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.
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.
12 months ago I started providing CAD design services to Planes of Fame, Chino to assist them with the restoration of a P-39 Airacobra. This aircraft is a static display restoration so we had some latitude in the manufacturing of the various parts. This included 3d printing (which is done by a professional company), sheet metal work, and vacuum forming.
The CAD models generally are for small complex geometry parts as you can see in the examples posted below. When preparing these models everything is double-checked against blueprints and where necessary the aircraft itself to ensure correct profiles.
I have over 40 years of experience in Engineering Design and drafting; in fact, I actually started my career as a draughtsman on the drawing board. I question everything and take nothing for granted with a focus on detail and accuracy. So when I prepare the 3D CAD models I also create a 2D dimension drawing as a checking mechanism to ensure accuracy and also as a reference for the engineers. If the 2D drawing is actually produced specifically for manufacture instead of being a design check it will incorporate all required standards references, tolerances, and material specifications.
3D CAD modeling of parts for aircraft can be quite complex and can take a long time to complete. The smallest part here would normally take about 3 hours. That may seem a lot of time for small parts that appear to be quite straightforward but often it is the reference geometry not visible that takes the time, whether that be the curvature of the wings or fuselage. Then everything is checked and documented.
The P-39 Restoration project still has a long way to go which I feel privileged to be part of.
If you require parts developed in 3D CAD for your project then please don’t hesitate to drop me a line at hughtechnotes@gmail.com.
I would like to take this opportunity to wish everyone a Happy New Year.
The ordinate dimensional study for the f4F/FM2 Wildcat will now be ready in January. This will include dimensional information for all the rib, strut, and frame profiles fully documented in 3D CAD, 2D drawings, and Excel spreadsheets. Probably the most accurate dimensional study available.
In January I will be taking this project and the P-39 Airacobra to the next level. The plan is to fully 3D model in CAD all the primary structural components for the wings, flaps, ailerons, elevators, rudder, fuselage, empennage, cowl, and landing gear; and then produce a 3D printed scale model at either 1:15 or 1:10 scale. The F4F empennage is already partially fully 3D modeled in CAD which gets us off to a good start in the New Year.
These models will be printed on an Elegoo Saturn MSLA printer capable of producing a 0.02mm accuracy. The resin I will use will likely be PLA with a 10% mix flex resin to minimize brittleness. This is an ambitious project and will take most of the year to complete.
Many of the components are thin-walled profiles which may have to be adjusted to suit the scale of the printed model. Some testing will be done to find the minimum thickness to achieve model integrity and maintain dimensional accuracy.
This project is something I have been thinking about for a long time which is only now possible with the incredible accuracy achievable by the latest 3D printing technology. The final 3D CAD model; suitable for 3D printing; will NOT be available publicly but I am open to the idea of private sponsors.
As usual, all inquiries to hughtechnotes@gmail.com
I have taken a break from the wing development whilst I await more information. So I have switched my attention to resolving the Canopy layout for the F4F/FM2 and true to form I have yet another bunch of questions. I often wonder how on earth they actually managed to build this aircraft.
First of all, we have a layout drawing showing the canopy dimensions…at first glance, it would appear that this will be a straightforward task. However, this is not the case.
We have a number of key dimensions that don’t quite add up…the dimension at “1” is shown as 29.25″ and the dimensions at “2” is 29/875″ but when you compare that with the offset dimensions from the Fuselage Station locations at “3” and “4” there is absolutely no way that “1” and “2” can be correct. The depth dimension at “5” is presumably along the line that would otherwise be defined by the dimensions “1” and “2” but as those dimensions are incorrect then what is this actual dimension relating to?
So I need to figure out what is going on here and therefore I thought I should check the track locations which should provide clarity and verification.
We do have a drawing that details the track components but there are no setting out dimensions for the track relationship to the fuselage. The only other drawing that shows the track is the Structural Assembly drawing…alas that does not help either. The fuselage section above the cockpit shelf is as shown highlighted in yellow. It shows the track and a number of frames that in my opinion are very important aspects of the design but what you see is the only information we actually have. You would think that something as important as a canopy track would be critical to warrant a detailed layout showing the correct alignments and setting out points…there is nothing there! I literally sat here one day reviewing every single drawing in my archive…all 8775 of them to find useful information.
It gets even more interesting as we continue this quest.
