"Jim Wilson" writes:

> David Luff said:
> 
> > Can anyone clarify the function of the "Boost cut-out EMERGENCY control"
> > mentioned in the manual.  The name implies that it cuts the boost
> > completely in an engine emergency.  However, the text implies that it
> > overrides the BCV for extra emergency boost:
> > 
> > "If it is desired in an emergency to override the automatic boost control,
> > this control can be cut-out by pushing forward the small red-painted lever
> > (14) at the forward end of the throttle quadrant.  The lever is sealed as a
> > check against inadvertant operation."
> > 
> > Can anyone confirm either one or other of the possible functions of this?
> > 
> > Cheers - Dave
> 
> That quote actually makes sense to some degree.  The term I've seen used is
> "war emergency power" which is basically just used to escape a bad under fire
> situation.  You are given 7 minutes of it IIRC.
> 
> But the automatic boost control I do not understand.  The supercharger is
> described as "two staged",  but what you are suggesting is that each stage is
> automatically and continuously adjusted through some sort of relief to
> maintain sea level pressure. 

To maintain sea-level-pressure *plus* a certain boost level mandated by the throttle 
position - eg 29.92 + ~13 = ~42inHg for the 9psi (~13inHg) rated boost (throttle 
position just before the take-off position gate) of the Merlin XII. 
 
> 
> In contrast, my take was the second stage kicked in automatically at a
> particular altitude or ambient pressure (note this is manual in our p51d
> model).  The purpose being to step up the pressure to make it possible to
> maintain normal sea level operating conditions (a gross adjustment that is). 
> The p51d cockpit comes with a manifold/throttle lever,  so I guess my
> questions is,  if I am wrong, how does such an automatic control work?
> 
> I have access to a real live p51d pilot via email so if we can get questions
> together I can probably forward them and get some answers.  Note however I
> will be out of town for a few days starting tomorrow, so there could be a delay.
> 

I think the engine described in the manuals (Merlin XII) was fitted with a single 
speed supercharger, whereas the engine in the p51d (Merlin 61 or Packard equivalent) 
had a two-speed supercharger.  For each speed of the Merlin 61 the automatic boost 
control would try to maintain a given absolute pressure (I think).  I've got a graph 
of power vs. altitude for a typical WWII 2 speed supercharger in a book somewhere.  
The power rises slightly from sea level to about 10000 ft as the exhaust backpressure 
drops.  It then starts to drop more steeply as the boost from the first speed reaches 
it's limit.  The after a small drop the switch to the second speed is made, and the 
power rises slightly again with altitude until the second stage boost limit is 
reached, at which point it drops off steadily with altitude.  Note that the switchover 
altitude is higher than that at which peak 1st speed power is made after the power has 
dropped off slightly - this is because the higher supercharger speed at speed 2 
requires more engine power to drive it and the switch is made at the crossover of the 
two powers.  Thus there are actually two local maxima in the power vs. altitude trace. 
 I had wondered about your Ctrl-b to switch over - all the references I can find have 
it as automatic.

Note also that the Merlin 61 is often described as having 2-speed, 2-stage 
supercharging.  In this case the supercharger is phyically made of two separate 
stages.  However, this is an engineering issue transparent to the pilot.  It is the 2 
supercharger drive speeds that are switched by the switchover valve, and within each 
of those discreet speeds the automatic boost control attempts to maintain constant MAP.

I *think* - I'm quite open to correction on all these points!

You can take it from this that supercharging in JSBSim is fairly imminent BTW ;-)

And I'll have to take my leave from this discussion shortly - I'm imminently off to 
the expo...

Cheers - Dave

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