Negative G Dives and Miss Shillings Orifice

Old carbs are complicated and IMO, prone to various maladies that include internal corrosion, degradation of rubber parts, sunken floats and all manner of previous abuse.
I found that the Daimler-Benz / Bosch Fuel injection on the big DB engines was reliable and straightforward. It has mostly mechanical parts that function with strong forces, there is only one diaphragm, that is easy to inspect, and the internals are largely full of oil. The injection pump and fuel control unit does need setting up on a rig with mechanical drive and plumbing but, Diesel rack type pump rigs can be adapted to suit.

The old WW2 carbs have been involved in many vintage aircraft incidents in recent decades, they need a lot of care and maintenance.

Eng

Here are some scans from TM 1-407 dated 4 Nov 1941

Engineman said:

The subject of aircraft Float carburettors and WW2 aircraft is a very involved topic.
However, simple replies for your questions are:

German tactics that caused particular problems for Merlin fighters in the BoB are generally recorded as the German Bf 109 simply pushing into negative G to enter a dive
and the Merlin cutting-out when trying to follow into negative G.

The RAE Restrictor allowed continued running of the Merlin during short periods of negative G.

All the fighter aircraft in the BoB had limited ability to operate under negative G due to fuel and oil systems not being designed for longer than a short duration period of
negative G. Of note, the Bf 109 would be likely to lose fuel supply from the fuel tank after approx 10 seconds of negative G.

Calum Douglas has published more detail on the subject and his website has good information.

Eng

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Hi Engineman, thank you for this

Fatboy Coxy said:

Hi Engineman, thank you for this

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That's OK.

The point about how long the WW2 engines could produce power under continued negative G even with a suitable WW2 fuel injection is worth consideration. The oil systems generally were not suitable for continued negative G, most oil supply systems relied on feed from the bottom of a tank and most engine scavenge systems relied on oil draining into a sump area. So, oil pressure was usually lost within a few seconds of negative G being applied. Engine limitations would caution pilots to minimise engine running with low oil pressure, but for fighter aircraft, short periods of low oil pressure during fighting were allowed. Specific aircraft manuals may state limits.
For Fuel supply problems, the fuel supply in the pipes and filters might last a short while, maybe 10 seconds on a Bf 109 at high power. The time to recover fuel feed, after engine fuel starvation, with positive G restored was more than 10 seconds. So, negative G manoeuvring with power was probably limited to less than 10 seconds, unless you didn't mind losing engine power. That time is sufficient to push over into a steep dive, or even past the vertical, but it is unlikely to be sufficient for a negative G outside loop.

Eng

Engineman said:

he oil systems generally were not suitable for continued negative G, most oil supply systems relied on feed from the bottom of a tank and most engine scavenge systems relied on oil draining into a sump area.

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I looked into that a while back. From the P-40 to the F-15 and every fighter in between the pilot's manual says No More Than 10 Sec Upside Down. Of course, Thunderbirds aircraft were modified.

By the way, aircraft piston engines generally have much larger oil quantities than you would expect, so that there is more in the sump/tank to prevent interruptions in the oil flow, as well as to aid with the cooling.

MIflyer said:

I looked into that a while back. From the P-40 to the F-15 and every fighter in between the pilot's manual says No More Than 10 Sec Upside Down. Of course, Thunderbirds aircraft were modified.

By the way, aircraft piston engines generally have much larger oil quantities than you would expect, so that there is more in the sump/tank to prevent interruptions in the oil flow, as well as to aid with the cooling.

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Just quoting the Bf 109 G with DB 605 A as an example, the oil tank (full) held 36L with around another 20L in the process of circulating in the engine and oil cooler. However, there was only 3L of oil between the oil tank and the oil pump and with circulation at about 1L/sec that gave 3 seconds before oil pressure would fall.
Of course, the loose oil inside the engine would continue to lubricate the majority of parts. But, the critical crankshaft bearings needed pressure oil for their oil film lubrication, and damage would soon start to occur to them after loss of oil pressure.
Interestingly, very late versions of the Bf 109 fitted with the DB 605 D engine incorporated an extra centrifugal oil de-aerator pump that recirculated de-foamed engine oil after it passed through the oil cooler, and mostly bypassed the oil tank. This system did provide some improvement in oil de-aeration but, it still required to be fed by the engine oil scavenge pumps and the oil tank, both of which lost feed when inverted and so, was not a fully inverted-flight oil system.
The Bf 109 fuel system remained feed limited during negative-G, with the loss of power after about 10 seconds of negative-G remaining a limitation on power during negative-G flight.

Eng

MIflyer said:

I looked into that a while back. From the P-40 to the F-15 and every fighter in between the pilot's manual says No More Than 10 Sec Upside Down. Of course, Thunderbirds aircraft were modified.

By the way, aircraft piston engines generally have much larger oil quantities than you would expect, so that there is more in the sump/tank to prevent interruptions in the oil flow, as well as to aid with the cooling.

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In most turbine engines the primary purpose of the oil is to cool the bearings with lubrication the secondary requirement.

MiTasol said:

In most turbine engines the primary purpose of the oil is to cool the bearings with lubrication the secondary requirement.

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Well, in the turbines I dealt with, turning at 60,000 to 75,000 RPM, bearing lubrication was pretty darn important. But those were not engines.