It’s easier to go Mach 2 at 50000 feet than it is at 25000 feet. Or so I hear.
In: Physics
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True air speed, which is the speed you’re travelling through the airmass itself, and on a day with no wind, will be the same as your ground speed. Besides used for calculating Mach speed, and estimates for points/destinations (adding the wind component), it isn’t really used.
Mach number is for an aircraft a limiting factor when it comes to compressibility at lower altitudes, to high a mach number, and the aircraft starts doing strange things.
Lower altitudes, Indicates/Calibrated airspeed is limiting factor, exceed the maximum CAS, and you may rip off the wings.
Yes Mach is the speed of sound through a medium.
The speed of sounds at different altitudes is different so the speed you need to travel chnages
It’s down to the temperature rather than altitude itself… Mach speed will be higher @FL500 than @FL250 as it is colder @FL500. The colder it is, the lower the speed of sound is. But when you reach above certain point, it will first stay the same for a while and then start changing the other way… Mach number will tell you what is the ratio between your speed and the speed of sound at your altitude (or to be more specific… in the outside temp at that altitude)
Check out the graph [Temperature and speed of sound change with altitude](https://upload.wikimedia.org/wikipedia/commons/thumb/9/9d/Comparison_US_standard_atmosphere_1962.svg/1280px-Comparison_US_standard_atmosphere_1962.svg.png)
Air density has a big effect.
At lower altitudes, the air is more dense – this makes it harder to travel at speed, as the plane needs to push more air out of the way, which requires more fuel to do.
At higher altitudes the air is a lot thinner, so the plane can cut through the air much more easily.
As an analogy, think about moving a knife through a jar of honey – it is hard work, and the honey resists the movement of the knife quite a lot. This is like moving at low altitudes.
Then try moving your knife through a bowl of water – you can swish your knife about with no effort at all. This is like moving at high altitudes.
The this is the main reason why commercial jets fly at around 35,000 feet – this altitude is a good balance between flying high enough to reach the thinner air and save fuel, while being low enough that the life support equipment needed to keep passengers comfortable and safe isn’t too prohibitively expensive.
Mach is the speed of sound. The speed of sound changes as the air pressure changes.
You say it’s more difficult to do Mach 2 at 25,000 than 50,000. That is because they are different speeds. At 50,000 Mach 2 would be about 1320mph, at 25,000 Mach 2 is around 1380mph.
Aerospace engineer here. “Mach” is not a set number – it is a ratio of your speed to the speed of sound in the air you are in.
Sound requires molecules to bump into each other to transmit. Generally, at higher altitudes, the air is thinner and it is more difficult for air molecules to bump into each other. Therefore, the speed of sound goes down with altitude.
If you’re traveling at 800 kts at a lower altitude where the speed of sound 800 kts, you’re travelling at Mach 1. If you’re traveling at 800 kts where the speed of sound is slower, let’s say 400 kts, you’re at Mach 2.
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