Quote:
Originally Posted by =XIII=Wedge
Function of the Automatic Boost Control
My understanding was that the original reason for the throttle being design with an automatic boost control was to reduce the workload on the pilot.
It allowed them to set the desired boost pressure with the throttle lever and the ABC would ensure that that desired pressure setting was maintained up to the rated altitude.
Without it the pilot would constantly have to keep adjusting the throttle to maintain the same boost as the plane changed altitude.
Using the boost cutout disables this function.
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This is correct. This are exactly the points mentioned in the Merlin II/III engine manual.
Some notes:
You discribe a "variable datum type" automatic boost control. Early production Merlin II engines had a "fixed datum type" automatic boost control (however even the manual mentions that these are replaced by the "variable datum type"). As you already wrote with a "variable datum type" boost control each throttle setting gives you always the same boost (up to rated altitude). The "fixed datum type" had the follwing drawback:
AP 1590B Merlin II and III Aero-Engines
Quote:
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240. (...) With the fixed datum type of automatic unit for which the control has been set, maximum permissible boost is obtained soon after the throttle is opened past the slow-running position. In consequence of this, below rated height and particularly near the ground level, it is only possible to open the throttle a small amount with the result that a considerable degree of lost motion in the cockpit throttle operating lever occurs.
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What the "modification" to the boost control does is simply limiting the boost pressure to rise above +12lbs even if the boost control was disabled. However my understanding is that you don't have the benefits of the "variable datum type" boost control anymore (as long as boost control cut out is enabled). In later Merlin marks this was changed and the "boost control cut out" didn't really disable the boost control but it did set a higher boost for each throttle setting.