3d Cad

Sopwith Pup:Wing Brackets

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SopwithPup: Wing Brackets

This was not meant to have been a study in its own right, but out of curiosity I couldn’t help but wonder if there was enough information to actually build an accurate 3D model.

I was also curious why I had received a number of help request emails from my friend about this particular aircraft…so I decided to have a closer look. His latest query was regarding brackets similar to the one I mentioned in my previous post but specifically the centre section connecting brackets to the wings.

The left bracket belongs to the centre section and the right bracket is the connecting bracket for the wing that slots into the centre section bracket.

sp-009

The bracket dimensions are such that the centre bracket sits proud off the centre spar whilst the wing bracket is embedded in the wing spar, so technically they should just fit into one another without too much problem!! That’s the theory but the reality is it doesn’t quite align with expectations.

sp-03

This image shows the actual clear dimensions within the top and bottom rib flanges which replicate the perimeter dimensions of the wooden centre spar. In order for the centre section bracket to connect to the spar we would have to notch the top and bottom rib flanges to get it to fit. The horizontal dimension can vary (highlighted) but we will be restricted by the vertical dimension. I can’t imagine why anyone would want to notch the top and bottom flanges as this diminishes its strength. Plus there’s another issue with this…

sopwith-pup06

This preliminary model shows the problem where the centre spar is actually set back one inch to facilitate the incoming connecting bracket from the main wing. Ideally, we need to fully assemble the centre section and have it fitted to the aircraft and aligned prior to fitting the wings, but how can this be done if we can’t screw the rib flanges to the spar? I think in this instance I would shape the wooden spars in such a manner as to facilitate fitting of the flanges and mating with the wing spars.

I have done some research on this and it appears to be a known issue with some clever blokes just redesigning the connectors to make it work better or tapering the wing spar to good effect as shown below.

sdasmpup

It looks as though the wing spar is tapered with a smaller bracket sized to fit within the centre bracket. That would work and likely an improvement implemented in the workshop. A very rough preliminary study could look something like this…

…it does need a lot more work but I don’t have a lot of time to develop it further right now!

The design in many respects seems a little rough and ready, but we have to remember in those days they were under a huge amount of pressure to get these aircraft built and get them into the field. The life expectancy of these aircraft was only six weeks so replacements had to be shipped out in rather a quick time.

No disrespect either to Tom Sopwith and his engineers, these things actually flew rather well regardless of the vagaries of the design and what may seem to be annoyances to us may well be things they would naturally deal with in the workshop without any hassle.

It is very tempting to continue developing the Sopwith Pup but to do so efficiently would require setting out the basic geometry for the entire aircraft, identifying the anomalies and determining suitable resolutions as close as possible to the original design intent. I’m not sure I have the time nor the inclination to do so.

This has been a welcome distraction from the P-39 Airacobra project and will likely feature in a few more posts as I will surely continue to receive help requests from my good friend.

Bell P-39: Progress Update

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Bell P-39: Progress Update; Comparison

Progress to date has focussed on the main inner fuselage development with additional modelling to the top cockpit glass.

Just for comparison and to give you some idea of scale and context I thought it may be prudent to bring together a photograph of the P-39 and the CAD model, that are roughly shown from the same viewpoint.

p-39_airacobra_2006-06-15

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Ordinate Observations:

I mentioned before that we don’t have an ordinate plan for the P-39 as the main ordinates are incorporated within the Bell part drawings themselves. One of the key objectives for this project is to create an ordinate plan for the main fuselage to ensure that everything matches perfectly. Typically for all manufacturers of this era, the Bell drawings are accurate to 1/64 inch (0.4mm) in some cases but more generally dimensioned to only 2 decimal places of an inch that occasionally results in some minor alignment issues.

An example is as follows:

The upper structure for the cabin has ordinates setout for defining the contour of the main structure which overlaps the fuselage outrigger as shown. The fuselage outrigger profile does not quite match either the dimension nor the curvature in this instance.

If we look at the ordinates for each part; as stated on the original drawings; we can see the difference is exceptionally small although well within the manufacturing tolerances.

WL (waterline) 12: Cabin noted as 16.98in  –  Fuselage noted as 17.006in

WL (waterline) 16: Cabin noted as 16.26in  –  Fuselage noted as 16.286in

The difference is only 0.026in which equates to 0.6mm. Admittedly some ordinates are given to the outside of the skin, others are not and it’s tempting to suspect that the variation is due to this. The skin though is 0.04in almost twice the difference.

Working with CAD these variations are quite obvious and ideally need to be sorted otherwise we end up with all sorts of interferences with adjoining components. This makes it rather interesting and challenging in order to derive a satisfactory model.

In this example the curvature analysis shows this point close to being negative curvature in the left image based on the ordinate value of 12.88in. We know that this dimension is a decimal equivalent of 12 7/8 inches which at 3 decimal places gives us 12.875.

Changing the value thus to 12.875in smoothes the curve in line with expectations.

