Fuel feed and negative G, worldwide.

[greybeard]

I think we all read a billion times about early Rolls Royce Merlin handicapped against Bf 109 during Battle of Britain by its own float-type carburetor...

But what about the rest of the world?

Today, for the first time, I read on Wikipedia:

"Negative G engine cut-out was not a problem unique to Merlin-powered fighters since many other pre-war aeroplanes also used carburettors containing a float chamber. For example, in 1942, the United States acquired a flyable, Japanese Mitsubishi A6M Zero, known as the Akutan Zero. When test-flown, this aircraft was also found to experience engine cut-outs, due to the float-type carburettor of its Nakajima Sakae engine. The Americans devised tactics to counter the Zero in combat that took advantage of this and other results of testing the aircraft."

and this unfolded a lot of questions for me, substantially converging to just one: how did behave under negative G's all other powerplants, from Klimov M-105 to Pratt&Whitney R-1830?

After some fierce research, I think to have understood that, in short, three types of carburating devices were in use during WWII: float carburetors, pressure carburetors and fuel injection. Only the first one was subjected to cut-off under negative-G condition. And this should have been true for ALL aircraft feeded by float carburetors.

If so, I wonder why a Yak-1 pilot never complained, fighting against a Bf 109 which could perform a sudden dive, not allowed by his Klimov? Fear of Stalin? Maybe! But why his French collegue, flying a D.520 with same carburetors, never wrote of such an inconvenience? And even German pilots experiencing flight capabilities of the French fighter, didn't mention this drawback in respect of their fuel-injected mounts, they were accustomed with.

The above quote let suppose that F4F-3 (which mounted a pressure carburetor - Bendix Stromberg PD-12E) pilots were instructed to perform steep dives to evade Zero's, that is an absolute novelty for me. I would just see confirmation of my guess extended to all engines known as feeded by float carburetors, to set them in a more complete picture against their historical counterpart, like so often made for Hurricane and early Spitfire versus Bf 109.

 

[XBe02Drvr]

A float carb aircraft engaged with a fuel injected one need not be at too great a disadvantage if it has a fast enough roll rate. Your target disappears under your nose, you quick roll inverted and pull for the shot. Where this doesn't work so well is if your target is a heavier fast-diver (P-40, F4F, BF-109, etc) and you are a lightweight floater (Zero, Spitfire, etc), specially if you're operating at speeds where your ailerons are stiffening up. Then his superior acceleration will have him at extreme range and going away by the time you can bring your guns to bear.
In the case of the Zero it was further complicated in that at diving speeds, torque, P factor, and wing warp combined to make high speed rolls and turns to the right difficult to impossible. Thus the escape from a Zero attack was: full throttle, push over HARD, accelerate past 300 mph, roll right HARD, watch him overshoot you, reverse back on his tail, aim over his head, and shoot. The one thing a Zero did well from that situation was zoom climb. If you shot AT him, by the time your tracers got there, he'd be gone. If you missed and he pulled up out of your sights, GET GONE FAST! From that situation a Zero could do an unbelievably quick loop into your six.
As for YAKs vs BF-109s, I don't know much about the particulars, but bear in mind most of the fighting in that theater was down in the weeds, which didn't leave much diving room.

 

[greybeard]

Thanks for your comment.

About quick roll inverted, I think it was a significant handicap even with the best assumptions: two seconds to roll inverted (best roll rate at best relevant speed of a Fw 190A) means, at an indicative speed of 300 km/h, to lose more than 80 m, not to mention pilot's time of reaction.

Anyway, my question was more technical than tactical. Why is so hard to know this important feature of historical aircraft? It is even irritating while reading over and over about Spitfire vs. Bf 109 during BoB, not to have info about the rest of the WWII air warfare.

In the meantime, though, it looks the Akutan Zero did not have problems with negative G cutout: its carburetor was incorrectly installed by allied technicians.

Also, in another thread of this forum was ascertained that the Solex-Hispano 56SVC carburetors of D.520 caused misfirings and then cutout in inverted flight. I guess the K-105 carburetors of the Klimov M-105 powering the Yak-1 had same problems, given they were license copies of the French one.

Continued research let me understand that float carburetors were not all identical from this point of view (like I supposed earlier), but there were differences in behaviour according to brand and model: some Bendix and Zenith models look as having some arrangement to deal with the problem. Also feature of fuel tank should be taken in account, though, since a carburetor capable to work inverted doesn't do much if not fed...

Lastly, I think this list of pressure carburetors (please search toward the bottom of page), making a liaison with engine, let establish which aircraft could have had no problems with negative G: they are mostly American with some British one; would be great to have for the rest of the world.

 

[XBe02Drvr]

I think the reason the information isn't readily available is because most haven't viewed it as significant. With relatively few exceptions the world of air combat has, until the jet age, been primarily a positive G world. Design concepts to combat tactics, the focus has been on positive G performance. Where header tanks with G-sensitive pickups and pressure carbs could be added to existing designs without excessive compromise, it of course would be done, but in the nature of icing on the cake, rather than a major upgrade.
Similar obscurity exists with regard to the progression from fixed pitch to two-position to constant speed propellers at the outset of WWII, and the rapid increase in fuel octane ratings, despite the huge performance gains these advances entailed. I suspect a lot of the people who relate these stories view these
details as "nitpicking and rivet-counting" and not of interest to their readers.

