In article ,
(The Revolution Will Not Be Televised) writes:
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.

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. 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). The pressure ratio of a
supercharger impeller is dependant on the speed at the impeller rim.
The power consumed by the supercharger is a function of its Pressure
Rise and the amount of air flowing through it. The supercharger, by
virtue of it being driven at a fixed ratio of the engine speed, will
try to compress the incoming air to its Pressure Rise for that speed
no matter what. 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. Of course, the power consumed
by the supercharger is the same that it would be at 2,000', so there's
a net decrease in available power, due in the most part to the higher
air temperatures at lower altitudes, in a throttled, supercharged,
engine. (Merlin 24, +18/3000R at Sea Level, 1620 HP, +18/3000R at
2,000', 1640 HP.)
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.

What happens is that when you increase boost, all other things being
equal, you decrease the FTH for the new power setting.

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. At low altitudee, the excess
compression of a high-altitude supercharger requires quite a lot of
restriction, so there's a lot of power going to waste. As I mentioned
above, power is proportional to pressure ratio, and pressure ratio
depends on rim speed, which is a function of the impeller diameter and
its rotational speed. Slowing the impeller down (lower gear) is one
way to get more power at low altitudes (At or below the new, lower,
FTH) Changing supercharger gears is a rebuild depot job, though, and
the gears themselves are difficult to make. Replacing an existing
supercharger impeller with a smaller one is something that can be done
at an airbase's engine shops.

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. For the low
supercharger gear, the critical altitude would have been about 1,000'
below sea level - It didn't have the pressure ratio to deviver more
than +20 boost at sea level -, but the High Blower gear could make +25
at, iirc, something around 5,000'. There was some thought of
re-rating V-1 chasing Mosquito Nightfighters to take advantage of
this, but the increased likelyhood of an engine failure was deemed too
hazardous to risk at night, and/or in bad weather, so they were rated
to Merlin 24/25 levels, at +18.

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).

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

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.

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. (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.)

[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.

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 supercharger gearsets are hard to build. They have
to be very precise, and they're under a lot of stress. Single-speed
engines mean half the gears. The bombers needed the extra low
altitude power for takeoff, so they got the 2-speed engines. They
also were the first recipients of variable pitch and constant-speed
prepellers for the same reason - they needed the performance, and
there just weren't enough to go around.

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.

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

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.

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 also have A&AEE Report ref: 4493/-A.S.56/42, released in November
'42, on testing the Spit V/Merlin 45 combination re-rated to +16
boost from +9. The test flying was done from July-Sept 1942.

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.

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.

Weather being the presence of clouds. The airplane will, in general,
perform better with lower temperatures. Of course, at altitude, the
temperature is going to be about the same, no matter where in the
world you are. (Well, somewhere around 35-37,000', but the differences
are a lot less marked at 20,000' than they are at Sea Level) There
was a not unreasonable view among Bomber Command crews that the higher
you flew, the safer you were. Even with an assigned height, they'd do
everything they could to get as high as they could. 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!_."

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].

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. 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.

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.

Quite so.

--
Pete Stickney
A strong conviction that something must be done is the parent of many
bad measures. -- Daniel Webster