P-39

Technote: P-39 and P-38 Updates

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Technote: P-39 and P-38 Updates

An update on some recent work I have done for the P-38 Lightning and P-39 Airacobra. For the P38 Lightning, I now have the Boom Tailend interface with the Empennage and for the P-39 Airacobra, the new work includes the Auxiliary Fuel tank, Wing and underside panels at the Centre Section.

P-39 Airacobra Wing Layout and Aux Tank:

I was doing some research into the various closed penetrations on the underside panel as shown in the photograph on the right. So I modeled this panel to get a clearer idea of what was happening in this area as marked “A” in the underside view and front view images above. The 2 oblong holes are actually openings that normally would have a curved reinforcement which I understand would be used for the Auxiliary Fuel tank pipes and hoses. The Teardrops are for domed covers, which you can see more clearly in the first image view.

The Square cutout towards the rear of the panel is for the exhaust Flap and the slot to the front is for a removable panel that houses the Auxiliary Fuel tank mounting. The Aux fuel tank itself was well documented and was an interesting model to develop…I still have the fuel cap and vent pipe to add along with a few bracing struts to complete.

Following this exercise, I decided to further develop the wing layout. Although the CAD work for the wing was well-dimensioned with outlines for the Wing plan, Front Beam, Rear Beam, and Aux Rear Beam there was not much information on the actual rib profiles. We know that at STA 1 (22″) from the center of the ship the rib profile is a NACA 0015 and at the wing tip this is a NACA 23009 profile (204″ outboard). Other than that we have virtually no ordinate information for the ribs except for a partial profile at STA 7 +7.

The arrangement for the wing has been a subject of debate on several forums mainly regarding the construction of the Wing Tip. Usually, when there is a change in the rib profile the change occurs at the intersection of the wing tip and main wing however in this instance it is located at the extreme point of the wing tip. So the surface model is based on a loft between the 0015 profile at the root and the proxy 23009 profile at the extremities. This loft reveals an interesting caveat related to the evident wing twist and alignment of the Leading Edge.

Clarification on the location of the different NACA profiles was actually found in the NACA Report L-602 on the Flying Quality of the P-39 which defines the relative positions of the profiles. The caveat I was talking about relates to the wing twist…normally when we think of Wing twist or Washout we visualize the rib rotated about the 30% or 35% chord with the leading edge dropping and the trailing edge lifting slightly…but that is not what is happening here. The entire 23009 rib drops from a static position at the trailing edge towards the leading edge…the rotation is roughly 1.257 degrees. This results in a continuous leading edge downward alignment all along the length of the wing from the root to the tip.

As this is most unusual I was able to check the resulting surface model against known dimensional information for the beams and the partial profile at STA 7 +7 which matches. I still have to model the wing tip which has an interesting upward curvature.

P-38 Lightning Boom Tail End:

Another challenging aspect of the P-38 Lightning was determining the geometry for the Boom Tailend…essentially the intersection of the Boom and Empennage. We do have the lines of intersection for the Vertical Stabiliser, Horizontal Stabiliser, and the end of the boom but we don’t have any dimensional information for the curved profiles though we do have drawings that give us some idea of the profiles.

This was surprisingly difficult to get right and to be honest this final version is the result of 3 different attempts to achieve a viable solution. At first, I attempted to draw the Boom section, and stabilizers then fill the void with a surface patch to naturally define the curved fillets…with a few guidelines I managed to get a reasonable result but I incurred a few anomalies with the finished surface which I couldn’t correct. The second effort was more structured with a number of contours traced from the available drawings as a reference to gauge the curvature and then try again with surface patches but this time is broken down into quadrants, top 2 sections, and bottom sections…this was better and very close but again I had a few surface deviations at the leading edges.

Finally, I decided to have a look at using variable radius fillets…although I had already tried this unsuccessfully I changed my approach slightly which gave me good results. The fillets I used initially were tangential which caused a few problems where they met particularly on the top surface…what was happening was a sharp edge developing where the fillets intersected…so that was no good. It also mattered in which order the fillets were generated.

