The primary project for 2024 will be the F4F/FM2 Wildcat development. I aim to have a highly detailed structural model at either 1:10 or 1:15 scale 3D printed by the end of the year. Due to the requisite accuracies, this will be MSLA resin printed. My work is simply to produce the most dimensionally accurate aviation models in 3D CAD and accordingly fully documented.
I have recently started building the Landing Gear for the F4F which is shown below; this is the axle part # SP597. The image on the right is the forged model which is derived for machining into the part on the left.
Another example is again the Landing Gear; this time the Lower Drag mechanism. Through exhaustive research, I can go from an almost illegible blueprint to a clear sketch on the right. This is why I do what I do.
The other aircraft I will be revisiting is the P-38 Lightning as some aspects of that project warrant further research. For both aircraft, I will be visiting the collections at RAF Cosford and Shuttleworth later this year to hopefully fill in some of the blanks.
The projects will also involve updating my blueprint archives to make it easier to search for drawings initially by renumbering all 8000 plus drawings inclusive of drawing numbers. I have already started this for the F4F Wildcat which was helped enormously by some clever folks on YouTube. https://youtu.be/I9ffWZ_Bt6o?si=OEog79e-XaRUZz7K
The first portion of the numbering sequence is the original scan reference followed by the actual drawing number. The An Parts library will also be updated with additional conversions for use in other CAD systems.
2024 will no doubt be a busy year for me with the 1:10 scale printed model being the biggest challenge.
I hope that you will continue to support my endeavors throughout this year. Happy New Year.
Interspersed throughout this blog are many examples of Technotes describing techniques and problem-solving primarily for 3d CAD modeling. Many of the part examples shown are actually created to address another major issue with Assemblies.
It is not uncommon for the assembly drawings to be either unclear or simply void of key information that would help establish relationships between sub-assemblies or parts. In many examples, it is simply that the reproduction of the microfilm prints is not sufficiently clear to comprehend what is going on, otherwise the omission of basic dimensional relationships.
For the P-51 Mustang, I fully developed the rear Landing Gear mechanisms to clarify what the heck was going on as the NAA Assembly drawings details were obscured.
It is too often the case that general assembly drawings tend to be nothing more than an illustrated parts list with few key dimensions that define locations or relationships between the individual parts. This is also true for many of the sub-assemblies. For the P-51 Tailwheel sub-assemblies, I also developed 2D detail drawings showing key dimensions and parts lists. Ideally, I would have developed presentation drawings showing the exploded views of each of these assemblies to provide further clarification…perhaps a project for the future.
In the case of the P-38 Lightning, I have developed the Landing Gear assemblies to check the ordinate dimensions… which by the way are good. I now have the Coolant Radiator assembly which was again developed to check ordinate data but also for the same reasons as I did the models for the P-51 Tailwheel.
Typically the general assembly pictorially shows the sub-assemblies without any key dimensional information to define the location or part relationships and similarly, the sub-assembly for the clamp is not that much better. This is important stuff as occasionally they are the only reference material we have to help define ordinate data that is missing from the archive blueprints.
The Coolant Radiator is compromised by wrong dimensions as well…the top clamp cover, for example, had dimensions for the connection to the rod with the part drawing showing conflicting locations for different views of the same part.
The problem here is the connecting bracket item 224045 cannot possibly be 1″ from the edge of the cover plate whilst the overall dimension of 6 7/16″ prevails. I initially had located that bracket at 1 inch which seemed to be correct at the time because it fitted the part profile but when I introduced this into the assembly drawing it would not correctly align with the radiator. However, when I revised this using the 6 7/16 inch dimension it worked. That connecting part also caused more problems because the face of the part is machined 1/64″ which is not taken into account when positioning the part in the assembly.
Accumulatively this resulted in the overall width of the clamp assembly being smaller than it should be. This only came to light when I modeled the 234183 almost inconspicuous part as the stated dimension of 9.25″ did not fit with my initial layout..my first thought was this may just be an oversight but when I tried to align the main support frame (in gray) it did not align correctly. I went through everything and realized that the machined face of the corner parts connecting to the rod as shown may not have been taken into account and when removed the alignment was better and the 9.25-inch dimension on the strap was now correct. I am convinced that there should be spacers/washers between those connecting parts but this is not apparent on the assembly drawings. There remains a small discrepancy of 0.8mm which I am unable to account for….as this mainly relates to a clamp mechanism that will be compressed on assembly it was probably not deemed important but when you are trying to establish baseline dimensions it is actually very important.
