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From: George Huxtable (no email)
Date: Fri Oct 08 2004 - 09:17:24 EDT
Thanks to Ken Gebhart for an interesting account of the effect of "Dutch
roll" of aircraft on bubble sextants.
I know nothing about aircraft stability or control systems. But it seems to
me that to affect a bubble sextant must require something more than simple
rolling itself; perhaps it's the CONSEQUENCES of the rolling that affect
the bubble.
Consider a navigator taking a bubble sextant reading, presumably from
somwhere on the aircraft's centre-line. He sees the horizon moving up or
down, with respect to the wingtips, by a degree or so, every minute or so.
But to the navigator, that's no more than tilt, which doesn't affect the
levelling of his bubble with respect to the Earth's gravity.
As I see it, to affect the levelling, there would have to be either-
1. Some yaw in the aircraft's direction, which caused it to deviate from
its straight-line path, a conequence of the roll itself, perhaps, or else a
consequence of correcting the roll by adjusting the control surfaces. This
curved path would then be related to a sideways force accelerating the
liquid to one side or the other, causing the bubble to tilt to left or
right alternately, as the aircraft oscillated, with the same frequency as
the roll oscillation. Not the tilt, not the resulting yaw itself, but the
effect of the resulting deviations of the aircraft from its straight-line
path, and the sideways accelerations necessary to return it to its original
course.
2. (Perhaps less important, in my view.) Some speeding or slowing of the
aircraft as a consequene of the roll, or as a consequence of correcting the
roll by adjusting the control surfaces. This would cause the liquid surface
to tilt in the fore-and-aft direction. Because tilt of the aircraft to the
left or the right must have identical effects on aircraft speed, if this is
a relevant mechanism, I would expect it to shift the bubble at twice the
frequency of the roll oscillation.
Any up-and-down motion of the aircraft, deviating from its straight-line
path, would not affect the tilt of the bubble, except if the speed were to
change as a consequence.
To summarise, it's only the horizontal ACCELERATIONS, not the tilts, which
will directly affect a sextant's bubble, but the tilts (and their
correction) can be the CAUSE of the accelerations.
Have I got that right, I wonder?
George.
=======================
Ken Gebhart wrote-
The purpose of averageing
>mechanisms on aircraft sextants is to compensate for what is called "dutch
>roll" which every aircraft is subject to. It is a sort of wallowing motion
>which anyone on even a 747 will have experienced, especially if seated in
>the rear section. A navigator using an aircraft sextant will see a star, for
>instance, begin to rise, and have to chase it with drum movement, and then
>begin to fall. Usually this motion will cause him to go up maybe a full
>degree in elevation both up and down. Obviously, if he were to take a "spot
>shot" anytime during this excursion, it would be subject to up to a degree
>of error. However this motion is quite regular, and repeats its cycle every
>minute or so. Thus, averaging over a standard period of two minutes was
>deemed to be sufficient to give him an average reading during the sighting.
>The need for this averaging has nothing to do with inaccuracy of the
>sextant, or the speed of the aircraft, but only for compensating for the
>dutch roll.
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contact George Huxtable by email at , by phone at
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