Wednesday, June 22, 2005

Spinning Things

Ezekiel saw a wheel a-spinning, way in the middle of the air.

In response to a question on engine outputs, I'll talk about gyroscopic instruments and what makes the gyroscopes spin.

The airplanes I fly use compasses that are not hugely different from the compasses that got Cabot and Cartier across the Atlantic to Canada. A magnetised component seeks to align itself with the Earth's magnetic field, indicating the orientation of the observer on a compass card. The compass is prone to large errors while accelerating or turning and and is hard to read in rough air. For this reason, the magnetic heading is transferred to an instrument which is not north seeking, but instead keeps track of where north is by remaining stable in its orientation while the airplane turns around it. The instrument is called a heading indicator. The pilot must update the heading indicator on the real heading periodically throughout the flight, but in return doesn't need to squint at a bobbing compass for every heading.

Another instrument, the attitude indicator, gives a cockpit indication of the aircraft's situation with respect to the horizon: nose up, nose down, and/or banked left or right. This is useful when the real horizon is obscured by darkness or clouds.

Both these instruments maintain stability through the operating principle of the gyroscope: a wheel inside that spins so fast that it maintains its orientation instead of turning with the airplane. It is gimballed to allow it to do that, and the motion of its turning relative to the case around it are transferred through ingenious levers to the face of the instrument. A third instrument also contains a gyroscope, but it takes advantage of the principle of gyroscopic precession--the tendency of a gyroscope to change its motion around a different axis than the one where a force is applied. This post is not about these instruments, nor about the gyroscopic principles, but merely about what keeps the gyroscopes spinning inside.

Early gyroscopic instruments were powered by a venturi tube mounted on the outside of the airplane. Some of them are still out there. The tube looks like an old fashioned car horn, flared out at the front and then narrowing in the middle. As the airplane flies (it doesn't work on the ground) air is forced by the pressure build up at the forward flared part to accelerate through the middle part, which causes a pressure drop in the middle part. A line leads from the middle of the tube to the instrument panel, so the pressure drop creates suction through the line. The suction force is applied tangentially to the gyro wheel in such a way as to keep it spinning. The rim of the gyro has little dimples in it to take advantage of the suction. It's like using a vacuum cleaner to pick up dead fish. That's not a very useful analogy but it's too hilarious not to link, and does at least document that suction can do work.

That method of spinning the gyros has certain weaknesses, like the fact that you can't test to see if the instrument is working until after you've taken off, or that the venturi tube is sitting on the outside of the airplane where the rain and snow and ice are. The airplane I plan to be certified on in a couple of months has never used that method to keep its gyros turning, but has used every other method I know of, so its history shall be my explanation.

The early serial numbers used pneumatically driven flight instruments, I believe simply directing the 18 psi regulated bleed air against the gyro wheel at an appropriate angle. That is to say, they would blow instead of suck to keep the gyros erect. They, um, stopped doing that because of moisture problems. I think I can relate.

Later they routed the bleed air through a venturi, just like the kind that used to be mounted on the outside of airplanes, and used the suction thus created to drive the gyros. They seem to have reverted to bleed air pressure for serial numbers fifty-eight to one forty-nine but those persistent moisture problems drove them to try something else. For a while the accessory gearbox drove a dry suction pump and then finally they settled on electric gyros, using 115V AC. The turn coordinators, as a backup, usually run off 28V DC, from the L and R bus bars for the L and R side of the cockpit (the pilots have one each).

My prayed-for future employers probably own aircraft from every serial number range, most of which have been refitted with avionics that the manufacturer didn't dream of, but if I'm familiar with what might be there I have a better chance of knowing what might or might not work in what kind of failure.


Yellowbird said...

Nice description, but the fish link was more than I cared to finish this close to lunchtime. I recently had Yellowbird's attitude indicator replaced. Naturally, I kept the old one so I could take it apart and see how it works. Even for a 1970's Cessna-standard instrument, it's a pretty impressive piece of engineering.

Aviatrix said...

Okay, okay, "the local absence of a balancing force for the atmospheric pressure, causes the atmospheric pressure on the side opposite the suction to PUSH the gyros around."