The Revolution Will Not Be Televised wrote:

On Sat, 13 Sep 2003 19:58:15 GMT, Guy Alcala
wrote:

Okay, I've got Price's "The Spitfire Story," which is very helpful. Here's
what Wing Commander Tuttle, former head of the PRU, told Price about the
handling of the hand-modified PR.1Ds (normal 84 gallons forward, 114 gallons
in the wing L.E., 29 gallons behind the pilot, plus two cameras further back
(but no radio):

"You could not fly it straight and level for the first half hour or hour after
takeoff. Until you had emptied the rear tank, the aircraft hunted the whole
time. The center of gravity was so far back you couldn't control it. It was
the sort of thing that would never have got in during peacetime, but war is
another matter."

What may be barely acceptable for a PR bird flying solo in VFR conditions by
experienced pilots not making any radical maneuvers, is definitely
unacceptable for formation or combat flying by less experienced pilots.

[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.
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. 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.

Note that this
configuration included up to two cameras positioned further back in
the fuselage than the fuel tanks, which I assume could only have had
an even more detrimental impact on the CoG than a 29 gallon fuel tank.

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). And no guns, but fuel in the L.E.s, which (unlike the gun
armament) is all forward of the datum, moving the Cg forward compared to the
fighter.

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. It was recommended to send them
to Malta singly, but Park preferred two so that if one had problems, the other
would have some idea where the a/c went down, so they might have some chance of
rescuing the pilot.

Equally,
the RAF did regard the instability involved in the 75 gallon rear
fuselage tank in the IX airframe to be tolerable when the operational
need was great enough. Tuttle's comments are identical to Quill's and
Havercroft's on the instabilty problems casued by rear fuselage tanks
in the IXs they tested with them.

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). 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. 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.

Later, the production PR.1Ds had the aft tank removed, the radio reinstalled,
and the L.E. tanks enlarged from 57 to 66.5 gallons each side, to improve the
handling (L.E. tanks were forward of the datum). They also got somewhat
heavier Merlin 45s.

They also began to get drop tanks underneath the fuselage to extend
range.

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. No radio (mounted
aft of where the cameras would be) moves it forward again. And quite early, they
got external fuel, in 30 gal. blister tanks under the wings.

The CoG problems this caused were
certainly unacceptable for peacetime flight, but then - by the RAF's
standards - so were the rear-fuselage tanks in the later Spit IXs and
XVIs, which were ordered not to be used without direct orders in the
post-war Pilot's Notes.

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.

This is also reflected in the decision to
delete the rear-fuselage tank in the RAF's Mustang IIIs. All of this
stems from an institutional attitude and the extent to which it could
be influenced by operational neccessity.

Certainly a factor.

The fact that late production Spitfires did get rear fuselage tanks
which the RAF considered compromised aircraft stability too much for
normal peacetime operation is instructive. If we were to take
Tuttle's comments about peacetime operations as gospel for credible
wartime developments, none of this development in regard of rear
fuselage tanks would have happened at all.

Sure.

What this counterfactual stems from was the level to which the RAF was
prepared to modify peacetime levels of aircraft stability to meet
operational needs. We know their historic position, but in this case
we are positing a greater operational need for range, in which case we
need to consider what would be a credible reaction. We can all agree
that the Spitfire (and especially the Spit V) had serious constraints
involved in increasing internal tankage, and specifically in the rear
fuselage. I believe it is instructive to observe how far the RAF were
pushed down that route historically, and extrapolate on a reasonable
basis from that.

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

Guy