Eventually, I figured why not try G2 fillets and see if that worked…I am always wary of using G2 fillets due to some bad experiences using them before but I was running out of ideas and I was keen to find a workable solution. I started with variable G2 fillets at “1” and “2” with several control points to control the curvature and avoid folding the surface at the leading edges. After some fine-tuning, this worked out well for the first 3 locations. The remaining fillet for the Vertical Stabiliser did not go quite so well as it was impossible for the CAD software to give me a G2 variable fillet…so this one ended up being tangential. Perhaps with a bit more tweaking, it may have achieved a G2 fillet but I had spent many hours on this and I needed to make a decision.

There is a very very slight edging but it is almost unnoticeable on the final product. The final curvature of this model matches well with the guidelines extrapolated from the drawings and I am satisfied it is a very good representation of the Boom Tail End.

I hope you find this article useful and as usual any inquiries please get in touch at hughtechnotes@gmail.com

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Technote: P-39 Airacobra Exhaust Stack

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Technote: P-39 Airacobra Exhaust Stack:

This rather small unassuming item is at first glance a straightforward little model that actually turned out to be a huge headache. I spent several days working on this model which will all be explained in this article. Hopefully, the solutions I found can help you.

The first hurdle was the Bell Drawing 12-614-001… several key dimensions were illegible and a complete end section was non-existent. The first task was to develop what I do know to help determine what I needed to know…that in itself took an inordinate amount of time but eventually managed to get that sorted. In addition to the sections shown on the drawing I needed to include a control sketch to control the dimensions of the eventual loft activity…this was essentially an ellipse with fixed height and variable half-width.

The side/outside lines of the Exhaust Stack are fixed profiles so it will be the inner profile that will change to make sure that the intersections of the 2 pipe exhausts were correctly located in the centre of the element.

You will notice the side profile sketch is separate from the main model sketches, this just makes it easier to see what I am doing as for the most part, it was mainly for reference.

At this stage, everything is quite straightforward as all I had to was loft the 2 pipes by selecting each of the profiles as shown and then trim the surfaces to give me a base model. A small tip: for this to work correctly and ensure the alignment with the external lines and allow for expansion internally it was necessary to loft using the Centreline option…the centerline holds the loft shape between sections normal to the centerline.

This is where everything got crazy. The main body part of this stack has no joint seam and is quite bulbous…so what I had to do was adapt this base model to form the bulbous surface complete with an internal curve.

As you can see in the image on the right the real item shows the bulbous main body part and the generous inside curve. By the way, some may have observed the real item looks shorter than the model…this is a puzzle…some of these exhaust items are indeed short and this is further noted on the Bell drawing as a dotted line! I suspect this may be linked to the engine used but as yet I do not know for sure.

I initially created the short version thinking that this was normal until I found out that most are actually longer versions. That was the start of many frustrations to come as that first batch of models was quickly scrapped and started again. That bulbous bit though is the main problem.

In order to achieve this I had to derive a solution that filled the void between the 2 tube lofts and at the same time provide an internal curve consistent with the real product. What I opted to do was simply trim and then remove the inner surfaces and then blend the remaining void with a surface patch.

My initial effort was to remove the centre section according to the natural divisions along the centre of each tube and the sketch plane. I tried variation after variation on this, adjusting tangency strength and G2 for each side, I even adjusted the dimensions on the control systems to make minor corrections. I eventually ended up with something reasonable and we got it 3d printed. The second image above is the latest model incarnation which I will explain below.

Two immediate issues are quite visible on these printed models…the initial curve between the 2 tubes is far too tight and almost looks as though it has folded. The second is the surface continuity…okay admittedly I was unsure about the short and long versions of this stack so the end extensions were a separate model part…though I should note that the surface should have been tangential and it’s clearly not.

Coming back to the cad model above…the second image shows what needed to be done. In this one you can see the cutout in the main body is an ellipse which gave much better results and a larger smoother curve between the tubes.

What was happening with effort 1 was that choosing the centre of the tubes at 2 and 3 (which by way was a logical choice) was actually restricting the area in which the inner curve could form, resulting in small deviations in surface accounting for the folded look. Essentially by selecting the centre lines I was actually creating self-imposed restrictions. It took me a while to figure that one out and many hours of work.

Further the mere fact that this patch includes sharp angular corners also did not help as the patch stretches to accommodate those corners again leading to imperfections. Finally, I decided to forego that first attempt and ended up using an ellipse profile that extended beyond the centrelines of the tubes. As there were no sharp edges continuity of the surface patch outline was good and ended with a smoother patch surface that was G2 compliant. There is a small additional patch at the end of the stack…there was a natural seam there which I needed to get rid of.

