No - 87 octane limited the engine to +6.5. In this period, the boost control cutout was a safety feature to permit the pilot to manually control the throttle valves if the ABC failed. Fully opening the throttle at sea level static would give about +17 psi boost. This would be very bad for the engine.

Of course, in theory you could operate the cutout and then manually set the throttle to provide any arbitrary amount of boost that the supercharger was able to deliver at your altitude.

So a Spitfire or Hurricane pilot in 1938 could have overboosted their engine anywhere up to about +17 on 87 octane. But this would have rapidly damaged the engine and would have been completely against the instructions in the Pilot's Notes etc.

I have seen no evidence that anybody did this deliberately (though doubtless somebody did it by accident, because if it's possible to do something silly then somebody probably will). However, it was obviously known that +17 would result from operation of the cutout in its initial state because this is clearly set out in documents which I have cited in my thread on the subject of the effects of operating the boost control cutout in Spitfires & Hurricanes using 87 octane fuel.

With 100 octane fuel, the engine could safely operate at +12. The boost control cutout system was then modified by drilling a bleed hole so that operating the cutout would now provide no more than +12 psi boost. This meant that the cutout could be used to provide a regulated level of emergency combat power instead of operating simply as a safety device.

The cutout could be wired "off" with thin wire, so that operation of the cutout would provide a tell-tale for the groundcrew. This was therefore a very elegant solution.

Climb power for the Merlin XX is +9 psi at 2850 rpm. This is the most likely source of your +9 figure.

Not at all. See above.

MS = Medium Supercharge
FS = Full Supercharge

Supercharger efficiency depends upon where you are on the compressor map. You'd generally plot non dimensional flow vs pressure ratio and include constant speed lines.

What you find is that centrifugal impellers are pretty forgiving machines, and will operate over quite a wide range of flows and pressure ratios at any given speed without surging.

Really you only care about tip speed for 3 reasons:

• Structural strength
• Supercharger power consumption
• Charge temperature

So ideally you want to run the engine with a wide open throttle and turn the supercharger at the lowest speed at which it is able to deliver the pressure ratio required to give the boost you want at your current ambient conditions.

But this would be too complicated for 1940.

So you compromise.

The Merlin XX had a 2 speed supercharger drive gearbox, so 2 compromise speeds were available instead of 1 for the earlier engines. This allowed better overall performance, though it didn't have any direct impact upon peak power.

Farman patented the gearchange mechanism; RR just licensed the IP. IIRC there were a couple of other gear change mechanisms on the shelf, and some of the Packard engines may have used a different one for production reasons.

In any case, it was an IP licensing thing rather than a case of Farman having specifically produced technology with the Merlin in mind.

Merlin without supercharger = Meteor. It would give something in the region of 650-850 bhp. It's quite hard to be concrete because Meteors were generally reconditioned (ie crashed, non-airworthy) Merlins and so they were never going to perform "as new".

~S

The Merlin III was allowed the use of 16lb boost on the Sea Hurricane IB, so I doubt that 17lb boost would be instantly fatal with 100 octane fuel, but with 87 octane bad things would happen.

You mean just like he said? He never mentioned 100 octane fuel, he is specifically referring to 87 octane fuel in that scenario. Why is it that whenever anyone mentions a Merlin engine could blow up you have to jump in to save the day and explain to all us mere mortals why the Merlin was perfect.

-1

It does not work, yes you can move to center the gunsight but as soon as you turn the TRACKIR on, it centers again automaticaly to normal view.


What a bunch of rubish! :(

Pls take a close look at your doc - Merlin XX. I hve here maximum perf calibration with max speed being plotted. It does not look like anything like a climb setting

Oh and it does come from RR..


I don't see the sense of what your wrote here. You might hve pasted half the sentence from your original doc here ;)

physically given that the SC is accordingly dimensioned (flows and struct) , the heat generated is what will drive the overall efficiency. And what drive away that heat : the flow of air (or air mixture) that is pushed by the impeller blades.

Hence there is a max amount of calorific E that a s/c can work with. Raise the oct -> you'll reach sooner the the max sustainable value

This is why I said that Max boost level hve very little chance to be reachable at an alt where the impeller hve alrdy to compensate for lower air density if the eng was not designed for

etc... etc...


Modern supercharger : yes. At the time of BoB hummm that's a pretty acrobatic assumption

pls see the above

I wld hve been pls to see a Farman 220 with 4xMerlins. Damn just to see that huge parasol wing being blown away by the engines airstream :rolleyes:

By the way, most patent are cross border documents. There is no shame using a Patent from an other country. Aviation is full of this. let us remind that if Aviation pioneer did not get inspiration from each other all over the world, we wld still cross the pound on steaming boat.

