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View Full Version : Aero carburettors work how?



Rising Sun*
12-06-2007, 03:37 AM
I understand auto carbies.

If they're turned upside down the float arm is going to cut off the fuel and the engine will starve.

How did WWII aero carbies avoid this to allow flight in all attitudes?

Man of Stoat
12-06-2007, 03:43 AM
They didn't, inverted flight was extremely limited with a carburettor equipped engine. The "break" manoeuvre used by the RAF just before attacking mitigated the problem somewhat, however.

Man of Stoat
12-06-2007, 04:00 AM
You also weren't going to get much machine-gun fire out of a Spitfire or hurricane of the same period when inverted , because the Browning machine gun ejects by gravity out the bottom

Rising Sun*
12-06-2007, 06:10 AM
What about centrifugal force?

I suppose the venturi effect will keep dragging fuel out of the fuel bowl as long as the pistons are maintaining the air pump effect, but in, say, a sustained high angle bank isn't the fuel going to get dragged away from the bowl outlet and starve the motor?

How did they manage a barrel roll with consistent fuel supply?

Man of Stoat
12-06-2007, 06:24 AM
As I understand it, you've got enough grace (and a fuel pump) to do it, but try to stay inverted for more than a couple of seconds and you are going to have issues.

Rising Sun*
12-06-2007, 06:36 AM
As I understand it, you've got enough grace (and a fuel pump) to do it, but try to stay inverted for more than a couple of seconds and you are going to have issues.

Fuel pump with conventional carbie will only fill bowl until float arm stops it.

Invert carbie and float arm stops fuel, but fuel in bowl is now in what used to be empty top of bowl, above (now below) float arm, with bottom of fuel bowl empty, which creates problems for fuel supply derived from anywhere under where fuel would normally be in roughly horizontal position.

Rising Sun*
12-06-2007, 06:37 AM
Maybe there's some sort of non-return valve in the setup?

Uyraell
02-11-2009, 09:39 PM
Maybe there's some sort of non-return valve in the setup?

Wiser heads than mine may dispute this, but as I understand it, depending on the model of Merlin in question, a small amount of air is bled off from the carb intake manifold and used to pressurise the outer lining of the dual-layer fuel line before the pump in order to give a few seconds' worth of fuel delivered under pressure past the float.
For barrel rolls it doesn't matter in the least, because centrifugal force assures that gravity is momentarily treated as a constant.

Late model Merlins and most Griffons were injector fed, though the carb air was delivered under pressure, which is why Merlins/Griffons are not usually classed as "Fuel Injected" engines as the DB 601 and onwards is.

As to non-return valves in the setup, there were, but there were then also dumpvalves inline with same.

Source of this info, live face to face conversation with an ex merlin mechanic/fitter, who worked on Merlins and Griffons, RAF, WW2.

Regards, Uyraell.

Canberra Man
09-29-2009, 07:48 AM
The carbs in the hurricanes and Spitfires did have gravity problems which where somewhat alleviated by a young lady at Rolls Royce, who devised a restrictor in the carb line that reduced the problem until injection came in. It was refered to jokingly as. "Miss Pennie's orrifice!"

Ken

pdf27
09-29-2009, 07:52 AM
Umm... IIRC her name was Tilly Shilling, but the rest of the statement is correct...