Inventor

Grumman Goose: Hand Crank Gearbox

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Grumman Goose: Hand Crank Gearbox

It is not common for blueprints to be almost illegible, and without a Parts catalogue, understanding the mechanisms and operations of assemblies like Gearboxes can be challenging. This was the case with the Tail Wheel assembly I built for the P-51 Mustang and, of course, the current work in progress, Landing Gear Hand Crank Gearbox for the Grumman Goose.

I became captivated by this unique gearbox upon discovering its remarkable dual function: it not only raises and lowers the main landing gear but also manages the tail wheel’s movement. However, delving into the blueprints left me with more questions than answers regarding its intricate operation. Intrigued by its complexity, I decided to construct a working model and evaluate its operational characteristics firsthand.

The Gearbox consists of a central shaft featuring an ACME thread along which the Traveler Collar for the tail wheel moves. Additionally, it includes a bevel gear that powers the main landing gear struts, as illustrated. At the base, the ratchet lock offers two positions: one for raising and the other for lowering the landing gear.

I am eager to explore the operational parameters and the criteria for calibrating this gearbox to ensure smooth operation and timing. The available blueprints and installation manuals do not clearly outline how this setup is configured, so I will need to rely on some trial and error.

To successfully complete this assembly, we still need to finalise several crucial details, particularly the assortment of nuts, bolts, and washers. Fortunately, I have access to an extensive library of parametric parts, ensuring that I can efficiently source the exact specifications required for this project.

Developing these assemblies requires a significant investment of time and effort, but I believe this investment is invaluable. Often, manufacturers’ documentation is either unclear, incomplete, or entirely absent, which can create challenges for maintenance and operational staff. By constructing detailed CAD assemblies, we create a visual representation that not only clarifies the intricacies of the components but also serves as a critical resource in the field. This practice can facilitate more efficient troubleshooting, enhance understanding of the system’s functionality, and ultimately improve the overall safety and effectiveness of operations. By proactively addressing these documentation gaps, we ensure that maintenance teams are better equipped to perform their tasks with confidence and precision.

In previous articles, I shared my aspirations to develop a 1/16th scale RC model based on this project. I realised that this gearbox configuration could serve as inspiration for creating a scaled version that would operate using a single servo to raise and lower the model’s main landing gear and tail wheel.

Update: 28th Jan 2026: Spur Gears

The Spur Gears and Splines dimensions are shown as “over pins”, the diameter of which are 0.140 in.

CAD software generally does not facilitate this type of dimensioning for gears, so first we have to determine the important gear parameters using online calculators like this one at Zakgear.com:

The Diametral Pitch is 12 (number of teeth/pitch diameter), which we then input into the CAD gear calculator. To match the calculated diameters from the Zakgear website, we need to adjust the Addendum to 0.800.

By overlaying the CAD data onto the Zakgear data, we achieve a good match. It may only require microdimensional adjustments within stated tolerances to ensure perfect alignment for a correct setup.

Restoration Insights: The Risks of Working from Blueprints

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Restoration Insights: The Risks of Working from Blueprints

Restoration projects…is working directly from blueprints a good idea?

A company I know is currently restoring a P-40N aircraft, and I came across several posts where they highlighted concerns about the alignment of the fuselage frames. The misalignment was approximately 1/8 inch (3.175 mm), which is quite significant. From their posts, it seems they are working directly from the blueprints.

Throughout my experience in the industry, I have encountered occasional dimensional errors in the blueprints of nearly every project I have been involved in. This recurring issue fuels my passion for my work. I strongly believe that dedicating time to meticulously developing these designs in CAD is essential for uncovering any anomalies before fabrication begins. This proactive approach not only enhances the accuracy of the final product but also ensures a smoother assembly process. However, I recognise that this level of diligence may not always be feasible due to various constraints.

