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aerobubba(at)earthlink.ne
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 Posted: Mon Jun 19, 2006 3:27 am Post subject: TC vs T&B |
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Hi Bob-
RE: ok you guys,
i dont understand the difference between a turn and bank indicator and a
turn coordinator. could someone please explain. thanks in advance, bob noffs
As a practical matter, the T&B needle displays yaw rate only, and a TC will
show yaw and / or roll. A result of this is that when flying in other than
smooth air, the TC will start to dance around and be much more difficult to
use well than the much more steady TC needle.
Glen Matejcek
aerobubba(at)earthlink.net
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dennis.glaeser(at)eds.com
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| Posted: Mon Jun 19, 2006 6:40 am Post subject: TC vs T&B |
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One of the main reasons the TC was created is to provide immediate
positive feedback for recovery from unusual attitudes.
A T&B indicator remains pegged as long as the turn rate is at or above
the maximum rate it indicates. Many years ago, researchers found that
pilots using a T&B would initially respond correctly to recover from an
unusual attitude with a high rate of turn. But quite often they would
not wait long enough for the turn rate to decrease and unpeg the needle,
and would subsequently reverse their initial response, aggravating the
unusual attitude. The conclusion was that the pilots needed some
positive feedback to let them know they were doing the right thing.
So the TC was created with it's canted gyro so that, no matter how high
the rate of turn, the pilot received positive feedback when the proper
recovery control inputs were used. It was credited with dramatically
improving partial panel unusual attitude recoveries.
If you want to do 'precision' partial panel flying, the T&B is the way
to go. That is what it was designed to do, (way) back when it was the
only gyro instrument in the panel. The TC was designed for a different
requirement.
If you don't religiously practice needle, ball and airspeed flying, and
you want something to help you stay right-side up in an emergency, I'll
suggest that the TC is the way to go.
Dennis Glaeser
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William.P.Dube(at)noaa.go
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| Posted: Mon Jun 19, 2006 6:57 am Post subject: TC vs T&B |
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The difference is the angle of the gyro gimbal inside. For the
traditional turn and bank indicator, the gimbal axis is in line with the
direction of flight (like the crankshaft axis.) In the Turn Coordinator,
the gimbal axis is inclined so that a bit of the roll information is
combined with turn information.
http://en.wikipedia.org/wiki/Image:Turninst4.jpg
Bill Dube'
Glen Matejcek wrote:
| Quote: |
Hi Bob-
RE: ok you guys,
i dont understand the difference between a turn and bank indicator and a
turn coordinator. could someone please explain. thanks in advance, bob noffs
As a practical matter, the T&B needle displays yaw rate only, and a TC will
show yaw and / or roll. A result of this is that when flying in other than
smooth air, the TC will start to dance around and be much more difficult to
use well than the much more steady TC needle.
Glen Matejcek
aerobubba(at)earthlink.net
|
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nuckollsr(at)cox.net
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| Posted: Mon Jun 19, 2006 7:19 am Post subject: TC vs T&B |
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At 07:23 AM 6/19/2006 -0400, you wrote:
| Quote: |
<aerobubba(at)earthlink.net>
Hi Bob-
RE: ok you guys,
i dont understand the difference between a turn and bank indicator and a
turn coordinator. could someone please explain. thanks in advance, bob noffs
As a practical matter, the T&B needle displays yaw rate only, and a TC will
show yaw and / or roll. A result of this is that when flying in other than
smooth air, the TC will start to dance around and be much more difficult to
use well than the much more steady TC needle.
|
Yes, the T&B is a pure yaw rate sensor and indicates
rate of turn about the yaw axis only.
A Turn Coordinator has the axis of the rate gyro canted
off-vertical by some amount . . . typically 30 degrees.
This drops the sensitivity to yaw by cosine of 30 (sensitivity
factor of .86) and inserts a sensitivity to roll by the sine
of 30 degrees (sensitivity factor of 0.5).
This feature was found useful because single axis wing
levelers performed poorly in terms of ride quality
when they were made privy only to yaw rate. This is because
with feet off the peddles, a turn is preceded by some
component of roll. If roll could be held at zero, then
a yaw component would be held at zero too. Ride quality
went up markedly when the autopilot was capable of anticipating
impending yaw by making it also privy to present roll
component. Of course, one could design an autopilot with
two rate sensors, one for yaw and one for roll. Then signals
from the two could be mixed electronically at what ever
proportions made for the best ride quality (like .86/.5) and one
might preserve the pure yaw rate sensing feature of
the T&B as both pilot display and yaw rate sensor for the
a/p.
