Aviation

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-38 Lightning Engine Cowl

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Technote: P-38 Lightning Engine Cowl:

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.

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Technote: P-38 Lightning Coolant Rad.

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Technote: P-38 Lightning Coolant Rad Scoop.

My latest endeavour is to model the Coolant Rad Scoop and later on the Engine Cowl for the P-38J. This is the Coolant Rad Scoop which was very challenging. There is not a lot of dimensional information on the drawings for this scoop which is larger and wider than the previous versions.

I would say this finished model is probably as close to the real thing as I can get given the complete lack of decent information. The Lockheed drawings for this scoop are largely predicated on known ordinate information which unfortunately is not available in the microfilm archives. What we do have though is a 5″ grid overlaid on the drawings…this in itself is a puzzle because what they have done is divide the drawings into 5″ square grids which may or not be relevant to end views and cross sections…so using the grid as a positional aid is inconsistent.

There are of course good references to the Stations which help a lot. One of the key decisions is interpreting what is an arc radius and what is a spline…I made some decisions on this early on and opted for a circular profile of the inlet and the second frame and beyond that a spline with ordinates at every 5″. It was a close match to the profiles on the drawings but if it is what the designer intended I have no idea.

Scaling digital copies of the drawings and using them as a background for building CAD models is not something I am keen on doing. I did write an article way back on scaling in X and Y directions…I shall get the link and post it here.

Fortunately, this model is being used for a CFD study so microdimensional accuracy isn’t required. The final model is what is best described as a close approximation…I don’t do close approximations…this is the exception…though I may have to undergo a similar exercise for the engine cowl!!

Inventor is probably not the best CAD product for serious surface modelling that is dependent on dimensional information. Sure they have the usual lofts, patches, sweeps and of course freeform. Freeform is a very organic feature that can work with other surface-derived types but it does not regenerate when that sketch geometry changes; a serious omission which I understand is on the Autodesk to-do list. Even the standard Loft feature is flawed.

For example, if you have 2 sketches that contain concentric profiles (like the ends of a tube) this cannot be lofted in Inventor…it just cannot be done. The other issue I have with this command is when you loft using guidelines or rails. No matter how precise your modelling there will be times this will not work…so you redo the lines over and over again…double checking everything and eventually it may work. Yet if you use the sweep command using the same profiles and rails it will work…so there are some serious issues with lofting that Autodesk really need to fix.

I think Autodesk need to take a leaf out of the Dassault workbook…I believe it was in Solidworks 2010 that Dassault decided to revise all the commands and features within the product…resolving glitches, adding functionality to existing functions and generally cleaning up the product. The main fear of the media at that time was whether there was enough to tempt users to upgrade…that was a stupid concern if a product is better and everything works as it should of course that is an absolute no-brainer, folks will upgrade and they did.

Even though Inventor has a number of glitches, I quite like the product and it is generally rather good but I do think it could be a lot better. When something does not work as it should then you can spend hours just developing workarounds to achieve the end result…time for a product cleanup.

I actually prefer Solidworks but Dassault does themselves no favours when it comes to product accessibility. You can’t just download a 30-day evaluation copy whereas Autodesk has a better approach with accessibility to their products. In fact, to get a 30-day evaluation of Solidworks you have to sit through a meeting with their sales rep and only then will they load it onto your computer for you…this is a real pain that you can’t just go online and download a copy. They do have an online access portal but for folks like me, that is not convenient. I don’t have time for sales reps, all I would want to do is buy online and download without the sales crap…you can buy Autodesk products online but not Dassault.

Getting back on the subject, what I wanted to mention is surface modelling. Generally, there are a few conditions for generating surfaces with Direction, Tangency or G2. If you are lofting or creating a sweep from a sketch you won’t have the latter 2 options but if you use a surface edge as a base for a loft you will get Tangent or G2 options. I like the option of G2 but comes with restrictions…it can cause problems with applying fillets (particular variable fillets) and surface offsets..so if you plan to do these late on in the model development stick to tangency. Variable fillets will not give you continuity with G2 surfaces.

When using guidelines or rails to control the curvature of a surface loft please consider using them judiciously. As I mentioned in the previous article overuse of constraining elements can create problems with the eventual surface generated. In the first image above I have several guidelines drawn but only a few have been selected…this gives you options so that can pick and choose between the various guidelines to see how the eventual surface evolves so it is worthwhile spending the extra time having these available…it does help.

