Vacuum Gauge
What is the proper reading for Vacuum during run-up/ in flight, in a 140? I have seen 5 +/- 0.1 and also 4.5 - 5.2?
thanks
Tom
thanks
Tom
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Ranges vary generally from 4.5" to 5.2". Your regulator should keep it in that range at cruise power.
Gyro performance can be sluggish below the lower limitation and they can be overly rigid and can overly react above the maximum limitation. Higher suction can also decrease gyro life.
It's a range for normal operation and nothing more.
> Some 140's STILL have original gyro types as
> installing
My Twin Comanche manual shows the operating vacuum range of these older instruments as 4.5" to 5.0".
> Gyro performance can be sluggish below the lower
> limitation and they can be overly rigid and can
> overly react above the maximum limitation.
>
Explain the highlighted comment please GM
The above information came out of original "artificial Horizon" owners information pamphlet. It also states excessive vacuum will cause premature bearing wear.
Review rigidity in space and displacement of a spinning object.
For further information or disagreement contact your local instrument manufacturer or repair facility.
Rigidity in space is the gyroscopic principle that AI and DG use to operate. I don't understand how something that designed to be rigid (not move) in space could ever "overly react".
Asking my "local instrument manufacturer or repair facility" to explain your statement is just a cop-out on your part. If you said it, you should be able to explain it. Or simply admit that you don't know and it just something you remembered reading somewhere.
> Or simply admit that you don't know and it just something you
> remembered reading somewhere.
Dex, take a good look at Lindsay Lohan's fingernail.
Jim, excessive vacuum and "the higher RPM the better" statement is incorrect. If you had a pneumatic turn and bank (operates under precession principle) and did not have the reduction needle valve to limit vacuum to 2.5 inches at the inlet, the instrument would not be accurate with 4.5" to 5.0". Likewise an attitude indicator or heading indicator (operates under rigidity) spinning with vacuum excessively out of its operating range will either be sloppy at the low end or excessively rigid and inaccurate in flight reasonably above the high end. This has been discussed in instrument training handbooks and maintenance training handbooks since WWII.
Surely you have had a spinning top toy as a kid. Push the plunger a couple of times (low RPM); push the edge of the toy and it will become unstable and wobble. Push it a half dozen times to spin it up good and apply a good force to it and it will resist deflection and dart off to the side (over reactive). If a gyroscopic instrument is spinning at a higher than normal RPM it will resist deflection to a greater degree and also apply excessive force creating excessive wear to it mounting points which are the needle bearings or bushings.
If you feel that adjusting your vacuum regulator above the operating range for grandma is better than be my guest. I don't.
edited: sp and double sentences.
> If you feel that adjusting your vacuum regulator
> to 5.5" or more for grandma is better than be my
> guest. I don't.
I never said or even implied that it was "better" to run vacuum higher than specified. I only wondered how it would cause the gyros in Tom's 140 to "overly react".
So, youre playing the vacuum T&B card (I saw this coming a mile away). What you say is true, but, (in typical GM fashion) totally irrelevant to the discussion.
But you claimed you found the statement in an "artificial Horizon owners information pamphlet"?... That just doesn't make any sense to me, Jim or anyone else
I hope the have the equavilant from the maintence training division Monday.
Gyroscopic systems and instruments
General
Most aircraft have several instruments that are traditionally driven by mechanical Gyros. These instruments assist in flying and navigation of an aircraft. These instruments are the Attitude Indicator (also known as the Vertical Gyro), the Directional Gyro, and the Turn and Bank Indicator. Aircraft also typically have a compass, and in some cases a Flux Valve (also known as a Magnetometer) to which the Directional Gyro is connected or slaved to cancel long term drift. If the aircraft does not have an electronic Flux Valve, then the Directional Gyro or DG has to be manually reset to the compass reading during straight and level flight (when the compass is accurate) on a periodic basis. In most light aircraft the Turn Coordinator (TC) is electrically driven. Usually the Heading Indicator (HI) and Attitude Indicator (AI) are vacuum driven.
