Merlin negative G cutout too quick?

[SEE]

Originally Posted by Blackdog_kt

Well, it seems more forgiving than IL2 to me, at least the engine doesn't completely turn off. ......

I actually like it quite a lot the way it's done in CoD. Between the individual aircraft quirks, the extra details and restrictions and the all-around higher level of challenge in all flyables.

+1

[klem]

Originally Posted by winny

I don't think it's wrong, any ammount of negative G in early Spitfires causes the engine to cut with very little delay. That's why it was such a problem. There is no inbetween.

If anyone can prove otherwise I'd like to see it.

I also just read an account where a RAF pilot deliberately nosed down to produce a cloud of black smoke to fake being hit when he was bounced.

Yes but it is happening intermittently in level flight give or take a little turbulence/undulation. I'm sure that never happened. The pilots reported the problem when they positively nosed over to dive. It's mentioned in many of their bios but no mention of problems in normal flight or even during normal descent but it is happening on CoD. I doubt it would have been cleared if that was the case.

[IvanK]

Spot on Klem. Normal Flight inputs such as setting up a descent shouldnt result in -Ve G cuts.
A healthy push to say -0.5G okay but anything between say 0.1 and up G should be okay.

[PE_Tigar]

I agree with Klem. I guess the effect starts now with anything less than 1G. In my around 300 hours of flying, most of which I've spent flying atmospheric piston engine airplanes with carburetors pretty much similar to WWII design (no neg G capability), I haven't seen this happening, not in heavy turbulence, nor in powered stalls (which is a zero G maneuver on full RPM).

[winny]

Originally Posted by klem

Yes but it is happening intermittently in level flight give or take a little turbulence/undulation. I'm sure that never happened. The pilots reported the problem when they positively nosed over to dive. It's mentioned in many of their bios but no mention of problems in normal flight or even during normal descent but it is happening on CoD. I doubt it would have been cleared if that was the case.

If it's doing it just from bumping around then you're probably right about it.

[Viper2000]

Originally Posted by PE_Tigar

I agree with Klem. I guess the effect starts now with anything less than 1G. In my around 300 hours of flying, most of which I've spent flying atmospheric piston engine airplanes with carburetors pretty much similar to WWII design (no neg G capability), I haven't seen this happening, not in heavy turbulence, nor in powered stalls (which is a zero G maneuver on full RPM).

I can show you plenty of YouTube videos of people doing silly things in 172s etc at zero g where no engine misbehaviour is evident, and I can tell you that back in my younger days I played map catching games* in a Grumman AA5 at zero g on a couple of occasions without the engine missing a beat.

What I haven't done is taken the carburettor apart to investigate its design, so I can't tell you whether the lack of a zero g cut was by accident or design. I don't recall reading anything about carburettor performance in the POH for the non-aerobatic types I've flown; I suppose that it's not considered to be relevant information.

However, I have managed to find a video of some rather foolish people (no HASSLE checks**) obviously pushing into negative g in a 172, which does produce what sounds like a lean cut:

http://www.youtube.com/watch?v=HE64UwGGOZE

There are actually quite a few YouTube videos out there of people confusing zero g with negative g and getting these sort of engine cuts, which perhaps explains why most rental aeroplanes are so clapped out. Of course, this would also imply that airframe and engine safety factors are being rapidly consumed by people incapable of judging what they're doing to the aeroplane, which isn't a happy thought...

OTOH, genuine zero doesn't produce misbehaviour in this video:

http://www.youtube.com/watch?v=cwwlk...eature=related

Of course, this sort of comparison is hardly scientific because not all 172s were created equal, and even if they were, they certainly aren't Merlin powered! But perhaps it can inform the discussion by analogy if drawings of the carburettors concerned can be produced for comparison with the particular carburettors the simulator intends to model.

That, of course, is the other important detail; there were several different carburettors which might be fitted to the Merlin, so it's important that we are specific as to which one we're trying to match, because clearly an early SU carburettor will behave differently from for example an RAE anti-g carburettor.

My understanding is that all of the Merlins which saw service during the height of the Battle were made at the Nightingale road factory, and so they're probably more likely to have a consistent set of ancillary components than later engines which were built at a dazzling array of Rolls-Royce and shadow factories in both the UK and USA. So arguably our task is easier than would be the case for later engines, provided that we can find the required source material.

*Place map on instrument panel glare shield. Pitch up 20-30º, then push to zero g and catch the floating map between your teeth before getting uncomfortably close to VNE.

Obviously, very slight negative g is required to get the map off the dashboard (say -0.01 or something), but once it has floated up a couple of inches you obviously have to stay almost exactly at zero if you're going to catch it. So I never pushed deep into negative whilst playing the game; doing so would probably be unwise in a non-aerobatic aeroplane, though I'd be more worried about the lubrication system than the carburettor TBH. Of course, I've pushed into negative g in aerobatic aeroplanes, but I can't say I've ever been a fan of negative g; it always used to give me a headache...

**Yes, I know most people would say HASELL/HELL, but the alternative is another one of the "interesting" habits I've picked up over the years; I find it easier to remember HASSLE because the checks are a pain. So, before starting aerobatics:
Height - sufficient for recovery/legality/insurance, whichever is greater
Airframe - capable of safely executing the manoeuvre intended in its current condition (snags, weight & CoG etc).
Straps/Security - straps tight, no loose objects in the cockpit, especially near control runs. Positively identify strap quick release box and parachute quick release box, since mixing them up in case of emergency would be terminally embarrassing.
Situation - not over built up areas, close to danger areas, restricted airspace etc.
Lookout - clearing turns and all that jazz, making sure to check both above and below. Set lights & transponder as required.
Engine - set power required, ensure throttle friction nut tight, check instruments for abnormalities (Temperatures, Pressures, Manifold Pressure, rpm)

Then for subsequent manoeuvres:
Height
Engine
Location
Lookout

[Deadstick]

I don't think it's wrong, any ammount of negative G in early Spitfires causes the engine to cut with very little delay. That's why it was such a problem. There is no inbetween.

