Vacuum system question

[a-v-8er]

Had a student ask why the gyroscopic instruments work by suction and air being directed against the rotor vanes instead of blowing air onto the vanes under positive pressure with a compressor (or pump) installed before the air enters the casing?

I honestly don't know, so here are some elaborate thoughts, but if someone knows feel free to correct me.

*With the pump located past the instruments there less chance that debris or a malfunctioning pump would damage the instruments.

*Seems to me that the suction system is more simple with ambient air being directed in to each instrument and an overpressure relief valve in case the pump would provide too much suction.

*Since the pump is an engine accessory, the engine speed determines the suction and if a pump was installed prior to the instruments maybe the positive pressure entering the instruments wouldn't have anywhere to go if rapidly increasing engine speed, "over pressurizing" the casing. The vacuum pump can operate more efficiently to yield reliable gyro speeds at several different engine speeds.

Anyone, care to give this a more thorough explanation? I feel like I'm just guessing here...

 

[SelectZoneFive]

I think you should smack your student and across the top of his head and say in a loud voice (Lee Emry style) "I'm the one who asks the questions around here, thank you very much!!!"

Also, there are more suction jokes than blowing jokes.

And finally suction pumps are cheaper than blowing pumps.

 

[310]

your ideas are good- in addition maybe it started out with vacuum because of the use of venturi tubes to produce vacuum a long time ago- just a guess but i bet avbug will know.

 

[DoinTime]

I really don't know either but maybe the vaccum system offers more protection to the instruments incase of a pump failure. If parts started flying off of pressurized systems pump you may end up with small pieces of debris in your intstruments but in a vaccum system they would just be thrown out to ambient air.

Maybe the vaccum system has benefits when being operated in variable air pressure enviroments?

Maybe its just a throwback to venturi style vaccum systems?

Maybe bud light is less filling?

 

[A Squared]

they are that way because the stuff coming out of a vacuum pump isn't all that clean, if it's a dry vacuum pump, there is graphite dust from the vanes, and when it self destructs (which they do about every 500 hours) big chunks of graphite come out. If it's a wet pump the discharge has a fine mist of engine oil in it. Neither graphite bits nor engine oil is good for gyros.

The other "reasons" suggested don't hold true. It is just as simple to make a pressure relief valve for a pressure system as for a vacuum system. The instruments are no more likely to be damaged by excessive pressure as they an equivalant amount of excessive vacuum. Really, the gyro doesn't care whether it's being "sucked" or "blown"....all that it cares is that the air pressure at it's outlet is less than the air pressure at it's inlet. It's just as happy and will spin just as fast when the inlet pressure is 35"hg and the outlet pressure is 30"hg, as it will when the the inlet pressure is 30"hg and the outlet pressure is 25"hg. (ok, for you pedants the relationship isn't exactly linear, but it's pretty close)

regards

 

[avbug]

You're all right, for all the reasons given. In truth, both positive pressure and negative pressure pneumatic instrument systems exist. Some airplane use positive pressure output pumps to pressurize deice equipment, and use the same air pressure through the various instruments to make them operate. In most cases, positive output pumps (air pumps, vs. vacum pumps) still use negative pressure (suction) at the instrument, by means of a venturi and jet pump. Some aircraft use both.

The early vacum instruments were operated using suction from external venturis, arranged in the slipstream of the propeller. These venturis had obvious issues with external contamination, such as icing. They were also very dependent on airspeed and engine speed (prop airflow).

The use of an engine-driven vacum pump was a natural improvement over the venturi tube (there's nothing wrong with external venturi tubes; they worked very well for a long time, and still work well). Suction powered instrument systems still were suction powered, only the suction source was an engine accessory, rather than the external slipstream.

The only chance of vacum pneumatic instrument system contamination from debris is the filter itself, or from air line deterioration. This differs from placing a positve pump supplying the motive airflow directly to the instruments, in which both the filter and the pump itself presented a threat to the instruments.

Something you may discuss with your student, that isn't often covered, is the meaning of the vacum pressure you're reading. It's easy to visualize why too little vacum pressure might result in instrument errors. This is simply the result of insufficient gyro speed. However, what about too much suction?

One might assume that too much suction might mean that the instrument is going too fast...after all, too little makes it go slow. However, that's not the case. An underspeed condition for the gyro will exist with too little, or too much vacum. Excessive vacum occurs due to a constriction in the system. It may be a filter blockage, or a line blockage. The effect is similiar to placing one's hand over a vacum cleaner hose. Pressure drops, and the suction value reading goes up, but far less air is flowing, and as a result, the gyro isn't receiving the motive flow to make it turn.

It can be reasoned, therefore, that weak suction is the result of a pump or vacum leak, while high vacum readings are the result of a system blockage of some kind. Both result the same; unreliable instrument indications. This is important because a pilot who doesn't understand this principle may be duped into thinking that where low vacum is unacceptable, excess vacum is fine and at least the instruments will be up to speed. Not the case at all. Both present the same hazard, and the instrument student needs to be made to understand this.

I point this out, because an insidious gyro or vacum failure is extremely dangerous (as opposed to a rapid or instant failure, which is far more recognizable and dealt with). The student who understands the potential is likely to take corrective action by covering the instrument or going to partial panel, rather than attempting to follow a slowly failing instrument.

Overpressurizing the instrument case isnt really an issue, regardless of weather negative or positive pressure is used, when considering engine RPM. Pressure and vacum are limited by regulators, rather than being a direct result of engine RPM, and therefore the issue is with the correct function of the air regulator, rather than engine speed.

The vacum pump will never produce too much suction. It produces a maximum value, and this will only decrease with time and wear. The amount of suction reaching the instruments is determined by the pressure or vacum regulator.

 

[knelson]

Suddenly, I am a pre-solo private student again that sits back and sas, "man, how does this guy know so much about this?" Great post Avbug, Thanks for the lesson.

Kevin

P.S. Most people charge about $30-$40 an hour for that nowdays.

Thanks again

 

[avbug]

You're welcome, sir. However, that information was taken directly from the back of a box of vacum crunch cereal (the raisin kind, not the stuff with the little colored marshmallows).

 

[SelectZoneFive]

Hey Yoda,

How come you know so much?

 

[TXCAP4228]

Ask avbug

your ideas are good- in addition maybe it started out with vacuum because of the use of venturi tubes to produce vacuum a long time ago- just a guess but i bet avbug will know.

I read the question and thought the same thing. Maybe we should have a "questions for avbug" forum.

 

[mckpickle]

Av Bug I gotta ask. Form where do you derive all your knowledge. Your post have become well respected!

 

[DC9stick]

Just to add to Avbug's post.

There is no suction in the instrument case as you would have sucking on a bottle with no place for more air to come in. The air to drive the rotor comes through a nozzle directed onto a vane or cup on the rotor, it makes no difference if this air is blown through the nozzle (pressure pump) or sucked through the nozzle (suction pump) the air goes in and the air comes out.
Beech used pressure systems for years and probably still does it's all a matter of engineering preference. Some manufacturers prefer to use suction for instruments and pressure for deice.