On Tue, 16 Sep 2003 23:01:46 GMT, Guy Alcala
wrote:

[I've got that book meself]

But this is precisely the argument against any kind of rearward CoG
movement implied by any rear fuselage tanks.

Not any kind, just the kind that can't be used in combat or in formation.

The problem is that we don't know the CoG travel involved in any of
these changes of equipment and trim. The only loading charts and CoG
details I have are for later versions of Spitfire. Nonetheless, it is
clear from the diagrams I've seen that the PR IV SLR with the 30
gallon rear fuselage tank had two cameras and their mountings,
occupying the entire fuselage at that point, mounted further to the
rear than the 29-gallon ferry tank. I don't know the weights involved
with the cameras, but in the case of the 29 gallon tank this was
mounted in the fuselage immediately behind the pilot: unless the tank
went under the pilot's seat there was no place with less of an impact
on rearwards CoG travel to put it.

Now, I fully accept that 200lbs of extra weight in the rear fuselage
in any position would make the handling of a Spit Vc deteriorate, but
I find it hard to believe that this was not a creditable proposition
within operationally tolerable limits, especially at a time when tail
ballast was being increased to test heavier Rotol propellers.

Remember, we're trying to extend the combat radius, with range being essentially
irrelevant. If you've got to burn the fuel off before you enter combat and the
only advantage is to extend the range/endurance, it's easier to just carry a bigger
drop tank.

To a point: the critical issue is the handling deterioration involved
in both cases. We know that a stock Mk IX was considered acceptable
for manoevering with 30 gallons or so consumed out of a 75 gallon rear
tank, and although this involves an aircraft with a different CoG
loadout and weight distribution, we know the RAF was prepared to
tolerate 30-40 gallons worth of weight with a detrimental impact on
longitudinal stability to increase operational radius in 1944-45.

I'm advancing the 29-gallon tank scenario because this offers the RAF
precisely this capacity in mid-1942 in plenty of time for further
exploitation in the summer of 1943. R&D workThe 29-gallon rear tank
and 90-gallon drop tank combination had already been done and offered
an "off the peg" solution for increasing the 12 Group Spit V wing's
escort ranges when this first became an issue. I'm not arguing that
this was an optimal solution, but I do think it was a possible and
credible one given the fact that it was an already-existing historical
equipment option.

I'm starting my speculation from the point of "what was actually
available" with Fighter Command escorts for US strategic raids. The
best-placed units for escorting attacks on Willhelmshaven, Bremen and
Hamburg were the 12 Group squadrons at Ludham, Coltishall, etc. These
units were considered second-line to the squadrons in 11 Group which
had priority for the Spitfire IX, and so an initial escorting solution
might have to be found which maximises their potential before major
reallocation of Mk VIIIs or anything else takes place.

Obviously, if you're positing the major change in strategic direction
under ACM Kramer in September 1943 rather than from the ground up in
June, as I am, then the whole issue of increasing the range of the Mk
V is moot.

Having said that, some figures from AVM Stickney might be interesting
to indicate precisely how much ******** I've been talking so far.

The extra internal fuel would allow you to go a bit further without
carrying a drop tank, but that's a relatively minor (but real) advantage.

Whatever we do, the Spit escorts beyond the Zuider Zee/German border
are going to be less effective than the P-38s or P-51s because of the
internal fuel limitations, but even a "drop external tanks, one or two
passes and then home" profile is better than no escort at all.

There's no question that MK VIII-style increased internal tankage is a
better option, as the wing leading-edge tanks and enlarged forward
tanks give greater endurance in combat trim than any external tankage
could provide.

[aftwards CoG in PR IV Spits]

But no radio, which appears to have been even further back, and IIRR of equal or
greater weight than the cameras (I saw the camera weight listed somewhere, and have
the radio weights).

