The A2A Simulations Community

Maybe some of you guys may find that flying the Spit becomes a little boring because there is really no
goal but to take off and land without killing yourself and / or the aircraft.

At least in the Stratocrusier, you have people to land safely and a job to keep.

So what I did, is set myself up to take a cross country flight across the U.S. in the Spit and land it whenever my fuel gets low.
I always set the weather to "real weather" and so I never really know what's going to happen.

For instance I left JFK in New York, fat and snappy, with good VFR weather. An hour later, the weather closed in, (I'm talking Soup) and I was nearly
out of fuel, and no airport would let me land since they were all IFR. So I looked up the nearest airport and "illegally" landed there with almost zero visibility.

It was raining and I kept the canopy closed to stay dry.

Since I always save my flights, I pick up where I left off and so I had to wait until the real weather cleared before I could take off again.
I am now in Chicago and have taken off and landed three times along the way with zero damage to the plane.

To me this "make believe" cross-country flight actually adds some tension to flying the spit, and gives one a better
sense of immersion since my goal is to continue on to LAX, then take a southerly route back to New York all with the same aircraft.

To answer some old questions...



As far as I know, the system was a crude version of the automatic throttle on the P-51. It restricted boost by limiting the max throttle position at any given altitude. This device was located between the throttle lever and the carburettor. There's a whole page about it here: http://www.enginehistory.org/Piston/Rolls-Royce/R-RmerlinABC/R-RmerlinABC.shtml
The holes to restrict the system were not actually in the cylinder head but on the cutout mechanism itself, as seen here: http://www.wwiiaircraftperformance.org/ap1590b.jpg



No, there is no secondary valve of any kind nor would you need one. Max boost, overboost, the supercharger can't tell the difference. All it does is multiply the pressure at its inlet by its rated pressure ratio. The only problem I see from that standpoint is possibly stripping the drive gears since the power to the blower increases with boost, although that is quite unlikely. As for the carburettor and the throttle connected to it (carbs have throttles downstream of the venturi), they only see ambient pressure so there is no issue there in regards to overboosting.

As for a sudden throttle cut... The carburettor is upstream from the supercharger (which is why it only sees ambient pressure) and as such the supercharger will always be multiplying the pressure of the air downstream of the throttle plate. If you suddenly cut throttle from wide open at max RPM you now have a vacuum entering the blower which gets multiplied to the minimum boost at that RPM (centrifugal blowers make more boost at higher RPM).


(Larger version here: http://farm2.static.flickr.com/1136/5137237496_e1a4eb4a71_o.jpg)
EDIT: This is a late model Merlin (61 I think), just to be clear.

There is no boost spike or compressor surge because the throttle is at the beginning of the whole system. Instead there is only a sudden vacuum in front of the blower which will not harm the engine. Turbocharged engines have boost spikes upon sudden throttle cut because typically the compressor is upstream from the carb/throttle (throttle sees boost) so when it shuts the boost from the still spinning turbo stacks up on the now closed throttle plate. I have never heard of this damaging the throttle but the compressor(s) will surge and the turbo(s) may sustain damage.

As an aside, throttles upstream from the supercharger aren't just for safety, they're great for fuel economy, too! If the carburettor were after the supercharger it would run at (and consume) full power regardless of throttle setting but putting it upstream allows the carb to throttle both the engine and the supercharger and so the supercharger's power requirement drops with reduced throttle, thus saving fuel.

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That would make sense. My guesswork was based on


Your version of it being a throttle opening limiter instead of a "vent" also makes more sense to

...because should it really vent air out of the system, it should still be able to give +0 psi of boost at sea level which means Merlin would not become mere ballast but could actually provide sufficient power for safe landing.

If the spring that applied counterforce to the piston controlling a throttle limiting device snapped, BCO would be fully and constantly applied and throttle plate would jam shut, making Merlin useless even at low altitudes.


Makes sense since holes in cylinder head spell disaster. Even holes in intake manifold tend to spell trouble no matter how small the holes are (intake leak => super-lean & rough idle).


So it's like venturi -> throttle -> supercharger -> BCO valve (that controls the throttle, thus forming a negative feedback loop that stabilizes MP to fixed 6.25 psi) and then finally -> intercooler -> intake manifolds. Makes sense. I was making way too many assumptions of how it's arranged in turbocharged planes (though, even them might have venturi and throttle in conventional order). For engines with supercharger downstream of carb, venturi indeed before throttle, just as they are with non-supercharged car engines. In this configuration the carb doesn't even need to "know" whether supercharger is installed or not.

