Sunday, March 13, 2011

A Zebra is Its Own Opposite

Kind of funny how "sucks" and "blows" ought to be opposites, but for fellatio, undesirable situations and even airplane systems, they are pretty much the same. The pneumatic system on the Screaming Whippet produces unregulated bleed air ranging up to around 175 p.s.i., regulated 31 p.s.i. bleed air, regulated 18 p.s.i. bleed air and about 4.4 to 5.8 inches Hg suction, all by pulling hot compressed air out of the second stage compressor diffuser.

The way I look at it, the pneumatic system consists of two parts, the part that can be turned on and off with the bleed air switches, and the part that is supplied with bleed air whenever either engine is running. Bleed air tees into these two parts right after leaving each engine. The main bleed air supply to the air cycler and the pressurization system passes though left and right electrically-operated cockpit controllable bleed air valves. Even if those valves are selected open, they are spring-closed when the engine is not running, so open in response to bleed air pressure. The bleed air valves receive DC power from the non-essential bus.

The remainder of the bleed air goes through a smaller line and through check valves to supply everything else with bleed air all the time an engine is running. The two lines combine after the check valves and then divide again, with some supplying a 31 p.s.i regulator to pressurize the hydraulic system, the only system that uses air regulated at that pressure. The rest goes through an 18 p.s.i. regulator. Here there is a connection to the cockpit deice pressure gauge and a 21 p.s.i. pressure relief valve. Some 18 p.s.i. air is drawn off to the pneumatic door seal, some to an air ejector pump, to create suction, and some goes to the distributor valve. All these things are clever, and I will talk about them.

The door seal is clever because when you turn the handle to close the door, not only are you poking bayonets in to align the door, and engaging catches with door frame microswitches that report on whether or not they are properly latched, but you are opening a valve connected to the 18 p.s.i. bleed air. Once the engines are started and bleed air is available, that bleed air seals the door, so you can't turn the latch handle until the engines are shut down. Child safety locks, except that there isn't a button to press on the door frame to disable the feature. One problem is that sometimes moisture in the air can freeze that valve in the open position, so that if the engines are started with the door open (because one crew member is still outside the aircraft to disconnect the GPU) then seal inflates and it becomes impossible to turn the handle to the closed position. Advice we were given for this situation was to either shut down, then start both engines on batteries with everyone inside, or to rapidly operate the deicing boots in an attempt to lower the bleed air pressure enough to wrestle the door shut.

The ejector pump is clever just because it's an ejector pump, I mean how cool is that to create suction from blowing? Ejector (or jet or jet transfer, or injector--same thing) pumps are interesting, and fairly common in aviation, but not necessarily part of everyone's experience. It's a little bit counterintuitive, but if you're willing to accept that the force of something hurtling through a tube creates suction at the junction with other side tubes, then you can be happy like me. The intimate relationship between a thing and its opposite always fascinates me. Refrigerators, prisms, trees growing out of dead logs and ejector pumps rock my world. The air that runs the ejector is dumped overboard, having done its job. The suction line has connections to the pressurization controller, the pneumatic relay and the dump valve, plus there's the expected vacuum regulator, which has a filter, suction indicator and a suction warning switch for the low suction annunciator. I always think that should be high suction. Negative numbers are fun. They are the opposite of positive numbers, but except for a little line, they are the same. Also, I looooove zebras. Black and white both at once and shaped like a horse. There are no zebras in the suction system, but there is a connection to the distribution valve mentioned two paragraphs ago.

The distribution valve is like a four-way intersection with a traffic light (but sadly no zebra crossings). The four streets that meet there are 18 p.s.i. regulated bleed air, suction, wing boots and tail boots. Depending on how the valve is positioned, suction or pressure goes to the wing and/or tail boots. When the deicing is off, suction goes to both. During the auto cycle suction goes to one and pressure to the other, then reverse, then back to both suction. And on the manual deice setting, pressure goes to all boots.

I tried to make this make sense of the earlier story about the inlet anti-ice getting bleed air from two sources, but none of the pneumatic diagrams in my manuals show it. I think it is unregulated bleed air taken before either the bleed air shutoff valve or the 18 psi regulator.

The groundschool went well. Management pilots did a little them-on-one with each of us, answering questions. They intelligently identified the weaknesses in my experience, and I acknowledged them and asserted how I would overcome them. Or at least that's how I remember it. Maybe I just drooled and picked my nose. They scheduled training for the first two, and will call next week when they are through and it's our turn.

5 comments:

Rhonda said...

Ejector/jet pumps are also sometimes called eductors. They can be anything from a simple tee to a venturi with the side port at the narrowest point to a fancy "non-clogging" design (useful for solids that would clog a basic tee or venturi).

If you remember high school chemistry, you may remember the faucets that had a port out the side which splattered water everywhere when you were trying to wash up. If you hook a hose up to that side port and turned the faucet on full, you had a cheap and easy, if slow, vacuum source (with occasional water backsplatter) without needing to buy and maintain extra equipment. That was a simple tee shaped eductor, powered by water.

Anoynmous said...

Distribution valves are fun to think about. The manually-operated ones in swimming pool filter systems have a component called a "spider gasket".

SkinnyDennis said...

Ejectors/jet pumps/eductors function due to the Venturi effect.
http://en.wikipedia.org/wiki/Venturi_effect

Which is related to Bernoulli's Principle. I heard somewhere this has something to do with the ability of aircraft to fly. ;^)

Jimmy said...

These posts illustrate why I could never move up from flying a humble Navajo... I can barely understand the systems of anything larger lol.

Thanks for taking the time to write all this out for us 'Trix. Long days are good for something I suppose...

Web said...

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