Okay, if you're going to tangent, this is something I can speak to.
There are several factors that go into agility in the classic dogfighting sense. Two of the big ones are turn rate and turn radius.
Turn rate is the degrees per second the nose tracks in a line. The higher the turn rate, the less time it takes for an aircraft to change direction a given amount of degrees.
Turn radius is used to derive how big your turn circle is...the smaller the turn radius, the smaller the turn circle.
Now, I'm going to blow your mind here:
In a level banked turn, any aircraft, going the same speed, pulling the same G, will describe the exact same flight path through the air with the same exact turn rate and radius. It does not matter if it is a Cessna, 747, or F-16.
Now, while you muse about that, some aerodynamics.
Wings generate lift through angle of attack (AOA) and camber. AOA is simply the angle the wing faces the wind...stick your hand out the window in a car. Tilt your hand up a little and notice how it gets pushed up...more angle, more force up. Also, this relates to speed...the faster you go, the more lift is generated for a given AOA.
When the AOA is too high, the wing stops generating enough lift because the air on top is too turbulent. This is called a stall, and is not strictly speed related (ie you can stall at well above minimum level flight airspeed). Also, when wings are generating lift, they are generating drag...which has to be countered by more thrust to keep the speed the same. You can reach a point where the drag is so high, due to the AOA being so high, that you can't generate enough thrust to keep your current airspeed unless you lower the AOA (which may mean losing altitude, backing off the turn, etc).
So, summary - AOA is used to generate lift, which can be used vertically to change altitude, horizontally to change direction, or some combination for both (a typical banked turn). Too much AOA, and the wing stalls. More AOA = equals more drag = more thrust to keep the same speed.
Now, lets talk about instantaneous verses sustained turn rate, and something called corner velocity.
Instantaneous and sustained turn rate are pretty self explanatory. Turn rate can also be described as 'G onset rate'. One of the highest instantaneous G onset rates aircraft was the Cessna T-37 Tweet...a slow, twin engined side by side jet trainer, with a onset rate of 17G per second. Which meant of any aircraft in the inventory, it could initially snap its nose around far faster than anything else...but because you quickly hit the airframe limit, human limit, and lost so much speed so quickly, you didn't get much out of it. One turn and you were done...out of airspeed and ideas. Hawgs (A-10s) have very good initial agility...you can get a good snap turn out of it...great to defeat a firing solution. But if you aren't careful, you had no speed left to do much else...and not enough thrust available to get back up to speed quickly.
Corner velocity is an important number for a fighter...it typically represents the speed the fighter can hold its maximum sustained turn rate at its minimum turn radius. Below this speed, a fighter will hit its AOA limit (stall) before it hits its G limit...it isn't rating its nose as fast as it could. Above this speed, its rating its nose at the G limit, but its turn radius is bigger and its turn rate is slower. This is why typical fights in modern 4th generation fighters have been around 400 knots, because that's where your corner speed tended to be...too slow and you weren't tracking fast enough...too fast and your turn radius gets huge.
Newer 4.5 and 5th generation fighters overcome this with thrust vectoring, relaxed stability, and tons of power...which means you can have a lower corner velocity. This translates into increased agility due to higher turn rates and smaller turn circles.
There are of course fundamental limits at the low end here...if your turn radius is so small that you don't move enough in the pipper to make a difference in the firing solution...you're still toast.
It also means an SR-71 is not agile in any sense of the term at speed...its turn radius is simply gigantic. However, to defeat a missile at that altitude, that may be all that you need.
FC
