Air Publication 1590B
Volume I, 2nd Edition
October, 1938 (Reprinted October, 1939)
Merlin II and III Aero-Engines
Quote:
Atomatic Boost Control
General
239. All Merlin III and the majority of Merlin II engines are fitted with a variable datum (or progressive) type of automatic boost control unit. Early Merlin II engines were fitted with the fixed datum type of boost control, but these will eventually be replaced by the variable datum unit.
240. The variable datum automatic boost control prevents the boost pressure from arising above normal boost at all altitudes below and up to rated altitude. It consists of a device which provides a superimposed control over the hand-operated throttle, thereby relieving the pilot of the need for constant observation of the boost gauge to maintain the boost pressure within the limits. With the fixed datum type of automatic boost 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 ground level, it is only possible to open the htorttle a small amount with the result that a considerable degree of lost motion in the cockpit throttle operating lever occurs. With the variable datum type of automatic unit the variation in boost pressure is proportional to the throttle opening, so that the lost motion between throttle lever and butterfly valve is eliminated and the movement of the pilot's throttle lever is a definit indiction of the boost pressure obtained; moreover, up to rated altitude, any given position of the pilot's throttle lever will always give the same boost pressure.
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03-31-2011, 11:57 AM
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