1/18 Scale P-51B 3D Print Build

[JayW]

Here - this post ought to be a fun read for you.

 

With the installation of the painted and weathered wing/fuselage fairings, the forward fuselage is basically finished, and I can strip the masking from the transparencies and place the actual Malcolm hood on there instead of the other one (meant to take the wear and tear of building this monster).  At the risk of "spilling the beans", a couple of shots of Cripes A Mighty's fuselage at its current level of completion, warts and all:

 

 

 

Canopy open:

 

 

I was relieved the transparencies didn't get mucked up by the masking as they can sometimes do.  And no paint leakage either.  Yay!!  It is true that the Malcolm hood can be displayed either open or closed.  But it takes a bit of heavy-handedness to do it - the canopy rails just do not slide well over the rollers.   A good bit of friction no matter if I lubricate it with dry lubes or not.   And the more I mess with the canopy position, or the more I swap out canopies, the more wear that is imparted on the rollers.  Already, if you look for it, a few of the rollers are getting worn down some.  We can't have that, as much work as I put into the roller bars.  So when this project concludes some months from now, I think I will select a position and just leave it there.  I'll take a vote when the time comes, and perhaps yield to the desires of the majority of my followers.

 

Next on my list would be the (brass rod re-inforced) landing gear lower struts - they have patiently waited long enough methinks.  The upper struts have done really well being subjected to the rigors of continued major assembly of wings and fuselage.  That is, outside of the torque link lugs I broke off, requiring a delicate couple of surgeries.  Now, with all that wing and fuselage assembly finished, I think I can relatively safely install the lower struts and the torque links.  Been waiting forever to do this and I was excited! 

 

Pictures:

 

3D printed details first, of a set of torque links, along with both sets of LG fairing links, and the fairings themselves (I was multi-tasking).  

 

 

For now just pay attention please to the torque links; I will get to the fairings in a minute.  The torque links were designed and printed many moons ago; finally it was time to paint and install them.  A shot of the assembly process:

 

 

 

 

My multi-use round exhaust stack protector being used as a relatively stable mount for this now huge assembly.  Glad I made that part.  The lower struts were first bonded in place, taking great care to assure good alignment and proper oleo compression.  Hope I did OK there; seems good to my eye.  On the left, torque links are dry fitted with .025 inch diameter plastic "bolts", to check the fit.  Later after assuring a good fit, the bolts are cut to size, reinserted in lug holes and bonded in place, one by one.  Then Meng nuts and bolt heads are added.  Kinda like the real thing....  On the right, the process has been completed.  It's ready to go.  

 

Close-ups:

 

 

 

 

Am very pleased to say all the tiny lube (zerk) fittings are still intact.  So fragile.  Also note the gently bending and twisting black brake hose, made quite a while back from wire rubber insulating cover.  Signature P-51 stuff.

 

And with wheel/tires dry fitted:

 

 

 

 

 

Anyone who has followed some of my builds knows I am cuckoo-for-cocoa-puffs when it comes to aircraft landing gear.  So I poured my heart and soul into these P-51 MLG.  I think they are going to be OK as long as I keep from breaking one or both.  They are pretty stout though - you should recall all the metal re-inforcement they have internally.   Which gives me pretty good assurance the gear will not sag over time.  Hope you like them. 

 

And as you saw above, work has begun on the fairing doors.  The fairing was designed in Rhino of course.  I created the 2D outline of the gear doors on the wing reference plane, as defined on the appropriate engineering drawings, and then projected the outlines onto the wing lower contour.  And I was off to the races:

 

  

 

The inner stiffening elements were a bit challenging, as usual.  Here I show the parts "hot off the presses":

 

 

 

I tell you - designing and making these fairings per drawing in this manner just takes all the guess work out of it.  I see too many Mustang models where the MLG fairings are too long or too short relative to the struts, and/or not oriented correctly either in a side view or a front view.  I made my parts like the drawings that define them.  That holds true for the fairing links as well.  So I can pretty much guarantee an accurate installation, when the time comes.

 

These couple of shots are sneak peeks of what the fairings will look like in place (or nearly so - dry fitted and a bit wobbly for now):

 

 

 

 

Pretty fancy contours eh?  Next you see these fairings, they will be skinned in aluminum and have invasion stripes on them, and then will sit for quite a while.  Were I to install them now, I would not be able to squeeze in the tires.  Which immediately tells me that in real life, changing a P-51 tire involves disconnecting the LG fairing, or inflating the tire after attachment to the strut, not before.  Anyway I cannot permanently install the tires/wheels until the aft fuselage and tail gear are completed - so that I can properly align the tire flats and bond them in place.  At that time the fairings and their links can be permanently installed.

 

Next post you will see skinned, painted and weathered MLG fairings, and I believe the beginnings of Rhino design work on the aft fuselage. There is going to be a period here where I spend alot of time in Rhino instead of making parts.  Gotta get that aft fuselage and empennage designed at long last.  Unless maybe 🤔 the bomb pylons.....  Hmmm.

 

Take care and thanks again for tuning in.   

 

 

 

  

 

 

 

Edited March 29 by JayW

[Shoggz]

Great update! My vote will be for an open canopy to see all the juicy goodies in the cockpit.

[Oldbaldguy]

While we are all enthralled by this here project, there are some viewers who will nod knowingly when you speak of things like torque links without having a clue what a torque link is or what it does - most airplanes have them, but what they do for a living is not common knowledge.  Would you care to elucidate please, professor, seeing as how you made these little gems from light and ether, and tell us not only why your Mustang needs torque links but also how they work?  Much obliged.

[JayW]

On 3/28/2025 at 6:54 PM, Oldbaldguy said:

tell us not only why your Mustang needs torque links but also how they work?  Much obliged.

 

Well certainly.  Apologies for any confusion.  Another picture of landing gear torque links, just for comparison:

 

 

 

The red arrow points to the torque links.  And the Mustang's:

 

 

 

They are also known as scissor links.  Note that each link has a wide joint where attached to the cylinder, and a narrow joint where attached to one another, giving a triangular shape that all torque links have.  This provides side load capability.  These links are needed on any landing gear that do not steer and are oleo type.  That is - a shock strut where two telescoping cylinders are used (usually they are air/oil where compressed air serves as the spring, and the oil flowing through small orifices serves as the shock absorber).   For any substantially large and complicated aircraft this is always the case for its main landing gear at least, and the Mustang is no exception.  The reason for torque links is simple - two telescoping cylinders can rotate about their common centerline relative to one another unless constrained from doing so.  Just by inspection of the picture above, it is clear the torque links provide that constraint.  And at the same time they allow the cylinders to telescope.   

 

The term "torque" link is often used because any force on the landing gear that tries to rotate the lower strut cylinder relative to the upper cylinder (like say a taxi turn or a discontinuity on the air strip) imparts a torque.  And the pair of links resists that torque, and transfers it from the wheel and lower cylinder to the upper strut cylinder and then up into the landing gear support in the gear bay.  

Edited March 30 by JayW

[JayW]

20 hours ago, JayW said:

There is going to be a period here where I spend alot of time in Rhino instead of making parts.  Gotta get that aft fuselage and empennage designed at long last. 

 

Soon as I say that, I remember other stuff to do first!  How about the centerline gear doors?  Huh?    That's a bit of a project.  And I think next post you will see progress on that front too.  

[JayW]

Staying on task, the landing gear strut fairings were skinned, painted, and weathered:

 

 

 

Situated on the gear strut (dry fit):

 

 

 

I am fired up about that!  As I stated previously, I will wait until the tires flats can be oriented and wheel/tires epoxied in place to final install the strut fairings.  And to do that I have to add the aft fuselage and tail gear!  Gonna be a while. 

 

Also design work has begun in Rhino on the inner gear doors.  Thought I would take you through the initial steps.  

 

At the beginning of this build, some will recall, I defined the basic planes of the airplane - fuselage station zero, cowl station zero, buttock line zero (the aircraft center plane), water line zero (or the fuselage ref line, or the FRL), the wing reference plane (WRP), wing station zero, etc), in full size.  And then I commenced to create the various surfaces that define the P-51B.  Including the wing:

 

 

The wing reference plane, or WRP, sort of splits the wing into an upper surface and a lower surface, and the plane is oriented 5 degrees from horizontal - that is the P-51's wing dihedral angle.  The WRP is the most important geometrical element of the wing.

 

The landing gear door outlines are defined on their engineering drawings in a plan view perpendicular to the WRP.  An example from drawing 73-33301 - Landing Gear Door LH:

 

 

I have circled some of the defining dimensions for the door edge outline.  It is shown in the stowed position, and as I mentioned the view is perpendicular to the WRP.

 

I can take those dimensions and create an outline of the door in Rhino, on the WRP full size:

 

  

 

I did same for the strut fairing, which is also shown above. 

 

From there, I can project that outline onto the wing lower surface:

 

 

 

Then I can split the wing lower surface using the projected outline as the splitting element:

 

 

And that surface is the beginning of the door design in Rhino.  The hard part comes next - detail design.  Here I show my initial progress from last evening and part of today:

 

  

 

 

The last shot shows the door (and its mirror opposite) rotated about the hinge lines 83 deg which is full open. That circular dish on the inside surface of the door was a big PITA, and I am not totally happy with it.  Just part of the fun.  Lots more detail is coming - the hinge fittings, the attachment lugs for the door actuator, latch fittings, etc.  And once complete, I will scale to 1/18, and print it (and its RH opposite).  That's going to be the subject of my next post no doubt.  

 

Til then.

 

Edited March 30 by JayW

[JayW]

So something has been disturbing me ever since I installed the landing gear lower struts.  The rake angle doesn't look quite right.  I am a stickler for getting landing gear geometry just right, and pride myself in doing so on my models.  So I took a picture and did some measuring:

 

 

 

Sure enough - the rake angle is too severe, and by kinda alot.  Was harder to detect until the lower struts were installed.  The Rhino models are right on WRT to this angle.  So what went wrong...  Assuming the wing-to-fuselage attachment is correct (a good assumption), the parts involved are as follows:

 

• Upper strut detail
• Sta 75 LG support rib and rotation box
• Wing front spar

 

The LG upper strut:

 

 

A one-piece 3D printed part, identical to the Rhino model, where the angle between the strut itself and the trunnion it rotates about is the critical feature.  This is almost certainly not where the error lies.

 

The Sta 75 LG support rib and box:

 

 

Also a one-piece 3D printed part, identical to the Rhino model.  The orientation of the large bore in the rotation box, and the mounting face of the rib are the critical details.  And also almost certainly not where the error lies.

 

The front spar:

 

 

 

My front spar parts are integrated into the wing box, except in the splice zones where they are hanging out in the breeze.  The landing gear support rib is placed squarely in that splice zone between the inner and outer 3D printed wing box parts.  I now recall I had some twisting deformations with the spars in the unsupported splice areas, and I relied on the separate inspar ribs you see above, and some good clamping force, to get the twist alleviated.  Note one of the ribs has "75" written on it - that one directly backs up the LG support rib and box.  So it is here where assembly is required and some variability can be introduced.  It looked OK to me at the time, but where else could the error be?  As a reminder, here is how the LG support rib mounts to the front spar face:

 

 

 

I am relying on the spar face to be dead on accurate and not twisted.  Any angular difference from nominal will directly affect the landing gear rake angle.   My splice design did not really lock down the spar face angle all that well.  And I am all but convinced this is the source of the error.  The LH and RH gear angles are not quite identical, which suggests the LH and RH spar faces have differing but similar deformation (a twist). 

 

So, leave it alone, or try to do something about it?  I think it is not noticeable to most observers.  But the most important one - me - sees it.  These parts are hell for stout, and we are way beyond disassembly - if I could, I would add tapered shims the rib-to-spar interfaces.

 

I think I might build a rake angle correction fixture - here is the concept:

 

    

 

This fixture would not be very hard to do accurately.  It's just big.....  It will allow the model in its current state of completion to be supported at its aft end by a stout 3D printed buttress bolted to a base plate, and with the model's LG brake housings lying on the base surface.  Two other buttresses are bolted to the base plate in front of the gear struts, and are designed to just barely make contact with the strut at the two unused forward facing lugs:

 

   

 

I can shim here (the orange part), which forces the gear strut to bend aft.  And I can of course control the thickness of the shim.  Nominally, a shim of about .06 inch thickness gets the gear back to the right rake angle.

 

Thing is - the model is stiff.  And I cannot have a fracture or disbond.  That would be a disaster.  I'm sure the struts and the rib can stand that .06 shim alright, but the struts are just going to spring back when I remove the model from the fixture.  So what I would like to do is to gradually over-deflect it by using thicker shims (maybe up to a .12 shim), and leave it for days at a time and see if I can get some permanent set somewhere.  Not sure where....  Remember the upper gear strut is heavily re-inforced with aluminum tube.  Aluminum will take on permanent set before a fracture.  And as we know, even the cured resin can take on some permanent set with time.  But must avoid a fracture.     

 

It's a little risky.  Anybody have any thoughts?

