#1 Jet of World War II

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

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

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

In article , The Revolution Will Not
Be Televised writes
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.

Interesting point, any sources for this. I haven't heard about
increasing armour for daylight ops, or trading bomb load for fuel. The
B1 Specials had virtually everything not nailed down stripped out, lost
their armour and most of their guns. The Lanc achieved its greatest
bombload in 1944-5 by daylight.

The 50s in the rear turrets were IIRC fitted only as a pair instead of
the quad 303s.

--
John

On Wed, 23 Jul 2003 13:49:11 +0100, John Halliwell
wrote:

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.

Interesting point, any sources for this. I haven't heard about
increasing armour for daylight ops, or trading bomb load for fuel.

Look at the trend evident in the Rose turreted B.1/B.IIIs, and the
engine installation in the Lanc IV which directly influenced the
Lincoln (two-stage Merlins with armoured, annular radiators), let
alone the massively increased armament in the Lincoln (e.g. twin 20mm
Hispanos in the mid-upper turret). The only way to get more fuel in
the Lanc for Tiger Force operations was in the bomb-bay, which had
obvious implications for the bombload carried. This just represents a
gradual and evolutionary change in operational emphasis.

The
B1 Specials had virtually everything not nailed down stripped out, lost
their armour and most of their guns.

And consisted of one squadron. Two, if you count 9 Squadron.

The Lanc achieved its greatest
bombload in 1944-5 by daylight.

But I think the future development pattern was clear, and in favour of
increased defensive resources.

The 50s in the rear turrets were IIRC fitted only as a pair instead of
the quad 303s.

Sure, but this still represented an increase in effectiveness at
daylight engagement ranges.

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

The Revolution Will Not Be Televised wrote:

On Wed, 23 Jul 2003 13:49:11 +0100, John Halliwell
wrote:

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.

Interesting point, any sources for this. I haven't heard about
increasing armour for daylight ops, or trading bomb load for fuel.

Look at the trend evident in the Rose turreted B.1/B.IIIs, and the
engine installation in the Lanc IV which directly influenced the
Lincoln (two-stage Merlins with armoured, annular radiators), let
alone the massively increased armament in the Lincoln (e.g. twin 20mm
Hispanos in the mid-upper turret). The only way to get more fuel in
the Lanc for Tiger Force operations was in the bomb-bay, which had
obvious implications for the bombload carried. This just represents a
gradual and evolutionary change in operational emphasis.

snip

The 50s in the rear turrets were IIRC fitted only as a pair instead of
the quad 303s.

Sure, but this still represented an increase in effectiveness at
daylight engagement ranges.

And let's remember that each .50 weighed 65 lb., while each .303 weighed 24
lb. As a perfect example of the apples to oranges comparison I'm talking
about, the 8 .303s carried by the typical Lanc weigh a total of 192 lb.
while the 13 .50s carried by the typical (in late '43) B-17G weigh 845 lb.
Oops, there goes 653 lb. of bombs/fuel right there, and then we've got to
carry extra fuel to haul that extra weight back from the target. And we're
ignoring the extra weight and drag of a ball turret and waist guns plus the
gunners and their equipment, which means carrying extra fuel to haul all
this extra weight both ways, which the night bombers didn't have).

Guy

In article ,
John Halliwell writes:
In article , The Revolution Will Not
Be Televised writes
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.

Interesting point, any sources for this. I haven't heard about
increasing armour for daylight ops, or trading bomb load for fuel. The
B1 Specials had virtually everything not nailed down stripped out, lost
their armour and most of their guns. The Lanc achieved its greatest
bombload in 1944-5 by daylight.

The BI(Specials) had 1500# stripped out to them, _and_ a special
clearance for a maximum weight of 72,000# vs. 65,000#, and Merlin 24
engines vs. the normal Merlin 22s to allow them to get off the ground
with a 22,000# Grand Slam on board. The Grand Slam missions were all
fairly short-ranged, and very heavily escorted. The missions, and the
aircraft that flew them, should not be confused with normal daylight
operations.


The 50s in the rear turrets were IIRC fitted only as a pair instead of
the quad 303s.

