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Why are gyros driven by vacuum and not high pressure air?

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Why are gyros driven by vacuum and not high pressure air?

Old 3rd Jun 2016, 10:28
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Why are gyros driven by vacuum and not high pressure air?

Apologies if this question has been answered before - I couldn't find it.

A friend of mine asked why vacuum (suction) is used to drive the gyros in small aircraft and not high pressure air. I felt I couldn't give a definitive answer.

Is it because it's lighter? Cheaper? More robust? More tolerant to leaks? And while I'm at it: Why don't we use electric instead? In one of our Cessnas, we have a backup electric gyro but the main in still vacuum-driven.

Thank you for your answers - I'm looking forward to learning something new today.

/N
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Old 3rd Jun 2016, 11:07
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Obviously, if the gyros were powered by pressure, they would run backwards. ��
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Old 3rd Jun 2016, 11:24
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It's a lot easier to get a consistent vacuum source from a 4 stroke engine than a pressure. I would also imagine the temperature of the air must be considered. PV=NRT. High pressure generally means high temp as well.
It's good to see some of the experimental electrical instruments getting FAA approval for retrofitting to certified aircraft. The MTBF of these seems much better than the vacuum ones..... the only problem is keeping a power supply.
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Old 3rd Jun 2016, 11:41
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The original gyro instruments were driven by "vacuum generators" (a convergent/divergent cone pair in the airflow behind the propwash) which could be relied upon to produce "vacuum power" as long as the engine was running or the aircraft had airspeed - far more reliable than a separate pressure pump. The later instruments may be powered by manifold vacuum, but it's a similar reliability thing - as long as the engine is running here will be manifold vacuum; it's not dependant on a separate pump and its drive belts/gears/pixies to keep working.

Of course it's also cheap and light, and that helps too!

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Old 3rd Jun 2016, 11:48
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If you blow air into an instrument case, you somehow have to exit the exhaust. If you suck air in, the intake and exhaust are taken care of?
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Old 3rd Jun 2016, 12:16
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Dunno..I'd offer one wild guess that if you "blow" the air in goes through the pump before the instrument and so perhaps there's more of a chance of introducing contaminants into the instrument than if the pump is on the other end of the process.
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Old 3rd Jun 2016, 12:25
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introducing contaminants

I was taught that anything that "pushes" air would/could introduce its lubricating oil into the instrument, contaminating the gyro's needle/pivots (whatever that part is called!).
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Old 3rd Jun 2016, 15:57
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When flight instruments, especially the vacuum-dependent gyros (AH and DG) were introduced c. 1930 , the simplest, lightest-weight (including connections) source for motive power for the gyros was a venturi tube outside the airframe a few inches from the instrument panel, providing suction. "So simple even a Piper Cub could do it."

https://upload.wikimedia.org/wikiped..._venturi_1.JPG

Much lighter than a pump casing and pipes, that had to hold pressure, and reach from the engine through the firewall. And, of course, aircraft engineers understood suction and vacuum (Bernoulli, Venturi, etc.).

Once vacuum drive became the standard - and worked - why fix something that ain't broke?

Remind your friend that in aircraft engineering, the first question in planning a new system is not "What's the best way we can do it?", but always "What's the lightest, smallest way we can do it?"