For example, if you are building the fuselage frames and one of those is 3mm out of alignment, you naturally assume that it is incorrect. That may not always be the case because, as the assembly progresses, there may be factors that are as yet unclear that influence this misalignment, or it could simply be a mistake. You won’t know for sure until all the parts are assembled.

Consider for a moment the following example from the Grumman Goose Tail Wheel blueprints.

I have intentionally highlighted the revision box to indicate Revision H. This revision specifically documents the change in dimension from 6.5 inches to 6.25 inches. If we examine the other dimensions, the blueprint specifies that the centre axis for the fork should be set at a 45-degree angle. Additionally, the key setting out dimension is 5.25 inches, measured horizontally to the intersection of the vertical axis and the centre of a 1.25-inch radius.

This immediately rings an alarm bell…to achieve a 45 degree fork with the dimensions shown, you would expect that 6.5 inches is in fact correct and that in this case the 6.25 inch is not. But yet it was the only purpose in this revision to record a change to 6.25 inches.

The tilde “~” indicates that this dimension is approximate, but for this to be a revision would suggest that the actual dimension is closer to 6.25 inches than it is to 6.5 inches.

To ensure all key dimensions align with the blueprint, particularly noting that the 6.25-inch measurement is approximate, the setout for the Tailwheel Fork should follow the above depiction. However, we now have a concern: the vertical post is meant to extend to the diagonal intersection and be welded to the curved plate’s interior. As shown in Detail B, the edge of the posts is too close to the fork’s edge, while the blueprint indicates they should be positioned further inward. Additionally, the actual component, seen in the following screenshot, reveals that the heel of the fork is more bulbous than the blueprints suggest.

There was a reason for the 6.25-inch revision, though we do not know it at this time. Therefore, in order for this to be correct and meet all criteria, something other than the 6.25-inch dimension should change.

Honestly, I’m not sure what the correct answer is here. Unless I can physically get my hands on the real thing, this will likely remain a conundrum. I will retain the CAD design as it is for now, which serves my intended purpose to demonstrate the deployment parameters of the Tail Wheel and provide clarity on the assembly configuration.

I recognize that the dimensions in most blueprints are generally accurate, with only a few exceptions. When budgets and schedules are tight, it may not be practical to explore entire assemblies in CAD before fabrication. However, in cases where discrepancies are identified, I recommend examining all relevant assembly components in CAD. This will help in identifying the correct solution and understanding all influencing factors before making any changes.

Grumman Goose Project Updates

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Grumman Goose Project Updates:

Before I dive into the exciting updates about the Goose, I would like to take a moment to address the recent posts regarding the SU-31 RC project. I’ve dedicated considerable effort to this project and have now brought it to a natural pause. I’ve revamped the SU-31 page, where you can explore the latest advancements, including the availability of detailed CAD designs in both full-scale and intricate 1/16-scale models. I encourage you to take a look!

I am currently working on a series of updates to the Grumman Goose project. This will include full surface modelling and comprehensive assemblies for the Landing Gear and Engine Nacelle.

The surface panelling is being implemented in a series of carefully planned stages to effectively accommodate the significant variations in surface contours that occur along its length. To achieve optimal curvature continuity for the surface panels, I have undertaken the modelling of multiple fairing contours, each meticulously designed to ensure a seamless integration with the underlying structure. This approach not only enhances the aesthetic appeal but also ensures structural integrity, as it allows for precise adjustments that align with the dynamic shifts in the surface geometry.

The Landing Gear will be fully modelled, including detailed working mechanisms that will later be the driving parameters for a deployment simulation.

I am currently exploring various options for replicating the components as high-quality 3D prints. This initiative is part of a future project aimed at demonstrating operational criteria in a tangible, physical form. I plan to utilise advanced 3D printing techniques and materials to ensure accuracy and durability in the prototypes. Additionally, I will conduct thorough testing to assess their functionality and performance. This approach will not only enhance the visual presentation but also provide a practical, hands-on experience.

As a basic test to check the viability of the project, I 3D printed the front cover of the secondary gearbox to see how it worked out.

