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

[patricksparks]

2 hours ago, JayW said:

 

Well shoot!   So this B-17C - is it done?  What does it look like? 

No, sadly the 17 was put into a box of doom a while ago, but it will probably see the light of day again now that I have a significant amount of months of 3D drawing and printing a mountain of parts(good and bad...). Looking forward to your progress on your new project, I have really been shifting my modeling to projects using the 3D route, it definitely opens up so many possibilities!!!

Pat

[JayW]

Alright - these test parts are very important.  This model is supposed to be primarily made from 3D print parts, so as to assure great accuracy of shape.  But what I am finding is that my parts have inaccuracies if not outright flaws.  Much of this was unexpected, and could be operator error.  By that I mean I am not printing parts off as well as other real experts could - due to the resin I use, or the machine settings I have created (exposure time, lifting speeds, layer thickness, aliasing), or how I am orienting and supporting the part during the printing process.  But it also could be I am expecting too much of the 3D printing process.  I am asking for some serious exactness.

 

Take a look at my prop spinner parts - I already learned from test parts, and have applied some lessons learned.  Also Alain suggested I print the nose upside down, which I wanted to try.

 

Recall the test parts:

 

 

Terrible fit, wavy edges.  Part of the terrible fit, BTW, are from some subsequently discovered small but nonetheless serious clashes with the prop roots.  I fixed that for the "production" parts.  Those wavy deformed edges are serious:

 

 

If I do not get a handle on that, this whole project is going to be a no-go.  Peter (Airscale) has been having similar issues with his 3D print work and is working on it in parallel.  Somehow that has to be addressed - all my parts are going to have edges of course.

 

The "production" parts:

 

 

Note there are two nose halves and one aft half.  The nose half to the right was supported in a traditional way, tilted about 45 deg with nose up, so that all the supports were on a portion of the aft edge.  Compared to the test part which was supported the same way, I only increased the number of supports to try to get rid of the local waviness.  It worked in that there is no waviness.  You can see remnants of those supports along the edge.  The other nose part was printed upside down at a slight tilt, where all the supports were on the nose surface itself.  That ought to leave the aft edges pristine.  You can see the little marks the supports made on the outside surface after snipping them off and sanding smooth, just off the "pointy end".  The third part of course is the aft half of the spinner. 

 

To my great dismay, the traditionally supported nose half has a large deformity in the region where it was supported:

 

 

No waviness, but a giant mismatch with the aft half of the spinner.  This part goes in the trash can.  Why oh why did this happen?  Well, clearly it was stretched and deformed during the printing process.  That corner was the first to be printed.  And as more and more layers are then added, the part gets heavier and its weight is pulling on those first small layers.  Also, after each layer is baked on, the build plate is lifted to allow resin to flow back underneath it so the next layer can be created.  There is some stiction force associated with that motion, which also pulls on all the layers previously laid down.  One or both of those phenomena might be causing this.  Perhaps if I spun it 45 degrees so that the first layers are not coincident with the prop root cutout?  Then the first layers should be more robust.  I dunno...  Or, maybe my exposure time (3.5 seconds per layer) is insufficient to give a good hard material?  Again, dunno.  That is the recommended exposure time for this stuff at 5 microns layer thickness.  The aft half, which was oriented and supported in a similar manner, suffered the same fate although less severe.  It is, however, a bit out-of-round as a result.  That part will also go in the trash can.

 

Then there is the other nose half, which was printed upside down:

 

 

Now THAT'S what I am talking about.  Thanks Alain - good idea!  Good edges, decent match up with the aft half.  That is a useable part.  It has a bit of a rash from the supports:

 

    

 

But some putty or CA will cover that up.

 

So I think I am about 80% of the way there.  A few more shots that seem to indicate a more hopeful situation:

 

 

 

 

The prop blades are just laid in there loose for now.

 

So, I have a useable spinner front half, and an un-usable aft half although quite close to being OK.  Next post I hope to show a useable aft half which is perfectly round at its aft edge (currently it a bit off), and not wavy or deformed.  And then I can declare victory and move on with some assurance that this 3D print process will actually work OK. 

 

Bear with me, some may find this a bit painful to follow.  But it is here where I can determine whether or not this project will succeed.  Stay tuned.    