The forward section of the canopy has no location information so there is no context as to where this actually resides in relation to the fuselage. Furthermore, although we do have the dimensions for the windshield itself there is absolutely no setout information for the side and top glass surfaces. This is again an area that will require full 3D development, similar to what I had to do with the horizontal and vertical stabilizers. However, I have run into problems with that as well. At Sation 2; the key to getting this correct; is an offset dimension (highlighted in yellow) which is noted as 2.781″ or 2.834″ depending on whether you take into account insulation….so ideally in an “as fitted” condition you have to wonder what the correct fitted dimension should be.
As you can see I have started the 3D development of the cockpit and canopy to hopefully realize pertinent information from individual part drawings and fitting details to determine the missing information and verify the setout for the canopy. This is a lot more work than I anticipated but other than just giving up on this project it is my only option.
I have also reached out to various companies and organizations to try to source more information that will help establish the key parameters I am currently missing. This can be expensive and the reason why I rely heavily on your support so that I can find the answers to these important issues.
I am very close to finalizing the ordinate/dimensional study for the FM2 so it would be a real shame to give up at this stage.
Please help fund these projects so I can find answers for you. Get in touch as usual to hughtechnotes@gmail.com
I have given this article the title of “Questions 2” as moving on from my previous post on the FM2 wing, I have identified another anomaly that I cannot explain…another chapter in the development of the FM2.
This relates to the wing trailing-edge ribs that cover the main flaps. Normally, when you set out a win,g the wing chord is divided into percentages which would then provide a straight line segment on the surface when lofted between root and wing tip profiles.
For the Wing Trailing Edge rib profiles, this is not the case. At the wing 80% chord line, the top surface calculates with minimal deviation, as one would expect, similarly at the extreme wing tip. However, between 85% and 90% rib chords the deviation is not as expected. The contours at 85% and 90% trace a curve where you would expect a straight line.
What appears to be happening is that the wing TE ribs are dimensioned at various stations from the main Rib Sta 0. At stations 48, 52, 56, and 60, the offset dimension from the Baseline is the same for each rib at each of those locations. The end result is that the wing top surface is actually perpendicular to the wing root chord and does not follow the transition lines you would expect on a conventional wing loft. The transition lines at chords 85% and 90% are curved as you can see from the calculated offset tables below, which would normally be expected as a straight line.
At 80% and 95% chords respectively the wing top surface is for all intents and purposes a straight line as you would expect. The residuals column in the above tables shows the necessary correction offset for the selected point to align with the calculated Best Fit Line in millimeters. It could be argued that the offsets are no greater than +/- 1mm, which is not very much, but the flap ordinates are as shown and could have easily been dimensioned to a 1/64th inch had the draughtsman intended to show something other than they did. This alone demonstrates deliberate intent. So far I have identified alignment issues with the Flaps and Ailerons in my previous post, and this anomaly just adds more questions.
I know that this plane was originally conceived as a BiPlane, which explains the 5 datum lines we have for the wings and I am curious whether that design decision introduced a number of key aspects from which these questions have arisen. The truth is at this stage, I do not know, though the Flaps can possibly be explained; everything else is a mystery.
I have searched and read many forum discussions on the FM,2 and as far as I kno,w none of these issues have been identified or discussed… even the fact that the wing tip NACA profile is not a typical 23009 I suspect should have raised some red flags. Identifying and finding answers for design issues like this is part of the reason why I do what I do.
Since my last post, I have further developed the Wing layout which has revealed a number of key considerations that you may be interested in.
Wing Trailing Edge:
Other than a noted offset on the rib drawings there is no definitive alignment specified for the Wing Trailing Edge. What I found was the Wing Trailing Edge rib profiles were reasonably accurate from which I could determine this alignment.
The component shown in green is the Alcoa K14403 standard Grumman profile for the trailing edge. When I developed each of the wing TE profiles (white) there was a minuscule variation in the alignment, so I needed to determine the best-fit line through those points using Linear Regression Analysis. I could just have easily selected 2 random points from the wing TE profiles which would have been okay but I like to get this stuff right.
By using Linear Regression there is no guesswork or random selection it simply analyses the point coordinates and calculates a line that best fits all these known points. As we have 11 coordinate points to analyze the end result will be an accurate placement of a Trailing Edge line that represents the collection of known coordinate points. The column named Residuals is the offset from the known coordinates to this line. As you can see the max offsets are in the region of 0.3mm…well within normal fabrication tolerances.
Having now established a correct Trailing Edge I checked this against the flaps (cyan) to see how well the assembly aligns with this newly defined trailing edge. I noted a deviation of 2.2mm on the outboard edge towards the wing tip.
