100-octane fuel in the RAF in 1940

[Mori]

The situation is at once both complicated and simple. (...)

A big thanks for the detailed explaination! In a nutshell, the data G.Bailey got was data used to compare the fuels at the time, but the way tests were done make the result irrelevant.

I also find it curious that the difference between British 100 octane fuel and American Military 100 octane doesn't seem to be mentioned in the article. The American suppliers were supplying fuel to British Specifications. The US would not accept US Military 100 octane fuel of 1940 and the US Military would not accept British 100 octane fuel of 1940.
British 100 octane fuel of 1940 would allow more boost than the US Military 100 Fuel. But that is a subject that has gone over in many other threads.

That's another problem. This article from 1936 states that octane rating depends on the measurement method. Two methods exist in the US, which more or less give a difference of 5, ie what is rated 87 under one method is rater 92 under the other. The Bristish octane rating is yet another method, but I haven't found comparisons with the US method.

I only read 10-15 articles so far; authors giving the measurement method of the octane rating they talk about are rare indeed.

 

[pbehn]

I cannot imagine any conversation with Park and Dowding where they agreed that a reduction of one minute in climb to 20,000ft and in top speed of 30MPH was insignificant, even if it was only for a limited time period or limited altitudes. 30MPH could be the difference between life and death in combat which always descended to the ground. One minute is half of the two minutes they wanted their planes to get off the ground in.

 

[Deleted member 68059]

I hope we can all go home now, this appears to be pretty categorical proof that "all operational fighter and bomber stations" were not only stocked with aircaft "approved" for use with 100 grade (which we already knew), but that also, the only way it could be stored was the REMOVAL of the old 87 remaining. This was it says complete by the end of June 1940.

 

[Mike Williams]

I do not know if +12 lbs boost was used for climb to any degree during the BoF or BoB, but if so there would have been a significant increase in ROC (+800 fpm?) and decrease in TTH. Just using it for a few seconds during maneuver would have given a significant short term performance increase in ROT (Rate Of Turn) and ability to perform the various acrobatics like rolls and loops without stalling.

This account comes to mind - P/O Roger Hall, No. 152 Squadron, 4 September 1940, (Roger Hall D.F.C. Clouds of Fear, (Hodder and Stoughton, London, 1977), pp. 85-86.)

"We were traveling at full throttle and climbing at nearly three thousand feet a minute in the general direction of the enemy formation, which was just visible high up above

p.s. great work with the graph!

 

[Shortround6]

That's another problem. This article from 1936 states that octane rating depends on the measurement method. Two methods exist in the US, which more or less give a difference of 5, ie what is rated 87 under one method is rater 92 under the other. The Bristish octane rating is yet another method, but I haven't found comparisons with the US method.

British 100 octane fuel could use up to (but not exceed) 20% aromatic compounds.
Aromatics give the fuel a better knock rating when running rich and are the major reason for ratings like 100/130 fuel.
American 100 octane fuel could contain no more than 2% aromatic compounds.
Aromatic compounds did not get along with natural rubber very well and US fuel lines, gaskets, seals etc tended to start leaking. This is before self sealing fuel tanks showed up.

The British had noted the better rich mixture response of aromatic fuels when using 87 octane fuels. Some of this depends on which oil fields you are sourcing your oil from and how much trouble you take to refine it. The British wanted the better rich mixture response (more boost before knocking or detonation) even though they couldn't measure it in the 1930s or even 1940. When the Performance Number scale was developed and old stocks of 1939-40 fuel were tested in Britain the British 100 octane fuel of the BoB was found to be about 100/115-120 depending on the batch. There are a number of different aromatics and exact blends do give different results.

The US 100 Octane fuel showed no real increase in rich response. It was pretty much 100/100 although depending on exact batch it could be 100/98-104. Yes, running rich could actually decrease the knock rating of the fuel under test on occasion.
This was a lot more important than the difference between 87 and 92, although that is a lot more than most people realize because the the octane rating scale is not linier.
The difference between 87 and 92 is about 14% rounded up.

