Originally Posted by
gums
Salute!
@FDR, et al....
I would bet that the 'bus is statically stable in pitch in the direct "law". Not "speed/AoA neutral" stable In other words, take out the "protections" and have the stick command control surface movements directly. Relaxing the stick to neutral returns the surfaces to whatever "trim" posiiton existed before moving the stick. Gotta look back at the FCOM for that mode/law WRT trim, but think it was control surface position as we had in planes since Glenn Curtis, Boeing brothers and Kelly.
The Viper also appeared neutral WRT speed/AoA when close to one gee flight, but like the 'bus, that was due to the FBW control law and not basic aero characteristics, pitch moments, etc. Using our pitch override function when in a deep stall resulted in "direct", proportional movement of the stab, just like the 'bus. In short, we could tell HAL to go take a leap until AoA below 30 degrees.
Gums sends...
It is statically stable, with a mild phugoid around the speed trimmed by the driver on the THS wheely thingy, being dynamically stable. Thats in direct law. By design, the C* doesn't trim to any speed as such, so it is neutral. Boeing (and your Viper I guess) using a speed trim component, C*U are statically stable in normal mode. The Boeing without confusers is still stable, your Viper, has the single use alternate landing mechanism.
To get to be unstable statically (and dynamically too...) apparently takes a large loading error such as the positioning of an MRAP through the aft pressure bulkhead of a B744F. Once you get to the neutral point, it is all pretty interesting, but not necessarily for very long. Investigated a 744F incident that went back more than 10% aft of the envelope, it was still just forward of the neutral point but dang if those elevators didn't get a workout. On landing the aircraft 4 pointed its way onto the grass, NLG was about 6' shy of the concrete. On the takeoff the nose gear came off the ground without control input at 56KCAS... and the plane rotated off the ground by itself before V1, should have sold tickets for the airshow. The plane had a bank on all by itself which was why the tail didn't strike the ground, it was jacked up by pivoting around the wing gear. What the crew did then was a story unto itself.
I have been out the back of the envelope but also out the front, the forward one ended up having to split the flaps to be able to gain a trim for landing. 6.5 tons from the aft cargo got put in the fwd cargo, which doesn't sound like much... rotate took about an extra 3,000' of pavement which happened to be available, with full elevator and continuous ANU trim as soon as the rotate didn't work as advertised.
Off topic: Came across the photo of your Viper with the "roll enhancing feature", that is up there with the IDF's F15 clipped wing for a surprise happy ending.