Spitfires saddle tank

[Krusty]

The fuselage tank on Spits was not often used either because of the CG issues.

Doesn't seem to be modeled correctly in AH, then.

[Casca]

Moving the CG foward relative to the center of lift increases longitudinal (pitch) stability.  Stability can be static or dynamic but I'm only considering static here which can be described as the initial response after a control displacement.  Controlability and manuverablility are related factors. When the CG is at the forward extreme the aircraft is positively stable; after a control displacement it tends to return to the previous condition.  This can have a deleterious effect on manueverability as the aircraft can become too stable.  To visualize the extreme case of positive static stability consider a bar dart, a very stable object but not very maneuverable.  Some year models of the Cessna Cardinal actually had a slotted stabilator lifting down because folks were unable to flare at forward CG loadings and were pranging nosegears.  Most aircraft are designed to be operated with some degree of positive static longitudinal stability.  It makes them easier to fly as they tend to correct themselves after a displacement.

If the CG is approximately co-located with the CL the longitudinal static stability approaches neutral.  The initial response after a control displacement is to remain in the displaced condition.  This can enhance manueverability and in some cases can be advantageous.  If you are flying an aerobatic routine you want the airplane to stay where you put it.  Maneuverabily is enhanced because more of the wing lift can be used to turn the airplane due to the fact that it is not required to support the downforce on the tail (in addition to the aircraft weight) necessary to keep the nose up.

If the CG is behind the CL the longitudinal static stability starts to be come negative.  The aircraft will continue to diverge from an initial control displacement.  This is a manueverable condition and also will generally yield the highest top speed.  There is no downforce on the tail so the wing can generate sufficient lift to remain airborne at a lower angle of attack (and therefore induced drag).  The problem is that with a CG behind the CL recovery from stalls or spins becomes difficult or impossible.  I once took a tiny boyscout for a ride in the front seat of a Schwiezer 233 and was too lazy to walk across the field to get the ballast block.  We were nose high and climbing with full foward elevator at the takeoff but accelerated enough to regain elevator authority with a few moments.  No huge drama but a good lesson in the effects of rearward CG.

[Guppy35]

Doesn't seem to be modeled correctly in AH, then.

Cause the Spits in AH don't have it Krusty.  not the DT, but a fuselage fuel tank that was fitted behind the cockpit that held 75 gallons.  I don't know that I've ever seen it in a photo of a combat bird.  I have seen photos of the installation, and ML417, a restored Spit IX has it installed.

I wouldn't want it added to an AH bird personally.

[Krusty]

Ahhh.. thought that was the term for the DT, my bad. Kinda looks like a saddle stand, only upside down.

[Guppy35]

The slipper tanks on the Spit were really the first conformal fuel tanks, in particular the 30 gallon.

[Angus]

Fuselage tanks, and you have the Spit21. A bird with contra rotating props, similar performance as a XIV (but not the torque) and long legs. Quad cannons?

[Baumer]

According to the pilots notes I have (see previous post in this thread) the Spit IX, XI, and XVI all could have the fuselage tank fitted for ferry purposes. It also sounds like it was a pain in the rear to fly with, "takeoff only from a long smooth concrete runway" or "except in an emergency, do not attempt to land with more than 30 gallons in the fuselage tank".

[moot]

Thanks Casca

I don't understand, though, why CG aft of CL would tend towards higher top speed.

[Casca]

Thanks Casca

I don't understand, though, why CG aft of CL would tend towards higher top speed.

If the CG is forward of the CL the aircraft is nose heavy.  You have to push down on the tail to hold the nose up.  The wing does not know if it has to carry an increased load because there is a downforce on the tail or a fat chick in the cockpit.  All it knows is that it has to operate at an increased angle of attack to support the airframe in straight and level flight for a given airspeed.  The increased angle of attack causes an associated increase in drag; specifically induced drag or that component of drag associated with the production of lift.

At the aft CG loading the force on the tail is up, that is to say the tail is actually lifting with the result that the load the wing sees is decreased.  The wing can operate in S&L at a reduced angle of attack and reduce the drag penalty.  This is the reason that canard planform aircraft operate more efficiently, everything is lifting.

[Guppy35]

Fuselage tanks, and you have the Spit21. A bird with contra rotating props, similar performance as a XIV (but not the torque) and long legs. Quad cannons?

Only a couple fitted with contra rotating props, not operational that way.  Didn't have the fuselage tank either.  The 4 cannons and the redesigned wing were the big dif over the XIV, but performance wasn't that much greater.  91 squadron had em before the war ended.  No air to air in them though, patrolling over the hook of Holland mainly from England.

[moot]

I understand, thank you.

[Angus]

Only a couple fitted with contra rotating props, not operational that way.  Didn't have the fuselage tank either.  The 4 cannons and the redesigned wing were the big dif over the XIV, but performance wasn't that much greater.  91 squadron had em before the war ended.  No air to air in them though, patrolling over the hook of Holland mainly from England.

Oh, I thought they saw service like that. I knew about no air-to-air, but by that time the LW was hard to find.
I have an account from Neville Duke when he was testing one and the prop mechanism broke. I can type that one up for you if you do not have it Guppy

[Die Hard]

Planning on flying to Malta? 

I might!

 

[Guppy35]

Oh, I thought they saw service like that. I knew about no air-to-air, but by that time the LW was hard to find.
I have an account from Neville Duke when he was testing one and the prop mechanism broke. I can type that one up for you if you do not have it Guppy

Seems like I remember that one.  One of the Spit XII drivers I got to know, later became a Supermarine Service Test Pilot after his combat tour.  He did the production tests on the 21 and his most eventful flight was taking off in a Spit 21 with the Ailerons reversed. Tough to have to fly it the opposite of normal just to get it back on the ground

[Angus]

Duke's problem was that the pitch of both parctically went flat. So he could barely make it to the airfield with full power. What a silly sight it must have been, an aircraft flaring in for a short 3-point landing with the engine on absolute full power