The three Gyro instruments, Attitude Indicator, Directional Gyro and Turn and Bank Indicator are gyro driven. What does gyro-driven mean? A gyro is a spinning wheel (mass) that obeys the Laws of Physics. The spinning wheel is spun up either electrically (electric gyros) or via air flow (vacuum gyros) to high rotational speeds and a high angular momentum. The spinning wheel is mechanically isolated from the casing of the instrument thru a series of gimbals. Due to the conservation of angular momentum, the spinning wheel will try to maintain its orientation, via the gimbals, as the outer casing moves. The outer casing is of course connected to the airframe. The gimbals, move by the amount the aircraft has rolled, pitched, or changed heading, and in some cases directly connect to the display. The display provides an indication of the aircraft attitude. In the case of a remote gyro and also with many electric gyros, the gimbals provide an analogue electrical output proportional to aircraft orientation change.
gyroscopic principles
Any spinning object possesses gyroscopic characteristics. The central mechanism of the gyroscope is a wheel similar to a bicycle wheel. It's outer rim has a heavy mass. It rotates at high speed on very low friction bearings. When it is rotating normally, it resists changes in direction.
The gyroscope exhibits two predominant characteristics:
Rigidity in Space
Precession
rigidity in space
The gyroscope resists turning. When it is "gimballed" ( free to move in a given direction) such that it is free to move either in 1, 2 or 3 dimensions, any surface such as an instrument dial attached to the gyro assembly will remain rigid in space even though the case of the gyro turns. The Attitude Indicator (AI) and the Heading Indicator (HI ) use this property of rigidity in space for their operation. The HI responds only to change of heading. The AI responds to both changes in Pitch and in Roll.
precession
Precession is the deflection of a spinning wheel 90 ° to the plane of rotation when a deflective force is applied at the rim. If a force is applied the top of the rim (the plane of rotation), the precession (turn) will be 90° in the horizontal plane to the left. The Turn Coordinator (TC) uses this precession property. For example, then taxiing on the ground, the Turn Coordinator will move, with the small airplane in the instrument showing a bank, even though the aircraft is level. The banking of the small aircraft presentation indicates only that the aircraft is turning.
the vacuum system
The Attitude Indicator (AI) and the Heading Indicator (HI) in light aircraft are usually driven by a vacuum system. The principal components are shown below. Not shown are auxiliary devices such as valves, filters etc. A pump provides the vacuum to the AI and HI through a system of vacuum lines. A Vacuum Gauge is attached to the lines which gives the pilot an indication that adequate vacuum is being generated.
gyro power sources
Air or electricity supply the power to operate gyro instruments in light aircraft. If the directional indicator and attitude indicator are air-driven (as they generally are), the turn-and-slip indicator is electrically powered. The advantage of this arrangement is that if the vacuum system (which supplies air) fails, the instrument pilot still has the compass and the turn indicator for attitude and direction reference, in addition to the pitot-static instruments.
1. vacuum power system: Air-driven gyros normally are powered by a vacuum pump attached to and driven by the engine. Suction lines connect the pump to the instruments, drawing cabin air through the filtered openings in the instrument case. As the air enters the case, it is accelerated and directed against small "buckets" cast into the gyro wheel. A regulator is attached between the pump and the gyro instrument case to control suction pressure. There is normally a vacuum gauge, suction gauge (See the Typical Suction Gauge figure, below) or warning light. Because a constant gyro speed is essential for reliable instrument readings, the correct suction pressure is maintained with a vacuum pressure regulator.
The air is drawn through a filter, to the instruments and then to the pump where it is vented to atmosphere. The pilot should consult the aircraft operating manual for specific information with regard to vacuum system normal operating values. Low gyro rotation speeds cause slow instrument response or lagging indications, while fast gyro speeds cause the instruments to overreact in addition to wearing the gyro bearings faster and decreasing gyro life.
2. electrical power system: An electric gyro, normally used to drive the turn coordinator or turn-and-slip indicator, operates like a small electric motor with the spinning gyro acting as the motor armature. Gyro speed in these instruments is approximately 8,000 rpm.
Aircraft that normally operate at high altitudes do not use a vacuum system to power flight instruments because pump efficiency is limited in the thin, cold air. Instead, alternating current (a.c.) drives the gyros in the heading and attitude indicators. The a.c. power is provided by inverters that convert direct current to alternating current. In some cases, the a.c. power is supplied directly from the engine-driven alternator or generator.
