Why the heck did they design it that way?

[Ivan1GFP]

Using air racers, especially the 1946-47 ones, as a benchmark doesn't actually tell us much. Many of the racers just used what they could buy cheap as surplus and sometimes it what they had flown in the war. They familiar with it. It took a few years to sort out what was competitive and what was not, and the hobby guys without much money tended to get left behind. Spitfires were more than a little hard to come by in US scrap and surplus sales. You could spend more shipping a surplus Spitfire from England than it cost to by an entire US fighter (with fuel still in the tanks) so yes, foreign aircraft were scarce.
By 1948 it took deep pockets and or sponsorship to get to the front of the pack although a few hobby flyers still showed up.
The Airacobra was as close to factory racer as you could get. Prep work was done in a Bell hanger by Bell employees in their off duty time with consulting done by Bell engineers. It may not have been paid for by Bell but no other race team in 1946-47 had that level of technical expertise.

Hello Shortround6,
Yes, I agree. Comparing racing configurations is a bit of a stretch, but in later years it does illustrate a bit of the limitations of the airframes.
As I see it, a NACA 2200 series airfoil does have some nice handling characteristics, but isn't really optimal for high speed, and speed was the only point I was trying to make that seems to have gotten a really good argument going.
The racing Cobras were an interesting story but one has to also remember the outcome despite the technical expertise and resources available to the team:
These folks were improvising just like everyone else. Their engines and propellers were surplus King Cobra units. Jack Woolams, the primary pilot, died from an apparent structural failure when testing Cobra 1. This conclusion is supported by the additional bracing added to Cobra 2 piloted by Tex Johnston who was the eventual winner of the race that year.

Quite a few of those racing planes ended up as smoking holes in the ground.

The Problem with using the later racers as indicators of actual WW II performance is that many of the later ones were extensively modified. In fact even some of the last of the early racers were a far cry form stock condition.
despite this a Spitfire Mk XIV, stock except for gun and armor removal, finished 3rd in the 1949 Tinnerman Trophy race behind a F2G-1 Corsair and a P-51K

I had forgotten about that appearance of the Spitfire. If you look at the speeds achieved, they are somewhat indicative of the point I was trying to make though:
F2G: 386 MPH
P-51: 379 MPH
Spitfire: 359 MPH

As you had commented on, modification are much more extensive today than in the past with much larger budgets.
We see Griffon powered Mustangs such as Red Baron (RIP) and R-3350 powered Bearcats and Corsairs. We see Hawker Sea Furies.
What we don't see is Spitfires, Hellcats, P-40s, P-38s, Thunderbolts and other airframes that may have made for great fighters but have limitations for absolute speed. While one can argue that Spitfires are a bit hard to find, if they really had a potential advantage, someone would most likely have found a rebuildable one to use as a racer. Just look at the Super Corsair (RIP) to see where it started from.
The point as before is that for the same amount of engine power, a Spitfire is only about average for speed. I figure from a speed standpoint, it has about the same potential as a P-40.

- Ivan.

 

[pbehn]

Are the Griffon Mustangs faster?

 

[pbehn]

The Spitfire was a much lighter airframe which is where a lot of its performance came from. I am actually pretty sure it actually could get airborne but maybe not in a reasonable length runway. If you want to take that argument a little further, then how about a 100 HP engine that would easily get a Fokker biplane airborne but has no hope of powering a heavier aeroplane?

If you really want to compare airframes, then take a look at what has been run in the "Unlimited Class" air races. An Airacobra won in 1946, the P-40Q has raced though it crashed, Corsairs, Bearcats, Mustangs, Sea Furies race these days. Many of those planes don't have Laminar Flow wings either. When was the last time you saw a Spitfire race? Does it even have a chance of being competitive there? I don't believe we have ever seen a Hellcat or a regular P-40 race either.

.

