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Instruments 249
With all the rotating parts, there is
bound to be friction, which will
cause some errors in the readings.
Others include acceleration error, found
during forward movement (as in a
takeoff) which gives a false climb to
the right – this is because of the
pendulous mounting - the heavy
bottom of the (suction) gyro suffers
from inertia and creates an
imbalance between misplaced
centres of gravity (roll error) and
closing one of the suction ports
(pitch error) – the effect is similar to
the compass. The resulting forces
precess 90° away for false readings.
Deceleration shows a false descent
and roll to port (exam question). A
climb to the right indicates pitch and
roll errors.
Centrifugal force created during a
turn will also displace the mass of
the heavy bottom of the instrument,
but modern designs minimise
turning errors at low rates of turn.
Heading Indicator (DGI)
This is used to give a stable heading
reference free from the errors found
with a compass. It works in a similar
way to the artificial horizon, except
that the gyro is horizontally
mounted. The casing turns round a
tied gyro, which has a compass card
mounted on it. The spin axis is
horizontal, and operates in the
yawing plane. It is also air driven.
To help with re-erection after
toppling, the mass of the gyro is
spun with air jets coming from twin
sources, very close to each other.
When the gyro does not lie in the
yawing plane, one jet will still be
pushing the gyro round, but the
other will strike the rim of the gyro
(A) and cause a precession force at
the top (B) which will tend to reerect
it:
Apparent drift can be found by
multiplying 15° by the sine of the
latitude to find the error in degrees
per hour. The sign will be negative in
the Northern Hemisphere because
the gyro under-reads as the Earth
rotates. DGIs are calibrated for
particular latitudes with a latitude nut
that introduces a precession force to
counteract apparent drift - out of the
box, this is usually for 45°. Assuming
no other influence, operating North
of the preset latitude will cause the
DGI to under-read. Going South
will make it over-read. When wound
outwards, the latitude nut exerts a
greater force against the balance
weight on the other side of the inner
gimbal, where it is mounted.
250 Canadian Professional Pilot Studies
When it comes to transport wander,
flight West will cause over-reading,
and Eastward under-reading in the
Northern Hemisphere.
Because of wander (real or apparent
- see above), unless you have a slaved
compass (meaning automatic), you
should align this instrument with the
magnetic compass every 15 minutes
or so, remembering, of course, to do
it in level, unaccelerated flight. You
may get erroneous readings if the
aircraft adopts unusual attitudes.
Turn Coordinator
This is actually a combination of two
instruments, one power driven, and
the other not. A small aircraft tilts to
indicate whether you are banking, so
it is a useful backup to the artificial
horizon, especially since the gyro is
electrically operated and not affected
if the suction system fails (although
it gives you a rudimentary indication
of bank, turns without the other
instruments are done with timing).
The instrument is sensitive to yaw
and roll, because the gyro's axis is
tilted upwards by about 30-35°.
When the wings in the little aircraft
hit one of the lower marks you are in
a Rate 1 turn, which takes two
minutes to go through 360°, making
3° per second (you can also add 7 kts
to 10% of your airspeed to get a
rough guide to the bank angle
required). Underneath is a ball in a
clear tube containing fluid, for
damping purposes, called an
inclinometer: It is subject to gravity
and centrifugal force, and will be
thrown one way or another if the
aircraft is not in a coordinated turn.
In a slip, the rate of turn is too slow
for the bank, so centrifugal force will
be less, and the ball will not be
thrown out so much (exam
question). It will therefore be on the
inside of the turn (decrease the angle
or increase the rate to correct):
In a skid, the turn is too fast, so
more centrifugal force causes the
ball to be displaced more, to the
outside of the turn:
Correction is the opposite of the
slip, above (exam question).
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