Gary:
Thank you for the post.
Why do you do CelNav? Why do we like the Bygrave? Why do some of us think about sextants? Most, I think, just like the intellectual challenge. Many of us might never go to sea. And if they did they'd be crazy not to take the GPS along. And I'd never fly in a 747 that depends on CelNav.
I also like making things, and making them well. A usable gyro, mechanic or otherwise, would exceed my capabilities by far, a bubble spirit - even a DoubbleBubble - would not. And I would be thrilled if I could build a bubble spirit that is significantly more stable than the ones known sofar -- no matter the practical use.
As far as practical navigation: gyros have such superior properties which never could be matched by any bubble spirit. They can seek North for once, a spirit level never could.
Do I sense,
though, that you have come to agree with the concepts of Bush's idea?
I'd be delighted.
H
--- On Thu, 8/20/09, Gary LaPook <glapook@pacbell.net> wrote:
From: Gary LaPook <glapook@pacbell.net>
Subject: [NavList 9571] Re: The development of bubble sextants
To: navlist@fer3.com
Date: Thursday, August 20, 2009, 2:33 AM
The Bush patent is intriguing since is needs no power and, though
complex, is is much simpler than the devices actually made to perform
its function, so there must be a problem with the concept somewhere. In
the same era as the Bush patent, Sperry was creating gyro instruments
for airplanes and, eventually, the autopilot. These all used gyroscopes
to sense level. Then later, in the 1950s, inertial navigations systems
were invented for use by our submerged nuclear subs which used
gyroscopes, not the Bush concept. Then in the '60 inertial navigation
systems were created for the Boeing 747 which finally got the sextant
mounts removed from transocean aircraft. Needless to say, the B-747
used a plethora of gyroscopes since it had three completely separate
inertial nav systems for redundancy. Gyroscopes are still in all
aircraft although the most sophisticated now use "ring gyroscopes" with
no moving parts, only light running around a circular light pipe.
Gyroscopes also went to the moon.
It is also quite likely that the sophistication of the Bush device
wasn't worth the effort. The common (read cheaper) bubble sextants as
well as the more expensive and less common pendulous mirror sextants
provided the all the accuracy that was practical and needed so there
was no reason to "polish the cannonball" as we say in the artillery.
I don't know what you are trying to accomplish. Many inexpensive bubble
sextants are available on ebay and provide one or two minute accuracy
when used on the ground and should be accurate enough to find your
backyard. If you need greater accuracy then use a marine sextant with a
bowl of mercury (gasp...from those worried about HAZMAT) or some other
less useful liquid to establish level and you will get accuracies well
below one minute. If you need maximum accuracy then get a mercury
theodolite (gasp...mercury again) and you will get accuracies measured
in seconds.
Or, just buy a hundred dollar GPS.
gl
Hanno Ix wrote:
George:
You wrote below:
But now, with the shunting
engine, start moving the flatcar backwards and
forwards. The pivoted mirror
will respond to the acceleration forces, and it
will start swinging. To
reduce that effect, you could add a sort of dumbell
arrangement, like a see-saw,
with its centre of gravity aligned with the
pivot. tThat would act like a
trapeze artist's balance pole, to increase the
moment of inertia. Now you
have a system with a long period. Its response to
short-period perturbations
will be reduced by the enhanced inertia. You can
make the period as long as
you like by increasing the dumbell weights, or
decreasing the weight and
spacing of the ballast.
And you are totally right - so far. However: you cannot accelerate
forever in the same direction, with the same amount. Pretty soon you
will reach top speed and then there will be no more acceleration -
other than gravity! The mirror will dampen its oscillations and go back
to equilibrium - i.e the horizontal position - while the train keeps
going with constant speed from then on. Until, of course, the train
slows down with neg. acceleration - causing the opposite movement of
the mirror.
The trick is to find out what in the particular application the biggest
acceleration is, and how long it can possibly last. I bet shipbuilders
can give us rather good estimations of the roll/pitch frequencies of
vessels from size boat to
size oil-tanker. I guess, as
I said before, we are talking about 1/10 Hz to 1 Hz with exceptions on
both sides.
If the cut-off frequency of the lowpass is low enough, say 1/10 of the
roll/pitch frequency, the acceleration will barely have caused the
mirror to move before accelerations in the opposite direction occurs,
thereby cancelling almost the first one.
Howver, there is one more problem: In our design the mirror will be
deflected while the
acceleration goes on. If the engine not simply accelerates, but it does
so differently with each expansion of the steam, you will see the jerks
in the mirror movement. Solution: put another lowpass filter in front
of the first one. Ugly, but easy to understand: let the pivot of your
pendulum slide softly on a rail that is attached to the car and
oriented in the direction of the accelarations, here front to back.
Connect the pivot with a soft spring to the point where is was fastened
to before. Make sure this secondary system is sufficently damped.
Now, short jerks cannot be transfered to the pendulum anymore, only
those slower accelerations will be transferred that really affect the
overall speed of the train. The mirror will move much slower and in a
mode commensurable with the train movement, and it will, in average,
maintain now its pointer much closer to the gravity vector.
I think I correctly described the line of thought that leads from the
original spirit level to the Bush patent.
Please, forgive my lapses of precision in my explanations. I hope you
can follow the argument nevertheless.
Regards
H
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