The final model is shown above and is a very much improved surface with good continuity and a generous internal curvature between the tubes. One final point is the mounting plate… which is not dimensioned on this Bell drawings because it is a contractor-supplied item. I searched through many drawings and found something similar…so I cannibalised the dimensions from that drawing to create the mounting plate. It turns out that this was a good effort as it actually aligns with the engine block.

Surface modelling is rarely this complex but occasionally you will come across something equally challenging. If something is not turning out the way you had hoped or expected just check to see if your choices are imposing restrictions on how the surface is created. When you are trimming or splitting surfaces try to minimise sharp edges and instead opt for curved circular solutions.

Update: 7th October 2022:

Just received word that the newly revised Exhaust Stack model has now been 3d printed…just another 11 sets to go. Apparently, the time for printing and ultrasonic cleaning will be about 4.6 days. They look good…check out the awesome curvature on these prints.

Technote: P-39 Airacobra Carb Scoop

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Technote: P-39 Airacobra Carb Scoop: New Project:

Following a recent inquiry about the P-39 Airacobra, I was asked if I had a model for the P-39 Carburettor Air Scoop. At the time I didn’t, though I did have some preliminary outlines that were done as part of the dimensional ordinate study. So I decided to get stuck in on this new project and see what can be accomplished…by the way did I mention this will be used on a real aircraft. The template moulds will be 3d printed and used to form the aluminium plates.

When you look at a photograph of the Carb Scoop it looks deceivingly uncomplicated however it turns out this part is surprisingly complex. The main body part itself is challenging with the curved profiles and filleted interfaces, the curvature of the fuselage; which by the way is not actually documented anywhere and the transitions from one frame to another to achieve smooth curvature. The internal duct comprises many varying profiles…the profiles tend to be rectangular with different corner radii throughout culminating in what can be described as a slot profile for the Air Scoop inlet.

I have been working on this for a few days now studying various modelling methods to achieve the most accurate and consistent results. So far I have the main scoop body and the fuselage skin modelled. The internal ductwork is set out on a sketch, though I will still have to define a number of intermittent profiles to ensure I get that right as well. Overall there are 12 individual parts for this assembly all detailed on one drawing…so some interpolation of design intent and cross-referencing with a few external drawings is essential.

At the Scoop inlet, there is a small lip that I have yet to model. The drawing has very little information on this so I decided to model the scoop without the Lip and then I will have to sculpt the curved form from extrapolated model information. That Lip at the end of the day will probably look inconsequential but the development work for such a small item cannot be understated. I shall update this article as work progresses.

An overview of the underlying geometry sketches for the CAD model. Point “1” is the fuselage skin…as mentioned the ordinates for this profile do not exist on available Bell drawings. So I work with what I know, namely the fuselage frames fore and aft and below. These were surface lofted and then the profile at “1” was patched to align with curvatures of known surfaces. using tangency and G2 on selected edges.

Point “2” is the scoop outline sketches, in which I made continuous elliptical profiles with an additional circular profile at the very tip below the fuselage plate surface.

The blue lines at “3” are the lofting guidelines, absolutely essential to getting this right. I initially skimped on this, instead, I attempted to just loft and use G2 or Tangency adjustments…it did not work well…so if you are doing this don’t skimp on these guidelines. Once the scoop body was lofted there was trimming to do with the flange plate (it was the offset from the fuselage surface) and then the 2 items were stitched. This provided an edge to which a variable fillet was applied. Cautionary note on the variable fillet…when initially applied don’t try and create adjustment points all at once…take your time, just create one pair at a time making micro adjustments and let the model regen and repeat.

The other key consideration is that the Bell drawing dimensions are generally only accurate to 1/32″ and 1/16″ (0.8mm and 1.6mm respectively). This will invariably impact the eventual quality of the end product when using CAD so it is important to understand where and how you need to compensate.

Update: Internal Duct:

I have now modelled the internal duct which has a partial concave curve on both sides to allow clearance to the main scoop housing. That was a real pain to model and to be honest, to achieve continuity with the duct curvature I simplified it slightly.

