Negative G Dives and Miss Shillings Orifice

[Fatboy Coxy]

Hi all, I'm looking for more information on what Miss Shilling's Orifice allowed the Merlin engined Spitfires and Hurricanes to do regarding Negative G Dives.

As I understand it, the RAF entered WW2 without a fuel injection aero engine, instead using a carburettor. This short coming was amply demonstrated when dog fighting the Me-109, the German aircraft could simply put the nose down and dive into an inverted negative G dive, while if the chasing Spitfire or Hurricane followed, the float camber of the carburettor would flood and the engine choke and stall, a highly undesirable situation in the middle of a dog fight.

So, their counter was to go through the slower manoeuvre of a half roll to chase. Now given the Me-109 had the tactical advantage of being faster, while the Spitfire and Hurricane had the advantage of a tighter turn, the Me-109 could simply dive away and escape. And for the chasing RAF aircraft, they could easily lose sight of the German aircraft in its dive.

Ok, so the German aircraft gets away and lives to fight another day, disappointing from the RAF point of view I thought but not the end of the world. However, while trying to find more information on this subject I come across a thread by beaupower32, Negative G Drive, see Negative G Dive. And in his post is a comment made by the German Ace Erich Hartmann, I've only posted the bit relevant to my thoughts. ""Once I was in a duel with a Red Banner flown Yak-9, and this guy was good, and absolutely insane. He tried and tried to get in behind me, and every time he went to open fire I would jerk out of the way of his rounds. Then he pulled up and rolled, and we approached each other head on, firing, with no hits either way. This happened two times. Finally I rolled into a negative G dive, out of his line of sight, and rolled out to chase him at full throttle. I came in from below in a shallow climb and flamed him."

Which got me thinking that maybe the Germans didn't all just dive away, some may have pushed forward into a negative G dive and then performs an outside loop to get on the tail of the RAF pilot, who at this point may have felt the combat was over. Given their speeds, and the manoeuvre, I'd expect there to be some distance had opened between them but perhaps that was happening?

Reinforcing my idea was the fact that when a small working party from RAE with Beatrice (Tilly) Shilling, the inventor visited RAF squadrons in early 1941, to refit the simple restrictor, see a BiffF15 post here March until October of 1940: fighters' ranking they were very well received. The idea that now they could chase a Me-109 who could out dive them anyway didn't seem reason enough, but if it meant they could follow the Me-109 and ensure it didn't perform a loop to reattack them did seem reason enough.

So questions

Am I correct in thinking that German pilots during the BoB were performing outside loops instead of just diving away

To what degree did Miss Shillings Orifice allow the Hurricanes and Spitfire to perform negative G manoeuvres, long enough to follow the Me-109

And was there any consequences to having performed a negative G dive for the aircraft. As I understand, the pilot can manage up to 7 positive G's but only about 3 negative G's.

Kind Regards

Fatboy Coxy

 

[don4331]

The easy one: The early Merlins used a Skinner Union float Carburetor (OK, I was curious and had to look up what S.U. stood for)

So, in the carb, the "float" keeps the fuel at a constant level (not too high, not too low) in the "bowl" allowing it to be accurately metered into the engine. When there is too much fuel in the bowl, the float rises and stops additional fuel from entering the bowl. When it gets lower, the float again allows fuel to enter.​

Aside: The reason you have a updraft carburetor is - should the float sink (they were often made of brass and if not completely sealed would fill and no longer function) the carb would pour excessive fuel into the intake. With updraft carb, it just pours onto the ground. With a downdraft carb, it would flood the engine. Having the fuel gauge just rapid go for empty is preferable to having the engine stop cold...

When you commence a negative G pushover, the fuel/float rise in the bowl and shut off fuel to the carb. No fuel in the carb and the engine shuts down....​

Ms. Schilling's orifice, allowed the correct fuel amount, for full throttle, to pass when in a high negative G situation.​

​

So, in answer to question 2: It allowed the Hurricanes and Spitfire to perform negative G maneuvers long enough to follow the Me-109 that the tactic was no longer a guaranteed get out of parachute card.​

Note: While WWII fighters can fly for short periods of time inverted, they are not acrobatic planes designed to be flown at all altitudes. There are lots of system (fuel, oil, etc) that don't take kindly to negative g's.

Now opinion:

I would be cleaning out my shorts after getting surprised, shot at and having to perform a negative G pushover. It takes a Hartmann to continue the maneuver into a full outside loop (recognize, while your attacker has lost sight of you, you have also lost sight of him). Far better, to gain a little altitude/speed and re-engage from a position of your choosing.

