The Link Trainer, Including Crab

When @She posted Dam Busters +80, I was reminded of a visit to the Bomber Command Museum near Calgary, Canada. I was impressed by their collection of aircraft, including the Lancaster bomber. They had a number of other interesting things, but honestly, the thing that made the most lasting impression on me was the Link Trainer.

Yeah, most of us have seen these little toy-like things, approximately airplane-shaped with stubby pretend wings and a tiny cockpit for a crew of one. They were perched on an articulated mount in order to move in response to the manipulation of the controls. They were early flight simulators used to train aircrew how to navigate using only their instruments.

My interest started with stories told by my Uncle Al. He had joined the reserves before WWII and was on an army rifle team, shooting at distances up to 1,000 yards. When the war started, he expected to be made a sniper and decided to find another job instead. He wrangled a position as a Link Trainer instructor and was posted to La Junta, Colorado.

I never knew any details about Link Trainers. It turns out the inventor, Edwin Link, used his knowledge of pumps, valves and bellows used to build organs at his father's factory.

Per Wikipedia:

The base contains several complicated sets of air-driven bellows to create movement, a vacuum pump that both drives the bellows and provides input to a number of aircraft instruments, a device known as a Telegon oscillator that supplies an 85 VAC 800 Hz sinusoidal reference signal to the remaining pilot and instructor instruments, and a wind drift analog computer.

The AN-T-18 (Army Navy-Trainer) has three main sets of bellows. One set of four bellows (fore and aft and both sides of the cockpit) controls movement about the pitch and roll axes. A very complicated set of bellows at the front of the fuselage controls movement about the yaw axis. This Turning Motor is a complex set of 10 bellows, two crank shafts and various gears and pulleys derived from early player piano motors [!]. The Turning Motor can rotate the entire fuselage through 360-degree circles at variable rates of speed. A set of electrical slip ring contacts in the lower base compartment supplies electrical continuity between the fixed base and the movable fuselage.

The third set of bellows simulates vibration, such as stall buffet. Both the trainer and the instructor's station are powered from standard 110VAC/240VAC power outlets via a transformer, with the bulk of internal wiring being low voltage. Simulator logic is all analog and is based around vacuum tubes.

But another interesting item is the external instructor’s desk, because that is where everything is monitored and controlled. Those are rarely preserved, but the Bomber Command Museum has a complete one, in working order.

Wikipedia again:

The second major component is an external instructor's desk, which consists of a large map table; a duplicate display of the pilot's main flight instruments; and the Automatic Recorder, a motorized ink marker also known as "the crab". The crab is driven by the Wind Drift computer and moves across the glass surface of the map table, plotting the pilot's track. The desk includes circuits for the pilot and instructor to communicate with each other via headphones and microphones, and controls for the instructor to alter wind direction and speed.

The instructor can see the air speed, altitude, “landing path” (glide slope?), and vertical speed.

The instructor can vary wind speed and direction, unknown to the trainee.

The coolest thing to me is the crab. The combined input of both the trainee and the instructor is recorded by the crab. It drives across the map table, leaving a trail of ink on the surface. The ink shows the path of the flight and reveals whether the trainee reached his way-points or destination. It records any errors and their magnitude. In bright red!

Link Trainers were sold internationally, including to Germany, before the war. During the war Ed Link produced 10,000 units, cranking one out every 45 minutes. I was thrilled to finally see a complete Link Trainer system. I always had the impression they were kind of hokey, but I am now impressed with their technical sophistication.

The Link Trainer instructors weren’t required to be pilots or even officers. Uncle Al was a junior enlisted man and had never flown. At some point his superiors were persuaded that Link Trainer instructors should know how to actually fly, and so he received flight training. He spent the entire war stateside, and as he repeated to me a number of times, there were “no promotions this side of the oceans.” He ended the war trained to fly multi-engine aircraft, but never made NCO.