> On Wed, 2003-12-24 at 05:51, David Megginson wrote: > > Cameron Moore wrote: > > All other things being equal, a plane that flies twice as fast > > (say, because of heavy wing-loading) > > needs twice as much time and four times as much space > > to make a change in its flight path -- that's why a little Cessna or Piper > > can start its landing flare over the runway itself, while a transport jet > > has to start flaring at least a half mile back (pulling up the nose at the > > last moment would only change the attitude in which the jet smashed into the > > runway). > > Airliners aren't that sluggish ... the flare is initiated below 50 ft > AGL and that is definitely over the runway.
I agree with David. Visual attitude is mostly unusable for such a discussion because it is dependent on approach angle, angle of attack and changes in the wing chord. It is unsafe to fly final in a light aircraft with a 3 degree slope because the best glide angle is 7 degrees, steepened by any local wind. A failure of the single engine would result in an off-airport crash. In contrast, transport aircraft are multiengine are can maintain the 3 degree approach with no trouble following an engine failure. Light aircraft landing visually therefore tend to be very nose down. Large aircraft tend to have leading edge slats, unlike light aircraft, so that the wing chord mostly lowers and lengthens for approach config. Light aircraft only have flaps and the transition from takeoff flaps to approach flaps is mostly adding drag, a small amount of length and a lot of angle change on the chord. A light aircraft flying in gusty conditions, with reduced flaps, will have less change in the chord and the visual attitude is _much_ more nose up. In fact, for a no-flap landing, a big part of the challenge is being able to see where you're going so you have to use a faster short-final speed and the like. Cessnas C172/C152 and similar aircraft are very draggy, normalized to their momentum, so a speed change can occur over a short distance. A low drag aircraft, of any size, takes a lot longer to slow down. For example, it is hard to slow an AA5B from approach to final speed. THe need to keep turbine engine RPMs up makes this even more difficult for transport aircraft, so they start to transition from approach speed to their landing speeds when there is still a mile (300 ft agl) to go. The speed reduction causes a considerable change in pitch attitude. The touch down zone for instrument approaches is 1000 ft down the runway, with the threshold crossing height therefore being 50 ft. There are six seconds of flight time from the threshold to the aim point, and additional seconds from there to actual touchdown. That is plenty of time to do the roundout (remove vertical speed) and the flare (remove excess forward speed) before touching down. Hope that helps ... NB. Our flight models should be good enough to represent all those factors correctly, so you could verify all that description in the sim. _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