The airframe is probably better able to take negative g's than the pilot... Most WWII aircraft are built out of isotropic material (metal) which have basically the same strength in all directions.

Don't try it in Mosquito, wood (and the design) product an anisotropic (stronger in one dimension).) Same would be true of most bombers - while you have to design for some negative (zero) g's load (that turbulence you just flew threw that bounced every thing off the ceiling), it isn't factored to the same amount.​

 

[MIflyer]

The float type carb on the Merlin produced two effects in a sharp pushover dive:

1. Lean Cutout = the fuel in the bowl of the carb (ever taken apart a carb?) floats up and thus is not down where it can be sucked into the cylinders. The answer to this was to put a standpipe inside the carb so that there was a path to allow the fuel that floated to the top to be sucked into the cylinders.

2. Rich Cutout = when normal G conditions returned the bowl of the carb had overfilled with fuel, resulting in an excessively rich mixture and poor performance until it can be consumed. Miss Shilling's Orifice was inserted in the fuel line and prevented too much fuel from coming into the carb.

All US fighter aircraft used pressure carburetors that injected the fuel into the "eye" of the mechanical supercharger and thus were not susceptible to the Merlin problem. Later Merlins were equipped with pressure carbs.

By the way, while the Germans used direct cylinder injection, the use of a carburetor gave the advantage of cooling of the air-fuel mixture due to the heat of vaporization of the fuel, resulting in a denser fuel-air charge and higher power. However this could also produce carb ice, although that would not be likely to occur with pressure carbs.

 

[MiTasol]

If you want more detail on how most of the various carbs worked see Aeroplane Maintenance and Operation series of books from early 40s volumes 1, 18 and 27. From memory the carbs on the Bristol Hercs are not covered - and they are a very complicated device.

Also I cannot remember if the Holley injection carb is covered - it is the diagram on page 37 above.

 

[Fortress2aZ9A]

Good evening all. Some time ago I was visiting a company - I think it was called Flight Engineering - in Leeds UK, which specilised in rebuilding wrecked Merlins to ground running or for static display. Unfortunately, Peter retired a couple of years ago. During that visit he showed me a 'Tilly's Shilling' , which he said was the first time he had seen one in about 40 years of dealing with Merlins. I have attached some pictures I took of it, at the time.
Regards
Mike D.

 

[MIflyer]

When the RAF went to American style pressure carbs I assume that Miss Schilling's invention was removed.

 

[Nerd1000]

More in-depth coverage of the Merlin Carb issues:

https://www.calum-douglas.com/article-1-rolls-royce-merlin-carburettor-development/

 

[z42]

The tragic irony is that Beatrice Shilling had been involved in the development of a RAE fuel injection system before the war, but that wasn't accepted into service /production due to the antagonistic relationship between RAE and RR. And the AM evidently also thought there was nothing wrong with the float carburetor.

So Miss Shilling became famous for developing a quick not so good but better than nothing stopgap solution to a problem she and her colleagues had solved properly years ago!

 

[Engineman]

Hi all, I'm looking for more information on what Miss Shilling's Orifice allowed the Merlin engined Spitfires and Hurricanes to do regarding Negative G Dives.


So questions

Am I correct in thinking that German pilots during the BoB were performing outside loops instead of just diving away

To what degree did Miss Shillings Orifice allow the Hurricanes and Spitfire to perform negative G manoeuvres, long enough to follow the Me-109

And was there any consequences to having performed a negative G dive for the aircraft. As I understand, the pilot can manage up to 7 positive G's but only about 3 negative G's.

Kind Regards

Fatboy Coxy

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

 

[Engineman]

When the RAF went to American style pressure carbs I assume that Miss Schilling's invention was removed.

The RAE restrictor was only fitted to UK Merlin engines with the basic SU float type carburettors. Some aircraft may have been modified later with a different UK float carburettor
that incorporated the RAE anti-G modifications. In those cases, the RAE Restrictor was removed.
Pressure or Injection carburettor fitted engines never had a RAE Restrictor fitted, they did not need it.

Eng

 

[Engineman]

From memory the carbs on the Bristol Hercs are not covered - and they are a very complicated device.

The Bristol Hercules engines were fitted with various Hobson float carburettors until at least 1943. Later production saw the introduction of various versions of the
RAE Injection system, later called the Hobson Injection carburettor.