However, since the goal was to produce the maximum
performance for minimum cost and parts count, making
the rate sensor sensitive to both components of rotation
by canting the axis of the gyro, the desired ratio of
display sensitivity could be achieved by controlling
the off-axis angle with a single sensor.
As it turns out, a human pilot's ability to smoothly
control turning rate benefited from the same mixing
of sensitivities in the same display. This discovery
ushered in the era of low cost, single sensor autopilots
that were really two-axis devices due to the mechanical
mixing of roll and yaw stimulus.
I'm not in a position to debate the value of one
display over the other for the purposes recovering from
an upset condition. Plenty of articulate debaters have
offered their arguments for one side or the other.
It may well be that polluting the pure yaw display
with a component of roll makes recovery from upset
more problematic . . . but the design goal of folks
who were pondering the low cost a/p problem over 40
years ago was to prevent upset from happening in the
first place.
This meant that optimizing the display for both
automatic and manual maintenance of heading was a
useful thing to do. It reduced pilot fatigue, improved
quality of ride for passengers and reduced probability
of upset when two-axis sensitivity was combined onto
the single display.
I used to ride safety pilot for one of my co-workers
at Videmation who would do his obligatory currency
approaches with the gyros covered. He argued that
to be truly 'current' one should be skilled at the
most challenging presentation of the task. I've
watched him shoot many approaches (without timing
assists from ATC) using needle-ball-airspeed and
mag compass.
Today, it's quite possible to craft a small servo
that contains a solid state rate sensor, a micro-
controller and a GPS engine all in one package.
Hook up 14v through an on/off switch. Attach mechanical
output to the aileron mechanism. Install and attach
GPS antenna. Install three wires to a pair of
push-buttons on panel labeled RtTurn and LtTurn.
A bill of materials for such a product could be
under $100 which means they could probably retail
for under $1,000.
When ON, device holds present course +/- one
degree. Tap one of the push buttons, you get
one degree increment or decrement of present
course. Press and hold either button and you
get standard rate turn in that direction. This is
95% of everything I ever wanted an a/p to do.
With two such devices installed in a system crafted
for failure tolerance, one could easily demonstrate
an ability to fly any maneuvers called for by ATC
or your instrument flight plan without visual
references to any panel displays for attitude. Further,
you would have system reliability equal to or greater
than any of those mandated by the regulators.
This is the vision of possibilities for the future
that are practical and attractive only because of
the availability of low cost, solid state rate
sensors, very simple stepper motors, low cost GPS
engines, and jelly-bean micro-controllers. And
yes, the rate sensor would be canted off-axis
by some amount that offers best ride quality
with a minimum of software.
This would elevate Mooney's vision for flight
safety in IMC to new heights for a fraction of
the costs that were required to implement Positive
Control 40 years ago.
Debates about "requirements" for flight in IMC
are a separate issue that has lost sight of the
mission and was never plugged into the quantum jumps
in capability and value that consumer products enjoy.
By the time committees crafting new requirements
can quit arguing about it and publish new rules,
the technologies they're considering are already
old-hat.
Yes, there will always be "requirements" to be met
so that the pests will go away. After that, you can
move on with what ever personal goals you have for
improvements that can stand well above what's required.
Bob . . .
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BobsV35B(at)aol.com
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| Posted: Mon Jun 19, 2006 8:34 am Post subject: TC vs T&B |
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In a message dated 6/19/2006 9:42:44 A.M. Central Standard Time,
dennis.glaeser(at)eds.com writes:
One of the main reasons the TC was created is to provide immediate
positive feedback for recovery from unusual attitudes.
Good Morning Dennis,
Your explanation of the origin of the canted gyro instrumentation may have
merit, but it is certainly NOT the way my ancient brain recalls the facts.
I am not claiming my version is the only correct one, but here goes!
The canted gyro was first used in an autopilot designed by a Chicago based
college professor. It was never placed in a high production, though quite a few
units were built and later sold for parts by a wholesale house in southern
Michigan.
It used a stock T&B mounted at an angle. I believe his first choice was to
use a forty degree angle. Later adopters of the idea used other angles.
Brittain and Century were among the many autopilot manufacturers that
adopted the Canted gyro for their low cost single axis autopilots. When some of the
Century engineers went off on their own to start S-Tec, they used the canted
gyro and the company still does so.
After a couple of years of production, the idea was promoted that if it
worked to make an autopilot smoother and more responsive, why not make a
presentation of the same information directly to the pilot?