This is pure fanboyism. The Spitfire is a lightweight design because it had to be, with the early Merlin engines and propellers it had to take off in a reasonable distance. The P-51 airframe is approaching a ton heavier than the comparable Spitfire MIX. You discount the fact that it was in service in 1939 while the Allison engined Mustang appeared in late 1942 as immaterial. You emphasise its performance at low level with the Allison engine ignoring the fact that it didn't perform at all at high levels. When fitted with a Merlin engine the P-51 had good performance but the Merlin was a bigger engine, the Griffon was just a bigger engine. The P-51 as mustang I,II,III or IV was never the top performer in the RAF at any altitude at any time during the war. It did have prodigious range when fitted with an extra tanks inside and out but the drawback to that is the size and weight of the fuselage. The British had need of front line fighters before the Autumn of 1943.

 

[buffnut453]

I had forgotten about that appearance of the Spitfire. If you look at the speeds achieved, they are somewhat indicative of the point I was trying to make though:
F2G: 386 MPH
P-51: 379 MPH
Spitfire: 359 MPH

The ONLY thing that indicates is that, on that particular day, these were the results achieved for those 3 specific airframes. I'd like to know the composition of the entire race, that way we'd know if/how many P-51s and Corsairs were beaten by the Spitfire to achieve this 3rd place position.

While one can argue that Spitfires are a bit hard to find, if they really had a potential advantage, someone would most likely have found a rebuildable one to use as a racer.

Because finding a Spitfire and getting it to the States was a lot more expensive and harder than just finding an unused or slightly used P-51 or F4U that were sitting in their thousands at airfields across the US. Cost and availability are key drivers that cannot be ignored in this context.​

The point as before is that for the same amount of engine power, a Spitfire is only about average for speed. I figure from a speed standpoint, it has about the same potential as a P-40.

So you base that on one race, from an example that hadn't been extensively modified from its wartime configuration and power performance? And the Spitfire has the same speed potential as a P-40? Please feel free to carry on figuring in your fantasy world of miniscule sample sets. ​

 

[Shortround6]

Most of these air races had numbers too small to signify much. This particular race/event only had 9 entrants, two of which dropped out during the race and two did not start.
The Spitfire did beat, rather handley an F-5 (P-38) and a P-38. which were down under 330mph. A P-51A dropped out on the 7th lap and another P-38 dropped out on the 3rd lap.
The SOHIO event also had 9 entrants, all finished.
Fastest was that highly modified Mustang at 388mph,
2nd was one of Cleland Cooks F2Gs at 384mph
3rs was a P-63 at 381mph
4th was a P-51D at 376
5th was a P-51D at 373mph
6th was a P-63 at 359mph
7th was a P-51D at 349mph
8th was a P-63 at 330mph
and last was a P-39Q at 329mph.

What can we "learn" from this?
1. The P-39Q was about as fast as a P-63
2. P-63s were faster than P-51Ds
3. Corsairs with over 3000hp are faster than Mustangs with under 2000hp?

Btw 1949 saw the only other entry of a non American plane in the National air races of this time.
In the Bendix (Cross country from Rosamond Dry Lake, CA.) to Cleveland Ohio, Mustangs finished 1,2,3 with a MK 25 Mosquito finishing a distant 4th.
A Martin B-26 failed to finish before the deadline after landing for repairs and an Seversky AT-12 dropped out in Gran Junction CO.
The Mosquito suffered from failures of the oxygen system and one engine near the end of the race.
The winner

" Joe DeBona
an ex Air Force pilot flying the Bendix for the third time, flew a P-51B Mustang sponsored by actor
Jimmy Stewart. His P-51C was refinished with no less than 48 coats of primers and gloss cobalt blue
paint. Polished to a high shine, the paint job reportedly added 8 mph to the speed of the aircraft.
A special propeller was installed. It was designed to increase the speed of Air Force Mustangs up to
10 mph but rejected because its service life was too short. Joe's wet wing racerfavored by tailwinds
of 28 to 33 mph and great navigation brought him into the winners circle at Cleveland."

again from 1949 National Air Races
He did average 470mph.

What does this tell us?

