# P51 Empty weight increase from B/C to D model



## oldcolt357 (Feb 8, 2021)

Hoping for some clarification regarding P51 empty weights. Depending on the source, I typically find the razor back B/C models listed at about 7000 lbs. empty, and the D bubble top listed at around 7,600 lbs. Does the empty weight include the guns? If so I'm guessing the modified wing (strengthened?) with the 3rd. gun would gain about 100 lbs. each. But added guns aside, to gain 600 lbs. from razor back to bubble top seems like a lot. 
Please let me know what I'm missing, thanks.

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## tomo pauk (Feb 8, 2021)

Summoning 

 drgondog

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## Shortround6 (Feb 8, 2021)

Empty weight does not include weight of the guns, They are included in "basic weight" or empty equipped. 

AHT gives the empty weight of a P-51B/C as 6988lbs (close enough) with a basic weight of 7325lbs (61lbs trapped gas and oil, 270lbs for the guns and 6lb of pyrotechnics) 
With fuselage fuel tank basic weight went to 7580lbs. 

The Ds had an empty weight of 7205lbs and basic weights of 7673 to 7781 depending on fuselage tank, guns, bomb racks and rocket launchers. 

The D would have extra weight due to the extra .50 cal guns over and above the weight of the guns. The empty weight would include the gun mounts, charging systems, heaters, etc. 

Make sure the sources are comparing the same weight, empty is not empty equipped.

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## jimh (Feb 9, 2021)

I'd have to look it up but I believe there was a difference in the weight of the different dash numbers of the Merlin. -3 , -7 , and -9

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## oldcolt357 (Feb 9, 2021)

Shortround6 said:


> Empty weight does not include weight of the guns, They are included in "basic weight" or empty equipped.
> 
> AHT gives the empty weight of a P-51B/C as 6988lbs (close enough) with a basic weight of 7325lbs (61lbs trapped gas and oil, 270lbs for the guns and 6lb of pyrotechnics)
> With fuselage fuel tank basic weight went to 7580lbs.
> ...


Thanks, this is what I was looking for.

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## pbehn (Feb 9, 2021)

oldcolt357 said:


> Thanks, this is what I was looking for.


There were other detail changes to the P-51D. Like a bigger "kink" in the wing caused by the undercarriage I think. Planes always gain wight because people stuff more into them like the gun sight and tail warning radar. The bubble canopy looks sleeker but that doesn't mean lighter, open top sports cars frequently get heavier because they have more bracing inside.

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## drgondog (Feb 9, 2021)

I extracted the Data in my book from NAA spec for each Mustang from NA-73 through the P-51D/K

Dean's AOHT was 'off' from NAA published data but not by much.

The Empty WT delta between B and D is 162 pounds which was airframe related mods including reduced total for cockpit enclosure/turtleback and the -7 vs -3 engine (6988 vs 7150). The trapped fluids, gun sight and 85 gal tank weight are the same pushing Basic Weight without ammo to 7120 vs 7282 pounds. 

The Gun/bomb rack WT delta between B and D, from Empty WT plus fluids and 55 gal tank,, is 140 pounds which is extra guns plus slight difference in the bomb rack weight (7422 vs 7728 pounds)

The Ammunition delta is 200 pounds. 
Total B vs D - no fuel but with pilot and oil and ammo - is 8132 vs 8638 ------>506 pounds

Full Internal fuel load - same. For 269 gallons - Wt = 1614 pounds. 
Final Fully loaded Internal combat weight between P-51B/C and P-51D/K is 9,746 and 10,252 pounds respectively.

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## Peter Gunn (Feb 9, 2021)

drgondog said:


> I extracted the Data in my book from NAA spec for each Mustang from NA-73 through the P-51D/K
> 
> Dean's AOHT was 'off' from NAA published data but not by much.
> 
> ...


Bill,

I was skimming through some rather poorly researched tome the other day about the Mustang and a rather irrational question came to mind.

Would there have been any advantage to retaining the dive brakes from the A-36 in the regular production runs of the P-51B, C and D? Or were they just extraneous weight and another widget that might complicate maintenance etc. with no real benefit? After all the P-51 did pretty well as a ground attack aircraft as is so would they have added, detracted or had no effect in your opinion?

Thanks.

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## drgondog (Feb 10, 2021)

Peter Gunn said:


> Bill,
> 
> I was skimming through some rather poorly researched tome the other day about the Mustang and a rather irrational question came to mind.
> 
> ...


IMO - the HQ-AAF Plans Division and Close Air advocates decided to produce the A-36 for several reasons:
1.) there was a political pressure on Arnold to acquire CAS mission specific capabilities to support tactical Army operations behind enemy lines. The threat environment was already deemed too hostile for USN Dive bombing aircraft employed by AAF (A-24, etc)
2.) There was a cadre within Plans that believed the Mustang was superior to P-39 and P-40 and were anxious to keep NAA production for Mustang intact - contrary to MC who wished it dead'.
3.) NAA already anticipated the political and military requirement for high performance low level attack aircraft in the form of a dive bomb cap[able Allison powered A-36 ready for Plans from late 1941 through Feb 1942 when Arnold and Spaatz visited NAA.
4.) All that said, the AAF doctrine of 1941/1942 changed to multi role fighter capable of air superiority over the battlefield. Capable of carrying bombs and rockets but not requiring dive bomb precision.
5.) The P-51A was deemed capable and ordered before the first A-36 flew. The P-51B was requested as an option to convert P-51A production order at any time, setting the stage for the P-51B to be the CAS fighter for Tactical Command to replace A-36, P-51A (and P-39 and P-40) as the multi role glide bobing, rocket carrying tactical air superiority fighter.

