The test mount had taken Silas the better part of the previous evening to build, working alone after June had left for the hospital and the Tinkery had settled into its nighttime quiet. It was not an elegant piece of work but it was precise where precision mattered. A steel baseplate bolted to a section of heavy workbench, with a vertical riser machined from solid bar stock and a horizontal arm extending from its top that held the cyclorotor in exactly the attitude it would occupy on the finished Skiffer: shaft horizontal, rotor axis parallel to the floor, the whole assembly oriented as though gravity were operating normally and the craft were hovering six feet above the test bench.
The accelerometer was his own design, adapted from a component he'd originally built for a gyroscopic stabilization experiment in 1891 that had worked correctly and been commercially irrelevant. It consisted of a small brass pendulum suspended in a damped oil bath, with four contact points arranged at the compass positions inside the housing. Any wobble or runout in the motor shaft during operation would induce a corresponding movement in the pendulum, which would register on the indicator dial mounted to the side of the riser. Under ideal conditions the needle would sit dead center. Under acceptable conditions it would move within the inner ring. Outside the inner ring meant a problem with the bearing, the shaft, or the rotor balance, and the motor in question did not go on the Skiffer.
The flags were June's contribution. She had arrived that morning with a length of light cotton strips cut into twelve equal strips, and had tied one to a thin wire stake at each of the clock positions around the test mount - twelve short poles driven into a ring of cork board laid around the motor, each flag hanging limp at rest and responsive to even modest airflow. The arrangement gave them an immediate visual read on thrust direction without requiring either of them to put a hand anywhere near a spinning rotor.
The dial she held was a repurposed compass card mounted to a flat piece of board, the compass rose replaced with her own handwritten indicators. Twelve positions. A pointer she could rotate with her thumb. It was connected by a length of insulated wire to the small servo motor that drove the cyclorotor's blade pitch linkage - a simple but effective arrangement that let her command thrust direction from a safe distance while Silas managed the main motor throttle.
They had the first cyclorotor mounted and wired by mid-morning.
~*~
Silas brought the motor up slowly, watching the ammeter clipped to the power lead and listening to the pitch of the rotor. He'd spent sixty years learning what healthy machinery sounded like and what it didn't. The cyclorotor was a more complex sound source than a simple propeller - the blade articulation added its own frequency to the mechanical signature, a periodic modulation that rode on top of the motor's fundamental tone. He listened for anything irregular in that modulation: a catch, a hesitation, a variation in the period that would suggest a linkage binding at a specific point in the rotation cycle.
The motor reached operational RPM and held. The sound was clean.
Around the mount, eleven of the twelve flags hung slack. The flag at six o'clock - directly below the rotor - was snapping horizontally on its wire stake with considerable enthusiasm, the cotton strip fully extended in the thrust airflow.
"Thrust confirmed downward," June said, her eyes on the flags. She was standing at the nine o'clock position, clear of the rotor arc, the direction dial in both hands. "Accelerometer?"
Silas checked the indicator. The needle was inside the inner ring, slightly biased toward the two o'clock position - within tolerance for a component they hadn't been able to replace with new stock, and better than he'd privately expected. "Acceptable," he said. "Shaft is clean. Proceed with direction check."
June rotated the thumb control on the dial. The servo motor on the linkage assembly made a small, purposeful sound, and across the rotor the blade pitch began to shift - each blade feathering through its articulation point at a slightly different angle, the eccentric cam profiles doing the work that three notebooks of calculations had specified. The transition took about four seconds. The six o'clock flag slowed, lost its urgency, dropped to half extension. At three o'clock, the flag began to stir, then extend, then snap with the same confidence that six o'clock had shown a moment before.
"Thrust rotating," June said. "Three o'clock. Flags confirm."
She continued the rotation. Two o'clock. The linkage reached its mechanical limit and held - the blade pitch at maximum positive displacement, the thrust vector angled upward and to the right at approximately thirty degrees above horizontal. Silas watched the flags at one and two o'clock and noted that the distribution was clean, no significant spill to adjacent positions, the thrust cone tight and well-defined.
"Upper limit," June confirmed. "Two o'clock, linkage at stop." She paused, examining the flags. "Clean."
She reversed the dial. The thrust walked back through three, through six, continued to nine - the opposite side, the airflow now pushing horizontally to the left - and on to ten o'clock, the lower rear limit, before she brought it back to six and held it there.
"Full range," she said. "No binding, no hesitation at any position." She looked up from the flags and across the mount at Silas. "So far so good."
Silas nodded. He had been watching the accelerometer throughout the direction sweep and the needle had remained inside the inner ring at all positions - the shaft loading changed as the thrust vector rotated, and a worn bearing would have shown it. This bearing was not worn. Or if it was, it was not worn enough to matter.
He reached for the throttle control.
"What are you doing?" June's voice was immediate and sharp.
"Margin test," he said, without changing his tone or his pace. His hand was already on the rheostat. "We've confirmed operational function at rated RPM. Now we test to ten percent above."
