Merlin engine limits & failure behaviour

[41Sqn_Banks]

Originally Posted by Viper2000

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.

According to AP 2095 Pilot's Notes General:

Note 2. - Merlin Engines.-The throttle valve cannot open fully unless the throttle lever is advanced to a certain point on the quadrant. It is therefore necessary, when climbing, to advance the throttle lever when the boost begins to fall.

So sounds like this behaviour is correct.

• 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.

AP 1590B Merlin II and III Aero-Engines:

In its unmodified condition, the boost control cut-out valve permitted the boost pressure to rise very considerably. Mod. Merlin 154, however, limits the pressure to +12 lb. per sq. in. for use under emergency condition (see para. 359A et seq.).

So a Merlin that wasn't modified for +12 boost the boost control cut-out will disable the boost control. A modified engine however will not disable the boost control but increase the allowed maximum boost to +12.