Aviation CAD TechNotes

Project Cad Technote: Sheet Metal Bending in CAD

August 30, 2015September 16, 2015Hugh1 Comment

Project Cad Technote: Sheet Metal Bending in CAD.

Sheet Metal Work is an interesting subject to which I could no doubt devote an entire blog to. Fortunately for us we don’t have to as this topic is covered in detail by the many professionals working in this industry.

However understanding some of the key principles is imperative to ensure that our CAD models created from the aviation manufacturers drawings are correct as the dimensions given do not always suit the CAD development process.

One particular aspect relates to something the Sheet Metal guys refer to as the Outside Setback. The Outside Setback is the distance from the apex of the outside mold lines to the tangent point of the outside radius. When the sheet metal is bent the inside radius pulls the edge of the material away from the apex of the bend.

Typically on many occasions we will have a developed profile for the part which is to be bent to the required profile with only a few dimensions noted to achieve this including a bend coincidence point and angle.

The image on the left is indicative of many situations that arise when working with the manufacturers drawings. It is not unusual for a dimension to be given to the projected point at “A” which understandably is important to ensure the part mates properly with another.

However in Inventor; for example; we only have selections at 1,2 & 3 for “folding” a part from a development sketch and no option to define the stated “Dim” to the point of coincidence; which therefore may not provide the desired result. We may of course have the angle, material thickness and usually the inside radius.

Its not practical to select points 1 & 2 but it may be possible to use point 3 if we know the OSSB dimension.

In Inventor this is the middle option from the sheet metal fold dialogue. In this case we have specified the complimentary angle (97 degrees).

In order for this to work we need to calculate the dimension OSSB. The smart guys in the sheet metal industry have this stuff all worked out and have an easy equation that we can use to ensure consistent accurate results.

B> denotes the complimentary angle which must be less than 170 degrees.

IR is the Inside radius and MT is the material thickness. (the dot in the middle by the way is multiplication).

From this equation we derive the value for OSSB which we will deduct from the Dim value provided on the drawings, thus giving us the correct location of the fold line at point 3 above.

In this example the dimension from the manufacturers drawing is stated from the hole center, which has been adjusted to locate point 3 by deducting the value OSSB.

It works perfectly and we now have a folded bracket from a development plan that complies with the stated drawing dimensions.

I should note that some CAD products take this into account and provide the necessary options for developing this folded model but where we have limitations a touch of maths goes a long way to achieving the desired result.

In this example the hole is very close to the bend causing a slight deformation. This could initially be drilled to a smaller diameter and reamed after bending or we could simply use a smaller bend radius; if permissible!

Update: Mustang P-51 Project & Operation Ark

August 20, 2015November 21, 2020HughLeave a comment

Update: Mustang P-51 Project & Operation Ark

The Mustang P-51 project is on hold whilst we review the CAD systems we will use for Operation Ark. To date we have utilized both the Autodesk Inventor & the Dassault Solidworks for our projects and research. We have another contender for the project which is Solidedge, until recently this was not a viable option but the latest version ST8 exhibits many of the features we would need.

Operation Ark will be a long term project requiring many man hours of work to research and build literally thousands of models, so it makes no sense to have different CAD products for this project. There is also a cost consideration as the project will rely entirely on goodwill and donations to support our efforts and assist with CAD software.

Collaboration technologies and access to rendering farms for final processing of the CAD data are also key considerations. We have received offers of support from a few fellow enthusiasts to help with the Cad model developments and rendering; the latter being from Bilby…thank you very much for your support. Some comments from fellow enthusiasts:

From Alan “I love your Operation Ark initiative, and would be more than willing to play a role in any capacity.”

From John; “ARK is an extremely important project and I congratulate you on your vision.”

From Beaufort: “…I am really impressed with what you do and I can see that massive amount of time that you put into it. I also love the design specifics of these aircraft…”

Operation Ark Project Status:

This project is attracting a lot of attention, with many positive responses as noted including suggestions of alternative aircraft for consideration. One of which is the Lockheed Vega , which is a unique aircraft and was; in many respects; ahead of its time.

This is actually a good example for Operation Ark as the only remaining examples are located in the USA with only one flight worthy example, though further research would suggest that number could well be 2. The location alone excludes a large number of enthusiasts from actually ever seeing one either as a static exhibit or in flight!

That is part of what Operation Ark is about, removing geographic constraints and bringing access to everyone; the complete aircraft with everything modeled right down to the nuts and bolts. An exact replica in 3D that can be interrogated online as assembled or as individual components. We are also contemplating extending this to include additive or 3d printing technologies to build a half size replica, making the parts available to interested parties.

But this is only one of the aircraft being considered and whilst a likely candidate for selection; specifically as we have access to the manufactures drawings; our preference would be for one that does not exist or has only 1 example in existence like the Ta152.

