Saturday, February 1, 2014
The Evolution of Control
If one could agree that you look to the displays in the cockpit to see the heartbeat of your aircraft, then the flight control system is surely where you’d look to find its temperament. The flight control system is only one element of the aircraft control loop, but without a doubt it’s the element most important to the pilot. It is through the flight controls that the pilot will work to accomplish a flying task, and so a pilot’s opinion of how nicely an aircraft flies will depend largely on the characteristics of its flight control system. Helicopter flight control systems can be divided into two general categories: “reversible” and “irreversible.” A reversible control system is one that is directly connected to the control surfaces. In other words, if you were to move the control surfaces by hand, you would see the corresponding cockpit control move in unison. Reversible systems may incorporate some mechanical advantage or balance system to reduce the aerodynamic forces felt, but do not isolate them from the cockpit controls.
Reversible control systems were the only method of control on the earliest models of helicopters, and continue to be in use in some of the smaller, lighter models today. This is due in large part to their use of rotor blades with symmetrical airfoils. Since the center of pressure (i.e., lift) essentially stays in the same place along the blade chord of a symmetrical airfoil as angle of attack changes, the system can be designed so this point is on or just behind the feathering axis, minimizing blade pitching moments and keeping aerodynamic feedback forces manageable.
As the capabilities of the helicopter expanded, so did the requirement for more efficient blades. The non-symmetrical airfoil has since become the standard, necessary to achieve today’s design goals. But the center of pressure on a non-symmetrical airfoil moves back and forth on the blade chord behind the feathering axis as the angle of attack changes. This creates large changes in blade pitching moment as the blades rotate. This nose-down moment is transmitted to the hub, pitch change rods, and swashplate, and is the force that pushes back at you, making it impossible to manipulate the controls of larger helicopters without help. An irreversible control system incorporates hydraulic or electro-mechanical servos, which provide control forces strong enough to overcome the aerodynamics of non-symmetrical blades. In this type of system, moving a control surface by hand would not produce an equivalent movement of cockpit control, since the controls are not connected directly to it, but to a valve (hydraulic) or a transducer (electro-mechanical).
It doesn’t stop there. Stick forces may have to be adjusted to more closely match the size or weight of the machine. Too heavy, and the machine can feel sluggish or have difficulty performing “high gain” tasks. Too light, and it can be “twitchy” or easy to over-control. Stick breakout force and centering qualities may have to be adjusted to give a good sense of the trim position, without being too high that a “notch” is felt as the stick is moved through trim during a roll reversal. The beep trim rate and/or lag may have to be adjusted to provide adequate fine control and cues for an IFR-certified helicopter.
Beyond these few examples lives the world of the automatic flight control system or AFCS, which generally can enhance the pilot’s experience, lower workload, and add autopilot capabilities. Since any system is susceptible to failure, provisions must be in place for such a possibility. Some hydraulic control systems can essentially become a mechanical (i.e., reversible) system with the loss of hydraulic pressure, and be hand-flown, albeit with unbalanced yet manageable aerodynamic forces to deal with. Other systems likely incorporate a second system for redundancy to allow continued aircraft control in the event of a control system failure. Hydraulic or traditional electro-mechanical helicopter flight control systems are now giving way to full digital fly-by-wire systems, in which there exist no mechanical linkages between the flight controls in the cockpit and the control surfaces. These multiple-redundant systems promise weight savings and greater options through software. Of course they also come with their own host of new problems to solve, and new ones to be discovered. But that is the beauty of pushing the bounds of aviation – it forces the evolution of not only the helicopter, but of us as well.