Carb flooding ( discussion, not bug report! )

[Richard Dastardly]

I don't want to post this in a bug report, I want a bit of a discussion on carb flooding & how much the problem was solved at various points in the war.

 

Early on there really was a big problem with negative Gs and merlin carbs ( as anyone who's flown the Mossie for more than 20s will find out ), solved in two stages - firstly was Miss Shilling's Orifice ( I'm really not making that up ) which was a restrictor in the fuel flow that wouldn't allow more than the maximum possible flow - obviously this is still going to run rich at times. Secondly and later there were pressurised carburettors.

 

The restrictor was installed everywhere by mid 1941, what engines are we getting? as for the pressure carbs, I don't know - which is one reason I started this thread!

[WarbossPetross]

37 минут назад, Richard Dastardly сказал:

I don't want to post this in a bug report, I want a bit of a discussion on carb flooding & how much the problem was solved at various points in the war.

 

Early on there really was a big problem with negative Gs and merlin carbs ( as anyone who's flown the Mossie for more than 20s will find out ), solved in two stages - firstly was Miss Shilling's Orifice ( I'm really not making that up ) which was a restrictor in the fuel flow that wouldn't allow more than the maximum possible flow - obviously this is still going to run rich at times. Secondly and later there were pressurised carburettors.

 

The restrictor was installed everywhere by mid 1941, what engines are we getting? as for the pressure carbs, I don't know - which is one reason I started this thread!

Wikipedia says our Mosquito FB Mk VI (first flight in 1942) had Merlin 21, 23 and 25. Bendix-Stromberg pressure carbs didn't come until 1943, and only for Merlin 66, 70, 76, 77 and 85, i.e. we ain't getting them. So yeah, as it is we're at the mercy of Ms Shilling and a little half-barrel roll before we dive.

[Richard Dastardly]

The flow restrictor should stop flooding on a little forward push of the stick ( unless you're at idle, I guess ), all you'll get at most is the fuel flow at max throttle - what it won't do would be let you fly inverted for long. Kinda curious if whatever Mossie they looked at behaves like what we have now.

[Gunfreak]

Stupid question, so this is the thing, that when you pull a little hard back on stick the engine "stops"

[Richard Dastardly]

7 minutes ago, Gunfreak said:

Stupid question, so this is the thing, that when you pull a little hard back on stick the engine "stops"

When you nose over & get negative G, the engines die for a bit because the carburettors aren't working properly - they get flooded with fuel & can't provide a proper mixture to the engine. Was a big issue in BoB era aircraft & a minor fix was found pretty fast.

[Olddog]

What kind of negative G's are we talking about? I'm instantly getting cut outs and flooding just bumping the stick forward the tiniest bit, something like .01 neg G.

Edited September 17, 2021 by Olddog

[Gunfreak]

54 minutes ago, Olddog said:

What kind of negative G's are we talking about? I'm instantly getting cut outs and flooding just bumping the stick forward the tiniest bit, something like .01 neg G.

 

 

Yes, same here, at first I thought I had done something wrong in the start up, but I tried from a hot start. Same thing happens. 

[Richard Dastardly]

2 hours ago, Olddog said:

What kind of negative G's are we talking about? I'm instantly getting cut outs and flooding just bumping the stick forward the tiniest bit, something like .01 neg G.

 

That's why I started the thread - I've never flown anything with a Merlin ( to no surprise ), but I've enough engineering & sim work background to be curious about the situation. The release notes do say they'll be tweaking the behaviour in this area but I'm not sure it should be doing it at all, hence asking for more details about what fixes were applied when. The RAE restrictor was basically a washer stuck in the fuel pipe, a ridiculously simple device that wouldn't be a problem fitting to anything. Suddenly getting full fuel flow might be a problem when there's not much air pressure, but when you're already at full throttle low level it's not going to change anything at all.

That is if they fitted them to FB VI. I can't think of a reason not to, but I don't know - hence asking.

[No1sonuk]

I'm glad this has come up.
Having flown the P-51 quite a bit without noticing this issue, I though I was doing something wrong.
I guess the P-51's Merlin has one of those mentioned solutions to this issue.

[Morat]

It's bad enough to cut the engine on the downward side of a roll, which I don't remember being mentioned by anyone who flew the real a/c.

[Krupi]

I was taken aback by this myself, as WarbossPetross has already stated a bit of research shows that the Bendix Anti G Carbs were not on the Merlin 25.

 

Now I knew that they came later in 1943 however I had assumed that they were retrospectively fitted to older Merlins that were still in production... clearly however that does not appear to be the case.

 

That said we have had confirmation on Discord that the current starvation starts after 1-1.5 seconds, this will be increased to 3-4 seconds. 

 

 

Edited September 17, 2021 by Krupi

[Krupi]

26 minutes ago, Morat said:

 don't remember being mentioned by anyone who flew the real a/c.

I was thinking this, but then the books I have read have been cantered around the Bomber Version which had the Merlin 71 and therefore didn't suffer from this issue (I have just started on an Autobiography of a Merlin 25 Navigator so I am intrigued to read his thoughts on this!).

Also these aircraft were never flown at altitude and so the need to dive was non existent. I think that it is just a bit too temperamental at present, lets see what it is like when the cut out is delayed to 3-4 seconds.

Edited September 17, 2021 by Krupi

[Morat]

Yes, early days!

[Bozon]

Yes, the current behavior seems a bit exaggerated - engines cut out even when reversing a roll with an unloaded plane, or when coming over the top of a (quite moderate) hill.

[Richard Dastardly]

I can't find anything definitive but I'm pretty sure this thing should be fitted with the restrictor - again, it's a washer and there is literally zero reason not to fit one - so the engine should not be cutting out in normal maneuvering. I'm a little curious whether the restrictor would mean an engine at idle would manage a burst of full power or if it'd just run rich for a bit if you were violent about it. Anyone familiar with carburettors? that's a bit of a dying art.

Edited September 18, 2021 by Richard Dastardly

[grafspee]

Carb flooding is secondary thing after plane was put to negative G.

First thing what happens is that fuel in float chamber is pushed up, main jet supply is at the bottom of the float chamber so engine experience almost instant cut out, in mean time fuel being forced at float chamber ceiling pushes float down opening fill valve. Restriction valve probably can prevent carb flooding but it is impossible to prevent fuel starvation at beginning of the negative G.

Carb modification prevent flooding but carb still is not operational under negative G, so engine cuts out until G returns to positive. This way engine can recover much faster.

Edited September 18, 2021 by grafspee

[SuperEtendard]

The Merlin 25 in the Mosquito FB Mk VI was based on the Merlin XX (used in the Hurricane Mk II), this description indicates the Merlin XX shared the same carburettor as the Merlin 45, which was the Merlin used in the Spitfire Mk V with the stop gap Shilling's orifice to help with the negative G cut out issue.

[grafspee]

On 9/18/2021 at 10:32 PM, SuperEtendard said:

The Merlin 25 in the Mosquito FB Mk VI was based on the Merlin XX (used in the Hurricane Mk II), this description indicates the Merlin XX shared the same carburettor as the Merlin 45, which was the Merlin used in the Spitfire Mk V with the stop gap Shilling's orifice to help with the negative G cut out issue.

You can help with negative G but you cant fixed it in float type carb.

[Jandar]

 

Edited September 23, 2021 by Jandar