camber
Dec-17-2013, 02:26
Ages ago I wrote a detailed post on the British Merlin boost control and the boost control cutout in the ubi forums and was ably helped out by 41Sqn_Banks, lane and others. I thought I would summarise the original post here updated with the extra info that was posted. There is lots more great documents scanned into the original thread if you are interested:
http://forum.1cpublishing.eu/attachment.php?attachmentid=8264&d=1326414965
I have a background in control engineering and my interest is: how did the real boost control and override work exactly? What is being measured, what is being manipulated and how?
In modern process control, we have sensors (measuring the process), actuators (intervening in the process), and computer logic driven control loops feeding electrical information between the two. I am fascinated with 1940s and earlier control engineering, where to control a parameter like actual boost at a rated boost setpoint the control loop is a purely mechanical system where the sensors, actuator and control logic are real interconnected parts like valves, pistons and control linkages.
The funny thing is that the handle marked "throttle" in the Spitfire really is not a direct throttle control at all. The handle is the input of a mechanical control loop that maintains a boost (manifold pressure) setpoint, based on where the handle is. The control loop output is the throttle valve position (between carb and supercharger). The pilot does not actually have direct throttle valve control unless he pulls the boost cutout switch.
The problem with giving the pilot a direct link between his "throttle" handle and throttle valve is that a fixed gear supercharger powerful enough to give altitude performance will develop way too much manifold pressure on the ground unthrottled. 87 octane fuel can only tolerate around +6psi boost without engine-damaging predetonation, however the Merlin II/III supercharger can develop around +20psi at full throttle valve opening on the ground. The BF109 DB601 actually uses a fluid drive to alter the supercharger speed instead of fixed gear, an arguably more complicated but elegant strategy.
American planes of the same period had direct throttle valve control, so pilots had to carefully set appropriate manifold pressure (by not opening the throttle valve too much) and then continuously adjust manifold pressure (which would decrease as the plane gained altitude). Analagous to a contant speed prop or automatic temperature control, combat performance improves if you take routine workload away from the pilot. For this reason the British gave their Spitfires and Hurricanes automatic boost control. But how did it work?
The best explanation for a suitable boost control I came across was this document:
Rolls-Royce Merlin Automatic Boost Regulator, Courtesy of Dave Birch and the Rolls-Royce Heritage Trust (http://www.enginehistory.org/Piston/Rolls-Royce/R-RmerlinABC/R-RmerlinABC.shtml)
I recommend a full read of the document if interested, but here is a simplified version of the system with some of the detail stripped away (Figure 1). Basically the boost cylinder valve is plumbed so that a relatively high pressure (from the supercharger output itself) is routed to one side or the other of a piston attached to the throttle via linkage. When actual boost is equal to rated (desired) boost, both sides of the cylinder are sealed and the piston stays where it is. There is more detail about how this is actually done in the original document. The piston has a linkage which alters the mechanical relationship between the pilot's throttle lever and actual throttle plate. This is a rather simplified version, in fact in the real Spitfire the linkage is done through a differential gear but with equivalent mechanical logic.
http://i406.photobucket.com/albums/pp142/wjhawaii/spithrottle_sm.jpg
There was actually two versions of the boost control, fixed datum (Merlin II) and variable datum (Merlin III and retrofitted Merlin II). The essential difference was that the fixed datum could only control at rated boost (+6.25psi for 87 octane). The variable datum was better because it could control boost at any intermediate value below rated boost based on the pilot handle position. If you selected +2psi using the handle and climbed, boost would remain controlled at +2psi with the controller automatically opening the throttle valve as height increased.
Boost cut-out override (BCCO)
The Boost control override did not originate as an emergency power setting, but was adapted to be so by the ingenious Brits. In original form it was just a way of disabling the boost controller in case of malfunction, thus making the system directly link the pilot handle to the throttle valve and giving him the ability to set any boost the supercharger was capable of (but without control, boost would change with altitude).
CloD shows the Spitfire red tab rotating a little cam allowing the throttle handle to go further, which is not the actual case and confuses the red tab with the throttle gate which appears as an additional overboost system on the (reality) Spit II. In fact the red tab in Spit I/II pulled a cable which opened a channel around the valve, which applied suction to the valve piston and forced it to the left in Figure 1 and stay there, thus disabling the controller. The Hurricane is correct in that the red tab is replaced by a knob that pulls the cable (the "tit").
