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Lathework for Gnomes

Asquith

Diamond
Joined
Mar 3, 2005
Location
Somerset, UK
Gnome01-1.jpg


The topic of Gnome radial aircraft engines comes up occasionally, and Franco has posted some information about machining the cylinders. These were turned from a solid steel billet, to leave a wall thickness of just 1.2 mm (0.047”).

I came across this photo of a cylinder undergoing the final stages of machining the O.D. in a French factory in 1910. This shows a multi-gang tool in a Herbert No. 16 turret lathe. The tools could be rotated to present a new cutting edge, and they could be replaced individually. They worked upside down, with the lathe running in reverse.


Two of the threads referring to Gnome engines are linked below, but dial-up users beware – quite a few photos.

http://www.practicalmachinist.com/ubb/ultimatebb.php/topic/11/2199.html#000011

http://www.practicalmachinist.com/ubb/ultimatebb.php/topic/11/1242.html#000005
 
While this accounts for some of the finning, up at the plugs and the bosses for the rocker arm the fins had to be milled because they come to an abrupt end with a radius from a plain milling cutter. We have a 160hp Gnome at the museum. I have thoroughly eyeballed it trying to figure exactly how it was made. Interesting to see these were turned, as I figured they would have used the milling for all the fins since they had the upper ones to do.
 
Fascinating, thank you for the photo. I've
always wondered how those were made, did they
do each fin at a time, or what. Now I know.

The next mystery - what's the 67 lb weight
doing on the cross slide????

Jim
 
Probably helping to keep chatter at bay. I'd imagine this could be a chatter nightmare with that many (essentially) cutoff tools in the work. With onyl .047 of wall chatter would probably washboard inside and outside cylinder surfaces.
 
I think the 67 lb weight is the original billet they are machined from. The finished piece in front also has a weight maarked on it, it looks like 5 lbs, but that seems kinda light.
 
Judging from the shape of that 67lbs, thats a billet of cylinder before work began on it, note the 5lbs on the finished cylinder as a comparison.
 
Mike C.

Have you seen the photos of the various stages of Gnome cylinder manufacture at:
http://www.pilotfriend.com/aero_engines/engine_specs/gnome_rotary.htm

It looks as though the spark plug boss does not appear until the final stage of manufacture. Wonder if the cylinder is completely machined on the lathe, then the spark plug socket added later (welded in??).

The several photos I have of Gnome cylinders show the exhaust valve, seat and rocker assembly as being screwed into the top of the cylinder as a complete unit, so any milling other than removing a small circle from four fins for the plug socket might have been confined to this separate assembly. I'd be interested to know if your engine shows any sign that the plug socket might be another separate assembly added after all the turning work was finished.

By the way, scroll down to the bottom of the page in the link where the specs. of the 160 Gnome appear. I'd hate to be buying the fuel and castor oil for one of these. For comparison,quoted figures for the 80 HP Gnome are 7 to 8 (imperial) GPH of fuel, and 1 1/2 to 1 3/4 gal. of castor oil.

franco
 
How come the billet designation? That is either a casting or forging regardless of it being solid. Billet to me means bar stock.

John
 
franco, I have not seen that yet. I'll try to have a look tomorrow. I'm at home on dial up, so I'll have to go into work to look at it (I'm off burning vacations days). I have to go in tomorrow anyway to do some side work for one of the guys that works with me (jeep axle bushing).

I did not realize the heads screwed on. I figured it was just one piece. The head is DEFINITELY once piece with the plug hole bosses and rocker arm supports being integral. You can see the curved surface where the plain cutters were stopped and withdrawn in precise locations. It's not a gang mill cut either. They are contoured so that each one ends in a different place around the plug boss.

These are just bizarre engines. The more I learn about them, the weirder they get. I have actually threatened to get the one we have running. One of our volunteers came across a copied manual on Ebay so we have it all right there. Just need a prop and a mount, the rest is on board (mags, interrupter, etc...)

So fuel consumption was about double a typical 1940s 80hp air cooled flat four (Cont A-85). Wow. And running 7-8:1 on the oil. I have heard tales of the pilots' gastrointestinal difficulties from flying behind these engines. No wonder.
 
The 67 lb lump is the piece received by the lathe operator, rough machined from bar. The final weight is 5 lb 5 oz.

The first operations are to rough machine, drill and bore, then turn round and rough machine the fins.
Remove and anneal.
Finish turn the top three fins.
Remove to mill these three fins, and electrically weld the spark plug boss in place.
Complete other lathe operations, including finish boring (leaving enough for grinding the bore), thread chasing for the exhaust valve housing, and finishing machining the rest of the fins (this is the operation illustrated).

A reminder of what it looks like…..

Aero103.jpg


Seen in the Science Museum in London.
 
Asquith,

Many thanks for that explanation, and for the original photo showing the machining of the fins. Now it all makes sense. I did once see a sectioned Gnome, but it was at the age of about ten in 1944, and the details are somewhat hazy. I remember the details of the inlet valve in the piston crown though.

It does seem to be a wasteful way to make a cylinder completely from solid rather than forging the metal roughly to shape first. I do remember reading somewhere that the American produced cylinders on the licence built rotaries were made from forgings, but were, at least initially, more expensive to produce than the cylinders machined from solid.