Which brings us to your question "Dive Brakes for Merlin Mustang"?

No. the weight growth from Mustang I to A-36 Basic Weight with Guns/ammo grew from 6,723 to 7, 135 -- of which 150 pounds was dive brake associated.

Additionally the parasite drag increase was not insignificant for the dive brakes.

There was no mission statement for P-51A, P-51B,P-38 or P-47 for dive bombing, or any post WWII requirement from USAF.

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## Peter Gunn (Feb 11, 2021)

drgondog said:


> IMO - the HQ-AAF Plans Division and Close Air advocates decided to produce the A-36 for several reasons:
> 1.) there was a political pressure on Arnold to acquire CAS mission specific capabilities to support tactical Army operations behind enemy lines. The threat environment was already deemed too hostile for USN Dive bombing aircraft employed by AAF (A-24, etc)
> 2.) There was a cadre within Plans that believed the Mustang was superior to P-39 and P-40 and were anxious to keep NAA production for Mustang intact - contrary to MC who wished it dead'.
> 3.) NAA already anticipated the political and military requirement for high performance low level attack aircraft in the form of a dive bomb cap[able Allison powered A-36 ready for Plans from late 1941 through Feb 1942 when Arnold and Spaatz visited NAA.
> ...


Thanks for the concise and VERY informative answer. I think I might have had a sneaking suspicion in the back of my mind WRT something along those lines but wasn't really sure. It was one of those "HUH... What if?" moments you have whilst reading on a specific subject.


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## jimh (Feb 11, 2021)

Most of the A-36 dive brakes ended up getting wired shut. They had a tendency to creep open, which for obvious reasons is not good. They also take up a good chunk of the wing. I noticed, while flying Betty Jane, that it liked to be 5-10mph faster during landing than a D model, even though the D is heavier. It doesn't seem like a lot, but the B got pretty mushy down around 105mph. At speed, the B and the D fly identically. To be honest, I like the reclined seat of the B better, as well as the cockpit layout. The D throttle quadrant is nice with that big grip, but getting to the trims takes a lot of getting used to.

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## drgondog (Feb 12, 2021)

jimh said:


> Most of the A-36 dive brakes ended up getting wired shut. They had a tendency to creep open, which for obvious reasons is not good. They also take up a good chunk of the wing. I noticed, while flying Betty Jane, that it liked to be 5-10mph faster during landing than a D model, even though the D is heavier. It doesn't seem like a lot, but the B got pretty mushy down around 105mph. At speed, the B and the D fly identically. To be honest, I like the reclined seat of the B better, as well as the cockpit layout. The D throttle quadrant is nice with that big grip, but getting to the trims takes a lot of getting used to.


A question, Jim. IIRC, the only difference in Warbird B and D with all guns and armor stripped, internal fuse tank removed - and using the same engine, is about 160 pounds (D heavier empty) - unless one or the other is both equipped for, and carrying a passenger. Do you have a similar 'feel' for all the B's (without passenger) 'over the fence' prior to landing? Coupled with mods to put second seat in Betty Jane, carrying a Bubba behind you would push the weight over a non bubba carrying D or similar B/C. 

The NAA Specs I sourced for P-51B-1 and D-5 weights did not include a.) structural beefing up of wing for the improved three hinge aileron of the B-5 and D, b.) both the DFF and Reverse Rudder Boost tab. The B-5 and subsequent were much closer in empty weight to the D-5, both carrying a 1650-7.

I suppose FAA require DFF but don't know if every Warbird Merlin Mustang has reverse rudder boost or the TO Installed WS 75 doublers or the Horizontal tab doublers when the metal elevators were installed. The structural modifications for the B-1 and B-5 and C-1 to install the 85 gallon tank were incorporated in all subsequent B-10/C-5 and subsequent - and remained in the B/C even after the cells were removed. Betty Jane with mods to carry second passenger also added more weight in the Aft Frame.

There is a small but significant in difference in weight of the -9 over the -3 and -7 and variable drag imposed by external bomb racks. The B bomb rack has about 2 times the drag of the 106 and 122 racks. Other factors affecting aero and feel are the multitude of times each airframe you have flown has been rebuilt over its original and previous factory existence - with potential issues to spec from NAA factory tooling built originals.

That said - you are far more qualified to discuss feel, the above were just a few things that came to mind.

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## Peter Gunn (Feb 12, 2021)

jimh said:


> Most of the A-36 dive brakes ended up getting wired shut. They had a tendency to creep open, which for obvious reasons is not good. They also take up a good chunk of the wing. I noticed, while flying Betty Jane, that it liked to be 5-10mph faster during landing than a D model, even though the D is heavier. It doesn't seem like a lot, but the B got pretty mushy down around 105mph. At speed, the B and the D fly identically. To be honest, I like the reclined seat of the B better, as well as the cockpit layout. The D throttle quadrant is nice with that big grip, but getting to the trims takes a lot of getting used to.


Excellent synopsis, thank you. And I'm sooooooooooooo jealous.


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## drgondog (Feb 12, 2021)

On the A-36 flying today, I can think of no reason for the Dive Brakes to ever be functional. During WWII Training Command issued a directive to wire them shut after as few fatal accidents. The primary single issue was failure to deploy the brakes BEFORE initiating the dive.

In combat, however, the 27th and 86th both used the A-36 as intended and designed with excellent accuracy. Howard 'Pete' Decker was a pilot in the 27th, and went to HS with my dad. Pete is named and quoted in Straight Down. He stated to me that the A-36 was far superior to both the P-40 and P-47 in CAS that he flew after the 27th ran out of A-36s. I don't know much about CBI ops.

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