Allison-Merlin Mustang wing position changes

[72modeler]

After reading all of the discussions on this topic, I did some research on the P-51 SIG website and found conversation on this subject as well as quotes from an NAA document dated 12/3/42 that seems to confirm the general concensus regarding the change in wing location/fuselage depth  of the Allison and  early Merlin Mustangs. According to one very highly regarded Mustang authority, (Who is also regarded as being responsible for the most accurate Mustang drawings) the wing was located 23.5" below the fuselage reference line on Allison Mustangs and was changed to 26.5" below the fuselage reference line on B models and subsequent. (I am guessing the more informed among you will know what that means- I am assuming it is a horizontal reference line maybe drawn through the center of the  thrust line?) Another post quoted an NAA document dated December 3, 1942 that stated that the wing was being lowered 3" beginning with the P-51B variant to increase ground clearance as well as to allow coolant piping to be run above the wing surface, which used to form the  cockpit floor, thus requiring a new flat floor to be constructed above the new coolant piping. Makes sense to me that the wing had to be lowered to allow this.

 

This would appear to confirm the observations made by some of you that there was a visible difference in the location of the lower canopy sill between the Allison and Merlin Mustangs- lowering the wing by 3" would increase the distance between the lower canopy sill and the top of the wing. I also pulled out all of the drawings I had that I felt were pretty accurate or were acknowledged to be so, and there was no measurable difference in the height of the canopies, or the height of the fuselage, measuring from a horizontal line drawn at the trailing edge of the wing at the root to the rudder post. I think this also tends to confirm that the P-51B fuselage was deeper, but due to the lowered wing and larger radiator/oil cooler housing, the increased depth was mostly confined to the   area below the wings. Three inches is only around 1-1.5mm in 1/72 scale, right? But it does seem to be noticeable!

 

I pulled my Condor, Special Hobby, Academy, and Frog Allison Mustang kits, as well as my Monogram, Hasegawa, Revell, and Academy P-51B's to begin to compare them to drawings and each other as well as the Hawkeye Designs wing; when I have had a chance to shake and bake, I can post what I find out, if you think that would be helpful, or at the least would also allow you to examine my results to see if I got anywhere close to the mark. I do not have the Brengun or KP kits, but am well aware of their strengths and weaknesses. I kinda have an idea on how I would proceed in building an Allison or Merlin early Mustang, but will hold off on comments until I get through with the autopsies. Let me know if you trust me enough to begin the quest!

Mike

 

PS- Don't even get me started on Spitfire Mk XII's!

[stevehnz]

3", slightly under 1.1mm ( 1.058). Not a lot to be sure but enough to make a difference to the look.

Steve.

[Troy Smith]

15 minutes ago, 72modeler said:

I can post what I find out, if you think that would be helpful, or at the least would also allow you to examine my results to see if I got anywhere close to the mark.

 

15 minutes ago, 72modeler said:

Let me know if you trust me enough to begin the quest!

 

As with anything like this Mike,  if you document your findings of comparisons, that alone is a great service,   as this is an area of doubt, confusion and thus controversy,  as recent threads on the matter have shown,   I'm sure there are many members who find this  very helpful.

I've read some right tosh about various kits over  the years, and often the same "facts" get repeated without further checking.

cheers

T

 

 

[Beard]

18 minutes ago, 72modeler said:

PS- Don't even get me started on Spitfire Mk XII's!

 

Please do (but do the Mustangs first).

[72modeler]

2 hours ago, Beard said:

 

Please do (but do the Mustangs first).

Beard,

In 1/72 scale, if you've got the Paragon Mk XII conversion and an Eduard Mk VIII week end edition or overtrees, then you're home free!

Mike

[Gmat]

There is also the Gartex 1/72 P-51A with a resin fuselage to go with the Hasegawa P-51B wings.

 

Grant

[brewerjerry]

Hi

    random thought

   

     have you compared the P-51 parts manuals to see if the same part numbers are used or different part numbers for the areas in question on the different versions 

 

  it might be worth looking into it if you really want to find out the differences between versions

 

        admitted it wont give measurements but it should confirm which panels are different 

 

   cheers

    jerry

[Bedders]

Mike,

 

Interesting post and I'll be eager to see the results of your studies of the kits. 

 

Will be equally eager to engage on Mk XII Spitfires in due course!

