Jan. 25, 2019, ©. Leeham News: We have covered the pitch stability for an airliner including some well know problems around deep stall.
We now continue with other frequent problems as the aircraft covers a large envelope in speed and altitude.
As a subsonic airliner flies at or faster than its cruise Mach (0.785 for a 737ng/MAX) the transonic effects on the wing affects pitch stability, Figure 2. The increased rearward movement of the supersonic area on the wing’s top with increased speed will move the area of low air pressure further back on the wing.
This will move the center of pressure of the wing further back, causing a nose down pitch moment. The changed flow over the main wing can also affect the flow field for the horizontal stabilizer. The horizontal stabilizer is in a downwash flow field caused by the main wing. This increases the Angle of Attack (AoA) for the stabilator as its flying as an upside-down wing to generate a stabilizing pitch up moment.
The downwash over the tail often decreases with increased Mach. The local negative lift AoA then decreases on the tail, causing the pitch up moment from the horizontal stabilizer to decrease. This can worsen the pitch down further from an increased Mach.
The phenomena is called Mach tuck and it’s dangerous as a pitch down of the nose increases the Mach further if not controlled. Early high-speed aircraft had big problems with Mach tuck and it was a major reason for the introduction of the pitch trim by an all moving stabilator.
Aircraft with fixed stabilator and elevator-only pitch up authority had problems countering Mach tuck (the elevator run out of pitch-up authority). The stabilator was therefore made movable in the forward end and the whole horizontal stabilator could be trimmed nose up to counter Mach tuck, leaving the elevator free to accepts the pilot’s inputs for fine control in pitch.
Airline OEMs introduced automatic Mach trim to counter the Mach tuck when the speed increased with jet airliners. The Mach tuck countering trim on the 737 is called Mach trim and it starts trimming nose up from Mach 0.615. The trimming is automatic in the background both when flying manually and with engaged Autopilot.
The sensitivity of the aircraft to pitch control through movements of the elevator changes with speed and altitude. At low speed the sensitivity is low and at high-speed it’s high. The sensitivity also changes with altitude, where high-speed at low altitude makes the aircraft very sensitive to elevator movements.
Low altitude flight at high speed (=high dynamic pressure) is the area for Pilot-Induced-Oscillation, PIO, a dangerous coupling of the aircraft’s pitch movements through the Pilot via the control column to the elevator. At the high dynamic pressure situation, the pitch control channel is too sensitive and the aircraft can pitch oscillate in a divergent mode until it breaks.
An airliner designer has to protect the aircraft from over control by the pilot at high speed while giving him adequate pitch authority at low speed. This requires a variable gearing of the elevator movement versus stick/yoke force. At low speed, the elevator shall have a large movement from a control column input, at high speed a small movement.
This is difficult to achieve with mechanical controls as variable gearing gearboxes can be unreliable. In-service solutions use cam curves or varying diameter cable pulleys to obtain some variable gearing. The 737 uses a varyiable pitch force system called the Elevator feel system to stop Pilots overcontrolling at high speed. The force of a roller against a cam is regulated by aircraft speed and altitude, making the controls forces heavier at high speed.
The need for a variable flight control gain with speed and altitude was the primary reason civil airliners changed to Fly-By-Wire flight controls. As civil airliners are not allowed to be unstable in any axis the reason for using the first airliner Fly-By-Wire flight control system on the Concorde was the need to vary the flight control gain as the speed went from 160kts airfield speeds to Mach 2.0 cruise speed and altitude went from 0ft to 55,000ft.
There are further control problems we need to handle at low speed. The 737ng has a low-speed control system called Speed trim which we will talk about in the next Corner.