And were much more effective, being both harder hitting and
longer-ranged. Late model Lancasters, notably the Tiger FOrce
aircraft intended for the invasion of Japan, and the Canadian Mk X,
also had 2 .50 cals in a Martin top turret, as well.

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

The Revolution Will Not Be Televised wrote:

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

snip much

[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.

Odd that you should mention that, as Middlebrook ("The Nuremberg Raid") says
that a/c of all the Groups on the mission (with the exception of No. 1 GP;
see below), whether Lanc or Halifax, were evenly assigned to one of four
cruise heights -- 20, 21, 22, or 23 thousand feet. Naturally, some a/c were
unable to get that high or anywhere close to it. One crew flying a very sick
or tired Lanc couldn't struggle above 12,000 ft. but pressed on regardless;
as it turned out it probably saved them on that mission because the fighters
were up in the stream several thousand feet higher. A few a/c proved capable
of much better when the Jagdwaffe got into the stream and the experienced
crews decided that rigid adherence to assigned altitudes was stupid under the
circumstances, and decided to get the hell out of it by climbing (and more
than a few got rid of some bombs to lighten the a/c). One crew in a
brand-new Halifax (like all Halifaxes by that time, carrying an
all-incendiary and thus lighter load than the Lancs, to improve their
altitude performance) was delighted to discover that they were able to get up
to 26,000, and cruised happily along over the carnage a few thousand feet
below. The exception was 1 Group, whose philosophy was to carry max. bomb
loads on every mission, and who asked if they could fly at 13-16,000 feet to
take advantage of some predicted cloud at that level. They were granted
permission to do so until IIRR they reached the Rhine, at which point they
were supposed to climb to the same heights that everyone else was (supposed)
to be at. Unfortunately for them, the predicted cloud didn't show up.

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].

In addition to the engine differences, there's the considerable difference in
fixed weights (carried both to and from the target) due to extra crew, armor,
guns, turrets etc., and the extra fuel required for formation assembly (an
extra 1/2 to 2 hours before setting out), climb to higher cruise altitude and
flying in formation (throttle jockeying). When Pete and I went through the
exercise of turning the RAF heavies into day bombers a few years back, it was
apparent that using Lancs in formation by day against the same targets that
U.S. heavies were attacking, and with the same equipment, procedures and
tactics, would require a decrease in bombload of between 2,000-5,000 lb. to
reflect this, even assuming we refit them with two-stage Merlins to give them
sufficient power at altitude (and those engines each weigh ca. 200 lb. more
than the single stage jobs, which decreases the useful load for the same
MTOW, or requires an increase in MTOW and thus a decrease in climb
performance, higher accident rates, etc.). In short, design and equip them
to do the same job and they'll do it with payloads within a couple of hundred
pounds (either way) of each other. The whole Lanc vs. B-17 argument is just
ludicrous.

Guy

Subject: #1 Jet of World War II
From: Guy Alcala
Date: 7/23/03 4:40 AM Pacific Daylight Time
Message-id:

Hence early jettisoning of bombs to reach bombing height, or climbing
after bombing. This is the sort o


You mean they had to dump their bombs to get to bombing altitude?? Once they
had no bombs what is the point of getting to bombing altitude???

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.

Only if they had enough fuel left to burn up in a climb. We rarely did.


Arthur Kramer
Visit my WW II B-26 website at:
http://www.coastcomp.com/artkramer

In article , Guy Alcala
writes
In short, design and equip them
to do the same job and they'll do it with payloads within a couple of hundred
pounds (either way) of each other. The whole Lanc vs. B-17 argument is just
ludicrous.

The crucial point though, is that you're assuming the Lanc would have to
follow US practises and fly in the big box formations. Whilst this may
have suited the B-17 with machine guns sprouting everywhere (although I
don't think it was appropriate before P-51D escorts were available), it
may not have suited the Lanc. You simply can't enforce US practises on
UK types in an attempt to 'equate' them with one another.

One Halifax squadron removed the nose and mid-upper turrets, armour,
flame dampers and various other bits, the lighter weight and less
restricted engines flew higher, faster and their losses were reduced
significantly.

--
John