XP-51F/G vs P-51H length.

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BarnOwlLover

Staff Sergeant
925
323
Nov 3, 2022
Mansfield, Ohio, USA
I know that the P-51H was at least a foot longer than the XP-51F/G it was developed from. The added length from what I read was added to try and offset the directional stability problem that the Merlin Mustangs had that became more pronounced in the F/G during testing.

My question is where was the length added. The distance between the tail and the trailing section of the radiator scoop/duct seems about the same (and the ductwork seems to be about the same length). However, for sure, the nose on the P-51H was longer (engine was moved forward several inches for CG reasons). Thus, were was the added length added, and about how much fore vs aft?
 
I know that the P-51H was at least a foot longer than the XP-51F/G it was developed from. The added length from what I read was added to try and offset the directional stability problem that the Merlin Mustangs had that became more pronounced in the F/G during testing.

My question is where was the length added. The distance between the tail and the trailing section of the radiator scoop/duct seems about the same (and the ductwork seems to be about the same length). However, for sure, the nose on the P-51H was longer (engine was moved forward several inches for CG reasons). Thus, were was the added length added, and about how much fore vs aft?
Front and back. The distance from tip of spinner to 1/4 chord line in wing
P-51B/D = 133 7/16"
P-51H = 139"

Distance from 1/4 chord line in wing to reference rudder datum
P-51B/D =253.87
P-51H = 261"
 
One other thing I noticed is that the engine on the F/G seems to sit further back than on the B/D (based on engine cowling TE relationship to the wing), though the length was similar (a couple of inches longer) than the D model. Is this true, or are my eyes playing with me?
 
What follows is the product of my own research as I try to create models of the Lightweights in 1/72.

The D length was 32' 2.5", and the F length was 32' 3". So basically the same overall (for 1/72 purposes). But the F was shorter in front of the wing than the D (the firewall having been moved backwards compared to the D), and longer behind the wing than the D, as the rear fuselage/radiator/tailwheel area was a new design. Apart from the propellers, the Gs (NA-105A) started out externally identical to the Fs (NA-105), but then got fin fillets and latterly got the big new tail unit, like the one on the H and J, which gave the Gs another couple of inches between the tailwheel well and the rudder post. I have cutaway fuselage drawings of the early G and the J, which show the additional frame at that point. But other than that the F, G, H and J were all the same length behind the wing, with the big changes for the H being at the top of the fuselage to create the smaller bubbletop. The H also had the firewall moved forward once again, to give it similar length to the D in front of the wing. (As far as I can see the angle of the firewall on the lightweights was more vertical than on the D.) The J (NA-105B) used the airframe of the late G, with that long Allison engine which made it 33' long overall. The H (NA-126), with the Merlin and the firewall moved forward, was 33' 4".

It's worth remembering that the J only flew well after the H, and was essentially an Allison engine mule using the G airframe. I suppose they could have used an H as the basis for the J but perhaps the firewall positioned further forward would have made the aircraft unstable with the very long Allison engine. Remember that the P/F-82 had the fuselages further lengthened behind the wing, presumably to compensate.

I have a question of my own. The fin on the F and early G is a different shape to that on the D, being slightly more vertical to give a bit more area. But height is similar to the D and the rudder is similar in shape to the D, albeit metal-covered rather than fabric. What I don't know is whether the original horizontal stabilisers on the F were the same as those on the D, or different. Can anyone help with that? I imagine that the stabilisers on the late G and the J were the same as on the H.

Justin
 
What follows is the product of my own research as I try to create models of the Lightweights in 1/72.