(Upstream) turbocharged carburetted systems are one thing I don't understand. Regular carb use ambient pressure as a input (to pressurize fload chamber)... and with carb upstream of throttle and supercharger pressure just before venturi is equal to that. Or that's probably how SU carbs do it as they're self-altitude compensating. With positive venturi pressure (i.e forced induction upstream of carb) the reference pressure to fload chamber has to be taken... from right before the venturi, I guess. If turbocharger is upstream, placing venturi upstream would probably still be better because pressure is more stable before throttle plate than after (even though theoretically both should be possible). And that positive pressure carbs might have slower throttle response as it might take time to equate pressure between reference point before venturi and the float chamber, requiring slower throttle movements to maintain correct mixture (and that added to turbo-lag)... Nah, let's not discuss that any deeper. It's very off-topic because that's no longer a discussion about Merlin. Could continue that in P47 or B-17 or off-topic section instead or see if there's a pre-existing topic on positive pressure carburetting.

Thanks for everyone who has participated. I think I understand Merlin engine management a lot better now. intake manifolds. Makes sense. I was making way too many assumptions of how it's arranged in turbocharged planes (though, even them might have venturi and throttle in conventional order). For engines with supercharger downstream of carb, venturi indeed before throttle, just as they are with non-supercharged car engines. In this configuration the carb doesn't even need to

I'm glad it all makes more sense, unfortunately, it's all wrong.

You need to stop dismissing what the guys who know how this thing works just because it doesn't make sense to you. The BCO DOES vent actual air to atmosphere. It is, in effect, a throttle limiting device, however it is NOT limiting the travel of the throttle lever or the butterflies of the throttle body. This is what he was trying say to get you to understand it so it made sense to you, that it ACTS as a throttle limiter, not that it IS a physical prevention to the throttle lever operation.

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Really? In the article I posted it describes the device as doing precisely that, moving the max throttle plate position based on altitude.



Is it possible that more than one type of BCO system was used over the Spit's history?

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r4y30n
No, it is not possible it is absolutley CERTAIN. Once, as I believe you and I have discussed before, the interlinking between boost, throttle and mixture was achieved, the system changed and became a fully throttle limiting device with a bypass circuit. Eventually, even RPM was linked unless you overrode it. The "override tit/lever/switch " disappeared and was effectively controlled by the gate. Nobody is suggesting that pressure/air is being vented from the individual cylinders.

What some people here are forgetting is that all the time the A2A Spitfire was in development so was the A2A P-51 Mustang. All the original Operating, Installation and Maintenance manuals for the SPECIFIC types of Merlins were examined in detail. The differences between them (and incidentally the diff between RR and Packard) were analysed fully. The Spitfire I and II, (Merlin III and XII) are not the same as a Spitfire IX , Merlin 66 or a Packard 266, nor are they the same in 1938 as they were in 1945. There are generations of difference which can be seen if you follow the progression from one to the other. In fact the Merlin IIIs and XII's had a similar number of modifications as the Spitfire itself had. My Merlin II manual contains supplements for gear up to a Merlin 66 (IIRC) as bits of those systems were used to update even a Merlin II.

There seems to be, still, a basic misunderstanding as to what Accusim DOES. It is not a "clever set of XMLs that trick FSX". It is the CREATION of the actual systems. The Merlin is "built" virtually and RUNS virtually. The throttle does not GIVE rpm and horsepower. The throttle lets a amount of virtual fuel (which varies with alt, temp etc) into the virtual system which then generates APPROPRIATE virtual horsepower given current pressure height, OAT and other factors (engine condition, compression, temps etc). It does not just tell FSX that at 20,000ft, 2400rpm this Spit flys at 240mph. It is deriving that performance. So an older engine in well maintained condition will likely give less speed than a run in, newish, immaculate engine.

In addition the Merlin you will see in the P-51 is NOT the same virtual engine with the "horsepower" factors fudged upwards. It is it's own engine, developed FROM the Merlin III, as was the original, and does include a fully detailed boost limit/mixture/altitude sensitive system. The increased engine hp come FROM the structure, size, arrangment and increased efficiency, not from "increasing the table of values".

