Okay, I post this because of a letter to the editor I read in Automobile Magazine. (note: the letter is talking about an F-16 jet that was featured in the magazine.) The author of the letter was asking about the magazines claim that Mach 2 equated to something like 936MPH. He stated that he believed that since Mach 1 is equivalent to the speed of sound, Mach 2 should equate to about 1500MPH, since the speed of sound is about 761MPH.
That sounded right to me, so the answer given boggled my mind. Paraphrasing, the response read:
“A Naval engineer gave us that figure. Since the speed of sound varies by altitude, the figure given is adjusted.
What!? We have a unit of measure that varies by altitude? Yes, indeed we do. From the US Centennial of Flight Commission’s page:
The Speed of Sound and Mach Numbers
The Mach number (M) refers to the method of measuring airspeed that was developed by the Austrian physicist Ernst Mach. It is used to indicate flight velocities in high-speed flight and is related to the speed of sound. The actual speed of sound varies depending on the altitude above sea level because sound travels at slightly different speeds at different temperatures, and the temperature varies according to altitude. At sea level, the speed of sound is about 761 miles per hour (1,225 kilometers per hour). At 20,000 feet (6,096 meters), the speed of sound is 660 miles per hour (1,062 kilometers per hour).
If an aircraft is traveling at one half the speed of sound, it is said to be traveling at Mach 0.5. A speed of Mach 2 is twice the speed of sound. Because the speed of sound varies, a particular speed at sea level expressed as a Mach number would be faster than the same speed at 30,000 feet (9,144 meters), which would be faster than the same speed at 40,000 feet (12,192 meters). In other words, Mach 2 at sea level is a greater number of miles per hour (or kilometers per hour) than Mach 2 at 30,000 feet, which is a greater number of miles per hour than Mach 2 at 40,000 feet. When an aircraft reaches Mach 1, it is said to “break the sound barrier.”
The following breakdowns have been generally accepted to classify speeds:
M less than 0.8 subsonic
M = 0.8 to 1.2 transonic
M - 1.2 to 5.0 supersonic
M greater than 5.0 hypersonic
I think we need a better unit of meaure than the Mach numbering system. I mean, imagine if the “inch” was tied to altitude. It would be completely useless.
What the hell. A unit of measure shouldn’t be tied to a variable.
/rant
July 23rd, 2005 at 9:22 am
I agree - but it appears that they classify it by altitude ONLY because of the temperature, meaning it has nothing to do with pressure or other external forces, so then it irritates me that they don’t just classify it by temperature. Of course, I’m not a scientist so there probably is some much more valid purpose for using altitude as the variable…
July 24th, 2005 at 9:01 am
Mach isn’t a unit of measure. It’s a dimensionless number. It’s a ratio. An Inch is a unit of measure.
Saying “a unit of measure shouldn’t be tied to a variable” is like saying we shouldn’t have sines, cosines, and tangents.
Mach is just your speed measured relative to the speed of sound, whatever the speed of sound happens to be in your surroundings. This is handy to know because as your altitude changes, the speed of sound changes, and if you don’t want to break the sound barrier, or if in fact you do want to, it helps to know how fast you have to go at your given altitude.
It’s just weird to think about the mach number being based on altitude, but it does make sense really. Think about it, time used in MPH or KPH calculations is also variable depending on your velocity.
July 25th, 2005 at 8:26 am
Okay, so perhaps I overspoke. But I still think is very strange to list a vehicle’s top speed by a number that can vary by altitude.
Saying that the F-16 has a top speed of Mach 2 is ambiguous, and it is really only accurate at one elevation, which was not given.
And why would we use multiples of the Mach number to describe speed? I fully understand the need to know what the speed of sound is at any given altitude, so figuring Mach 1 seems important enough. But why do we ever talk about Mach 5, for instance?
July 25th, 2005 at 10:31 am
First of all, saying an F-16’s top speed is Mach 2 is not ambigous at all. It’s also no accurate at only one elevation. It’s accurate at all elevations, that’s why they give a number that factors elevation into it. If your speed, divided by the speed of sound in the conditions you are travelling equals 2, you are travelling at Mach 2.
They list an aircrafts top speed by mach number because it’s crucially important. It’s not the speed at which the jet engines could theoretically thrust the aircraft, it’s the speed that the aircraft has been designed for. In other words, if you go faster than what it’s designed for, the compressability effects will add up, and quite likely make the plane uncontrollable due to the way the airstream is broken up around your plane. You could lose thrust, lose control, or any number of bad things just because the aircraft wasn’t designed to fly in those parameters.
So if a plane goes Mach 2 at sea level, It’s travelling roughly 1523mph (calculated at the impossible 0 feet altitude - but it’s an example). At this point you have reached the maximum conditions that the aircraft has been designed for. However, if they said that the max speed of the aircraft was simply “1523 mph” and you were flying at 30,000 feet, going 1523 mph, you would be travelling at Mach 2.244, which would be outside the specifications, and likely to fuck up your plane, because it simply wasn’t designed to fly in an aerodynamic situation like that.
July 25th, 2005 at 10:35 am
As a small followup, it’s the reason that an F-16 that maxes out of Mach 2 looks totally different than say, a space shuttle which needs to maintain control at roughly Mach 25.
It’s the same principle as with a car that can make way more power than it can keep on the road…only in this case the road is “air”. Just because on paper your car with twice as much horsepower should go twice as fast as mine, it doesn’t mean that it will when it hits the point your tires no longer grip.
July 25th, 2005 at 10:58 am
fuck mach numbers.
July 25th, 2005 at 11:05 am
hippy.