The D length was 32' 2.5", and the F length was 32' 3". So basically the same overall (for 1/72 purposes). But the F was shorter in front of the wing than the D (the firewall having been moved backwards compared to the D), and longer behind the wing than the D, as the rear fuselage/radiator/tailwheel area was a new design. Apart from the propellers, the Gs (NA-105A) started out externally identical to the Fs (NA-105), but then got fin fillets and latterly got the big new tail unit, like the one on the H and J, which gave the Gs another couple of inches between the tailwheel well and the rudder post. I have cutaway fuselage drawings of the early G and the J, which show the additional frame at that point. But other than that the F, G, H and J were all the same length behind the wing, with the big changes for the H being at the top of the fuselage to create the smaller bubbletop. The H also had the firewall moved forward once again, to give it similar length to the D in front of the wing. (As far as I can see the angle of the firewall on the lightweights was more vertical than on the D.) The J (NA-105B) used the airframe of the late G, with that long Allison engine which made it 33' long overall. The H (NA-126), with the Merlin and the firewall moved forward, was 33' 4".
Before diving into details, what is your source pool of data - NAA or 'other'.For example 105-00001 Three View General Assy or something else? Is the regerence 1/4 chord location at FS 99? or?
It's worth remembering that the J only flew well after the H, and was essentially an Allison engine mule using the G airframe. I suppose they could have used an H as the basis for the J but perhaps the firewall positioned further forward would have made the aircraft unstable with the very long Allison engine. Remember that the P/F-82 had the fuselages further lengthened behind the wing, presumably to compensate.
The H was lengthened both in front of firewall and aft of wing. 6" more in front of A.C. and 7 inches lnger aft of A.C. than the B/D. The J had modified empennage from F/G and H (larger H.S)
I have a question of my own. The fin on the F and early G is a different shape to that on the D, being slightly more vertical to give a bit more area. But height is similar to the D and the rudder is similar in shape to the D, albeit metal-covered rather than fabric. What I don't know is whether the original horizontal stabilisers on the F were the same as those on the D, or different. Can anyone help with that? I imagine that the stabilisers on the late G and the J were the same as on the H.

Justin
The H.S. on G was different from H and H/F different from J.
 
Before diving into details, what is your source pool of data - NAA or 'other'.For example 105-00001 Three View General Assy or something else? Is the regerence 1/4 chord location at FS 99? or?

The H was lengthened both in front of firewall and aft of wing. 6" more in front of A.C. and 7 inches lnger aft of A.C. than the B/D. The J had modified empennage from F/G and H (larger H.S)

The H.S. on G was different from H and H/F different from J.
Thanks for these clarifications. My sources are secondary rather than primary ones, and as I'm looking down the telescope from the UK I'm quite distant from the original sources. Also I'm a modeller rather than an engineer hence my probably rather woolly descriptions. So if there are good 3-views of F, G, H and J out there, I'd be very keen to see them.

Justin
 
Justin, I have spent a fair amount of 2023 trying to figure out how to build a LW Mustang model. Backdating a good P-51H model is the only way to do this, short of scratchbuilding, so I've been trying to figure out how NAA got from the XP-51F to the P-51H so that I can undo all those changes. I think I have a pretty good answer now. This may belong in the Modeling section, but since I haven't actually completed the model yet, there didn't seem to be a good place to put it there.
I recently built a 1/48 scale XP-51J based on the excellent Modelsvit P-51H kit and my own cowling, canopy, turtledeck and spinner.
The relationship between the the B/D and F is interesting, but from a modeling standpoint it is moot since the P-51F was a clean-sheet design that was just based on the general configuration of earlier Mustangs. The P-51G/H/J were all based on the XP-51F, with the H being the most radical change.
I agree that the aft fuselage of the P-51H was basically the same as the XP-51F, except that the upper back was higher on the P-51H. There were also some detail differences between them such as the access panels.
The firewall was moved forward about 12-13" for the P-51H, but I haven't been able to find any factory inboard profiles for the P-51H to confirm the actual distance. As Dan Whitney mentioned in a related post, his book "Vees for Victory!" does include an inboard profile for the XP-51J, which was key to figuring all this out. It's also a fantastic book in general.
The cockpit was moved forward 12-13" along with the firewall to make the P-51H. The nominal height of the pilot's seat did not change, although a cursory look at the P-51H might give you that impression (it did to me). In the P-51H they raised the upper back and shrank down the canopy from the XP-51F. You can see this in the attached overlay of the XP-51J inboard profile and the Modelsvit P-51H drawing (which appears to be a very accurate drawing).
I think you are also correct in that they changed the angle of the firewall by just a few degrees. Note that the top sections of the LW Mustang firewalls are vertical, not slanted, so there is a slight kink in the panel line.
Regarding the differences in vertical stabilizers and rudders for the LW Mustangs, I still haven't quite figured that out. There's a lot of distortion in the photos and few are high quality enough to measure panel lines. It appears to me that the P-51H vertical stab/rudder had a greater chord than the earlier LW prototypes, but I haven't been able to quantify it.
The last photo shows a comparison of the XP-51J and P-51H, both starting with the Modelsvit 1/48 P-51H kit.
- Eric
 

