Merlin engine limits & failure behaviour

[Viper2000]

I've been experimenting with the Merlin as part of zero g cut investigation.

I find its behaviour quite strange.

For example, take a Rotol Hurricane and try climbing at less than full boost; say +4 psi and 2800 rpm, being careful to keep temperatures within limits.

• You'll find that constant boost is maintained until roughly the rated FTH.
• You can then open the throttle and maintain boost to some higher altitude.
• Eventually you'll run out of throttle movement.
• If you then operate the boost control cutout, you'll be able to go a bit higher again.
• Engine failure will follow on swift wings, despite the fact that boost, rpm & temperatures haven't gone into the red.

This is questionable for several reasons.

1. The boost control cutout just disables the ABC; it can't magically increase the FTH of the engine.
2. The engine itself doesn't care where the cockpit throttle is; it cares about boost and rpm; there is therefore no reason for the engine to be more stressed at say +4 psi and 2800 rpm with the boost control cutout operated and the cockpit throttle firewalled than at sea level with the cockpit throttle partly closed; indeed at altitude the engine's life is somewhat easier because the intake manifold temperature will be considerably lower.
3. With a double-acting ABC, the throttle position for +4 should remain constant irrespective of altitude; the ABC should progressively open the throttle valve during the climb to maintain constant boost until I reach the FTH for +4 psi and whatever rpm (rpm matters because supercharger pressure ratio varies as roughly the square of rpm). When the boost starts to fall off, opening the cockpit throttle should make no difference, because the ABC should already have fully opened the engine throttle valve.
4. With a single-acting ABC as fitted to really early engines, there would be very little throttle movement at sea level before the +6.25 normal operating limit was reached; we'd then need to progressively open up to maintain boost throughout the climb, because the automation would only close the throttle to enforce the boost limit, not open it to maintain boost (hence "single acting"). We can therefore rule out the possibility that the intention was to model a single acting ABC.

Also, I suppose that this has already been reported, but the mixture control in the Hurricane is very strange.

Rich mixture should be forward, not aft, and the mixture handle should be ahead of the throttle handle. This means that moving the throttle handle forward would automatically move the mixture handle towards the rich position, protecting the engine from detonation/preignition.

Throttling back would leave the mixture rich, so the pilot would have to manually lean the engine. However, the degree to which the mixture could be leaned would remain a function of cockpit throttle position.

Incidentally, this arrangement is strongly suggestive of a double acting ABC, because it would only protect the engine if cockpit throttle position directly correlates to a given maximum boost, such that a fixed mixture handle position would always provide a sufficiently rich mixture to guard against detonation/preignition.