Although it is hard to find references on this, it is easy to see how the BCCO could become an unofficial emergency power switch. A pilot could pull it and try for a bit more boost than the rated 6.25psi, and hopefully get a bit more power without damaging the engine.
Modified BCCO Overboost
When the British went to 100 octane fuel which could tolerate +12psi boost, they adapted the BCCO from it's original purpose (disable boost controller) to be an official overboost switch, and they did it by drilling a single hole of precise size into the system at squadron level. This hole changed the cutout action so instead of totally switching off the controller, it incremented the upper controllable boost level from +6.25psi to +12psi. Of course the supercharger is not spinning any faster, so this is only possible if you are low enough with sufficient rpms for the supercharger to deliver +12psi boost (full throttle height is lower for +12psi than +6.25psi). It was a very impressive system modication, really. The hole was on the suction side so it it did not vent pressurised fuel-air to the atmosphere (which would not a good safety feature even without incendiary bullets around).
The throttle gate overboost system
The real Spit II had (in addition to the BCCO red tab) a take off power gate on the pilot handle that gave +12.5psi. This seems a bit strange, an alternative overboost system that seems redundant! This is the "gate method" alternative overboost system described in the Rolls Royce document above and summarised below:
The gate system does not alter the boost controller action like the (modified) BCCO red tab, it actually overrides it. Effectively it allows a maximum throttle valve position to be held even with the controller piston fully to the right in Figure 2 (i.e the controller piston is sensing "boost too high" and is trying but being disallowed to close the throttle). In the case of the Spit II gate this throttle position is calibrated to give +12.5psi at sea level.
http://i406.photobucket.com/albums/pp142/wjhawaii/spithrottle_b.jpg
The advantage of this system is that it is real take off only overboost. In normal conditions (take off gate, red tab not pulled), the Spit II would take off with around +12.5psi depending on airfield altitude, which would immediately decay by itself as the plane climbed even if the pilot didn't touch the handle. It would decay back to +9psi (Spit II rated boost) at which point the controller regained effectiveness, and boost would be maintained at +9psi in the climb until such point the supercharger could no longer deliver it.
Anyway feel free to comment,
Cheers, FS~camber
http://forum.1cpublishing.eu/attachment.php?attachmentid=8264&d=1326414965
I have a background in control engineering and my interest is: how did the real boost control and override work exactly? What is being measured, what is being manipulated and how?
In modern process control, we have sensors (measuring the process), actuators (intervening in the process), and computer logic driven control loops feeding electrical information between the two. I am fascinated with 1940s and earlier control engineering, where to control a parameter like actual boost at a rated boost setpoint the control loop is a purely mechanical system where the sensors, actuator and control logic are real interconnected parts like valves, pistons and control linkages.
The funny thing is that the handle marked "throttle" in the Spitfire really is not a direct throttle control at all. The handle is the input of a mechanical control loop that maintains a boost (manifold pressure) setpoint, based on where the handle is. The control loop output is the throttle valve position (between carb and supercharger). The pilot does not actually have direct throttle valve control unless he pulls the boost cutout switch.
The problem with giving the pilot a direct link between his "throttle" handle and throttle valve is that a fixed gear supercharger powerful enough to give altitude performance will develop way too much manifold pressure on the ground unthrottled. 87 octane fuel can only tolerate around +6psi boost without engine-damaging predetonation, however the Merlin II/III supercharger can develop around +20psi at full throttle valve opening on the ground. The BF109 DB601 actually uses a fluid drive to alter the supercharger speed instead of fixed gear, an arguably more complicated but elegant strategy.
American planes of the same period had direct throttle valve control, so pilots had to carefully set appropriate manifold pressure (by not opening the throttle valve too much) and then continuously adjust manifold pressure (which would decrease as the plane gained altitude). Analagous to a contant speed prop or automatic temperature control, combat performance improves if you take routine workload away from the pilot. For this reason the British gave their Spitfires and Hurricanes automatic boost control. But how did it work?