Part #9632 front cover. Printed on an Elegoo Centauri with 0.12 layer height using PLA+ filament. The surface was surprisingly smooth with good dimensional accuracy. Eventually, I will print all the internal gears and check operational criteria.

The engine nacelle is still very much a work in progress, which I will feature in a future post. Following the example of the SU-31 project, the Grumman Goose will also be available in a 1/16 scale version suitable for RC projects.

For reference, this is the Landing Gear Assembly Drawing #12600.

Landing Gear Deployment Positional Representations:

This drawing, created in Inventor, utilises positional representations in the assembly to illustrate the Landing Gear deployment.

Goose Bumps!

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Goose Bumps!

The Grumman Goose project is both challenging and frustrating; it is definitely not a straightforward aircraft to work on. I have primarily focused on updating the empennage, which includes the vertical stabilizer, horizontal stabilizer, rudder, and elevator. During the development of the ordinate study, I observed discrepancies in the documented locations of various components. Let me explain what I mean.

Upon reviewing the CAD drawings on the left and comparing them with the Maintenance Manual diagram, I noticed that the level of the ribs varies by 1/16 of an inch. This discrepancy caused me immediate concern, and I began to wonder where I might have misinterpreted the Grumman drawing data. Therefore, I felt it was necessary to review and verify the information.

Initially, we do not have any reference location information on the Rudder Layout drawing. Normally, you would expect reference dimensions to the fuselage centerline or a fuselage station reference, but there are none. We do, though, have locations of the Hinges on other drawings for the Station bulkheads and Fin layout which in turn will help derive location information for the Rudder.

The first image above is the bulkhead layout at Station 36, which specifies the centre of the hinges 1, 2, 3, and 4 relative to the Fuselage Ref Line.. The second image is the bulkhead at Station 33, which shows the dimension of 65 13/16″ to the top of the Lower Rib on the Vertical stabilizer Fin.

I am looking to verify the dimensions and locations of the rudder ribs and hinges in relation to the Fuselage Reference Line. To accomplish this, we will start with the information we have and determine what additional information we need. The first image confirms that the CAD drawings for the rudder accurately depict the positions of the hinges. The second drawing further supports this; the “Top of Rib” location refers to the lower rib of the fin which includes the locations of the hinge centers. At this point, we have established the correct locations of the rudder hinges from two different sources.

Having determined the hinge locations, we know that the ribs for the rudder are offset by 5/8″ on either side of those locations, which allows us to derive the final levels noted on the Rudder Layout CAD drawing. Does this mean that the Grumman drawings, and therefore the CAD drawings, are correct while the manuals are incorrect? Yes and No…let me explain…

The first image is the Lines Diagram for the Vertical Stabilizer Fin Ribs. In the Table of Offsets, you will notice a list of dimensions from the “Root,” with the first rib specified at 10 7/8 inches. If we overlay these dimensions onto the CAD drawing, we observe a 1/16-inch discrepancy to the top of the first rib. However, all other sources, including those mentioned above and additional references not listed, such as the fuselage Lines layout, indicate that the top of the rib is correctly positioned in the CAD model (second image), contradicting the information provided in this Table of Offsets.

So what is going on?

We should take into account the revision history of the Grumman Goose development. If you examine their drawings, you’ll notice that they have made numerous revisions, some of which are labeled with letters as late in the alphabet as “R.” That indicates a significant number of changes.

I believe that various details have changed over the year, with the more prominent aspects being updated while the less prominent drawings remain unchanged. Regarding the manuals, it seems they were created early in the project, and it may have been considered too labor-intensive to update the level references. This aircraft is quite complex, and I can only imagine the effort involved in both its development and the ongoing updates to its design.

Whenever a small anomaly becomes apparent, I will make an effort to gather information from other drawings to verify the final result. This is one reason why these Odinate studies take so much time; it is crucial to ensure that the final study represents the most accurate dataset possible. If I were building a Grumman Goose replica, I would be using my datasets.

