|
If I
ever decide to fly in circles for a long time, I will be sure to buy a
Chelton.
Chuck Jensen
Rick writes:
"... some
EFIS systems (especially the inexpensive ones) derive their attitude depiction
data from GPS and pitot data. In other words, if you lose GPS or your pitot
(blockage, icing, etc.), your ATTITUDE depiction will become INACCURATE.
<snip> The most startling thing that I found out during my research is
that the FAA CERTIFIED, Chelton system does have the problem of inaccurate
attitude data with the lost of pitot input. Their manuals state that the
attitude indication can be off as much as 2.5 degrees when pitot data is not
available....>>>
2.5 degrees ??!!??!!
Horror of horrors!
Rick's post proves two things. One is that a little
knowledge is a dangerous thing. The other is that if you start with a
conclusion you can find facts that "support" said conclusion.
Now for
some real facts and reason. Most mechanical AH gyroscopic instruments rely on
the gravity to determine which way is up. The influence of this "gravity
vector erection" is small compared to the forces that maintain the gyroscope
"rigid in space". Imagine a pendulum hanging in a jar of honey. If
you roll the jar, the pendulum will point in a new direction for a while
but will, over time, return to pointing straight down. Likewise, if you roll
into a coordinated turn with a mechanical gyro AH and hold the turn for
several minutes the AH will eventually show level flight when you are still
turning. Your inner ear also has this property, but for different
reasons.
When your are in a turn the centripetal force vector and the
gravity force vector combine to create an apparent gravity vector, which is
not vertical (with respect to the earth).
Mechanical gyro AHs, and
apparently GRT, rely on the assumption that, on average, aircraft fly straight
and level.
Now suppose you were a cleaver guy and you knew the
velocity of the aircraft and the rate of turn (degrees per second) you could
calculate the centripetal force vector and then subtract that from the
apparent gravity vector to get the actual gravity vector. You would know which
way is up regardless of how long you maintain the turn. Chelton uses a
proprietary algorithm to do this very thing and delivers the resulting
superior performance. The other benefit of this is that the Chelton
ADAHRS is the only low cost solid state AHRS that does not have a "kill
maneuver" that will cause significant attitude errors (e.g. low rate climbing
turn).
Will the Chelton attitude accuracy degrade with loss of air
data? Yes, but in it's degraded state it still has better performance that its
competitors over its entire operating range. Rick's assertion that his
GRT AHRS is superior to the CFS ADAHRS because it does not have an air data
"problem" is like saying a car without wheels is superior to a car with wheels
because it does not have a potential flat
tire "problem".
The bottom line is that the
Chelton system has undergone extensive certification testing and has
proven its performance. All the other wannabes can claim to be "just as good
or better" but until they prove it empirically you should not take
the claims as fact.
Having spent the last 14 years designing, building
and flying cockpit flight information systems and having the experience of
having thousands of these systems deployed I can say that it is never a good
idea to fly actual IMC without mechanical backups and that most experimental
EFIS systems only work great when you don't need them, although many fail that
test as well.
Regards
Brent
Regan
|