On Fri, 18 Jul 2003 01:04:53 -0400, (Peter
Stickney) wrote:

[snip more useful info]

I've just been poring over a stack of A&AEE Test Reports on various
Spitfire V adn Spitfire IX variants, and one fact quickly becomes
apparant. Both versions of the Spitfire had their most efficient
cruise speed between 160 and 170 mph IAS. The difference in True
Airspeeds comes from the fact that the 2-stage supercharger of a 60 or
70 series Merlin allowed that 170 mph IAS to be maintained at about
30,000', rather than 16000'. RPM for RPM, adn lb of boost for lb of
boost, the 2-stage engine wasn't as powerful as the single stage. The
extra compressor did allow that lower power to be maintained to a much
higher altitude, though, which made a difference.

There's also the possibility of increasing boost below FTH, e.g. with
higher-octane fuels for chasing V-1s in 1944 (e.g. Merlin 66's being
uprated to +25 psi from +15 psi). But none of that applies to
cruising speed settings, and for all I know such pressures might be
attainable by de-restricted single-stage engines like the Merlin 45,
when the cropped impeller version could go up to +18 psi from +12 psi,
and also when this had already been done with the move from +9 psi to
+12 psi with the introduction of 100 octane on the Merlin III.

Sure, but if you're flying at the same engine settings in a Spitfire V
and a Spitfire IX at the same altitude (above FTH for the M46), the IX
will go faster, even when the FS supercharger is almost identical
(Merlin 46 vs Merlin 61). The cruising regime was governed by engine
rpm and supercharger boost, and different engines produced different
outputs at different heights at the expense of differing fuel
consumption. In this case, I know this would be down to differences
in the height at which higher boost pressures could be maintained.

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).

True Airspeed at that point will be about 215 mph. Above
that height, boost, and thus power, drop off.

Sure, I can percieve the criticality of the attainable boost level,
which indicates the level which the supercharger can compress air
beyond the natural density at that height.

A Merlin 61 powered Mk IX can maintain +7 up to about 33500', where
that same 170 mph IAS is now a True Airspeed of nearly 300 moh TAS.
You;re not getting more power, you're getting more altitude. That may
sound like splitting hairs, but at altitudes below the single-stage
engine's Full Throttle Height, the performacne for minimum fuel
consumption is the same.

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.

[increasing supercharging output below FTH]

You can increase boost, using the excess supercharger
capacity to develop more boost, and thus more power at lower
altitudes, but you have to be careful about that, or the engine
becomes unglued pretty quickly. The usual solution was to introduce a
lower supercharger drive speed, as outlined in my example of the
COrsair engine at the beginning of the post.

Or the Merlin 20 series.

This led to the 40 series Merlins
for the Spitfire V, which were basically Merlin XIIs with the
supercharger and high-speed gearset of teh Merlin XX.

Actually, my impression was that there were two varieties of Merlin 40
series: the Merlin 45, which was a Merlin III with Hooker's more
efficient Merlin XX supercharger inlet housing on a Merlin III, and
the Merlin 46 with the Merlin XX supercharger housing and gearing, but
with the MS or low-altitude supercharger gear deleted. This lead to a
slightly different FTH and output against height for the two variants.
Not that this changes anything in your explanation.

It was felt
that the low altitude penalty in the SPitfire V with a Merlin 45
wouldn't be that bad. In the event, engaging 109Fs across the Channel
and in North Africa, it was found that they needed more power at low
altitudes, and the engines were re-rated accordingly, from 3000 RPM/+9
originally, with an FTH of about 18,000', to 3000 RPM/+12, and later,
3000 RPM/+16, which gave 1470 HP, but at onlu 9300'. The two-speed
engines allowed you to have your cake and eat it, too.

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.

Another factor I just thought of would be the power being soaked up by
a second mechanical supercharger at lower heights, which would affect
the relative efficiency (well, it would do that at all heights, but I
presume the inefficiency concerned would be maximised at lower
atltitudes where the full supercharging wasn't required to reach the
maximum pressure permitted for full power. Another assumption...).

A good one,

I got there in the end......

One of the things this brings into focus is the actual operational
performance of the Spitfire V. One of the little-publicised reasons
they had a hard time on offensive operations in 1941-42 (aside from
the obvious issues of tactical positioning and advantage) was that
they were flying at relatively low speeds, and the low-revs, weak
mixture cruising speed of the Mk V was similar or even slower than the
Mk I or II. The power advantages it had over the earlier Merlins were
only apparent at higher altitude or rich-mixture, high rpm settings
which were only relevant once combat had been initiated and weren't
relevant to the relative tactical positioning beforehand. This
changed over time, as the RAF formalised higher cruising speeds as a
tactical doctrine by 1942-43.

[Merlin 66 vs 61]

Yep. There's another factor, too. SInce the gear ratios of teh
Merlin 66 supercharger are lower, it's overall pressure rise is lower.

Ah! Yes! Precisely what I was searching for, thanks. The gearing
issue is also germaine to the Merlin 45/46 differential I was on about
earlier.

[Lanc altitude on ops]

While it doesn't get talked about much, the Lancaster was a fairly
height-limited machine. The usual heights on a raid into Germany were
between 15,000, and 20,000', depending on the amount of fuel burned,
and the particuar airplane.

By the winter of 1943-44, planned height bands for Lanc squadrons in 5
Group (which I have researched) were often 20-22,000 feet. The actual
bombing height was rarely much over 20,000 feet, and often a lot
lower, depending on weather and individual aircraft characteristics.
On operational conditions, with a full load, and winter weather to
deal with, they really did have difficulty getting over 20,000 feet.
The most common bombing heights (excluding exceptions like the
Peenemunde raid) seem to have been around 18,000 feet.

After shedding the 5-7 tons that it was
delivering over the target, it would certainly have been able to
return at a much higher altitude than it went in.

Hence early jettisoning of bombs to reach bombing height, or climbing
after bombing. This is the sort of real context that tends to get
ignored in the interminable and ahistoric B-17 vs Lancaster
nationalist posturing. One of the trade-offs for the Lanc's higher
bombload was lower operating altitude [although this was also down to
the different engine outputs at different hieghts].

On the other hand, I have seen log entries which give route timings
which can only be explained by atypically strong tail-winds, or more
likely the crew either cutting corners en route or increasing the revs
to get home quicker with a little polite fiction in the records for
when the squadron navigator leader became suspicious about their
marginally early returns. So I need to factor in the original context
before making some more characteristic sweeping assertions.

Well, there's always the oldest truism of Air Combat: "On the way in,
I'm working for the Air Force. On the way out, I'm working for
myself". Since the night bombers didn't hold to any formation, and
the higher you were, the harder for the interceptors and flak to find
you, I'm sure they made for Breakfast and Bed in th emost expedient
manner. I'd say they earned it, too.

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 (demonstrated by
returning several minutes earlier than the preceding aircraft, e.g.
taking off as the ninth and returning first or second consistently
with time on target within expectation - i.e. making up any time on
the egress route, not on the way in), and this might have been a
factor in their higher survival rates. Sitting on the fringe of the
stream was dangerous as you began to lose window and RCM coverage, but
equally cutting corners and increasing speed made you a slightly more
difficult intercept proposition for nightfighters which had precious
little speed advantage and were directed to the head of the main
stream.

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