|Quoting smartt1982 (Reply 3):|
I still cannot understand how moving the elevator to a more nose down position enables more up stab trim!!
I just happened to find the operation and description sections of the flight control chapters I printed out many years ago for the 767 and 744. The Q-feel operation is broadly similar for both aircraft, however, the 767 being a twin is more relevant to the discussion, and interestingly, neutral shift is mentioned as something that is only fitted to the 762. Anyway, the basic idea is the following.
Just below the cockpit floor, is a spring loaded cam wheel riding upon a heart shaped (cardioid) cam, with neutral being in the valley of the two lobes of the cardioid. This provides a base level of resistance to the control column. Column movements are mechanically transmitted to the tail section where they act upon aft quadrants, the aft quadrants in turn transmitting mechanical inputs to the elevator power control actuators.
The elevator feel and centring units are connected to the aft quadrants and modify the forces upon them, which in turn, alters the feel of the control column as experienced by the pilots. The feel and centring unit hydraulically adds resistance to the aft quadrant for increasing airspeed, and increasing nose down trim (stab leading edge up). The elevator feel and centring unit also adjusts elevator neutral position according to a direct mechanical input of the stab position.
The exact amount of feel force added is determined by the feel computer, which receives two inputs, a direct mechanical input of the stab position, and a pitot pressure (Q) input. These inputs allow the feel computer to supply a regulated hydraulic feel pressure to the feel and centring units. This regulated hydraulic feel pressure is also fed to the stab trim control modules.
The Q input is applied to bellows which expand with increasing airspeed. This acts upon hydraulic valving in the feel computer such that regulated hydraulic feel pressure rises rapidly with airspeed. This regulated hydraulic feel pressure is fed to the feel and centring units, the end result of course, being added control column resistance as experienced by the pilots.
The stab position input acts upon a spring loaded, movable stop in the feel computer. Once the bellows have expanded sufficiently to contact the spring loaded stop, further increases in bellows expansion meets added resistance. The end result is a reduction in the rate of increase of metered hydraulic feel pressure.
The 767-200 feel and centring unit adjusts the neutral position of the elevator system according to a direct mechanical position input from the stabiliser. For stab positions of +2 to -6.4 degrees, there is no effect on neutral elevator position. From -6.4 to -12.5 degrees stab position, the neutral position of the elevators moves from streamlined to 2 degrees trailing edge up. This is done in order to increase the effective nose up trim of the aircraft as a whole.
There is no mention of Mach tuck compensation in the 767MM - either that or I didn’t print it off - but I’d say that the 737 elevator / stabiliser system is broadly similar. Anyway, according to the 737 classic FCOM available from smartcockpit;
“Mach trim is automatically accomplished above Mach 0.615 by adjusting the elevators with respect to the stabiliser as speed increases. The flight control computers use Mach information from the flight data computers to calculate a Mach trim actuator position. The Mach trim actuator repositions the elevator feel and centring unit which adjusts the control column neutral position
Mach tuck causes nose down pitch, thus I assume elevator trailing edge up is used initially to counteract this, followed up by stabiliser movement in order to return the elevators to the streamlined position. Obviously, the Mach number during take-off is too low for any of this to happen.
Anyway, assuming the 737 elevator / stab control and operation system is similar to the 767, it appears the whole point of FCNSE is to get the stabiliser out of the neutral shift region, which theoretically allows the pilots to use the full elevator movement range from streamlined to trailing edge up, which is obviously beneficial for engine out situations on take-off.
The thing I don’t understand is they need to forgo some of the elevator trailing edge up movement range in order to allow the stabiliser to move out of the neutral shift range in the first place!
Time for a 737 driver / engineer to chime in!