Spit IIa is now so much more inferior to the 109

[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.

[camber]

Originally Posted by Crumpp

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.

Dear Crumpp,

I think this is incorrect about the Spitfire boost control cutout. It actually had quite different roles pre and post modification.

The original boost cutout was a true "cutout" in case of failure of the boost controller (which controls at +61/4psi). It gives full throttle valve control back to the pilot, enabling him to get +20psi boost (full unthrottled supercharger output) at ground level if he desired. Of course with either 87 or 100 octane this would not be a good idea.

There is no documented use of the original boost cutout as a combat boost system I am aware of, but it would be possible. The pilot would need to be very careful not to exceed a boost level that caused predetonation or engine damage, and the boost would continuously change with rpm, height and throttle position.

That document giving 10.55 psi boost is very interesting. As Banks suggested, it appears to be the boost attained with full throttle and (unmodified) boost cutout pulled at height. The height is about right. Interesting in that such a high boost was usable at all in a Merlin III on 87 octane.

However it is very unlikely that pilots were ever authorised to use the unmodified boost cutout as emergency power because the risk of instant degradation of engine performance was high (especially at low altitudes)

When the boost cutout was modified it was actually no longer a "cutout". It became a boost setpoint changer from +6 1/4 to +12 psi. It then became authorised as a practical combat boost (in conjunction with 100 octane fuel) that Dowding could fret over.

camber