The majority of the Bell P-39 drawing dimensions are in fact very accurate, with the first example above being the exception rather the rule. This is an update of the ordinate developments for the fuselage which is derived from multiple part drawings.

p-39-airacobra-fuselage-ordinates

Bell P-39 Airacobra Blueprint

The Bell P-39 Airacobra archive of drawings is very comprehensive, comprising in excess of 10,000 good quality drawings. Probably one of the better quality archives available, for further details send me an email to HughTechnotes@gmail.com

Bell P-39: Creating Wing Fillets

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.Technote: Bell P-39 Creating Wing Fillets.(Inventor 2017)

Wing fillets are probably one of the most complex aircraft items to model as they need to follow the curvature of both the wings and the fuselage shell. Invariably we have many offsets to contend with and variation in angular alignment of the flanges.

The following images are typical of the manufacturers drawings with an ordinate table listing the X,Y ordinates and angle of the flange at each point.

As usual we would start with marking out what we know; in this case the ordinates points from which we create the reference geometry.

P-39 Wing Fillet1

The reference geometry in this example is the 2 splines for the flanges connecting to the fuselage (left) and the wing (right) with a horizontal base line for the lower flange.

We then check the curvature of the splines to ensure we do not have negative curvature; adjusting the handles to negate this where necessary.

These Fillets are full of tangent and perpendicular dimensional oddities that can sometimes be a real pain to achieve satisfactory results .

Previously we would create a work plane (tangent) at each node and individually sketch the required flange construction lines set to the correct angular value. This was a lot of work and a heck of a lot of sketching. Thankfully Autodesk have introduced some nice functionality to the 3D sketch environment in Inventor 2017 making this task so much easier with provision of logical constraining options and associations.2016-08-14_15-19-34

In Inventor we have various planar constraining options as shown. The top one is to constrain a sketch element to a surface and the lower ones are parallel constrain options to the main work planes.

We would still create the work planes tangent to each point as before; I have shown one for clarity, then we simply move straight into the 3D sketch environment to model all the flange construction lines.

We first need a reference base line constrained to the tangent spline work plane and also be parallel to the main work plane YZ.

P-39 wing fillet 3

We then sketch the flange line, constrain to the tangent spline work plane and dimension to the reference line as shown at 95 degrees.

P-39 WING FILLET 5

It really is a simple case of drawing a few lines and just using the planar constraint options to ensure correct tangency for developing the flange guide lines. Furthermore you don’t even need to project geometry from the 2d sketch as you place the line it will automatically connect to a point on the 2d sketch.

We continue doing this for all the ordinate points as shown then surface loft the flanges and apply a surface patch to create the main body. I should note that the surfaces shown have already been trimmed to the extents of the part.

It is very tempting at this stage to stitch and then thicken to achieve the finished part, however in my experience occasionally the transition of sharp corners introduces anomalies along the edges which can be negated if we first apply a fillet prior to thickening.

P-39 Wing Fillet2

To finish the part after thickening, I converted to a sheet metal part and added a flange to the base at 7.5 degrees, a few holes and that’s it done. There are some flange holes still to be modelled which will be done later when the other connecting parts are modelled and checked for alignment in the assembly.

Progress Update:

The following image shows a typical interface check between the P-39 wing and fuselage:

P-39 Wing Location

…and here the Radiator Intake Duct, preliminary alignment:

P-39 Rad Intake Duct

This radiator intake duct was an interesting development as the Bell chaps had provided both the tangential and the exterior dimensions at 2-inch intervals; on plan and elevation; which collectively are projected to form the profiles at each station. The white sketch at the bottom of the image shows these dimensions on the side elevation, with the curved lines depicting the tangent lines. I checked the curvature of this line and I only needed to adjust 2 dimensions by a minuscule amount to correct for negative curvature.

Update July 2022: New Revised P-39 Ordinate/CAD Dataset:

For all inquires please get in touch: hughtechnotes@gmail.com

Bell P-39 Airacobra: Fuselage

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Bell P-39 Airacobra: Fuselage

This is an update on the P-39 project. I have actually been drifting between this and the P-51 Mustang as a number of inquiries have come in regarding the ordinates and various questions on the Oil Cooler model and landing gear mechanisms; which has been an interesting diversion.

Getting back on topic, I thought it may be prudent to write a quick update on what I am doing with the P-39 Airacobra and where I hope the journey will take me.

I have of course continued working on the ordinate data spreadsheet which is derived from the part drawings themselves. This serves as a check whilst I am developing the structure. The 3D models are being developed in context, i.e the individual part models are located to the 3D spatial ordinates relative to a single datum so when I plug these into the assembly they will import to the correct 3D location thus negating the requirement for constraints.

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This is the first time I have worked this way as I usually just model the part and then constrain to the corresponding items in the assembly, but this is usually dependent on the quality of the assembly scans to clearly identify and ensure correct alignment of the parts. As we all probably know these scanned files are the most likely to have problems with legibility. In many respects having the part files modelled relative to ordinates in 3D space ensures that the parts line up correctly and I don’t have to worry too much about the quality of the assembly scans.

