Breaking the Bronco

On its first flight, the X-15 immediately ran into a control problem. The B-52 dropped the vehicle into an unpowered test, and pilot Scott Crossfield glided to a landing at Edwards. At first, all had worked smoothly for the brand-new, strange, and heavy plane. But as Crossfield brought the X-15 in for a landing, its nose pitched up sharply, catching Crossfield off guard. He pushed forward on the stick, but the vehicle overcor-rected downward. Then he pulled up again, but the nose came up too far. Crossfield later recalled, ''Now the nose was rising and falling like the bow of a skiff in a heavy sea. I could not subdue the motions.''12

Crossfield was in a scary situation that would become known as a PIO—''pilot induced oscillation.'' In effect, the human-machine combination was feeding back on itself and becoming unstable. This had nothing to do with the pilot's skill, but was inherent in the nature of the system. In fact, Crossfield's skill saved the airplane and his life, as he managed to touch down on the runway at the bottom of one of the oscillation cycles. The landing gear was crushed, but otherwise the program was saved.13 Like the Wright brothers, Crossfield used a horse analogy: ''The X-15 had an independent and contrary personality of its own. I was determined to break that bronco before it got me. I won.''14

Though easily solved, Crossfield's problem with the new plane stemmed from the design of the X-15's control systems. The ''systems view'' of flight controls that engineers had adopted during the 1950s treated the human operator like a component that could be modeled like any other device.15 Borrowing from electronics, engineers used the term ''gain'' to describe one aspect of those components. The term originally refers to an amplifier, and the gain is nothing more than the volume control—how much the amplifier amplifies, and how much output there is for a little bit of input. From a systems view, the airplane itself behaved like an amplifier: the control system amplified a little movement of the stick to a large movement of the aircraft. The pilot has a gain as well—for a given perception of the ''error,'' the pilot will make a movement to correct it, but how much? What is the pilot's ''gain''? Put another way, how sensitive should the aircraft's control system be? Should just a slight force on the stick produce a slight change in attitude, or a great excursion? These were questions that Gilruth had defined in his flying qualities research, and the right answer varied for each aircraft.

To answer this question the X-15 designers built a simulator to model the aircraft. A simple idea: put the pilot ''in the loop'' in the simulator, then adjust the gain until the aircraft becomes easy to fly, until its flying qualities are satisfactory. But there was a problem, which led to Crossfield's PIO: when flying the real aircraft to a real landing, his ''gain'' was not the same as when he flew the simulator. In fact, it was much higher. Unconsciously, the pilots behaved differently during flights than in simulation. The stress of flying, the concentration, the very real possibility of losing one's life, made the pilot a more sensitive controller in a real airplane than in a simulation, causing the system to oscillate. As with many such feedback problems this one was easily solved by changing the ''gain'' on the control system—a bit of tuning. But the episode illustrated the importance and the pitfalls of simulation as a tool in development, and the subtle ways that it interacted with the control systems and the role of the pilot.

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