A modified Mustang (wet wing) with special finish and special propeller flying at an unknown altitude with tail winds can beat a Mosquito flying at unknown altitudes with unknown tailwinds (pilots choose their own specific routes and cruise altitudes) that suffers equipment failures.

 

[Shortround6]

From the 1946 race;
Paul Mantz sought advice from Kelly Johnson of Lockheed on how to the wing the Bendix cross country race. Johnson advised to him "wet the wing". Fill all openings, seal the inside of the wing and fill it with gasoline. Mantz may or may not have thought of the trick of wrapping dry ice around the fuel filler hose to chill the fuel as the tanks were filled. In any case the modified Mustang held 875 gallons without drop tanks.

 

[Ivan1GFP]

This is pure fanboyism. The Spitfire is a lightweight design because it had to be, with the early Merlin engines and propellers it had to take off in a reasonable distance. The P-51 airframe is approaching a ton heavier than the comparable Spitfire MIX. You discount the fact that it was in service in 1939 while the Allison engined Mustang appeared in late 1942 as immaterial. You emphasise its performance at low level with the Allison engine ignoring the fact that it didn't perform at all at high levels. When fitted with a Merlin engine the P-51 had good performance but the Merlin was a bigger engine, the Griffon was just a bigger engine. The P-51 as mustang I,II,III or IV was never the top performer in the RAF at any altitude at any time during the war. It did have prodigious range when fitted with an extra tanks inside and out but the drawback to that is the size and weight of the fuselage. The British had need of front line fighters before the Autumn of 1943.

Hello Pbehn,
Don't you think name-calling is a bit rude?
I am finding this discussion extremely strange because it all started with an observation (correct or not is for each to decide on his/her own) that the Spitfire wasn't a particularly fast aeroplane for the amount of installed power. I find it particularly funny that you accuse me of being a "fanboy" when the reality is that I have a much higher opinion of the Spitfire as an air-superiority fighter than I do of Mustangs in general and the Merlin Mustang in particular. Why does this speed issue bother you so much?

As for Griffon Mustangs, I believe the racing versions were reaching into the low 500 MPH range at very near Sea Level.

The ONLY thing that indicates is that, on that particular day, these were the results achieved for those 3 specific airframes. I'd like to know the composition of the entire race, that way we'd know if/how many P-51s and Corsairs were beaten by the Spitfire to achieve this 3rd place position.

I am actually in agreement with you on this point. Shortround6 already posted the rest of the results.
Everyone was a bit faster than stock but the question is how much faster.
With stock aircraft I would have expected the spread to be less but the same order of finish with a very close call between the Mustang and Spitfire.
Please note though that it was a Griffon Spitfire that raced and not a Merlin Spitfire.
Pylon racing is at low altitude and at low altitude, a Merlin Spitfire is surprisingly slow.

Because finding a Spitfire and getting it to the States was a lot more expensive and harder than just finding an unused or slightly used P-51 or F4U that were sitting in their thousands at airfields across the US. Cost and availability are key drivers that cannot be ignored in this context.

This may have been true for the early post war races, but is much less true today. That is why I mentioned the "Super Corsair". It started off in pretty poor condition before all the work was done to it. If the Spitfire had some speed advantage, the cost would not stop people today.​

So you base that on one race, from an example that hadn't been extensively modified from its wartime configuration and power performance? And the Spitfire has the same speed potential as a P-40? Please feel free to carry on figuring in your fantasy world of miniscule sample sets.

​

I don't know if you saw the beginning of the discussion but the original comparison was between a Spitfire Mk.IX and a P-51B. Both aircraft are running engines that are pretty near identical and yet there is about a 30 MPH speed difference. The comparison with a P-40 came about because at one point the P-51 and P-40 used identical engines as well. The difference there is that the earlier Allison Mustang was a bit more slick than the later Merlin versions, so draw your own conclusions from that.

- Ivan.

