David Luff said
 
> "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 a t 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
> 

All you ever wanted to know about a Merlin with 2 speed, 2 stage
supercharging is here:

http://www.unlimitedexcitement.com/Pride%20of%20Pay%20n%20Pak/Rolls-Royce%20
Merlin%20V-1650%20Engine.htm

Except exactly how the boost contol valve worked :-)

Regards

Vivian Meazza



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