On Wed, 23 Jul 2003 00:24:11 -0400, (Peter
Stickney) wrote:

If I may, I'd like to put a bit of context to the above paragraphs.
I've been noting that you may have a misapprehension about what "Full
Throttle Height is.

No, I was trying to distinguish between FTH (end of barometric
throttle limitations designed to prevent over-boosting at low
altitudes in denser air) and peak supercharger efficiency - as I did
for the Merlin 45. However, I wasn't doing it consistently, so you're
quite correct to pick me up on it where I don't.

There is no single Full Throttle Height for any
engine, or airplane. The Full Throttle Height is the height that, for
an airplane with fixed-speed supercharger drives, like, say, a Merlin,
or an Allison, that it's no longer necessary to restrict the flow from
teh supercharger into the engine to prevent it from over-boosting.
(Also called teh Critical Altitude).

I'm afraid I don't follow that - my understanding is that the
regulation will vary between engines (due to manufacturing tolerances,
etc) and climatic conditions (e.g. with climatic variation in air
density over altitude), but that the same type of engine with the same
baromatric restrictor will hit the FTH at approximately the same
height.

FOr example, the low speed gear of a Merlin 24
produces +18 psi of boost at 2,000'. At sea level, it would produce a
bit more than 20 psi boost, but the flow is restricted to keep the
boost at +18 at sea level for takeoff.

This is where the higher octane fuel comes in: overboosting becoming
tolerable within certain limits when predetonation of the fuel can be
avoided.

The pressure ratio for the Merlin 24 supercharger in low gear (MS
Gear), was about 2.3. In high gear, the same blower produced a
pressure ratio of a bit more than 3, but at the cost of about 140 HP.
So excess compression in a supercharger isn't a good thing. It eats
up power that would otherwise go to the propeller.

I suspect this is why the Merlin 46 never totally replaced the Merlin
45, and it's why the Merlin 60 series was re-geared to produce the 66.
An "automatic" gearing might have been a better solution, although I
suspect that's my ignorance speaking.

The reason for the cropped supercharger impellers, BTW, was that it
was a simple way to reduce the amount of power required to run the
supercharger at low altitudes.

I see that. Same as changing the rear sprocket on my motorbike
instead of changing the gear ratios in the gearbox.

Replacing an existing
supercharger impeller with a smaller one is something that can be done
at an airbase's engine shops.

The LF Vbs seemed to get their modified engines from MU's or from RR
at Hucknall. Or Cosford, thinking about it.

Oh, one other thing. It wasn't used very much, but the Merlin 24/25
was also re-rated to use +25 boost with 150 PN fuel.

I didn't know that.

Ah, but you're not flying at the same engine settings above the Merlin
46's FTH. The Merlin 46 can maintain +7 lbs of boost to about
18,000'.

Yeah, but I was thinking of crusing speed settings (typically 2,650
rpm, + 3.75 or +4 psi for max continuous weak mixture cruise). This
setting is common between the Merlin 45/46 and Merlin 61/63/66, and
could be maintained over 20,000 feet (albeit not much over 20,000
feet) for the Merlin 45 (with the lowest FTH and lowest supercharger
peak of the lot).

At +4, a Merlin 45 would peak at about 18,000',

In terms of power output? I'd agree, which is why they cruised slower
at 20,00 feet than the Merlin 60 variants did.... which is where I
came in..... :-)

a Merlin 46 at about
22,000. That _4 leam mixture cruise isn't a given, though.
Later 20 series Merlins, the 22, 23, 24, and 25, and their Packard
equivalents, had carburetors that would allow max lean mixture
operation at +7.

I'm not sure how often this was used, though. The "minimum revs,
maximum boost" mantra seems to have left +4 as the most
commonly-referred-to option on operations, but that's just based on my
own readings, which I can't claim to be authoritative.

The 60 series Merlins did as well. You're right
about the 40 series engines, but the 50 series sengines had new carbs,
(Which, among other things, weren't susceptible to the -G cutout of
the earlier engines, and may have been able to cruise at the higher
boosts.

Some of the 45's and 46's got them (RAE and SU negative G carbs) as
post-production tests, with unimpressive results.

The 2,650 rpm +4 psi regime is quoted as returning 56 gallons per hour
on the Merlin 45/56 Spitfire V Pilot's notes, and 71 gallons per hour
on the Mk VII/VIII/IX Merlin 61/63 pilot's notes. There's a slight
differential in that the Merlin 45 boost figure is actually 3.75 psi,
but otherwise the consumption figures seem higher than I would expect
even including the extra power being sucked up from the crank output
by the second stage impeller. But this is speculative on my part. I
assume thanks to your explanation that the height would provide the
missing factor here, and the Merlin 63-engined Spit IX would cruise at
those settings, but higher and faster than the Spit V: same or
similar IAS, but different TAS.

Right. What it comes down to, in terms of range, is Miles Per
Gallon. A 2-stage Spit is burning more fuel per hour, but its
covering more ground.

Higher crusing speed on the same engine settings *at a different
altitude* giving a similar range but shorter endurance.

(Actually, as far as range goes, I think that
it's a wash - the higher fuel consumption gets countered by the higher
cruise, so MPG stays about the same.)