Eng

 

[MiTasol]

The Hobson is the one that is a very complicated device with many moving parts including a small impeller to create the pressure differential as opposed to the throat providing the differential like on the US pressure carbs.

 

[MIflyer]

A friend of mine told me that the engine on his Toyota Tundra had a fan in the intake, sort of a low grade supercharger.

 

[MiTasol]

From Pressure injection fuel metering carburettor | Science Museum Group Collection. It is amazing how little shows up on the net regarding this device.

 

[Engineman]

The Hobson is the one that is a very complicated device with many moving parts including a small impeller to create the pressure differential as opposed to the throat providing the differential like on the US pressure carbs.

The Hobson RAE Injector is quite a complex system, but I would suggest most similar systems are also complex.
The Hobson Injector system has an unusual Plessey fuel pump that actually uses a main 4-vane rotary pump, coupled with a small centrifugal impeller pump. The Impeller is used to provide the pressure head onto the fuel as it flows into the fuel metering section of the system. In the Bendix Stromberg PD18 (or RR 8D/44) Injection Carburettor system, the fuel pump is illustrated with a more usual spur-gear pump with pressure regulation by a spring loaded relief valve that can spill back to its inlet.

Both the Bendix Stromberg and the Hobson systems use differential air pressures to influence the fuel flow to match the airflow requirements. The Hobson Injector uses "Boost pressure" and "Exhaust back pressure" (which will be approx static air pressure).
In contrast, the Bendix Stromberg PD18 has, Venturi that provide "Venturi suction" and Impact tubes that provide "Air impact pressure". Additionally, Chokes in the inlet provide "Air intake pressure approx" to the Pressure/Temperature Mixture control unit.

I enclose some illustrations of the Hobson Injector system and the Bendix Stromberg PD18 type system.
As said, both systems are complex.

Eng

 

[Engineman]

From Pressure injection fuel metering carburettor | Science Museum Group Collection. It is amazing how little shows up on the net regarding this device.

View attachment 827589

Yes. Possibly the best (later) information is in the original Hobson manuals. The Hobson RAE Injection manuals only seem to start with the later War period injection systems for the Napier Sabre IV and Bristol Hercules 100 series, onwards from 1943. The earlier detail is piecemeal from various original manuals and books, such as the biography of Beatrice Shilling "Negative Gravity". Official records of the RAE and other aircraft testing reports provide other scattered detail.
The story of the development of the carburettors for the Rolls Royce Merlin family is likewise thin. However, a comprehensive review of Merlin carburation chronology is available on Calum Douglas' website, as linked at post #7 above. IMO, well worth reading.

Eng

 

[MiTasol]

I always found the Bendix PD carbs easy to understand and work on and the Hobson carbs on the Herc's the opposite. The article on Calumn's site is excellent but does infer that the RAE did not favour injection carbs which is not correct - as we know they developed and flew an injection carb in 38.

Thanks for the heads up on Shilling's book "Negative Gravity". Unfortunately the sole copy available at present at is well beyond my budget at $202.

I know of a copy of the manual for the Hobson carb for the Alvis Leonidis but unfortunately the owner will not loan or copy it.

 

[MIflyer]

Is selecting that type of carb to go on an engine an example of Hobson's Choice?

 

[Engineman]

The article on Calumn's site is excellent but does infer that the RAE did not favour injection carbs which is not correct - as we know they developed and flew an injection carb in 38.

The RAE did develop that "RAE" Injection carb, from a technical proposal by AA Griffith in the early 1930's. However, we have to remember that the RAE was a subordinate technical branch of the Air Ministry and followed Ministry direction. The progress of the RAE Injection carburettor development was very slow, Rolls-Royce declined to be involved and Bristol don't seem to have been that keen. The first production version for the Bristol Hercules did not appear till 1943.
There was a definite hiatus in the development of British aero-engine fuel system technology in the 1930's, despite the positive and innovative inputs of Ricardo, AA Griffith, Beatrice Shilling and others, the overall attitude of the Air Ministry and the RAE was not generous or a high priority, until about 1941.
Bear in mind, in the mid 1930's Germany perfected Fuel Injection and USA developed their own Pressure carburettor.

Eng

 

[MiTasol]

It will probably surprise you that on the B-25's I preferred the Holley carbs. They just kept on keeping on on the B-25 we had that had Holley's but the two with Bendix gave trouble. On every other type the Bendix pressure carbs were trouble free, including on the TBMs. Probably just Murphy screwing with me.