Measurements for a Creagh Osborne 5/17 air compass

Jeff Brooks · 9 June 2024, 09:15 AM

Hi All,
does anyone have measurements or drawings for a 5/17 air compass?
I was thinking of trying to make one or have one made.

MattS · 9 June 2024, 11:51 AM

Hi Jeff,

Happy to spend an evening photographing and taking measurements off mine. I don't wish to take it apart yet, but if exterior info is useful, PM me.

Hopefully someone has a full set of details.

Cheers
Matt

cvairwerks · 11 June 2024, 09:00 AM

Jeff: Some measurements here:

https://compassmuseum.com/aero/aero.htm#C-O

Some more data here:

https://www.theaerodrome.com/forum/s...ad.php?t=34845

Might be worth contacting the Admiralty Surface Weapons Establishment and see if there is anything you can obtain copies of, from the archives of the Compass Department/Compass Directorate.

Jeff Brooks · 11 June 2024, 10:13 AM

Quote:

Originally Posted by cvairwerks

Jeff: Some measurements here:

https://compassmuseum.com/aero/aero.htm#C-O

Yes, I have looked here thanks
The measurements given are overall dimentions and they really don't explain how/where they are measured.

I did get some directions from MattS which I hope to make some sort of drawings from.
When I am done, I'll post them here so I can get feedback from others who have a compass.

Quote:

Originally Posted by cvairwerks

Some more data here:

https://www.theaerodrome.com/forum/s...ad.php?t=34845

Might be worth contacting the Admiralty Surface Weapons Establishment and see if there is anything you can obtain copies of, from the archives of the Compass Department/Compass Directorate.

I saw this one too thanks.
Most of the pictures on that thread are gone.
Also not too many actual measurement there either.

Jeff Brooks · 17 June 2024, 10:30 AM

Hi All,

MattS & I have been corresponding about the various measurements on the 5/17 Greagh-Osborne compass.
Matt has a compass and has been feeding the measurements to me.
We have been focusing on the externial dimentions so far.

I thought I would post them here for others to see.
I am hoping that other folks who posess a 5/17 compass can compare and confirm measurements.

Here are the first drawings.
This is a General Assembly drawing to identify the various parts.
I'm sure this drawing will chage as we get more information.

Here is part #1, the bowl for the compass.
It looks to be made of brass and turned on a lathe.
We aren't sure of the wall thickness yet.

This is part #2 the flange on the end of the bowl.
I believe that it is made of brass and is sweated onto the end of the bowl.
I don't have a good reference for what size screws go in the screw holes, if someone has a compass and knows the thread dia & pitch ... please pass it on.

I am still filling in the measurements on the other drawings and I will post them here when I am done.

P.S. this isn't CAD.
I draw them out by hand and scan them, then I touch them up in MS paint.

Jeff Brooks · 17 June 2024, 11:59 AM

Jeff Brooks · 17 June 2024, 08:56 PM

Can anyone confirm if the Creagh-Osborne 5/17 compass was filled with fluid or was it an "Air" compass?
There is a screw on the back of the bowl that I think would either be used to fill the bowl with fluid or to drain moisture.
On the Bamberg compass, it was liquid filled to dampen the engine vibrations and had a bellows on the bottom to prevent bubbles from forming.
I am wondering if the 5/17 had something like this on the top.

I think the large cylinder on the top is a corrector magnet

MattS suggested that it might have something to do with equalizing air pressure as the compass goes through altitude changes.
I don't see a hole on the bottom (inside) of the cylinder to allow air to move through it.

I also don't see any magnets in the brass tubes that cross at 90 degrees.
Does anyone here have any knowledge about this part of the compass works?

CMawson · 18 June 2024, 01:00 AM

Hi Jeff,

I have a 5/17 compass on my Camel, I will measure it and add dimensions to the drawings and send them back.
It will be a week, or so, before I can do it.

The screws that hold the glass retaining ring are 7BA thread.

The compass is filled with fluid, of 0.75 specific gravity. I have a manual for them and it specifies the fluid to be a mix of ethanol and distilled water.
The cylinder at the top carries the corrector magnets in the tubes. I don't have any for mine so I will have to experiment with modern magnets. The corrector magnets are held in position with wooden dowels of the correct length to keep the magnet in place.
The lower part of the top cylinder is an expansion chamber, to cater for the expansion and contraction of the compass fluid at different temperatures.
I will dig the manual out, scan it and send you a copy.

Chris

Jeff Brooks · 18 June 2024, 05:15 AM

Quote:

Originally Posted by CMawson

Hi Jeff,

I have a 5/17 compass on my Camel, I will measure it and add dimensions to the drawings and send them back.
It will be a week, or so, before I can do it.

The screws that hold the glass retaining ring are 7BA thread.

The compass is filled with fluid, of 0.75 specific gravity. I have a manual for them and it specifies the fluid to be a mix of ethanol and distilled water.
The cylinder at the top carries the corrector magnets in the tubes. I don't have any for mine so I will have to experiment with modern magnets. The corrector magnets are held in position with wooden dowels of the correct length to keep the magnet in place.
The lower part of the top cylinder is an expansion chamber, to cater for the expansion and contraction of the compass fluid at different temperatures.
I will dig the manual out, scan it and send you a copy.

Chris

Very helpful, thanks!
I am looking forward to seeing the manual.

Here is the details for the 7BA thread.
https://britishfasteners.com/threads-ba

I am also looking for measurements or other details about the inside of the compass.

MattS · 18 June 2024, 04:17 PM

I can see all those internal parts inside mine, but loose and looking the worse for wear.

Really interesting details Chris, my guess on the purpose of those top holes was somewhat flummoxed by finding tubes under the cover with no obvious purpose. Big thanks for that info. Also keen on that manual when you find it.

I had a quick look on the internet for the manual, no luck but did find this site which has a pretty good photo of a manufacturers lable for H Hughes & Co.

https://compassmuseum.com/aero/aero.htm#C-O

Also has the front page of a manual written by Creagh-Osbourne "The Magnetic Compass in Aircraft", 1915. Searching for that (but not finding it) led to finding "The Magnetic Compass for use on Land", 1915, same author. Found this which contains some of the text.

https://www.compasslibrary.com/produ...s-on-land-1915

More useful perhaps is the "Manual British Naval Service, Navigation, Magnatism and Deviation of the Compass", 1917 which contains a useful description of the compasses in use (not the 5/17 but the slightly earlier models which are similar in important details) and their inner parts, in particular in PDF part 2.

https://ncisahistory.org/wp-content/...1917_Part1.pdf

Change "Part1" to "Part2" for the second half.