The sides of the duct are shown on the Bell drawings as being flat from the base level almost to the scoop inlet itself; merging to a point just past the horizontal breaker bar. I tried various methods of doing this but failed to achieve a good result…even using freeform curves…mathematically it is not to be! I settled for a smooth loft of the various sections to ensure that at the very least I could still achieve the partial concave surface and a smooth shell.

The front curved edge of the scoop inlet has a weld seam which is shown in the centre of the edge on the Bell drawings. I decided to move that joint further inward because an extended flange may reduce the installation clearance when installing the duct.

The curved plate you see will be cut back to finalise the flange for the scoop, which I shall leave until the main parts are all modelled. More updates to follow.

Duct Vane and Scoop Ring Stiffeners:

All inquiries as usual to HughTechnotes@gmail.com

Update 11th Sept 2022: Almost There:

Looking good and is deadly accurate. Trimmed the flange for the scoop main body and added the fuselage frame stiffeners. I still have a small panel door, forward lip and a few miscellaneous items to finish. One more day should do it.

The Main Body Sorted: Just the Lip to model and add to the finished model.

If you require a professional design and draughting service for your projects then please do not hesitate to drop me a line. Providing professional engineering, draughting (time served, old school) and modelling services in CAD since 1985. Fully conversant with Geometrical Tolerancing, Geometric Dimensioning and Tolerancing (GD&T), ISO Geometrical Product Specification (ISO GPS), BS 8888 and mechanical specification.

Email: hughtechnotes@gmail.com

Update 12th Dec 2022:

I have just received word that the P-39 Carb Scoop has just come off the 3d printer after 4 days. They printed this in 2 parts as you can see. Still a lot of finishing work to do before they have a chance to fit it onto the P-39.

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Technote: P-39 Airacobra Update Horiz Stab.

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Technote: P-39 Airacobra Update Horiz Stab.

In a previous post, I covered the significant new model for P-39 Airacobra. This model is fully inclusive of all aspects of the aircraft. Within this post, I mentioned the extensive study involved in determining the layout for the Horizontal Stabiliser; the dimensions of which were unclear in the available blueprints

https://hughtechnotes.wordpress.com/2022/05/18/technote-bell-p-39-airacobra-updated-model/

I was particularly keen to establish verification for the leading edge angle and though I had written to a number of organisations that have the P-39; surprisingly none of them took the time to either acknowledge or indeed reply…which of course was disappointing. From my experience, the industry is normally very supportive with regard to technical inquiries.

I revisited the documentation I do have and established that relevant information was included in the NACA Wartime Report L-602 which gives the chord length at Sta 49.25. It turns out; from my initial assessment; that the dimension at “2” was barely 2mm out and the Leading Edge angle is now 16.7796 degrees.

I mentioned in my last post that this latest study is available now which also includes the original model; which was more of a 3D modelling exercise than a dimensional study.

The P-39 Airacobra new CAD/Ordinate study is an impressive project.

All inquiries as usual to; hughtechnotes@gmail.com

P-39 Airacobra Recovered from the Black Sea

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P-39 Airacobra Recovered from the Black Sea

“SIMFEROPOL, September 25. / TASS /. An expedition of the Russian Geographical Society (RGO), together with the Russian Ministry of Defense, lifted a Bell P-39 Airacobra fighter from the bottom of the Black Sea from the regiment that took part in the air cover of the Yalta conference of leaders of the anti-Hitler coalition in 1945, a specialist from the Expeditionary Center of the Ministry of Defense Anatoly told TASS on Friday Kalemberg.”

As many of you know I am a huge fan of the P-39 Airacobra. So I was particularly interested to read these reports of the recovery of this P-39 from the Black Sea. These are the links to the articles I have read so far from various news publications.

Links to the articles:

https://tass.ru/obschestvo/9550353

https://www.dailymail.co.uk/news/article-8772705/Second-World-War-era-Bell-P-39-Airacobra-crashed-1943-raised-depths-Black-Sea.html

If anyone has any further information to share on this project please drop me a line and I will feature updates in future articles; perhaps even progress on the restoration.

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

2016-09-07_00-09-49

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: Cockpit Glass

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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.

P-39-1

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!

Bell P-39: Wing Trailing Edge

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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.

xyz

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.

 

Bell P-39: Fold Over Flange

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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.

P-39 COOLER MAIN5

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.

P-39 COOLER 4

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.

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:

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