Been watching the dialog on "simple, cheap and low-cost"
attitude indicators. Many folk have already picked up on
the shortcomings of many low cost "attitude indicator"
offerings . . most notably from the ultra-light contingency.
In support of understanding and the exercise of good options
I offer the following:
"Attitude" is generally understood to describe the
positioning of an aircraft with respect to earth surface
(the mean surface, ignoring things like mountains, canyons
and potholes . . . sea level is about as close to mean
surface as one can physically demonstrate). There are
four basic means for deducing attitude (1) local reference
(2) universe reference (3) inertial reference and
(4) gravity reference.
All four of these systems are utilized to various
degrees in the cockpit of most airplanes.
(1) LOCAL REFERENCE is your eyeballs . . . the visual
contact with the outside tells us how well we're doing.
Sources for error: sloping terrain, obstructions to
perception - fog, white-out, glassy seas, etc.
(2) UNIVERSE REFERENCE is gyros . . . unless acted upon
by some outside force, a gyro will point at some fixed
point in space. Obviously, as the earth rotates, a gyro's
indication will change depending on latitude of your current
location -and- direction the gyro was pointed when it was
first spun up. Attitude gyros are fitted with pointing
mechanisms that use gravity reference to keep the critter
right side up. Heading gyros (unless slaved to a magnetic
compass) simply drift as the earth rotates. Hence, they
require occasional annual updating to match the magnetic
compass.
Sources for error: Friction in rotor and gimbal bearings,
wind drag inside the instrument (remember, the thing
turns at 10,000 rpm - 2" diam rotor has perimeter of 6.3
inches. The SURFACE speed of the rotor is over 80 feet per
second!) and mechanically induced errors from
erection/calibration systems. For example, if you
fly in a coordinated 30 degree banked turn for about
15 minutesand then level the wings, you'll find that your
attitude gyro has picked up some error while trying to
erect to a new "gravity" vector.
(3) INERTIAL REFERENCE says, "if I turn at 3 degrees per
second for 10 seconds, I'm now 30 degrees difference from
where I started. Hmmmm . . . this is obviously limited.
This is how your turn coordinator works. It's a rate
sensor that has no idea where you're pointed right now
but it does know how fast your heading is CHANGING.
Angular rate gyros are core components of an inertial
navigation system along with linear rate sensors
(accelerometers). These systems must be initialized
---From a known baseline. Like sitting on the ramp before
engine start. The systems may also use data from other
parts of the navigation system (iron gyros, keyboard
input by pilot, pitot-static transducers, GPS, VLF Nav,
Loran, etc.) to decide where we are and what our current
attitude is.
Once the system is "stood up" it keeps track of linear
accelerations (feet per second per second) and rotational
rates (degrees per second) in millisecond slices to deduce
our current position and attitude. The turn coordinator on
the panel is a rate sensor which requires you to INTEGRATE
readings with time to deduce a change in heading.
Sources for error: Most modern inertial sensors have zero
moving parts so things like friction and windage are no
problem. No-moving-parts sensors have calibration drifts
with respect to temperature but this is small and can be
calculated out. The largest source of error is time dependent
and based on limits of sensor resolution and magnitude of
uncertainty in calculations. Fortunately, the short term
stability is VERY good and other sensors such as radio nav
aids and iron gyro platforms can be combined with inertial
sensors to paint and update a VERY accurate navigation
picture for the duration of flight.
The needle portion of turn coordinator is also a gyro but
it turns at a much lower speed than the attitude and heading
gyros. Further, it's constrained with a series of springs
that forces the gyro maintain a fixed position with respect
to the airplane. As you CHANGE direction, the gyro tugs on
the springs with a force that is PROPORTIONAL to your RATE
of turn. Obviously, coupling the gyro frame to a pointer
allows one to display turning rate on the face of the
instrument. This makes your turn-coordinator an INERTIAL
REFERENCE device.
There are important differences between Turn Coordinator
(airplane roll axis displayed on face) and the older Rate of
Turning (single, fat, upward pointing needle) instrument.
A Turn Coordinator's gyro axis is canted up from horizontal
by a few degrees to make it slightly sensitive to ROLL.
The old Turning Rate instrument is insensitive to roll rate.
This enhancement of instrument makes for more accurate
"no gyros" maneuvering.
(4) GRAVITY REFERENCE is a important part of the nav sensor
system but certain things must be known for gravity to be
a useful parameter for display. First, you cannot be
changing direction (turning) because a coordinated turn can
present a new and totally false picture of where the center
of the earth is lies with respect to your aircraft.
This is why the needle and ball are combined on our panels.
To deduce that wings are level, both needle (turn rate)
and ball (gravity vector) values must be zero.
Sources for error: The ball in our turn coordinators
is about as free from error as any device in the airplane
. . . when the ball is centered, you can believe that
the gravity vector is parallel with your yaw axis. Now,
if the rate of turn is ALSO zero, you can believe that
the gravity vector represents an accurate position with
respect to the center of the earth.
--------------------------
With a little thought you can see that as pilots we use a
smoothly integrated combination of local, gravity, and
inertial inputs combined with other senses (sound and stick
forces) to maneuver an airplane. Gravity and inertial senses
in your butt and inner ear are subject to the same confusion
factors that plague instruments. Loss of local sensing is
best replaced with stable platforms (iron gyros) but it's
very possible to get by with a manual integration of
inertial and gravity vector data (needle and ball) to keep
the dirty side down and the pointy end forward.
Obviously, devices offering only gravity referenced displays
cannot be depended upon as sole indicators of aircraft attitude.
THE FUTURE:
A number of manufacturers are offering solid state rate and
linear accelerometers. These may be combined with "slow"
sensors like GPS and LORAN to mathematically deduce everything
you ever wanted to know about where you are, where you're
going and whether or not you're right side up. The trick
is to design a system that can take data from a variety of
simple sensors, deduce what is needed to know and display it,
and be capable of falling back on less accurate but equally
stable modes of flight in case of certain sensor failures;
an electronic reversion to needle-ball-airspeed-position
mode from an attitude-position mode. The micro-controller
combined with the new families of low cost sensors will
make it all possible.
WRAP UP:
Low cost "attitude" displays are not. The turn coordinator
is probably the simplest, most reliable alternative to iron
gyros. On anything less than a CAVU day, the T/C should be
a dispatch item (don't go flying without it) and it should
enjoy two power paths for energy to keep it running. The
critter won't do you any good unless you are practiced in
it's use. Shoot some non-precision approaches under the
hood (make up your own GPS approaches to some little airport
. . or even a stretch of straight highway . . . you just
need some kind of runway-like ground reference) and get
someone to ride shotgun for you several times a year.
It's fun, it's enlightening and it could help you avoid
bending an otherwise perfectly good airplane!
Regards,
Bob . . .
AeroElectric Connection
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(o o)
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