When you do run into problems with surface modelling using Lofts or Sweeps occasionally it helps if you delete that surface and replace it with a Fill Patch…the reason for this is that you have more control over each edge of a surface patch that you would not otherwise have with those features.

The Scoop actually turned out quite well…it was a frustrating journey to get to this point but it is worth it.

Update 20th Oct 2022:

I decided that it would be prudent to also develop the earlier variant Coolant Rad Scoop for the P-38 D, G, and H models.

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Aviation Photography

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Aviation Photography:

I mention in my Bio that I used to indulge in Photography which I got into in the late 70s. In those days it was mainly portraits, fashion and hairstyles…I did okay and had many images published at that time. I ventured into many fields of photography throughout the years; including industrial; which of course was an advantage for me also working as an engineer…I had ready access to exciting material.

Photography though was always a part-time interest, something I was passionate about but not something I pursued commercially after my stint in the late 70 and early 80s (even then only part-time). Actually, when I got married and had kids the focus was entirely on them; which of course was amazing times. I photographed everything and everywhere we went as a family…great memories.

I also enjoy photographing aircraft. These were taken at RAF Leuchars in Fife, Scotland at the last ever International Air show before the base was transferred to the army in 2015. These were all taken with the Sigma DP2M camera…one of the Foveon sensor range of cameras. The Sigma camera was legendary for the incredible detail they could capture, though admittedly they were a real pig to use!

I will look through the archives and process some more and include them in future articles here. I have these online and are available for download here…full-size images (roughly 4704×3136) in Jpg format.

Aviation Photographs Download Link

Technote: AN115 Shackle Model

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Technote: AN115 Shackle Model:

Working my way through the various AN Standard components I thought it may be prudent to write up a quick Technote on modelling the AN115 Shackle.

At first glance, this seems to be a fairly straightforward item to model, however, getting the transition from the flat section to the curved ring is rather tricky. When you view this item on Google and look through the various images of the finished product there are visibly small variations in how this has been interpreted. Needless to say that I have my own interpretation to achieve a smooth integration while ensuring the integrity of the finished model.

First of all, ignore the bulge on the right-hand side…for some part numbers this area is elliptical and not round…so for development reasons I have purposely exaggerated the profile to test the iPart creation.

What I was looking for was to achieve a smooth transition along the edges of the flat portion to merge with the round profiles of the ring section. My first attempt was simply to have a curved edge at point “1” that was tangent to the ring and the edge of the flat section. However, that did not achieve a good result because the top and bottom surfaces of the flat section coincide with the ring at different points which created a small twist when lofting the sketch profiles.

I found the best way of solving this was to introduce a small horizontal line tangent to the ring section profiles. For the inner sketch, this was only 0.2mm which translates to just over 2mm for the outer sketch. The points shown as “1” and “2” are the centre points projected from the ring centre line which is the start point for those sketched horizontal lines.

Now when we loft the 2 sketches we have a good square edge for the flat section…by the way, I should note that the flat section is initially lofted as a separate solid because we still have one step to do before we merge the solids into one. After lofting the flat sketches we still have to trim the resulting solid to follow the centre of the ring surface.

This is simply done by extruding a surface from the centre line of the ring as shown and then trim back the excess from the flat section model. Now we merge the 2 solids and we end up with a shackle that has a smooth transition from flat to ring without any twists or surface anomalies.

The bulge is still there on the final image…that will be gone once I finish filling out the table with the various part number dimensions.

So sometimes when you are modelling a complex item like this it often helps to introduce a minor feature (in this case a small 0.2mm line) to ensure that lofting and extrusion activities provide the desired end results.

AN481 Clevis Rod End:

Another tricky item to model is the AN481 Clevis Rod End. There are 4 variations on the same model that comprise 2 sets each with a narrow gap and a wide gap. Technically it is possible to create all variations in one part file and use Suppress and Unsuppress options to exclude or include features…however, I decided not to do that because it can be a real pain adapting the model if the regeneration does not quite work the way one expects it to. The model is not that complicated so it was just as easy and to be honest much tidier to create separate part files for each variation.