 

[pbehn]

Hello Pbehn,
Don't you think name-calling is a bit rude?
I am finding this discussion extremely strange because it all started with an observation (correct or not is for each to decide on his/her own) that the Spitfire wasn't a particularly fast aeroplane for the amount of installed power. I find it particularly funny that you accuse me of being a "fanboy" when the reality is that I have a much higher opinion of the Spitfire as an air-superiority fighter than I do of Mustangs in general and the Merlin Mustang in particular. Why does this speed issue bother you so much?
As for Griffon Mustangs, I believe the racing versions were reaching into the low 500 MPH range at very near Sea Level.
.

The discussion is strange because the topic is why was the Spitfire designed as it was. It was designed as it was because of what was available and known in the mid 1930s and what it was required to do. You continually throw in the P-51 which was designed a generation later, the laminar flow wing was as much a new technology as the constant velocity prop, two stage supercharger and 130 octane fuel are. You cannot have a P-51 with laminar flow wings in 1939 the engines and aerofoils didn't exist. It is you who has the obsession with speed, you completely dismiss rate of climb, harmony of controls and turn performance then leap onto "reno racers". By the time the P-51 B entered service both the UK and Germany had jets flying, by the time the P-51D entered service both the British and Germans had jets in service, what relevance has a reno racer, all the Reno prop planes are slower than a Meteor and Me262 If you are quoting the speed of Griffon engined Mustangs at Reno, then Reno isn't at sea level, it is about 4,500ft.

 

[pbehn]

One eternal mystery to me was the US introducing fighters with "birdcage" canopies long after the Spitfire and Fw190 were in service with "Malcolm hood" and type bubble canopies, even as a layman the advantage is obvious.

 

[Ivan1GFP]

Hello Pbehn,

I do see the points you are trying to make, but you are also forgetting that my original reply was to a comment about speed of the Spitfire. The Mustang was only brought in as an illustration. As for the "not invented yet" descriptions, I believe you are incorrect about mot of those and that some of them are somewhat irrelevant.
1. Constant Speed propellers were in service long before the war started and yet the early British fighters didn't have them.
2. Perhaps the specific Laminar flow airfoils used on Mustang et al. were not yet used, but contemporaries of the Spitfire used other more "Modern" airfoils. Basically this was a choice. Perhaps there was a reason most of the folks used a 23000 series airfoil? Other folks also made some rather interesting and unusual choices.
3. Multi-stage superchargers may not have been a common thing but the Spitfire made a name for itself without one.... at least to start.
Every other common fighter started that way as did Mustang and P-40.
4. As for 100 octane gasoline, a fellow name Jimmy Doolittle did a fair bit on that subject well before Spitfire existed.

I am actually not particularly obsessed with speed. I just pointed out an observation and got a surprisingly energetic objection. The most interesting thing was that the objection was not so much against what I had actually stated but against what I had NOT stated. The racing aircraft only were brought into the discussion to suggest that if an airframe is seen to have good speed potential, the experts would probably have used it.

....
It is you who has the obsession with speed, you completely dismiss rate of climb, harmony of controls and turn performance then leap onto "reno racers".

I actually wasn't doing a general comparison between two aircraft. Why are you assuming (incorrectly) that I am dismissing the other factors when I didn't? This wasn't a discussion of merit. It was only a simple assertion about relative speeds of two aircraft with the same engine.
Now keep in mind that I am not really disagreeing with your assertion about qualities of Spitfire, but that isn't really the entire story either.
If you read Captain Eric Brown's evaluations, you also might note that he thought the FW 190 had excellent control harmony.
Tactical evaluations of Spitfire Mk.IX versus FW 190A give the maneuverability advantage to FW "except for turning circles".
There is also a minor issue of Longitudinal instability in the early model Spitfires that was addressed in later models with changing the aerodynamic balance of the elevator. Since I haven't flown a Spitfire personally, I can't claim great personal knowledge, but there is a video of Mark Hanna (RIP) flying a Spitfire Mk.IX very low and constantly making control corrections.
The point here is that every aircraft has its issues.

