David Megginson wrote: > Note that some fighter aircraft, like (I think) the F-4, are > inherently unstable, and if they're modelled correctly we won't be > able to fly them at all by direct controls: we'll need to work though > a fairly sophisticated FCS.
The F-4 is stable. It's actually much older than you think -- it started life as the McDonell F4H before the 1962 unification of aircraft identifications. The idea of using statically unstable aircraft wasn't practical until the advent of cheap computers in the late 70's. I believe the F-16 was the first operational unstable aircraft (it was certainly the first in US service, anyway). This feature gets hyped up more than it should, IMHO. The advantages to having an unstable aircraft are that you can hold it at a much higher peak AoA. Stable aircraft tend to run out of elevator authority somewhere around the stall. If you want to hold a higher AoA, you need a bigger elevator, which adds weight and drag and is generally a pain for the designer to deal with. If you have a computer that can wiggle the elevator for you, you can move the wings forward (ahead of the c.g.) to the point where only a comparatively tiny elevator is needed. Basically, it's a weight/drag optimization, nothing more. And the FCS doesn't have to be too terribly complicated. To first approximation, you would just "simulate" stability by computing a target AoA from the stick position, trim, and airspeed and using elevator deflections to seek to that. I strongly suspect this is how the F-16A works, although I know it has a bunch of "modes" for safety and usability reasons. There really wasn't too much computer available in the mid 1970's to do much else. [Random aside: I was recently reading a description of the E-3 Sentry from the 1970's when it was introduced. They described the on-board IBM computer in glowing terms: 760 KHz (yes, kilohertz) cycle times, with 111 kwords of core memory. Wrist watches toast that this these days. The F-16 was from about the same generation, and I'd be really shocked if its computer was bigger than this one. There's only so much software complexity you can fit in something like that.] Andy -- Andrew J. Ross NextBus Information Systems Senior Software Engineer Emeryville, CA [EMAIL PROTECTED] http://www.nextbus.com "Men go crazy in conflagrations. They only get better one by one." - Sting (misquoted) _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