The project though is entirely dependent on the availability of the original manufacturers drawings and specifications, which is our current priority!

Even when we do have access to materials they first have to be evaluated, which incurs a cost for scanning of microfilm archives and then reviewed for completeness. This process is rather costly but ensures that we don’t commit to a particular aircraft that we can only partially build. Usually where we have incomplete datasets we will endeavor to source the missing data elsewhere before we actually exclude the aircraft from selection.

All the research and work published here to date has been done voluntarily in the hope that it will help other enthusiasts.

Topology optimization and additive manufacturing

August 17, 2015August 18, 2015HughLeave a comment

Topology optimization and additive manufacturing

This is probably a slight divergence from the main subject of Historical Aviation though it does align well with the underlying research philosophy relating to the how and the why of how the aircraft designs develop from concept to manufacture.

Engineers have always strived to maximize efficiency with materials, reduce weight and improve manufacturing…the shape and form being integral to that same desire.

Thus today we have many tools at our disposal that have evolved to interrogate and simulate our designs before they become a real world object. In recent years Topology Optimization, an industry term that Wikipedia defines as “A mathematical approach that optimizes material layout within a given design space,” could be a critical motivator to create industrial designs more efficiently with less material…an ideal environment for the aircraft designer.

In my day job as an engineer I am always looking for new tools to help me make my designs better and more efficient. In my studies of historical aircraft designs I attempt to get into the designers mind and understand the motivations and inspiration for the aircraft designs.

So I am naturally curious about the many influences that impact the design process and the tools available to do that job.

This is a link to a youtube presentation on Topology Optimization..just because it is; for me personally; an incredibly interesting technology…enjoy!

Solidthinking Inspire; Topology Optimization

Republic XP-47J Superbolt

August 8, 2015August 12, 2015HughLeave a comment

In my endeavours to research the forgotten aircraft of the war eras and the remarkable people that designed, built and flew them I came across this article on the Republic XP-47J Superbolt.
What is important; apart from the fact that this aircraft recorded the highest speed in level flight for a propeller-driven aircraft in 1944; is the tribute noted in the response column by the daughter of the pilot; Mike Ritchie who made that historical flight.
Perhaps the XP-47J should be added to the list for the Operation Ark project.

Old Machine Press

By William Pearce

In mid-1942, Republic Aviation Corporation initiated a design study to lighten their P-47 Thunderbolt fighter for improved performance. The Thunderbolt had been steadily gaining weight as the design matured, while comparative enemy aircraft, like the Focke-Wulf FW 190A, were much lighter. Republic officially proposed a light-P-47 to the Army Air Force (AAF) on 22 November 1942. On 1 April 1943, the AAF gave Republic a letter of intent to purchase two light-weight P-47s, and the contract was officially approved on 18 June 1943. This new aircraft was designated the XP-47J.

An early image of the Republic XP-47J before the Superman nose art was applied. Note the cooling fan vanes around the spinner inside the cowling.

As with all P-47s, Alexander Kartveli was the main designer of the XP-47J, and he was assisted by Murray Burkow. The XP-47J was similar in appearance to a P-47B, but it was…

View original post 1,100 more words

NAA P-51D Mustang: Fuselage: Conics

August 3, 2015November 21, 2020Hugh1 Comment

NAA P-51D Mustang: Fuselage: Conics

In the preceding article I had some fun with polynomials and how they could be useful for determining a smooth fit spline for the development of the Mustang fuselage. As a follow up to that article I wanted to share some research relating to conics.

The Mustang P-51 was the first aircraft to be completely defined by conics. The designer Edgar Schmued worked with Roy Liming to mathematically analyze the Mustangs shapes, tangents and curves. Conics were used by NAA as far back as 1932 though many of the techniques and equations we use today however were not actually in use until 1959.

The Bézier curves for example were based on the Bernstein polynomial which had been known since 1912 but its application for graphics was not understood till much later. Bézier curves were widely publicized in 1962 by the French engineer Pierre Bézier, who used them to design automobile bodies at Renault. The study of these curves was however first developed in 1959 by mathematician Paul de Casteljau using de Casteljau’s algorithm, a numerically stable method to evaluate Bézier curves at Citroën.

So I started to wonder how did Edgar Schmued and Roy Liming actually apply conic principles and what methods did they use for the Mustang design!

The documentation I have available for the Mustang Wind Tunnel models gives us a clue at the geometric construction for the fuselage frames. The designers used smooth conic sections with key parameters controlled by longitudinal shoulder and slope control curves. The longitudinal curves defined fullness and tangency values for the conics from forward to aft of the fuselage. The P-51 designers found that this technique allowed them to accurately control sectional areas to secure the required effects for lift, drag, stability, and overall performance.

