B-26 Engineering Data/Technical Information (From Martin's Own Blueprints)

Maty12

Airman 1st Class
281
280
Nov 6, 2019
Hello all!

I decided I need a place to compile all my findings from analyzing the many technical drawings I scanned from the Smithsonian's microfilm collection. It would be pretty difficult to find a more confusing aircraft to research or model, because of decisions made by Martin, the USAAF/USAF, and persistent myths that get repeated very often, and I want to spare any future researchers/modelers from getting confused or stumbling upon incorrect info. I plan on uploading some spreadsheets at a later date to replace my outdated and incorrect table found elsewhere on this forum. They will be:
-A table of all stations for the fuselage, wing, ailerons, wing flaps, nacelle, fin, rudder, stabilizer and elevator, detailing what parts were located where for each B-26 model.
-A table describing the production models listing serial numbers along with who's been converted into what.
-A table showing how many different versions of each subassembly existed, and what models used which.
-A table listing all B-26-MA and B-26B squadron assignments as best I've been able to find.
-A table describing the "B-26 Skeletons", a concept I'll explain later in this post and which I think will be very helpful for any future modelers and kit builders.

First things first: Defining Terms
Talking about the B-26 is by its very nature confusing, so I will do my best to avoid using conflicting terms. I will also be doing my best to use the language used in the technical drawings. I will also avoid using fractions when talking about dimensions because I find them annoying and they're harder to use in 3D modeling software.

Production Models
-B-26: the term I'll use for the rest of this thread when referring to all models of the aircraft, or nonspecific models.
-B-26-MA: the term I'll use when referring to the first production model, of which 201 were built. Martin and the USAAF just call this the B-26, which makes thing confusing. People (myself included elsewhere) usually call it B-26-MA, and while I have not found any official use of this name and the B-26 predates the USAAF's use of production suffixes (-MA, -MO), a way to refer to specific models is needed for clarity's sake so it'll have to do.
-B-26A: the term Martin uses to describe the A and A-1 models, because they are identical other than the engine
-B-26B: the term Martin uses for all short wing B-26B models (B-26B-MA through B-4), but only the short wing models. The Aft Aft Fuselage subassembly was significantly redesigned to accommodate the new twin-gun turret, but the fin, stabilizers, rudder and elevators remained the same. Last 141 B-26B-4s had new waist gun positions with sliding hatches, requiring a redesign of the Aft Fwd Fuselage. All B-26B-4s had a new longer landing gear strut that was mounted lower on the fuselage, I'm still researching its specifics as the only drawings I've found seem to contradict photographic evidence.
-B-26B-MA: the term I'll use to refer to the first production block of the B-26B, of which 308 were built. Again the same problem as the B-26-MA necessitates adopting unofficial terminology.
-B-26B1 Quick Fix (QF): Martin's term for the long wing B models fitted with the same manual turret as the short wing models (B-26B-10-MA & B-15-MA). Despite changes to the Aft Fwd Fuselage subassembly (again), the Aft Aft Fuselage of the B-26B was maintained. I'll describe the long wing modifications in more detail in the Skeleton section.
-B-26C Quick Fix (QF): Martin's term for the all C models fitted with the same manual turret as the short wing models (B-26C-5-MO & some B-26C-10-MOs). These C models are identical to their B1 equivalent, the 30 first C-5-MOs being the only unique ones due to having the old hydraulically actuated main gear doors and 47" main wheels as opposed to the new mechanically linked doors and 49" wheels fitted to all other B-26B1s and B-26Cs.
-B-26B1: Martin's term for all long wing B models, though also used for the B models fitted with the Bell M-6 or M-6A electric tail turret (B-26B-20-MA through B-55-MA), which along with the "Quick Fix" name for the other models leads me to believe those were a stop-gap until the M-6 turrets could be fitted. Aft Aft Fuselage was once again redesigned to accommodate the turret, as was the rudder for the same reason, having a shorter chord. Stabilizer, Elevator and Fuselage Aft Fwd Section stayed the same. These modifications reduced the B-26's length from 698.5" (58' 2.5") to 675" (56' 6"). Manuals sometimes refer to this model as the B-26B-1, which is very confusing because of...
-"B-26B-1": The term used by the USAAF for short wing B-26B models bound for the MTO that were modified by Martin's Omaha Modification Center.
-B-26C, B-26F, B-26G: Pretty straightforward, Martin's terms for all C, F, and G production blocks. F and G models are also called "Twisted Wing", because the wing and everything attached to it (nacelle and landing gear) was rotated 3.5 degrees to increase takeoff performance. I don't know what point they were rotated around, but suspect either the center or bottom of the Wing Center Section's Rear Spar. I'm not very knowledgeable on the F and G models. Sharp-nose (not shark-nose, as sometimes stated) ailerons were introduced on the 51st B-40-MA, I am not sure whether any B-26C-40-MOs had these. They were fitted to all subsequent models and required modifications to the Wing Trailing Section.
-XB-26D: Again, straightforward, the one B-26-MA modified to test hot air anti-icing.
-B-26E Bomber: Martin used the term B-26E for all models they proposed as the "next model" after the B1 & C models. This proposed model would have revised the Nose Cone, Fwd Fuselage, Aft Fwd Fuselage, Rear Bomb Bay, Wings and Rear Nacelles. B-26B-40-MA 42-43319 served as the prototype for this configuration, it was not adopted.
-B-26E Strafer: This proposed model would have revised the Nose Cone, Fwd Fuselage, Aft Fwd Fuselage, Wings and Rear Nacelles. B-26B-15-MA 41-31672 served as the prototype for this configuration, it was not adopted.
-XB-26E Turret: Much less ambitious than the Bomber and Strafer designs, this is the very unofficial term I use to talk about B-26C-5 41-34680, which had the deck turret moved from the Aft Fwd Fuselage to the Fwd Fuselage, towards the rear of the Radio Operator & Navigator's compartment. This configuration was also not adopted.
-XB-26E (XB-26F): Unofficial term I personally use to talk about B-26B-40-MA 42-43459, which served as the prototype for the B-26F.