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Justin, I have spent a fair amount of 2023 trying to figure out how to build a LW Mustang model. Backdating a good P-51H model is the only way to do this, short of scratchbuilding, so I've been trying to figure out how NAA got from the XP-51F to the P-51H so that I can undo all those changes. I think I have a pretty good answer now. This may belong in the Modeling section, but since I haven't actually completed the model yet, there didn't seem to be a good place to put it there.
I recently built a 1/48 scale XP-51J based on the excellent Modelsvit P-51H kit and my own cowling, canopy, turtledeck and spinner.
The relationship between the the B/D and F is interesting, but from a modeling standpoint it is moot since the P-51F was a clean-sheet design that was just based on the general configuration of earlier Mustangs. The P-51G/H/J were all based on the XP-51F, with the H being the most radical change.
I agree that the aft fuselage of the P-51H was basically the same as the XP-51F, except that the upper back was higher on the P-51H. There were also some detail differences between them such as the access panels.
The firewall was moved forward about 12-13" for the P-51H, but I haven't been able to find any factory inboard profiles for the P-51H to confirm the actual distance. As Dan Whitney mentioned in a related post, his book "Vees for Victory!" does include an inboard profile for the XP-51J, which was key to figuring all this out. It's also a fantastic book in general.
The cockpit was moved forward 12-13" along with the firewall to make the P-51H. The nominal height of the pilot's seat did not change, although a cursory look at the P-51H might give you that impression (it did to me). In the P-51H they raised the upper back and shrank down the canopy from the XP-51F. You can see this in the attached overlay of the XP-51J inboard profile and the Modelsvit P-51H drawing (which appears to be a very accurate drawing).
I think you are also correct in that they changed the angle of the firewall by just a few degrees. Note that the top sections of the LW Mustang firewalls are vertical, not slanted, so there is a slight kink in the panel line.
Regarding the differences in vertical stabilizers and rudders for the LW Mustangs, I still haven't quite figured that out. There's a lot of distortion in the photos and few are high quality enough to measure panel lines. It appears to me that the P-51H vertical stab/rudder had a greater chord than the earlier LW prototypes, but I haven't been able to quantify it.
The last photo shows a comparison of the XP-51J and P-51H, both starting with the Modelsvit 1/48 P-51H kit.
- Eric
The P-51H fuselage extended the fuselage 13" aft of cockpit. The Firewall location remained the same, the wing moved back (IIRC 5.8") to account for the empennage movement (from CG) while maintain stability derivatives. The firewall is about FS 99 in all, but the 25% chord for the H is further aft by nealy six inches.

The wing and empennage dimensions are the same for XP-51F and XP-51G, the same wing dimensions for XP-51F/G/J & H but different empennage variations dimensionally for the J & H (I have to find the XP-51F Spec to be certain about J change).

The P-51H emennage was larger than F/G/J in every respect, dimensionally

The wing and H.Stab airfoil sections are the same but not the chord length between H and F/G/J horizontal Stab.

The RH fuel tank is the same 105 gal, but the F/G and J have 75 gal removable LH, and the H has 100gal tank.

ALL had carb return line to RH tank in contrast All Mustangs prior to XP-51F feeding overflow to Left Main.