If you tried to use US 100 Octane fuel in a Merlin at 12lbs of boost you might very well blow the engine up. There was a lot of back forth between the British and the Americans during 1941 and 1942 about fuel standards to get the joint 100/130 specification/s but that is after the time in question. But what you could do with 100/117 fuel and what you could do with 100/100 when in combat is rather obvious.

However the often told fable about the US 'saving' the British with quick shipments of "US" 100 octane fuel is a myth. The British may have been sourcing Fuel from US refineries but it was not US spec fuel.

On the subject of the constant speed props and take-Off.
You don't have the constant speed prop you are screwed.
With the two pitch prop they took off at around 2700rpm (?) and shifted to Coarse pitch at about 2,000ft and 170mph. Engine RPM dropped from 2850rpm to 2080rpm.
If you tried to fly much faster than 170mph at low level you would over speed the engine.
You might have had to use reduced throttle for take-off. You might have over spun the propeller on runway and lost thrust until the plane got to take-off speed.

The DH props were good for about 20-22.5 degrees of pitch change. The Rotol's had 35 degrees of travel. Maybe they didn't use all of it?
But the DH props, until they got converted, were pretty bad. After they got converted things were a lot better but there were still limits?
If you want full speed (max coarse) you are limited to how much fine pitch you have. Over revving or using max boost may give the sound of power but actually slow you down trying to take-off. The Prop can only transmit so much power at low speed, it is slipping at a poor angle.
A bit like tires on a slippery surface. Too much power at slow speed just makes the tires spin.

 

[Mike Williams]

On the subject of the constant speed props and take-Off.
You don't have the constant speed prop you are screwed.
With the two pitch prop they took off at around 2700rpm (?) and shifted to Coarse pitch at about 2,000ft and 170mph. Engine RPM dropped from 2850rpm to 2080rpm.
If you tried to fly much faster than 170mph at low level you would over speed the engine.
You might have had to use reduced throttle for take-off. You might have over spun the propeller on runway and lost thrust until the plane got to take-off speed.

There is the peculiar case of Sgt. R. C. Wilkinson who preferred the fixed pitch prop. No thanks, I'd pass on that for the CSP any day.

Sgt. R. C. Wilkinson, 3 Squadron, 14 May 1940 Kenneth James Nelson, C.D., Spitfire RCW, The Wartime Exploits of Royce Clifford Wilkinson O.B.E, D.F.M. & Bar, Hignall Printing Ltd., Canada, 1994). p. 6.

 

[RCAFson]

That's were I stand now, correct. There does not seem to be more recent litterature about 100-octane than Bailey's 2008 article, so I'll stick to his conclusions.

But also learned quite a few things on related topics, like 100-octane during battle of France.

Here's an RAF test report on a Hurricane1 showing the speed improvement at 5000 and 10000ft:

http://www.wwiiaircraftperformance.org/hurricane/hurricane-l1717-cal.jpg

 

[Mike Williams]

Here's an RAF test report on a Hurricane1 showing the speed improvement at 5000 and 10000ft:

http://www.wwiiaircraftperformance.org/hurricane/hurricane-l1717-cal.jpg

That's about 30 a mph increase for a Hurricane with the increase in boost from +6.25 to +12 boost. Even a Defiant picked up around 30 mph with the same boost increase to +12 lbs:

 

[Shortround6]

In general the 100 octane made no difference above 18,000ft (and darn little there) in 1940.
However the Germans were not trying to bomb the radar stations and airfields from 18,000ft or above in the early part of the BoB.
They were also not trying to bomb the North Sea or Channel Convoys from 18,000ft.
In a tight turning fight the fighters could loose 2000-4000ft per 360 degree turn so even fights that started at over 18,000ft could descend to well under 18,000ft in under 2 minutes.
Using high boost also increase fuel consumption considerably so it did have to be used with a thought to getting home.

The Merlin III gave about 880hp at sea level and 1030hp at 16,250ft with no ram and about 955hp at around 8,000ft at 6lbs boost.
Getting 1310hp at 9,000ft was a very useful boost indeed. Especially for the Hurricanes even if the power went back t 1030hp at 16,250ft as the plane climbed.

Without actual timed climbs to altitude using the higher boost settings trying to proof anything from standard climbs doesn't show anything.

