The plane decided to land
Imagine: you’re a pilot, you’re flying, you’re not hurting anyone, you’re fixing a Primus stove, and you decide to gain altitude. To do this, you need to accelerate a little and at the same time slightly increase the pitch, which is what you are doing. By increasing the thrust with the throttle, you simultaneously slightly pull the control lever towards you. Everything goes well for the first few minutes, but then the lever moves completely independently and with enormous force to the “toward” position all the way and freezes there. Congratulations, you have become familiar with a phenomenon called “elevator overcompensation.” While you're frantically trying to push the stick back to its normal position and your plane is going into a sustained spin, you've only got a few minutes to hit the ground, so let's take a closer look at this phenomenon.
First you need to decide on such a thing as the center of pressure. Wikipedia says that for an airplane wing (and the elevator is almost a wing), the center of pressure is the point where the line of action of the aerodynamic force and the chord plane of the wing intersect. That is, a point on the wing profile where, conditionally, the aerodynamic force acts (the resultant of all the forces that you draw on an airplane flying in the air). In general, the center of pressure can move, and it changes its position depending on the speed of the aircraft and the angle of attack (let's save the rest of our sanity and not analyze its movement).
The second thing necessary to understand what is happening is the hinge moment. In aviation, this is the moment that acts on the control (the elevator in our case) relative to its axis of rotation and which is created by the aerodynamic force. Normally, the hinge moment is directed against the steering wheel deflection and tends to return the steering wheel to the neutral position. That is, the elevator is deflected to the balancing position, deflected around the center of rotation in which it is fixed and around which it rotates. Somewhere on the steering wheel there is a center of pressure where the aerodynamic force acts. If the center of pressure and the center of rotation are NOT the same, the force will create a hinge moment. This moment tends to return the steering wheel to a neutral position. If nothing is clear, don’t despair, it’s normal, I’ll put a picture at the end. And one more important, but not always obvious nuance: the hinge moment depends on the angle of deflection of the steering wheel and the flight speed (in general there is a high-speed pressure, but why do we need such difficulties), the greater the angle and the higher the speed, the greater the hinge moment.
We continue our introduction. In the early days of aviation, airplanes were small and flew slowly. The hinge moments on the rudders were small, and the average pilot could easily overpower them simply with the power of his muscles (the picture of a pumped-up pilot in an 18+ flight uniform is unlocked without SMS and registration). In those good old days, all rudders were connected to the control lever and pedals (yes, pilots have pedals, almost like on a car) mechanically, by a system of rods and rockers, that is, stupidly with cables, and the pilot felt all efforts from the rudders directly with his hands/feet . But time passed, people’s appetites grew, and so did flight speeds. The moment came when the forces on the steering wheels increased so much that even a weightlifter could not hold the control lever. The aircraft control system included first boosters (hydraulic boosters), and then electronics. It quickly became clear that flight speeds (and the magnitude of hinge moments) grow much faster than the power of hydraulic drives. And even if the drive reaches the required power to keep the steering wheel in the required position at supersonic speed (forty-ton elevator drives from Sushki are approved), they stupidly become too huge. And you have to choose: either a powerful drive = heavy plane = lousy acceleration, or a small drive = low power = won’t be able to hold the steering wheel, you have to limit the speed. Two chairs, both bad.
Are you still here? Let's continue. The engineers were not going to put up with two bad chairs, they made a third, good one: they invented ways to compensate for hinge moments on the steering wheels. There are several of them: axial, horn, internal compensation, servo compensation and the use of a trimmer. Let's briefly go over these methods.
Axial compensation consists in the fact that the axis of rotation is not located at the end of the steering wheel, but a little further, so that part of the steering wheel remains in front of the axis of rotation. Then, when the steering wheel is deflected, the front part of the steering wheel seems to deviate in the other direction, and a moment of the opposite sign is created on it. It has gained universal love for its ease of design and good aerodynamics.
Horny compensation is similar to axial compensation, but here a smaller piece of area remains in front of the axis of rotation, similar to a horn, hence the name. This piece creates a compensating moment. It is also simple to implement, but worsens aerodynamics, especially at large steering angles.
Internal compensation is common mostly on the ailerons. In this case, the piece of profile adjacent to the axis of rotation of the steering wheel remains empty and is divided by a flexible sealed partition (diaphragm) into two cavities. A pressure difference arises in the cavities, acting on the diaphragm and creating a compensating moment. It does not introduce any disturbances into the flow, which is especially valuable in super- and hypersonic conditions, but it limits the range of rudder deflections, especially on a thin profile.
Servo compensation is the use of small deflectable surfaces on the trailing edge of the main rudder. A sort of steering wheel on the steering wheel. Trim compensation is one of the types of servo compensation; it differs in that it is used in steady-state flight conditions and completely zeroes out the hinge moment (all other types of compensation only reduce it, but do not completely remove it).
So here we are. Axial compensation has one interesting and extremely dangerous side effect: if for some reason the center of pressure creeps onto the compensator (the very part in front of the axis of rotation), trash will begin.
Firstly, the steering wheel (and the control lever too) instantly flies to its extreme position.
Secondly, the force sign on the control lever immediately reverses. Now, in order to move the lever from the “pull” position to the neutral position, you need to PUSH it, whereas normally the lever itself tends there and you need to PULL it. This makes it very difficult to control and can lead to disaster.
And thirdly, by the time the pilot understands all this, the plane will already reach critical modes (corkscrew – ground – coffin – cemetery).
Congratulations, you have met Ms. Overcompensation. And the steering wheel, accordingly, became overcompensated. Overcompensation is extremely dangerous crap, and they try to avoid it by all possible and impossible methods. As I once heard in lectures (I can’t vouch for the reliability of the information) that Sukhoi planes are equipped with incredibly powerful drives that can overcome possible overcompensation, but the MiG is more cunning, they build the rudders in such a way that at low speeds (and small hinge moments) they are overcompensated, but then they are easily overpowered, but at high speeds the steering wheels become normal. This scheme allows the installation of weaker and lighter drives. Ideally, of course, it is better to avoid overcompensation at all, but it depends on your luck. The flight of a fighter is an unpredictable thing.
Enjoy your landing.
Author: Liza Gladysheva