 

    

 

 

 

 

Edited September 15, 2023 by JayW

[LSP_Kevin]

Excellent progress, Jay! With your first spinner cone, rotating the part so that the heavy supports are not coincident with the blade roots would likely yield better results, as you suggest. With the second one, I recommend filling the support divots with more of the same resin, and curing it with a UV torch, rather than using CA or putty. This will fill like with like, with no risk of differential sanding. If the resin filler still seems tacky after you've applied and cured it, just give those spots a quick rub with a cotton bud doused in IPA before sanding.

 

Kev

[Woody V]

Boy, you crazy.... 
In other words "SUBSCRIBE"

[JayW]

I have all manner of good news to report. 

 

First the bad news.  Recall that I have been trying to make spinner halves that are essentially solid, with holes for the prop roots and prop shaft.  Like this:

 

 

Also recall that I have been having maddening deformity issues especially at the point of the initial print layers - a stretching of sorts.  Also out-of-roundness.  Ugly.  Well, I tried a couple more times with revised and enhanced supports - hours of printing.  Still failures.  

 

So I tried a completely different approach - thin shelled spinner halves with a separate prop hub (since the spinner halves would no longer be able to support the prop roots).  And, bulkheads for mounting the spinner to the hub.  BTW, this is how the actual spinner is on P-51's:

 

 

 

It was suggested by Maxim61 and Peter (Airscale) that the aft half of the spinner be printed directly on the build plate rather than supported in mid air by a hundred supports (perhaps that would address the deformity issues).  So I tried that:

 

 

 

What a beautiful part.  The shape is perfect.  The challenge would be to free it from its sacrificial base without fracturing it.  And I fractured it.  But, it was repairable with thin CA, so no harm no foul.

 

And here are the other new concept parts, fresh off the printer waiting for the post-curing station (the simplified hub and two bulkheads):

 

 

Here is the collection of useable production parts, post cured and ready to go, for the very first component of this P-51 build:

 

 

Assembled:

 

 

 

 

Victory.  Gone are the deformity issues.  I am a happy camper, although it disturbs me how much pain and suffering it took to get these parts right.  Hope in the future I don't have such trouble, but I am learning.  Wish I knew what color the spinner will be - then I would just finish it.  Note that these prop and spinner parts are common to the B/C and the D.  So in theory I do not have to decide what I will do yet.  In theory....

 

Kevin suggested that I could cover the little pimple marks from support removal by dabbing on uncured resin, and then curing it with a UV pen.  Well Kevin - sunshine works too.  Thanks for that excellent suggestion - look at this:

 

 

That nose cone was a bit ratty with pimples and now it is practically perfect.  A coat of primer will hide any remaining (very tiny) imperfections.  BTW - that big hole at the forward end (.125 inch dia) is going to be filled with plastic rod and sanded to shape to simulate the rubber plug on the actual aircraft.

 

Meanwhile - continued surfacing in Rhino 7.  More of the engine cowl area, and importantly - the B-model specific inboard extended wing leading edge.  Pictures:

 

      

 

 

 

 

The P-51 expert knows that the -B/C wing has a different (significantly smaller) inboard leading edge extension than the -D.   I had ordinate point data for the -D, but nothing on the -B/C.  So I did some intensive study of drawings (wing rib cross-sections, etc), and came up with a damn good approximation.  Interesting how it droops going inboard....yup - it really does.  If any of you aerodynamicists can explain why the -D has that over-extended leading edge, please explain!  I cannot figure it out...  Look at the difference:

 

 

I believe y'all have seen the last of 3D printed parts for a while, unless I just get sick and tired of the surfacing project.  I have very tough surfaces to come - wing fillet fairings, the radiator scoop, the fin and h/stab fairings, and the cockpit enclosure (including BTW the Malcolm hood).   At this point, I have run out of excuses not to do a -B/C.  Unless something unforeseen happens, I think the decision has been made. 

 

Speaking of the Malcolm hood, look what Mike McM got for me:

 

 

 

I have some reason to believe that fishbowl cross section for the hood is accurate.  It looks to be a simple circle.  Who'da thunk.  So that takes a bunch of guess work out. I like!

 

Don't go away as I continue to surface!

Edited September 9, 2023 by JayW

[Antonio Argudo]

2 hours ago, JayW said:

If any of you aerodynamicists can explain why the -D has that over-extended leading edge, please explain!  I cannot figure it out... 

