Both ground speed and mach. Why is that?

 

My guess would be: No Oxygen??

You're having trouble at 5,000 feet? And at 6,500 feet? The air is pretty dense down there for supersonic travel. If you actually mean FL500 and FL650 (50,000 and 65,000 feet), then it might be related to the very low air density and the low engine power output at those altitudes, but since I don't generally fly in those regimes, perhaps someone who has a lot of experience there will chime in.

 

PLEASE IGNORE ground speed. That is airspeed modified by wind, and has NOTHING at all to do with how the aircraft flies, so if mach is affected, then by definition ground speed is also. Mach is exactly the same as true airspeed (TAS), except that it is expressed as a percentage of the speed of sound, so it's just another way of calibrating true airspeed, such as knots, kilometers per hour, feet per second, etc. Of course the mach calibration varies mostly with temperature, so that may be affecting some things you see, too.

Yeah, FL650... Oops! I missed typed my post. I usually fly at FL500 and I can achieve mach 2.35 very easily. Once I pull past that altitude and go to FL650, the F22 struggles to achieve mach 2.35. I'm guessing less air as well. Temperature up there is like -99 F or colder.

My guess would be: No Oxygen??

 

 

 

Well, I'm sure there is oxygen, just not that much. The aircraft.cfg says its top altitude it can fly is FL650.

The aircraft.cfg says its top altitude it can fly is FL650.

 

I presume that it lists that as its service ceiling? The service ceiling is the altitude at which the aircraft can climb 100 feet per minute. So it's at a point in its performance envelope where stall speed, best rate of climb speed and cruise speed are very close together, and its maneuvering ability is very minimal.

Mach is NOT a particularly good indication of TAS: it varies directly with air temperature, and 0.80 IMN is a lot faster at low altitude/high OAT then it is at 35,000 ft MSL and very cold temperature.

Mach is NOT a particularly good indication of TAS: it varies directly with air temperature, and 0.80 IMN is a lot faster at low altitude/high OAT then it is at 35,000 ft MSL and very cold temperature.

 

Very true, but it nonetheless actually IS A MEASURE of actual airspeed (rather than groundspeed, IAS, CAS, etc.), just as the other units do, even though its primary purpose is for aircraft safety and certain performance parameters, and its lack of consistency when it comes to navigation makes it fairly useless. And that is part of what I was trying to get across to those thinking of it as a useful measure for speed, in the same sense as TAS. Actually, I probably should have said that it is a ratio of TAS to the speed of sound, rather than a direct measure of TAS, though it's based on TAS.

 

So as you indicate, mach 2 at sea level is much faster than mach 2 at 35,000 feet (or mach 2 at 15ºC is much faster than mach 2 at -40º or -50ºC). Without calculating right now, a figure I recall from my (much) younger days is that mach 1 at sea level is about 760 mph, and mach 1 at 35,000 feet is about 660 mph, under standard conditions. Those figures may not be precise, but they're not far off. That 100 mph difference is what made it stand out in my mind.

Lnuss, just checking, you say: "mach ... though it's based on TAS."

Was that a typo perhaps? Is it not based on IAS?

Is it not based on IAS?

 

Nope -- it's based on TAS. More accurately, it isn't really calculated FROM TAS, but the ratio is that of ACTUAL speed through the medium (air, water, whatever material).

 

From the Wikipedia article (they say it better than I can):

 

 

The biggest reason that mach is used at high speed cruise is that shock waves and other pressure phenomena behave the same at the same percentage of mach 1 (mach number), even though the ACTUAL speed may be different one time from another, by 100 mph or more, depending (primarily) on the temperature difference, and it will change as you go through temperature changes even on the same flight, if you maintain the same mach number.

 

The above referenced article gets even deeper.

Mach 1 is commonly held to vary from 760 mph at sea level to 660 mph at 36,000 feet - in ISA conditions.