Regarding birdcage canopies:
That is a goofy thing isn't it? At least from a tactical standpoint. From an aerodynamic standpoint, the Malcom hood had a small speed penalty.
There is no question what the better design was for a fighter but at least on the Spitfire, I like the appearance better without the Bubble Top.
Perhaps this question should also be asked about other aircraft designed during the same period. The Hurricane never got anything better, and Tornado and Typhoon didn't start with anything particularly good either. Airacobra started with a good design as did Hayabusa and A6M. It also took a long time before Spitfire and Corsair got the more modern Bubble canopy. I wonder what a Me 109 would look like with a Bubble Top.

- Ivan.

 

[pbehn]

Hello Pbehn,

I do see the points you are trying to make, but you are also forgetting that my original reply was to a comment about speed of the Spitfire. The Mustang was only brought in as an illustration. As for the "not invented yet" descriptions, I believe you are incorrect about mot of those and that some of them are somewhat irrelevant.
1. Constant Speed propellers were in service long before the war started and yet the early British fighters didn't have them.
2. Perhaps the specific Laminar flow airfoils used on Mustang et al. were not yet used, but contemporaries of the Spitfire used other more "Modern" airfoils. Basically this was a choice. Perhaps there was a reason most of the folks used a 23000 series airfoil? Other folks also made some rather interesting and unusual choices.
3. Multi-stage superchargers may not have been a common thing but the Spitfire made a name for itself without one.... at least to start.
Every other common fighter started that way as did Mustang and P-40.
4. As for 100 octane gasoline, a fellow name Jimmy Doolittle did a fair bit on that subject well before Spitfire existed.

I am actually not particularly obsessed with speed. I just pointed out an observation and got a surprisingly energetic objection. The most interesting thing was that the objection was not so much against what I had actually stated but against what I had NOT stated. The racing aircraft only were brought into the discussion to suggest that if an airframe is seen to have good speed potential, the experts would probably have used it.
.

1 Yes, but not on fighters, and it was peacetime, Rolls Royce and Bristol formed a company to develop their own, De Havilland also started developing them.
2 The P51 aerofoils are not truly laminar flow, they were just better than previous developments.
3 Yes the single stage supercharger did OK in 1940 it was outclassed in 1941/42 by the Fw190. It was the Mk IX with the two stage supercharger that took the Spitfire back to the frontline in 1942 and the P-51 a little over a year later.
4 Not 100 octane 130, the Merlin ended on 150 with the Hornet

 

[pbehn]

I actually wasn't doing a general comparison between two aircraft. Why are you assuming (incorrectly) that I am dismissing the other factors when I didn't? This wasn't a discussion of merit. It was only a simple assertion about relative speeds of two aircraft with the same engine.

Regarding birdcage canopies:
That is a goofy thing isn't it? At least from a tactical standpoint. From an aerodynamic standpoint, the Malcom hood had a small speed penalty.
There is no question what the better design was for a fighter but at least on the Spitfire, I like the appearance better without the Bubble Top.
Perhaps this question should also be asked about other aircraft designed during the same period. The Hurricane never got anything better, and Tornado and Typhoon didn't start with anything particularly good either. Airacobra started with a good design as did Hayabusa and A6M. It also took a long time before Spitfire and Corsair got the more modern Bubble canopy. I wonder what a Me 109 would look like with a Bubble Top.

- Ivan.

But you were, you repeatedly talk about "The Spitfire" why not start at the Spitfire Mk21 and work backwards?

 

[Shortround6]

The British air ministry (or the dolt in charge of propellers) should have been summarily shot for either treason or criminal malfeasance.

Rotol and De Havilland were making variable pitch propellers pretty much in the face of official disapproval. Fedden ahd Hives knew they were needed and would be needed in numbers when the war came.
It wa not only fighters. way, way to many British bombers got two pitch propellers years after not only invantly variable but constant speed props were available.
Way, way too many British multi engine aircraft had to put up with putting the prop on a dead engine in course pitch and using a propeller brake rather than being able to feather the prop on the dead engine. Not all constant speed props are fully feathering.
And many British planes had to use props with a limited amount of pitch change. 20 degrees is not really enough for fast aircraft.