Mr. Google says  "The B may of enjoyed a bit more maneuverability being 900lbs lighter with a normal load than the D. But then the D had 2 more . 50"s than the B and a thicker wing so the guns could sit upright, where the B they were canted at an angle and caused problems with the feed system"

Edited September 8, 2023 by Antonio Argudo

[JayW]

33 minutes ago, Antonio Argudo said:

and a thicker wing so the guns could sit upright,

 

Antonio I have read this more than once over the years.  Yet, I see no indication that the basic wing contours are any different between the two.  I wish I had the -B wing ordinate drawing (I only have the -D) so I could do a point-by-point compare.  Instead I did some scaling from wing rib drawings to see if ribs are deeper on the -D.  And they do not appear to be so. They are the same.  I don't know how 3 fifties got crammed in there versus slanted 4 fifties, there it is. 

 

So with the trimmed down rear fuselage on the -D, if skin, frame, and stringer gages were the same, I estimate about 40 pounds less airframe weight for that part of the D model.  Everything else being the same that results in some amount of nose-down pitching moment compared to the B-model.  Perhaps that's a big deal for the aircraft's balance??  Were that the case, that nose-down pitching moment would need to be offset.  Extending the inboard leading edge would provide a nose-up pitching moment.  Just spitballing here....     

[TAG]

Hey dudes,

 

Re: the canted Ma Deuce's in the B/C wing, as Jay pointed out the wing/airfoil was exactly the same in all models, including the X73 prototype, Mustangs I & IA, P-51, A-36, P-51A/B/C, and P-51D/K . Apparently the canted gun mount goes back to the belt-fed Hispano 20mm cannon armament option that the USAAF chose not to accept after the -A model, for some unknown reason they just never changed the mount on the B/C wings, unlike the D/K's which had their guns fully upright on a new mount. The B/C's canted .50's would inevitably jam from the G-forces being applied during aerial maneuvering in combat, but in the words of Lt. Col. Richard Turner, CO of the 354th Fighter Group, "this deficiency was corrected in short order by a modification figured out and installed by our own ingenious and hard-working ground crewmen. They liberated some ammo-booster motors from a B-26 group nearby and rigged them on our fighter’s ammo belts which for all practical purposes eliminated all such stoppages in the future. In the later model P-51D there were six guns, but all were mounted upright, effectively solving the former problem."

 

As for the Malcolm hood, according to Col. Turner "this modification gave the best visibility that I was ever to enjoy from the cockpit of a fighter. I could actually lean out of the cockpit and look under and behind my own fighter’s tail area, a very comforting bit of insurance to have available during a large helter-skelter mixed up dogfight. The P-51D which followed had a one-piece plastic canopy that approached this visibility, but which did not quite equal it."

 

Wow! Who'd a thunk it, right? Guess that Malcolm hood was literally like a fishbowl!

 

Here's a pic of Lt. Col. Turner and "Short-Fuse Sallee", his -D model Pony.

 

Here are the excerpts of Col. Turner's testimony from "Fighting Mustang", authored by William Hess in 1970. It's a fascinating read!

 

And just for good measure, here's a photo of two restos, a B/C and a D/K flying in tandem, good plan views of both wings right next to each other for comparison:

 

Hope that helps!

- Thomaz

 

 

 

 

 

[MikeMaben]

15 hours ago, JayW said:

Speaking of the Malcolm hood, look what Mike McM got for me:

 

 

Please give all credit to Mr. Jumpei Temma. Project looking great Jay.

[Cap'n Wannabe]

You can get airfoil types from here.  The P-51 B/C/D used the exact same airfoil, ie the NAA/NACA 45-100, other Mustangs such as the V and VI used the NACA 66-(1.8)15.5 at the root and the NACA 66-(1.8)12 at the tip.  What those numbers mean is beyond me except that they're something to do with chord vs max thickness.

[JayW]

16 hours ago, TAG said:

Here are the excerpts of Col. Turner's testimony from "Fighting Mustang", authored by William Hess in 1970. It's a fascinating read!

 

Thanks Thomaz!!  Pretty sure everyone enjoyed that read.  So, part of why I am so enamored by P-51's is that my late uncle Henry Rudolph was a Mustang driver and served in the same fighter group as Dick Turner - 354th FG.  However he was in the 353rd fighter squadron whereas Turner was in the 356th.  Lt. Rudolph flew several aircraft, one being a P-51B (wish I knew the serial number - I would make it my subject if it had the hood) and more than one P-51D's.  One of the last ones was P-51D-20NA serial number 44-63685, buzz letters FT-J.  Here it is, supposedly, in March of 1945, after "coming to grief" in a training accident resulting in the death of another pilot (not Hank).  