Progress Update 18th March:

A few screen shots showing the latest updates to the JRF Goose. The wing has been completely rebuilt with all dimensions verified.

Technote: P-39 Door Handle CAD Solution

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Technote: P-39 Door Handle CAD Solution:

This little part at first glance seems fairly straightforward, but there are a few caveats.

It has been a while since I specifically wrote a CAD solution Technote, and this seemed to be an ideal subject for surface modeling and 3D sketching. The dimensions define the outline for the front view, which is fine, and the plan view, which details a thinning of the handle cross section.

The thinning of the handle occurs in a specific plane as indicated in the plan view, while the front view maintains a consistent full depth diameter. Before diving into the modeling process, it’s important to pause and consider how to approach this design. Typically, my first step involves sketching out what is already known, which helps clarify the information we still need to gather. This initial sketching phase is crucial for laying the groundwork for an effective modeling strategy.

In each case, you’ll notice that these profiles are not closed. The base lines shown in the front view are defined as construction lines, and the end curves in the plan view are also intentional. This design choice allows the main profile lines to be used later for creating a Loft and for selecting a 3D Sketch Intersection. The center line of the arc in the front view will serve as the second selection for this 3D sketch. Additionally, note that the curves in the plan view are elliptical.

The purpose of the 3D Intersection sketch is to define guidelines for the eventual loft. Using the 3D sketch feature, we first select the center line from the front view and one curved edge from the plan view sketch. The resulting intersection will serve as the 3D path for the loft. This process needs to be repeated for both sides of the handle. The ellipses that will form the ends of the loft are created in a separate sketch from the previously mentioned plan view. This keeps them as distinct entities.

Hold on a moment; where did the ellipse in the middle of the arch come from? If we simply loft the two end profiles of the arch, as shown earlier, we can create an acceptable model, but it won’t be ideal. In the second image, where both surfaces are overlaid, you can see that this approach tends to create a diamond-like cross-section in the center. While this is not entirely incorrect, incorporating the ellipse in the center of the arch results in a much better finished surface, ensuring good continuity, as demonstrated.

Once we have the arch lofted surface, we extrude the centre section circle to match the surface contours.

We then use this extrusion to trim the underside of the arch surface, apply patch surfaces to fill in the ends of the arch and this centre section. Then stitch everything together and we have the main solid model.

Apply a fillet as shown to the underside; note the fillet in this case is better selected as a tangent fillet and not a G2 curvature. It is often tempting to overuse the G2 fillet option as the perceived notion is that it creates a smoother finish, which by the way is correct, though in a case like this it tends to sharpen the fillet corners which is not good. Something to watch out for when applying fillets.

To finish up we add the holes as specified, fillet the ends of the arch (a good opportunity for a G2 fillet) and add the part identifier. The final part should look something like this:

In summary, when developing surface models, it’s beneficial to explore your options and start by creating sketches that support your plan of action. Consider using 3D intersections to define loft paths, and incorporate additional geometry as needed to maintain the circularity and continuity of the final surface.

This part is ready for manufacturing, which will probably be 3D printed for this static display restoration.

Typical Design Workflow:

Usually I would initially receive an inquiry via email from companies like Planes of Fame for a 3D CAD model of a specific part or assembly. Typically, the request includes a brief description of what is needed and not necessarily the actual part number. In this instance, it was for “the handle for operating the window glass.” I then searched through my archives to locate this item, reviewed the part’s blueprint, and checked which parts or assemblies it connects to ensure I have all the relevant information.

I will make every effort to start working on the CAD model as soon as possible, regardless of the time of day, to minimize any delays. For example, I received an inquiry about a part at 9:17 PM last night for the “P-39 Throttle Control Mount.” Following the established procedure, I was able to begin working on it relatively quickly on a Friday evening. The finished part (#12-631-027) was completed and submitted on Saturday at 11:17 AM. The final design included both the original 3D CAD model and a fully dimensioned 2D drawing, which is essential for verifying that all dimensions conform to the original blueprint.