The Part catalogs generally are your first port of call when developing these assemblies but they do not contain the key dimensions you need so these 3d CAD models are essential to achieve clarity. Incidentally, while we are talking about part catalogs it is important to understand what parts belong to which version of the aircraft. For the P-38 Lightning, the first few pages list the version and serial numbers which in turn are listed elsewhere where a Usage code is assigned. In this case the “e” is essentially the P-38H and the “bv” is the P-38J. The P-38 Part catalogs tend to show the version variations on one page; which can be really daunting; whereas others may show the version differences on separate pages…so you have to be attentive.
As I mentioned at the beginning of this article the main purpose of these assembly models is to achieve clarity and to check dimensional relationships. I think this is very important stuff that would certainly benefit from exploded views in conjunction with clear assembly 2d drawings.
As usual, get in touch if you can help support my work. hughtechnotes@gmail.com
The P-38 Lightning Ordinate/Dimension study is now finished after 7 long months. Initially, I had planned on doing this study in 3 months; working night and day; but alas due to the complexity of this aircraft this drifted into 7 months.
All areas of the aircraft have been studied, and modeled with all known, and henceforth many previously unknown dimensions collected and recorded in a comprehensive spreadsheet.
All bulkhead and rib profiles are generated for the wings, ailerons, elevators, horizontal and vertical stabilizers, rudder, fore and aft booms, fuselage, cockpit, and flaps. The latter was a challenge as the Lockheed drawings were unclear about the relationship of the flaps to the wings…however, after some research, I was able to resolve this issue to determine the exact positions of the Wing and Center Section flaps. The flap details are fully dimensioned now on 2d Acad drawings…that was the last hurdle.
Further to the Ordinate study I also have full 3d Cad models for the Nose and main Landing Gear Assemblies.
For further details get in touch: hughtechnotes@gmail.com
The P-38 Fuselage development has been a real challenge. When I first started this project it seemed to me that the fuselage was well documented with stacks of ordinate information and therefore should be a fairly straightforward model. The front section and the Cockpit enclosure are actually quite well documented but the Aft section and the mid-fuselage section; forward of the cockpit; most definitely are not. After more than 6 weeks this part of the project is still very much a work in progress.
The Cockpit enclosure: There are glass profiles for the cockpit enclosure but they are from the XP-38 early model; which on inspection; in comparison to the little-known information for the later production models suggest they are very close but do vary by 1.2mm but only on the side profiles. I suspect the glass was thickened slightly when they started production. I have to work with what I have and in the absence of sufficient information on the production models’ glass dimensions I have opted for a compromise. As only the side dimensions change with the top profile and interface with the fuselage remaining the same I think working with these profiles in conjunction with the structural elements of the production P-38s will work out quite well.
Ref 3rd image: In the 3rd image I have highlighted the location of the profiles at Station 123, 126, and 154. Sta 126 and Sta 154 are absolutely critical in setting out the cockpit enclosure and yet they are not documented nor do we have the drawings listing those dimensions. However, we do have the dimensions at Sta 123. On the windshield drawings, there is a note that states the profile at Sta 126 is the typical profile for the windshield moving forward…logically you would think therefore Sta 126 will match the profile at Sta123. I checked this and it is close but because we also have the glass profile at Sta 126.093 any minuscule deviation will have a profound impact on the curvature when eventually this is lofted. To be sure of maintaining good curvature continuity I lofted all the center section glass profiles and extended the edges by 12mm and then trimmed this resulting profile at Sta 126 and Sta 154. This gave a good result and to check I then swept the Sta 126 profile along the line of the Windshield center line and examined the profile with the known profiles at Sat 123 and of course at the interface with the fuselage. The variance was something close to 0.03mm…that is good enough for me which now ensures good curvature continuity throughout.
Aft Section; As mentioned we don’t have very much ordinate information for the Aft Fuselage Section which will require extensive research of all parts drawings from which we can extrapolate individual points that hopefully will be sufficient to fill in the blanks. The image on the left is a good example where I have drawn the various profiles for the fillet tangent to the fuselage and the wing.
Most of the drawings for the Fillets include the Tangent Points for the Fuselage and the Wings which I included in the model that now collectively gives us a reference line for the side of the Aft Fuselage at the top and bottom of the wing. Each fillet curve was checked against the ordinate surface for the wing and adjusted accordingly taking into account the skin thickness; these were also checked against known bulkhead profiles in this area.
