October 11, 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 continue our discussion of pitch stability augmentation systems when we have a mechanical (“fly by steel wire”) pitch control system.
In last week’s Corner we went through the flight cases when a pitch augmentation is necessary if the base aircraft has a pitch moment curve like in Figure 1.
With feedback based FBW we have the perfect tools to fix problems like the reduced stability region in Figure 1. The aircraft measures its behavior and auto-corrects any misbehavior. It has flexible and powerful tools to do this and system redundancy to allow these tools to be active in the needed cases are there.
For an mechanical flight control system, it’s harder. We discussed how any gearing ratio change (giving a control surface deflection ratio changes) based on for example flight speed is difficult to achieve. But we need different deflections of control surfaces at low and high speeds as the dynamic pressure, Q, which gives us the control forces, changes.
Further, if we use the normal pitch channel which controls the elevator to get these forces, we take away elevator authority from the pilot which he might need in an emergency or when countering an augmentation system gone awry.
If we can’t use the elevator we soon land on using the horizontal stabilizer trim system for our augmentation.
One could argue we should find a base aircraft solution to the problem, like an aerodynamic fix. Such a fix would probably look like the ventral fins on the underside of the aft fuselage of a Learjet or on a Boeing 737 Wedgetail. But these fixes are there at all times, costing weight and drag and by it, fuel consumption in normal flying.
As discussed the flying in this pitch AoA domain is very improbable. An airline pilot should not have been there in his career. The installation of an aerodynamic fix is, therefore, not optimal if it degrades efficiency in normal flying. If we can devise a fix that goes active if needed but is not present when not needed it’s better.
So we soon end up with the trimming of the horizontal stabilizer as our tool. It’s a tool that is free for use in manual flight as it’s so powerful the pilot will not use all its authority to trim the aircraft in his manual flying.
We will detail what we need from the trim system in this Corner, then we discuss how to achieve these needs in the next Corner.
With the requirements outlined above, we will in the next Corner discuss how we can implement such a system, what components fulfill our needs and what trade-offs we will have to make.