September 13, 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 flight control laws of Boeing’s 777/787 and Airbus’ A220 last week.
Now we continue with Embraer’s fourth-generation FBW, the one for the E-Jet E2 series.
We discussed Embraer’s first-generation FBW for the E1 E-Jets in previous Corners. It’s an analog open-loop FBW resembling a classical flight control system but with control authority gain scheduling based on dynamic pressure and stall protection.
The flight laws for this first generation FBW was designed by an external supplier, Honeywell. For the second generation FBW for the Legacy 500 Business jet Embraer decided to take the design of the flight laws in-house. It created it’s own flight laws team including the team coding the laws for the digital FBW computers.
The resulting Legacy 500 FBW is known for its nice flight controls with full protection from stall and over-speed. The next project for the FBW team was Embraer’s new tactical transport, the KC-390. Here the capability of a full authority FBW is used to ease the tactical flying for the pilots.
The replacement for the E-Jet, the E-Jet E2 implements the fourth generation FBW for Embraer. It’s an implementation which retains Pilot commonality with the E1, yet draws a number of advantages from its closed-loop feedback based architecture.
The digital FBW implementation was used to improve the efficiency of the aircraft in areas of cruise efficiency but also take-off and landing performance. For cruise efficiency improvement, the static stability of the aircraft was reduced as the aircraft’s static and dynamic stability is at all times augmented by computer-controlled flight laws, Figure 1.
This is possible as computerized flight laws are active both in Normal and the backup Direct mode. Direct mode is invoked when the system detects malfunctions for sensors like Angle of Attack sensors, Air Data sensors, the Inertial platforms or a Flight Control computer (it has three computers, all dual-processor designs with self-checking). Despite failing sensors or computers, its redundancy is such the FBW keeps pitch damping despite being in a degraded mode.
The always-on pitch augmentation allows an efficiency improvement by moving the wing further forward for reduced static stability and by it, less tail downforce drag, Figure 2.
The reduced force needed in turn allowed a reduced horizontal tail stabilizer surface, reducing weight and wetted area drag. In total the relaxed static stability contributed a 1.5% improvement in aircraft efficiency, Figure 3.
The E2 FBW also implements wing load alleviation through simultaneous deflection of the ailerons to mitigate gust loads, by it reducing aircraft weight further.
The common Pilot type rating between the E1 and E2 E-Jets limited the team in what Pilot assistance it could implement but the E2 contains:
In summary, the E2 FBW is an important component in the improvement of the efficiency of the E2 generation of the E-Jet and it gives the Pilot additional comfort and safety over the E1 FBW.
In the next Corner, we will discuss how the different Flight Control solutions handle the flight augmentations needed for modern airliners like the 737 MAX MCAS system.