 

Justin

[Jamie @ Sovereign Hobbies]

10 hours ago, 72modeler said:

After reading all of the discussions on this topic, I did some research on the P-51 SIG website and found conversation on this subject as well as quotes from an NAA document dated 12/3/42 that seems to confirm the general concensus regarding the change in wing location/fuselage depth  of the Allison and  early Merlin Mustangs. According to one very highly regarded Mustang authority, (Who is also regarded as being responsible for the most accurate Mustang drawings) the wing was located 23.5" below the fuselage reference line on Allison Mustangs and was changed to 26.5" below the fuselage reference line on B models and subsequent. (I am guessing the more informed among you will know what that means- I am assuming it is a horizontal reference line maybe drawn through the center of the  thrust line?) Another post quoted an NAA document dated December 3, 1942 that stated that the wing was being lowered 3" beginning with the P-51B variant to increase ground clearance as well as to allow coolant piping to be run above the wing surface, which used to form the  cockpit floor, thus requiring a new flat floor to be constructed above the new coolant piping. Makes sense to me that the wing had to be lowered to allow this.

 

This would appear to confirm the observations made by some of you that there was a visible difference in the location of the lower canopy sill between the Allison and Merlin Mustangs- lowering the wing by 3" would increase the distance between the lower canopy sill and the top of the wing. I also pulled out all of the drawings I had that I felt were pretty accurate or were acknowledged to be so, and there was no measurable difference in the height of the canopies, or the height of the fuselage, measuring from a horizontal line drawn at the trailing edge of the wing at the root to the rudder post. I think this also tends to confirm that the P-51B fuselage was deeper, but due to the lowered wing and larger radiator/oil cooler housing, the increased depth was mostly confined to the   area below the wings. Three inches is only around 1-1.5mm in 1/72 scale, right? But it does seem to be noticeable!

 

I pulled my Condor, Special Hobby, Academy, and Frog Allison Mustang kits, as well as my Monogram, Hasegawa, Revell, and Academy P-51B's to begin to compare them to drawings and each other as well as the Hawkeye Designs wing; when I have had a chance to shake and bake, I can post what I find out, if you think that would be helpful, or at the least would also allow you to examine my results to see if I got anywhere close to the mark. I do not have the Brengun or KP kits, but am well aware of their strengths and weaknesses. I kinda have an idea on how I would proceed in building an Allison or Merlin early Mustang, but will hold off on comments until I get through with the autopsies. Let me know if you trust me enough to begin the quest!

Mike

 

PS- Don't even get me started on Spitfire Mk XII's!

 

It's a horizontal datum line, and on some aircraft it MAY coincide with the thrust line though. Sometimes it would be the beefiest fuselage longerons that touch the datum line.

 

For something like you are about to embark on, I suggest you disassociate yourself with any reference to the thrust line. The thrust line MAY coincide with the datum/reference line but it doesn't have to. This is especially important because the Allison and Merlin are different engines with thrust lines at different levels on the engine itself. North American MAY have made a conscious effort to with engine mounts to keep the resultant thrust line the same, but unless you find data to confirm that it's an assumption.

 

What I am getting at is that you should avoid measuring the model fuselages with reference to the centre of the propeller hub because that too MAY have moved when changing from Allison to Merlin.

[FinnAndersen]

I did look at what I had of pictures of Allison and B/C Mustangs with the canopy down yesterday and there is a difference.

 

With the canopy closed, you need very good eyesight to spot the difference on photos.

 

Question now, is how you can modify, say the Brengun, to move the wing up that 1 mm. (and if it's worth the trouble)

 

Running for cover...

 

Edit: Changing the position of the propeller from the Allison to the Merlin would be a major modification and would dramatically change aerodynamics. I don't belive NA did that.

 

/Finn

Edited October 6, 2017 by FinnAndersen

[Work In Progress]

So, between the A and the B they changed the engine, the depth of the main structural body of the fuselage, the position of and configuration of the intake and cooling system, the entire design of the ailerons, and you don't think they would have made a 'major modification'?  Those were all major modifications. And every one of the known changes has a significant impact on the aerodynamics.

 

Think Supermarine didn't move the the Spitfire thrustline when they put the Griffon in? They did, you know... it's much lower in the airframe. Which is why a XII has to have a smaller diameter prop than a IX.

 

The Allison Mustang thrustline is different from that of the Merlin Mustang thrustline. Assemble your side-on photos. Run a rule straight down the centre of the exhaust stacks and note that the extended line passes below the tailplane. Now do the same for any factory Merlin Mustang. The extended line comes out higher, just about on the chordline of the tailplane.

[FinnAndersen]

18 minutes ago, Work In Progress said:

...

The Allison Mustang thrustline is different from that of the Merlin Mustang thrustline. Assemble your side-on photos. Run a rule straight down the centre of the exhaust stacks and note that the extended line passes below the tailplane. Now do the same for any factory Merlin Mustang. The extended line comes out higher, just about on the chordline of the tailplane.

You may be right, but the angle of the engine does not necessarily prove that the position of the propeller was changed.

 

Digressing, the Griffon Spitfires trustline were canted downwards; I'm not that much into aerodynamics to explain why, but believe me, there was a reason.

 

[Work In Progress]

I give up. Believe what you want.

[Fernando]

Hi, everyone,

 

The fuselage in the "production" Merlin Mustangs is deeper, TMK, by all accounts. Just look at the Merlin conversions made in the UK using Allison fuselages. The nose is faired in in the undersurface.

 

Fernando

[Jamie @ Sovereign Hobbies]

29 minutes ago, FinnAndersen said:

You may be right, but the angle of the engine does not necessarily prove that the position of the propeller was changed.

 

Digressing, the Griffon Spitfires trustline were canted downwards; I'm not that much into aerodynamics to explain why, but believe me, there was a reason.

 

 

Changing thrustlines is never ideal but often the only practical solution to re-engine when the engines are different shapes.

 

WIP is correct. The thrust line of the Griffon Spitfire IS lower. Much lower infact. That's why the Griffon used shorter propeller blades - there was no ground clearance left.

 

Lowering the thrust line has the effect of increasing the aircraft's tendency to pitch up when power is increased. That is why the Griffon Spitfire has a lot of geometric downthrust built in - so that the propeller tries to pull the nose down as power is increased to counteract the pitch-up tendency the lowered thrust line gave. It didn't fully work but it was better than it would have been without downthrust. Any pilot with time on Griffon and Merlin Spitfires will tell you that the Griffon needs a trim change with any throttle changes. The Merlins are much more straightforward.

 

Engineers know all this stuff, but engineers also know that all engineering is a compromise and you never get to keep everything ideal.

 

When trying to measure whether model kits reflect a wing moved by 3 inches when an engine was changed, it would be foolish to presume the thrust line was preserved simply because the idealistic preference would have been to do so.

[Seahawk]

10 hours ago, 72modeler said:

In 1/72 scale, if you've got the Paragon Mk XII conversion....

First catch your unicorn!  (But I agree.)

[gingerbob]

2 hours ago, FinnAndersen said:

Edit: Changing the position of the propeller from the Allison to the Merlin would be a major modification and would dramatically change aerodynamics. I don't belive NA did that.

I think it would be harder to change the engine and force the prop to stay in the same place!  Look at early vs. "late" P-40s- two generations of Allisons, very different geometries between core engine and prop-shaft.

 

There is no doubt about the 3" change for Merlin Mustang wing position.  Another bit of visual evidence is the "joggle" at the aft end of the wing root fairing- hopefully I can make a photo appear to show what I mean:

 

 

That's because the longeron aft of that is in the same position it always had been.  Loosely speaking, the aft fuselage structure is unchanged (there ARE detail differences to structure). 

 

Some people have said that the spinner is also a different diameter between Allison and Merlin Mustangs, though (superficially) they look about the same to me.  I don't think I've gone looking for measurements to confirm that, though it is quite possible it came up on the SIG.

[FinnAndersen]

I rest my case

 

Clearly there are a lot of knowledgeable members here to correct me when I'm guessing wrong, and thank you for that.

 

What I could use now was a nice little drawing superimposing the Allison and Merlin highbacked fuselages...

 

/Finn

[Courageous]

After everything that has been said, and their is a 3" difference between the A & B wing, how does the modeller correct it...if they want to?

 

Stuart

[ColFord]

And diagrams from the NAA Technical Manuals showing key dimensions and weights, including the diameters of the propeller spinner for the Allison engined versions and the P-51B/C Merlin engined versions.  Merlin engined was bigger.

 

P51 Dimensions by Colin Ford, on Flickr

 

P51 BC Dimensions by Colin Ford, on Flickr

 

I have yet to find the appropriate diagrams in the relevant NAA technical manuals, to be able to identify if there has been any change in the quoted thrust line from the fuselage datum line between the Allison engined Mustangs and the Merlin engined P-51B/C.  My search continues.

 

 

 

 

 

[FinnAndersen]

RE Colins drawings above:

 

A spinner 25", B spinner 29" across, but fuselage depth aft of cockpit unchanged at 66".

 

Interesting...

 

 

Edited October 6, 2017 by FinnAndersen

[72modeler]

6 hours ago, Jamie @ Sovereign Hobbies said:

 

It's a horizontal datum line, and on some aircraft it MAY coincide with the thrust line though. Sometimes it would be the beefiest fuselage longerons that touch the datum line.

 

For something like you are about to embark on, I suggest you disassociate yourself with any reference to the thrust line. The thrust line MAY coincide with the datum/reference line but it doesn't have to. This is especially important because the Allison and Merlin are different engines with thrust lines at different levels on the engine itself. North American MAY have made a conscious effort to with engine mounts to keep the resultant thrust line the same, but unless you find data to confirm that it's an assumption.

