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Frequently
Asked Questions
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Question:
How long should my gyro last?
Answer:
Well, that depends. At TGH we have seen gyroscopes that have lasted and
worked properly in aircraft for a number of years and then we have seen
gyros that have failed in just a few days. This holds true for gyros that
are factory new as well as for gyros that are overhauled.
How
well do you treat your gyro?
One of the primary factors in determining the life of a gyro is how carefully
it is handled during transportation and during installation into the aircraft.
A gyroscope is an EXTREMELY delicate instrument. Dropping a gyroscope a mere 1/8th inch onto a bench top causes sufficient force to “Ball Dent” the units primary bearings. This means that the balls within the main race of the bearing cause an impression of themselves to be created on the inside of the bearing race due to the force of being dropped. The bearings within a gyroscope can be operating at speeds from 12,000 RPM up to 25,000 RPM, depending on the type of gyro. Dents on the inner race will cause friction as each ball within the bearing passes by the dent. Friction causes further wear on the bearing. Further wear causes further friction and so on until an avalanching effect occurs. Friction is the main cause of gyro precession and ultimate failure of the unit.
Also you don’t have to drop the gyro to cause ball denting. Rough landings can have the very same effect. This is evidenced by the high occurrence of this type failure on training aircraft. Students just learning to fly often have bumpy touchdowns, this results in gyros that have less than expected life spans.
Did
you replace those hoses and filters?
We often get vacuum operated gyros that have failed due to ingestion of
carbon dust. The source of carbon dust is the vacuum pump. The vanes within
a dry vacuum pump are manufactured from a material very much like pencil
lead. As the vanes rotate around the pump housing they a re moved outward
by centrifugal force. The edges of the vanes rub against the pumps inner
housing and wear down, just like a pencil. As a result of this wear the
vanes generate carbon dust. When the engine is shut down, the carbon dust
is sucked into the aircrafts gyro hoses and filters (remember the pump
created a vacuum within the gyro, when the pump stops the vacuum must
be filled). If your filter is no longer operating efficiently that dust
will go into your gyro. Many times pilots tell us, “But I just put
a new filter in”. Perhaps you did, but did you change the hoses?
The carbon residue builds up and lines the hoses as well as the filter,
particularly when there has been a catastrophic pump failure, like a shattered
vane. If you don’t replace the hoses as well as the pump you are
exposing your gyro to premature failure. The hose costs about $25.00 plus
labor to install, the gyro costs hundreds or perhaps thousands, depending
on the type, to repair or replace. So the next time you replace that filter
think about doing the vacuum hose as well.
Complexity
of the Gyroscope
While the cost of a gyroscope is directly proportional to the complexity
of the unit, the lifespan is often inversely proportional to the complexity.
Simply put, the more parts there are the more it costs and the more likely
it is that one of those many parts will fail. Gyroscopes that have meter
movements in them, such as Power Flags or NAV Flags are significantly
more delicate than gyroscopes without those options.
Inactivity
Yes, not using your gyroscope can cause it to fail as well. The bearings
within the unit are lubricated. If the gyro is not spun up every now and
then, the lubricant will collect at the bottom of the bearing and can
harden or dry out. It is recommended that you spin up the gyro every 30
days or so and allow it to run for at least 10 to 30 minutes in order
to redistribute the lubricant.
In
Summary
The lifespan of your gyroscope is very dependent upon the environment
in which it is used and on how well you maintain your aircraft. When we
overhaul a gyroscope at TGH we totally disassemble the unit and replace
all of the bearings with factory new FAA approved bearings. We dynamically
balance the rotor and gimbal assemblies. We perform a calibration utilizing
test equipment that is traceable to the National Institute of Standards
and Technology. We then allow the unit to run-in for hours before performing
a final Acceptance Test Procedure. We are confident enough in our work
to provide you with a full 1-year warranty on parts and labor. However
even allowing for all of this we cannot accurately predict the lifespan
of any one unit. You should ideally realize hundreds of hours of use,
but that is totally dependent on how the unit is treated and the environment
in which it is used.
Question:
My DG works fine in straight and level flight but experiences precession
during a turn what is going on?
Answer:
Most likely you have a faulty compass that you are comparing with the
DG. A DG that is experiencing precession will do so in both a turn and
during straight and level flight, not just during one or the other. The
way to determine whether it is your compass or your gyro that is causing
you difficulty perform the following procedure:
1.
Turn your aircraft to a specific heading as indicated by the compass,
for example due North or 0 Degrees.
2. While flying straight and level at the selected heading, set your DG
card so that it is in agreement with the compass.
3. Perform a standard rate of turn for 90 degrees. Take note of the error.
4. Perform a standard rate of turn back to your original heading that
was chosen in step 1.
5. Did the DG return to the original heading?
If the DG returned to the original heading the problem is not your DG. The problem is with your compass. The DG will not unprecess but will continue to precess in the same direction.
Question:
How much vacuum is needed for my gyro to run properly?
Answer:
In most vacuum gyroscopes the rotor will actually start to spin up at
around 2.5 InHg (Inches of Mercury). However with that low of a vacuum
the gyro is not spinning fast enough to provide accurate and reliable
information. Typically you should set your regulator to be at 5.5 InHg.
Question:
At what voltage level will my electrically operated gyroscope become unreliable?
Answer:
All electrically operated gyroscopes have an allowable tolerance for the
input power. Within those tolerances the unit will operate reliably, outside
of those tolerances the unit may appear to operate however the information
it is providing may not be accurate. The accuracy of a gyroscope is directly
proportional to the speed of the gyro rotor. When calibrations are made
on the unit they are made at a specific rotor speed depending on the design
of the unit. The actual speed at which the rotor is currently operating
is dependant on a number of items, such as the condition of the bearings,
proper rotor balance, and input power.
Raising or lowering the input power will cause the rotor to run either
faster or slower. This results in affecting the accuracy of the gyroscope.
Listed below are typical tolerances for the standard operating voltages,
however you should always read the information provided by the manufacturer
for your specific gyroscope.
14
VDC systems – Typical tolerance is from 11.5 VDC up to 16 VDC
28VDC systems – Typical tolerance is from 26.5 VDC up to 31 VDC
115 VAC systems – Typical tolerance is from 110 VAC up to 120 VAC