Good luck and GREAT skill required to kit bash any of the LW from a P-51H accurately scaled model.


The cocpit floor of the P-51H was raised relative to the Thrust line and fuselage datum line to achieve the
 
Thanks for these clarifications. My sources are secondary rather than primary ones, and as I'm looking down the telescope from the UK I'm quite distant from the original sources. Also I'm a modeller rather than an engineer hence my probably rather woolly descriptions. So if there are good 3-views of F, G, H and J out there, I'd be very keen to see them.

Justin
Justin - the pilot operating manual are good sources for H. The Specifications Reports are the bible with total dimensional data for all assemblies and components. My co-author has the NA-105 5503-F Specification for X/F on microfilm but NA-8033for XP-51J is for NA-105B.

There should be a pilot operating manual for XP-51G and G but I have never seen either one.
 
The P-51H fuselage extended the fuselage 13" aft of cockpit. The Firewall location remained the same, the wing moved back (IIRC 5.8") to account for the empennage movement (from CG) while maintain stability derivatives. The firewall is about FS 99 in all, but the 25% chord for the H is further aft by nealy six inches.

The wing and empennage dimensions are the same for XP-51F and XP-51G, the same wing dimensions for XP-51F/G/J & H but different empennage variations dimensionally for the J & H (I have to find the XP-51F Spec to be certain about J change).

The P-51H emennage was larger than F/G/J in every respect, dimensionally

The wing and H.Stab airfoil sections are the same but not the chord length between H and F/G/J horizontal Stab.

The RH fuel tank is the same 105 gal, but the F/G and J have 75 gal removable LH, and the H has 100gal tank.

ALL had carb return line to RH tank in contrast All Mustangs prior to XP-51F feeding overflow to Left Main.

Good luck and GREAT skill required to kit bash any of the LW from a P-51H accurately scaled model.


The cocpit floor of the P-51H was raised relative to the Thrust line and fuselage datum line to achieve the
This has most of the info I was looking for, except for one or two things. Did the P-51H's cockpit or seating position move backwards, or is that due to some other change (the detachable panel between the windscreen and engine cowling seems to be longer on the H than the F/G/J), and did the seating height change between the F/G/J and the H, the latter which had a higher seating position vs the P-51D to help with vision and defection shooting?
 
This has most of the info I was looking for, except for one or two things. Did the P-51H's cockpit or seating position move backwards, or is that due to some other change (the detachable panel between the windscreen and engine cowling seems to be longer on the H than the F/G/J), and did the seating height change between the F/G/J and the H, the latter which had a higher seating position vs the P-51D to help with vision and defection shooting?
No to backward movement of cockpit, but the wing did move back about 6 inches. The fuselage X-section of H greater than F/G & D and the relative floor/seat was also raised to accomodate the increased size of cooling matrices - which did enable better vision over the nose.

I have not been able to source powerplant drawings for the H to look at ADI storage for the 1650-9.
 
So now I'm wondering again if the nose moved forward slightly given the (seemingly) increased gap between the rear of the engine cowling and the windscreen, or if the windscreen changed vs the F/G? I also read in another thread that the cooling arrangement (mostly oil) changed between the F/G and H.

If the engineering drawings existed (or I knew where to find them or could obtain copies of them) for the F/G, H and D/K, I could at least make (albeit probably rough) comparisons on various arrangements. But I've only seen one for the XP-51J and one low-ish quality one of the G.

At least I know now what allowed the raised seating position on the H and also maybe the F-82 (the latter def. based on photos had it's wing moved back compared to the H). I've seen photos in a couple of books of P-51Hs and XP-82/P-82Bs with cowlings removed as well as mock ups of them. And there's photos of the XP-82 and P-51H that show the panel that can be opened to fill up the ADI liquid. Unfortunately, I doubt that'll help much with what you're looking for with where the ADI storage tank is.
 
