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  1. One of the frustrations of being a child and reading lots and lots of books on combat aircraft was that there would be impressive-sounding technical terms bandied about, but no explanations. Or if there were explanations I didn't understand them because I was a child. One of the terms that got thrown around a lot was "relaxed stability" or "artificial stability" or even "instability," and this was given as one of the reasons for the F-16's superiority. Naturally, an explanation of what on earth this was was not forthcoming, but it had something to do with making the F-16 more maneuverable. This is partially true, but relaxed stability doesn't just make a plane more maneuverable. It makes a plane better in general. Why is this so? Let's look at a schematic of a typical aircraft: There are two points of interest here; the center of lift (CL) and center of gravity (CG). The CL is the net point through which all aerodynamic forces acting on the aircraft pass. Various things can cause the CL to shift around in flight, such as the wing stalling or the transition to supersonic flight, but we'll ignore that for now. The CG is the net center of mass of the aircraft. The downward force of the weight of the aircraft will act through this point, and the aircraft will rotate around this point. The reason that this configuration is stable is that the amount of lift a wing generates is a function of its angle of attack (AOA, or alpha). AOA is the angle of the moving air relative to the wing. If the wing is more inclined relative to the air, it generates more lift up until it starts to stall. The relationship looks like this: Obviously this depends on the exact shape of the wing and the airspeed, but you get the idea. The lift increases as alpha goes up, but falls off after the wing stalls. This means that in a conventionally stable aircraft in level flight, anything that causes the nose to pitch up will cause the amount of lift to increase, but because the CL is behind the CG, this increased lift will cause a torque on the aircraft that will rotate the nose back down again. Thus, any disturbances in pitch are self-correcting. This is important because it means that a human being can fly the aircraft. If random disturbances were substantially self-magnifying, the plane would begin to tumble through the air. There's a bit of a problem though. Because the CL is behind the CG, the plane has a tendency to rotate downwards. So, to keep the plane level the tail has to apply a torque to trim out this tendency to rotate. The torque that the tail is applying is pushing downward, which means that it's cancelling out part of the lift! Keeping the tail deflected also increases drag. These problems would go away if the arrangement were reversed, with the CG behind the CL: However, this would make the plane unflyable for a human. But this is the 21st century; we have better than humans. We have computers. A computer (actually, an at-least-triply-redundant set of computers) and an accelerometer detect and cancel out any divergences in pitch faster and more tirelessly than a human ever could. The tail downforce becomes tail upforce. Also (contrary to wikipedia's shitty diagrams), the distance between the CG and CL is closer on unstable designs, so the trim drag of the tail is smaller too. OK, so unstable designs get a slight reduction in drag and a slight increase in lift. Why is that a big deal? Think of a plane as a set of compromises flying in close formation. Everything in aerodynamics comes at a cost. Let's take a look at how this principle can kneecap people trying to be clever. The quicker of you will have no doubt objected to my characterization of stable aircraft losing lift due to tailplane downforce. "But that doesn't apply if the plane is a canard design! The CG will be in front of the CL, but still behind the canards, so the canards will generate an upforce to trim the plane out! No need for fancy computer-flown planes here!" Yeah, they tried that. But the need for CG/CL relationships ends up screwing you anyhow. Let's look at a stable canard design (and one of my favorite aircraft), the J7W1 Shinden: Note that the wings are swept. Now, this is a prop-driven plane, so I can guaran-fucking-tee you that the wings aren't swept to increase critical mach number (I don't think the designers even knew about critical mach number at the time). Instead, the wings are swept for two reasons: 1) To move the CL back so that the plane is stable 2) to move the rudders back so that they're far enough behind the CG that they'll have adequate control authority. There are lots of reasons you don't want swept wings on a prop fighter. Since the thing is never going to go fast enough to encounter the benefits of them, in fact, the swept wings are almost entirely a negative. They reduce flap effectiveness and have goony stall characteristics. If you could get away with not having them, you would. But you can't. You can't because it's 1945 and the computers are huge and unreliable. Your clever dual-lifting-surface canard design's advantages are heavily watered down by the disadvantages imposed by the need for stability. That is the big advantage of instability. The designer has a lot more freedom because there's one less thing they have to worry about. This can indirectly lead to huge improvements. Compare a mirage 3 and a mirage 2000. The mirage 2000 is unstable, which adds some extra lift (nice, especially on takeoff where deltas really hurt for lift), but more than that it allows the designer to move the wings further forward on the fuselage, which allows for better aft-body streamlining and better area ruling. Instability doesn't allow for better area ruling per se, but it frees the designer enough that the could potentially opt for that.
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