I'm back, but an ahem, "flying visit" Trade Show season for us is
coming up, and I've just shaken off a 48 hour attack of the 24 Hour
Ebola...

In article ,
Guy Alcala writes:
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 managed to latch onto a few Spitfire Pilot's Notes.
Specifically, the Mk V/Seafire II/III, the Mk XII, the Mk XIV, and the
Seafire Fr. 46. (No Mk IX yet, dangit!) I'll gove more exact quotes
tomorrow, but I'll sum up a bit here, as appropriate.

The Spit FR. XIV is listed as having a 31 imp. gallon tank behind the
cockpit, and 2 13 imp. gallon tanks in the wing leading edges. Note that
this is only for the FR. XIV (Fighter/Recce), not the F. XIV (Pure
Fighter) or the P.R. XIV (Pure Recce). It's also worth noting that
the date of publication is early 1946, and at that time, the rear
fuselage tank is noted as "Blanked off and not to be used unless
specifically ordered by hte Operational Commander, and only for
special operations. In the Handling Section, it is noted that teh use
of the rear fuselage tank reduces stability, and the aircraft should
not be flown above 15,000' until it is empty. (That's not really a
problem with a Griffon Spit) The recommended fuel burn sequence it to
take off on the main tanks, switch to the rear tank at 2,000', and
when that's empty, (signified by the engine quitting) switching back
to the drop tank, if fitted, or the main tanks and transferring the
wing tank fuel as soon as possible.

So, it was there, adn it was a somewhat useful amount of fuel, but it
really wasn't considered a good thing.


[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.

Oddly enough, the Spit V notes has an appendix to th handling
instruction for the 29 gallon fuselage tank/170 gallon super drop tank
combination. A few salient points: The aircraft is restricted to
straight and level flight until the drop tank and the rear fuselage
tank are empty. There are a lot of warning about how setting the fuel
cocks wrong will case teh system to siphon fuel overboard. The
sequence of use was to take off on the main tanks, switch to the
drop tank and run it dry (indicated by the engine cutting out - the
Spitfure Fuel gages must have been designed by the same bloke who did
the MG oil gage. Lucas, I think his name was.) The the rear tank is
emptied, (Same fuel gage), then the mains. The drop tank may be
jettisoned at any time, as long as you're straight and level, empty,
full, or in between. Nothing but straght and level until the rear
fuselage tank empties.

So - while it appears that the 29 gallon tank wa only intended to be
used with the 170 gallon tank for the Malta reinforcements, there's
really no reason why it might not have been considered, with the
proviso that you couldn't fly a real formation, or engage in combat
until it was empty, or that it couldn't be used with one of the
smaller "combat" tanks. Note though, that the 90 imp. gallon tank is
also restricted to straight and level flight.

That's a bit more flexible than I thought it would be.



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.

Not only that, but a PR Spit wasn't expected to be maneuvering - It's
job was to be as high as possiblem where nobody had any maneuver
margin. They'd get to height as soon as possible after tankeoff, and
stay there. A good chunk of the aft fuel would get burned off in the
climb. It's also worth noting that the PR Spits had no disposable
load other than fuel. On the fighter Spits, the ammunition was aft of
the CG, (but not too much) and contributed to any stability problems.


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.

See above. I was rather surprised myself. The 90 gallon tank is
pretty much out as anything but a ferry tank, but an intermediate
choice of a 50 or 60 gallon tank would provide the same fuel capacity
over the 30 gallon combat tank as the aft fuselage tank would, without
much in the way of bad effects. (Didn't the Hurricane use a 50 gallon
teardrop or torpedo shaped tank? It might even be less draggy than
the 30 gallon blister.

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