By the way, where did you find the information on the details of cylinder manufacture? I did a quick search before my last post, and did not come up with anything useful - apparently I did not look far enough.

Regards,

franco
 
Is it me or is it strange that the photo shows a French Machine shop but the weights are in pounds and not Kilograms? I always thought the French in 1910 used metric unit of measure 100%.

I wondered if it was staged for photo for English or American (or other countires using imperial units of measure)viewers.

John
 
I'll get pics of the one at work. This is a bit different from ours, perhaps an earlier, lower hp model. I really do not think the plug boss is welded in on ours.
 
These motors pre-date the widespread introduction
of the metric system. There was a long
discussion here a while ago started when somebody
showed a set of german submarine tools that were
for sale, and said they had to be fakes as they
were SAE spec tools. Not so. English measures
and thread standards were commonly used at that
late date.

They fly rotary engines at Rhinebeck all the time
and they make quite a show of it, they get them
all juiced up before they light them off, and
have the plane pointed back at the audience. Of
course there is a huge cloud of castor oil smoke
that blows back at them.

The story was the pilots used to carry blackberry
brandy to minimize the effects of the castor oil
smoke, but that there were more than just a few
emergency landingst that were biologically caused
rather than by mechanical malfunction.

Apparently the MTBF on those motors was measured
in the tens of hours. After that it was time
for a teardown.

The intersting thing is that, while the Gnome
and the LeRhone rotary motors had the highest
power per weight of their time, they are still
a poor second compared with the aircooled radials
that came along by the late 30s.

If you compare the motor that flew Lindbergh
across the atlantic, it has a *higher* power
per weight number, and a *longer* MTBF by far.
This I suspect is a direct result of two things,
advances in materials, and better fuel, compared
to the WW1 motors. In fact those two things
alone are, IMO the factors that allowed the
lindbergh flight to happen. As an egineer he
understood that if one motor was not reliable
enough to move a plane across the atlantic, then
two or three of them were also doomed to fail.
And that, if one could get *one* motor reliable
enough, you only needed one of them.

Jim
 
Franco,
The description was in the magazine 'The Engineer' or 'Engineering'.

John,
The photo has been heavily retouched, and it may be that the weight was added in the darkroom back in the UK (although that on the finished cylinder does look as though it was written on there in chalk).
 
jim rosen,

I remember reading somewhere, probably in the 1917 Air Board Technical Notes, that the Gnome engines were all metric. I assume this applied to the French and British built ones. I have no idea whether the licence built US ones were converted to imperial dimensions and thread systems. It would be interesting to know what modifications were made to these engines for US mass production.

Bluchip????

franco

[ 12-22-2006, 10:02 PM: Message edited by: franco ]
 
Again, thanks to asquith for putting the photos
here. The cutaway of the gnome cylinder is
fascinating indeed.

I would love to see one of those run on a
test stand someday, up close. They were
probably some of the most advanced power of
their day.

Jim
 
Talking about metric units - in present-day France it seems normal to describe car and truck engine power in horsepower (ie cv or chevaux). This is in the advertisements for cars as well as general conversation. I don't know why metric units, eg 'Watts' haven't taken on.

Jim,
these French engines are described in metric terms by a book of the day, eg the Airboard Technical Notes from 1917 (still available from Camden Books) gives bore and stroke etc in mm. In fact the book appears to use metric dimensions for all clearances etc for all the engines described (including RAF V8, 120 hp Beardmore, though these engines had European origins).

Asquith,
thanks for the sectioned view and description, that helps.

There is a guy here in Auckland making a two-row Gnome, I think it is mostly done. These were made and used before WW1 apparently.

The Rotary Aero Engine by Andrew Nahum is a must for anyone who is interested in these engines - he includes some excellent photos from engine handbooks, one shows a guy with a bar fitting a cylinder head. I am sure all these handbooks are available from WW1 Aero (they have a huge list), but have been warned that they are photocopies of photocopies...
 
Just looked at the Gnome downstairs here at work. The plug bosses are DEFINITELY integral to the cylinder and not welded in. Got to run, but I'll get some pics next week while I am here.

Wonder where this came from? Was this on a placard with the engine in the museum? I have seen some curators take liberties with their "expertise", the best example of which is the Smithsonian website stating that the SR-71 has a titanium skin to protect the underlying aluminum framework. I can assure you there is no aluminum under there because I have seen the innnards.
 
Mike,

There were several different models of rotary made by Gnome, so it is quite possible there were different methods used for their construction.

The 50 hp Gnome appeared in 1908, this being the type with the inlet valve in the piston and 7 cylinders. The engine in Asquiths first photo looks like one of these, the photo comes from a 1910 article. I don't know how many types there were, but I see they also list an 80 and 100 hp of this type.

The Monosoupape was introduced in 1913, with 9 cylinders and I see references to 100 and 150 hp versions. In one of your posts you mentioned your engine has the inlet ports at the base of the cylinder, so I guess you have a Mono, although you mention two different horsepower ratings, 120 hp, 160 hp.

Then there were the other rotary designs by Le Rhone, Clerget, Bentley, and the German engines.

BTW, in the RAF V8 thread there is some surprise as to the capacity of the engine - these rotaries were even larger, 721 cubic inch (11.8 litres) given for the 80 hp Gnome, and 970 ci (15.9 litre) for the 150 hp Mono. They were large capacity engines although they are so bare and compact.
 








 
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