The second image; on the right; shows how we can also use the main longitudinal members in a similar fashion to help ascertain key dimensional information to assist with the development of the aft sections. The red lines are the longitudinal members where the part drawings contain relative dimensions to the Fuselage reference line and the Centre of the Ship. Again each of the dimensions was checked against known bulkhead profiles where the average variation was in the order of 0.012mm. It may seem too small a variance to be of any consequence but when I later have a need to use these lines when creating the surfaces they have to be exact…so in each case the point was adjusted to be an exact intersection with the bulkheads. Inventor is very fussy when lofting with a guideline with no room for error…so this has to be exact.
Ultimately the goal is to find as many part drawings as possible with dimensional information that I can use to eventually have enough data to build the relevant missing Aft Section profiles. Typically this will be the main longitudinals, the skin parts, and of course the fillet drawings. This is painstakingly slow work as virtually every part drawing in this area is being reviewed for potential data that will help me achieve this goal and there are a lot of drawings!
Similarly, the process will be the same for the fuselage area forward of the cockpit which again sadly lacks a lot of key profiles. The research is where the time is expended in developing these ordinate sets…so far for the fuselage alone, I have spent in excess of 6 weeks of continuous work to get to his point, and still a lot to do.
Finally, both the P-39 and the P-38 ordinate dataset models are updated with a new approach to how these datasets are being built. I still have the extensive Excel spreadsheets listing all known dimensions but for the model, each ordinate profile is now inclusive of a surface patch. What this means is that conversions of the model for use in other cad systems will now provide a surface plane as well as a sketch profile which helps the model builder very quickly create the bulkheads for these scale models.
The P-38 is almost complete with the Boom, Wings, Horizontal and Vertical stabilizers, Flaps, and Ailerons all modeled and recorded. The Landing gear is almost fully 3d modeled as well…which is great for those that are keen on super detailing their RC models. These models have also proven to be enormously useful for the Restoration groups one of which I already work with on a P-39 Airacobra restoration.
Update 14th June 2023:
I have been developing the key Aft center profiles at the top and lower part of the fuselage. This is actually quite exciting stuff as there are not a lot of pertinent ordinate dimensions for the Aft Fuselage so I resorted to building profiles from individual part drawings.
For each part sketch profile, I have extrapolated various curves to determine the center work points. What is exciting about this is the eventual lower fuselage curve (in magenta) is absolutely perfect…normally when you derive work points from half a dozen different parts in inches there is an expectation that the eventual curve would show the odd deviation…but it didn’t. The curvature analysis shows this to be absolutely spot on.
Update 23rd June 2023:
Fuselage Aft Assembly: Almost finished with the ordinate study for the fuselage Aft assembly. This work involved generating cross-section profiles from stringers, longitudinals, bulkheads and fillets to derive series of points from which to build the curved profiles. Each profile built is checked against the existing ones by lofting a new surface profile, then a sketch cross section generated to check the curvature maintains alignment with the existing profiles. This is done for every newly generated profile. Ultimately I will end up with the best-fit surface for the Aft Fuselage Assembly. All new points will be recorded in the Main Spreadsheet and fully dimensioned on individual drawings.
I had promised an article on the P-38 Flap CAD development as a follow-up to my earlier article on this topic…but I deviated slightly to address a question from a reader about Forged Parts.
Typically for all these aircraft Forged parts are the main element in the process of manufacturing complex parts that may be used in such applications as Landing Gear. Such is the case with the P-38 Lightning where we have the main support members that are machined forged parts.
I have touched on this briefly in previous posts: Technote P-39 Inventor Face draft and P-51d Mustang Tailwheel Down Position support. Those articles tend to focus on using the Face draft feature in Inventor and using Derived model parts to differentiate between model states i.e. Forged and machined. I should note that with the later versions of Inventor, it is possible to contain the various Model states in one part file but I prefer to use separate derived Part files. The reason is that they are in fact 2 very different manufacturing processes and the drawings for each model may be sent to different departments or indeed different companies. So it makes sense to keep them separate.
In the example above we have 2 components for the Main Landing Gear and the Nose Landing Gear. Both examples use the derived parts process as you can see. In this article, I wanted to cover some of the frustrating differences that you will likely encounter when building these models.