"Silas - "
"If it fails here," he said, "we know before we bolt it to a vehicle that someone intends to fly." He began advancing the throttle. The motor's pitch climbed, a tighter, harder sound than the operational note. The six o'clock flag pressed further against its stake. "If it fails at three hundred feet, we know ahead of time at a significantly less convenient moment."
"I know the argument." June had moved two steps closer to the mount, though she stopped well short of the rotor arc. Her voice had taken on a controlled tension - words chosen carefully because the words she wanted to use were louder than the situation required. "I sized those motors to the application, Silas. The spec is correct. Running them over spec damages the windings."
"Running them to ten percent over rated load for sixty seconds does not damage windings that are in good condition." He watched the tachometer. The needle climbed past the red line he'd marked at operational RPM, continued to the second mark he'd added in grease pencil at ten percent beyond. He held it there.
"Then reduce them."
"Not yet."
The motor held the elevated RPM. The flags around the mount were under more pressure now - the six o'clock flag fully horizontal, pressed flat against the airflow, the adjacent flags at five and seven beginning to show sympathetic movement from the expanded thrust cone. The accelerometer needle had moved slightly further from center under the increased load, but remained inside the inner ring.
June stood with her arms at her sides and her jaw set, watching the second hand on the workshop clock. Silas could see her counting. The Tinkery had been quiet all morning except for the sound of tools and the occasional conversation; now the only sound was the cyclorotor at elevated speed and the specific silence of two people not saying what they were thinking.
At sixty seconds, June said: "Are you satisfied?"
"Yes," Silas said, and brought the throttle down.
The motor decelerated through its speed range and stopped. The flags went slack. The Tinkery was quiet.
~*~
They dismounted the first cyclorotor together, unbolted the retaining collar, disconnected the power leads and the servo control wire, and set it on the workbench. Silas marked it with a small piece of tape on which he wrote 1 - PASS in pencil. June lifted the second cyclorotor into the mount and began the bolt sequence while Silas reconnected the power leads.
The second motor came up cleanly. The direction sweep was precise - if anything, slightly better than the first, the linkage response crisper, the thrust vector transition smoother. June ran the full range twice without being asked, her expression professionally neutral. The accelerometer was well inside the inner ring at all positions.
Silas reached for the throttle.
"Stop," June said.
He advanced it anyway.
"Silas." Her voice had gone past controlled tension. "I'm telling you to stop.You are going to wreck one of my motors, and if that happens the Skiffer does not fly. Do you not understand that? There is no contingency. I have no money. You have no money. If one of those motors shorts out because you insisted on being… on being… "
"Cautious?" he offered.
"I was going to say scared." She was facing him now. "The judges at the Exposition are not going to care about your margin tests. They're going to care whether the Skiffer flies. And we cannot demonstrate that it flies if you have burned up a motor on a test stand because you are too afraid to trust the engineering."
The motor held at ten percent over rated load. The accelerometer needle was steady.
Silas answered, "The judges at the Exposition will have engineers among them. Those engineers will ask about safety margins. They will ask what we tested and how we tested it, and if the answer is that we tested to rated spec and no further, they will draw their own conclusions about how the vehicle will perform in conditions that are not optimal." He watched the clock. "Thirty seconds."
"Other contestants probably won't even have gotten to this stage before they demonstrate."
"That may be. But other contestants have not asked us to stake their reputation on their work, and we have standards here that exist regardless of what other contestants do."
"Standards." She scoffed the word. "Standards that exist in the head of a man who hasn't completed a project since before I was born! Someone whose biggest idea is a complete-"
She stopped.
Silas said nothing. He watched the clock. Forty-five seconds.
The word that had been on its way out of her mouth had turned back, but they both knew what it was. The Tinkery's silence filled in around it.
"Not done well," she finished, and the substitution did not make it better.
Silas brought the throttle down. The motor decelerated and stopped. He did not look at her immediately. He looked at the accelerometer needle returning to center, at the flags going slack one by one around the mount, and worked hard to bring his own temper under control.
He had been ready to say something cutting.
The observation that she had come to him, after all, with a crashed vehicle and a three-week deadline; the reminder that the Skiffer's first attempt at flight had ended in a field somewhere with twisted metal and torn dreams. That she needed him.
He was aware that these things were true and that saying them would accomplish nothing useful and would hurt her, and that the hurt would translate into more anger, and that the anger would go out the door with her when she left and she would do something that confirmed all of his fears about what happened when she let her impatience get ahead of her preparation.
He had watched this sequence once before. He did not want to watch it again, but this time see the end result in a newspaper headline.
He let the breath out slowly through his nose.
"If you die," he said, "I will do what I can for your family. I will do everything I am capable of. But I am seventy-eight years old with a bad leg and a building full of things nobody wants, and what I am capable of is considerably less than what you are capable of or will be. So." He looked at her. "We test the margins."
The workshop was very quiet.
June had been moving toward the door, metaphorically speaking - he could see the process of the old argument assembling itself in her face, the preparation for exit, for the kind of departure that ends something permanently this time. And then his words arrived and the proces stopped and something else moved through her expression that was harder to name.
She looked at the second cyclorotor in the mount. She looked at her direction dial and she looked at the floor between them. Basically at anything but him.