Fuselage Group
-Nose Cone: Martin uses both this term and Nose Turret to refer to the Plexiglas nose cone, I prefer the former.
-Forward (Fwd) Fuselage: Martin's term for the forward fuselage subassembly, which extends from station 33 to station 230.75. Contains the nose gear and the bombardier, pilot, co-pilot, navigator and radio operator's stations.
-Pilot's Enclosure: Martin's term for the cockpit canopy.
-Center Fuselage: Martin's term for the center fuselage subassembly, which extends from station 230.75 to station 434.25. Contains the wing center section, the forward bomb bay and the aft bomb bay.
-Aft Fuselage: Martin's term for the entire aft fuselage subassembly, itself made of two large subassemblies, which includes the entire fuselage from station 434.5 onwards. and the tail turret.
-Deck Turret: Martin uses this term, Dorsal Turret and Upper Rear Turret to refer to the Martin 250CE electric turret located at station 460.25.
-Tunnel Gun: Martin's term for the ventral gunner's position. The gunner fires out of the camera hatch which extends from station 512.25 to station 548.375.
-Aft Fwd Fuselage: Martin's term for the subassembly that extends from station 434.25 to station 581.375. Contains the deck turret and waist and/or tunnel gunner's compartment (depending on the model). The fin attaches to the top of this section.
-Aft Aft Fuselage: Martin's term for the subassembly that extends from station 581.375 onwards. Contains the stabilizer's center section and the tail gunner's station.
-Fuselage Tail Cone: Removable endcap for the tail. Extends from station 693 to station 698.5 on B-26-MA & B-26A, or station 670.75 to station 698.5 on B-26B. B-26B1s, Cs, Fs and Gs did not need tail cones.

