Sopwith Pup: Technote

Sopwith Pup: Spar Clip Technote

The Sopwith Pup is a single seater biplane built by the Sopwith Aviation Company, another aircraft in my archive, though not one that I have done much work on. This is just a quick technote; so not a new project; my priority still lies with the P-39 Airacobra.

I received an email from a close friend and he asked if I could help him out with this model for the main spar clip, item number 1393-1 from the Sopwith drawings. The area in question was the cable lug at the base of this clip, which comprises 2 parts.

The problem related to matching the profile of the top part to the profile of the lower part, without extensive or complex modelling. For the lower part, I decided to use the sheet metal features to create this as a multi-body part which I would then use as a template to profile the upper section that is essentially an extension of the main model.

What he was trying to do was project a sketch from the each face of the lower part, extrude each sketch and then fit a bend to connect the two extrusions. He reckoned this was more complicated than it should be and asked me if there was better way of doing this.

He was actually not that far from achieving a simpler solution, he just needed to adapt the process a little bit.


In a previous article for the P-39 cabin glass I discussed the merits of selecting the solid surfaces as a means to modelling the jogged edges. I have used a similar technique here for developing the upper part of the lug.

Simply by selecting the top surfaces of the lower part as shown above; we then apply a thickness to this selection and opt to merge with the upper part as shown. There we have it; an exact match and fit between lower and upper lug parts in one step!.

It looks simple and often the best solution is, but occasionally it is easy to overlook the fact that we can manipulate the surfaces of a single solid model to create new separate parts without too much effort.


Squaring the Edge:

The Sopwith drawings for this part and many other similar parts are a little misleading given that they show the edges of these components as beveled. This is normally not good practice, particularly when metal meets timber. Ideally we need to square the edges to negate this problem and to facilitate the cutting of the developed sheet metal pattern.


These brackets are an awkward shape which requires some careful planning to ensure that the model is correct and can be manufactured. So to achieve this I occasionally use surfaces to set-out the basic cut profile shape and then thicken.

Thickening a surface model is actually a good way of working due to the thickness being applied normal (perpendicular) to the surfaces, thus by definition achieving a good square edge to the developed pattern.

As you can see in the image on the right the edges are square and easy to cut.

The other way of doing this is using the cut option feature from the sheet metal command.


By selecting the “Cut Normal” option in the dialogue this will ensure that each of the edges from this extrusion will be square to the surface when flattened.

Whilst we are on this subject; the weld seam at the top of this bracket is something I would consider improving by having a thin continuous metal strip either side of the seam instead of 3 smaller widths (top image) which may distort the metal, something like this (A):


Notice I have tidied up the bend at (B)…this gives a much cleaner profile when the draft angle is quite small. I should note that I don’t normally take liberties wth the manufacturer’s details, but occasionally exploring options to see how things could be improved can be quite an interesting exercise.

I should note that it is normally good practice to state on the 2D manufacturing drawing a “Break Edge” minimum size anyway for all edges even when square cut.

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Bell P-39: Progress Update

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.



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 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.



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Bell P-39: Cockpit Glass

Bell P-39: Modelling Curved Cockpit Glass (Inv 2017)

Modelling the Cockpit glass can be a challenge to achieve the correct curvature and create the inevitable jogged and profiled edges.

P-39 canopy

The Bell drawing lists all the ordinates to enable us to create the profile sketches from which to derive the required basic shape with two areas worth extra consideration in respect to the rounded corners and the jog along the perimeter edge.

We developed the initial extruded surface from the contour ordinates and then simply extruded a sketch to trim this surface to the basic shape.

P-39 c1

The first thing we need to do is to fillet the corners. In Autodesk Inventor we cannot fillet a single surface, though we could use various techniques to do this we decided instead to Thicken the surface an arbitrary amount ( it does not much matter how thick it is) and then apply a fillet of each corner of the solid which ensures correct tangency.

P-39 C2

The jog along the edges is a bit tricky, given the nature of the surface. One way of doing this would be to sketch the jog profile and sweep the profile using the edge as the path. We tried this in several configurations but the result was not consistent.

To solve this we need to consider what a solid comprises off in order to rethink our strategy. A solid is essentially a series of closed surfaces that are used to contain the solid properties. With this in mind, we started by offsetting the top surface to create a copy at the desired jog dimension inward. Along the edge of this new surface, we sketched a circle with a radius the same as the jog flat dimension and swept this along the perimeter of the new surface.

P-39 C20

By using a circle profile for the sweep we ensure that the resulting flange; which is trimmed from the copied surface; will be a consistent width throughout its length. Now we have a surface representing the exact dimensions of the jogged top face at 3/8 inch. We do something similar for the top surface which is selected from the solid with the circle set to a bigger dimension to facilitate the jog transition curves. This time simply trimming to remove the edge width.

p-39 c21

This gives us 2 surfaces, the lower surface for the top face of the jogged flange and the second, the actual main surface for the top of the canopy glass. To fill the resulting gap between the surfaces we used a patch surface.

