P-51 – Page 2 – Aviation CAD TechNotes

NAA P-51D Mustang: Standard Part Models & Specs

July 26, 2015November 21, 2020Hugh1 Comment

NAA P-51D Mustang: Standard Part Models & Specs.

I have revisited the standard parts I have been producing for this project to verify that the information is correct and in compliance with the latest National Standards and specifications.

As mentioned previously I will be developing the parts for Bolts, Nuts, Washers, Pulleys Turnbuckles etc…in fact everything that constitutes a standard component pertinent to aircraft manufacture.

The parts specified for the P-51 are universal which have been updated over the years and superseded with new part numbers. These parts are suitable for reuse on other projects, in particular the forthcoming Operation Ark project.

To raise funds to support the “Operation Ark” project I have decided to make these 3D Cad parts library available for a small cost.

The part above is the Clevis End (Part #AN161), dimensioned in accordance with the MS21252P 2007 specification. All sizes are incorporated within an Inventor iPart model and in a separate spreadsheet.

“As from 2007 the parts covered by dash numbers shown on AN161 are canceled after 10 December 1971. Steel, carbon and alloy MS21252 parts are inactive for new design. Use only 17-4 PH stainless steel parts for new design and replacement for comparable alloy and carbon steel MS21252 parts and AN161 parts. The canceled AN161 parts and alloy and carbon steel MS21252 parts cannot replace comparable 17-4 PH stainless parts and should be used until existing stock is depleted.”

The CAD 3D model parts include both the AN161 parts number and the MS21252 Part number for comparison. The 17-4 PH number is not included in the model but is listed on the accompanying spreadsheet.

Currently only a few parts are verified; please refer to the Resources page for updates as additional libraries are made available or if you have a special request for a library to be created then drop me a line.

For further details send an email to hughtechnotes@gmail.com

NAA P-51D Mustang: Tail Wheel Assembly: Update.

July 15, 2015November 8, 2019HughLeave a comment

NAA P-51D Mustang: Tail Wheel Assembly: Update.

I shall need to temporarily suspend further work on the assembly model as the remaining parts to achieve a full build are created in a later version of the Inventor cad program and therefore not compatible with the version I currently have access to.

So this is as far as I can go with the assembly, though one could argue that it may be worthwhile including the necessary bolts, washers, turnbuckles etc, but to be honest most of this is planned as the final components in the build. The main reason for this is to ensure that everything aligns properly and works according to the design intent before plugging in all those connecting bits!

I have some tidying up to do with the fuselage frames and to develop that library I was talking about for the aeronautical standard parts and components…so perhaps this may be the time to get this done.

I also plan to do some 2d detail drawings for some of this modelling to record some of the key information that I have had to research separately from the archive resource and create the Bill of Materials structure that complies with the existing NAA documents for the complete assembly.

The 2d drawings will also serve as a dimensional check as these objects were built in mm whereas originally they were designed in inches.

Its very hard to identify small dimensional discrepancies when just reviewing the 3d model!

So for now I probably wont be posting too much on the modelling side of things but may include some new cad technotes on the techniques I have used in this project.

NAA P-51D Mustang: Tail Wheel Retracting Hydraulic Cylinder

July 13, 2015July 16, 2015HughLeave a comment

NAA P-51D Mustang: Tail Wheel Retracting Hydraulic Cylinder.

Hydraulics is not something I have had very much exposure to in my varied engineering career, so it was rather interesting to build this Tail wheel retracting cylinder and learn some new stuff about the hydraulic designs of this era.

All the component parts are fully detailed in the NAA drawing archive enabling a complete cylinder to be built with the pipe fittings added from the Inventor Content library.

The Autodesk Inventor product has a very comprehensive standard parts library which includes a wide variety of pipe fittings and components. The elbows and reducers are from the Parker range which are sized correctly but slightly different in style to the aeronautical standard parts which would normally be used.

I did modify the hex head for the reducer to size correctly with the corresponding AN912 aeronautical part to ensure correct fitting with the cylinder interface.

When I have time available I intend to create a special library for all these standard components that will correspond exactly to the specified aeronautical standards.

The blue support brackets on either side of the cylinder should actually be fitted to a sheet metal formed channel, which I don’t have the details for. There is a drawing for the P-51B/C models which will be similar to what I need but the lower station frames in this area are slightly different. I can’t be sure exactly how the channel should be fitted so I emailed a few companies that have been involved in the restoration of P-51D Mustangs to see if they can assist with either photographs of this area or even better some drawings!

NAA P-51D Mustang: Project Cad Technote Multi Body Parts

July 7, 2015July 12, 2015HughLeave a comment

NAA P-51D Mustang: Project Cad Technote Multi Body Parts

The process of developing these drawings into accurate 3d models relies on maintaining the hierarchy according to the original NAA drawings, even if sometimes it gets a tad confusing when dealing with what constitutes a “sub-assembly” as I mentioned before.

The sub-assemblies I described as “Part Assemblies” as the assembly unusually comprises a fully detailed part inclusive of additional items like bearing, spacers etc.

I have reviewed my approach to how I deal with this and thought it may be prudent to write a quick note on this technique.

I am utilising the multi-part feature within Inventor for this, which allows you to model separate solid parts within a single part file and then create an assembly that comprises some or all of these solids.

This is a scrap view from the NAA drawing showing an assembly that has 2 configurations based on varying paired angles with spacers and rivets as shown.

Each of these items has a suffix added to the part number i.e -1, -2, -3 etc.

These images give you some idea of how I have modeled this, with the first image showing the configuration of items 2 & 3 and the second showing the configuration of item 2 & 4; all in one cad part file.

The beauty of working with multi body parts is that you only need one set of sketches that can be shared between all 3 parts.

The sketches are dimensioned exactly as the original drawing…I mention this because I would not normally dimension from the edge of an angle section (cut edge); its not really good practice!

The image on the left shows the feature tree within Inventor; listing the 3 solids with appropriate suffixes.

The part file name (at the top) comprises the NAA drawing number with a suffix noting the archive reference.

All I have to do now is create an assembly for each of the configurations and add the relevant spacers and rivets. This is done very quickly using the “Create component” feature. The assembly number will comprise the NAA drawing number suffixed with either a -1 or a -5 respectively.

Only assemblies created from a multi-body part will be suffixed with a numerical character, otherwise they will simply be suffixed with SA.

Using this technique we maintain the integrity of the NAA numbering system with an hierarchy that suits the CAD strategy.

In a previous post I discussed “as-fitted” parts; like bushes; that might be press fitted and and reamed thus dimensionally different from the manufactured part, so these will still be modelled within the part file to “as-fitted” state and not brought in as a component of the sub assembly.

NAA P-51D Mustang: Tail Wheel Project Update

July 7, 2015August 8, 2015HughLeave a comment

NAA P-51D Mustang: Tail Wheel Project Update

This project is growing arms and legs; every time I check back to the NAA documentation I find yet another part associated with the assembly in this area.

I am beginning to appreciate just how complex the interaction is with all the parts that share this very small space and wonder sometimes if I will ever complete this task!.

For this period of build I have had to revert to an earlier version of Inventor; which unfortunately means many of the parts already modeled cannot be included in the assembly build at this time as the version variants from a later release will not be compatible with this one. Also the material finishes are not as good as the Inventor 2016 as you can see below.

So I am focusing my attention on building the supporting elements for the Tail Wheel mechanics; including the fuselage frames local to this area.

The fuselage frames are surprisingly complicated to build, partly due to the limitations of the software but also due to the flanges having to align with the surface form of the main fuselage as shown. I mainly used the sculpting technique but found that it is not possible to apply a fillet to the edge of a sculpted solid that is derived from a spline curve, so these had to be added when creating the lofting sketches.

I have added a few parts (where I can) for the tail Wheel assembly; these parts in blue; and also an additional component in yellow which is a Support Assy – Rudder & steerable tail wheel control bell crank. This part by the way was a nightmare to build trying to get all the edges to align correctly with the sloping webs.

I have mentioned before the importance of having quality copies of the original materials to work with and this particular archive (from FlugArchiv) was done to a very high standard.

2015-07-07_02-06-47 Occasionally though you do get the odd drawing that is almost impossible to use but having gained some experience in developing these aircraft structures it was not too difficult to determine the missing information.

This is the one I have for the lower section of one fuselage frame.

NAA P-51D Mustang: Tail Wheel Assembly Update

June 27, 2015June 29, 2015HughLeave a comment

NAA P-51D Mustang: Tail Wheel Assembly Update

I rather enjoy building these Cad models and the challenges they represent, however I still have a lot of work to do with organising the archive materials and the document register; which I briefly mentioned in previous posts.

I have decided to adopt a BIM methodology for the organisation and restructuring of the archive datasets and to look at the options for managing the content and format of the tagged properties within the 3D CAd models.

This all needs to be done, so I will take a break from modelling the parts for a while to concentrate on developing the above. So on this last day I decided to tackle some of the smaller parts for the eventual Tail Wheel assembly for the P-51; which surprisingly were interesting parts to build.

#73-34189: Bell Crank: Steering Pulley #97-34172:Retainer: spindle lockpin

# 97-34191: Bracket Torque Tube Pulley #73-34515: Bell Crank – Retract position lock

There are also a lot of parts that don’t have drawings in this archive; mainly aviation standard parts for bolts, washers, seals and bushes for which I need to knuckle down and source the dimensional information so that I can build the requisite part model libraries.

There is still a lot to consider as I move forward with this project!

NAA P-51D Mustang: Tail Wheel Down Position Support

June 26, 2015September 20, 2018Hugh3 Comments

NAA P-51D Mustang: Tail Wheel Down Position Support; Derived Parts (Inv)

I mentioned in an earlier post that we don’t have many of the forgings/castings for this aircraft but the few that we do have are not stated as such in their description and thus occasionally overlooked.

In this case the forging/casting was noted in the NAA machining drawing; which I do have. This gives me an opportunity to explain one of the strengths of the Autodesk Inventor product, namely derived parts!

Derived parts are a powerful but easy-to-use tool that comes in two basic flavors: you can derive a part from another one, or you can derive a part from an assembly. Using derived parts, you can easily create machining models and drawings from an as-cast model, and you can create a mold from the same model.

These images show the casting model I created from the original NAA drawing #73-34162 for the Tail Wheel Down Position Support.

This model took me quite a while to do due to the creation of all the fillets which got a bit crazy sometimes and I ended having to redo them several times to get them the way I wanted.

The machining model is a separate Cad part file created from NAA drawing #73-34161 which has the casting body and sketches derived from the first Cad model above. I can now go about working on the derived model; creating the machined elements and holes; without affecting the original model file above as shown.

The great thing about working this way is that should the original casting model change then this will be propagated to any other cad part files to which this item is derived but conversely any changes in these Cad part files are not reflected in the casting model.

I still have a few minor details to finish this model but thought it may be prudent to touch on the derived part capabilities of the Inventor product.

Another use for derived components is when you only have Inventor LT (Lite version) which is a parts only product and unlike its big brother does not handle assemblies. Using the derived feature it is possible to create a proxy assembly for checking the alignment of parts as shown below.

This is the armor plating for the Mustang P-51 Firewall; with the top section modeled separately from the bottom section. In this example, I have derived the top part into the lower part file as a surface model to assess the alignment and curvature continuity.

NAA P-51D Mustang: Tail Wheel Progress Update

June 22, 2015June 5, 2021Hugh2 Comments

NAA P-51D Mustang: Tail Wheel Progress Update

I’ve been busy building the 3d models and working through the vagaries of the Mustang P-51 Tail wheel assembly. This is a selection of the new parts built to date:

Mustang P-51 Spindle — #73-34004 Housing Tail Wheel Spindle

Mustang P-51D Spindle — #73-34004 Housing Tail Wheel Spindle

Mustang Idler — #99-34153 Support: Tailwheel Declutch cable idler

Mustang Bell Tail Wheel Door — # 97-34502 Bellcrank: Tail Wheel Door

#73-31114: Support: Tail Wheel Shock Strut

2015-06-23_14-51-42 — #73-31114: Support: Tail Wheel Shock Strut

Mustang P-51 — #97-34120 Mast Assy – Tail wheel steering

This last image is a photo showing the Housing for the Tail Wheel Spindle with the model in a similar orientation and finish for comparison.

For the latest version of these models please refer to this article.

NAA P-51D Mustang: Tail Wheel Housing EndCap

June 20, 2015June 26, 2015HughLeave a comment

NAA P-51D Mustang: Tail Wheel Housing EndCap

This item #91-34005 is the cap to be fitted to the end of the tail wheel housing described in the previous post.

The drawing that this was based on details the finishing and machining from a forging. Unfortunately and is now seeming to be quite typical for the tail wheel area again I am missing key information…its not that the drawing is lacking the necessary detail for its intended purpose as it is based on an existing unit its just that I don’t seem to have the existing forging drawing as a reference.

The missing information; relating to diameters and material thicknesses; would have been on the forging drawing and the draughtsman did not really have any cause to replicate this information on his finishing drawing…though it would have been enormously useful!

However this is not always the case as depending on the draughtsman some do include this type of information as reference dimensions.

I have therefore interpolated the missing details as best I can based on the information available and where possible cross referenced against other drawings.

The forging drawing is actually listed in the NAA register but the archive record lists the wrong reference drawing so its likely that there is a copy of this forging somewhere in the archive!

For my initial purposes though I think the data is sufficient for the end goal of creating a working tail wheel assembly as part of the mechanism study.

NAA P-51 Mustang: Tail Wheel Housing

June 19, 2015January 7, 2021HughLeave a comment

NAA P-51D Mustang: Tail Wheel Housing

In my endeavors to develop the assembly and parts register I have made some progress determining the associations in conjunction with the listing in the NAA documents. I have started with the Tail Wheel assembly,which actually has 59 drawings for the sub assemblies and parts…it was surprising the number of drawings just for this one area!

This is a partial screen shot of the assembly register; again divided by sub-assembly level down to the individual parts.

Having collated this information I started work on building the 3D Cad models, starting with the Tail Wheel Oleo Housing.

This housing is perhaps the most difficult model in the series for the Tail Wheel and it has given me problems.

The main lower bracket struts are shown in the end view on this drawing as being tangent to the main body housing but the side view suggests that the alignment is lower than the centre and therefore not tangent. The main body is tapered from the middle of the main body so a tangent with the lower struts would be difficult to achieve…I know I tried it and the geometry was complex.

I wondered if this should be tangential as is typical of these types of castings. Sometimes its not too clear what the intent of the designer was and this drawing sure would have benefited from the inclusion of an additional section through the strut.

I couldn’t be sure what the intent was here and the plan view did not present any new information, however I did notice a small detail for one of the side brackets with a solitary single line that turned out to be a reference for the interface with the struts and confirmed that in fact this item is not tangent!

This is the work-in-progress model which shows the generated profile of the junction between the main body and the lower struts, which satisfies the main details of the drawing.

As it turns out the strut is tangent but only to the edge of the end of the main housing and not the main body itself. There is also a glimpse of a side mounting bracket which will be quite complex to model due to the nature of the curved profiles that will need to be developed individually and then lofted to define the final surfaces.

Update: Finished model for the Tail Wheel Housing item 91-34003. To complete this sub assembly I just need to add the bushings.