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

[Antonio Argudo]

🙂 glad to see the canvas boot printed, painted and in place Jay, looks awesome! another ticked box on your amazing build, cheers

[JayW]

So about that brass wire re-inforced tail gear post.  When I put weight on wheels, the tail gear post flexed visibly although within reasonable limits:

 

 

But my word - alot more weight is coming for that single dinky post to support - the entire h/stab and its elevators (like a whole smaller wing on its own), the fin and rudder, and the fillet fairings with back-up structure.  It is likely going to flex too much.  So it became pretty clear to me that I needed a strengthened tail gear.  Recall the current gear post has been re-inforced with an internal .032 inch diameter brass rod.

 

Here is a hard-fought-for .039 inch (1.0 mm) diameter music wire rod, formed to fit inside the gear post halves:

 

 

I was able to use my existing form block after a fashion, even though the rod is a bit thicker.  But man - it is soooo much more difficult to bend into shape than the thinner and softer brass rod was.  On my third try, I got a part that would fit fairly well inside the gear post halves.   However I had to re-design the gear post halves to have a slightly larger clearance trough in the middle, which necessitated a slightly enlarged cross-section at the middle portion to maintain decent wall thickness.  Fortunately, the outer diameters at the axle and where the post inserts into the gear oleo housing are large enough such that wall thickness was not really an issue.  Here:

 

 

The part on the left is the new one; the part with red paint is the old one.  It takes a discerning eye to tell the difference (hence the red paint).  I regard this as a major victory.

 

So what do I get for this strengthened gear post?  A bit of elementary engineering analysis for those interested:

 

The deflection of the gear post can be considered roughly equivalent to a simple cantiliver beam with a point load "P" on its free end, and fixed at its root end:  

 

 

So deflection or flex (y-max) at the free end of the post (e.g. at the wheel), under the weight of the aft end of the model, is (P x L to third power) / (3 x E x I).  Where:

P = the model weight

L = the length or height of the post

E = Young's modulus - a measure of a material's stiffness

I = area moment of inertia - a measure of the beam's cross section geometry like height and width, as they apply to beam strength

 

So to compare the .032 inch diameter brass re-inforced gear post to the .039 inch diameter music wire (steel) re-inforced gear post, the terms P (model weight) and L (height of the post) are the same for each case so can be ignored when comparing the two parts.  Also the strength of the resin post can be ignored, since its strength contribution is much smaller than the internal metallic rod.

 

E (Young's modulus) changes though:  Brass is 17 million psi, music wire is 30 million psi.  A stiffness increase of 30/17 = 1.76, or 76%.

 

I (area moment of inertia, or second moment of inertia) changes as well.  For circular cross sections "I" is defined as follows (the rods having a circular cross section):

 

 

 

Ix = Iy = (pi x r fourth power) / 4, where "r" is the radius of the cross section.

 

The increase in value "Ix" or "Iy", when the diameter of the rod increases from .032 inch (or radius .016 inch) to .039 inch (or radius .0195 inch)  = (.0195/.016) to the fourth power = 1.219 to the fourth power = 2.21.  You can readily see how a slight increase of a circular beam's diameter results in a large increase in stiffness.  BTW same goes for increasing the height of a rectagular beam.

 

Combine the increase factors for "I" and "E" and you get 2.21 x 1.76 = 3.88!!  Or an astounding 288% increase in stiffness.  That ought to cut down that flex alot.  

 

That is what I am getting out of the new part, quantitatively.  And a bunch of peace of mind qualitatively.  We need not worry about the strength of that tail gear post now.  Yay!! 

 

The new post in its proper place:

 

    

 

 

 

   

Edited June 6 by JayW

[geedubelyer]

My brain hurts 

[JayW]

4 hours ago, geedubelyer said:

My brain hurts 

 

Or, by switching from a .032 brass rod to a .039 music wire rod, I nearly tripled the stiffness of the gear post.  That's all I needed to say.  ☺️

[scvrobeson]

Oh gosh, it's like that engineering class I took in college.  As long as it works, I'm a fan of it.