 

What I am getting at is that you should avoid measuring the model fuselages with reference to the centre of the propeller hub because that too MAY have moved when changing from Allison to Merlin.

I guess I wasn't clear- my comment about the FRL being the thrust line was just a guess; I was counting on one of you who knew what the FRL was to join in and explain it to me, which is why I put in the disclaimer- thanks for the explanation, Jamie; when I start looking at kit parts and drawings, I will know what to look for!

Mike

 

Why couldn't the kitmakers have saved us all of this time and trouble and just given us an accurate model to work from? I'd rather be bending plastic than yanking out a ton of references and kits and trying to make some sense of it all!

 

Gingerbob- your photo of the 'doghouse' was outstanding! In reading all of the discussions related to the 'joggle' I never really understood what they meant, but your photo makes it all perfectly clear- you can sure see where the fuselage was deepened by 3" to accommodate the dropped wing, coolant plumbing, and radiator/oil cooler/intercooler! Thank you so much!

 

Re your P-40 comments- IIRC on the P-40D and subsequent variants, a spur reduction gear was added to the Allison that raised the thrust line by 6" I think, and shortened the nose by 18" but I will have to refer to my reference library to confirm the measurements, so don't hold me to those figures just yet!

Edited October 6, 2017 by 72modeler
added comments and thanks

[gingerbob]

2 hours ago, FinnAndersen said:

A spinner 25", B spinner 29" across, but fuselage depth aft of cockpit unchanged at 66".

 

Interesting...

Thanks, Colin. 

 

Finn, yes, that is interesting, but notice that the bottom point that they're measuring to is different, and (if I see it properly) is aft of that joggle on the second (Merlin) drawing.  I'm frankly a little surprised that they updated the drawing that well- there are certainly examples of recycled drawings that do NOT update some features.  So, either a coincidence, a shrewd plan to have that component still have the same overall depth (to fit the existing crates?), or perhaps even an overlooked dimension that is not, in fact, correct.  Probably not the last, but one can't assume!

 

Now, off to see if I can find a thrustline reference- I wouldn't have thought that it would be obscure...

 

OK, from A-36A and P-51D/K Erection & Maintenance Manuals (sorry, no pictures this time):

 

A-36A distance between Fuselage Reference Line and Wing Reference Line: 23.5 inches

     Thrust line 1.5 inches below (I think!) FRL, parallel to it.

     And also noticed... FRL to top of fin: 67 9/16"

     FRL to nominal ground plane (level): 73.5"

 

P-51D distance between: 26.5 inches

     Thrust line: no standoff from FRL shown (to center of mass of engine?) BUT 1 degree 45 minutes downthrust

     FRL to top of fin: 69 9/16"

     FRL to nominal ground plane: 76.5"

 

So, the FRL doesn't change position (I'm sure it is defined somewhere, but I didn't happen to see that), there are 3 inches difference both to the wing Reference and to the ground (tire), and the fin is (apparently) 2 inches taller*.  I wonder where those two inches come from?  [Yes, I know that's been discussed before, also...]

 

*EDIT: However, the fin and rudder part numbers are the same in A-36A and P-51D Illustrated Parts Books, which makes me think that the fin and rudder are actually identical, and only the measurements (numbers) have changed.  Curiouser and curiouser.

Edited October 6, 2017 by gingerbob

[John Thompson]

<sigh!> Where's Charles Neely when you need him? It's been a while since I've seen or heard anything of him - I hope he's not the late Charles Neely by now...

 

John

Edited October 6, 2017 by John Thompson

[72modeler]

5 hours ago, John Thompson said:

<sigh!> Where's Charles Neely when you need him? It's been a while since I've seen or heard anything of him - I hope he's not the late Charles Neely by now...

 

John

Tell me about it! He's the 'source' I alluded to in my earlier posts; Charles has forgotten more about Mustangs than all of us know- put together. he was very gracious in sharing references and drawings for the  project I am  slowly  working on to build XP-51F FR409, which was bailed to the RAF, using P-51H, P-51D, and scratchbuilt parts. He is a real gentleman and a class act, and I, too, hope his health finds him still alive and kicking!

Mike

 

 

John,

 

I just found a website for RC modelers on which Charles apparently is/was a regular contributor; lots of the same Allison/Merlin Mustang discussions! Charles stated in one of the discussions that the wing was indeed dropped 3" beginning with the P-51B, for the reason/s I stated earlier and also to fair the new carburetor intake trunking  that ran below the Merlin in a smooth line from the inlet to the wing. Makes sense! (Sure sorry now I never purchased a set of his Mustang drawings!)

Mike 

Edited October 6, 2017 by 72modeler
added additonal information