As usual for further information please get in touch at Hughtechnotes@gmail.com

AN116 Shackle:

The AN116 Shackle PIn is shown with a rounded head which I decided not to model due to the lack of detailed information for this pin. I tend to shy away from modelling components where there are no specific dimensions. I am not sure this is critical except where accessibility for tightening the pin is a consideration.

The new Parts Library should be finished at the end of August…I will advise.

New Project: Standard Part Libraries

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New Project: Standard Part Libraries

Many moons ago I started a project to develop libraries of Aeronautical standard parts according to the various National and International standards pertinent to aircraft design and maintenance noted in this article.

https://hughtechnotes.wordpress.com/2015/07/26/naa-p-51d-mustang-standard-part-models-specs/

Using the original standards from the wartime era and the updated, often replacement standards, I figured it would be a good idea to develop this project further. I am aware that there are many different CAD systems so it would be folly to just develop this for just one product.

The above products are currently available in the Resources Tab of this blog and though included with the Mustang P-51 Ordinate/CAD dataset are standard for many aircraft of this era and accordingly are available separately. This existing collection is already very comprehensive with over 300 parts modelled and listed, though these are in line for an overhaul and update.

Moving forward with this project I will develop the configuration spreadsheets exactly as per the original specification tables set out plus any additional dimensional data that will be required for modelling. This will be accompanied by a DWG file as a template to use when developing your own equivalent of an iPart. Essentially putting together a dataset that anyone can use regardless of what CAD system they are using.

Additionally, standard metal work profiles will also be developed and produced in a similar manner.

There is a catch: This will take a while to do and probably won’t be ready until October. Typically the study will comprise a basic dimensioned drawing exactly as per the reference Standard with accompanying spreadsheets. There will be separate spreadsheets for each part number in a collective Standard, though there may be only one drawing. For example AN21 THRU AN37.

The way to use this dataset; regardless of the CAD system; is to first develop the part model naming the parameters as defined in the spec (you can use the DWG for your sketch template). In this example, the first 2 columns are generic to the specific CAD system with the first column being a unique value. From LENGTH to Dim P, in the table, these are the main geometry parameters. The Hole1 column has values “Suppress” or “Compute” which is an instruction to exclude or include the hole. The Thread parameters are defined as a Designation and Class which are standard integral parameters; those names may vary accordingly. Typically in Inventor, they can be found in the iParts Author as follows:

Once you have your Part modelled, open the iParts author and set up the first line of the table…you just need the first line at this stage Close the Author and open the table in Excel and copy the contents of the provided spreadsheet data tables above… ignore the header/titles. The iPart table will now be updated with all the above variations. It does not matter if your part template is Metric or Inches as the part dimensions are predefined as inches and will automatically recalculate depending on your template standards. You can of course already do this with the existing iParts but they are not inclusive of dimensioned drawings…so you have a bit more work to do referencing the actual standards for parameter names. That’s what this project work is designed to do…essentially finish with full documentation.

These spreadsheets and CAD profiles will enable anyone to very quickly develop a standard library in their own CAD system…an important resource and time-saving endeavour. I should note the actual AN and MS standards are available online for anyone that wants to access them. I have provided a link below to my previous article on this subject.

https://hughtechnotes.wordpress.com/2022/02/17/technote-manufacturers-standard-parts/

Update 27th July 2022:

Blimey, this is quite an awesome task…I envy those that build the standard libraries in the many CAD systems that contain thousands of parts…this will definitely take a long time.

Many of the parts are relatively straightforward like Bolts, Castle Nuts, Clevis Pins etc that require nothing more than basic dimensioned drawings. Occasionally though many parts will require additional sketches to clarify the profiles, like this AN667 Terminal Fork End. Also in similar cases, the model will be dimensioned to As-Fitted/Swaged for use in assemblies. You can basically ignore the Scale as all the DWG versions of these drawings will be 1:1 according to the part number actually modelled.

This is a list of the Specifications I am currently working on. Many of these are updated versions of the existing standards available on the CAD Resources page. The updates include marginal improvements to the 3d models, additional data and verification of listed dimensions. The data sets also include dual part numbers where an item has been updated to a newer standard the new designation is noted alongside the old.

