A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Ken Gebhart
Date: 2014 Dec 4, 22:35 -0600
Hanno and Gary,
This is really very interesting. Chichester tried the Booth bubble (no sophisticated averaging mechanism) and failed, so reverted to standard marine sextant and low altitude, natural horizon sights.(with some difficulty as described). If you really want to do the Chichester test, perhaps you should have a go with marine sextant at low altitude, using natural horizons, or is this too dangerous? Fun though? Bet your modern altimeters are a lot better than Chichester’s broken relics?
Still think the Bygrave and possibly (to be proven) Brown-Nassau will turn out to be the best systems but the Doniol Hv quick method looks very good to me.
Let the contest begin!
Still like the idea of an annual LaPook prize for this (the most slick method, goose-shit or otherwise, more politely expressed.)
Await deep thoughts/ comments from the Sages.
I admire your guts! No, I was not planning on doing CelNav in a glider but trying to use my flying experience to understand what you did.
In the past I mistook your enthusiasm for the Bygrave as a hint of a plan to use it in a plane and I could not understand how one could possibly focus back and forth - mentally and literally - between piloting and navigating. It is still a mystery to me how Chichester did it. Maybe I'll find details in the notes you send me.
Some friends of mine and I, we are in a discussion about the dynamics of the bubble.
It seems to us, the bubble is only the best of bad tools to create a resemblance of an AH.
You will remember the discussions we had with George Huxley. The averager can do only a limited job of evening out, i.e. reducing the standard deviation of noise. First, you need to square the number of shots to reduce it by half. Secondly, the timing of the shots also matters since for true averaging you want to eliminate correlation within the noise and also any periodicities in the bubble movements themselves. This is a challenging engineering problem. Of course, I would be interested finding out how gyro sextants behaved in this regard - do you have any reports on that?
Actually funny, how easy it has become to model that with an AH based on modern gyro IC's that you find in model helicopters - life is simply too short.
Thank you for this detailed description.
On Wed, Dec 3, 2014 at 11:56 PM, Gary LaPook <NoReply_LaPook@fer3.com> wrote:
I'm a licensed glider pilot and I don't see how one could operate a sextant is the tiny cockpit of a glider. A Cessna has a much larger cockpit with lots of headroom so it not difficult using an octant. Most times I have been solo so the copilot seat was empty so I could place my stuff on it. Sometimes the copilot seat was stowed in the baggage compartment and a ferry tank put in its place so I could put my stuff on to of the tank. What really helped was the Polhemus which has a very compact plotting surface and is easily handled in a small cockpit. I started with the HO 249 on the seat next to me but later I programmed an SR-49 calculator to do the math. It would calculate the celestial data, determine the center of the "cocked hat," after advancing the earlier shots to the fix time. Then, based on the course and distance from the previous fix and an input for the heading and true airspeed, it calculated the winds and used these winds to compute the heading to use to the destination, the groundspeed and the ETA.
To take a sight with an averaging octant you preset it to the Hc, establish the heading, trim for constant speed, and at the planned starting time for the shot you look through the octant, line up the star in the bubble and push the start button. You then chase the bubble around with the star by moving the altitude knob, Since these are random accelerations the averager takes care of them. Airplanes and gliders are both stable in pitch so they can be trimmed to maintain a constant airspeed but they are both exhibit spiral instability so they will drop a wing and start turning.
Every 10 or 15 seconds you take the octant away from your eye, level the wings back on heading and then resume shooting the star. If the heading variations go both ways then the averager will deal with the deflection of the vertical caused by the changes in headings. You want to end up on the same heading at the end of the shot that you started with and this will most likely cancel out the variations by the action of the averager. If you are having real problems with the airplane staying on heading during the shot then turn the plane to put the object directly in front of you, two minutes off course will not cause a problem. Since the change in heading causes the bubble to slosh towards the wingtips, it results in a deflection of the vertical athwartship which causes errors in shots taken out by the wingtips but doesn't affect shots taken ahead or astern. We discussed these acceleration errors in the last couple of weeks and how to correct for them. So by moving the star to in front of you then the heading changes (wander) error goes away. There is an airpeed change acceleration error that DOES affect shots on bodies ahead of you but the plane is stable in pitch and airpeed and, even if the airspeed does change, the magnitude of this error in much less than the turning error. .
From: Hanno Ix <NoReply_HannoIx@fer3.com>
Sent: Wednesday, December 3, 2014 9:16 AM
Subject: [NavList] Re: Hughes explanation of Chichester's navigation.
first, thank you for this copy. This will require a long study.
May I ask a question a concerning the mechanics of CelNav
while flying? I flew gliders for a long time so I may have some
appreciation of the process.
Both, operating the sextant or the Bygrave requires 2 hands, right?
So how does one operate the yoke in the meantime to keep the plane
in straight and level flight?
Observing the plane's attitude and looking through the sextant at the
same time seems difficult but you probably trained yourself to do that.
I can also see you writing on a kneepad for the calculations.
On Tue, Dec 2, 2014 at 3:01 AM, Gary LaPook <NoReply_LaPook@fer3.com> wrote:
(img/129498.chichester navigation.pdf: Open and save or View online)
(img/129498.chart to lord howe typed.pdf: Open and save or View online)
(img/129498.chart to norfolk.pdf: Open and save or View online)