Why are gyros driven by vacuum and not high pressure air?
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From: 64N, 020E
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
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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From: Aus
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.
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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From: The Winchester
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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From: Augusta, Georgia, USA (back from Germany again)
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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From: Denver
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?
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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From: Aus
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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From: Tennessee
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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From: Enzed
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.
Psychophysiological entity
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From: Walton on the Naze Essex.
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.
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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From: Zulu Time Zone
A friend of mine asked why vacuum (suction) is used to drive the gyros in small aircraft and not high pressure air.
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.
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From: Miles away
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'!
Psychophysiological entity
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From: Walton on the Naze Essex.
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.
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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From: Ontario
I believe the first gyros, for turn indicators, were venturi operated. It was soon found, the hard way, that venturis were prone to icing (a dim memory says that an iced-up venturi was involved in Knut Rockne's (famous football coach) death in a Fokker). But (I think) the same instruments could be used with engine source vacuum. Electrical systems came along for navigation light and cockpit lighting, and then artificial horizons which if operated electrically gave redundancy, anyways the vacuum was more reliable and the turn indicator gyro didn't topple.
Some may have read of a certain Convair 240 (?) which took off from Boston circa 1950 +- a year or two in IFR conditions at night, with all gyros operated electrically, and suffered a total electrical failure. Luckily the captain was able to manage in a rough way with the magnetic compass and staggered along to the New York area where visibility improved and they found a lighted airport. I think this reinforced the idea of vacuum plus electrics.
Now of course electrical systems in aircraft are almost perfectly reliable.
Some may have read of a certain Convair 240 (?) which took off from Boston circa 1950 +- a year or two in IFR conditions at night, with all gyros operated electrically, and suffered a total electrical failure. Luckily the captain was able to manage in a rough way with the magnetic compass and staggered along to the New York area where visibility improved and they found a lighted airport. I think this reinforced the idea of vacuum plus electrics.
Now of course electrical systems in aircraft are almost perfectly reliable.
Psychophysiological entity
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From: Walton on the Naze Essex.
I hope so. Oddly, the other evening after posting on this thread, I found myself looking at some Pathe film that would never be shown to the public.
The British Eagle Viscount lost its electrics, and John Dawdy, probably following a cloud-break, snapped off the outer wings in a failed attempt to raise the nose. We were told then that the recorder wire was in 20,000 pieces.
I'd pictured the scene a thousand times, but the reality was quite different. Now I was looking at the FO's window and a bit of the roof. I'd spent many hours behind that glass. No punches were pulled in the photography the public were protected from. I suppose I must have taken a breath during the film but I felt as though time outside the scenes had stopped.
I'm mindful of the Egypt Air's hasty returns in the days leading up to the loss of the crash. The Eagle Viscount had a fault that no one could find - electrical intermittences are like that - but it had gone on a long time. One crew found themselves going into Malta at night not being able to see anything whatsoever, apart that is, from one bright red light telling them the electrics had failed. The captain (G Birch IIRC) told me he tried time and time again to reset the power, and suddenly it came on, I gather, just in time for the cloud break.
Different days, but even in our modern era electrical failures can be so subtle that they can lurk without revealing themselves for months. In those days, just one tiny physical fault, and now a fault and or a rouge digit lying fallow in seldom read code.
The British Eagle Viscount lost its electrics, and John Dawdy, probably following a cloud-break, snapped off the outer wings in a failed attempt to raise the nose. We were told then that the recorder wire was in 20,000 pieces.
I'd pictured the scene a thousand times, but the reality was quite different. Now I was looking at the FO's window and a bit of the roof. I'd spent many hours behind that glass. No punches were pulled in the photography the public were protected from. I suppose I must have taken a breath during the film but I felt as though time outside the scenes had stopped.
I'm mindful of the Egypt Air's hasty returns in the days leading up to the loss of the crash. The Eagle Viscount had a fault that no one could find - electrical intermittences are like that - but it had gone on a long time. One crew found themselves going into Malta at night not being able to see anything whatsoever, apart that is, from one bright red light telling them the electrics had failed. The captain (G Birch IIRC) told me he tried time and time again to reset the power, and suddenly it came on, I gather, just in time for the cloud break.
Different days, but even in our modern era electrical failures can be so subtle that they can lurk without revealing themselves for months. In those days, just one tiny physical fault, and now a fault and or a rouge digit lying fallow in seldom read code.