This part will likely be 3D printed for the restoration of the static display, so the 2D drawing serves both as a dimensional check and a reference for manufacturing. If the inquiry had required a metal casting manufacturing process, the drawing would include more detailed information about part machining and the tolerances necessary for a full-metal manufactured item.

If you’re looking to bring your ideas to life with accurate 3D and 2D CAD models for replica parts, I would love to help! Don’t hesitate to get in touch hughtechnotes@gmail.com

Technote: P-47 Canopy Contour Lines

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Technote: P-47 Canopy Contour Lines:

In a previous post, I discussed a minor discrepancy at the intersection of the canopy contour lines and the fuselage contours. This discrepancy is quite small, measuring around 0.3 mm, which is generally considered an acceptable tolerance. The purpose of these CAD/Ordinate studies is to provide the most accurate dimensional record for the various aircraft currently available, so it is crucial to ensure that these measurements are correct. However we must first understand design intent and check that the canopy contour ordinates are designed to match the fuselage contours.

Depending on the aircraft manufacturer, the canopy contour lines may not align exactly with the fuselage because the canopy surface is typically offset from the fuselage surface, which is reflected in the information provided. For the P-47 you can see the ordinate points are an exact match with coincident curves from the fuselage surface therefore the tangent line is actually defined by the intersection between the canopy contours and the fuselage contours.

Initially, when I started this study, I profiled all the ordinate points for the canopy and compared this with the fuselage surface, revealing a minor discrepancy. The thing is we don’t have to fully connect all the coordinate points for the canopy, just the points above the intersection line.

First, we need to define the actual definition of this intersection on the fuselage surface which will be transposed to the canopy model. We take the vertical dimensions from the fuselage centre as defined on the canopy ordinate drawing #89F11456 and create a sketch which will be lofted to split the fuselage surface. On the second image above you will notice a number of prominent points on the upper curve profiles. These ordinates are not shown on the early P-47D drawing but are shown the on the later P-47D and P-47N ordinate layouts.

Initially, I opted for a tangent spline curve to complete the main circular profile of the fuselage bulkheads as per the ordinate drawing thinking that the relevance to the finished profile was nonessential. However when I compared the first run of the canopy and fuselage alignment studies I found that it was necessary to include those additional ordinates which are now included in the spreadsheet record.

These images show I have opted to correct the minor discrepancy by only profiling the canopy to the actual intersection line. I should note the Canopy and Fuselage are separate CAD models which means I can derive the surface from the fuselage model and manipulate it as required in the canopy model without affecting the original. For each canopy station, I projected a section thru the fuselage surface which gave me a spline to which I could add a tangent constraint when profiling the canopy lines. The images show the initial interpretation of the canopy profiles and the corrected profile in red (construction geometry omitted for clarity).

Tech Tip: if we had instead derived the station sketches from the fuselage model and then projected this in the canopy frame sketches as an outline we would not be able to add a tangent constraint. This is a limitation with Autodesk Inventor when working with splines and the workaround is to project a surface cut section as I have done above.

For each canopy station, I am only sketching the ordinates down to the intersection line with the fuselage and adding a tangent constraint to the projected fuselage profile curve. Because we split the fuselage surface we will have a point at the split that we can use in the profiling of the canopy frames.

The actual skirt for the canopy obviously overlaps the fuselage surface and therefore we will have to define the edge relative to the tangent intersection line. As mentioned before we can manipulate the fuselage surface that is derived in the canopy model which means we can trim that to suit without impacting the fuselage model.

The tricky bit is ensuring that the edge of the skirt is exactly the same dimension from any point along the intersection line and this is how I do that.

The first thing to do is create a work plane perpendicular to the intersection line and draw in a partial curve and then sweep this along the intersection line path. The reason for this being a partial curve and not a full circle is because there is a tight radius at the front edge of the canopy which may not be possible to traverse using the sweep command if this was full circle.

