Guy Alcala wrote:

The Revolution Will Not Be Televised wrote:

snip

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.

And before I forget, the (high altitude) PR birds like the PR. 1D also lacked
gunsights, bulletproof glass, the gun camera, and the flare chutes/ditty box (cameras
went in thatspace). So far I've been unable to find whether the (high altitude) PR a/c
also removed the armor (the very early PR a/c probably lacked it from the start), but
it might make sense to do so. The flare chutes/ditty box are no great loss from the
fighter. With the exception of the gun camera, all of these are behind the datum, and
while individually small (except the armor), they do all move the Cg aft.


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.

Correction: We do have the Cg travel of some of the versions, but not in any
systematic fashion.

snip

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.

Checking the Mk. IX weight and loading chart at

http://www.fourthfightergroup.com/eagles/ab197.html

it's stated that the a/c tare weight includes 87.5 lb. of ballast in the tail.

Guy