Wing Group
-Center Wing:
The section of the wing located inside the fuselage. The spars in this section are connected to the station 230.75 and station 346.25 by shear webs and are straight and of constant section, perpendicular to the direction of flight when viewed from above. This section extends 46" from the fuselage centerline, to wing station 46.
-Outer Wing: The panel that makes up most of the wing. This section extends from station 46 to station 364 and can be divided into Nose Section, Main Wing and Trailing Section. The Nacelle and Wing Tip attach to this section.
-Main Wing: The section of the Outer Wing between the two wing spars, that the nose and trailing sections attach to. To this section attach the nose section, trailing section and landing gear.
-Wing Nose Section: The section of the Outer Wing forward of the forward spar, which is itself built of several smaller sections. Contains the landing lights and deice boots (if fitted).
-Wing Trailing Section: The section of the Outer Wing aft of the rear spar, which is itself made of two sections. The inboard section contains the wing flaps, while the outboard holds the ailerons.
-Wing Tip: The section of the wing extending from station 364 onwards. It contains navigation lights and the pitot tube, as well as one of the aileron hinge brackets in the long wing models.

Engine/Nacelle Group
-Nacelle: the term Martin uses to describe the entire nacelle assembly, including the engine hood. Stations are measured along the thrust line, with station 0 being the intersection of the centerline for the propeller blades.
-Engine hood: the term Martin uses to describe the engine cowling. It is split into five (5) components: two (2) upper panels (which include the carburetor ducts), two (2) side panels and one (1) lower panel (which contains the oil cooler duct). Extends from station 6.6 to station 53.59 (for the upper panels) or station 54 (for the lower panel). These panels are attached to the engine via two mounting rings, located at station and .
-Desert Cowlings: the term the B-26B manual uses for the engine hood upper panels fitted with enlarged air intakes to allow for installation of air filters. These were fitted to "B-26B-1s" and all production models from the B-3 to the G-25-MA.
-Venturi: the term Martin uses to describe the section of the nacelle aft of the engine hood and forward of the firewall (station 104.25), with holds the engine's accessories including the oil tank and oil cooler. It is split into five (5) panels: two (2) upper panels, two (2) center panels and one (1) lower panel. The two (2) fairings for the tailpipes are also a part of this unit and are located between the center panels and the lower panel. The upper panels extend from station 53.59 to station 103.65, the center panels from station 54 to station 114.74, and the lower panel from station 54 to station 103.65.
-Tailpipe: Martin's term for the exhaust stacks.
-Rear Nacelle: the term Martin uses to describe the assembly that extends from station 103.65 onwards. This section attaches to the wing outer panels and contains the landing gear doors.
-Nacelle Tail Cone: Martin's term for the removable endcap for the nacelles that extends from station 229.25 to station 262.16 on short wing models and from station 218.13 to station 262.16 on long wing models.

Skeleton
So, with that out of the way, what the hell is a B-26 Skeleton? Simple, it's what you get when you combine the smallest amount of information required to describe the airplane's geometry. For scale modeling, it's useful to figure out lengths and distances when modifying kits. For 3D modeling, it's a very helpful tool in order to accurately place all the subassemblies. These are still WIP but I plan on sharing the blender files for them once they are complete. Currently missing are the main landing gear, the nose gear doors and all other doors, the tail turrets, the wing flaps and the ailerons. I also have yet to add the arcs for the tips of the wings, stabilizer and fin. I also have yet to make a skeleton for the non-Quick Fix B1 & C or the B-26F & G, the latter two because I have very little information on then.

I will now use the skeletons as a visual aid to explain how the B-26's wings and empennage were enlarged. Black represents the B-26 and B-26B, while orange represents the B1/C Quick Fix. It's important to remember that Martin's priority was reusing as many components as possible, and keeping the attachment points the same.

First off, the wings. Martin kept the main wing's geometry the same, though might have reinforced it. The nose and trailing sections were replaced with ones with longer chord at station 364. The new trailing section was significantly larger and responsible for the most noticeable change in the wing's shape. Since Martin had to redesign this section anyway, they switched from split flaps to slotted flaps as recommended by NACA. The old wing tip was replaced by a new unusually long unit which extended from station 364 to station 426. This made each wing 36" longer, for a total increase in span of 72" or 6'. Keeping the main section allowed Martin to reuse not only it but also the nacelle, with the only change to this unit being where the tail cone starts, and a new tail cone. Some inboard nose ribs were also reused.
ww2 b26 thread 2.PNG


For the empennage, Martin had less constraints. As such, they essentially just bolted (well, riveted) on extensions to the bottom of the current units. The fin and rudder grew 20", while the stabilizer and elevator grew 24.25".

