It always makes me smile when I see the ‘F’ and the ‘B’ at the start of Bristol engine part numbers…

…but what (or who) does the ‘S’ stand for?

It is common to see ‘SFB’ at the start of wartime (and post-war) Bristol engine part numbers; I’ve always fancied that it may stand for ‘Shadow’, as in ‘Shadow factory’, to distinguish them from ‘genuine’ Bristol parts? Anybody know?

Fedden was sacked in September 1942, could it be pique that sought to place a Mr S over Mr F ?

Single cylinder test rig

[ATTACH=CONFIG]227874[/ATTACH]

I hope to see a Jupiter in the air one day, ideally powering a Gamecock.

This is an inlet manifold assembly, there must be other bits around?

Robert M

Does anybody have any detail of photographs of the single cylinder test rigs that Bristol engines set up to test Jupiters and other engines ? I presume this was something like an old single cylinder motorcycle engine with a strong crankshaft.

Ratios n Rithmetic

Magneto rotor turns at 9/8 engine speed…..I’ve never quite figured that one out!

I have never thought about this, but now is probably a good time to start.
In the photos below is the V12 Kestrel accessory gear arrangement, the gear ratios (maggies driven at 1.5 crankshaft speed) and the SC12 maggie with drive cog engaging rotor cog at 24 teeth to 74 or 1:3. I can’t do maths very well, so you need to check me, but I work this out as the rotor going at 0.45 crankshaft speed.

The last two photos are 9 cylinder Jupiter gear arrangement, with what appears to be maggies driven at 1:1 crankshaft speed. I cannot find anything as basic as the gear ratio diagram so thoughtfully provided by Messrs Rolls Royce. The last photo is the SC9 maggie with drive cog at 44 teeth engaging rotor cog at 100 teeth, roughly 1:2.3 Again, using my fingers, I think this is 0.44 crankshaft speed.

On both magnetos a four lobe can opens the points four times for each revolution of the magneto drive cog.

So, if this is right, for each revolution of the crankshaft, the rotor traverses 0.45% of 360 degrees, roughly 162 degrees.
In the SC9, if the pickups are offset at 40 degrees, this is four cylinders worth of traverse.
In the SC 12, if the pickups are offset at 30 degrees, this 5.5 cylinders worth of traverse.

What happens in one crankshaft revolution of a V12 or a 9 cylinder radial that would make sense of this ?
I am going off to stick my head in the fridge.

The iCentaurus

Chicken or the egg question.
Don’t we see more Bristol radials because the aeroplanes they went in are (more or less) extinct as fliers, or are the aircraft rare because of the engine….lack of spares or whatever?

In a world where there are still plenty of DC-3s (an older type with an older engine) flying with their P&Ws, there should be a few Bristol Freighters out there flying with their Hercules….

Steve Jobs established Apple then was ejected : Apple came close to bankruptcy. Steve Jobs returned to Apple and turned it into the most valuable corporation in US history. Steve Jobs died and I do not expect Apple to continue on an upwards trajectory. It will tread water, give off some sparks then slowly die. Fedden came into Bristols and turned it into a business where nearly 90% of its revenues came from engines in the 1930’s, in the midst of Depression. Fedden, like Jobs, was a product person, an insatiable spirit driving products ahead of the curve, dragging the rest of the world into a place it didn’t know it needed to be. Both men created strong, competitive, personal tension within their markets and between the companies that carried them on their journeys. It is obvious Bristols shot itself in the head once it sacked Fedden, it lost its key animating spirit. The gentler genious of Barnwell was extinguished once he smashed his aircraft into the ground. Bristols carried on, did what it was expected to do, but did not astonish the world; trod water, then died.

It is not just amazing that DC-3s are still flying, but astonishing that I can reach out my hand and touch two DC-2s within a few hours distance. The Bristol Aeroplane Company are Carthage, and Rolls Royce is Rome. Carthage has been destroyed,its walls rent and verily salt ploughed into its ruins. Probably deserve it. Fedden, however, deserves to be remembered by history, taken in with cornflakes by youth at breakfast, with his iJupiter, iPegasus, iHercules and iCentaurus.