I have, before, told the story of Scott and my conversation (I sure he won't mind me telling it again) re the Watts (weybridge) airscrew not giving proper performance above 13,000ft in beta. He looked at the prop and all seemed well. I showed him my schedule climb tests and they looked to have been performed correctly and were true to current performance...the answer, in the end was tracked to the BREATHING of the supercharger at above 12,000 to 13,000ft. That breathing ratio was changed and all of a sudden, the Spit performed correctly. THAT is the level of system modelling we are talking about...not, "Scott, I need you to up the climb figure above 13,000ft please".

The P-40 is not a merlin engine tweaked to pretend to be an Allison, it is an Allison engine built as such and modelled in the virtual world as such.

A quick google of "superchargers" and some figure tweaking is not at all how these models are built!! Scott and I would have spent more than 10 or 20 hours together on Skype over the ABCO-O on the Mk I and II. And that is just discussing the research and how to implement it. I've lost count of the hours of research done before and after that!



Darryl

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...Some say he never blinks, and that he roams around the woods at night foraging for Merlin parts...

Ah yes, I remember now. Anyhow thanks for setting the record straight!

On the note of that BCO, however... How does it actually work on the Merlin II, III and XII specifically?

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I'm at work and away from references..and all this was 12 months ago..but I can quickly give you the main features: (home now and have some more info...see edits)[/i]

1) Boost Cut-out failure leads to the engine being "choked". I think the confusion on apparent "direction" comes from linkages..much the same as people get confused on WHY you move the DeH prop control (bicycle pump) BACK to FINE but the later DeH or those when converted to CS units FORWARD to FINE. It is of course because that (the bike pump) cable runs directly from it to the "local" control at the prop, whereas on the later throttle body mounted lever the cable is BELOW the pivot point...so FORWARD actually still moves the "local" controller "back".


2) Holes in Cylinder of BCO vent excess pressure after 12lbs "modification" you reduce the amount of pressure "available" to push/resist(?) the piston to reduce excess boost on the "negative".
.. [i]Edit ignore following, unhelpful...:easier than "plugging" holes in the "other side" of the system. So rather than increase pressure on "plus" side,

3) Plugging holes edit: IN BCO cylinder leads to return to FULL natural boost ie around 18-19lbs available only after red tab on Mk I but available in takeoff gate on mk II.


4) Throttle travel is unmolested. Edit: confirmed

5) Aneroid in the system makes small (but not enough to constitute "fully automatic" adjustment for altitude that the P-51 has, edit: ie it is not as good early on as it later became)


6) With the Merlin XII the red tab would NOT be useful if throttle travel was reducible...(and a "gate" would do the same job...as it did on later models that used the system you describe). If throttle WAS reducible then 12lbs for takeoff could not be engaged without red tab being thrown (and it could) because throttle could not be advanced far enough to get into gate. There is no mechanical evidence I 've found in the GA's that a 3/16" move sideways of the throttle lever disengaged the BCO, so 12lbs was available from the system within the extra travel, ruling out restricted travel........On gated models thereafter (starting with the Mk V from memory) you are not permitted to throw the red tab with the throttle IN the gate as the full 18lbs results...it can only be activated at the "rated" stop. This is where we start to get into different mechanical arrangement for the BCO that lead next to the Mk IX (Merlin 6X, and 7X) type ideas.

The Mk V used a Merlin 45 initially and THIS engine was basically a single stage XX, so it had followed a slightly different development line.


7) The transition to a fully interlinked system started at a basic level with the early Spit IX, late Mk IXs were often missing mixture levers altogether and the prop became semi automatic and linked to throttle and boost I *THINK* starting with the Griffon Mk XIV..but I will stand to be corrected on that one. The mk IX had no finger tab (red) or any need for it. The gate did all of that work alongside the aneroid.


8 ) Later models used a "bypass", getting around the need for a red tab arrangement. Earlier models modified the way in which the BCO let fuel "in" to the system when the tab was thrown.

(EDIT : 9) The BCO maintains desired boost for any given throttle setting at all altitudes up to full throttle height...after which it maintained maximum boost available OR boost of throttle setting whichever is less.



Sorry I can't be more specific now.


Darryl

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...Some say he never blinks, and that he roams around the woods at night foraging for Merlin parts...

Brilliant, thanks a bundle! So, just to be clear, the early BCO moves your full throttle position acccording to altitude, yes? The lever will always go to the end of the gate but the throttle plate will not necessarily open all the way and we lose boost with altitude at anything less than the full throttle setting because the system can only set where the max position is at that altitude.

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Yes, that's right. I will have some more info for you soon, we just need to get a certain aircraft beta tested first

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...Some say he never blinks, and that he roams around the woods at night foraging for Merlin parts...