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 _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
