[simpits-tech] Gyro madness (Was: How 1 hour becomes 3+ (Progress update) )

Wed, 27 Feb 2002 15:51:18 -0600

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> Have you got a servo or a stepper motor controlling the bank? 
> If a servo how are you going to get 360 degree rotations out 
> of it?

I'm using servo's for both the pitch and bank. Each connected to seperate
controllers. The pitch it limited to +/- 45 degrees (theoretical limit, the
mathematical hammer I used to adjust the gauge clipped the range by a few
degrees). The bank is physically limited to +/- 90 degrees, however, initial
tests show the actual limit to be about +90/-65 degrees (at the -65 point
the servo literally keels over an extra 15 degrees).

This means the HSI is cannot handle / will behave strangely if I attempt a
roll or loop, however, the objective of the pit is more geared towards small
civilian aircraft for practice. 

I've been both impressed with the ease of design / use and disappointed by
the performance of my guages. They're a bit choppy (it appears they are only
updating about 10x / sec max - and that's with just 3 servos). The
theoretical limit is about 37x / sec (9600 bps / 8N1 / 3 chars per servo / 8
servos per controller). However, I'm currently running 2 controllers on one
serial line so that halves my max to less then 20 updates / sec which will
appear choppy. I am doing things like only updating gauges that actually
move, and limiting the number of updates per loop keep the 'important'
gauges more up to date (no sense knowing the exact amount of fuel in the
left tank if my airspeed is dropping 500'AGL while turning to final on the
runway....). I think I'm still doing something wrong in my software - will
have to check that out sometime.

As for the guts of an HSI, I'm always amazed at both the simplicity and
ingenuity that makes up mechanical instruments. For example, the first HSI I
got my hands on was a HUGE (6" diameter) circa WWII instrument that had a
'Cage' knob on the front which locked the display to show 'level flight'. I
assumed this was needed for calibrating the instrument at startup (how else
would the gauge be calibrated? ). Then I got the one I'm currently using and
it didn't have the 'Cage' knob - how in the blazes does a pilot ensure the
gyro is spinning level? Turns out, as the air passes through the gyro it
exits via 4 small ports at the bottom. These ports are partially covered by
little gates that swing freely. If the gyro is not level, one or two ports
will open up and the opposite one(s) will close (because the gates hang
down). This results in an offset of thrust from the ports which cause the
gyro to turn back to level. Very simple. Very cool! This could be very bad
if a gate were ever to stick - the gyro would start spinning, tempting you -
no, daring you to correct the spin...

This has made me start to wonder about gyro drift / errors and how
significant they are. For example, while accelerating the gates will tend to
be pulled back, this will cause the gyro to 'adjust' forward and the display
will show a higher pitch then reality. Same holds for a prolonged
uncoordinated turn, the gyro will start showing an incorrect bank. 

Presumably these correct themselves after establishing straight and level
flight, but what about the gyro compass? I imagine that when flying certain
directions, pitch the plane up/down will cause the gyro to drift left or
right, level out and the drift is corrected. What happens if after pitching
the plane up, you turn such that pitch no longer affects the gyro and then
level out. Now your gyro is 'permanently' out by a few degrees. I also noted
the autopilt feedback mechanism in my gyro compass. I would guess that when
pitching up or down, the autopilot may automatically adjust your course by a
couple of degrees - this seems like a bad thing...

And on and on and on my mind keeps wandering...

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<TITLE>Gyro madness (Was: How 1 hour becomes 3+ (Progress update) =
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&gt; Have you got a servo or a stepper motor =
controlling the bank? 
 &gt; If a servo how are you going to get 360 degree =
rotations out 
 &gt; of it?

I'm using servo's for both the pitch and bank. Each =
connected to seperate controllers. The pitch it limited to +/- 45 =
degrees (theoretical limit, the mathematical hammer I used to adjust =
the gauge clipped the range by a few degrees). The bank is physically =
limited to +/- 90 degrees, however, initial tests show the actual limit =
to be about +90/-65 degrees (at the -65 point the servo literally keels =
over an extra 15 degrees).

This means the HSI is cannot handle / will behave =
strangely if I attempt a roll or loop, however, the objective of the =
pit is more geared towards small civilian aircraft for practice. =

I've been both impressed with the ease of design / =
use and disappointed by the performance of my guages. They're a bit =
choppy (it appears they are only updating about 10x / sec max - and =
that's with just 3 servos). The theoretical limit is about 37x / sec =
(9600 bps / 8N1 / 3 chars per servo / 8 servos per controller). =
However, I'm currently running 2 controllers on one serial line so that =
halves my max to less then 20 updates / sec which will appear choppy. I =
am doing things like only updating gauges that actually move, and =
limiting the number of updates per loop keep the 'important' gauges =
more up to date (no sense knowing the exact amount of fuel in the left =
tank if my airspeed is dropping 500'AGL while turning to final on the =
runway....). I think I'm still doing something wrong in my software - =
will have to check that out sometime.

As for the guts of an HSI, I'm always amazed at both =
the simplicity and ingenuity that makes up mechanical instruments. For =
example, the first HSI I got my hands on was a HUGE (6&quot; diameter) =
circa WWII instrument that had a 'Cage' knob on the front which locked =
the display to show 'level flight'. I assumed this was needed for =
calibrating the instrument at startup (how else would the gauge be =
calibrated? ). Then I got the one I'm currently using and it didn't =
have the 'Cage' knob - how in the blazes does a pilot ensure the gyro =
is spinning level? Turns out, as the air passes through the gyro it =
exits via 4 small ports at the bottom. These ports are partially =
covered by little gates that swing freely. If the gyro is not level, =
one or two ports will open up and the opposite one(s) will close =
(because the gates hang down). This results in an offset of thrust from =
the ports which cause the gyro to turn back to level. Very simple. Very =
cool! This could be very bad if a gate were ever to stick - the gyro =
would start spinning, tempting you - no, daring you to correct the =
spin...

This has made me start to wonder about gyro drift / =
errors and how significant they are. For example, while accelerating =
the gates will tend to be pulled back, this will cause the gyro to =
'adjust' forward and the display will show a higher pitch then reality. =
Same holds for a prolonged uncoordinated turn, the gyro will start =
showing an incorrect bank.

Presumably these correct themselves after =
establishing straight and level flight, but what about the gyro =
compass? I imagine that when flying certain directions, pitch the plane =
up/down will cause the gyro to drift left or right, level out and the =
drift is corrected. What happens if after pitching the plane up, you =
turn such that pitch no longer affects the gyro and then level out. Now =
your gyro is 'permanently' out by a few degrees. I also noted the =
autopilt feedback mechanism in my gyro compass. I would guess that when =
pitching up or down, the autopilot may automatically adjust your course =
by a couple of degrees - this seems like a bad thing...

<P><FONT SIZE=3D2>And on and on and on my mind keeps =
wandering...</FONT>
</P>

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