> It's a chicken an the egg problem. Any aircraft can have quickness in > maneuverability with large enough control surfaces. But you can't > make the control surfaces too large and still intercept nuclear > bombers at Mach 2.
True .. though not so much "Chicken and Egg" as balanced design tradeoffs. > Thus, at some level this allows designers to put back elevator > "authority" that they had to remove to get the drag performance they > wanted. The idea of having agile aircraft able to pitch to very high > AoA's has never required a stability augmentation system. Many WWII > fighters were extraordinarily nimble in pitch. Hmm. I'm thinking of high alpha as something over 20 degrees. Even 50 or 60 degrees. Of course, this does not require a SAS, or a DFlCS. All you have to do is cut the engine and yank back on the stick :-) In any case, agility is being able to roll quicker than your opponent, or pitch up quicker. High alpha is not so much the goal as change in direction is. One interesting high alpha film clip I saw recently was the (Sukhoi?) forward swept wing demonstrator. The thing could almost hover in midair. The high alpha maneuvers were amazing. I was discussing it with a former F-18 driver and we agreed that it would be an easy target during those kinds of maneuvers. Not very useful for combat at the low speeds demonstrated given the current engagement practices... The point I was making in the paragraph I wrote (above) was that it was the neutral stability that gives you the agility in today's fighter aircraft (given the design trade-offs) but it requires computer control. Here's a small portion of the technical doc I wrote some years ago for inclusion into the F-16 DFlCS simulation model for Link: A flight control system of sorts has been present on all aircraft since the earliest attempts to fly were made. It is a human. The pilot commands a turn, for instance, by applying force to the stick. If the turn is not quick enough the pilot increases the turn rate. The pilot has received feedback information from the aircraft instruments, and has compensated for the slower than desired turn rate by commanding additional turn rate by applying additional force to the stick. Modern flight control systems use the same technique: comparing the pilot's command to actual aircraft dynamics, and driving the control surfaces to produce zero error between these two quantities Recently [note: this was written ca. 1990], fighters such as the F-16 have been produced which wander between neutral stability and instability. These aircraft are extremely agile. They require a highly capable, fail-safe flight control computer to keep from tumbling out of control. In such a system the pilot inputs a command through the stick, the flight control computer processes that command, and the computer then sends an electrical signal to an actuator at the control surfaces to implement the command. This is called fly-by-wire. The responsibilities of the modern flight control system are wide ranging. During straight and level flight, for instance, the F-16 flight control computer keeps the aircraft straight and level by taking the air data information supplied to it by the Air Data Converter (ADC) and aircraft dynamics data from rate gyros and feeding it back into the flight control loop. During this straight and level flight - assuming no pilot inputs are present - the aircraft basically flies itself, sending commands to the aerodynamic control surfaces based on air data and rates. Because the aircraft is statically unstable, and is traveling through a random and imperfect atmosphere, the control surfaces are in motion constantly. Another responsibility of the FLCS is to process pilot commands, and to drive the surfaces accordingly. When the pilot pulls back on the stick with a force of X pounds, for instance, he/she is commanding a pull up of Y g's [note: the F-16 FLCS pitch channel is a normal acceleration command type]. The flight control system takes the command, limits it if necessary, compares it to the actual current g force, and drives the tail surface until the actual and commanded values are equal. Jon
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