hurricane problems

[Viper2000]

Originally Posted by *Buzzsaw*

Sustained Combat setting for Hurricane when CEM is on:

+3 boost, full rich mix, 2800 rpm, full rad open, Prop pitch full fine


Full Combat setting for Hurricane when CEM is on: (use for 5 minutes)

+4 boost, full rich mix, 3000 rpm, full rad open, Prop pitch full fine


Emerg Combat setting for Hurricane when CEM is on: (use only for 1 minute at a time)

+6 boost, full rich mix, 3000 rpm, full rad open, Prop pitch full fine

In most cases, you are best to decide on your settings in combat, and leave them, just fly the plane, don't be playing with the boost and rpm unless you are in a difficult situation. I usually enter combat with settings on my 'Full Combat', then drop them to 'Sustained Combat' after 5 minutes, then back up to 'Full Combat' after a break of a couple minutes. I use 'Emerg Combat' only when absolutely necessary.

Remember when you reduce RPM's boost rises, when you increase RPM's, it drops, so you need to be careful and adjust your boost throttle when you adjust RPM. (pitch)

Couple of points.

1) You'd be more historically accurate in this period if you used 2600 rpm for max continuous; 2650 came later.

2) The relationship between boost and revs is a bit more complex than that.

The Merlin III has the original supercharger design, which suffered from some questionable design decisions made before RR hired SGH. It was not very efficient. Maximum performance was actually obtained at about 2800 rpm until the FTH for the desired boost was reached; above this altitude it was better to go to 3000 rpm to get more boost despite the frictional losses (and increased engine wear).

This is the main reason for the two different combat concessions in the Pilot's Notes; the central entry supercharger was considerably more efficient and therefore later marks of Merlin used 3000 rpm for combat power at all altitudes.

[I'm not very impressed with the massive boost increases associated with reducing revs, because it's quite hard to see where this can come from; the supercharger puts in kinetic energy from its tip speed (which is fixed at constant engine rpm), plus a component due to the radial flow along the vanes. This is then diffused into a pressure rise. There are therefore strict limits to the maximum supercharger pressure ratio obtainable at any given rpm. In reality, there might be a small increase in boost during a deceleration transient, but large increases are unlikely since the engine charge consumption varies directly with rpm, whilst the kinetic energy at the supercharger tip varies as the square of rpm. So in general one would expect boost to fall with rpm under normal circumstances - this might change at very low intake manifold pressures at or close to ground idle because of scavenging effects, but that's very much an edge case.]