Supercharging the eng was meant to keep power at alt (the rated alt) and increased the fighter perfs (smaller engine) where the bomber stream were expected . This is not NHRA supercharging although a direct benefit were a low perf improvement and climb rate to alt.

I hve arldy quoted the Merlin XX data with and without s/c (your 12lb thread)

You'd see that the s/c being driven by the eng shaft use a 7 to 20 % of eng power. Hence teh 1.3k SHP and 1.175k BHP or something like that

Anyhow, the team that did code those FM game have proved superior man skills in the field. I am looking frwd for the next released giving they can work without too much pressure away from 1C SC
(read Spit Complex )
~S

No. The efficiency is the ratio of useful work to input work. In general, we'd use the isentropic adiabatic compression work for the pressure ratio achieved by the system as the definition of the "useful work".

This makes no sense.

Better fuel = higher max charge temperature, all other things being equal (which they are for merlin development as the basic piston engine (bore, stroke, CR, valve timing, max rpm etc were the same throughout its production life).

The engine is flat-rated to a maximum safe boost pressure. The supercharger is designed to deliver this boost WOT at the FTH. Below FTH the supercharger is throttled.




Not really. Compare the ease with which RR developed their centrifugal flow jets with the massive difficulty they experienced trying to get the AJ.65 to work. It's all relative.

Modern compressors of all types will have better polytropic efficiency and deliver more work per stage, but the basic trends haven't changed.


I work with patents quite frequently, and hold a couple of my own. They're only cross border if you've got the money to chase the legal paperwork, and they only mean anything much at all if you've got the money to sue whoever might infringe... I don't see why you'd ascribe any particular moral value to the nationality of the inventor to whom a patent you've decided to license is attributed. Most engineers have better things to do...

Supercharging is really about making the engine smaller.

The non dimensional flow that a piston engine can handle at fixed rpm is basically constant.

Supercharging allows you to cram more absolute mass flow rate into a given non dimensional flow rate.

How you choose to rate the engine is another debate. But in most cases, people only flatrated because fuel quality prevented them from operating WOT at lower altitudes.

No, your data quotes the power consumed by the supercharger, and the power produced by the engine.

This is not the same thing as operating without the supercharger. Without the supercharger you can't get above 0 boost by definition. Hence you get something like 650-850 bhp.

Obviously the Supercharger consumes fewer horsepower than it adds via the boost increase it creates - otherwise nobody would bother!

Superior to what? It's not as though there's a whole lot of competition in the Flight sim market these days...

At first I wld like to say that I'm sry to all reader for this dual exchange.

Secondly I did not quote you to shorten those otherwise long answers that tend to be felt as walls in a forum pulling away the reader interested by the title of this topic

Let's go :

Adiabatic = when E is exchanged without loss of Calorific energy

Simply said the s/c depend of it's operating inside Temp that drive the efficiency of the compression

There is no adiabatic transformation. Isentropic is also an approximation to figure out the inside Temp that RR could not measure in 1940. But yes that what you plot. But who cares ?

Better fuel : higher piston head temp -> eng wearing. This is well known from car's tuner. You'd see a lot of interesting threads on that subject without over complicated words.

By the way the DB605 is no more than a revamped DB601 but with the same minor details that change and took so little time to arrange before being sent in full production

Wasn't WOT 5000 ft ? -> it's far from any rated alt - look like more for a naturally aspirated eng. I guess that the redesign of the intake was the partition they played here ;)

Yes yes you 'r right impeller are quite easy technology once you 've got the backup of strong industry supplying nice raw materials proof of any small glitch. Oh yes you'r right ... Of course the Russians, The Italians and the Japanese just might hve miss read the same early studies.

Si vis pacem para bellum : UK proved here her superior "governing awareness" (despite the Munich debacle)

RAF not NHRA :rolleyes:

Using copy/Paste as well :

Merlin XX !!

ALT(ft) SHP BHP (diff correspond to the power used to drive the supercharger)
15K 1267 1048
20K 1298 1073
20K+ 1362 1126
25K 1162 960
30K 945 778
35K 700 568

At 20K the eng is fed with sufficient amount of air to regain is low alt power. But it still hve to drive the s/c. This everything abt turbo-charging an eng.

FM : 1c far superior to any Modder (I am sry to hve to write that)

Patents ? I never doubt you was a talented individual. Well let's pat our back and share our patents together one day ;)

In terms of Bomber Command usage, the pilot's operating handbook for the Blenheim Mk.IV confirms the above points.

Actually, the Mk.IV used both types of fuel.

What happened?
1) They needed more range so they added two more fuel tanks, one in each wing (the outboard tanks).
2) This made the aircraft heavier so it needed more power to get off the ground safely when fully bombed-up and fueled.
3) Engines were modified and a boost cut-out installed, so that maximum boost could raised from +5 lbs to +9 lbs.
4) This needed fuel with more resistance to detonation (aka 100 octane), but it was also important for Fighter Command use.

So, what they did was load 100 octane only in the outboard tanks and use 87 octane in the inboard tanks.

When loaded for long range, takeoff was done on 100 octane fuel from the outboard tanks with the boost over-ride enabled and +9 lbs boost, then immediately after take-off they throttled back and switched to 87 octane from the inboard tanks.

When flying shorter range sorties only the inboard tanks were loaded with 87 octane and the normal maximum of +5 lbs boost was used, in order to let the fighter boys have more of the 100 octane supply.

Another consideration was that only the outboard tanks had jettison valves. So, they used the 87 octane fuel from the inboard tanks first, during the climb out and cruise, because in the event of an emergency they couldn't dump it. Then they switched to outboard tanks and 100 octane fuel for the remainder of the trip.

The ability to use +9lbs when running on 100 octane as an emergency rating would probably factor in tactical considerations as well, so maybe they switched to outboard tanks when in dangerous airspace or over the target/during the bomb run, just to be able to pull the boost cut-out and throttle up to +9 if they needed.

In case of a long-range run (like the raid on the Cologne power station on August 12th 1941), this would probably mean using 87 octane on the outbound leg, switching to 100 octane near the target, switching back to 87 octane once outside the "danger zone" (if any was still left in the tanks) and finally, switching back to 100 octane fuel for the remainder of the trip back home.

In any case, this could make up for a sizable part of fuel expense for both types of fuel, especially if we consider that during the BoB they were operating throughout the battle:
a) in the long range reconnaissance role (as far as Germany itself) and
b) attacking targets as far as Denmark.

After the description of their manufacturing restrictions and resulting operating procedures above, it's more or less clear that the more far-away a Blenheim target was the more 100 octane fuel would be used, since close-range targets would only need the inboard tank fuel load and could be flown solely on 87 octane fuel (less weight, less boost needed for take-off-->no need for 100 octane)

The difficult part here is getting a similar graph for Blenheim sorties and factoring in target range, so that we can actually know how much of the 100 octane fuel expenditure could be attributed to them.

I don't have the wealth of references some of you guys have, but i thought this might interest you and you may be able to dig deeper into it ;)

Salute

Thanks for the info Blackdog.

And of course, the Blenheim was only the smallest of the bombers which the RAF had available and was the only bomber to use 100 octane, but as you say, only in the outside wing tanks, and only for takeoff.

Bomber Command had 207 Armstrong Whitworth Whitley's the 33,000 lb heavy.

They had even more Hampdens, over 250, as well as close to 200 Wellingtons. All these bombers used huge amounts of 87 octane fuel, they flew missions all over Northern Germany, as far as Berlin, many times the range of the short hops the fighters made in their intercepts.

In addition, coastal command aircraft also flew constant patrols, over very long distances, the anti-U-Boat campaign was already a major priority.

All of this explains the heavier usage of 87 octane fuel.

Hallo David,
You asked a rather simple question and I am afraid your thread got kidnaped :D

I think the best book to have and read is:
Fighter combat: tactics and maneuvering by Robert L. Shaw
as it explains the differences between maneuveurability and speed and tactics against different types of oponents.
Despite that, there are some serious "deficiencies" in the CoD aiplane models (judging by the tenacity people argue about their 87oct vs 100oct fuel (I do not belong to this club)) and "deifiencies" in the game graphics engine (low quality ground, small size representation of enemy planes, head movement restriction) which will considerably hinder your capability of applying what you will learn in this book in practice in this game.
Usualy the best solution to solve the problem is flying in pairs (with a wingman) and everybody covers each others back. This is the only technick that always work, even against massively superior opponents (e.g. the "Thatch-wave" technik American pilots used in the Pacific as the only way to defend their Wildcats against the vastly superior Zeros) but it requires in most cases that you enter a squadron and train a lot in navigation, formation flying, deflection shooting and voice communications in order to perfection the art of wingman.

Sorry I could not give a quick and simple solution.

~S~