Laminar flow is relative, the Davis airfoil used on the B-24 was sometimes described as laminar flow and it was nowhere near the improvement the P-51 wing was.

there were two RR single stage superchargers in 1940. The Pre Hooker one used on the Merlin III, VIII, X and few others and the Hooker one used on the XX and the 45 and few others. The FW 190 might have been a bit less of problem had they mounted Merlin XX engines in Spitfires and allowed a bit higher boost sooner.

British 87 octane may have been a bit better than some other peoples. In any case British 100 octane during the BoB was actually 100/115-120. American 100 octane in 1940 was 100/98-102. That is right, some American 100 octane (lean) was less than 100 octane when running rich.

 

[pbehn]

The British air ministry (or the dolt in charge of propellers) should have been summarily shot for either treason or criminal malfeasance.

Rotol and De Havilland were making variable pitch propellers pretty much in the face of official disapproval. Fedden ahd Hives knew they were needed and would be needed in numbers when the war came.
It wa not only fighters. way, way to many British bombers got two pitch propellers years after not only invantly variable but constant speed props were available.
Way, way too many British multi engine aircraft had to put up with putting the prop on a dead engine in course pitch and using a propeller brake rather than being able to feather the prop on the dead engine. Not all constant speed props are fully feathering.
And many British planes had to use props with a limited amount of pitch change. 20 degrees is not really enough for fast aircraft.

Laminar flow is relative, the Davis airfoil used on the B-24 was sometimes described as laminar flow and it was nowhere near the improvement the P-51 wing was.

there were two RR single stage superchargers in 1940. The Pre Hooker one used on the Merlin III, VIII, X and few others and the Hooker one used on the XX and the 45 and few others. The FW 190 might have been a bit less of problem had they mounted Merlin XX engines in Spitfires and allowed a bit higher boost sooner.

British 87 octane may have been a bit better than some other peoples. In any case British 100 octane during the BoB was actually 100/115-120. American 100 octane in 1940 was 100/98-102. That is right, some American 100 octane (lean) was less than 100 octane when running rich.

As a law of unintended consequences maybe the early fighters developed for the RAF without constant speed props were lighter than they would have been and therefore better performing in the end?

 

[pbehn]

Just saw these great pictures
by Terry in the "Flying Legends 2018" thead.
View attachment 502510
View attachment 502511

Please note the angle of the flaps.
They are doing just about nothing for lift and are acting as air brakes (drag producers and/or lift spoilers).
This was rather common for flaps designed in the mid 30s. You still needed a big wing for take-off but the flaps kept you from floating down the runway for a long distance on landing and steepened the glide angle/approach angle.

For take of from carriers flying to Malta the landing flaps were modified to take off flaps by wedging them part closed, after take off the flaps were fully opend to drop the wedges and then closed.

 

[pbehn]

British 87 octane may have been a bit better than some other peoples. In any case British 100 octane during the BoB was actually 100/115-120. American 100 octane in 1940 was 100/98-102. That is right, some American 100 octane (lean) was less than 100 octane when running rich.

That sounds a bit complicated S/R obviously well understood by all parties in 1939?

 

[redcoat]

Because finding a Spitfire and getting it to the States was a lot more expensive and harder than just finding an unused or slightly used P-51 or F4U that were sitting in their thousands at airfields across the US. Cost and availability are key drivers that cannot be ignored in this context.​

Only 1 Spitfire was ever sold to a civilian by the British government and that was due to a lot of fudging of the governments policy guidelines on the sale of military aircraft, while the USA was happy to sell hundred's of P-51's to civilian buyers post-war

 

