Merlin negative G cutout too quick?

[IvanK]

This from AP2095 Pilots Notes General. These were supplemental Generic aircraft handling notes to the usual individual aircraft notes:



Talk about "a sufficiently hard push down" and finally a reference to a G value ..."usually around 1/2G"

Now a 0.5G push to do something like set up a descent is a reasonable push.

[Deadstick]

After a bit of searching I foundthe document source you quoted above: A.P 2095, PILOT'S NOTES GENERAL, PROMULGATED BY ORDER OF THE AIR COUNCIL , 2ND Edition , April 1943. FOR OFFICIAL USE ONLY.

A.P. 2095 is not type-specific. It is about explaining the different operating requirements of a wide variaty of operational aircraft at the time - and over time - like engine management or handling props.

So where does it mention the Spitfire 1? As you said they are GENERIC notes that might not specifically relate to the model.

Spitfire fans clutching at straws again? (JOKE)

[IvanK]

I am a 109 driver by choice. As I said they are generic but its a clue as to the numbers involved.

[klem]

I thought I would post this extract from the Accident Report that Viper gave us the link to. It refers to a Mosquito crash in which it was found that the later carburettor modification to overcome the negative G effect was negated by a fault in the carburettor that failed to sustain fuel flow to the carburettor chamber.

"In the event that the combined dynamics of the aircraft and float chamber fuel mass caused the floats to be forced towards their fully depressed conditions, then it is likely that the ensuing restricted fuel flow could cause a loss of engine power, as the residual fuel in the chamber would last only a few seconds."

This indicates to me that the engine cut would not be instantaneous and I simply don't believe that the aircraft would have been accepted if persistent cutouts or misfires were experienced in normal flight. It is certainly not mentioned once in the many bios I have read including those that talk about the nose-over dive cut-out problem.

Is it 0.5G, 0G, -ve G? 1 second, 2, 3? I don't know but it's not 0.25G and 0 seconds.

It needs some research but if there isn't any I'd be happy with -1G and 2 seconds to onset (rpm reduction) and a couple more to engine cut. After all, we're only interested in making normal flight and descent not give problems. When we nose over in combat we NOSE OVER because we wan't to catch to so-and-so - we ain't setting up a descent - and we can expect the engine to cut out. No point in splitting hairs over half a G and a second or two. And that should be acceptable to the Blues because they still get to cut and run. Sorry, make evasive maneouvres

Also I've never read of it being stated as a problem during those RL fast descents and landings made to reduce the risk of being vulched by prowling 109s.

[Moggy]

Originally Posted by Viper2000

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

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.

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.



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...

Here's the fuel consumption of a Hurricane Mk.I for comparison;

Originally Posted by Viper2000

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

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.

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.



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...

Bloody norah! nother engineer!

my humble knowledge comes from a thing called experience (in this specific case experience with Merlin and other aviation engines), which is something you can't learn on a manual or at university, but for the sake of science (and in order to pass my exams) I also had my fair share of theory books..

Anyway, I meant on average going from the early marks to the late ones, but regardless of that, the comparisons that I've seen here are somewhat out of place. Comparing an air cooled boxer engine of a Cessna with a liquid cooled V12 is a bit of a silly thing, since the engine have humongous differences.

As for the Merlin not being a delicate engine, I beg to differ, and seriously too. The most delicate part of the Merlin is actually the cylinder banks, where -because of the external cooling jacket - cracks and microcracks are hard to spot unless going with internal inspection. A radial engine can still fly with a damaged piston and/or cylinder, a Merlin simply can't. Damages to cylinder banks on Merlins are normally the reason why we have engine failures still today. You have to appreciate that the planes we have today use 60+ years old banks, not to mention that some pilots today still think you need 100% throttle for takeoff, while we normally takeoff at 75% to save on engine life and play it safe (and because there's SO much power!). Another cause of cracks is not paying attention to temperatures, which can cause further thermal fatigue to the banks, especially on a cold start.

The G loads don't affect only the carburetor btw, but the inlet manifolds as well, so cutouts can be more or less abrupt. I haven't had the chance to fly the sim yet, maybe a video would help understanding better.

gosh, I would spend hours at the pub (or at the hangar with a pint) talking about this stuff mate lol

[IvanK]

Sternjaeger what would be your opinion on how much of a push or reduction in G would be required in an early Merlin to cause it to cough ?

At present a smooth (like doing your best IF technique) lowering of the nose to say 10 degrees nose down for a descent causes the engine to cough.

Originally Posted by IvanK

Sternjaeger what would be your opinion on how much of a push or reduction in G would be required in an early Merlin to cause it to cough ?

At present a smooth (like doing your best IF technique) lowering of the nose to say 10 degrees nose down for a descent causes the engine to cough.

IvanK it's no guesswork, you need a G-meter to determine it properly. A sustained 0G or a minimum negative G load (-0.1) are enough to interrupt the flow. Despite what our scientist friends here say, the mixture gets to the cylinder inlet almost instantaneously. As you know it's not like cylinders burn mixture all at the same time, so one cylinder not receiving enough mixture for a full detonation is enough for a cutout sometimes. Again, please guys provide me with a video and I will be able to tell you (in my humble opinion) if the effect is overmodelled or not.

[IvanK]

Here is a small video. Starts with throttle at idle then to max Boost.
Then a slow descent initiation with max Boost set.
I would be interested in your opinion on the RPM needle Bounce as well as the cough which starts about 19seconds in:

http://www.youtube.com/watch?v=Py7lfQUxgRA

I figure (based on experience) that the descent entry is less than 0.2g decrement on 1G i.e by estimation G is never getting less than +0.8G ... pretty slow entry and still it coughs

[doghous3]

Just to add to IvanK's vid.

Crusing speed with various moves to invoke this issue. Full realism.

http://www.youtube.com/watch?v=L7bxmmh0VzE

Dunno if it's just me, but the problem seems worse with engine heat realism off.. so that you can't control the rad. Prolly just me. *shrugs*

Hope the video helps anyway.