3d Cad – Page 2 – Aviation CAD TechNotes

NAA P-51D Mustang: Project Cad Technote; iParts

July 15, 2015July 22, 2015HughLeave a comment

NAA P-51D Mustang: Project Cad Technote; iParts

When it comes to organising standard parts using a Cad system like Inventor there are various ways to achieve this. Initially I considered a custom content library or even an iLogic expression linked to a parameter spreadsheet but I settled on using iParts.

The main reason for this is due to the fact that I already have a plethora of data contained in many spreadsheets for everything from ordinates to document registers and at any one time one or more of these spreadsheets is usually open for reference. Therefore the iparts seemed to be the ideal choice by maintaining all the relevant data in a single Cad part file.

A simple example of this would be for the AN960 standard washers. I could have done something really clever here as the actual part number contains references to the physical sizes and properties of the washers and I had thought it would be great to link the naming convention to the parameters.

However there is no real benefit to be gained from this and would have added a level of complexity that’s quite frankly unnecessary for this type of component.

We have 3 dimensions that define the washer; the Outside Diameter (OD), Inside Diameter (ID) and the Thickness (Thk). We also have a material type but the Cad library will need to be updated to include the specifics of the materials for a P-51 mustang, which is another custom job; so I have ignored it for now!

The above sketch shows the expressions of the parameters defining the relationship of the values as declared in the parameters dialogue; this is where it gets interesting.

I should note that the template and default units for this model is millimeters. The standard units for the washers is inches.

This image on the left is the parameters dialogue box to which I first added some user parameters (1) set to “inch” units. I then created the cad model dimensional parameters (2) and linked those to the user parameters (1) with the units set to “mm” (3). The wonderful thing about this is that Inventor will adjust the values based on the unit type automatically; so just by changing the unit type the value will change accordingly, which is verified in the nominal value column (4)…great stuff!
This is the iPart creation dialogue, showing the table of values, input from the standard catalogs in “inches”.

Its very important that the original values are retained as “inch” units so that it is easier to check and verify the correctness of the information and traceability.

Tip: If I already had these values set-out in exactly the same format in excel I could just copy and paste the spreadsheet directly into the iPart table.

At some stage I will add the material values to the end of this table for each of the components listed. Some examples of iparts include the Locking Stud and Clevis Fork; colour coded to differentiate size..

The notion of working with different units is made so much easier by the capabilities of these cad systems. Essentially when inputting the dimensions in a model sketch the value of the dimensions will change if you select either inches or millimeters according to the default template units setup for the cad model; it will even work with fractions.

For example if you type in “3/4 in” for a dimension in a sketch based on the “mm” unit template then the actual value for the dimension will be “19.05 mm”.

Another example; 12 23/64″; for this you type in 12 leave a space then 23/64 followed by “in”…”12 23/64 in” gives us “313.928 mm”.

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 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-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.

These models and several others are now available on CGtrader

Messerschmitt: Bf109 Part

June 11, 2015June 19, 2015Hugh1 Comment

Messerschmitt: Bf109 Part

Scans of the drawings for the Bf109 have long been part of my archive, but I was always reluctant to spend anytime on this aircraft as many others have already done a great deal of work producing CAD data from these designs.

However and occasionally I come across an item that presents a challenge and this part is one of those.

8-10910-2501: Lower Frame mount.

This is a cast aluminium part with a 3 degree draft on all faces (refer second image showing cross section) except the lower edge and the top edge on centre.

The challenging aspect of this part has to do with the top face, where it was important to achieve tangency with the draft faces at each of the mounting holes and maintain this through to the centre portion where the face locally is perpendicular.

The material thickness also increases marginally at the extent of the 2 inward holes.

The front edge of this face has a center portion of a fixed radius; so to achieve the desired results based on the above criteria for the back edge I used a tangential spline. I could have just used a radius arc instead of a spline as the difference was only 0.2mm, but I am keen to get this stuff as accurate as possible.

It worked out rather well.