Eventually, the trade-off of high-speed drag vs. weight moved the suction source to the manifold from the external venturis. But the manifold suction was already there - free, gratis and fer nuttin' - compared to adding a dedicated, relatively heavy accesory pump. Plus gauges were already designed around "suck" and not "blow," and why redesign them?
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Old 4th Jun 2016, 00:38
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The early Bell rocket propelled research aircraft used pressure from the onboard nitrogen source to run the gyros. The exhaust from the gyro was dumped into the cockpit to maintain pressurisation. The nitrogen main use was to pressurise the propellant tanks.
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Old 4th Jun 2016, 02:28
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Not all GA air instrument are vacuum. Later beechcraft: Baron and Bonanza use pressurized air instead of vacuum.
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Old 4th Jun 2016, 06:31
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Correct. I flew a 1977 Beech F33 Bonanza in Chicago that had a pressure vacuum system based on an engine-driven pump. According to the POH.
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Old 4th Jun 2016, 06:54
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Some early aircraft use exhaust Venturi tubes to draw air through the gyros, cheap light & reliable. What more could you want ?
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Old 4th Jun 2016, 08:47
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PA 31 series has pressure driven gyros also.
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Old 4th Jun 2016, 09:01
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Just to be clear, the vacuum source for small piston aircraft is not engine vacuum, like cars of a few years ago. They use mechanical vacuum pump(s) driven from the accessory section on the engine, like the magnetos.
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Old 4th Jun 2016, 09:26
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I think you'll find the pressure mentioned as used in the Bonanza and Chieftain is s derived from the exhaust of a vacuum pump. Basically the same set up but instead of the pump being connected to the outlet of the gyros as in most aircraft the pump is connected to the inlet.
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Old 4th Jun 2016, 10:08
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Originally Posted by Tscottme View Post
Just to be clear, the vacuum source for small piston aircraft is not engine vacuum, like cars of a few years ago. They use mechanical vacuum pump(s) driven from the accessory section on the engine, like the magnetos.
That gets a "DOH! from me! I knew this, but the brain ran on in automatic when I typed the above.

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Old 4th Jun 2016, 10:12
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Shy, that's a keyboard, screen and some wallpaper you owe me!!


Me, a Heron, a very rainy day. Surely, that can't be water in the bottom of my Horizon . . . can it?!?!?

Yes. But, the aircraft had just been washed with something bubbly. Within ten minutes, the foam had reached 15% of the glass. When would it touch the gyro, with all its little buckets spinning so furiously?

Wow! That's pretty.

What did I learn from all this?

The vacuum replacement air came though a filter just at the back of the Horizon. It was bronze and about as much use at keeping out water as Mary O'Riely's breast was in a fight.

I also learned that when the gyro toppled, the foam filled the glass in a nano-second. Oh,a and that I can fly reasonably well on an old turn and slip indicator.
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Old 4th Jun 2016, 10:17
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A friend of mine asked why vacuum (suction) is used to drive the gyros in small aircraft and not high pressure air.
A good question.

anawat pointed out that some types use pressure instead of vacuum:

Not all GA air instrument are vacuum. Later beechcraft: Baron and Bonanza use pressurized air instead of vacuum.
...either way the same type of gyros and pumps are used so it does not matter in principle. I think that wiggy and LTCTerry are correct - less chance of contaminating the gyro if the pump is downstream as per the vacuum arrangement, therefore less filtration, simpler and less chance of ruining a gyro when the pump fails. Backed up by the fact those types using pressure have dry pumps. If they used wet pumps then oil would get to the gyros.
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Old 4th Jun 2016, 10:33
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I always understood the reason to be that if the exhaust from either vacuum or pressure instruments was blocked, then the former would be safe but the latter a potential 'bomb'!
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Old 4th Jun 2016, 11:00
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Subject close to my heart, this is.

I had the pleasure of meeting, flying with and later the friendship of Col. Carl Crane. After he'd tumbled out of clouds in an open cockpit aircraft - with a senator's son on board - he spent a lot of his life devoted to teaching instrument flying and inventing blind landing systems. He was still flying days before his death at 79 years of age. In that era, he was an honorary lecturer at Randolph AFB's Advanced instrument flying school, mostly talking about the old days of course. Some of the tales were fabulous.

To any aviation enthusiast, he's really worth a Gooogle.

I had a total instrument failure in a DC3 due to loss of vacuum. Well, not quite total, I'd learned that gyros took time to rumble to a full stop and while they were useless for horizons, the turn and slip still showed a tiny movement for quite a while. I got a cloud-break from that somewhere around Kirkwall.

You may have noticed, I've bellyached about taking tied gyros out of transport aircraft for about 40 years.
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