Originally Posted by
MLHeliwrench
Say the horizontal stab somehow became able to float up and down the screw shaft on its own - would the elevators act as a servo surface like how some aircraft have a smaller flight control surface move opposite the desired deflection of the main surface?
In this hypothetical scenario, pulling back the yolk would cause the horizontal stabilizer to deflect leading edge up. This would cause a dive.
Can anyone attest to the capability of full yolk back (under normal travel limits) to overcome a horizontal stab trimmed all the way leading edge up?
I concur with your suggestion that a floating stabilizer would be impacted by the elevators in much that same manner as a tab-driven trailing edge control surface. Airplane nose up elevator deflection puts loads on the stabilizer in the opposite direction and thus would drive a floating stabilizer in the airplane nose down direction. For small motions, elevator has about half as much pitch authority as stabilizer on a degree for degree basis. The story is not quite that simple, however. Below are a few factors to consider:
First is the question of where the stabilizer is positioned for trim. If the stabilizer were near its airplane nose down limit when trimmed and then went to that limit there should be enough elevator to counter. If, however, the stabilizer were a long way from its airplane nose down limit when in trim, having it go all the way to its airplane nose down limit would probably generate more nose down pitching moment than the elevator could counter.
Second is the question of elevator blow-down due to limited hinge moment capability. At approach speed the elevator control actuators are able to generate enough force to drive the elevators to their travel limits. At cruise (and faster) speeds elevator travel will be hinge moment limited as the actuators cannot generate enough force to reach elevator travel limits. This can become a serious issue in a dive as the higher the airspeed gets, the less elevator deflection the system can generate to command the needed pull-up to recover.
Third is the question of elevator effectiveness with the stabilizer at its full airplane nose down limit. With the stabilizer leading edge up (i.e., airplane nose down) it may disturb flow over the elevator in such a way as to reduce elevator effectiveness. This would be particularly true if elevator is at first able to generate nose up pitching moment to generate significant wing AOA needed to pull out of a dive. In that case the AOA seen by the tail will be that of the airplane (i.e., the wing) plus the angle of the stabilizer. If this is high enough, flow over the stabilizer will begin to separate severely reducing elevator effectiveness - particularly in the airplane nose up direction.
The bottom line is that having the stabilizer significantly out of trim is a bad situation. Herein lies the crux of why control systems that require detection and arrest of uncommanded stabilizer motion have requirements to do so within just a couple of degrees of errant stabilizer motion.