Changes P-51A to P-51B/C

[Zipper730]

From what I remember being told, the changes made from the P-51A (impressive machine even as it was, about 409 mph at 13000-15000', and 378 mph at 20000', twice the range of the Spitfire) included...

1. Replacement of the V-1710 with the Merlin-61 or V-1650-3 (the nose might have been reshaped); redesigned exhaust nozzle for the engines.
2. Redesigned radiator with enlargements and provisions made for the intercooler radiator, greater spacing between the splitter/gutter than before, possible changes in shape of the radiator intake itself
3. Larger diameter propeller with 4-blades to handle the extra horsepower

I'm basically talking about the P-51B prior to the center-tank installation, so I'm curious if ballast was added to the rear fuselage since the nose was a bit heavy, and I'm curious if the ailerons were modified off the bat, or at some later point in development?

 

[MIflyer]

The Merlin equipped P-51B had to have both the coolant radiator increased from 2.1 sq ft to 2.9 sq ft qnd a supercharger cooler of 0.645 sq ft added as well as the increased duct size required to hold those. The Merlin originally also required a fuel cooler be added, but tests proved that if the Bendix pressure carb eliminated that need.

So, even without the fuselage fuel tank there was some added weight well aft if the engine..

 

[drgondog]

First - the XP-51B was a hybrid, beginning with the P-51-NA airframe with 4x 20mm cannon. Some of the P-51A including the original shallow V top surface radiator intake scoop was installed and as MIflyer noted, the Harrison Radiator with increased radiator coolant matrix plus after cooler matrix imbedded was installed. The ducting dimensions changed to accommodate the larger radiator and the oil cooler was removed from its former 'donut hole position to a place forward of the Harrison radiator. The fuel aftercooler was incorporated in the Packard Merlin 1650-3/Merlin 61 engine upstream of the manifolds.

In parallel with the XP-78/XP-51B modifications for flight test, NA-102 P-51B-1-NA underwent several 'internal design changes, The wing was lowered 3" from the base WL,the carb intake and lower cowl were faired smoothly to the bottom of the new wing position, the longerons were beefed up, the ailerons were modified to rig for +/- 10, 12 and 15 degrees on a two hinge system. (The NA-104 P-51B-5-NA had a three hinge system with stiffened aft spar to reduce deflection at full play.)

The original Harrison Radiator was positioned so that the aftercooler matrix was on top 30% of the radiator. During flight tests the issues with the radiator cooling coils clogging up were discovered, and when solved , the 'duct rumble' issue surfaced during dive tests. When XP-51B #2 (41-37421) had 20mm guns removed ane airframe sent to Ames wind tunnel, Horkey and several NAA folks accompanied it. The first major change was dropping the upper lip of the intake scoop from the wing to create a larger gutter (with a few iterations to balance drag with improved cooling results, then removing the flow splitter, then angling the scoop. Gruenhagen's very fine book extracted the NACA report tables to explain the iterations. As the first P-51B-1-NA was ready to fly (completed save engine in March - before the XP-51B tests at Ames) on May 3rd, the first flight was with the 'old' vertical face front scoop. In addition to the intake duct changes derived from Ames, the Harrison Radiator was rotated 90 degrees to place the aftercooler matrix vertically and on the RH side due to lingering internal BL flow separation causing degraded pressure recovery along the top of the radiator. See Lednicer's report (posted in this forum) for details regarding why it had never been 100% solved in the production P-51B/D.

I'll have to look it up but I believe ~60 pounds of ballast mounted on the forward bulkhead of the tail wheel bay was provided for P-51B w/o fuel tank to better manage CG travel.

 

[waroff]

on P51B,C & D, there was a weight(64#) bolted on rear spar of the fin

 

[drgondog]

on P51B,C & D, there was a weight(64#) bolted on rear spar of the fin

thanks

 

[Ivan1GFP]

From what I remember being told, the changes made from the P-51A (impressive machine even as it was, about 409 mph at 13000-15000', and 378 mph at 20000', twice the range of the Spitfire) included...

1. Replacement of the V-1710 with the Merlin-61 or V-1650-3 (the nose might have been reshaped); redesigned exhaust nozzle for the engines.
2. Redesigned radiator with enlargements and provisions made for the intercooler radiator, greater spacing between the splitter/gutter than before, possible changes in shape of the radiator intake itself
3. Larger diameter propeller with 4-blades to handle the extra horsepower

I'm basically talking about the P-51B prior to the center-tank installation, so I'm curious if ballast was added to the rear fuselage since the nose was a bit heavy, and I'm curious if the ailerons were modified off the bat, or at some later point in development?

Hello Zipper730,
I believe Drgondog already mentioned this but folks may have missed it in passing.
The Fuselage of the B/C was around 3.5 inches deeper than the A series and earlier. It is most noticeable at the edge of the cockpit to the upper wing surface.
Note also that at 10,000 feet and below, the P-51A was the fastest version of the Mustang until the H models came along.

- Ivan.

 

[Skyediamonds]

I'm building a highly detailed Guillow's kit of the "D" variant Mustang & I'm learning a lot from you guys. Thank you very much.

For those who might be interested; see under models; Super detailing Mustang (current), S.E.5, & Wright Bros' Flyer (completed and super detailed).

As a modeler, I'm constantly amazed at amount of information this site has & I've been blessed with several of our fellow members offering suggestions & ideas as I go along.

Thanks again!

 

[Zipper730]

So to summarize

• The engine radiator was redesigned with the oil-cooler re-positioned, and the aftercooler radiator taking up 30% of the overall radiator, placed on the right-side and lying vertically?
• The radiator duct was redesigned with greater spacing from the fuselage to reduce turbulent flow into the engine, and other changes to improve cooling; the duct had a bifurcation with the bulk of it heading to the radiator, with a small split feeding the oil cooler on the ventral side?
• The wings were lowered 3.5", accounting for the upper fuselage being 3.5" higher?
• The aileron hinges on early P-51B's had a three-hinge system that could allow varying degrees of deflection, attached to a sturdier aft spar to limit the degree of deflection at higher speeds; this was later switched to a twin-hinged layout allowing for 10-12 and 15-degrees up/down.
  • I'm not sure I understand what you mean when you say the ailerons were mounted on a twin-hinge system. I'd probably need a diagram.
  • This was presumably to improve roll control across a wider range of airspeeds?
• Ballast consisted of 64 pounds of weight attached to the vertical fin's rear-spar?

 

[waroff]

Fuselage B,C& D higher 3 inches only.

to illustrate Drgondog's post:

note the Allison thrust line parallel 1.5" below the fuselage reference line
and Merlin on the fus ref but setting -1°45'