If anyone can prove otherwise I'd like to see it.

+1 on that.

The above post is an impressive demonstration of knowledge, and it is very interesting...BUT as mentioned above, the Cessna 172 is not a Spitfire.

I understand the need for explanation,and comparison, but you have to take into account at least 15-20 years difference in aero-engine technogical development between the Merlin Mk II and the Continental O-300(Early 172 engines).

From what I gather the problem of cut-out in the Merlin was not completely fixed until 1942 when pressure carburettors were introduced.

And as the CLoD manual states, pilots had to develop the tactic of half-rolling the Spitfire to chase the fuel injected 109s in negative G dives.

This seems to suggest to me that even a small amount of negative G was causing the cut out, or why else would the tactic be necessary?

..the amount of guesswork some of you guys do on this forum is astonishing sometimes..

On average a Merlin engine gulps an astonishing 3 UK gallons (almost 14 litres) per minute at take off and circa the half (roughly 6,5 litres) at 75% FEC.

It's literally like pouring petrol off a jerrycan on the ground.

A minimum negative acceleration that can occur also in turbulent air can cause a misfeed and an irregular detonation for such delicate but thirsty engines.

This was sorted with the introduction of new carburetor designs, but the early configs suffered from an instant cutout when being hit by negative Gs. Even a 0 G situation could cause trouble.

[effte]

Originally Posted by Deadstick

This seems to suggest to me that even a small amount of negative G was causing the cut out, or why else would the tactic be necessary?

The tactic was necessary as the Bf109 pilots pushed more negative Gs than the Merlins could handle. In short, it's anecdotal evidence showing it happened, but next to nothing about the hows and whens.

Specifically, it doesn't say anything about whether problems began at 0 G, 0.2 G, -8G or 0.95 G.

Has anyone found a good description of the inner workings and design of the early Merlin carbs? I'd like to see one to form an opinion on when we should realistically expect the engine to cut.

[Viper2000]

Originally Posted by Sternjaeger

..the amount of guesswork some of you guys do on this forum is astonishing sometimes..

Sure. Please provide source material that will save us the effort of guessing.

Originally Posted by Sternjaeger

On average a Merlin engine gulps an astonishing 3 UK gallons (almost 14 litres) per minute at take off and circa the half (roughly 6,5 litres) at 75% FEC.

Actually the Spitfire I Pilot's Notes quote 89 imperial gallons per hour all out at FTH, which is only about 1.5 gallons/minute.

You might see 3 gallons/minute from a Merlin 66 running at +25 psi, since it produced roughly double the shaft horsepower; but that's very much a horse of a different colour. Actually I seem to recall that somewhere I've got the accurate figures for the mighty RM.17 SM engine which was of course the thirstiest Merlin of all; but they're not at university with me.

Originally Posted by Sternjaeger

It's literally like pouring petrol off a jerrycan on the ground.

A minimum negative acceleration that can occur also in turbulent air can cause a misfeed and an irregular detonation for such delicate but thirsty engines.

Why exactly does the absolute flow rate matter? We care far more about dimensionless parameters such as FAR than we do about the absolute rate of fuel flow, which serves merely to tell us that Big Engine Is Big...

Actually, if you want to get into analysis, the Merlin should be inherently more forgiving of the failings of carburettors than a more modern naturally aspirated GA engine because the supercharger both vigorously mixes the charge and heats considerably. Therefore the fuel is considerably more likely to be fully evaporated and homogeneously mixed than would be the case for a naturally aspirated engine.

I would submit that the Merlin was not especially delicate; whilst its reliability was imperfect, especially during its early life, it was considerably better than many other engines (e.g. the Rolls-Royce Vulture, almost anything ever made by Napier, many early Bristol sleeve valve engines etc).

The early ramp or "penthouse" head engines certainly had trouble passing type tests, but of course we're not talking about them in this context, since all of the aeroplanes we're interested in are fitted with Merlin II or later engines.

Originally Posted by Sternjaeger

This was sorted with the introduction of new carburetor designs, but the early configs suffered from an instant cutout when being hit by negative Gs. Even a 0 G situation could cause trouble.

How exactly do you think anything with a carburettor could be "instant"?

The carburettor is quite some physical distance from the cylinders. The flow velocity in the induction system, other than at the supercharger impeller tip, is subsonic and thus decidedly finite. It therefore obviously takes some time for any leaning of the mixture at the carburettor to impact upon the mixture at the cylinders and thus the engine shaft power output.

So even if the leaning of the mixture at the carburettor was instantaneous upon reduction in positive g, any effect upon the engine clearly could not be.

But of course, the impact of g load upon the carburettor could not be instantaneous because it is caused by physical displacements brought about by inertial loads. So obviously there is a time lag involved here as well.

Actually, inherent time lags are one of the (many) arguments raised against the carburettor, especially for automotive applications where swift throttle response is considered important.

Did you read the accident report I posted, which contains considerable information on the history of negative g cut behaviour in the Merlin?

Did you watch the video I posted which shows the actual phenomenon in flight? You can quite clearly see and hear the lags involved.

Here is some more source material:

http://www.flightglobal.com/pdfarchi...%20carburettor

http://www.flightglobal.com/pdfarchi...%20carburettor

http://www.flightglobal.com/pdfarchi...0-%202734.html

There's probably a cutaway of the SU carburettor out there somewhere on the internet, just waiting to be found... I've almost certainly also got one in my library at home, but that's several hundred miles away...