July 25, 2019, ©. Leeham News: Last week’s Corner which dealt with Airbus’ issue with an updated A321neo Fly By Wire (FBW) and how it was unrelated to the issue of the Boeing 737 MAX, gives a good segue to a Corner series about the possibilities of FBW versus classical flight controls when it comes to tuning an airliner’s flight characteristics.
The two different control principles present the designer with very different challenges and possibilities.
The airliners of today fly a very wide flight envelope. The aircraft and its aerodynamics shall work in a linear way from zero altitudes and 120kts or Mach 0.18 (the speed when rotating for take-off or touching down) and to 41,000ft and 500kts or M0.88 (the typical maximum certified speed/Mach at altitude).
The forces the air exert on the aircraft is dependent on this speed range and the variation of the air’s density. At sea level the air weigh 1.2 kg/m3, at 41,000ft 0.3kg/m3.
If we hold our hand against the wind through the cockpit side window, the difference in felt pressure (the dynamic pressure, also called Q) varies from 2.3kPa/0.34PSI at takeoff to 9.5kPa/1.4PSI when cruising at the highest speed, a change of four times.
If we instead compare the takeoff dynamic pressure and the pressure at highest allowed speed when the aircraft flies at lower altitudes (the Vmo), the difference increases to six times.
It means an aerodynamic surface like the Elevator on the aircraft has an efficiency pitching the aircraft up or down which varies six times from the lowest Q to the highest. Yet, to the pilot, it shall take him about the same force to raise the nose at low speed as at high speed (really Q in both instances). As it’s six times more difficult to move an elevator at highest Q the designers of the high-speed jets were forced to isolate the Pilot and his yoke from the Elevator and give him an artificial force to work against. Moving the elevator was left to powerful hydraulic jacks, controlled from the Yoke over a system of wires, rods, and pulleys.
This takes care of the resistance in the Yoke when changing nose position, but it doesn’t solve the difference in Elevator travel required to generate an up or down movement of the nose. At low speed (a low Q situation) the elevator needs to move six times more for the same generated pitch force as at high Q situations (assuming linear aerodynamics)
The wide variation in needed elevator force and travel to move the nose of the aircraft at different speeds and altitudes must be solved by our flight control system.
When the speed envelope is modest, like for lower speed propeller aircraft, the problem can be solved with a mechanical system with aerodynamic assistance from balance tabs to keep the forces in check.
As the speed increases, we need a hydraulic system to isolate the Pilot from the forces. Our flight control system is now controlling the pressure working on the piston in hydraulic cylinders. It requires modest forces from the Pilot and his Yoke. But we haven’t solved the problem; the movement of the hydraulic piston needs to be large at low speed and minute at high speed.
Our flight control system needs to adapt its gearing Yoke-to-Elevator when Q changes. Changed gearing in a mechanical system over a wide range is difficult. It was the very wide gearing range needed for the Concorde (which had an even wider speed/altitude envelope) which took the designers to the first civil airliner Fly By Wire system.
With analog computers in the electrical control loop, the gearing of the system could be easily adapted based on Concorde’s Q. But allowing electrical circuits or computers to control powerful hydraulic controls is potentially dangerous. Any hiccup and the aircraft is in trouble.
While FBW opens up more possibilities for gearing changes (and other features) it puts very different safety requirements on our flight control system. The large authority to control the aircraft must be harnessed by a system which cannot malfunction.
More about this in the next Corner.