North American P-51 Mustang: Fuselage

June 2, 2015July 11, 2015Hugh2 Comments

North American P-51 Mustang: Fuselage

The drawing archive I have contains quite a large selection of legible fuselage frame drawings which I am collating according to the Station reference on the fuselage. I have a spreadsheet that lists all the Mustang drawings including the original drawing number, the scan image number and location within the archive.

Each fuselage frame at each of the designated stations may comprise 3 or more elements, which unfortunately are scattered throughout the many rolls of scans thus requiring some exhaustive work with the spreadsheet data-sets to sort the numbers and folder locations in order to identify and collate the required frames for each assembly per aircraft type.

One such frame was at station 216 which I decided to model; partly due to the fact I was getting fed up looking at and sorting spreadsheet data.

There are several methods to modelling this and whilst I was subject to the vagaries and still limitations of the Inventor product (Solidworks has more options for working with splines) I developed a workflow that obviates some of these limitations and also how the end product is finished.

One way of doing this is to simply create a surface for the main plane and then project a flange line along the edge to create a”folded” surface and then apply thickness but this method gave some unusual iterations in the smoothness of the fillets at the end of the profile. I found that the best way is to create surfaces for all six faces; the splines inside and outside, the top and bottom planes and the ends, then sculpt to create a solid.

I would then go out about creating the notches and cutouts in the solid and then shell the solid to the required thickness. This works very well and ensures the integrity of the original spline ordinate lines (which would have to be split to do this any other way). This method also maintains better continuity of the end fillets and curvature (image 2).

The frame drawings reference the mold line ordinates, which I have for the P-51 B/C Mustang variants.The P-51D is similar with the exception of the ridgeback on the main rear fuselage that has been reduced above the +10″ W.L.

Techy stuff: I mentioned a limitation in the Inventor software which relates to creating a line perpendicular to a spline. In Solidworks you just sketch the line and constrain it perpendicular to the spline, but you cant just do it this way in Inventor (as far as I know). What I did was use sketch construction lines to define the point of intersection with the spline that I wanted the perpendicular line to start from. As I already had a surface projected from the mold frame spline (for above construction) all I had to do was create a new plane perpendicular to this surface at the selected point. It was then quite simple to create a further sketch to define the line I wanted perpendicular to the spline at the correct location.

De Havilland DH82c Tiger Moth

May 31, 2015September 19, 2015HughLeave a comment

De Havilland DH82c Tiger Moth:

This is a selection of parts developed in 3D CAD for the Tiger Moth DH82c (Canadian variant). I had hoped that the original drawings and corresponding data sets would be sufficient to actually develop the entire aircraft, unfortunately I was stumped by the fuselage dimensions which remains incomplete. A few key dimensions were illegible on the drawing copies I had and although I tried to source legible information from various places I was unsuccessful.

I have though interpolated a solution for the rear fuselage based on available information that seems to satisfy the requirements for manufacture and assembly.

The archive drawings suggest that the setting out point between the front fuselage and rear fuselage members is coincident on the centre line. However this would not facilitate a flush connection with the bent plate connector, so I have offset the SOP to the face of the front fuselage upright, which does. Notice too the minor adjustment to the angles.

These setting out dimensions are not verified and the drawing should not be used for any other purpose than for reference. I don’t normally publish stuff without verifying the data but this fuselage has bothered me for a while so I thought it may be prudent to publish what I have in the hope that someone may provide the verifiable data I would need. The model shown is work in progress.

This has proved to be an interesting project and I progressed quite well with the tail and various other elements of the design. I hope to return to this project at some later date as it is a fine aircraft with many examples still flying.

North American P-51 Mustang: NAA Profiles

May 31, 2015July 25, 2016HughLeave a comment

NAA P-51 B/C/D Mustang:

The majority of parts created for aircraft like the P-51 are derived from a library of standard section profiles, not unlike the steel and ship building industries. Fortunately for this project we have an extensive collection of these original NAA standard drawings, numbering 208 in total.

To facilitate the long term goal of recreating the P-51 part drawings as 3D models and associated 2D drawings I figured it may be prudent to first recreate the standard profiles as 2D CAD profiles for this purpose.

So far I have drawn the first 24 profiles exactly as shown on the original drawings but with dimensions in dual format inch & mm…many more to go!