Getting back to the P-51H/F-82 ADI, I'm not 100% sure where the ADI storage tank is exactly, but I'm betting that it may be behind the supercharger, possibly also the firewall. There's a cutaway of a XP-82 that shows some tank behind the firewall next to a "socket". That's what I'm calling it for now because I originally thought that the door was a 24 volt attachment point. But seeing close ups of the XP-82 that was restored, it seems to be a door for something related to the ADI system. Both the Merlin F-82s and the P-51H also have a door on the right side of the engine cowling below the heater air vent that's also marked as access for something to do with the ADI system.

You're probably going to want to see if you can locate some engineering documents or maybe something else that might tell you where the tank is placed exactly, but based on what I've described above, that at present is my best guess.

I do think that on the Allison F-82s, the ADI tank was mounted under/alongside the supercharger duct trunking below the engine.
 
Getting back to the P-51H/F-82 ADI, I'm not 100% sure where the ADI storage tank is exactly, but I'm betting that it may be behind the supercharger, possibly also the firewall. There's a cutaway of a XP-82 that shows some tank behind the firewall next to a "socket". That's what I'm calling it for now because I originally thought that the door was a 24 volt attachment point. But seeing close ups of the XP-82 that was restored, it seems to be a door for something related to the ADI system. Both the Merlin F-82s and the P-51H also have a door on the right side of the engine cowling below the heater air vent that's also marked as access for something to do with the ADI system.

You're probably going to want to see if you can locate some engineering documents or maybe something else that might tell you where the tank is placed exactly, but based on what I've described above, that at present is my best guess.

I do think that on the Allison F-82s, the ADI tank was mounted under/alongside the supercharger duct trunking below the engine.

ADI tank on F-51H is located behind firewall, underneath cockpit floor. I remember this was shown on the later SAC from July, 1950.
 
Apologies for being out of touch in recent months - I've moved house twice and so it been a bit hectic. Some great information here on the LW Mustangs, and Eric/TbirdsAreGo, love that comparison of the Modelsvit and cutaway drawings and accompanying commentary. Rather foolishly I've promised over on Britmodeller to complete a 1/72 XP-51G by June - I hope I can live up to that commitment and hopefully we can be in further touch as it progresses.

Justin
 
Justin, I think the most expeditious way to build a 1/72 XP-51G would be to mash up the top of a P-51D with the bottom of a P-51H. I just finished building a gaggle of lightweight Mustangs in 1/48 scale and learned a lot in the process. I started looking into doing the same in 1/72 but frankly I'm kind of burned out on the Mustangs so I am pivoting back to my YB-24N project! The basic conclusion I came to is that if you take the upper mold line of the P-51D and the lower mold line of the P-51H you are getting pretty close. Since the P-51H is longer, keep the upper P-51D cowling and replace the lower part (from the exhaust pipes down) with the P-51H kit plastic. Here are some photos of the newer Airfix P-51D and RS Models P-51H kits that show how they compare. The vertical tail of the P-51D is the same as the XP-51G except for the dorsal fin, which you need to reduce in height. On the lower fuselage you want to keep the P-51H cowling, radiator duct profile, wing root, and tailwheel well. Use the P-51H horizontal stab, which has a longer span and longer root chord than the P-51D. Of course, use the P-51H wing and remove the outer two gun chutes. The landing gear covers are a little bit different than on the P-51H because the split between the main covers and the strut covers was moved a little bit. The XP-51G cockpit layout was a little different than the P-51H but in 1/72 that's not real obvious. Of course you will need to make the long canopy from scratch. If you get a resin Griffon Spitfire 5-bladed Rotol prop, you can heat it up and reverse the pitch for the XP-51G. Griffons and Merlins rotated in opposite directions and I doubt you can find a 5-bladed prop that spins in the right direction. Alternately, maybe find a 4-bladed Rotol prop for a Merlin Spitfire and add another prop to it. The hard part is matching up the top of one fuselage with the other, but at least you don't have to worry about panel lines because most of them are wrong for the XP-51G anyway. There are dozens of other differences, but they are minor. In any case, it ain't easy, but it can be done!
 

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