It is very important to get this stuff right, to ensure the part designations and representations are correctly defined in the assemblies. Have you ever tried to figure out assembly configurations from the NAA assembly drawings or picked your way through the Parts catalog just to identify a single connection for a clevis, nut and bolt, turnbuckle or whatever…it is time intensive. It was this desire to bring clarity to these assemblies that I created the P-51 Mustang cad models shown below, which incidentally was the catalyst that drove the development of these Part libraries.

Get in touch with any inquiries at the usual email. hughtechnotes@gmail.com

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

Technote: Inventor Sketch Blocks

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Technote: Inventor Sketch Blocks

I have uploaded a video showing the mechanism for the Main Landing Gear for the Lockheed P-38 Lightning. This was created using the Inventor Sketch block feature which is a great tool to understand how these mechanisms work and provides an opportunity to examine the operational relationships.

Landing Gear mechanisms are quite complex and at first glance at the drawings, it can be difficult to fathom how they actually work. One way of visualising this mechanism and understanding the extent of the operation is to use Sketch Blocks.

The way this works is that you first build your sketch; minimise constraints, and select the elements that form each of the components; whether that be hydraulic cylinders, linkages, axles etc. Then you would constrain them according to how the mechanism should work…in this example, the cylinder actuator rod is constrained to align with the centre of the cylinder and virtually everything else is concentric constraints at each of the nodes. There are a number of good Youtube videos that show how this is done.

The dimension shown is a “driven” dimension which will change according to the location of the operation. You could of course have driven dimensions for the angles to check the max and minimum inclination. The quality of the video is not great but you get the idea.

Video link: P-38 Main Landing Gear Operation

For better precision, it is always best to use the Simulation environment with relative constraints applied accordingly to confirm operational parameters but for a quick check on movement, the Sketch Block feature is a good solution prior to committing to modelling.

Update 16th June 2022 LG Hinges:

Have you ever wondered what the Main Landing Gear Door Hinges look like?…

Technote: P-38 Lightning Tailfin Rudder Calcs

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Technote: P-38 Lightning Tailfin Rudder Calcs

When I started this project the Lockheed drawings seemed to be quite well organised with the provision of a number of what I thought were key ordinate drawings. These appeared to be full of tabulated dimensions and associated formulas. The wing layout and dimensional information were well documented so it was logical to assume this pattern would follow with the other drawings. Unfortunately, this was not to be the case with the Empennage drawings which required a lot more work thus this blog article.

Having worked my way through the vagaries of the wing design and the forward Boom section I then progressed to the Vertical Stabiliser Fin and Rudder drawings. The first drawing in the batch I looked at was an ordinate layout drawing which on closer inspection only provided the location of the spars and struts…there was no information on the Leading or Trailing edge curved profiles. So I ventured to look at Assembly drawing #223026 to see what information I could glean from that.

Again it was just the main component locations and little or no information on the curvature. However, there was the drawing for the Rudder Tab and yes indeed it did contain information on the curvature. At this point, I should note that the Lockheed drawings include some sketches which contain chord profile information for both the wings and empennage…unfortunately 80% of those are illegible.

This sketch is the exception for the Fin/Rudder profiles at a specified WaterLine. This is where things got interesting because the chord dimension on this drawing did not match the dimension of the Rudder Tab at the same location after I had modelled it and furthermore did not match a comparative drawing in the Structural manual which also included dimensional information. It turns out that the Rudder and Tab Trailing edges are constructed in the same way as the main wing with an extended tab for jointing top and bottom sheet panels…which explains the dimensional variation.

The dimensions on the Basic layout sketch above and the corresponding information in the structural repair manual are actually relative to the rib chord and not to the finished edge.

As the above sketch was the only legible example of the requisite rib chord information I had to rethink my approach and reverse engineer the data on the Fin/Rudder’s ribs.

The Fin/Rudder rib drawings contain chord profiles for the ribs, though only partial I suspected that they may follow a standard format normally applied to rib airfoils i.e. percentage increments. It may seem an obvious comparison but in my experience, this is not always the case.

The drawing on the left is the partial profile information for the Fin/Rudder rib and the drawing on the right is the basic profile included on the Ordinate layout drawing I mentioned in the beginning. I surmised that if the Rib drawing follows the same convention as the Ordinate table with logical percentage increments it would be possible to determine the chord lengths of each rib.

