Modern aircraft are impossible to control
And this isn't even clickbait. Okay, partly clickbait: if you want to fly some passenger or transport aircraft, they will obey you. But modern fighters are not like that at all. Even experienced pilots cannot subdue their wild nature, and if it weren't for the dancing with a tambourine from shaman-engineers, they would fly much worse. And to understand why the worse a fighter flies, the better it is, a little immersion in theory is required.
To begin with, we need two remarkable points on the plane. The first is the center of gravity. Everyone knows about it, but I will remind you anyway: this is a point where the force of gravity of a given body is conventionally placed; its main property is that the force of gravity placed at this point does not create a torque. Therefore, if the center of gravity is exactly above the support, the body will remain in equilibrium on this support.
The second point is the aerodynamic focus. This is a point where you can conditionally place the lifting force and the lifting force will not create a moment. Like the center of gravity, only for the lifting force. These two points can be located differently relative to each other and the behavior of the aircraft depends on their relative position.
Let's consider the first case: the focus is located BEHIND the center of gravity, closer to the tail of the aircraft. Now let's conduct a standard test of aircraft dynamics: a small increase in the angle of attack (a gust of wind at the bottom of the aircraft), provided that the aircraft is balanced in straight and level flight. As the angle of attack increases, the lift increases (which is applied at the focal point), therefore, the tail of the aircraft will begin to rise, reducing the angle of attack (and with it the lift). That is, after a short-term disturbance, the aircraft returned to a stable position itself, without any additional actions from the pilot.
An airplane in which the focus is located behind the center of gravity is called statically stable. The main advantage of such an airplane is that the pilot can set the controls to the trim position and leave, the airplane will continue to fly and will cope with small disturbances itself (note: of course, no pilot will leave his seat during the flight, but no one forbade me to exaggerate). All transport and passenger airplanes are statically stable, this greatly increases their safety.
The second case: the location of the focus coincides with the center of gravity, the so-called static neutrality. In this case, with a small
the plane will not return to the trim position, but it will not leave it either. This plane will feel all the atmospheric disturbances, but it is still relatively safe and obeys the controls (and the winds too).
The third case, the most interesting: the focus is located IN FRONT of the center of gravity, closer to the nose. And now the same test, a small increase in the angle of attack. The lifting force naturally increases, only now it is applied in front of the center of gravity and raises the nose, increasing the angle of attack even more. If you are late in parrying the disturbance, the plane overturns. Such an aircraft is called statically unstable, and this is how it reacts to literally everything, to any atmospheric sneeze in its direction. Every second it tries to overturn in a random direction (because gusts of wind are also random). Now let's remember that the pilot is a human being, and he needs time to react to a change in the position of the aircraft. And let's also remember that there is a (hardware) delay in the control system, and it cannot be removed. And to top it all off: during the flight, the plane's center of gravity at least changes (the fuel is used up, the rockets are fired), which means that the reaction to atmospheric disturbances and to deflections of the control surfaces (to the control itself) will also change. The result: a person is physically incapable of controlling such a plane.
And yet, absolutely all modern fighters are statically unstable. When I learned about this, I had two questions:
1) How do they fly then?
2) Why the hell suffer like this?
Now, in order:
The answer to the first question was at the beginning of the article: statically unstable aircraft fly thanks to the efforts of engineers responsible for the automatic control system. These half-mad geniuses assemble multi-level systems of fantastic complexity and turn the aircraft into a dynamically stable one. Dynamically stable – it is statically unstable, but is felt by the pilot and controlled as stable. Shamanic dances with a tambourine and under fly agarics with coffee in front of a computer with a future SAU work wonders.
Now the second question: why the hell do we need all this? Why bother with a super-complex SPG and what are we paying for with safety? As it turns out, static instability gives very tasty perks for the military, which outweigh the disadvantages:
The first perk. In flight dynamics, the concepts of stability and controllability are somewhat antagonistic. The greater the stability, the more effort it takes to get the plane out of equilibrium, and therefore, the worse the controllability. Transport planes do not need to perform aerobatics, safety is more important for them. But for military aircraft, it's exactly the opposite. Statically unstable planes are ultra-maneuverable. The inability of an aircraft to remain in one position for any length of time greatly hinders long-term horizontal flight, but in an air battle it gives +100 to maneuverability and chances of survival.
The second perk. The focus of the plane also does not stand still, and the higher the speed, the more it creeps back, towards the tail. At subsonic speeds, this is not yet noticeable, but when the speed of sound is exceeded, it begins to play a significant role. If the focus creeps on an unstable plane, then such a plane will simply slowly approach static neutrality. But on a stable plane, the focus will creep even further and the plane will become too stable. It will simply stop responding to the controls until the speed decreases.
And the third bonus: additional lift. This is where it gets tricky, watch your hands. On a statically stable plane, the trick is
is located behind the center of gravity. Because of this, a nose-down moment appears during flight: the lifting force acts closer to the tail and lowers the nose. To prevent the plane from falling, the nose-down moment must be balanced by a pitching moment of the same magnitude (aimed at raising the nose). The balancing moment is created on the horizontal tail, which in the classic design is located on the tail. In order for the moment to be directed at pitching, the force on the GO must be directed downwards (to lower the tail and raise the nose).
Thus, part of the aircraft's lift (directed upward) is eaten up by the balance (the force on the horizontal stabilizer is directed downward). This is called balance losses. For transport aircraft, these losses are not critical, safety is more important there, and the wing is huge. But for supersonic fighters, the situation is the opposite: their wing is small and the lift is always in short supply (who wants to see the apogee of small wings and lack of lift – google photos of the Lockheed F-104 Starfighter). And here the unstable layout helps out: it has a focus in front of the center of gravity, the lift creates a pitching moment, and the balancing moment should be a diving moment and be created on the tail by a force directed upward. Thus, with a statically unstable layout, there are no balance losses, on the contrary, the horizontal stabilizer creates additional lift, which helps the aircraft fly.
So respect the fighter pilots, they fly something that can kill them at any second. And respect the engineers even more,
who, at the cost of their mental health, made such a flight possible and even relatively safe.
Author: Liza Gladysheva