[Shortround6]

actually it wasn't. The performance number scale hadn't been developed yet. They had no real way of measuring anything over 100 octane except to say 100 octane + 1cc lead, 100 octane +2 cc of lead and so on.
The British KNEW there was a difference in rich mixture response if the fuel contained a fair amount of aromatics. To get this unspecified result they required the 100 octane to be made with not less than 20% aromatics but not all aromatics act the same so fuel batch A with 10% automatic X and 10% aromatic Y might act the same running lean as batch B that had 12% X but 8% aromatic Z but batch B might be several percentage points better when running rich.
They had discovered this difference back with the 87 octane if not before with batches of fuel coming from different refineries or oil fields. Some batches allowed higher power or more boost when running rich than others but they had no way to test the fuel except in service engines.
The aromatics tend to dissolve certain rubber compounds so the US required fuel to contain NO MORE than 2% aromatic compounds. The US had to change a lot of gaskets and rubber components when they combined fuel specifications with the British.

Again, there is a lot of confusion about "american fuel" in the BoB, a lot of the fuel came from the "The Americas" which is not the same as "America". Think of England as America and the British Isles as the "The Americas" British had a large oil refinery in Trinidad refining both local fuel and fuel from South/Central America.

Pouring American 100 octane into a Spitfire and trying to pull 12lbs boost might have lead to a nasty surprise.

An awful lot of work was done on fuels in a short period of time in 1940-41.

In theory 100/130 fuel allows for 30% higher pressures (or more power) in the cylinders than 100 octane fuel. But since you need more power to run the supercharger to get the higher pressure and few other losses you don't get 30% more power to the propeller.

 

[pbehn]

actually it wasn't. The performance number scale hadn't been developed yet. They had no real way of measuring anything over 100 octane except to say 100 octane + 1cc lead, 100 octane +2 cc of lead and so on.
The British KNEW there was a difference in rich mixture response if the fuel contained a fair amount of aromatics. To get this unspecified result they required the 100 octane to be made with not less than 20% aromatics but not all aromatics act the same so fuel batch A with 10% automatic X and 10% aromatic Y might act the same running lean as batch B that had 12% X but 8% aromatic Z but batch B might be several percentage points better when running rich.
They had discovered this difference back with the 87 octane if not before with batches of fuel coming from different refineries or oil fields. Some batches allowed higher power or more boost when running rich than others but they had no way to test the fuel except in service engines.
The aromatics tend to dissolve certain rubber compounds so the US required fuel to contain NO MORE than 2% aromatic compounds. The US had to change a lot of gaskets and rubber components when they combined fuel specifications with the British.

Again, there is a lot of confusion about "american fuel" in the BoB, a lot of the fuel came from the "The Americas" which is not the same as "America". Think of England as America and the British Isles as the "The Americas" British had a large oil refinery in Trinidad refining both local fuel and fuel from South/Central America.

Pouring American 100 octane into a Spitfire and trying to pull 12lbs boost might have lead to a nasty surprise.

An awful lot of work was done on fuels in a short period of time in 1940-41.

In theory 100/130 fuel allows for 30% higher pressures (or more power) in the cylinders than 100 octane fuel. But since you need more power to run the supercharger to get the higher pressure and few other losses you don't get 30% more power to the propeller.

S/R I knew the answer I just cant explain it myself, in short "Its Complicated, extremely complicated", and trying and eventually succeeding in harmonising these fuel requirements was one of the many successes of the war, easily said and much harder to do.

 

[Shortround6]

S/R I knew the answer I just cant explain it myself, in short "Its Complicated, extremely complicated", and trying and eventually succeeding in harmonising these fuel requirements was one of the many successes of the war, easily said and much harder to do.

It is a bit difficult trying to condense several hundred pages worth of aviation fuel history into a few paragraphs. but yes, it was extremely difficult. It got much harder as the demand for fuel skyrocketed and choices had to be made about allowable levels of both lead and aromatic compounds in fuel as opposed to the amount of steel and building requirements needed by refineries to manufacture the needed fuel, one change in lead allowed was supposed to have saved enough steel to build ten destroyers if I remember right. The piping in refineries being a higher grade steel than freighter hulls.

Anda s most planners, they were trying to think 6 months to year ahead and not react to a shortage this week or next.