Here we see a scrap view from the wind tunnel models, showing clearly the development of the conic constrained by 2 tangent lines and a third Shoulder Point as a known point on the designed curve.

The intersections of lines extended from the Max Half breadth point and the Lower Ship Centre point illustrate a drafting technique for creating the finished curve for the lower section of a fuselage frame.

Taking this method further we can describe a curve using a series of extended lines to define any point on the curve as shown in my Cad drawing.

This is my interpretation of a technique for the drafting of a typical Mustang fuselage frame. I haven’t seen this technique applied to a full fuselage profile and whilst the design information I have suggests a similar approach by the Mustang designers I can’t verify that this was the actual technique used.

It is not possible within the scope of this article to go into the detail of this technique, but suffice to say that selecting only 3 points for the lower and upper sections contained within tangential lines provides the basis for accurately determining any other ordinate point on the particular curve. I have uploaded a short video on Youtube here: Drawing a Conic

This is actually a lot better than using the polynomial equations for frame geometry as they only give you a best fit approach based on the tabled ordinates; with limitations; whilst this construction technique will allow the flexibility of defining any point on the curve to an unprecedented degree of accuracy when created in CAD…it works!

So what else did these visionary guys do? I am really keen to further research the mathematical approach that Edgar Schmued and Roy Liming used in the other aspects of the aircraft design and uncover the methods that made the Mustang unique.

It is my hope that by sharing my research and developments that this will inspire others to also research the work of the designers from this era and hopefully in some small measure encourage support for our project “Operation Ark”.

Update: I must have spent a full day browsing through the archives to find more information that would assist with understanding the conics development and thankfully I came across this NAA lines drawing for the cowl on P-51C (NA-103).

This shows the development and tangent lines for everything including the shoulder lines and the fairing lines as well as the main profile contour lines.

Its very important to spend time verifying the information used for developing these designs to validate the research. Sometimes I could spend days just looking for small scraps of information just to verify one dimension, which happened quite a lot on the Ta-152 project!

Full profiles drawn in Autocad from comprehensive excel spreadsheet ordinate collections now available for download. See this article for details.

NAA P-51D Mustang: Fuselage Lines; Polynomials

July 31, 2015November 21, 2020HughLeave a comment

NAA P-51D Mustang: Fuselage Lines; Polynomials.

This evening I spent some time looking back through some old notes I had on fuselage design, particularly Conic sections and Setting-out design theory.

Checking through the archives for the Mustang P-51 we have a design set for the wind tunnel model with a line plan showing the Shoulder Points (SP) and the “point of convergence” where the upper line of the Mustang fuselage converges with the lower fuselage line and the Fuselage reference line.

The Wind Tunnel drawings are a quarter scale but are quite accurate.

Here we can see the “point of convergence” actually defined on the the wind tunnel drawing at the scaled sta 92. Technically station 92 does not exist as it is outwith the fabric of the WT aircraft, but for convenience I have defined it!

So with this in mind I decided to undertake an experiment to calculate the “point of convergence” with the fuselage ref line according to the manufactured ordinates.

For this exercise I used the upper line of the fuselage, shown here as X,Y values starting from Station 113 and created a line chart.

I applied an third order polynomial equation to the line chart with a scientific value to 5 decimal places to increase the accuracy.

I recalculated the values of the Y ordinate to check that the formula produced an accurate result; shown in red. As you can see the resulting values are very close to the original Y values.

The last X value is the projected value I want to calculate to achieve a “close to zero” Y coordinate thus by definition being the calculated “point of convergence”. This value is 9518mm (374.725 inches) which compares quite well with the Wind Tunnel drawings showing this to be 92*4=368 inches.

Should I recreate this exercise but instead use a fifth or sixth order polynomial equation I am quite sure the resulting value for the point of convergence would be closer yet to the scaled up wind tunnel value.

Normally for this type of exercise I would work with tangent lines and the start points of the upper and lower fuselage lines from predefined Shoulder Points.

This was a bit of fun just to demonstrate how we can use the power of spreadsheets and mathematical equations to assist with developing our Cad designs.

Update: I decided to play about with this a bit more and had a look at the fuselage lines for the Bf109. I don’t have the design “point of convergence” for comparison but decided to do it anyway to find the convergence between the Lower and Upper fuselage lines.

These points are measured from a ground datum at 800mm below the fuselage reference line.

The stations/frames are from 2 – 8 inclusive. As you can see the calculated values verify the existing ordinate dimensions with the projected “point of convergence” calculated at 4832mm from station/frame 2.

These are the fuselage lines on the vertical plane which in theory should share the same convergence point for the fuselage lines on the horizontal plane (technically plan of max width)…an exercise for some other time!

What is even more interesting is that a line equation can be used to generate a spline in both the Inventor & Solidworks cad products… as a check to verify the cad work this is enormously useful!