 

 

Edited April 2 by JayW

[Oldbaldguy]

The difference in the angle is based on what datum - waterline, center line of the fuselage, what?  I ask because I don’t understand how you come to the 2.5 degree difference without the model being complete - a datum has to start and stop at two fixed points to be viable.  I would counsel that you take a deep breath and back away for a day or two before trying to coerce parts into doing something they don’t want to for the sake of a perceived error that only you can see.  Two and a half degrees would move the centerline of the struts a total of how many tiny increments at the axle?  Will you - or any of us - be able to discern said difference with the gear doors on?  Once on its wheels, what dimensional difference would there be at the tip of the spinner if you don’t commit some sort of intervention?  Seems to me there needs to be a serious cost/benefits audit before you break out the hammer.

[JayW]

On 4/2/2025 at 6:31 PM, Oldbaldguy said:

The difference in the angle is based on what datum - waterline, center line of the fuselage, what?  

 

Yeah - a water line.  The horizontal red line in the picture is on a panel edge that runs parallel with the upper longeron centerline, which is on a water line.  The camera will give a bit of parallax, but not much.  I can also compare the rake angle with the angle of the firewall which is also on a diagonal.  The gear angle is supposed to be a couple degrees steeper than the firewall angle.  Look at the picture - they are darn near parallel.  So, not right.

 

I am going to try this, but I will also be gentle.  I am not really expecting a great correction, if anything at all, but if I get a degree or so I'll probably be happy.  Right now it is quite noticeable to me.      

Edited April 14 by JayW

[dodgem37]

Is the method of correction you're proposing the right way to make the correction?  Or the short way to make the correction?  The long way to make the correction may be the best.  Changing the insert angle to match your need is one option.  Offering a ball joint is another, though fraught with peril.

 

Good luck, Jay.  Always a pleasure looking in.

Sincerely,

Mark

[easixpedro]

Well, Jay you asked.  I think you're splitting RCHs - hairs. To me it's not worth it. But it's not my project, and you're building for you. The fact that you went that far to investigate and post about it, tells us the real answer.  

 

Good luck!

[JayW]

7 hours ago, dodgem37 said:

Is the method of correction you're proposing the right way to make the correction?  Or the short way to make the correction?  The long way to make the correction may be the best.  Changing the insert angle to match your need is one option.  Offering a ball joint is another, though fraught with peril.

 

Yeah Mark - good ideas for a time before I built it!  Not now.  At this point of the build, any kind of disassembly to get more at the root of the problem is impossible.  Had I anticipated this back when I was deciding on the landing gear support scheme, I would have provided for some sort of adjustability for the strut angles.  Like oversizing the bore in the rotation box in some manner, say.   Instead I just merrily and blindly made it like the real airplane is made - just slapping a big forged gear support rib (or in my case a 3D printed support rib) with its rotation box with the double canted rotation axis straight onto a spar face.  Figuring that will make it all perfect - no fuss no muss.  In the NAA factory in Inglewood back in the 1940's it worked fine, because the parts involved are all fixed in place by major tooling, and the machined parts were made to extremely exacting tolerances.  Whereas for me I am subjected to the vagaries of 3D printing imperfections.  Which have dogged this build in so many ways.  I think I still have things to learn about designing 3D print parts such that deformations like warping or twisting are more minimized.

 

I am going to follow through with this fixture idea, and play with it a little.  I really would like to steepen up that rake angle a bit, and I suspect the elbow on the upper strut (between the strut cylinder and the trunnion) might be able to yield a bit.  Alot of metal in that joint.  We shall see!  

Edited April 3 by JayW

[JayW]

14 hours ago, Oldbaldguy said:

Two and a half degrees would move the centerline of the struts a total of how many tiny increments at the axle?

 

Answer - 0.17 inch measured on the ground line, per Rhino layout.  It is discernable, and it's bugging me.  My brain looks at it and I think about "Easy Rider".  

Edited April 3 by JayW

[JayW]

And so the concept of a gear rake angle correction fixture becomes a reality:

 

 

The fittings:

 

 

 

With Cripes attached:

 

 

The shims:

 

 

The .07 thick shims squeezing on Cripes' struts:

 

 

 

So those two struts are under some fairly significant stress from those little shims.  I will let it stay that way for a day or so, see if I got any permanent set.  Then maybe thicker shims.  Relegated to the boneyard:

 

 

 

Will report out soon.

[brahman104]

G'day Jay!

 

Been a while since I've been on the forums and catching up on your progress. All I can say is simply WOW! Your dedication to getting things right is absolutely second to none, and I would argue, not even a real one!

 

Craig