Inaccurate performance data for BOB fighters in COD comparing to RL data

[TheGrunch]

It's definitely sensible to run-in engines at their 5-minute limit boost levels.

[madrebel]

because that was the only difference right?

[Triggaaar]

Originally Posted by JG52Uther

+1
I don't care if my plane is porked,as long it is historically porked to 1940's standards!

Likewise. Besides, if my plane is correctly porked, I can always switch sides

[TheGrunch]

Originally Posted by madrebel

because that was the only difference right?

These were the alterations made (dated March 20th 1940). After the alteration the only difference in actual operation was the modified boost cut-out control allowing the ability of the engine to be run on 100 octane fuel up to +12lbs. sq. in. The engine was still perfectly capable of being run on 87 octane fuel and indeed this would avoid the problem of lead fouling of the spark plugs if they had not been changed to the new ones.
You will note the last paragraph. Mechanics were not likely to run the engine up to +12lbs on the ground. Its use significantly reduced engine life.

[Viper2000]

Since the FTH for +12 was 10,000 feet or so, it would be pretty reckless not to ground test at +12 to ensure that Mod. No. Merlin/154 was embodied correctly given that it was a local mod; otherwise there would be no guarantee that +12 might not be exceeded, leading to rapid failure in flight.

Reverting to 87 octane would require that the 87 octane limits be observed; since operation of the cutout would now give +12 it would probably be wired shut; clearly in this condition the aeroplane would not be exactly combat ready.

Having filled the tanks with 87 octane again, they would probably require cleaning when reverting to 100 octane to ensure that the fuel reaching the engine met the performance standard.

Changing fuel is not quite so small an undertaking as many would imagine; aeroplanes are less forgiving than cars, and the consequences of engine failure are inevitably more severe.

[TheGrunch]

Given that as far as I am aware the 100 octane fuel was mixed by mixing the iso-octane with existing 87 octane fuel I cannot imagine that a tiny remnant of 87 octane would be something to get distressed about, however I will admit that the possibility of Mod 154 being performed incorrectly might be something they would test, agreed. However the since local mod would be performed on existing engines that had already seen use I am unsure as to what this would mean in terms of the treatment of the engine immediately after the mod.

[Kurfürst]

Originally Posted by TheGrunch

Sounds like that's an extreme case to me Kurf. I've looked at a few of them, but so far:

• 32 Squadron flew 60 and-a-half non-operational hours in the first week of August 1940, all of which were transfers to and from their forward base.
• 43 Squadron flew approximately 10 non-operational hours in the first week of August 1940, namely one transfer of six aircraft and their return and another transfer of seven aircraft, and three night practise flights.
• 54 Squadron flew approximately 106 non-operational hours of a huge variety of kinds (beware that one page of the ORB appears twice in a search for some reason).

There's no data for 56 Squadron in 1940 so I'm not sure which one you looked at. Anyway, won't be posting for a while, I hear the human body needs sleep.

My bad, 54 Squadron. But I agree this could be an extreme case. My point was rather to illustrate that there would be considerable fuel requirement for non-combat sorties as well, which is usually not being taken into account.

[TheGrunch]

Originally Posted by Kurfürst

My point was rather to illustrate that there would be considerable fuel requirement for non-combat sorties as well, which is usually not being taken into account.

Definitely agreed there. As it was stated earlier in the thread there was a lot of movement to and from forward bases which seems to have been the bulk of the non-operational flying for operational squadrons, particularly for squadrons flying into and out of Manston due to its exposed nature on the coast I suppose.

[fruitbat]

Hi, Kurfurst the page you linked showing 54 squadron on training flights was during one of the times they were posted north to Catterick on a rest period.

54 squadron had 3 rest periods north, once after Dunkirk, once in june and once end of july/start of aug if memory serves me right.

Catterick was not an operational base during BoB, being well up north, one can only speculate at what fuel was being used there.

It would seem logical that 100 octane was not really needed there....

[Viper2000]

Originally Posted by TheGrunch

Given that as far as I am aware the 100 octane fuel was mixed by mixing the iso-octane with existing 87 octane fuel I cannot imagine that a tiny remnant of 87 octane would be something to get distressed about, however I will admit that the possibility of Mod 154 being performed incorrectly might be something they would test, agreed. However the since local mod would be performed on existing engines that had already seen use I am unsure as to what this would mean in terms of the treatment of the engine immediately after the mod.

Isooctane is a specific chemical isomer. It was used as a reference fuel by Ricardo when he produced the Octane scale. The other reference fuel was n-Heptane.

So
0 octane = 100% n-Heptane
50 octane = 50% n-Heptane + 50% iso-octane
100 octane = 100% iso-octane

Fuels are evaluated by testing them on a variable compression ratio engine and then mixing a reference fuel to match their performance.

Note that this scale cannot measure any fuel outside of the range 0-100 octane. There is quite a lot of casual incorrect use of the octane scale for values >100, but this is simply incorrect.

A different scale (Performance number) is used to rate fuels superior to 100 octane; the performance number being the ratio of the power produced by the reference fuel running on the test engine without detonation to that produced by 100 octane running on the test engine.

This naturally varies as a function of mixture strength, so for example modern 100 LL will give approximately 100/130 performance number (weak/rich), whilst late war purple fuel could give 115/145 Performance number.

Care must be taken when comparing fuels to check exactly which rating scale is in use; the power output achievable from a fuel is not necessarily a linear function of its octane number, so you can't make a direct comparison between an octane number and a performance number. Additionally, there were several different test procedures in use which would produce different results from identical fuels (ie MON, RON etc).

Production fuels were not made in a chemistry lab by mixing individual chemical isomers together; this would have been impractical. The Germans were somewhat closer to this approach with their use of Fischer-Tropsch derived synthetic oils, but even then they didn't go to the lengths of blending up their fuels isomer by isomer, because it just wouldn't be worth the effort. About the closest that anybody has ever come to this kind of "chemistry lab" approach was JP-7 for the SR-71, and that was phenomenally expensive stuff...

Instead, production fuels were made by distillation of crude and the judicious use of additives such as TEL. AFAIK, 100 octane used a slightly different distillation range from 87, and there may have been some catalytic processing (eg cracking etc) to improve yields, but without making direct reference to a contemporary reference on the subject I couldn't be certain off the top of my head.

However, I'm pretty certain that the main difference between 87 octane and 100 octane was the amount of TEL used. This would have been a variable, because different wells produce different crudes which will produce different products at the refinery, so the amount of TEL required to meet the 100 octane standard would have been varied from batch to batch. Even today, the specification therefore just includes an upper limit for TEL content.

To get an idea of just how many chemicals are actually contained in real fuel, you can read the safety sheet.

In any case, if you don't clean the tanks when swapping from 87 octane to 100 octane then you'll end up with a mixture which will have some undetermined intermediate octane rating. Running at +12 on this mixture is not the same as running at +12 on 100 octane, and might cause pre-ignition or detonation.

It would therefore be advisable to flush the fuel system thoroughly with 100 octane before embarking on +12 running, and the obvious way to do that without taking the whole aeroplane & engine to bits would just be to ground run through say a tank full of 100 octane. Then you can be pretty certain that all the 87 octane is out of the system and it should be safe to start testing at +12.

You would then probably conduct a few minutes of running at high boost to ensure that the boost control cutout was really delivering +12.