Sperry vertical gyro for Boeing 747

The next time you're on a plane, here's something to take your mind off the unhappy baby in row 34 or the food cart that just ran over your foot--gyros! Used for inertial navigation and basic input to aircraft instrumentation of all sorts, the gyros are a critical aspect to flight and flight automation but as with all forms of technology, things move on. As such, we have for teardown here a Sperry Flight Systems Vertical Gyro surplused from a Boeing 747 aircraft.

This box was probably manufactured somewhere around the original rollout of the Boeing 747, circa 1970, a date supported by the 1975 retest tag marking on the gyro. While size and weight are always considerations in flight, the Sperry box is no lightweight, with a cubic case design measuring about 25 cm on a side and a weight in the neighborhood of 10 kilograms (roughly 20 pounds). While case and internal components make liberal use of cast aluminum, the collective effect is still one of a pretty chunky piece of avionics.

Click on image to enlarge.

Fundamentally, the Vertical Gyro is used in aircraft to measure both bank angle (roll) and attitude (pitch). The instrument name comes from the fact that at the center of the design is a spinning mass whose spin axis is aligned in the vertical direction. Beyond this, I'll take a crack at explaining how the Sperry box works with apologies in advance for the near-certain errors in references and descriptions of exactly what's what and what each piece is doing. I'm confident that there are some highly skilled folks in the audience to set me straight if and when I wander into the weeds.

Overall the instrument is internally divided into an upper bay that houses the electro-mechanical apparatus of the gyro proper while the lower half encloses all system electronics, the two separated by the floor of the gyro tub. A single DB-25 connector joins the two halves, and a single external Cannon connector provides interface to the aircraft.

As you might expect, two degrees of freedom equates to two axes of motion and a gimbal (pivoted support) is seen for each of the direction changes to be monitored (pitch and roll). Both of these gimbals reside in a meaty frame that in turn is mounted to the outer case with rubber suspension points, presumably to reduce the impact of small-scale box vibrations on gyro output. Given that the instrument is hard-mounted somewhere in the aircraft, the buzz and vibes from all the other parts of the plane carriage must be kept at bay.

Housed in the inner gimbaled structure is the spinning mass--or flywheel --quite literally the heart of the gyro. An armature is used to spin up the flywheel--and keep it spinning--to begin the gyroscopic process. The flywheel device is a surprisingly heavy but truly gorgeous piece of turned metal, chock full of various-sized drill divots to assist in achieving perfect balance after shaping. I've no idea what the spin speed of the flywheel might be but it's probably way up there (pilots know of the characteristic whine of aircraft gyros) so even a minor out-of-balance condition is intolerable.

In the most fundamental terms, the larger outer gimbal senses roll while the inner gimbal--which houses the flywheel assembly--senses pitch. As many may have learned from the "spinning bike wheel" experiment, the flywheel wants to keep it's orientation and while the aircraft and hard-mount gyro box go through pitch and roll changes, the flywheel wants to stay right where it is.