Hi everyone,
I am trying to gain a proper understanding of the operation of the automatic boost controller in early Merlin engines (single-speed supercharger) and how it should behave under various conditions.
As I understand it, the throttle lever in the cockpit has three ranges of its travel and interacts with the automatic boost controller differently in each one.
In the first, which extends from idle to a position yielding about +1 lb of boost at sea level, the automatic boost controller is completely out of the picture. In this range, given a constant throttle lever position, the throttle valve remains stationary no matter what, and fluctuations of RPM or ram air effect cause fluctuations in boost pressure.
In the second, the throttle lever is beginning to activate the automatic boost controller via mechanical linkage, and the farther forward the lever is pushed, the greater the range of supercharger output pressures the controller can compensate for to maintain a constant boost pressure. At the low end of this range, the ABC can compensate for supercharger outputs significantly higher that the desired boost pressure by closing the throttle valve, but has very little range of motion to open the valve, while the reverse is true at the high end of this range, which is against the throttle gate.
The third range is past the throttle gate to the full forward travel limit and effectively overrides the ABC, returning direct control of the throttle valve to the throttle lever since the ABC no longer has the range of motion to close the throttle. On a Merlin XII or early 45, at full forward this yields about +12.5 lbs at sea level, even though the ABC is trying govern the boost at +9 lbs. However, as altitude increases, the reduced supercharger output allows the ABC to close the throttle valve more and more until it is once again governing at +9 lbs.
There is also the automatic boost control cut-out function, which, when activated by the pilot, effectively tricks the ABC into governing boost pressure at +12 lbs (later increased to +16 lbs), assuming the lever is at or beyond the throttle gate, and has a progressively lesser effect on boost pressure as the throttle lever is pulled back until it has no effect at all once the throttle lever is in the first range where the ABC is not functioning.
Now, here are my questions:
1. Is this understanding correct?
2. Are there any changes in operation between different models - does it work the same on a Merlin III as it does on a Merlin XII or 45 or other model?
3. What happens when the throttle is moved from the unregulated first range to the regulated second range? I can imagine a scenario such as a high speed dive, where, if the throttle lever were in the first range, boost pressure would be unregulated, and could get quite high due to increased ram air. Advancing the throttle forward to a point where the ABC began working, it would seem that the boost pressure would suddenly drop as the ABC would be able to close the throttle valve to bring boost pressure back down to the value called for by the throttle lever's position. Is this correct? It seems strange that advancing the throttle lever could result in a decrease in boost pressure in some scenarios.
4. When automatic boost control cut-out went from +12 lbs to +16 lbs, did this have any effect on the third range of of throttle travel? With the cut-out not onengaged, would it still yield +12.5 lbs at sea level, or would it be +16 lbs? With the cutout engaged, would moving the throttle beyond the gate have any effect at all, and boost pressure would remain constant at +16 lbs?
Probably more questions later, thanks for any help!
Dan
I am trying to gain a proper understanding of the operation of the automatic boost controller in early Merlin engines (single-speed supercharger) and how it should behave under various conditions.
As I understand it, the throttle lever in the cockpit has three ranges of its travel and interacts with the automatic boost controller differently in each one.
In the first, which extends from idle to a position yielding about +1 lb of boost at sea level, the automatic boost controller is completely out of the picture. In this range, given a constant throttle lever position, the throttle valve remains stationary no matter what, and fluctuations of RPM or ram air effect cause fluctuations in boost pressure.
In the second, the throttle lever is beginning to activate the automatic boost controller via mechanical linkage, and the farther forward the lever is pushed, the greater the range of supercharger output pressures the controller can compensate for to maintain a constant boost pressure. At the low end of this range, the ABC can compensate for supercharger outputs significantly higher that the desired boost pressure by closing the throttle valve, but has very little range of motion to open the valve, while the reverse is true at the high end of this range, which is against the throttle gate.
The third range is past the throttle gate to the full forward travel limit and effectively overrides the ABC, returning direct control of the throttle valve to the throttle lever since the ABC no longer has the range of motion to close the throttle. On a Merlin XII or early 45, at full forward this yields about +12.5 lbs at sea level, even though the ABC is trying govern the boost at +9 lbs. However, as altitude increases, the reduced supercharger output allows the ABC to close the throttle valve more and more until it is once again governing at +9 lbs.
There is also the automatic boost control cut-out function, which, when activated by the pilot, effectively tricks the ABC into governing boost pressure at +12 lbs (later increased to +16 lbs), assuming the lever is at or beyond the throttle gate, and has a progressively lesser effect on boost pressure as the throttle lever is pulled back until it has no effect at all once the throttle lever is in the first range where the ABC is not functioning.
Now, here are my questions:
1. Is this understanding correct?
2. Are there any changes in operation between different models - does it work the same on a Merlin III as it does on a Merlin XII or 45 or other model?
3. What happens when the throttle is moved from the unregulated first range to the regulated second range? I can imagine a scenario such as a high speed dive, where, if the throttle lever were in the first range, boost pressure would be unregulated, and could get quite high due to increased ram air. Advancing the throttle forward to a point where the ABC began working, it would seem that the boost pressure would suddenly drop as the ABC would be able to close the throttle valve to bring boost pressure back down to the value called for by the throttle lever's position. Is this correct? It seems strange that advancing the throttle lever could result in a decrease in boost pressure in some scenarios.
4. When automatic boost control cut-out went from +12 lbs to +16 lbs, did this have any effect on the third range of of throttle travel? With the cut-out not onengaged, would it still yield +12.5 lbs at sea level, or would it be +16 lbs? With the cutout engaged, would moving the throttle beyond the gate have any effect at all, and boost pressure would remain constant at +16 lbs?
Probably more questions later, thanks for any help!
Dan