Forged Parts are notoriously complex and the Lockheed drawings tend to only provide the main dimensions and key elements often omitting small details that are likely to have been decided by the mold maker. To determine missing details I often build the models as a surface and then turn that into a final solid.
In the above images, this part had an elevated top and bottom section interspersed with a waveform for the main body. The 2d sketches were drawn outside the main part body to make it easier to visualize and manipulate the part data. This part used 3d intersection curves to generate a sweep path for the top and bottom profiles and the surface trim command to profile the main body.
Incidentally, although the sketches do not share the same space as the main model you can still select a single line from any of the sketches in order to trim parts and surfaces in the model…they do not need to be connected. I have often seen folks extrude surfaces from external sketches and then trimmings to that surface but you don’t have to do that…just select the line.
One of the key details that is not clear in this particular example was the protrusion just above the cylinder at the front of the model. All you have on the drawings is a line on elevation and 2 lines on the plan sketches..the specific details of how this small detail interfaces with the main body is down to interpretation. I modeled it with the flat upper surfaces tangent to the curved edge and applied a fillet to the intersecting sides. I did look at a number of variations but I think the end product is close to how it will actually be. This is the frustrating bit when trying to decipher designer intent with limited information.
Some of the complexity comes from how the drawings themselves depict the dimensions of the profiled sections. In the first image above we have the criteria shown as the center line of the section’s curved profile. The second image shows a different part however this time the dimensions are to the projected edge intersection of the curved profile. The third image is also similar where the dimensions shown are to the projected intersections. The final image is the Flap carriage arm with the dimensions shown to a dotted line which is not clearly defined on either the sections or the main views to determine what this actually is. After much deliberation, I deiced to interpolate this line as the projected intersection of the drafted sides with the top and bottom faces. I had initially suspected this was to the corner tangent but that would entail a very complex development process due to the varying corner radius.
As you look through the dozens of forged part drawing there are all sorts of variations on the theme with few consistencies. This is where you can spend a lot of time determining how these dimensions relate to the model and how best to incorporate this information in such a manner to keep the model as simple as possible. Consequently, it is not unusual to spend upwards of between 3 and 4 hours modeling the forged parts. I think for the most part where doubt exists to work to a projected intersection as the point of dimension…it will be a lot easier to model and saves a whole lot of frustration.
To give you some idea of progress on the Nose Landing Gear models:
In the latter 2 images, you may notice small differences which relate to the various model variances. I am modeling the P-38H and the comparison photo is the P-38J.
TechTip: Variable Fillets:
When modeling these complex parts often applying fillets can yield unexpected and undesirable results.
In the images above you can see how applying just standard fillets of different radii can result in quite an undesirable intersection between the flat plane and the circular node. What we need is continuity to achieve a smooth transition from one edge to the next as shown in the second image above. This can be achieved by using the Variable fillet feature.
Variable Fillets give us the option to vary the radius of the applied fillet. When you first apply the Variable Fillet you have a radius specified for the beginning and the end of the selection…you can apply additional points anywhere along the length of the selection to which we can adjust the radius at those points.
You can also add selection sets of edges to the original selection which have their own capacity for separate adjustment. To achieve our goal here for fillet continuity I have 4 selections: the top planar edge (1), the node circumference (2), the lower planar edge (4), and the remaining node circumference (3). It is important for each selection set fillet to have the same radius at each intersection to ensure continuity.
Each selection set is listed separately in the dialogue box and the way to adjust them is to simply select the edge selection as I have highlighted with the first one…this shows the applied points and values in the area below under the heading “Variable Fillet Behaviour”. I have added additional points to the planar fillets at 1 and 4 where the value is set to 2mm which then defines the radius between those 2 points. A small point worth noting is the diagonal draft parting line on the face of the round node that prevents selection continuity which is why we have 4 selections and not just one continuous.
It does not take long to do this and the end result is much more agreeable.
Just a quick update to share new and updated assemblies for the Wing Flaps, Centre Section Flaps, and the Horizontal Stabiliser.
This post was intended to be a detailed overview of the Wing and CS Flaps but I was keen to share progress on these main assemblies. I will revert back to the flap discussion in my next post.
An interesting point worth noting is the color coding for the Horizontal Stabiliser and Elevator. The Red ribs are exclusive to the forward Horizontal Stab area, and the Yellow Ribs are where the internal Horizontal Stab ribs and Elevator ribs share the same alignment.
