September 20, 2019, ©. Leeham News: In our series about classical flight controls (“fly by steel wire”) and Fly-By-Wire (FBW or “fly by electrical wire”) we discussed the FBW flight control system of Embraer’s E-Jet E2 series last week.
We have now covered examples of classical flight controls and their modern FBW counterparts. Now we discuss how these handle different stability augmentation needs like Yaw damping, Mach tuck protection or Pitch control improvements like the Boeing 737 MAX MCAS system.
We have described how airliners which cover a large speed and altitude envelope need assistance from a flight control system to provide adequate stability in all corners of the flight envelope in our Stability Corner series.
To avoid the uncomfortable yaw/roll phenomenon Dutch Roll all airliners implement a Yaw damper using the rudder to stop the airliner from wagging its tail while flying.
Further, at high speed, the aerodynamic center moves back due to transonic effects on the wing and we need Mach tuck protection. It’s implemented as an automatic nose up trim using the movable horizontal tail.
T-tail aircraft has a problem where the wing shadows the high placed horizontal tailplane at high Angles of Attack (AoA). Therefore, T-tail airliners/business jets implement a stall avoidance system called a Stick pusher, pushing the nose of the aircraft down if the pilot ignores stall warning and increases the AoA into the stall region.
Key to the Yaw damper and Mach tuck protection implementations for flight control systems are their control authority. For a Yaw damper, it’s typically 5% of the deflection authority of the aircraft’s rudder. It’s enough to stop the tail wagging which triggers Dutch Roll and is limited enough to not endanger the aircraft if there is a “hard-over” fault causing the damper to deflect fully in one direction. This simplifies the requirements on the damper system and also on the infra-structure supplying the damper with electricity and hydraulics.
The same goes for a Mach tuck trim system. Its trim authority is limited so if it has a failure it can’t endanger the aircraft. The pilots can use the elevator followed by manual trim to neutralize a wrongly executed Mach tuck trim.
The Stick pusher is more tricky. By design, a stick pusher creates a distinct nose-down movement of the control Yoke to avoid entering stall Angle of Attack. The pusher must be precluded from endangering the aircraft if it activates erroneously. This can be achieved by making the servo so weak the pilots can overpower it by pulling on the Yokes or by deactivation the pusher in certain configurations like at take-off or in the landing phase when full flaps are activated with an extended gear.
For a Pitch argumentation system like the 737 MAX MCAS it gets trickier still. The system is there to make the aircraft consistent in pitch control all the way until stall. Modern airliners often have a region of reduced pitch stability before stall, Figure 1. It’s caused by the aerodynamic center of the wings moving forward caused by today’s larger engine nacelles disturbing the flow over the wings at high AoA.
Depending on the needed augmentation at different parts of the flight envelope this system needs more or less authority. For a classical flight control system, this is more challenging to implement than for a FBW system. We will discuss why in the next Corner.