The best explanation for a suitable boost control I came across was this document:
Rolls-Royce Merlin Automatic Boost Regulator, Courtesy of Dave Birch and the Rolls-Royce Heritage Trust (http://www.enginehistory.org/Piston/Rolls-Royce/R-RmerlinABC/R-RmerlinABC.shtml)
I recommend a full read of the document if interested, but here is a simplified version of the system with some of the detail stripped away (Figure 1). Basically the boost cylinder valve is plumbed so that a relatively high pressure (from the supercharger output itself) is routed to one side or the other of a piston attached to the throttle via linkage. When actual boost is equal to rated (desired) boost, both sides of the cylinder are sealed and the piston stays where it is. There is more detail about how this is actually done in the original document. The piston has a linkage which alters the mechanical relationship between the pilot's throttle lever and actual throttle plate. This is a rather simplified version, in fact in the real Spitfire the linkage is done through a differential gear but with equivalent mechanical logic.
http://i406.photobucket.com/albums/pp142/wjhawaii/spithrottle_sm.jpg
There was actually two versions of the boost control, fixed datum (Merlin II) and variable datum (Merlin III and retrofitted Merlin II). The essential difference was that the fixed datum could only control at rated boost (+6.25psi for 87 octane). The variable datum was better because it could control boost at any intermediate value below rated boost based on the pilot handle position. If you selected +2psi using the handle and climbed, boost would remain controlled at +2psi with the controller automatically opening the throttle valve as height increased.
Boost cut-out override (BCCO)
The Boost control override did not originate as an emergency power setting, but was adapted to be so by the ingenious Brits. In original form it was just a way of disabling the boost controller in case of malfunction, thus making the system directly link the pilot handle to the throttle valve and giving him the ability to set any boost the supercharger was capable of (but without control, boost would change with altitude).
CloD shows the Spitfire red tab rotating a little cam allowing the throttle handle to go further, which is not the actual case and confuses the red tab with the throttle gate which appears as an additional overboost system on the (reality) Spit II. In fact the red tab in Spit I/II pulled a cable which opened a channel around the valve, which applied suction to the valve piston and forced it to the left in Figure 1 and stay there, thus disabling the controller. The Hurricane is correct in that the red tab is replaced by a knob that pulls the cable (the "tit").
Although it is hard to find references on this, it is easy to see how the BCCO could become an unofficial emergency power switch. A pilot could pull it and try for a bit more boost than the rated 6.25psi, and hopefully get a bit more power without damaging the engine.
Modified BCCO Overboost
When the British went to 100 octane fuel which could tolerate +12psi boost, they adapted the BCCO from it's original purpose (disable boost controller) to be an official overboost switch, and they did it by drilling a single hole of precise size into the system at squadron level. This hole changed the cutout action so instead of totally switching off the controller, it incremented the upper controllable boost level from +6.25psi to +12psi. Of course the supercharger is not spinning any faster, so this is only possible if you are low enough with sufficient rpms for the supercharger to deliver +12psi boost (full throttle height is lower for +12psi than +6.25psi). It was a very impressive system modication, really. The hole was on the suction side so it it did not vent pressurised fuel-air to the atmosphere (which would not a good safety feature even without incendiary bullets around).
The throttle gate overboost system
The real Spit II had (in addition to the BCCO red tab) a take off power gate on the pilot handle that gave +12.5psi. This seems a bit strange, an alternative overboost system that seems redundant! This is the "gate method" alternative overboost system described in the Rolls Royce document above and summarised below:
The gate system does not alter the boost controller action like the (modified) BCCO red tab, it actually overrides it. Effectively it allows a maximum throttle valve position to be held even with the controller piston fully to the right in Figure 2 (i.e the controller piston is sensing "boost too high" and is trying but being disallowed to close the throttle). In the case of the Spit II gate this throttle position is calibrated to give +12.5psi at sea level.
http://i406.photobucket.com/albums/pp142/wjhawaii/spithrottle_b.jpg
The advantage of this system is that it is real take off only overboost. In normal conditions (take off gate, red tab not pulled), the Spit II would take off with around +12.5psi depending on airfield altitude, which would immediately decay by itself as the plane climbed even if the pilot didn't touch the handle. It would decay back to +9psi (Spit II rated boost) at which point the controller regained effectiveness, and boost would be maintained at +9psi in the climb until such point the supercharger could no longer deliver it.
Anyway feel free to comment,
Cheers, FS~camber