Magnificent Magnetos

Versuch, no daggers for you my friend, just the compliments of the season.
Thank you for posting up the magneto and engine information, which gets closer to the mystery of the CSE 12- 12S and CSE-ES magnetos, which I think were custom builds for Napier, the first time I have seen the engine designer dog wag the magneto manufacturer tail !

My guess is that these magnetos were constructed on a similar principle to the two crankshaft Dagger engine they were placed on :
a central engine driven spindle geared to two new spindles driving through two piggy backed coils. It would not be hard to do this if you shifted the distributor into a separate device. What about the points. Perhaps they yin-yanged around a single lobe arrangement, necessitating a larger points cover. I have held one of these points covers in my hand and it is much larger than the standard points cover on the standard CSE magneto.

To prove or disprove this theory the driving ratio from the engine to the twinned magneto is required, then an assumption that the lobes opening the points were the usual 4 lobe arrangement, then follow this gearing arithmetic through to the distributor and engine.

In simple terms the Napier Dagger, with 48 spark plugs, had the equivalent of 4 x 12 cylinder magnetos stuck to it, optimized into 2 x 24 cylinder ‘custom’ magnetos.
It seems quite brilliant, typically Napier, typically introducing 200% more dependency on nothing going wrong….

Seeing as the knives are out , here is my parry….

Are you sure ?
Why are there four magneto switches on the left hand side of the cockpit of the Hawker Hector ? Was this a twin engined aeroplane ? Or did it use twinned magnetos ? How do you provide ignition to 48 spark plugs, simply !?

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Do you have a Dagger AP or handbook which actually describes the ratio of prop shaft to magneto and distributor drive? There is a little dedicated gear on the prop shaft which drives little bitsy magneto and distributor gears.

Well, the magnetos and distributors are separate units driven from the reduction gear which is 2.69:1. This number is close to 8/3 so I wonder if it should read 2.67.

The magnetos need to give 12 sparks per rev of the crank so they need to rotate a crank speed x 12 / number of cam lobes. What about 4 or 8 x the speed of the airscrew?
C /2.67 x 8 = 3C so x8 would give 12 sparks per rev with a 4 lobe cam with an 8:1 drive (or an 8 lobe cam with a 4:1 drive)

The 24 point distributors need to rotate at half crank speed so need to be speeded up by 2.67/2:1 = 1.33:1

When do I get the cake?

New Question

OK so that’s the Learner stuff. Now let’s get down and dirty. Who wants to explain how the magnetos on the Napier Dagger worked !

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Official Apology

Need to get the number of teeth right and consistent between posts! I demand a recount!

Smirky remind me never to divide a chocolate cake up in front of you ! In the interests of aviation safety and accuracy I acknowledge my counting was as accurate as a statement by the Federal Reserve of the United States estimating total government debt, sort of approximate…

You are correct, so I put back the beer I gulped!:very_drunk:

Then the rotor, running off the 2.5 revolutions of the engine to magneto drive gear, would need to be slowed down to 1 revolution of rotor, so we gear down 2.5:1. Say we use the same rotor cog as the v12 at 75 teeth, we would need a 30 tooth drive.”

Nope cos it was 24×3=72 for the V12 (unless it should have been 25×3=75?).

Need to get the number of teeth right and consistent between posts! I demand a recount!

Ahh Smirky, you get the beer ! But first I take the foam off the top, and maybe a gulp! You get some left in the glass for pointing out my errors. But this was too easy, it was all in the link :

“Understanding this relationship, it is possible to understand how the 10 cylinder Armstrong Siddely Serval would have been arranged out of the same magneto body, if it had a four lobe cam driving the points. Say 10 sparks are needed per 2 revolutions of the crankshaft (full cycle) and one revolution of the rotor (full cycle). Therefore 10 sparks/4 lobes equals 2.5 revolutions of the engine to magneto drive gear during two revolutions (full cycle) of the crankshaft, or 1.25 revolutions per single revolution of the crankshaft, therefore a 1:1.25 gear running off the crankshaft.

Then the rotor, running off the 2.5 revolutions of the engine to magneto drive gear, would need to be slowed down to 1 revolution of rotor, so we gear down 2.5:1. Say we use the same rotor cog as the v12 at 75 teeth, we would need a 30 tooth drive.”

I will post up a photo of the spindle cog to check our prognostications, then a tougher question ! Full pint’s worth.

3. AS Serval 10 cylinder radial
a) Crankshaft Revolutions : 2
b) Magneto drive gear ratio 1.25 (2.5 revn per full cycle)
c) Sparks generated from a four lobe breaker cam : 10
d) Distributor rotor revolutions with a 1:2.5 drive arrangement, being 60 teeth on a rotor cog and 24 teeth on spindle cog :

There are a couple of errors in the question!: 74 teeth should read 72 (3×24), 100 teeth should read 99 (2.25×44)