The approx 6 air m.p.g. figures in the Pilot's Notes for several
Merlin-engined types seems to confirm this.

[Low altitude supercharger gears]

Just so, or teh Merlin X before that, which was basically a bomber
engine (Whitley and Wellington, and some Halifaxes).
As to why they
just didn't build one type, I suspect that it comes down to allocation
of resources.

The critical and ever-pressing need to maintain exisiting production
lines with minimum alteration to ensure overall output didn't fall too
far. Hence the Merlin 45 (keep building Merlin III's with the same
supercharger gearing, just slap Hooker's new impeller housing on
them), the Merlin 46 (grab some Merlin XXs and do the same thing), the
Merlin 61 (stick a Vulture supercharger impeller in series with a
Merlin 20). Obviously, there were prop reduction gear changes,
different gearings, and so on, but the introduction of new types when
maximum output was required was a real problem.

One thing you might know about and I wanted to ask was the "built for
100 octane" label applied in certain documents to the Merlin X and
XII. Apart from possibly higher compression ratios at lower altitudes
following from increasing supercharger boost figures, what else would
this involve?

Essentially true - but as I remember it, there were a lot of new-build
Merlin 45s, so it wasn't just conversion.

They just kept one of the Merlin III production lines at Derby
running, I suspect.

The +9 to +12 psi increment was in 1940, with release of 100 octane
fuel to Fighter Command, while the next step seems to be in 1942-43
with +15 and then +18 psi becoming attainable, both down to the supply
of higher-octane rich-mixture PIN fuel.

I've a number of A&AEE reports on verious Spitfire V tests, most
conducted in '41, and they all use 3000/+9 as the Maximum rating.

I've seen the same ones, I suspect, but Quill gives a figure of +12
for 100 octane, and although I can't substantiate this from my own
notes yet, I believe this was mentioned as one of the specific
production reasons for the Merlin XII. However, all the figures I
have seen from A&AEE and even RAE reports in the same period give +9
(I can find higher pressures mentioned, but only in 1943 or at the
earliest 1942). However, these seem to concentrate on speed at height
and maximum continuous climb ratings, and I'm not sure they indicate
anything conclusive about higher boost pressures being used lower
down. I think the boost control had to be manually over-ridden to
achieve that. I'll comment further if and when I can actually produce
something to back this up in the months ahead.

It is true that Merlin XIIs, the engine on the Spit II, were re-rated
from +9 Max to +12 in late '40. It didn't happen in the Merlin 40s
until later.

That might be it, and I might be confusing the two, but I'm convinced
I've seen a reference for the Merlin III being similarly adapted in
1940.

A fully-loaded
Lanc or Halifax would generally start cruising somewhere around
15,000', drifting up as fuel burned off. I could easily see some
crews in the less-well performing airplanes shedding some weight, if
only to not be the lowest bloke in the bomber stream. As we say in
the North Woods, "I don't have to outrun that bear, I just have to
outrun _you!_."

A relative served in 61 Sqn at that time, and the bombing altitudes
over Berlin seem to have been about 18,000 - 19,000 feet, and I'm sure
they would have gone higher if they could. They also had a long, slow
climb across the enemy coast and into enemy-occupied Europe before
getting anywhere near that altitude, at a rate of climb of less than
100 feet per minute. Not something I'd like to try.

Right. The Night Bombers, not having to hold a formation out & back,
also could end up being a bit more efficient, as far as cruise goes.
Formation flying, especially if it becomes necessary to jockey
around. (Evasive maneuvering, or the leader's not being smooth) makes
it tough, with a lot of power changes.

Not to mention the lenghty assembly phases over England, which tend to
understate the Fort's capacity by consuming useful load (fuel) and
affect endurance and range in a manner the night bombers didn't. [OK,
they often had climbing legs routed over the land, but nothing like to
the extent that the day bombers did.]

You're also limited to the
performance of the slowest/lowest/thirstiest ship in the formation.
But... That was the only way to go by day, and that sort of formation
flying was impossible at night. Both the U.S.A.A.F. and the RAF ended
up with just what they needed, in order to perform complimentary tasks.

They evolved that way, from roughly similar airframes as a starting
point. Lancasters flying by day would soon develop heavier armour,
especially around the engines, less bombload in exchange for more fuel
to burn for higher height on the ingress route, and heavier armament
like .50 calibres in the rear turret - all of which they were adopting
by 1945, which cut into their bombload margin over the B-17.

Not that I'd push this too far, but at least one squadron ORB shows
that the more experienced crews were clearly learning to cut corners
on the egress route as their tours progressed ...

Quite so.

I did like the "following wind" claims in one log book where the same
crew had resorted to dog-legging across the route track on the way in,
evidently to reduce their ground speed and arrive on target at the
right time. Not something you'd do facing the sort of headwind they
would have needed to produce the kind of tailwind that got them home
20 minutes early.... with an aiming-point photograph.

Gavin Bailey

--

"...this level of misinformation suggests some Americans may be
avoiding having an experience of cognitive dissonance."
- 'Poll shows errors in beliefs on Iraq, 9/11'
The Charlotte Observer, 20th June 2003