Some experimentation was done and several different visual presentations
were tried. The result was tested and approved by the FAA to be substituted for
the standard T&B. Doing so saved panel space and reduced the number of
gyroscopes needed for flight.
Since it had already been approved as a substitute for the T&B, it was
decided to build the TC as a stand alone instrument even though it was not serving
a sensor for an autopilot.
I really do not believe that there was anyone searching for an improvement
over the T&B. It just became available because of it's use as an autopilot
sensor.
I installed them in all of my trainers because it was the "latest, finest
and fastest" available.
After a few years, I noted that pilots who had trained using the TC had more
difficulty handling partial panel on their annual IFR check rides than did
those who had been trained in the days of the T&B.
In addition, as the years went by, our industry started to have more and
more accidents where the pilot had lost the airplane following the failure of an
attitude gyro. It was obvious that the pilots were having difficulty flying
partial panel.
That did not seem to be a problem in the days when proficiency in needle,
ball and airspeed flight was common.
I realize that the increase in accidents may have had some other cause and
the fact that it all started happening after the industry started to switch
over to the TC may be just coincidental, but it COULD have been a factor.
I believe it was.
Other anomalies came to light and I now feel that switching to the TC
instead of the T&B was a BAD idea.
Too many reasons to go into on an electronically oriented list, but it is my
hypotheses that the problem is primarily one of presentation.
The TC looks too much like an attitude gyro. When one's mind is confused, it
is difficult to accept that the instrument is correct and the mind is wrong.
Autopilots don't have that problem, but most of us humans do.
The T&B looks like nothing else on the instrument panel. It tells us just
one thing. Either the airplane is turning or it is not. It never confuses us as
to which way is up or whether or not the wings are level.
If the aircraft is not turning, we will survive.
It makes no difference which way we THINK is 'up'.
If we stop the turn we will survive.
Happy Skies,
Old Bob
AKA
Bob Siegfried
Ancient Aviator
Stearman N3977A
Brookeridge Air Park LL22
Downers Grove, IL 60516
630 985-8503
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RDarrah(at)austin.rr.com
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| Posted: Tue Jun 20, 2006 10:05 am Post subject: TC vs T&B |
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I didn't think that how they worked made much difference, the big difference
is the presentation to the pilot. The turn nedle shows the direction and
rate of turn. Very hard to misinterperate. If it points to the left, you
are yawing (turning) to the left. If you made one with a canted gyro, it
would be just as usful to the pilot.
The turn coordinator looks too much like the attitude indicator. It, unlike
the attitude indicator, showes which way you are turning by showing a
banking airplane, while the attitude indicator showes your relation to the
horizon. These presentations end up being exactly opposite to each other
leading to easy misinterpratation of the turn cooridnator.
I'll take the turn needle every time.
New Bob (as apposed to Old Bob and the other Bob's)
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dennis.glaeser(at)eds.com
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| Posted: Tue Jun 20, 2006 11:06 am Post subject: TC vs T&B |
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Hello Old Bob,
I don't remember where I found the explanation I posted - I came across
it when I was teaching ground school at Parks College (in the late '70s
through early '90s). Saying 'one of the main reasons it was created' is
undoubtedly overstatement/embellishment based on inaccurate
recollection, but I clearly remember that the main point was the
'advantage' of the canted gyro being positive feedback for the pilot in
a high-rate turn, which is why it stuck with me. It makes sense that
it's roots are in autopilot design. Sounds like the stuff I remember
was part of the justification (or sales pitch) for making it a
stand-alone instrument.
Dennis Glaeser
-------------------------------------
Good Morning Dennis,
Your explanation of the origin of the canted gyro instrumentation may
have merit, but it is certainly NOT the way my ancient brain recalls the
facts. I am not claiming my version is the only correct one, but here
goes!
<snip>...
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BobsV35B(at)aol.com
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| Posted: Tue Jun 20, 2006 1:28 pm Post subject: TC vs T&B |
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In a message dated 6/20/2006 2:09:10 P.M. Central Standard Time,
dennis.glaeser(at)eds.com writes:
Sounds like the stuff I remember
was part of the justification (or sales pitch) for making it a
stand-alone instrument.
Dennis Glaeser
Good Afternoon Dennis,
As I mentioned earlier, when the instrument first became available, it
sounded like a good idea to me. It was only after a few years of observing the
results that I changed my mind.
Thanks for the additional information you have provided.
Do Not Archive
Happy Skies,
Old Bob
AKA
Bob Siegfried
Ancient Aviator
Stearman N3977A
Brookeridge Air Park LL22
Downers Grove, IL 60516
630 985-8503
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