While the old fin was held in place only by two bolts at the spar and a row of screws attaching its lowest rib directly to the skin of the upper fuselage, the new fin received a new bolt at the also new partial spar near the leading edge, which only extended until station 20 (formerly station 34). The lower section of the rudder was redesigned to still fit in the old fairing at the top of the aft aft fuselage (note that the orange and black lines intersect at the bottom of the rudder trim tab's trailing edge). The main priority for Martin was keeping the rudder's attachment points the same, particularly the torque tube contained in the aft fuselage.

The theoretical rudder hinge's centerline was thus kept, while the fin spar and nose of the rudder gained a distinct break/angle in their lines. It's worth noting that with this change Martin also changed the datum for the fin's stations from a line tangent to the fuselage centerline to the theoretical base of the fin, which used to be station 34 (which was also the same between all models). So while the fin gained 20", one must subtract 14" from the short-wing B-26s' stations to find the corresponding stations and ribs on the long wing models. Most ribs were the same as the older models.
ww2 b26 thread 1.PNG


The Stabilizer was trickier to modify because three mounting points had to be kept: the front spar, the rear spar and the elevator torque tube. Martin also wanted to avoid reshaping the fuselage frames entirely if possible. The solution was an extension that had constant section between the two spars, with only the nose ribs being tapered. The elevator was a much easier affair, with Martin simply "cutting" (reshaping) the ends of the trim tabs so that they lined up with the pre-existing fillet for the B-26B. With this approach, the entire Aft Aft Fuselage subassembly from the short-wing B models was kept with zero changes. A new fillet was still needed for the Aft Fwd Fuselage, but in this area the fillet was merely riveted onto the skin and not a structural component, thus easy to replace.
ww2 b26 thread 4.PNG


B-26B Tail (note that the angled/cut section ends at the elevator):
Tail B-26B 4.jpg


B-26B1 "Quick Fix" Tail (angled/cut section continues into the elevator, rounding off into the trailing edge with an arc of radius 5"):
Tail B-26B1-QF 1.jpg


Hope y'all find this informative!
-Matt
 
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Maty12

Airman 1st Class
281
280
Nov 6, 2019
Correction/addendum to my original post, which I've since edited:

I said the fin was attached to the fuselage via two bolts at the spar and a row of rivets attaching the lowest rib to the top of the fuselage. That rib was actually attached to the fuselage via forty 1/4" screws according to the E&M manual. This makes sense to me, as the unit has to be easily removable for repairs/replacement. The bolts were 5/8" units.

Also according to that manual, the wing outer panel is attached by four 1/4" bolts, two at each spar, and also by 3/8" bolts at the crest of each corrugation.

The stabilizer outer panels are attached by four 1/4" bolts each, two at the front spar and two at the rear spar.
 
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Juanita

Airman
64
31
Apr 7, 2014
Excellent work Matt.
It is always very interesting what is uncovered when you 'deep dive' into the technical aspects of an aircraft.
Your research will be very valuable to other folks who study this aircraft type.

Juanita
 

Maty12

Airman 1st Class
281
280
Nov 6, 2019
Excellent work Matt.
It is always very interesting what is uncovered when you 'deep dive' into the technical aspects of an aircraft.
Your research will be very valuable to other folks who study this aircraft type.

Juanita
Thank you, Juanita!

I should have more to post soon, still working on the skeletons and do want to make a post with more general information using actual dimensions

-Matt
 

Maty12

Airman 1st Class
281
280
Nov 6, 2019
Hi all,

Wanted to explain why I haven't updated this thread. I've been trying to use CADsketcher in Blender as well as regular mesh to make the skeletons, and these seem to disagree quite a bit at times. Martin's drawings have also been giving me a hard time, so it will be a while before I can provide information on things such as the arcs that make up the contour of the fin and rudder tips. In the meantime, there is information I can give:

The B-26 is a particularly confusing airplane for any modellers, because different dimensions are measured from entirely different planes of reference, the location of which is often not explained, and sometimes the plane itself isn't even mentioned. Dimensions for the overall airplane, the landing gear, the nacelle and the wing are all taken from the thrust line, while dimensions for the fuselage follow its centerline, and dimensions for the empennage have their own reference plane parallel/perpendicular to the stabilizer's chord plane. The location of these planes is completely ommited from the general arrangement/3-view drawings and has to be figured out from blueprints which often times don't appear relevant at all. The only drawing which shows the location the thrust line is the boresighting diagram for the guns, and the stabilizer from the drawings for the elevator torque tubes and empennage change diagram for the B1. That being said, here is that information:

Reference Planes (from CL Fuselage)
Thrust Line PlaneValueNotes
Angle
1​
From CL Fuselage
Fuselage Station of Intersection
453.52​
Approximately 453.451" in Thrust Line Plane
CL Propeller
143.58​
Measured in Thrust Line Plane, from point above start of CL Fuselage when CL Thrust is horizontal. This is the Reference Datum for the Engine Nacelle's Stations
Wing Chord PlaneValueNotes
Angle
4.5​
From CL Fuselage, 3.5 from CL Thrust. All wing dimensions not measured across Chord Plane are measured in Thrust Line Plane
Fuselage Station of Leading Edge
218.58​
Approximated to 218.128" in Thrust Line Plane
Height above Thrust Line at Root
19.9​
Measured at Leading Edge. Approx. 24.0039" above CL Fuselage
Wing Sweep (in inches)
56​
Leading Edge and Trailing Edge both have 56" of sweep (positive and negative, respectively), resulting in perfectly centered 1/3 taper from Root to Tip
Height above Thrust Line at Tip
28.5​
Measured at Forward Spar (12% of Chord)
Stabilizer & Fin Reference PlaneValueNotes
Angle
0.5​
Positive from CL Fuselage, Negative from CL Thrust
Fuselage Station of Intersection
617.065​
Intersection of Elevator Hinge CL along Ref Plane and Fuselage CL. Elevator Hinge is 36" aft of Forward Spar at Station 20.75, where Forward Spar is Fuselage Station 581.375
Stabilizer Root Station
32.5​
Above Ref Plane
Stabilizer Zero Sweep Line
60%​
There is Zero Sweep along Elevator Hinge (60% of Chord)
Fin Root Station
34​
Above Ref Plane
Fin Tip Station
145.5​
Above Ref Plane
Fin Sweep (Inches)
6​
Forward Sweep of Rudder Hinge from Root to Tip (55% of Chord)

If anyone wants clarification on these, let me know.

I'd also like to make a retraction. I get my information about specific models/production blocks from Tannehill and Baugher, and update it whenever I find either documentation, original drawings or photos that contradict that information. As such, some things have yet to be corrected. There are also things I'm still researching.

I was under the impression that there was more standardization that there really is between the B1 and C models, as most places list these as being the same, so figured the Bell M-6 tail turret was introduced with production block 20 for both the B1 and C models, but there are photos of B-26C-10s and C-15s with the turret. It is unclear to me when this change occurred in the Omaha production line. I will update the original post once I know more.

There is also the question of the nose gear. Any list of B-26 models will say that the nose gear strut was lengthened by 6 inches starting with the B-26B-4. For a very long time, I thought this was incorrect, as in my mind a longer strut would need a longer wheel well, and clearly the wheel well never changed size. I figured the entire gear assembly was simply moved forward and lower. Photos and measurements I got from Pat Rodgers confirmed that the mounting point was indeed moved, but I somehow missed the obvious until recently: it was also lengthened, and the new position allowed this. This is pretty obvious if one compares photos of the old gear to the new one. On the early models, the gap between the bottom of the oleo strut and the top of the wheel is very small, while on later models that gap is very large. The part that was lengthened was the half-fork that holds the wheel.
I don't have drawings for this version of the landing gear. I do have drawings for what's apparently a third version, which might be the version used in the B-26F/G (I have yet to confirm this), which is longer and mounted lower but not further forward. I do not have much information about this setup. The half-fork is angled backwards either 1" or 2.5 degrees, one of the two.

Will provide more info when I can,
-Matt
 
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