Have you ever wanted to become a pilot? Know that a goal without a plan is just a desire (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 aerodynamics courses. 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 in all its glory the letter M. What does it mean?
Very quick reference
The Latin letter M in aerodynamics textbooks is nothing more than a Mach number. It denotes the ratio of the speed of flow around an object (for example, an airplane) to the local speed of sound. It owes its name in aviation works to the Austrian scientist Ernst Mach. Scientific words look like this:
M = v / a
Here, v is the free-stream velocity, 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 his professional career is likely to have two questions. What is such a local speed of sound? Why do you need Mach number?
Ready to take off!
What is meant by the word sound? First of all, it is a wave. After all, the sound source creates perturbations in the medium that are transmitted to air molecules, and so on along the chain. Therefore, with increasing altitude, where the atmosphere is more discharged, the sound wave will propagate at a lower speed. Accordingly, the local Mach speed is present in the formula for the Mach number. All values for specific heights have already been calculated (special tables) - you just have to substitute. Free air velocity is measured using air pressure detectors (LDPE), 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?" Do not forget, you fly at high numbers M.
Three, two, one - let's go
Mach number in aviation (and not only) plays a huge role. Almost all pilots of civilian, military and space shuttles can not do without it. This parameter is so important!
When the aircraft moves in space, the air molecules around it begin to "resent". If the speed of the aircraft is low (M <1, ~ 400 km / h, subsonic aircraft), then the density of the environment remains constant. But, as kinetic energy increases, part of it goes into compression of the near-airspace. This compression effect depends on the force with which the aircraft acts on air molecules. The higher the flight speed, the more air is compressed.
At a transonic speed (~ 1190 km / h), small perturbations are transmitted to other molecules around the aircraft (it is easier to look at the wing surface), and at one fine moment, when at some point the speed of the incoming flow is compared with the local speed of 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 occur, only a 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, flight operation documents (RLE for domestic, FCOM for foreign) indicate the critical Mach number. This is the minimum value of M, at which the incident flow in any part of the aircraft reaches the speed of sound (Mkr). That’s the whole secret!
By the way, the most successful flying passengers of the Soviet Union traveled faster than modern ones. Do not believe?
New is long forgotten old
Old people are faster than young ones! And it's not a joke. One old plane, forgotten by all, was once the flagship of the USSR aviation. His name was TU-144. It was the first (and is) the world's supersonic passenger airliner performing commercial flights with a maximum speed of up to 2500 km / h. Although the flight career of the Tu-144 was short-lived, his fate was inextricably linked with the number M.
The second similar aircraft was the British-French Concord. It is noteworthy that they made the first flight with a difference of only two months. A good knowledge of aerodynamics will help passengers of commercial flights forget about long flights across the Atlantic. And flights of aircraft and spaceships will continue to inspire humanity to new discoveries.