In excel I created this spreadsheet with the Ordinate Table on the left and subsequent tables containing information from the Fin/Rudder Rib drawings. The first 2 columns in each table are the values as noted on the drawings and then to check my theory that they followed a logical sequence I calculated the third column which indeed returned a close approximation of the actual chord length. The fourth column is the new offsets calculated from the derived chord length in each case.

Having established that the rib profile is as I expected it is now possible to create ordinate points to profile the Trailing Edge and define the contours for the Rudder’s ribs. Remember we also have a tab extension to which we have to add an additional fraction of an inch to get the final trimmed profile. As I am calculating and applying the new information to the CAD model sketches I maintain a 2d view to check the overall dimensions to see how they compare.

I am only halfway through the development of the Fin and Rudder layout as shown but will continue the same process to ascertain the remaining curve sections. At the end of the day and similarly the same with the wing the 2d drawing will display 2 lines profiling the Trailing Edge, one which will be the 100% chord ordinate and the other the extended tab. By the way please don’t use any of the dimensions noted on this drawing…it is a study with temporary dimensions!

A lot of work still to do on this which will have to be done for all the spars and ribs to ascertain the correct curvatures of the Trailing Edges. Where occasionally you need to derive specific information it is often beneficial to look at opportunities to interrogate what information you do have to determine the information you need.

Update 26th May 2022:

After extensive study and listing of ordinates in stacks of excel tables, I have managed to verify the Vertical Stabiliser dimensions. The Basic or True Rudder line noted on the sheet drawings is defined by the 100% chord dimension for the ribs…this is an important change to the wing trailing edge. Anyway as I need to take a break I thought it may be prudent to provide this update for your perusal. Still some work to do for the top and bottom profiles and of course a general tidy up would be in order…it is still a work in progress!

I could have just accepted the dimensions noted in the Structural repair manual as the end result would have been close. However, it is important where there are slight variations between the manual, the ordinate sketch and the part drawings that every effort is expended to understand the design intent and derive a correct solution.

One further point of interest: the profile for the Vertical Stabilizer is close to being symmetrical about the vertical centre of the full length of the rib chords. I marked out the centres of each rib profile and found only a 3.6mm difference for the top section, however, the variation in the lower section (below WL 21) is considerably more at 19mm… which is too much even accounting for the fractional accuracy from inch measurements.

Update 10th July 2022:

My study of the P-38 Lightning is now finished. I have documented all aspects of the aircraft and compiled an extensive record of dimensions in a comprehensive Excel spreadsheet. The 3d CAD model is supported with dimensioned 2d layout drawings with all models available in native IPT, IAM forms as well as Parasolid XT and 3d DWG.

For more information get in touch, as usual, contact me at hughtechnotes@gmail.com

P-38 Lightning: Looking for Mold Line Drawings!

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P-38 Lightning: Looking for Mold Line Drawings!

I am looking for 6 Mold Line drawings for the P-38 Lightning. These drawings are the Cowl Mold Lines for the engine encasement.

Lockheed drawing numbers: 195072, 195081, 232543, 232544, 232545, 232764.

I can obtain a small number of key dimensions from the panel drawings which will not be enough to achieve an accurate full profile. I do hope someone has a copy.

I have tried all the usual sources for this information without success.

I can’t offer you much for the drawings but I am willing to share the comprehensive ordinate study and cad material when this project is complete.

Further Request: Photos of Wing Tip Required:

The wingtip trailing edge has a tab extension as a consequence of the connection of the top and lower panels. I am curious as to how this extension integrates at the extreme tip of the wing. If anyone has any close-up photos for the wing tip I sure would appreciate a copy.

Let me know if you can help. Email hughtechnotes@gmail.com

Update: 21st May 2022:

I have not had much luck with sourcing the above material. The Mold drawings would certainly have been enormously helpful in determining an accurate ordinate model. There is a Plan B, though it is going to be a fairly intensive search for every morsel of information that can be gleaned from the individual part drawings, manuals and reports that collectively will give me enough to achieve an accurate definition of the FWD Boom and Engine cowl surfaces.

An example would be the Scoop web plate profiles shown above to achieve some surface definition in those areas. I am currently working on the Landing Gear doors which will help define the lower surfaces. This is a lot of work which unfortunately means this will not be ready until much later in the year. I don’t do guesswork, if the ordinate point does not exist it is not on the model.

If anyone has any information that can assist me with these ordinate points, please, please do get in touch.