For the flaps, the main surfaces shown represent the cutout in the wing ribs…the information for this is rather sketchy but more on that in my next post.
Each of these new assemblies also includes new Basic Geometry fully dimensioned drawings in DWG and PDF formats. Soon to be added to the P-38 CAD/Ordinate dataset, drop me a line for details or check out the CAD Resources tab at the top of the page.
The P-38 project is a study I have been working on for a while. In previous posts, I have covered the development of the Boom, Empennage, and Dive Flaps…even did a video on Youtube for the latter. Many of these studies are designed to research the operational characteristics of the component parts and this new study of the wing Flaps is no exception.
Essentially the flaps are split on either side of the Main Boom, with one being fitted at the Centre Section and the other at the main wing. These are activated by hydraulically controlled push and pull tubes with preformed carbon steel cables.
The extent of the operation of the flaps is controlled by guide tracks at each end, which incidentally is where this part of the project starts.
These guides are machined solid from forged Alumimiun blocks. In the image above the machined part is shown with the original forging in the background. It comprises 2 tracks with the upper track just over an inch wide and the lower track slightly smaller at 0.835″ wide. The blue part at the extreme end is a separate stop block.
I am currently working through the variations of these tracks for each location and although some minor differences they are all based on one type of forging, part #235452.
The forging drawing is not too clear about the definition of the track at the left-hand side which appears to drift slightly from the main track center. Understandably the lower part of the track walls deviate to align with the edge of the forging and therefore the main upper wall portion will adjust accordingly. I have improvised in developing this area and now that the project has further progressed there are a few minor changes I would make should this part ever be required for actual production. At this stage, my primary objective is the operational characteristics that are unaffected by this as the end product is the machined component that is derived from this forging.
Update: 26th March 2023:
Making good progress on the Wing Flap mechanism with the Carriage Assembly now complete except for a few standard AN nuts and Washers. The Carriage Assembly also shows the track surfaces to demonstrate the correct relationships between the rollers and the track.
The second image above shows the adjustable roller at the front end of the carriage. This is achieved by the use of an eccentric bushing item #221832 which fits into the retaining locking ring item #221741 at increments of 30 degrees.
Update 30th March 2023:
Have spent a considerable amount of time researching and resolving macro dimensional variation for the Flap Track guides. When I talk about macro I am looking at close to 1/128″ or 0.2mm…but it is essential to get this correct. The dimensions are blanked out for obvious reasons as this stuff takes a lot of time to research and develop.
Finally located the Flap Track assemblies in their exact position on the wings. I will actually build the final assemblies as 2 separate items; one being the Inboard Flap and the other the Outboard Flap. For now, the initial plan was to get to a point where the tracks are accurately positioned and ready for the next phase which will be the Flaps themselves. The final stage will be a working simulation to determine operational parameters but we are a long way from that goal at this time.
The tracks are currently shown in the assembly as surface models which keep it simple, however, when I get to the stage of finalizing the assemblies this will be fully modeled. This part of the project was surprisingly complex to achieve and every dimension has been cross-referenced and checked against known data…at one stage I had over 21 drawings one at the same time.
To help establish the starting point for the Flap Crriage I have an outline sketch of the key runner positions to which I later constrain the carriage parts in an assembly. It is actually quite a useful technique to use sketches to help establish relationships when building an assembly.
In summary, it is often beneficial to use surfaces in lieu of solid models for clarity when building these types of models as it is so much easier to see the key relationships between the main elements. Also using sketches to help align component parts in an assembly is a good work method and can also be used later when creating 2d drawings.
This more or less covers the basic setup for the Flap Tracks and carriages…the next article will focus on the Flaps and the eventual final wing Flap assembly.
This is what I am working on now: the P-38 Lightning Dive Flaps. At the moment this is a multi-body part file which I will then extract as separate parts and then assemble. The plan is to also include all the mechanical components to analyze operational criteria.
I shall add to this post as this part of the project progresses and perhaps add some notes on Simulation within Inventor. I should note that the external panels are actually transparent to show the internal lightening holes depicted in these images.
Update Dec 7th 2022:
The mechanical components are now modeled and temporarily located in the Dive Flap assembly. It is actually quite a substantial mechanism that is currently missing the main hydraulic operating cylinder. That component is a contracted supply unit for which I do not have any details so the simulation will use a proxy component for purposes of evaluation.
