Oh god now you have done it...


JD
AKA_MattE

Thanks guys, just one qestion. At alt 17K and above, the manifold pressure in Clods Spit Mk1 drops significantly (less than 2 Ibs per Sq.in.) - full throttle - straight and level - The boost cut out has no effect as pointed out earlier in the thread. So, based on the above - this appears to be incorrect for that altitude?

The maximum boost is only +12lbs at sea level in emergency override in the Spitfire Mk I.

That is all the Merlin II and III were capable of handling and that was a definite over boosting of the engine.

Although not Merlin engines, I assume these engines were running above their maximum allowed boost!

http://www.youtube.com/watch?v=aUkXriHjQeI

http://www.youtube.com/watch?v=vq5cj...ture=fvwp&NR=1

Well at least it put a smile on my dial!

Cheers!

You got me curious so i set forth to investigate this by converting some units and making a couple of simple comparisons :-P

My personal conclusion is that while i can't be sure if it's accurate because i don't have original performance specs, +2 boost is nothing to scoff at in the wider sense of things.

First of all boost is measured in relative units, it's PSI above the normal sea level ambient pressure or in other words, zero boost means full throttle at sea level in a non-supercharged engine.

To put this in perspective, +0 boost would be equivalent to about 1 Ata (for Luftwaffe aircraft) or 29.92 inches of mercury (for USAF aircraft)

According to conversion tables, 1 psi = 2.036 inHG = 0.068046 atmospheres.

I don't know if Ata units correspond exactly to standard atmospheres, but here's how it looks thus far with relative boost measuring:
1 atmosphere = 29.92 inches Hg = +0 psi boost

If we measured absolute psi we'd get:
1 atmosphere = 29.92 inches Hg = 14.696 psi (absolute)

If this gets confusing, just keep in mind that boost is like setting the scale arbitrarily: +0 psi boost = 14.696 psi absolute


Following this train of thought we have:
+2 psi boost = 16.696 psi absolute (14.696 absolute + 2 from the boost) = 1.13609 atmospheres = 33.993 inHg

If we keep in mind that variants of late war fighters with turbo-charged engines like the P-47 were limited to about 42 inches Hg, getting the equivalent of 33.99inHg from a supercharged Merlin (no turbos involved) is not shabby at all.

Like i said i don't know how accurate it is, it's just that it doesn't seem implausible when making comparisons to later designs. I know this doesn't answer your question in a satisfactory manner, i was just in the mood for some investigating until someone with more accurate data can pop in ;)

So you throw the insults. I respond politely to calm you down and then you add another layer of xxxxxxxx ?

Where is your mum that I made an urgent request for some (boosted) slap on your pinky butt ? That also I hev done and was rather good :twisted:

Just to remind you after reading BlkDg post : btw 1940 and 1944 there was Four (4) years of harsh technological development. If they had such boosted Merlin, the MkII would hve been called a MkIX etc.. etc..

WHen dealing with history You need to put things in perspective or you end with acrobatic assumptions such as where we are now.

When dealing with a fleet of airplane at war you need to understand taht factory made does not mean fleet wide or you'll end to say that the F35 is actually in full service just because a couple have just been delivered to an instructor unit (both wings are still on guys ;)) .

And by the way I did alrdy put RR data on the forum. It does not say 17lb at all : 9 lb max op boost at rated alt. And it was in late 1940 early 41 (cant remember).

I would come back latter for the max boost and alt question but I think I (we) hve alrdy discuss this one alrdy. Just remind that that the boost (the supercharger) was not there for max low speed but for increasing the climb rate and maximizing the overall speed at alt. As I hve alrdy said this is not a car tuning. You don't plug the biggest supercharger to get the max peak power.

~S

PS: the FM of the spit is more of a concern for me
PSS : I am (wwe are) not historian(s). Instead of recreating evidences we shld stick to what are the data in the literature. There is enough to discuss on how any data cld impact the FM. Valec as alrdy post a realy good link to source out any perfs

Thanks for taking the time to do the maths Blackdog. I don't use the 'boost' because I have never really noticed any gains that are worth the risk of engine damage if you forget to disable it.

But I do look at the manifold pressure guage to check I haven't blown a gasket......:grin:

I have read Spit pilot accounts where they describe (in passing) enabling 'boost' or 'throttle through the gate' to get out of trouble and head for base.

If I fly straight and level - full throttle, with the boost cut out positioned for that extra bit of throttle movement - I don't see any increase in RPM, IAS, etc. That left me wondering what should I expect to read on my gauge at 18000ft for example and what performance gains I should expect to see or get.

There is a difference in 'would' and 'were'. ;)

Merlin III
1,310 hp at 9,000 ft, +12 lb/sq.in, with 100 octane fuel (5-minute limit).
1,440 hp at 3,000 rpm, +16 lb/sq.in boost, 5,500 ft in Sea Hurricane

Harvey-Bailey, A. The Merlin in Perspective - the combat years. Derby, England: Rolls-Royce Heritage Trust, 1983. ISBN 1-872922-06-6

The discussion is the Battle of Britain. Sea Hurricanes are a much later development and the boost increase came after some significant upgrades to the engine.

A 1940 Merlin II and III were only capable of +12 lbs and that boost was achieved by engaging the boost cut out.

The first of the Sea Hurricanes to see service with the Fleet Air Arm arrived in February 1941.

Yes be sure that is much later than the BoB.