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I had a quick question on the behavior of the Boost and RPM gauges when changing the RPMs in the Spit IIa using the constant speed prop.

When reducing or increasing the prop pitch lever during a cruise is there any effect on the Boost gauge which I am (correctly/incorrectly) interpreting as manifold pressure

Responses greatly appreciated

Yes there is.

A higher or lower RPM will change the amount of Boost that can be obtained at a given throttle setting in some situations. I have seen a long discussion about this somewhere but don't fully understand the technical aspects.

In some Spitfires (later Mk IX's and all the Griffon stuff) the throttle and RPM are interconnected and, in theory at least, I can't see that there would be these variations there.


In any event, to increase total power by increasing throttle and RPM, make sure you push up RPM first, then increase throttle to the boost figure you want.
To reduce power the same way, first pull back THROTTLE then decrease rpm. Then adjust throttle slightly to desired boost. This process protects the engine from overspeeds and/or excessive loadings.


regards


Darryl

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...Some say he never blinks, and that he roams around the woods at night foraging for Merlin parts...

Well, I try a short and simple answer to this question.

Due to the fact that we have a fixed, geared supercharger in the Merlin, it means that the obtainable boost pressure is linked directly to the RPM of the engine.

This means that at a low altitude, you could reach theoretically much more boost at max RPM and fully open throttle than the engine would sustain without damage.

In all cases below a certain altitude of max power at full RPM and full throttle, you must "kill away" the generated boost pressure with the throttle valve to get the desired boost. Think of it as an orifice, which must restrict the boost pressure generated by the Supercharger at a given RPM and at a given altitude.

The more modern Merlin variants control the Throttle valve automatically so that a dialed boost pressure is maintained at a given RPM and altitude. But, you must always keep in mind, the early Merlin supercharger has a fixed boost pressure ratio linked directly to the engine RPM, the only way around this would be variable speed supercharger like in the BF 109 (via fluid clutch), or multiple gears to change the speed ratio between the engine and the charger, like the two-speed one in the Mk IX / P-51.

So far a very simple explanation, in reality there is much more about it, depending on different points of efficiency of the supercharger at different speeds, air density, temperature etc...

Hope this helped a bit,

E.

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Basically the volumetric efficiency of a centrifugal compressor (as in the Merlin, Allison and all radials) increases with speed, meaning it moves more air per revolution the faster it spins. The engine, on the other hand, has a more or less constant VE (because of the positive displacement of the cylinders) so as the VE of the supercharger increases with added RPM the engine can't ingest all that extra air so it builds up in the manifold as added boost.

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Thank you!

That does help. The Automatic Boost Control on the Spitfire performs this function (governing total possible boost) low down. It is a piston, cylinder and spring arrangment. As pressure builds, the piston is forced back along its travel, venting excess and therfore acting as a "relief valve" to physically stop manifold pressure rising above the maximum.

Interestingly, whilst most people think that the Automatic Boost Cut-Out Override was a "war emergency power" setting, this is not correct, although it came to be used as that....Originally the override was there so that should the piston fail (and in doing so jam "open" , venting all pressure to the outside air) the override bypassed that system so allowed the manifold pressure to remain high, letting the engine continue to run, as opposed to choking it.

Now THAT meant that the pilot could destroy the engine with overpower....not a great issue in a normal "failure" as the pilot moderated the use of the throttle to nurse the aircraft home...but in a combat situation it could be deadly, with a stressed pilot banging on full throttle and therefore blowing up the engine in very short order.....an unrestrained early Merlin could actually produce around 18-19 psi, as ErlkOenig has said, much more than the engine can actually withstand.

To guard against this, the modifications that allowed the use of the feature as a "combat emergency" power setting, included the drilling of two small holes (of a very specific diameter) in the system that meant that the MAXIMUM boost obtainable, even when the over ride was thrown, was 12psi, any excess venting through those holes, to the atomosphere.

I know of at least one senior pilot (who flew Hurricanes) who breeched regulations and obtained the full boost the engine would generate, by the simple expedient of sticking matchsticks in the holes!!!! He was famed as having the fastest but most repaired/replaced Hurricanes in Fighter Command!

regards



Darryl

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...Some say he never blinks, and that he roams around the woods at night foraging for Merlin parts...

Priceless!

Now I had read somewhere that one other method of boost cut-out used on the Merlins actually restricted the throttle movement instead of venting excess boost, similar to the Packard system but simpler, from what I remember it adjusted the maximum throttle position from a sea level setting to critical altitude or so but couldn't really compensate for altitude like the P-51... Was this the type of system used on the Mk IXs or other late models?

Here's the page: http://www.enginehistory.org/Piston/Rolls-Royce/R-RmerlinABC/R-RmerlinABC.shtml

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As I understand it, yes. The later IX's and on used that sort of arrangement (not sure if it was that exact one). They also had mixture as fully automatic and prop pitch interconnected.

A gate was used on some for "Combat Boost". Interestingly, the total "throw' of the throttle lever seems to have remained relatively constant for all models, so pilots could probably "feel" about what % of throttle they were using without actually even looking at rpm or boost..at least after some experience.

In the later Mk IX's it translated to something like 100hp per 1/2 inch advance in the throttle. Now it doesn't take a lot of thought to come to the conclusion that winding up a late Merlin at 200hp an inch with your left hand must be an impressive experience!!


Darryl

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...Some say he never blinks, and that he roams around the woods at night foraging for Merlin parts...

Wow. Thanks for a wealth of info. I was experimenting with the Spit IIa and the constant speed prop and noticed that my rpm would rise and settle as I increase throttle but was curious to know if reducing RPM before throttle if I would see a jump in the boost gauge or if by increasing RPM, my Boost gauge would decrease if I didn't touch the throttle?

Just the opposite, in fact. Because of the nature of these superchargers they will make more boost at higher RPM. If you add some revs with your prop lever you will see increased boost without having changed the throttle. If you pull some revs then the opposite will occur, again without touching the throttle.

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