> But you claimed you found the statement in an
> "artificial Horizon owners information
> pamphlet"?... That just doesn't make any sense to
> me, Jim or anyone else
Here is some enlightenment from the Piper PN 753 711 gyro information booklet dated April 1969, page 4 that came with my aircraft:
The amount of drift of a directional gyro can be dependent upon geographical location on the planet. At the equator earth has zero effect. Drift increases up to 15degrees at the poles. To counteract this instrument technicians balance gimbal rings to minimize this error.
Recalibration is desirable for latitude change exceeding 50 degrees.
I suppose that doesn't make any sense to you either Dex, and yes, the T&B reference is applicable as it demonstrates why specific gyros require a specific RPM to function properly.
It's not rocket science to understand that a gyro such as an A/I designed to be operated at a regulated RPM of 20,000 for max efficiency should not be run at 23,000 RPM with expectations the same results.
> That just doesn't make any sense to me, Jim or anyone else
Amazing how you now speak for Jim and everyone else...so you think.
> I suppose that doesn't make any sense to you
> either Dex, and yes, the T&B reference is
> applicable as it demonstrates why specific gyros
> require a specific RPM to function properly.
>
You'd be surprised how much sense it makes to me GM. Let me explain it to you in my own words as the Piper manual is a bit over simplified (which is why you probably like it).
Gyro shops can purposely unbalance (not balance) one of the gimbals in the DG (not necessary in a slaved DG or HSI) to induce gyroscopic precession in order to counteract the natural tendency of the DG/HSI to drift because of the earths rotation. However, my understanding is the effect is zero at the equator and 30 degrees per hour (360/12) at the poles, not 15 degrees (per hour) as you quoted from the Piper publication.
But even this characteristic would not cause the DG to overly react if spun at a higher than specified speed. Only the T&B or TC would have that problem (but they arent vacuum driven).
While we're on the subject of oversimplification, what university is that handout from? It's got several errors that they should correct.
PilotKris
(PilotDex to GM)
> However, my understanding is the effect is zero at
> the equator and 30 degrees per hour (360/12) at
> the poles, not 15 degrees (per hour) as you quoted
> from the Piper publication.
PK, sorry, earth rotates 360° in 24 hours, so 15°/h
Markus
> You'd be surprised how much sense it makes to me
> GM. Let me explain it to you in my own words as
> the Piper manual is a bit over simplified (which
> is why you probably like it).
> PilotKris
>
> (PilotDex to GM)
Yes Dex, you are correct. I do like "over simplified" publications such as this Piper booklet and I do find them beneficial.
Throughout the years I've found simplification and even "oversimplification" of anything in this business produces excellent results finding it to be a far better teaching method than needlessly complicating matters.
edited: extra verbiage
> PK, sorry, earth rotates 360° in 24 hours, so
> 15°/h
>
> Markus
You are correct. I don't know what I was thinking
> Throughout the years I've found simplification and
> even "oversimplification" of anything in this
> business produces excellent results finding it to
> be a far better teaching method than needlessly
> complicating matters.
>
And that's a very "Airline" attitude GM. "Just teach what to do, not why".
In your "business", there is no "out of the box" thinking. Pilots aren't supposed to think at all, only follow procedures... to the letter. And when something really bad happens to an airliner (i.e.: double engine flame out just after takeoff), it's usually the pilot's GA experience that saves the day.
Part 121 flights are planned down to every last detail following very detailed instructions spelled out in the Co. manuals. From simplified and standardized routes to how/when the pilot can take a leak. Sometimes those procedures translate well to GA but lots of times they don't.
On every flight, GA pilots face a range of variables many times greater than airliners ever have to. That's why I don't just teach what, but also why.
PilotKris
I can't discuss the top in a "civil manner"? What like GM?
GM Wrote:
>"Dex, take a good look at Lindsay Lohan's fingernail"
I harbor no resentment at all towards airline pilots. The airlines have done a remarkable job at making air travel save and reliable.
But we don't fly Boeings or Airbuses and the challenges we face are entirely different. The procedures developed to fly them on scheduled routes don't always translate well to GA. The skills necessary to be a good airline pilot don't automatically make you a good Piper pilot.
The airline and ex-airline (and military) pilots I know who are good GA pilots realize that. They seek out those more experienced in GA for advice/instruction and don't walk around the airport acting superior and lecturing GA pilots about the proper "airline" way to do things.
PilotKris
BTW, I recommend the Pilot's Guide as a nice supplement to the POH.