P-39 Airacobra Fuselage

The P-39 main assembly drawings are actually not too bad as the image above shows. This is a scrap view of the fuselage Longitudinal, comprising many small parts all riveted together to form the assembly. The area in red is where I am working at the moment; which is a major node; just aft of the engine bay; where the many struts and braces overlap on both sides of the stiffener plate. The following image gives you some idea of the detail to which this is being developed.

P-39 Airacobra Fuselage1

The pilot holes for the rivets are unique to each individual part and just like the real process of construction these holes will be match drilled to all the other corresponding parts in assembly.

Modelling the complex parts and locating all those holes takes a lot of time but I believe the end result will be worthwhile. With this degree of accuracy you could just about build one of these aircraft from scratch!.

Quick Technote: P-39-01This is the lower level fuselage cross member that has a built in twist to align with the connecting frames at both ends. The model consists of 3 profiles with the 2 outer ones containing a small angular deviation in the centre at point A. Normally I would loft the profiles to create the finished surface but this projects the deviation throughout the length giving us 2 surfaces; which does not look good.

I therefore deleted the resulting 2 base surfaces and simply replaced them with a boundary surface. I’m sure you will agree the result is a much smoother gradation of curvature; that matches expectations.

 

 

The Journey

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The Journey:

This blog has been about the journey cataloging my passion for historical aviation design and construction. Its about the geometry; the ordinates and plans, about the designs and construction; from wood and canvass to full metal and alloy and the inspirations for the designs. The sheet metal work, the manufacturing, the mechanics, materials, electrics and hydraulics.

Its been an interesting time studying the different aircraft construction techniques and design methods. The different approaches to how different designers organise and develop the designs on the drawing board, sometimes accumulating 100o’s of drawings for a single aircraft…an admin challenge that even today would be quite daunting.

Not all my work has been published here, only a few examples that I think may be of particular interest. The evolution of the FW-190 to Ta-152, the various marks of the Spitfire, the early design characteristics for the Tiger Moth, the Mustang P-51 conic research and mathematical analysis culminating in a broad spectrum of research material that lays the foundation for the next chapter in my work.

I have learned a lot from this work which has been both challenging and frustrating. Its tested the limitations of my knowledge and the CAD systems we have come to rely on so much in our designs today.

Not many of the archive drawings sets I have are representative of a complete aircraft, often missing key information or simply illegible; though the latter sometimes can be overcome by studying other aspects of the design. I am often asked if I would consider creating an entire aircraft design in CAD that could actually be manufactured and whilst the answer is of course yes I would be reluctant to spend the considerable time required for any aircraft for which we have many flying examples.

Having said that Operation Ark was setup to undertake such a task for an extinct or rare aircraft depending on availability of sufficient design data. This work is still in progress and will take a while to resource, evaluate and fund such a project.

In the interim I have received a new set of archive material for an aircraft that was used extensively by Russia on the Eastern front which will be featured here in a few months time.

For now there wont be many updates but please do drop me a line as its always good to hear from the many readers of this blog about their own experiences in the exciting world of historical aviation.

NAA P-51D Mustang: Carb Air Scoop

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NAA P-51D Mustang: Carb Air Scoop

In an earlier post I discussed in some detail the progression of model development for the Carburetor Air Scoop (Lower Cowling) inlet and I mentioned that the final Air Scoop would be uploaded upon completion. Earlier Post : Air Scoop Prelim work:

It has actually been completed for awhile; I just forgot to upload it!

So here it is and if anyone has attempted to model a complex surface of this type you will understand how difficult this can be. Needless to say the Freeform T-Splines were invaluable in obtaining the correct surface.

The surface model is attached to over 300 ordinate points with numerous contour and fairing curves generated in preparation for the final surface modelling.

The data was first prepared in a spreadsheet; listing all ordinate points in mm and inch dimensions from which I extrapolated the 3D coordinates for each point; essentially creating a point cloud.

The ordinates were imported into Autocad, analysed and then the points grouped accordingly to define the contours and fairing lines.

This was then imported into Inventor and the surface painstakingly built up in each separate square grid attaching all the ordinate points. There was no easy way of doing this; I know I tried!

I am delighted to have finally completed this particular model having consumed many hours trying various methods to get it just right.

Project Cad Technote: Sheet Metal Bending in CAD

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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.

2015-08-30_12-39-11Typically 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.

2015-08-30_12-13-10The 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.

2015-08-30_12-19-11Its 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.

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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).

2015-08-30_12-25-18From 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

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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:

Lockheed_Vega_5

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.

WEB11667-2010pBut 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.

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

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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.

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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.

2015-07-27_20-05-59Currently 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

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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.

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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.

2015-07-23_03-13-17The 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.

2015-07-23_03-23-16The 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.