 

 

 

Matt 

[JayW]

The short nickname for the horizontal stabilizer, in industry speak, is h/stab.  Not sure if everybody knew that.  Much easier to say and type.  So that is the terminology you will see from now on.  The h/stab will be the focus of my work for a while, that is unless I decide to skin the aft fuselage first.  That aside, I feel I cannot proceed with the design of the h/stab parts without a better idea of what the fillet fairings are going to look like, and how they will mate with the h/stab surfaces, the fin surfaces, and the fuselage surfaces.  And in order to do that, I must develop the surfaces, or lofts, for the fillet fairings.  A task I have been dreading because they are just plain complicated and will severely test my Rhino surfacing abilities.

 

Here is what I have so far:

 

 

And yes it has been a test.  What you see there is about three days of anxiety, victories, defeats, do-overs, hand-wringing, outbursts of anger, and lost sleep.  The upper surfaces are not symmetrical, due to the 1 degree yaw angle of the fin/rudder.  The lower surfaces are symmetrical.  But I think I have made it through the hardest parts, although I am not very happy with some of the seams - in some cases they will show up as mildly sharp edges or depressions rather than smooth transitions.  More talented Rhino surfacers could smooth out those seams better than I.  But nothing I cannot handle down the road.  After all - often fillet fairings are done simply by applying putty and sanding it to shape.  The rear fairings will be easier (famous last words). 

 

Ultimately, these surfaces will be used to make backing parts similar to the parts I made for the wing fillet fairings, along with a form block also similar to the wing fillet fairings.  All for the purpose of making convincing looking metallic fairings.  I was very pleased how that all worked out on the wing/fuselage.  So I will try it again.  

 

OK - til next time.    

Edited May 12 by JayW

[geedubelyer]

As someone who's sum knowledge of 3D design amounts to zero this all looks extremely impressive Jay. 

 

Keep at it. 

[Hubert Boillot]

Just love the engineering notes ! Do you still do your calculations with a slide rule ? I found again mine a few years ago, and was pleased I still could do the maths with it  . And there is no battery that runs out just when you need it …

 

Hubert

Edited May 11 by Hubert Boillot

[Oldbaldguy]

On 5/11/2025 at 3:16 PM, Hubert Boillot said:

Just love the engineering notes ! Do you still do your calculations with a slide rule ? I found again mine a few years ago, and was pleased I still could do the maths with it  . And there is no battery that runs out just when you need it …

 

Hubert

Ha!  Me too!  I got mine from a retired math professor from Texas A&M.  Pretty sure we went all the way to the moon and back with those things while kids today can’t even tell time.

[JayW]

Last post I neglected to tell you where I was getting my empennage fillet fairing surface information from.  It comes from drawing 73-20002 - Fillet Assembly Empennage:

 

 

Points table and supplementary station curves.  That may look impressive, and it is somewhat helpful.  But it is also maddeningly incomplete.  Anyway I have used it to the extent possible.

 

Let me update you on some Rhino work - been doing alot of that.  I am finding for some reason that this empennage work is particularly fascinating and fun.  And challenging.

 

Last post I showed progress on the fillet fairing surfaces:

 

 

Note the gap between the fin and fuselage is only partially filled in.  Much remained to be modelled:

 

 

  Well this is what fills it in (surface highlighed in yellow):

 

 

Not really a fillet fairing at all, because it is in the sweep zone of the elevator.  So a different shape.

 

Here is a look at the fairings on a real airplane (Lope's Hope, and picture provided by Airscale):

 

 

That aft fairing very much in evidence.  Oh, ignore the fin strake.  Cripes didn't have one.  Here is the Rhino solid back-up part that will be used for the fairing: 

 

 

It will attach to the fuselage WL 10 deck like so:

 

 

The fin box will rest on top of that part.  I probably ought to add a locating post there to help assure an exact fin location.  I'll think about it.

 

And shown with other surfaces (not solids - yet):

 

 

I am getting a better idea what I want to do with the rest of the fillet fairing surfaces and the back-up parts I will need.  Got a plan.  Enough to proceed on the design of the h/stab.

 

Here is what I have so far on the h/stab.  It's been easy.  It will be made from a single internal frame with front and rear spars, and various ribs, and .025 inch thick skin panels.  Plus four major attach points for mating to the fuselage.  Not too dissimilar from what I did for the wing.  So not a LH and RH half - a single big plank that will barely fit on the printer's build plate (I checked).  Pictures:

 

 

 

I show the corresponding four attach point posts on the fuselage WL 10 deck:

 

 

Those four points are not very far apart from one another.  The two forward points are only a bit over 1/2 inch apart, for a stab that is around 9 inches in span.  In theory of course the stab will lie flat and horizontal; in practice, we shall see.  When the time comes, I will have to somehow measure it and "shim if/as required" I guess. 

 

The h/stab shown in place (with skin panels on LH side):

 

 

 

 

On my previous 1/18 scale models except the P-38, my h/stabs are all in halves, with tabs and slots for attachment, and particularly prone to breaking their bonds and becoming loose.  Also the modeler must take care to attach the halves straight (no dihedral, etc).  Surely y'all can relate.  Not going to be the case here.  

 

All right - up to date.  None of these Rhino parts are ready for prime time just yet.  But getting close.  Hopefully I can show parts next post.   Stay tuned!

 

 

Edited May 14 by JayW

[geedubelyer]

Epic! 

[JayW]

Very quickly I will be loading up the printer with STL files and printing parts.  Many parts.

 

Here is a shot of the big Rhino h/stab and fillet fairings layout model showing all of the new parts coming our way in the next week or so:

 

 

Some of the parts shown are ready to go.  Some still have some detail tweaking like panel lines or weight savings subtracts.  That work will go quickly.  By far the most difficult and time consuming work was done on the various fillet fairing solids.  There are eleven separate pieces.  And that doesn't include the fillet portions that are incorporated into the h/stab skins.   Hidden in the above picture is the full span h/stab frame.  It's really going to be something.

 

Very cool things coming your way at the "A55" end of the airplane .  Stay tuned!  

Edited June 6 by JayW

[JayW]

Before I post my progress to date on the empennage, let's take a little detour back into the gear bay.  Dodgem37 (Mark) graciously provided me with some very small springs - springs I just punted on when I was doing the gear bay months ago.  With some real live micro-surgery I was able to get these springs installed:

 

 

You will see two springs at the uplock hook, and one longer spring at the cable link for the gear door.  Welcome additions.

 

A right-side-up view:

 

 

Thanks Mark!  Several LSP'ers are putting their signatures on this big project - let me give kudos:

 

Peter Castle (Airscale) - great decals and valuable Rhino files on some parts

John Kim (Thunnus) - great nose art decals and stencils, and squadron letter stencils and of course the national insignia stencils

Antonio Argudo (Antonio Argudo) - digital design of several canvas parts (seat cushion, head pad, and lately the tail gear bay cover)

 

And now Mark with the springs.  It takes a village!

 

Now - empennage.  Last post I showed Rhino files, this post I show parts:

 

 

H/stab structural components include the full span frame, two nose skins, and four main skins.  The other smaller parts are all fillet fairing components, 11 in all.  I batted 1000 on the printing - all printed just fine.

 

The fillet fairing stuff is going to gather dust a bit, as I work on the h/stab and the fin.  Also skinning must take place on the aft fuselage, h/stab, and fin.  The h/stab components built up without much issue:

 

 

 

Elevator hinge fittings in evidence in that last picture.  I sure get lots of use from my saw horses....

 

 

Mounting points above.

 

I was a little concerned that the full span frame was a little flexy initially.  And it was warped a bit too (a "reverse" dihedral).  But with some massaging, and bonding on the skin panels, the h/stab is no longer flexy at all; now quite stiff.  And it no longer has any warp to it, at least not to my eye.

 

What is it all starting to look like in back - here:

 

 

 

No "shim if/as required" needed.  The stab looks as level as I could hope for.  

 

Next post has to involve aluminum skinning, although the h/stab tips need to be done too.  Ought to be fun.  

 

Edited May 20 by JayW

[LSP_Kevin]

Superb, Jay!

 

Kev

[scvrobeson]

Looks spot on Jay!

 

 

 

Matt