A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: averaging devices on sextants
From: Ken Gebhart
Date: 2004 Oct 8, 21:00 -0500
From: Ken Gebhart
Date: 2004 Oct 8, 21:00 -0500
George is absolutely correct in his analysis of horizontal accererations affecting the bubble. I did not completely describe the phenomonon of dutch roll completely. It is a combination of roll and yaw that exists. This is more pronounced in swept wing airplanes at high altitudes such as the 707 and upwards (and for George, the Comet). These airplanes have yaw dampers installed to minimize yaw excursions. In the early days when this was not as fully understood, there was one instance of yaw damper failure that resulted in a 707 that actually that slung its outboard engine pods completely off of the airplane. George is also correct about speed variations. Going back to my previous description of taking "spot shots" by trimming the airplane and taking hands (or autopilot) off, the requirement is that the heading and speed of the airplane does not change during the sighting. Ken ----- Original Message ----- From: "George Huxtable"
To: Sent: Friday, October 08, 2004 8:17 AM Subject: Re: averaging devices on sextants > 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. > > ================================================================ > contact George Huxtable by email at george---.u-net.com, by phone at > 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy > Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. > ================================================================ >