Evening, Folks,
I’ve recently acquired this old aircraft RPM gauge. It’s a big ‘un being 4 inches across the glass face and 5 1/2 inches overall diameter. It looks to be post-WW1 to me, though I would like an informed opinion. General feeling is it’s from a bomber type, being so bl**dy heavy! (about 1kg) and, perhaps, a nacelle-mounted one, which is why it’s so big? Scale is 600 – 3500 RPM.
Not much detail on the face. Here’s what there is:
RPM
hundreds
14A . No 49,431
S SMITH & SONS Ltd
LONDON
4 – 1
The lower figures are probably the drive ratio.
[ATTACH=CONFIG]235592[/ATTACH][ATTACH=CONFIG]235593[/ATTACH][ATTACH=CONFIG]235594[/ATTACH]
Over to you…
By: powerandpassion - 4th March 2015 at 11:17
AP1275 1930 “General Instrument Equipment for Aircraft” Sect 1, Chap 1 :
“Mark VI Revolution Indicator
As the dimensions of aircraft increased, it was found that standard types of revolution indicated(sic) needed exceptionally long flexible drives, more particularly in the case of multi engined aeroplanes whose engines were carried in nacelles on the wings at some distance from the pilots cockpit..trouble was experienced when the flexible drive exceeded 30 ft..there arose a demand for dials for the use of the engineer..led accordingly to the development of a large dial indicator.
The MkVI has a dial of 5 inch diameter, graduated 600 – 2600 RPM, movable lubber mark, weight 3lbs 12 ozs.”
3500 RPM is associated with V12 engines, contemporary Hawker Hinds with Kestrels had 3000 – 3400 RPM Mk IX indicators, which AP 1275 1937 shows came after the Mk VI. As aircraftclocks has pointed out, perhaps this was a Mk VI special order.
If the guts of the dial stayed the same as a 2600 RPM unit and only the dial face changed, you would only have to alter the engine output ratio for the needle to traverse approximately 300 degrees around the dial, which is a product of the engine gearing ratio for output to the flexible drive. So what apart from the V12s output this ratio to the RPM indicator ? Armstrong Siddeley Serval ? Saro Cloud flying boat ?
I wonder if this is an early thirties gauge from multi engined type using RR Kestrel, HP Heyford, Short Singapore, Supermarine Southhampton ?
Nice gauge.:)
By: Arabella-Cox - 3rd March 2015 at 20:55
If you’re referring to my photo, TA, then I can’t see an A or a V at the 3 o’clock position.
There are a few scratches which had me confused for a while – or at least until I put my glasses on!
Anon.
By: Trolly Aux - 3rd March 2015 at 14:34
the “A” Rober refers to
A
–
V
Is on the back I can see it in the 3Oclock position
By: Trolly Aux - 3rd March 2015 at 14:33
the “A” Rober refers to
A
–
V
Is on the back I can see it in the 3Oclock position
By: aircraftclocks - 3rd March 2015 at 14:24
This is a Smiths model AV.503 engine speed indicator.
The standard calibration was 60 to 2600 RPM. When you placed an order you needed to state the calibration required, the particulars of the engine and if dial markings were to be luminous or non-luminous.
The Mk V is the standard version of this model.
All the other models of indicators have a 14 or 14A printed on the dial face. So it looks like this one is a civil one.
By: Arabella-Cox - 1st March 2015 at 18:01
Robert, there are no such markings on the back of the gauge, but there is this graticule symbol of sorts, which I have seen on other instruments.
What I thought may have been a figure 1 and also an F turned out, on closer scrutiny, to be just scratches.
Anon.[ATTACH=CONFIG]235635[/ATTACH]
By: Robert Edward - 1st March 2015 at 10:04
Mike, I think that units intended for military aircraft use were stamped with an “A” and an arrow, as shown on the Smith and Sons MK. V. Does yours have this marking?
By: Arabella-Cox - 28th February 2015 at 07:59
I looked in the 6A section of my 1924 AP1086 up to AL7 and the only two RPM gauges it mentions are MkV 4″ and MkVI 6″ with a range from 600-2600RPM
By: Arabella-Cox - 27th February 2015 at 21:59
It’s made of aluminium alloy so I am presuming it’s an aircraft instrument.
Anon.
By: Rosevidney1 - 27th February 2015 at 19:49
Could it have once been fitted in a tank, I wonder?
By: Arabella-Cox - 27th February 2015 at 19:20
It looks like a Mk. V type instrument. 4″ Dial would suggest panel mounted, not engine nacelle mounted (they were the Mk. VI indicators with 6″ dial).
May not be ex-Air Ministry as the standard range was 600-2,600 r.p.m. but Smiths sold them worldwide with the standard claim
“Calibrations and gear ratios to suit all requirements”
They were still advertising them in the mid 1930s.
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February 27, 2015 at 6:05 pm