"The motors," she said, and her voice had changed - the heat gone out of it. "I didn't size them with the margin. I should have, I can see that now. But I'm scared, Silas. If one of them fails on the stand… I have nothing left. No money for replacements. The Skiffer would be finished. I would be finished. My family…" She looked up. "What do we do if a motor shorts out?"
He reached past her and picked up the marking tape, tore off a strip, wrote 2 - PASS and put it on the second cyclorotor.
"If that happens, " he said, "I can rewind it,"
She blinked. "You can rewind motors? Since when? "
He shrugged," I got a book on it to try my hand. It's pretty straightforward once you get the hang of it. And I acquired a winding machine a few years back. A small manufacturer in the Ironbound District was closing and sold off the equipment at liquidation prices. I thought it might be useful." He set the tape down.
"Even if we need centrifugal contacts, I have a box of those somewhere - I'll have to find it, but it's here. Don't worry about the motors, June. If one fails on the stand, the stand is where we want it to fail. We rewind it and we know what we have." He met her eyes. "We don't have money. But we have the skills and the time and the will to make money unnecessary. The Skiffer will fly. We'll make it so."
She was quiet for a moment. Some of the tension had gone out of her shoulders - not all of it, but enough.
"Just," she said, and stopped, and tried again. "Just take it easy on them. Winding a motor is all very well, but if we don't need to do it, let's not spend the time."
"Agreed," Silas said. He meant it. He had stated his position on margins and would not retreat from it, but there was no satisfaction in running a motor to destruction. "Ready for the third?"
~*~
The third cyclorotor went into the mount and behaved impeccably. Direction control test: clean. Accelerometer: well inside the inner ring. Operational RPM: steady and smooth, the blade articulation producing the clean periodic modulation that indicated properly functioning linkages. When Silas advanced the throttle to the ten percent mark, June stood with her arms crossed and her eyes on the clock, counting seconds with visible but controlled impatience.
At sixty seconds he brought it down.
She was already reaching for the tape roll.
She marked it 3 - PASS and set it with the others.
The fourth cyclorotor was the last. June lifted it into the mount with the gentle care she'd shown all four of them - they were matched components, built to the same design, and she treated each with the same attention. Silas connected the power leads. June connected the servo wire and confirmed the linkage articulation by hand, working the direction dial through its full range slowly with the motor unpowered, feeling for any resistance or irregularity in the mechanism.
"Linkage is free," she said. "Ready."
Silas brought the motor up.
The sound was wrong inside the first three seconds. It wasn't dramatically wrong - no catastrophic signature of something failing immediately, but there was a wrong in the way hummed, its resonance discordant. A slight roughness in the fundamental tone. A periodicity that did not quite match what the other three had produced.
He glanced at the accelerometer. The needle was moving this time, not wildly, but with more freedom than its predecessors, oscillating slightly rather than settling to a steady position.
Maybe it needed a new bearing? No, the sound want mechanical, it was deeper than that.
He kept the throttle where it was, watching both the tachometer and the accelerometer, waiting for the motor to settle into its operating temperature and see if the indications stabilized.
They did not stabilize.
The roughness in the tone did not increase, but it did not smooth out either. The accelerometer needle held its oscillation, a small regular movement that spoke of something slightly out of balance in the rotating assembly, a winding asymmetry, possibly, or a rotor that had not been dynamically balanced to the standard of its counterparts.
He was about to say something to June when the motor said it first.
The sound changed. A single sharp crack, simultaneous with a flash of orange light from the winding slots in the motor housing, and then a smell; copper and insulation and the acrid ozone attribute of a winding that had opened under current. The motor decelerated abruptly as the running circuit interrupted itself. A puff then a thread of smoke rose from the housing.
June hit the power disconnect before Silas had finished reaching for it. The shorting hum disappeared. The motor coasted down to silence. The flags went slack. The smoke thinned and dispersed in the air of the Tinkery.
They stood looking at the fourth cyclorotor in the mount. The motor had not reached operational RPM. The margin test had never begun. The winding had opened during the operational run, at load levels June had calculated as normal, and the motor had reported this fact in the most direct way available to it.
Around the mount, eleven flags hung limp on their wire stakes. The twelfth, at six o'clock, turned once in a slow eddy from the residual airflow and was still.
Silas looked at the motor for a measured moment. Then he looked at June, who was looking at the motor with an expression that had gone past alarm into a look that said it was both exactly as bad as anticipated and somehow more manageable than the anticipation had been.
He dusted his hands on his trousers, "Right," he said. "Back shortly."
He picked up his walking stick from where it leaned against the workbench and moved off into the interior of the Tinkery, heading for the section near the old electrical equipment displays where he was reasonably confident he had stored the winding machine, and the box of centrifugal contacts was almost certainly nearby.
Behind him, he heard June pull a stool to the bench and sit down, and then the sound of her opening her notebook and beginning to write.
Good, he thought. That was the right response to an unexpected problem. Write it down. Work out what it meant. By the time he got back with the machine, she would have the winding specification ready.
He made his way between the shelves, the walking stick marking the floor in its familiar rhythm, and did not look back.