P-39 CX

We have trimmed the surfaces of the solid body thus breaking the solid cohesion leaving a number of orphaned surfaces which can now be deleted. To finish we would stitch the surfaces and then thicken to the required amount.

p-39 c12

To achieve a smooth transition when applying a patched surface between 2 surfaces a good result can often be achieved by using the tangency option relative to each joining surface. In this particular instance, the patch size was too small to do this so instead we applied fillets to achieve the same results.

A Note on Curvature:

P-39 Canopyx

It is absolutely critical to manage the curvature of the sketch profiles prior to lofting to ensure the best possible surface. This usually requires marginal adjustment to the ordinate dimensions; generally fractions of a millimetre; to achieve a good result.There is a small shoulder on this glass panel thus accounting for the slight edge deviation. To improve further the definition of the finished surface we can convert to a freeform surface which will derive a new surface with G2 curvature.

P-39 Cockpit Glass

Another Quick Tip:

Sheet metal flanges are restricted in Inventor to straight edge segments whereas with Solidworks we can actually create a curved flange where there is continuous tangency. One workaround in Inventor is to sweep a profile along the edge of the sheet metal part to create a flange or alternatively use the Ruled Surface feature.


This feature provides a few functions for extending surfaces either perpendicular or tangential to an existing surface. In this example, we simply select the default and create a perpendicular edge without requiring additional sketches.

Thicken the resulting surface, convert to sheet metal part and apply a traditional flange!

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Bell P-39: Wing Trailing Edge

Technote: Bell P-39 Wing Trailing Edge Calculation.

The root wing profile for the P-39 is based on the NACA 0015 (4-digit series).

p-39 wing TE

The Bell P-39 archive contains ordinate data for the fuselage, tail, stabilisers, cowls and so on but sadly the main ordinate plan for the wings is missing. However, we do have some ordinate data including a mid wing profile section and of course the front, rear and aux beams. We also know the root wing profile is based on the NACA 0015 which collectively provides enough core information to develop the wing structure.

The “baseline” NACA 0015 has a non-zero trailing edge thickness relative to the chord length. Just working from the generic geometry formula we end up with a large trailing edge thickness which is greater than that specified by Bell.

The baseline NACA 0015 airfoil is described by the function:2016-08-23_04-27-34

In order to achieve a degree of control over the resulting trailing edge thickness we only need to adjust the fourth coefficient in the polynomial slightly.


The above amendment will give a zero thickness at the trailing edge. The actual value we were looking for was 0.03in radius which was achieved through trial and error with the fourth coefficient value set to 0.1024.

  • x = coordinates along the length of the airfoil, from 0 to c (which stands for chord, or length)
  • y = coordinates above and below the line extending along the length of the airfoil, generally defined as either yt for thickness coordinates or yc for camber coordinates

The final profile was checked against known ordinates from the fuselage data.

The information here was sourced from a white paper written by WeiHei, Francisco Gomez, Daniel Rodriguez and Vasilis Theoflis.


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Bell P-39: Fold Over Flange

Technote: Bell P-39 Fold Over Flange.(Inventor 2017)

This a quick technote to highlight an issue that we sometimes come across with creating flanges in Inventor when one part is sloping away from the other.

The part we are working on is shown on this scrap view from the Bell drawings. This flange is folded over onto a sloping top plate from the side plate that is at an angle of 105 degrees.

P-39 Oil Cooler Main1

The issue relates to the reference edge selections that will determine whether or not we obtain a smooth transition from the side plate to the new flange.


When I first did this I selected the outside edge of the side plate to align the flange sketch. This was not satisfactory due to the notches; that are perpendicular to the side plate; influencing the creation of the eventual flange bend which gave us a rather awkward and untidy bend transition…definitely not good.

So I recreated the sketch; this time aligning with the inside edge of the side plate; which resulted in a smooth transition bend to both notched areas as shown below.


Occasionally when creating flanges the selection of which edge is referenced can make all the difference in achieving a satisfactory result. Use the sheet metal Face command to create a flange based on a 2D sketch as we have done here.

I should note that those notches are bigger than they need to be at this stage. I normally develop these complex models using a generous radius until I have completed the construction. Once I have achieved a satisfactory model and everything aligns correctly then I can go back and adjust these notches to a minimum size.

Progress Update:

I have included the rear fuselage section contour lines for reference. Will probably have to leave this project for a few weeks as I really need to spend some time sorting out my garden that is slowly resembling a jungle!

P-39 Aug21

37mm Gun Mount & Rudder Cable Guide Pulley.

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Bell P-39: Creating Wing Fillets

.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.


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.

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Bell P-39 Airacobra: Fuselage

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.


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.


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