Another example of application would be for the frames or station profiles.

In this example I have applied a polynomial equation to a set of ordinates for the top section of station 300 for the P-51 Mustang.

This needs a full profile as an arc to achieve an accurate result, which I’ve applied as a sixth order polynomial…you cant get much more accurate than this with Excel!

Ideally we would wish to extend this arc to the max width ordinate, which would add another negative ordinate (below the base line) to the graph…for some unknown reason Excel finds it difficult to compute an acceptable polynomial with 2 sets of negative values, so I would have to transpose the ordinates accordingly.

The Mustang ordinates induce a minuscule negative curvature on the top rear fuselage frames when you create a CAD profile just using the ordinate values from the NAA drawings. Its not detrimental in anyway but it is rather annoying…so to obviate these issues I could utilize a polynomial solution to adjust the ordinates to get a positive curvature. The adjustment is micro millimeters, but hey that’s the way that CAD works.

Another Update: Out of curiosity I recalculated; to a higher degree of accuracy; the upper fuselage line for the P-51 and contrasted that with a similar calculation for the lower line of the fuselage.

The calculated point of convergence of both lines based on a 4th order polynomial to 5 decimal places is at 9375mm and slightly above the fuselage reference line at +18mm. Factoring in error based on the original ordinates being accurate to 1/16th inch and possible error as a consequence of a higher order polynomial I think this is a reasonable result. Its interesting to note the variation with the results we got before.

This is certainly closer to the expected values based on the wind tunnel data. The squiggly line by the way on the lower part of the fuselage is the plotted max half breadths; which is rather interesting!

Confirmation; have received confirmation that the intended point of convergence for the upper and lower fuselage lines is at Sta 368, which is at 9347.2mm…this is great!!

All CAD profiles included in the P-51 Mustang Ordinate Package now available. Refer promotion here.

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: Project Cad Technote; Smart Parts Vb

July 23, 2015August 8, 2015HughLeave a comment

NAA P-51D Mustang: Project Cad Technote; Smart Parts Vb

I was looking at options for routing the cables in the tailwheel assembly. There is potential for a lot of ancillary routing for pipes and cables yet to be done in this assembly so I have deliberately shied away from the adaptive parts (which I am not keen on) and the typical pipe and cable routing functions.

Also the cables are comprised of end terminals and many are sleeved for part of their length, which would mean having to route several times if I was to do this using the routing functions.

What I really wanted to do is have a sub assembly that contains the cable with all its bits in one sub assembly file but using the coordinates from the assembly to ensure correctness.

Extracting point coordinates from an Inventor assembly is not that straightforward requiring as in this case a vb solution, but first I had to define the key points.

I use the term “smart parts” and what this entails is for the parts or sub assemblies to contain additional geometry that will assist with other modelling activities like cable routing.

The image on the left shows the cables in this area with 2 key points 1&2 highlighted that are replicated in the 2 archive images. They define the straight section of the cable sleeve that is below and above the cable clips; the locations of which I have incorporated as points in the component sub assembly (last image). This sub assembly does not sit vertically in the assembly, the final position and orientation being determined by other factors which influences the final routing of the cable sleeve.

I have done something similar with the connection at the other end towards the left of the first image. At this stage I now have 4 points that determine the extent of the cable sleeve.

The next step was to go to the main assembly and extract the X,Y,Z coordinates of the four points from the fitted components.

I first select these and run a visual basic routine to extract the coordinates of each point and create a csv file which I import into excel which in turn is imported into a separate Cad part file.

It was then simply a case of running a spline through all four points and sweeping the sleeve profile.

The great thing about this is that the coordinates are relative to the origin of the main assembly so when I import the cable sleeve into the assembly I only have to constrain to the origin planes and it fits perfectly.

The cable itself will be done later in a similar manner which would be added to the sleeve part file as a multi part item or sub assembly using the sleeve centre line as part of the routing.

So no adaptivity, no complex pipe or cable routing just simple association through coordinate translations. The parameters of the sub assembly can be linked back to a spreadsheet so if the route changes I just re-extract the point coordinates and update the spreadsheet, which in turn will update the model.

To me this is a very tidy solution and maintains the integrity of the modelling hierarchy in accordance with the NAA register.

Using additional content in part files to facilitate other activities is very useful for examples like this and in fact any part that is associated with piping or cabling systems, particularly where you have cable clips or supports that need to be considered.

I should note that the extent of the cable sleeve is not exactly as shown in the first image due to the termination part not yet being modeled so I used something that was close at hand to demonstrate this principal.

If you would like a copy of the VB routine then please drop me an email and I will send it onto you.

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.

Share this:

Share this:

Share this:

Share this:

Share this:

Share this:

Share this:

Share this:

Share this:

Share this: