Spit IIa is now so much more inferior to the 109

[Kwiatek]

Originally Posted by Ernst

Are the SPITFIRE FMs at UP 3.0 RC4 yours?

Probably not my FM's ( for 109s, Spits, Fw 190s, P-51 etc) was up to UP 2.X versions.

[NZtyphoon]

Originally Posted by Crumpp

It also shows maximum boost at emergency power at 12,500ft as +10.55lbs at 3000 rpm for the Merlin III when you use boost override or pull the tit on 87 Octane.

Absolutely NOTHING to do with the use of 100 Octane fuel.

Actually the modifications to the boost control cut out had not yet been instigated in June 1939; the modifications to the boost control cut-out were needed before the engine could reach +12 lbs boost. This is full power test proving that the engine was strong enough to withstand high boost pressures, and that relatively high pressures could be achieved on 87 Octane fuel. Once again this is a pre-war document which proves nothing germain to Crumpp's "case".

http://www.wwiiaircraftperformance.org/ap1590b.jpg

http://www.wwiiaircraftperformance.o...ol-cut-out.jpg

[Seadog]

RR was using 100 octane fuel for testing and development from 1937 onward:

Appendix IV

The Merlin and 100 Octane Fuel

Questions have been asked on the early use of 100 octane fuel and in particular on its influence during the Battle of Britain. Until 1937 the Merlin had been confined to 87 octane fuel to DTD230, because it was felt that in the event of war 100 octane. which was being developed by the Americans, might not be available to the British. This anxiety arose from the American Neutrality Act. which could prevent supplies being shipped to this country. Probably as the result of a paper by Rod Banks in January 1937, the Air Ministry agreed to proceed with engine development to take advantage of high octane fuel.

At that time the American 100 octane did not suit the Merlin because it lacked a good rich mixture response. Esso undertook the development of a suitable fuel, using 10% aromatics, and the driving force behind this was Dr Bill Sweeney whose fuel mix became known as Sweeney's Blend. Three months before the start of the war an Esso tanker Beaconhill delivered a full cargo of the special 100 octane fuel to Britain and by March 1940 the decision had been taken to switch Fighter Command to this type. Bomber Command changed over early in 1941.

The effect of 100 octane was to allow the Merlin to run at 12lb boost putting up the power of the Merlin III from just over 1000 hp to 1300 hp. However, this high power was obtained at between 8 000 and 9 000 ft and above this altitude, at a max combat power rpm of 3000, the boost and, therefore. power advantage was progressively declining. On 87 octane fuel and 6lb boost, using 3 000 rpm, the maximum power was 1030 hp at 16 000 ft. At this point on either fuel the engine was giving the same power, so above this height 100 octane fuel offered no advantage. The majority of the air fighting in the Battle of Britain was at 18 000 ft and above and the engine in most common use was the Merlin III. The gain in performance from 100 octane was entirely at lower altitudes. Before the end of the Battle Spitfire IIs with Merlin XIIs were in service, with the supercharger gear ratio increased from 8.58 to 9.09:1 giving a better full throttle height at 12lb boost and a small number of Hurricane IIs fitted with two-speed Merlin XXs. with ratios of 8.15 and 9.49:1 for MS and FS gear, these engines could take much greater advantage of 100 octane fuel and in the case of the Merlin XX were capable of maintaining 12lb boost to over 20 000 ft at 3 000 rpm, thanks to the new central entry supercharger.

This set the pattern and without 100 octane fuel the further power development of the Merlin would not have been possible. As an example the two stage blown Merlin 66 was capable of over 1600 hp at 16000 ft using 3000 rpm and 181b boost. The pioneering work of Esso to produce a suitable 100 octane fuel was the key to the high power Merlins in all spheres of operation and it was not until 1944 when 150 grade fuel became available that further advances in boost pressure to 25lb were made, allowing over 2 000 hp to be used in squadron service.

The opening paragraphs of this appendix are the result of information supplied to Michael Evans, Chairman of the Rolls-Royce Heritage Trust by Alexander Ogston, Historian of the Wings Club in New York, who has had a lifetime in fuel technology and a conversation which I had with Rod Banks shortly before his death.

The Merlin in Perspective, p87.

[NZtyphoon]

Originally Posted by 41Sqn_Banks

It's interesting to note that publications from 1938 and 1939 give maximum take-off engine speed of 2850 rpm for both Merlin II and III. Publications from 1940 give 2850 for Merlin II and 3000 rpm for Merlin III. The increase is not associated with the use of 100 octane fuel.

This most likely has a great deal more to do with the adoption of two speed and CS propellers; in 1938 the majority of the Hurricanes and Spitfires were still using the fixed (coarse) pitch two bladed wooden props, so the maximum allowable rpm was fixed at 2,850.

The Merlin III had the universal prop shaft allowing de Havilland or Rotol propellers to be fitted, and the availability of two-pitch and especially CS props would have allowed for higher rev limits with less strain on the engine than a fixed-pitch unit. It also makes sense that in wartime some restrictions are lifted to allow for maximum power. One of the reasons 100 Octane fuel was originally adopted was to allow greater take-off power.

[41Sqn_Banks]

The Hurricane I Pilot's Notes mention that during take-off 2850 RPM will not be achieved with fixed-pitch propeller.
IIRC the Merlin II was only able to take the DH propeller (2-pitch and later converted to CSP) and the Merlin III could take both. Some early Merlin II were not able to take a CSP at all as they didn't have a connection to drive the CSP unit. I can't remember where I've read this ...

[Crumpp]

CS props would have allowed for higher rev limits with less strain on the engine than a fixed-pitch unit.

No they won't allow for higher rpm. It is also not less strain on the engine. CSP's are much harder to aerodynamically balance while a fixed pitch is very easy and will run with less vibration. RPM is a function of the engine speed and reduction gearing. You can change those items to increase rpm to adjust for different propellers. If you mount a fixed pitch propeller that is grossly underpitched for the application, you will also see an rpm increase and very soon have a big bill to pay.

You control the manifold pressure and rpm in a CSP unit. Fixed pitch, you can only control the rpm and manifold pressure is irrelevant.

That is why the RAF amended their definitions in 1937!

With a two position fixed pitch, the Spitfire pilot is controlling rpm and not manifold pressure. The airplane is equipped with a manifold pressure gauge but it serves as a diagonistic tool only. He flys the airplane by rpm setting.

If you read the Spitfire Mk I Operating Notes, the maximum rpm is the same for all propellers.

It seems very likely that RAF pilots were authorized to "pull the tit" on their aircraft using 87 Octane fuel with a lower manifold pressure boost gain. It appears to be independant of 100 Octane fuel use.

[Crumpp]

The Hurricane I Pilot's Notes mention that during take-off 2850 RPM will not be achieved with fixed-pitch propeller.

Exactly why you cannot draw blanket conclusions from engine instructions! All installations are different, even with the exact same engine/propeller combination.

The Spitfire Operating Notes distinguish between the Merlin II and Merlin III by rpm. The Merlin II is restricted to 2850 rpm and the Merlin III to 3000rpm. In a dive, both engines can momentarily achieve 3600 rpm. The run up can be deciving too as the engine is not under an airload.

[Crumpp]

I just reread the Spitfire and it does make a distinction between the Rotol and DeHavilland propellers on run up. The Operating Notes on a Merlin engine make no such distinction.

The difference in rpm is probably due to lack of airload on run up unless Rotols were not mounted to Merlin III's.

[41Sqn_Banks]

Originally Posted by Crumpp

Exactly why you cannot draw blanket conclusions from engine instructions! All installations are different, even with the exact same engine/propeller combination.

Sorry forgot to mention that the Hurricane I Pilot's Notes are from 1939. I think it also give only limits for Merlin II an not III.

The following publications give 2850 RPM for Merlin II and III:
Notes for Pilot's on Merlin II and III - 1939
Merlin II and III Aero Engine, 2nd Edition - 1939

The following publications give 2850 RPM for Merlin II and 3000 RPM for Merlin III:
Notes for Pilot's on Merlin II, III and IV - 1940
Merlin II and III & V Aero Engine, 2nd Edition A.L. 4 - 1940
Spitfire Pilot's Notes - 1940

[TomcatViP]

it has more to do with the airstream ard the prop, hence the aircraft speed in that case (fixed pitch).

Max rpm is more a function of admissible eng wear. It always a trick for eng manufacturer to increase rpm to raise the number of HP available.

I think tht latter in the war, the Merlin's limits were raised to 3k rpm [confirmed - see above].

There is nothing hidden here: just like you ride your bicycle, you can't reach your max rpm at high gear starting from rest.

The Hurri had a thicker wing, so it didn't need so much speed at T.O as the Spit. Hence the T.O pitch setting wldn't need to be as lean as tht one for the spit. If the Merlin was tuned for both aircraft, it wld seems normal tht the max admissible rpm for the Merlin wld hve been fixed as a ref for the Spit and then wld hve differed slightly with the Hurri.

This are only supposition. Take it only as it shld be.