 

 

 

I have learned that one cannot hang his hat on the buzz letters to identify aircraft, as they were used more than once on different aircraft after the original one was lost or retired.  Who knows if this is really "Sissy Mana".  But the six kills align with his record.  Also, there is some confusion as to which aircraft carried the name "Sissy Mana" (which was named after his sister - my mother).  It is believed by some that it was 44-63885 as opposed to 44-63865.  And 63885 survived the war and is actually a restored flying aircraft today painted up as "Tempus Fugit".  Some of you know already I have been hunting far and wide for pictures of the nose art of "Sissy Mana" should any exist.  Were I to find it, I would immediately switch over to a -D model and use it as my subject for this project, and once finished present it to my still living mother with her nick name painted on the cowl panels, should she (and I) live so long!  

 

Late in his life, Uncle Hank described some of his adventures with the 353rd.  I believe he did not enjoy speaking of his victories, but he freely described his harrowing escapes.  Just as Col. Turner describes, Hank also survived a compressibility dive early in his combat days with the group, recovering from the dive with an altitude margin that brings back bad dreams decades later.  He also claimed he was shot down in a protracted one-on-one dogfight that devolved into a low altitude duel over occupied Belgium (or Holland or France?), survived the belly landing, evaded a strafing run by the victorious German pilot, evaded capture by the Gestapo, and was later delivered by the underground to his unit.  But curiously there seems to be no record of it.  Some other exciting stories too, some perhaps embellished a bit by a sentimental and a bit senile old man.  What a way to spend one's twenties.

 

  

 

 

Edited September 9, 2023 by JayW

[JayW]

16 hours ago, TAG said:

And just for good measure, here's a photo of two restos, a B/C and a D/K flying in tandem, good plan views of both wings right next to each other for comparison:

 

Look at the difference in the inboard leading edges!  Changing that from the -B would have been a big deal.  So there had to be an important reason to do it, and I am betting it was a Center-of-gravity versus Center-of-lift thing.  

[Ralph-D]

Hi Jay,

 

do You know this link:

 

https://hughtechnotes.com/2016/06/04/mustang-p-51-bc-ordinates/

 

Your prop design is great, can You tell me the referens  / doc pages you got the design data for the Hamilton prop.

I have an acout at aircorpslibrary and can check by my self.

 

Regards

Ralph

[JayW]

18 hours ago, Ralph-D said:

Hi Jay,

 

do You know this link:

 

https://hughtechnotes.com/2016/06/04/mustang-p-51-bc-ordinates/

 

Your prop design is great, can You tell me the referens  / doc pages you got the design data for the Hamilton prop.

I have an acout at aircorpslibrary and can check by my self.

 

Regards

Ralph

 

Ralph!!   I did not know.  I have contacted Hugh to see if he can help me with the -BC inboard wing leading edge ordinates - the only data thus far that I don't have.  Thanks.

 

As for the prop - get ready for a whirlwind.  I wish I had access to actual design data, but Aircorps Library does not have (at least I couldn't find it).  What they do have though is an overhaul manual that contains geometry data.  Go to the "Props" section, click on "Hamilton Standard", then go down to the document "Hamilton Standard Blade Repair Manual", part no. 130, revision date Oct - 1947, page count 204.  On page 10 of the document is a table for various blade designs.  Go to blade design number 6523A - this is the basic prop blade for the P-51BCD.  Note the shank size is "D".  You will find the shank size dimensions on pages 36 and 37.  Note also in the remarks column "6487A with AMS 4130 alloy".  It is that 6487A blade that is in the geometry tables (not the 6523A).  Also note that geometry data is listed as pages 155-160.  And also page 193.  But that is wrong - actually it is page 189, not page 193.  Always look for data for blade 6487A.  

 

I would encourage you to read the whole manual (except the geometry data that doesn't apply to our blade type).  That way you will better understand the terminology.  That is what I had to do to make sense of it all.

 

All this data is for the basic blade - nothing for the cuff.  For that I obtained some information from the P-51 SIG back when I could access that website (I no longer can for some reason).  Let me send you a note with that data.   Soon.

[Ralph-D]

Hi Jay,

 

Welcome and thank you for the detailed information + note later.

Do you know or have the document "Mustang Modeller & Enthusiast". I have it and send PM if you would like to have it.

There is also information (sketch) about the propeller in there and on occasion I will compare it for interest.

 

Many years ago the Swiss Aviation Museum exhibited one P-51 prop, I took photos and there I discovered why the spinner cut has a shape of 8 (because the root part is oval and not round).

 

The P-51 SIG is closed for good, unfortunately, I was there too.

 

BR

Ralph