Vacuum versus pressure

[Jeroen Hoppenbrouwers]

On smaller aircraft, various gyro instruments are powered by vacuum (suction power). What is the main reason the design uses vacuum instead of pressure?


Jeroen

[Mundyas]

It is a good question? I am not sure I have the answer but at least I will start the ball rolling!

If you lose suction power / failure you will eventually have no Airspeed Indicator and Direction Indicator. So you will have to fly with a limited or partial panel which is OK if you have done the training or have an instrument rating.

I can only think with a suction failure / vacuum pump the instruments will take a while to run down and whist flying you might notice the zero reading and have some time to adjust. And could it be that it is just simpler having an engine driven suction  pump.

A lovely flying day here just certainly not IFR today!

[Garry Richards]

On smaller aircraft, various gyro instruments are powered by vacuum (suction power). What is the main reason the design uses vacuum instead of pressure?

Don't know whether this is relevant to aircraft instruments or not but with suction you have the pump on the output side of the device rather than on the input side. This results in smoother airflow through the device. This principle is used in pedal powered pump organs for that reason.

[Jeroen Hoppenbrouwers]

One other reason that I can come up with myself is about filters. With suction, the chance of having one filter clogged that blocks the whole vacuum system seems smaller (many input filters, one per instrument, and basically no output filter).

[Hardy Heinlin]

I think the chance of a tube adapter being disconnected is lower if the connection is being sucked rather than pressed.


|-|

[Jeroen Hoppenbrouwers]

Nice how a "logically" irrelevant difference can indeed make for several good reasons to choose one over another.

[Jeroen D]

Interesting question and I must admit I really don't know the answer. What I do know is that vacuum system are more difficult to trouble shoot then pressurized ones when it comes to leakages. On a pressurized system it's relatively easy to find the leak. On a vacuum system you usually have to resort to blocking parts of the system and see if the vacuum holds

Never the less, lots of cars use vacuum systems as well. Not so much instruments but for actuation. (e.g brake booster, but also for the auto transmission, door locks, headlight adjustment, seat adjustment etc.)  For a while in the car industry pneumatic actuators were considered to be more reliable than electric ones. And possibly lighter as well.

In fact Mercedes copied several 'aviation based systems' into some of their car in the '60 - 80's. Mercedes being Mercedes re-designed every single component, but still the ideas/concept were supposed to come from the aviation industry.

There are a few things that might be relevant for aviation instruments and vacuum. Vacuum systems need very little air treatment other than particle filtering. With a pressurized system you tend to have additional filter for moisture/condensation and in many cases lub-oil. Could it be that vacuum is relatively simple, light and cheap compared to pressurized systems?

Jeroen

[Zinger]

1. An airflow pressure pickup for driving gyroscopic instruments, is likely to produce more drag than a suction pickup.
2. An airflow pressure pickup collects particles present in the airflow with the air, whereas the suction collector is positioned 90 degrees to the flow, and due to particle inertia is less likely to collect such particles during flight.

[Hardy Heinlin]

An electrical analogy:

In conventional devices and buildings an electric circuit usually has the following sequence:

Power source
Switch
Consumer
Ground

This is, for example, practical when replacing broken lamps, so that there is no dangerous power at the open lamp connectors when the switch is off.

On the Boeings (and probably other aircraft) the sequence goes like this:

Power source
Consumer
Switch
Ground

E.g. the indicator lights are always on the "hot" end, regardless of the switch status.


|-|ardy

[Zinger]

And usually 5 VAC nowadays. Every circuit is protected by a circuit breaker BTW.

[Jeroen Hoppenbrouwers]

The Boeing system seems to try to have power distribution switches including circuit breakers at the power side of the consumers, and logic/operating switches at the ground side. Given that many indicators are connected to a bright/dim/test box, lots of stuff gets very complex very quickly. Keeping power (both levels) on one side and on/off logic on the other may help here.

[John H Watson]

There are actually quite a few variations on Boeings.

Most high-powered stuff will be:

Power source
Relay (controlled by a switch),
Consumer
Ground

There are
 
"Power Source
Switch
Consumer
Ground"
-types

e.g. Wing Bleed Isolation Valves, Autopilot Disengage bar (shutting off power to the servos), etc.

By the way, most switchbutton or annunciator bulb removals involve pulling the (mostly plastic) rectangular switch cap (where the bulbs are held) away from the switch assembly, completely isolating the bulbs from the power.

Regards
JHW

[JRBarrett]

The main reason vacuum is used is to prevent contaminating the delicate inner workings of the instruments. The vacuum pumps used on typical general aviation aircraft are "dry" pumps, which contain no oil. The pump has vanes made of graphite, which is self-lubricating, and resistant to the heat of compression.

The drawback to a graphite-vane dry pump is that it sheds carbon particles in operation, which would find their way into the gyros and eventually cause problems. Thus, the pressure side of the pump is vented to the atmosphere within the engine compartment, while the suction (vacuum) side goes to the instruments.

Usually, there is only ONE filter, located behind the instrument panel. It is typical to daisy chain instruments together, especially if the aircraft contains only a single directional gyro, and attitude indicator. The filtered cabin air flows into the first instrument, spinning its gyro, then on to the second, which in turn connects to the suction line going to the engine-driven pump.

It is also standard for there to be a suction gauge in the line to alert the pilot of a pump failure - which is all too common in graphite vane dry pumps.

A multi-engine aircraft is more complicated, in that there is a vacuum pump on each engine - and often, more instruments (i.e. on both pilot and co-pilot sides of the panel.) In this case, there will usually be a vacuum regulator to set maximum vacuum, as well as to provide isolation between the two engine pumps, so that just one can drive all the instruments if the other pump should fail.

In a multi-pump, multi-instrument configuration, the instruments typically all connect to a common vacuum distribution manifold, rather than being daisy-chained.

Jim Barrett