Navigational Computer MKIII

There were two varieties. The type in my pics above was the navigators type, you can see the remains of two of the four feet on the top pic, where it lay on the chart table. This was because most use was done on the circular window where wind speeds and direction were fiddled. One then turned the contraption over to use the circular slide rule on the bottom to calculate times and distances etc., as well as determining true airspeeds and heights with the little scales in the windows.
The other type had that section on the hinged lid and with leg straps, for use by a pilot who had it strapped to his knee. These were in the main used by the PR pilots, I cannot begin to comprehend how these men managed 4 hour plus sorties over a very hostile continent, and maintained accurate navigation in the very confined space of a Spitfire cockpit.
There was a later variation that came with the jet age. In this the cloth band was replaced with a plastic slider which could be reversed to give a greater range of speeds. This slid through another plastic gimmick with a 360 rotating window to do the same job.

As Bill has pointed out above, look for the serial number. Most AM stuff has the serial in for format / which often helps date things.

Mine is slightly different, having a hinged section which contains a small note pad for navigation notes / observations, actually has some notes written on pad, but I THINK in Biro? so post ww11 used?
It is an early, all metal one with full makers name-address ‘Henry Hughes and son ltd London’, it also has ref no. 6b/145. the RAF reference, also AcrownM, and ser no. 4144/39, so its early, 1939
It has the short all metal, tube form pencil holder, the later ones had a Bakalite holder and its rare to find one that isn’t chipped!
Originally it would have had leg straps, but I THINK personally that on some of these, they were probably removed unofficially? when the larger heavy bombers came in?
It has TWO control ‘knobs’ on the right edge, one of which turns an inner ring which has compass points on and ‘True Course’ at the top.
I think the later ones had all manufacturing details, ADDRESSES! removed as other RAF equipment did for war time use?
For some reason, the ‘plastic’ window is loose and doesn’t seem to fit, poss shrunk?……..might have a go at disassembly one day!
Nice item to have and display.
Oh yes, its MK111D.

If you’re feeling brave; release the eight small countersink headed screws. This will release the two top plates which in turn, release the rotating 360° ring. You can then lift out the continuous cloth belt. This should be ink-stamped on its inner face with a date of manufacture.

I have a Navigational Computer IIID*, written inside is P/O Willhouby 464 Sqdn. 464 Sqdn, as well as being RCAF, also supplied crews for the Ameins Raid and was disbanded in 1945.

That’s the beast all our modern ones came from.

I have received this reply Keith. I think it pretty much answers my initial question:

I can only give you a partial reply.
I don’t know how to establish date. The American Dalton company patented the ‘wind arc slide’ on an endless cloth belt inside a square box in 1936 and this formed the RAF MkIII computer. This was certainly used right through the war and I have not seen a date quoted for the end of manufacture.

I do know that models A,B,C and D were made to strap on the leg. A&B was for slow speeds, C&D for higher speeds. In each case the first (A & C) were based on an isothermal model of atmosphere and the second (B & D) on an ICAN atmosphere.

Model F was made without a strap but with some scales on the reverse and made to stand alone on the chart table.

Models G & H do not appear in my 1944 RAF manual of navigation so there is a slight pointer to date. H appears to be stand alone on chart table for ICAN atmosphere – I presume G is for Isothermal Atmosphere.

The * indicates that the circular scale is rotated by grasping it. In the original Mk III the scale was rotated by a knob at the side but this probably proved too complicated.

The calculations based on altitude use standard atmospheres. The Isothermal atmosphere assumes that there is no change in temperature with altitude (not sure why, because it clearly does change). The ICAN atmosphere specifies a standard drop in temperature per 1000ft. ICAN is International Commission for Aerial Navigation and dates back to 1924.

Hope this helps,

Keith Hope-Lang.

Thank You Keith

Yes thats the one.

Mine is not in as good condition as that one, nothing missing and works just fine (yes I can use it JUST) it’s a little stressed in general and been there.
Can anyone put a year to them? they must have some kind of code.

Now I remember, known as the Dalton computor and invented and developed by an American, Philip Dalton in the 1930s. Adopted by the RAF and used during WWII.
Here is the animal, note the correct English spelling of computor, not the idle American version.

ICAN may have been the International Convention on Aerial Navigation (aka the International Commission for Air Navigation), the forerunner of ICAO. Under one name or another ICAN operated from 1903 to 1945. One of the many things the participants had difficulty agreeing on was what it was called, which may explain why it’s a bit difficult to find on Google. It’s various pronouncements are often referred to by the names of the places where the various conventions were held, notable events being in London in 1912 (which allocated the first radio callsigns), Paris in 1919 (air sovereignty) and Chicago in 1944 (which established it’s successor!)

Thank you for your reply, However!

How can I tell the age of a Navigational Computer MKIII? not a rare item I know, however a nice thing to own.
The one I have is a MK3 III H REFNo 6B/250.
It is the no lid type and only one winder. Usual AM mark with a P. Says I.C.A.N Calibration on the back. I can usually work out the year of RAF items, this one has me stumped!

I am certain this type of device was used in ww2. I know the Dambusters film isn’t 100% accurate, however you can clearly see one being used on the way to the dams. Isn’t it strange that very little information is available on Google? I don’t even know what ICAN stands for.
I suspect the letter after the MkIII is a clue to the year.

Mine is Stores ref 6B/214 and is a Mk111F, it also says it is Isothermal calibration. I guess this is an earlier one, I don’t know when the ICAN system came in but guess that was post war. If that is any little help to you