With the props we can "assume" that the increase in climb performance for the constant speed props will carry over to the combat boost climbs or show a similar increase in performance.

 

[Greyman]

Calculations from Henning 'Hohun' Ruch suggested an improvement of about +800 ft/min from jumping to +12 boost.

If I use the comically elementary method of simply extrapolating boost and climb curves I get roughly the same.

Not directly related -- but giving an idea of the relationship of the two gains -- the Spitfire V gained about +37 mph and +1060 ft/min when going from +9 to +16 boost (test up on wwiiaircrafterformance here).

 

[Mori]

Hey Mori,

Here is a marked up version of the Spitfire Mk I graph showing how to read the various lines and numbers. If you have any more questions please ask.

The hatched area in the lower right quadrant of the speed envelope indicates the effect of the increase in boost.

Note that the +30 mph speed increase mentioned in Mike William's post#55 is indicated in the graph.

There are no TTH (Time To Height) or ROC (Rate Of Climb) values in the graph, only speeds for Best Climb, Maximum Economic Cruise, and Maximum Speed (at max available boost. The Best Climb Speed uses a different mph scale than do the Maximum Economic Speed and Maximum Speed.

View attachment 738001

I do not know if +12 lbs boost was used for climb to any degree during the BoF or BoB, but if so there would have been a significant increase in ROC (+800 fpm?) and decrease in TTH. Just using it for a few seconds during maneuver would have given a significant short term performance increase in ROT (Rate Of Turn) and ability to perform the various acrobatics like rolls and loops without stalling.

A big big thanks for these detailed comments. They clarify the curve effectively!
Frankly, that the thing to watch on this graph is the small penciled lines on the bottom right wasn't obvious to a newcomer, especially when one also needed to get that "+12 Lbs" means "100 octane fuel".

If I understand properly - and I may well not -, the benefit from the 100-octane boost is most tangible at low height (below 12 000 feet) and disappears completely above 18,000 feet.

 

[Mori]

The British wanted the better rich mixture response (more boost before knocking or detonation) even though they couldn't measure it in the 1930s or even 1940. When the Performance Number scale was developed and old stocks of 1939-40 fuel were tested in Britain the British 100 octane fuel of the BoB was found to be about 100/115-120 depending on the batch. There are a number of different aromatics and exact blends do give different results.

All this is new to me and I would be very thankful if you could cite the source since it is of direct interest to what I am doing.

 

[Mori]

Hey Mori,

Here is a marked up version of the Spitfire Mk I graph showing how to read the various lines and numbers. If you have any more questions please ask.

The hatched area in the lower right quadrant of the speed envelope indicates the effect of the increase in boost.

Note that the +30 mph speed increase mentioned in Mike William's post#55 is indicated in the graph.

There are no TTH (Time To Height) or ROC (Rate Of Climb) values in the graph, only speeds for Best Climb, Maximum Economic Cruise, and Maximum Speed (at max available boost. The Best Climb Speed uses a different mph scale than do the Maximum Economic Speed and Maximum Speed.

View attachment 738001

I do not know if +12 lbs boost was used for climb to any degree during the BoF or BoB, but if so there would have been a significant increase in ROC (+800 fpm?) and decrease in TTH. Just using it for a few seconds during maneuver would have given a significant short term performance increase in ROT (Rate Of Turn) and ability to perform the various acrobatics like rolls and loops without stalling.

A big big thanks for these detailed comments. They clarify the curve effectively!
One last question: how do you read that the

If I understand properly - and I may well not -, the benefit from the 100-octane boost is most tangible at low height (below 12 000 feet) and disappears completely above 18,000 feet.

 

[Juha3]

...
If I understand properly - and I may well not -, the benefit from the 100-octane boost is most tangible at low height (below 12 000 feet) and disappears completely above 18,000 feet.

yes, in the case of Merlin III. The benefits were under the full throttle height of the engine used. As has been said one could benefit the benefits of the higher boost only up to the height at which the supercharger could deliver that boost.

Ps. One point more about the constant speed propeller, it not only increase the performance, but it also eased the pilot's workload. So the pilot had one less worry to distract him from the actual combat.