Hi Powerandpassion,

the market for these is booming ! if you make the BTH SC/AV12 caps I will take 4. Andy

Andy, do you mean these new fangled caps used on those awful, modern engines like Merlins !?

Timing, Cunning and Reinventing

A cunning plan, except one needs to be able to time the points to coincide with the passing of each pick up on the dizzy. 😉

Rather than trying to reinvent the wheel and attempt to manufacture a distributor cap, I would talk to these guys…
http://www.antiquedistributorcaps.com/distcaps.htm

Ahh indeed I have many cunning plans, except I keep jamming my fingers when I close the laptop and walking into glass doors…it is hard to be cunning in historic aviation, because you keep competing with the ghosts of a room full of mathemeticians, stress calculators, draftsmen and wily old shop foremen, let alone design genii. They lived with the consequences of their actions, which was often a test pilot in a coffin, so things were pretty well worked out. You try all sorts of short cuts and keep returning to their design solutions and material choices, and when you decipher these, there is often a sweet moment when your mind shifts into a higher gear and you get something that is rare to feel in this modern world of designed obsolesence and Chinese metallurgy, which is admiration.

Timing a mag seems logical on these babies. I have put a naked SC12 next to an SC9 in the photo and all that is different is the drive cog for the rotor arm. On the SC9 it is approximately 60 teeth and on the SC12 it is approximately 72 teeth. The lobe driving the points is the same four hump thing. So all you have to do to turn an SC9 into an SC12 is press up the appropriate rotor drive cog in the bakelite press, another piece of tooling. Probably you would CNC it out of a modern engineering plastic these days. The same bakelite cap fits on each body, but the more I think about it I would be offended by using a 12 cap and leaving 3 HT lead entries unused; what I really need is an SC9 cap to pattern off. The same magneto bodies were used for SC14 (Armstrong Siddeley Panther) and SC10 (?).

The mob doing the antique distributor caps do a wonderful job, but use phenolic resin only and low pressure casting, which might not be as strong as the original bakelite with all its fillers. Good for popping the bonnet on a car or stinkboat, probably no good at 20,000 feet. Again the ghosts of the past would haunt me, and the custodians of safety in the present would require too much convincing to cost defensibly shift to new material choices for short run production. Mostly I want to learn how bakelite was made, then get into instrument cases, switches and other such good things, go mad with the press..

Relightflynn9,

I have seen SC9-3 with the coupling for auto ignition advance, so would love to know if yours has it too. Picking the logic of magneto ‘improvements’ seems to be a matter of laying them side by side and trying to figure it out. If yours has a bakelite distributor cap, I would love to get my sticky fingers on it to pattern off. As you do, there is a high temperature, high pressure bakelite press sitting in the back shed, as I figure one day it will need to be used for these type of things. The distributor caps seem to have been made in a two step process, with an intermediate step allowing the location of a brass fretwork carrying the HT current from the rotor pickups to the HT lead connection. Apparently the bakelite was mixed with asbestos for additional fireproofing, so as well as holding off from licking radium dials we now have to not cut up distributor caps from old magnetos and sniff them.

In a 1944 lecture by Fedden to the Royal Aeronautical Society he states that “55% of all RAF aircraft in 1939 were equipped with Bristol engines”. These included the Pegasus and Mercury, which covers everything from Lysanders to Blenheims running nine cylinder engines which may have used BTH SC9 magnetos. In addition a great variety of civil aircraft, mainly airliners, used Bristol engines, so point to any 9 cylinder air cooled radial from the period as the home for your SC9 and you will be half right ! If you hitch your thumbs in your braces and announce that this is the SC9 used in the Wallace flying over Everest expedition then there will be very few folk who could say otherwise, unless somebody has a sneaky photo of that particular setup.

HT lead ends

[QUOTE=Robert Edward
I hope to see a Jupiter in the air one day, ideally powering a Gamecock.

This is an inlet manifold assembly, there must be other bits around?

Robert M[/QUOTE]

Here are some HT lead ends for Jupiter.

[ATTACH=CONFIG]227877[/ATTACH]