Before I get around to doing that I must first locate it on the underside of the wing…that will need to be partially modeled with local ribs and struts in order to define the fixing bolt locations.
Update Dec 8th 2022:
Another day, another update. I now have the rib at Station 146 and 158 modeled primarily to assist with positioning. The Lockheed layout drawing for this assembly is not included in the archive so I had a lot of research to do to get this thing in the correct position. What I noticed is an access hatch on the underside of the wing which this assembly effectively replaces so that was useful in this task, though I had a lot of rivet holes to position to get it right!
You will notice that the assembly now has the operating cylinder…I was lucky to find a drawing for this that provides much-needed data for maximum and minimum travel. Actually, I call it an operating cylinder but technically it is a linear actuator gearbox.
When I inserted this item into the assembly it provided a positional check on the main assembly.. which incidentally was perfect.
A few bits to tidy up and then move on to a simulation…I shall upload a short video to Youtube when that is done.
Update 9th Dec 2022:
I have cobbled together a quick video showing the operational aspects of the P-38 Dive Flaps. I am working on a more detailed video which won’t be ready for a few days…so check out this short version for now and let me know what you think.
The technical bit: Each of the 3 flap panels comprises 3 layers of sheet metal with a total thickness of 0.18 inches or 4.572mm. The mechanical assembly is fitted between Ribs at Station 146 and Station 158 and driven by a Linear Actuator. The Dive Flaps were designed to be retrofitted to P-38 Lightning prior to P-38J which explains the location of the main support frame which aligns with the hole centers for a wing access panel and the forward panel fitted forward of the main beam. For reference, I have included a selection of design operational data and some close-ups of the mechanism. that you may find interesting below.
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
Yet another challenging aspect of the P-38 Lightning is the Engine Cowls for the P-38J and earlier variants. As before with the Coolant Rad Scoop, there are very few ordinate dimensions so this will require a similar workflow by developing what we do know to help determine what we need to know.
Part of this development includes the Shroud Air Intake Scoop which will provide some key data for determining a partial profile for the top section of the cowl.
The engine cowl above is for the P-38J, I also have another work in progress for the earlier variants. Common to both forms is the Shroud Air Intake which is the main subject of this article. As per normal practice, I tend to first develop all the sketch profiles according to the drawing information…this is not always ideal for the CAD modeling environment but it is important reference material to ensure the final model is compliant.
The sections described in the drawing show a gradual curve intersecting arcs that form the scoop…early on I determined that these will need to be separate sketches as they will be modeled separately and then combined. The first arc section is important for sweeping the scoop duct and then of course building the profiles of the leading edge, so this was actually created as a full circle. The remaining profiles are retained as arcs.
This profile is a swept feature using the centreline (1) as the guide curve making sure that the orientation is set to Fixed and not Follow Path. All the arcs are set perpendicular to each other so it is important to make sure the swept profile follows this alignment.
Another important consideration for making the leading edge a full circle path is when building the actual leading edge. I elected to build sketch profiles at each of the circle quadrants to account for the variations in the curved swept surface. The edge profile actually extends inside to form a lip which I offset from the main surface by 0.1mm… sometimes if the surfaces are coincident this causes problems with later editing.
That worked rather well and gave me a smooth curved leading edge when lofted. A quick point to note is the loft does not do circular paths so the profile at D was duplicated and selected separately to complete the full circle.
The second image above is the fillet applied to the stitched surfaces of the scoop main body and the curved plate. This is a variable filler as I wanted to control the eventual curvature around the leading edge. Regardless of how careful I was to ensure perfect tangency at the leading edge, sometimes this is not always possible and micro variations can result in a slight imperfection which prevents the edge selection continuity for applying the variable fillet.
So what I did was select the edges as separate sections within the same command which you can see at E, F, and G. To define each of the edges you first select the edge from the top panel and then adjust the variables in the panel below. By doing this within one command Inventor will adjust the finished fillet to be continuous along all 3 sections.
The final model is rather good and very accurate. The key thing is to think ahead as to how you will model these objects from the outset so it is well worth taking your time to get this right.
Update 28th Oct 2022:
Just about finished with the P-38H Engine Nacelle…just a few items to add.
Just to give you some idea of the complexity of the underlying geometry: P-38H and P-38J Overlaid…
Update 6th Nov 2022:
This is the updated version of the Left-Hand boom on the P-38H Lightning.
Aviation CAD TechNotes 