Have you ever wanted to be a pilot? Know that a goal without a plan is just a desire (the words of the great classic Antoine de Saint-Exupery). It is worth noting that he was not only a writer, but also a professional pilot.
Absolutely all people associated with the sky take courses in aerodynamics. This is the science of the movement of air (gas), which also studies the effect of this medium on streamlined objects. One of the sections of aerodynamics is the features of flight on supersonic aircraft. And here the student will see the letter M in all its glory. What does it mean?
Very brief reference
The Latin letter M in textbooks on aerodynamics is nothing but the Mach number. It denotes the ratio of the flow velocity around an object (for example, an aircraft) to the local speed of sound. It owes its name in aviation works to the Austrian scientist Ernst Mach. In scientific words it looks like this:
M=v / a
Here, v is the speed of the oncoming flow, a is the local speed of sound. It is worth noting that in foreign sources, the speed of the object is used, in contrast to domestic literature. A person who does not encounter this in professional activities is likely to have two questions. What is the local speed of sound? Why do we need a Mach number?
Ready for takeoff
What is meant by the word sound? First of all, it is a wave. After all, the sound source creates disturbances in the environment, which are transmitted to air molecules, and so on in a chain. Therefore, with increasing altitude, where the atmosphere is more rarefied, the sound wave will propagate at a lower speed. Accordingly, it is the local speed of sound that is present in the Mach number formula. All values for specific heights have already been calculated (special tables) - you just have to substitute. The oncoming flow velocity is measured using air pressure receivers (APS), which are installed on all aircraft. Now we have all the data, which means we can easily calculate the Mach number. A fair question arises: "Why not just use the flight speed?". Don't forget, you fly high M numbers.
Three, two, one - let's go
Mach number in aviation (and not only) plays a huge role. Almost all civil, military and space shuttle pilots cannot do without it. This parameter is so important!
When an aircraft moves through space, the air molecules around it begin to "disturb". If the aircraft speed is low (M<1, ~ 400 km/h, subsonic aircraft), then the ambient densityenvironment remains constant. But, as the kinetic energy increases, part of it is spent on compressing the airspace around the aircraft. This compression effect depends on the force with which the aircraft acts on the air molecules. The higher the airspeed, the more the air is compressed.
At transonic speed (~1190 km/h), small perturbations are transmitted to other molecules around the aircraft (easier to consider the wing surface), and at one fine moment, when at some point the speed of the oncoming flow compares with the local speed sound (M=1, namely the flow, the aircraft can fly at a lower speed), a shock wave arises. Therefore, the difference in the design of fighters is so obvious: their wings, tail and fuselage, compared to subsonic aircraft.
On aircraft flying with M<1, but at high speeds (modern passenger liners), this situation can also happen, only the transition to transonic speed will lead to a stronger shock wave, a significant increase in drag, a decrease in lift, loss of control and further fall.
For such aircraft, the flight operation documents (AFM for domestic, FCOM for foreign) indicate the critical Mach number. This is the lowest value of M at which the oncoming flow in any part of the aircraft reaches the speed of sound (Mcr). That's the whole secret!
By the way, the most successful flying passengers of the Soviet Union traveled fastermodern. Don't believe me?
New is long forgotten old
Old people are faster than young ones! And it's not a joke. One old plane forgotten by everyone was once the flagship of the USSR aviation. His name was TU-144. It was (and still is) the world's first commercial supersonic passenger airliner, with a top speed of up to 2,500 km/h. Although the flight career of the Tu-144 was short, its fate was inextricably linked with the number M.
The second similar aircraft was the British-French Concorde. It is noteworthy that they made the first flight with a difference of only two months. A good knowledge of aerodynamics will help commercial passengers forget about long flights across the Atlantic. And the flights of aircraft and spacecraft will continue to inspire humanity to new discoveries.