 

[Mori]

Re-reading the above comments as well as the 2008 article by G. Bailey, I see no contradiction. Bailey did underline the benefits of 100-octane along the lines of the data presented in this thread.
Then he compared the 100-octane benefits with the variable propeller. He found that the later was way more significant, and concluded that all in all the impact of the fuel was "marginal, and perhaps even insignificant" because another, major innovation was introduced at the same time.

I suppose (and correct me if wrong) that pilots could not control the propeller pitch, but could decide when to go full throttle. So it's all natural that pilots' narratives of air combat only ever mentioned the +12lbs boost, never the propeller.

 

[pbehn]

Re-reading the above comments as well as the 2008 article by G. Bailey, I see no contradiction. Bailey did underline the benefits of 100-octane along the lines of the data presented in this thread.
What he did was comparing the 100-octane benefits with the variable propeller. He found that the later was way more significant, and concluded that all in all the impact of the fuel was "marginal, and perhaps even insignificant" because another, major innovation was introduced at the same time.

I suppose (and correct me if wrong) that pilots could not control the propeller pitch, but could decide when to go full throttle. So it's all natural that pilots' narratives of air combat only ever mentioned the +12lbs boost, never the propeller.

There was no difference however marginal that was considered insignificant in 1940. Initially the RAF used a variable pitch prop that had two positions fine and coarse, then they got a constant speed prop which automatically adjusted the pitch to keep the engine at a constant speed. Initially the Germans had a variable pitch prop which was adjusted by the pilot, this is a real high skill and difficult in combat. The first ROTOL props used in the battle of Britain were not the end of the story. Another was produced that was tried out on a couple of Mk I squadrons and fitted to the Mk II. The MK II was a more powerful but slightly different plane, on most metrics of speed at altitude it appeared to be worse than the Mk I, but this was because it was optimised to perform better overall, which it did. There were experiments into how few flush rivets they needed on a Spitfire without loss of performance. A Spitfire was flush rivetted 100%, then split peas glued to the heads and speed noted as they were gradually removed. This optimised performance AND production, flush rivets cost more and took longer to fit. Nothing at all was "insignificant" at the time. They may have chosen to have BP glass and cannon which cost in terms of performance, but that was a considered choice of what was more significant.

 

[Juha3]

...I suppose (and correct me if wrong) that pilots could not control the propeller pitch, but could decide when to go full throttle. So it's all natural that pilots' narratives of air combat only ever mentioned the +12lbs boost, never the propeller.

The idea of CS propeller was that it changed its pitch automaticly. I cannot say on the early Rotols from top of my head but IIRC the VDM propellers (licenced Hamilton Standard type) used by Germans were CS but at least in those used later in Bf 109Gs had also a manual use option.
Pilot could not do anything with a fixed pitch propeller either but he had to change pitch with a two position variable pitch propeller. Take off and climb used one setting and level flight another. One could kill himself and/or wreck his plane by taking off with wrong setting and had to watch his engine speed while diving for not to ruin his engine by overspeeding (too high rpm)

 

[DerAdlerIstGelandet]

You confuse me for someone else. This may explain why your behavior is so defensive. The site administror(s) know from my IP I don't have another account. Besides, I'm sure I make EFL grammar mistakes that others don't. You are not such a Sherlock Holmes after all. But it seems you routinely trigger fights with fellow members of this site...

It would have been so easy to just say "this doc is from AIR 2/1234". You would have been the nice, open-minded, confident gentleman. And what could I reply?

Mori, people get defensive when others are not open with their intentions. Keep that in mind.

 

[DerAdlerIstGelandet]

Calum, it's clear you don't want to share any information. But why don't you go parasite another thread?

I could play the contempt game too, hitting masters-degree-of-not-disclosed-university-which-happens-to-be-deep-in-the-UK-ranking-and-let's-talk-about-global-rankings and never-been-employed-in-a-large-company-because-lacking-basic-social-skills. But would that help me understand anything about 100-octane?

And what does this type of character attacking have to do with furthering this discussion?

Usually when someone wants help they don't resort to personal attacks unless they have an ulterior motive.

I suggest you tone it down.

 

[bf109xxl]

Has anyone read this book? Is it worthwhile?