Longitudinal stability. A down-force stab is self stabilizing (aerodynamically) to a pitch input, an up-force stab is not.
In a big jet you can trim with fuel, and with the A/P’s helps you need minimum lift either way from the elevator (lift = induced drag, you want to minimize it). The stab’s airfoil is still cambered for force downwards.
The (in)famous GAL 56.
Inone of the Eric Brown books he describes a late 1940s glider as “the worst aircraft I have ever flown” (forget the name and my books are in a container somewhere in the Atlantic). It was an early swept wing design and, like the Draken, would pitch up into a stall. The difference being that this would typically happen close to the ground, and with no engine to help recovery. It killed just about everyone who tried to fly it, except Brown, including the factory chief test pilot.
It would be very interesting to read/watch the “real” story about the J35 from those who designed and (test)flew it. As a little boy, driving through Sweden on family holyday we sometimes saw these Drakens landing and taking off from the main road (although I guess one could argue if the runway was used as a road, or the road was used as a runway 
). Later the Viggen was designed to use considerably smaller stretches of road.
No necessarily, it could be easily stable in pitch in normal alpha ranges and unstable at high alpha.
Correct. The Draken is stable from 0-20°, unstable from 20-45° and superstable above 45°.
That is not as much a cobra maneuver as much as recovery demonstration (or probably flight testing chase videos) for the superstall that happens at high altitude and low IAS – which, if not recovered rapidly and aggressively would result in all sorts of unpleasantness (flameout, ejection, even death).
It was a “feature” known of since the development of the Saab 201 technology demonstrator.
Ted wrote:
At a guess I think this is what is happening. i.e. the CP is moving forward in front of the CG or at least much closer at high alpha.
Exactly. There is no way physics will allow such a violent pitch up (a bit beyond 90 degrees) if the configuration is stable in pitch. I don’t think this “feature” was intended, but it is a result of the wing planform. Very cool maneuver though
Ted wrote:
If your C-172 or TB20 did this at the stall we would have to update the training syllabus.
I’m quite certain the J35 didn’t do that by itself in a stall…
Airborne_Again wrote:
The J35 was not unstable in pitch. If it was it couldn’t have flown without a full-time flight control system which didn’t exist in 1955. It did have a lot of pitch authority, obviously.
No necessarily, it could be easily stable in pitch in normal alpha ranges and unstable at high alpha. A fundamental problem with any swept wing is that if the tips start to stall the CP moves forward. At a guess I think this is what is happening. i.e. the CP is moving forward in front of the CG or at least much closer at high alpha.
This aircraft I am guessing is like more modern aircraft with those long leading edge extension along the fuselage like seen on the F18 which are capable of very high alpha.
If your C-172 or TB20 did this at the stall we would have to update the training syllabus. The video is quite impressive
LeSving wrote:
Come to think of it. The Saab J35 Draken was one of the first aircraft, if not the first aircraft of all that was instable in pitch.
The J35 was not unstable in pitch. If it was it couldn’t have flown without a full-time flight control system which didn’t exist in 1955. It did have a lot of pitch authority, obviously.
Come to think of it. The Saab J35 Draken was one of the first aircraft, if not the first aircraft of all that was instable in pitch. This enables the so called cobra maneuver where the aircraft pitches up dramatically in an instant to an AOA of 90 degrees, then lowers the nose and continue on. Not to be repeated until some version of the Su-27 did it in the 1990s ? or something AFAIK.
at approximately 1:50
