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 7, 23:08 -0500
From: Ken Gebhart
Date: 2004 Oct 7, 23:08 -0500
Gentlemen, Sorry for the late reply as I am traveling. 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. The idea of averaging sextant readings using a marine sextant, (ie. instead of using a bubble), really has no basis because acceleration has no effect on it. To digress, the subject has additional historic interest in that dutch roll is really a combination of having either an autopilot trying to hold the aircraft straight and level, or having another pilot trying to do so. In the 1930s Wiley Post and Charles Lindbergh made a Pacific crossing during which they were evaluating a bubble sextant for air navigation. On their return, they recommended an averaging device be added. Probably, Charles told Wiley to "hold the airplane straight while he took a sight". However, had he told Wiley to trim the airplane straight and level, and then to not touch the controls, he probably could have taken acurate spot sights by himself. and not have been hampered by dutch roll, or accererations induced by Wiley. Ken Gebhart ----- Original Message ----- From: "Paul Hirose"
To: Sent: Thursday, October 07, 2004 6:43 PM Subject: Re: averaging devices on sextants > Alexandre Eremenko wrote: > > > > I am trying to imagine how a "mechanical averaging" > > device could possibly work, on what principle. > > The U.S. military A-12 bubble sextant (early 1940s) has a thumb lever > which puts a pencil mark on the edge of the plastic altitude setting > knob. You push the lever periodically during the observation run. When > the run is complete, turn the knob so the pencil is at the midpoint of > the marks. Then read the "average" on the vernier altitude scale. > > Three identical knobs are provided with the sextant so you can use > them in quick sequence for a 3-star fix. > > The AN5851-1 (also called the A-14) sextant of WW2 is much more > sophisticated. A clockwork mechanism in the base accumulates the > average on a counter (like an odometer) during a 2-minute run. The > manual explains the averager's operation in detail. I don't have one > of these sextants so I haven't paid close attention to what the manual > says, but I can send it (5 MB PDF) if you want. > > One disadvantage of this averager is that the length of the > observation is fixed at 2 minutes. The mechanism automatically stops > and drops a shutter across the field of view. If a cloud covers the > celestial body before that, the average will be bad. > > In the A-15 (same sextant with a different averager) that limitation > is removed. A ball and disk integrator keeps a continuous running > average. > > The principle of the integrator is simple. Imagine a tennis ball with > a pencil through its center. Sharpen both ends of the pencil and hold > it between left and right index fingers so the assembly can spin > freely. With the pencil horizontal, let the ball touch the center of a > rotating phonograph turntable. It doesn't rotate. If you shift it left > of center, it rotates one direction. Right of center, the ball rotates > the opposite direction. > > In the sextant averager, winding the clockwork drive positions the > ball at the center of the disk. Also, a pointer which records ball > rotation is reset so it points to a zero mark. > > When you press the start trigger, the disk begins turning at constant > speed. At the same time, a clutch connects the altitude knob to the > ball, such that knob movement above or below the starting altitude > moves the ball to one side or the other of the disk center. > > When you stop the averager, the disk stops turning. The altitude knob > connects to a different part of the averager, so now it directly > rotates the ball. Turn the knob to return the ball rotation pointer to > the zero mark. This removes any accumulated ball rotation, and also > sets the sextant to the average altitude of the observation. > > A dial on the averager displays half the duration of the observation. > You add that value to the start time to obtain the mid time of the > observing run. > > Periscopic bubble sextants had the same kind of continuously > integrating mechanical averager. I've also seen them with digital > electronic averagers. Those averagers were probably retrofitted years > after the sextants were manufactured. > > > There are plenty of bubble sextants available cheap on eBay. But the > sellers are generally clueless on operating the instruments. If you > want the averager tested, you'll have to walk the person through the > process! >