 

[stona]

Did any of the other carburettors mentioned feature a device comparable to the R.A.E. restrictor (Miss Shilling's orifice)? If they did then the problem of negative g, at least for short periods in combat manoeuvres would be mitigated.
Cheers
Steve

 

[greybeard]

Wes,
interesting point of view about world of air combat seen as a "positive G world". Nevertheless, negative G exist and who considered this attitude negligible faced inferiority problems like early Spitfire. I think that until when articles and books are assembled on the basis of what already stated, it's hard increasing knowledge (not to mention the risk to feed misinformation). I think that as often as possible, authors should start from scratch, leaving their comfortable homes and newspaper offices and going on site, looking for original documents. Nowadays Internet and forums like this increase immensely potentiality, having the common sense to scrap garbage and avoiding peril to consider technical details as "rivet counting".

Steve,
so far I read in detail only about Miss Shilling's orifice. Those about Bendix and Zenith carburetors are just hints; anyway I don't know on which aircraft they were mounted and ignore if their fuel tank could work inverted. I guess some fighters had just from the start (like, maybe, the Zero), or got later (like middle Spitfire) or never (like Dewoitine 520) similar devices to allow negative G for a short time, while prolonged negative G, as well as inverted flight, perhaps was definitely allowed only when bladder type tanks entered in service (together with pressure carburetors or fuel injection).

 

[pbehn]

Isnt the altitude of combat important? It was noted as a problem on the Spitfire and Hurricane during the BoB but not before in France at Dunkerque or in the Channel, if you start combat at a few thousand feet or lower bunting over into a dive isnt really an option.

 

[XBe02Drvr]

Bladder tanks aren't the only way to provide negative G fuel flow. Another common way was via a header tank in the fuselage that was fed by gravity from wing tanks through a system of check valves that only allowed inward flow and was kept full by "slosh factor".
Another limiting factor for negative G operation is engine lubrication. When you go inverted, the loose oil in the engine falls away from the scavenge pumps, limiting your inverted endurance to how long it takes the pressure pump to empty the oil header tank. 90 seconds in the T-34 I used to fly. Which was a LOT longer than I was comfortable with! (Do you know how LONG 90 seconds is when all the blood's in your head and every scrap of cockpit debris is stuck between your helmet and the canopy?)

 

[stona]

Funny thing time

As an imaginary experiment, try firing three shots from a bolt action rifle (like, say, a Carcano) in 7.1 to 7.9 seconds (Warren Commission, first shot missed) or 8.6 seconds (if the first shot was at frame 155 of the Zapruder footage and missed) and you'll find it goes slower than you think.

Cheers

Steve

 

[camman]

I think that pbehn is on to something. Most air battles on the eastern front were fought at low altitudes, hence why the Russians were able to make good use of planes such as the P-39, which was less useful in U.S and British service at higher altitudes. Perhaps they didn't complain of carburetor problems because they were never using their planes in ways where this problem would manifest itself. I also wonder if the volume of the float bowl could make a difference. The S.U. Carburetors used on the Merlin had very small float bowls, so I'm thinking that it wouldn't be long before the bowls would completely fill up and the engine would flood. If the bowl was larger, it might be a longer period of time before the engine floods.

 

[pbehn]

I thought the problem with the Merlin was just high negative "g" causing excessive fuel flow, the Spitfire and Hurricane could fly inverted so it wasnt just negative "g" but the level of it, this only became a problem when confronted by a fuel injected adversary.

 

[greybeard]

Ah! Header tanks and lubrication! Right! They were important too.

About Russian pilots and low level combat, of course this latter negates any advantage derivating from a steep dive, although I read that they flew at low level as evasive maneuver, trying to induce German pilots to crash into some ground obstacle. But when the Russians were on the offensive, how they could keep their advantage against a fuel injected enemy, able to push instantly his fighter in a dive? Perhaps I've too the answer: just yesterday I was reading on "Red Phoenix Rising" that over Northern Romania in 1944 they mostly flew P-39, always higher than German fighters (according to Lipfert memories), and all P-39 were fed by a pressure carburator of the Bendix Stromberg PD-12K family...

 

[Marcel]

Some types were not in front line for long enough for this to develop as a major complaint. For instance the Bristol Mercury of the Fokker D.XXI had this problem as well (well documented in Dutch literature), but fought for only 5 days against the Germans. Also much of the fighting was low level. And I think a D.XXI would rather choose to out-climb than to out-dive a BF109. In Finland they fought the Russians which also had this problem with the floating carburetor, so none had the advantage.

 

[XBe02Drvr]

Ah! Header tanks and lubrication! Right! They were important too.
About Russian pilots and low level combat, of course this latter negates any advantage derivating from a steep dive, although I read that they flew at low level as evasive maneuver, trying to induce German pilots to crash into some ground obstacle.

