Allison-Merlin Mustang wing position changes

[Bruce Archer]

Everybody needs to realize the Allison Mustangs were different aircraft from than Merlin Mustangs. Yes, they looked like each other but when you actually start to measure things, they are very different.

 

Bruce

[Graham Boak]

The same wings and much the same fuselage, allowing for changes due to the more powerful engine including a larger cooling system.  Less than happened to other aircraft without quite so much fuss.  The key item to understanding this is the raising of the central and rear fuselage relative to the wing.  The problem is making people aware of it.

 

I do wonder if the rising of the fuselage was required in order to fit in the larger radiator?

[KRK4m]

However, I would avoid the term "raising the central and rear fuselage", because this in some subconscious way suggests a change in the contour of the fuselage spine and a smaller elevation of the vertical stabilizer above the ccanopy top. And here, there is none of these things - above the FRL and the thrust line, both the Allison-versions and P-51B/C  have the same height of canopy (36 2/16") and fin (67 9/16"). Even the location of the tailwheel has not changed in relation to the FRL, as lowering the wing by 3"(while keeping the same main U/C legs and wheels)  increased the parking angle from 12*45' to 13*36'. Thus, the reason for lowering the wing was not a deeper radiator (although, of course, further lowering its lower edge would inevitably result in contact with the ground), but a different design of the air duct to the compressor.

Well, Allison has this duct above the engine, and in Merlin, the duct goes under the crankcase and then turns straight up in front of the firewall. This is clearly seen in the side photos of the XP-78, which have an old (slender) fuselage with the original (high) wing location, making the lower part of the Merlin cowling protrude a few inches down, spoiling the aerodynamics of the lower fuselage contour.

On the other hand - as far as I remember (I sat in a Mustang once in my life and it was over 30 years ago), the upper surface of the wing is the cockpit floor there. So, did the pilot in the P-51B/C sit 3" lower (in relation to the windshield and canopy), or do the Merlin versions have the seat and rudder pedals moved upwards?

Cheers

Michael

[gingerbob]

No, a plywood floor was added on Merlin Mustangs.

[Graham Boak]

Michael, I don't agree with your interpretation of what I said.  The centre fuselage includes the cockpit and canopy, so there is no implication of change in the spine nor tail unit.  Quite the contrary - the difference is in the belly, with a deeper curve to the radiator area.

 

I was careful to say raising the fuselage relative to the wing.  I agree that in design terms it is the swing that has been dropped, but away from the drawing board and looking at the aircraft, it is the fuselage that appears (is) deeper.  Yes, there is a deeper design to the intake but the question still remains of whether the new radiator would actually fit into the old fuselage.  Mustang Mk.X clearly had a very different cooling system which may imply otherwise, but this aircraft is interesting in itself but better left to a different discussion.

[gingerbob]

It fit on the XP-51B!