How much were they? In my LR Vc scenario, the radio stays where it
is, the oxygen bottles get displaced downwards and the top half of the
clear perspex frame behind the pilot now has a fuel tank. I'm not
sure what impact this would have on the pilot's armour plate: I'm
pretty convinced it would be impossible to move it aft to protect the
tank without an unacceptable impact on CoG travel.

I don't think the PR Spits are a valid indicator of what was tolerable
with a fighter profile - e.g. the V's with 29 gallon rear fuselage
tanks (and 170 gallon drop tanks) used for reinforcement flights from
Gibraltar to Malta in October 1942. Now these were not in operational
fighter trim, but they were to be flown by squadron pilots.

Two at a time, not in squadron/wing formations, and not having to take off in IFR
conditions and climb above clouds on instruments.

Yeah, but this was also with the 170 gallon overload tank; I don't
think we can take it as a comment on what the 29 galllon tank was like
in isolation.

[instability eliminated in Mk IXs after 30 gallons used from 75 gall
rear tank]

Ah, but we agree that the IX's had a more forward Cg to start with, and we don't
know the degree of instability (which is what we're waiting on Pete for).

Precisely.

BTW, I
think I now know why the Mk. IX/XVI with cut down fuselage had 66 gal. in the rear
fuselage tanks vice 75: a simple space issue.

Yep.

The photo in "The Spitfire Story"
(pg. 162 in the copy I have) showing a Mk. IX with aft tanks shows that they're
stacked directly behind the pilot's back armor plate, and the upper one extends
almost to the top of the aft cockpit glazing Presumably the bubble-canopy a/c
tanks were lower.

Yep.

I think it's instructive that until alternative fuel tankage
could be provided in the wings and externally, they did adopt a rear
fuselage tank, and that was in an aircraft with an existing CoG travel
worse than the fighter versions.

But was it? After all, with no armament, the Cg moved forward.

And moved back with wing tanks....

No radio (mounted
aft of where the cameras would be) moves it forward again.

But the cameras moved it back again....

I agree, we can't really conclude this without some more information
on the CoG travel involved in the V and IX.

Unfortunately, we don't know the degree of instability (and Cg travel) of the
various versions, since they apparently found measuring such rather difficult at
the time. It seems to have been a question of phugoids and nothing else, which
would leave a lot up to pilot interpretation.

Exactly. But from a service which ordered single-engined fighter
pilots up at night in 1940-41 with no radar or landing aids worth the
name, or ordered a Hurricane squadron to fly off a carrier in May 1940
with no prior experience, or ordered the first Typhoon wing into
action when the engines were still blowing up and the tails falling
off, I think we can assume a certain degree of ruthlessness when it
comes to a perceived operational requirement.

[snip evidence of unprovoked agreement]

On that basis, the use of the 29 gallon rear fuselage tank can't be
ruled out on the basis that the RAF preferred not to use it. If we
left it to RAF institutional preference alone there wouldn't have been
any rear fuselage tanks at all, but then nor would there have been PR
Spitfires to start with.

I think the Mk. IX could probably have gotten away with a 29 gallon tank or
something in that range, even with the original tail.

Actually, they were getting the 75 gallon tank even without the
broad-chord tail.

I have serious doubts that
the Mk. V could have.

I can see the rationale, but I'm talking about less than half the
weight.

Tuttle's comments imply negative longitudinal stability with
ANY fuel in the tank, and that means it is of no help to us for increasing fighter
combat radius. We could put the same fuel in a larger drop tank (at somewhat
higher drag, to be sure), say 120 gallons; the handling will be better than with
the rear tank, and it's a much simpler solution.

I see the point but once again I'd urge the constraints of actual
production availability. The 90 gallon tanks were available in
quantity in time for this: I'm sure they could have built other
tanks, but the strategy would have had to be made to work with what
was realistically available first.

Gavin Bailey

--

Another user rings. "I need more space" he says.
"Well, why not move to Texas?", I ask. - The ******* Operator From Hell