Of course they would take advantage of any available obstacle to scrape off a pursuer, but that's not why they were down there; down in the weeds was where the action was. There was very little use of strategic air power on the eastern front; it was all about the war on the ground. Stukas and Sturmoviks, JU-88s and PE-2s, busting tanks and beating up the troops and the supply train. In that environment survival was a matter of low and fast, specially if your opponents could do high better than you could. Negative G is not a major player in that scenario.
Cheers
Wes

 

[greybeard]

... Bristol Mercury of the Fokker D.XXI had this problem as well (well documented in Dutch literature)
...
Russians which also had this problem with the floating carburetor, so none had the advantage.

Well! Two further tiles of the puzzle! Thanks!

 

[stona]

I'm not sure what 'down in the weeds' means exactly. Most claims by 'jagdflieger' in the East were at altitudes from practically ground level up to 5,000m (about 16,500ft).
Even this upper figure is probably lower than the average in the West. I'd estimate the average altitude for claims in the East to be somewhere close to 2,000m. If that is 'down in the weeds' then I agree It's certainly low!
Cheers
Steve

 

[pbehn]

I'm not a combat pilot but surely if you are at 15,000ft or lower, going into a dive just ensures that your adversary has height advantage when you level out so you must be sure you get away. On the eastern (indeed any) front the lower you were the closer you were to ground fire which was pretty indiscriminate.

 

[XBe02Drvr]

As I mentioned in an earlier post, diving to escape an opponent works only if you can out-accelerate him in the dive. The closer the diving performance of your two aircraft are to each other, the longer a dive you'll need to make a clean getaway, thus the higher a starting altitude you'll need to succeed. Otherwise you wind up in the uncomfortable situation pbehn described above.
Considering that the primary mission of fighters was the protection of their tank busters and troop pounders down low and the destruction of the enemy's, it makes sense that most of their combat would occur down where that was happening. Think 2000 feet, not 2000 meters. This doesn't leave much room for negative-G pushovers and long diving getaways. If you're flying a MiG vs a BF-109, your options are a turning fight or going vertical. Lack of negative-G capability isn't a factor here.
The kills at up to 5000 meters had to be cases of top cover engaging top cover. Bear in mind, every aircraft devoted to top cover is one less available for the primary mission of killing (or protecting) the mud pounders.

 

[greybeard]

In general I agree about low level fightings in the East: both sides conceived air force as a tactical one, especially Russians, and, as such, aimed to CAS. But there were exceptions; I think following quote from book I mentioned earlier can help (Helmut Lipfert memories of combats over Yassy, June 1944):

"My wingman and I carefully approached the front at 7500 feet (2500 meters), being alert to the presence of Il-2s reported in the area. Suddenly, we sighted them, flying 3000 feet (1000 meters) below us. As a precaution, I looked around one more time before attacking the Il-2s and spotted two enemy Airacobras with their distinctive red noses coming at us from out of the sun. I quickly instructed my wingman to stay close. I waited and acted as if I had not noticed the approaching fighters. When the Russians were in firing range, we made a tight turn. The two Airacobras shot past as they were unable to turn with us. The Russian pilots, however, did not abandon the fight. They pulled up and came down at us again. They made one attack after another, forcing us to a lower altitude. Soon the two enemy aircraft became four. Now in the face of these alternating attacks, we were denied any opportunity to fire. My wingman was even hit several times. I then rammed the throttle forward, pulled the stick back into my stomach and spun down from 3000 feet (1000 meters) to almost ground level. I pulled out and raced away to the southwest... omissis... I decided not to let the Russians catch us so easily a second time, so we climbed quickly to around 13500 feet (4500 meters). But even at this higher altitude several Airacobras jumped us from above and gave us a scare... On the third sortie we climbed to 18000 feet (6000 meters); I had never encountered Russian aircraft at a higher altitude. But once again we failed to even reach the front. All I saw were Airacobras above. We were forced to flee the area. Now I had really had enough, and following a steep Split-S I made for my home with my wingman as fast as I could."

I think Russian aircraft set to perform such kind of escort (or ambush) not by chance were Airacobra instead of Yak or Lavochkin.

I would like to add that a float tipe carburetor starts to cause misfires also at zero G, therefore even an enemy's shallow dive may cause difficulties to the pursuer.

 

[XBe02Drvr]

I think Russian aircraft set to perform such kind of escort (or ambush) not by chance were Airacobra instead of Yak or Lavochkin.

Sounds like the Soviets had numerical local air superiority here, with more available aircraft, but less experienced pilots. They may have "handpicked" an Airacobra outfit for this particular encounter, but much more likely in the general chaos of war it was a chance meeting.
Interestingly, the Germans chose to spin down to a lower altitude rather than dive, entirely a positive G maneuver, and one that doesn't show much respect for the ACM proficiency of their opponents. Any combat-savvy fighter pilot would be waiting at the bottom to pounce on them as they recovered from their spins.
This does point to one superiority of the German aircraft; they could sustain a stabilized spin. In an Airacobra with its aft-mounted engine and its tail-heavy proclivities, that would be fatal. Unrecoverable flat spins were the Achilles heel of the Cobra.
Cheers,
Wes