When this is done it is a simple exercise to trim the derived fuselage surface to obtain the skirt surface.

By creating a curved sketch and sweeping along a curved profile we ensure that at any point along this path, the distance to the resulting edge is exactly the same. A similar technique will be employed to develop the finished edge of the glass panel models.

I still have some work to do on the windscreen portion of the front canopy and then I will fully model the structural components.

Republic/Ford JB-2 Update

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Republic/Ford JB-2 Update:

The JB2 project is progressing quite well, with most of the structural elements in place. I will be doing a lot more detail work on the surface skin and, of course, adding the main support elements for the engine structure. In the interim, I thought it may be prudent to post a few images of the project for your perusal.

Comments or inquiries as usual to hughtechnotes@gmail.com.

Update 21st Jan 2025:

Technote: Accurate Label Placement

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Technote: Accurate Label Placement:

For instrumentation Panels, the location and size of text is very important to ensure clarity. This is usually well documented on the manufacturer’s blueprints so it is essential we get this right. In Inventor for example and I am sure it is equally similar in the many different CAD programs the key is the Text justification…let me show you.

First of all a quick update on the P-39 Restoration progress. Much of the recent discussions revolved around fabrication and 3D printing. As mentioned in the previous article this restoration is a static display for which many of the parts will be 3D printed, although the key aluminium panels will still be fabricated as such. The very latest part to be issued for fabrication is this small Switch Box on the Radio Console.

A surprisingly complex box which will be 3D printed and the Nameplate will probably be CNC. The dimensions of the main box are not defined on the Bell drawings so I had to interpolate from the known information and other Bell references to determine the final dimensions. This took into account the clearance from the Drive Shaft connecting flange which is in very close proximity to this box. This also fits quite well into our discussion here on Label Text Placement.

Typically on the Bell drawings, for example, the panel drawings include the height and location of the Label text similar to the following.

The way we do this in Inventor is by using the Text justification feature in the text editing box.

In the first image, we adjust the justification using the icons at “1”. If the dimension to the text label is to the bottom of the text we set the vertical justification to the bottom and if the placement horizontally is to the centre we centre the justification. When you exit the Text editing box a Text outline box is shown in dotted (this is optional so make sure you switch that on). The appropriate edge of the dotted line frame automatically aligns with the justification of the text entities. This dotted outline can be dimensioned and constrained as you would any graphic sketch entity. The second image shows some examples of how the dimensions of this outline relate to the justification.

It is not unusual for the overall width of the label text to also be specified in which case the “Stretch” value can be adjusted accordingly, entity “2”. At “3” we set the font and height, make sure you have the text highlighted in all cases or these adjustments will not be applied.

Interesting to note that the text outline can be useful if you require a frame around your text. The dotted lines can be changed to normal sketch lines and extruded or embossed as required.

There are a lot of features in the text editing dialogue which I may do as a technote further down the line but for now, to get the label text in exactly the right locations this is the way to do it.

F4F/FM2 Wildcat Landing Gear Update

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F4F/FM2 Wildcat Landing Gear Update:

I have been busy with the Landing Gear CAD model for the F4F/FM2 Landing Gear assembly.

These images give you some idea of the progress to date. This is quite a challenging project due in part to the poor quality of a few drawings but also to the ongoing checking of dimensional relationships between the parts. Most notable is the forward Drag Link Support where you can see several red lines which is a visual indication of stated minimum and maximum tolerances. Also on this part, it is worth noting that the top pair of main holes are at 4.0625″ x/centres whereas the lower pair is at 4.1557″ x/centres…a minor variation but obviously critical dimensions.

The roller chain sprocket is a calculated profile to suit the specified roller chain; there is a smaller sprocket yet to be added to the Retracting Mechanism gearbox. This part of the project will take a while to complete and it will eventually also include the Engine mounting frame.

Technote: 3d Modeling to Clarify Assemblies

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Technote: 3d Modeling to Clarify Assemblies

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