Flaps:
In the image above you can see how the flap assembly does not align exactly with the wing trailing edge. My first impression was that I had made a mistake with the model, so I rebuilt it resulting in the same deviation. So I checked the location of the hinges…they are dimensioned to 4 decimal places of an inch so for all intents and purposes they are exactly located. Further research reveals that there is a return spring on these flaps and I think what is happening is the flap layout is deliberately set out this way so the flap first engages with the wing at the control cylinder end and then the return spring engages closure with the outboard end…hope that makes sense. Grumman has used this type of spring mechanism to engage the closure of wing surfaces elsewhere at the wing folding mechanism.
I believe the geometry for the flaps is correct however my dilemma is whether or not to adjust the alignment to align perfectly for the future purpose of design analysis…and of course should there be any interest in the development of an RC model. One to ponder.
Wing Folding Web:
On the inner wing stub section, there is a sloped web plate attached to 3 triangular gussets. This is basically the mating plane for the wing stub and the main wing assemblies at the wing folding joint. This is one area that is not so accurately dimensioned…when you develop the triangular gussets there is a slight variation in the edge slope that this web plate is fitted to and similarly, the profile of the web plate is also marginally out. We are talking about fractions of millimeters but it does matter. I developed this area in a separate assembly where the wing ribs were lofted and then the triangular ribs and web plate were sectioned. Incidentally, the second image above is the only drawing (#7150645) that indicates the slope of this web plate at 50 degrees. You can also see the numerous datum lines that we have for setting out this wing that I mentioned in previous articles.
The mating portion of the outboard wing that engages with this web plate is the spring-loaded assembly I mentioned above…I have yet to do that part…will probably feature in a future article.
Wing Folding Hinge:
Just a quick update on the Wing Folding Hinge. I have this fully dimensioned now as an ISO View, Front and Side elevations which enables alignment checks with associated ribs and web plates. It is important that the rear face of the main spar aligns with the center of the hinge so these dimensions help establish this correct relationship.
Wing Tip:
The wing tip sketch profiles are now drawn but there appears to be a slight mismatch with the wing tip rib profile at Sta 222. The Trailing Edge at 55/64″ below the Chord LIne was also puzzling as it did not align with the Trailing Edge line mentioned above. Again my first impression was that I made a mistake with the rib profile…drawn again…same result. I then checked the alignment with the Aileron assembly and whilst the wing rib TE aligns with the Aileron TE the Aileron does not align with the Wing Trailing Edge line.
This one is a bit more difficult to comprehend as there is no logical reason for the Aileron to essentially drop toward the Wing tip…yet the wing tip rib and aileron align well. Again I checked the hinge locations and they are exactly where they should be. I have been in touch with a number of museums and restoration companies to see if they have an explanation and also requested photographs along the edge of the aileron to visually examine the aileron alignment. I will get back to you on this one. By the way, I also carried out a linear regression analysis to determine the exact reference line locations for each aileron rib as a check.
This aircraft is surprisingly complex and whilst there may be perceived anomalies that at first cannot be explained there is usually a good reason for being the way they are. For example, the leading edge of the horizontal stabilizer has a negative camber towards the tip, essentially the leading drops….this is most unusual.
Finally, to make things even more puzzling the wing tip rib profile is not actually a NACA 23009…it is close in profile but it does match exactly…I believe this is a modified NACA 23009. Once I have all the ribs modeled according to the Grumman drawings I will calculate the wing rib ordinates to double-check the profiles…that will be a real pain and time-consuming thing to do as the ordinates are at 4-inch and 2-inch intervals along the chord and not by chord percentage as one would expect…so I need to transpose that data from the cad models to develop the equations for checking.
I have spent an incredible amount of time developing this wing, perhaps more than any other aircraft study I have done. This design is very complex and keeps throwing up small anomalies that at first are difficult to comprehend…it does require a lot of research to figure out the reasons why.
Update 17th Sept 2023:
Wing Rib Ordinate Check: As mentioned above I have now carried out a check on the wing rib profile ordinates. Normally I would do this the same way as I calculated the wing rib ordinates for the P-38 Lighting but that is only applicable when you know for certain the root and wing tip rib profiles. The main point of this exercise was to determine the accuracy of the FM2 wing tip profile which is apparently different from the stated NACA 23009 profile.
I resolved to do this using Linear Regression Analysis from plotted points on the 15%, 25%, 50%, and 60% chord planes. These percent chord planes actually have to be determined separately because the wing rib ordinates on the Grumman drawings are incrementally spaced at 2″ and 4″ intervals which of course does give us the straight-line projections we need.
Typically I did this for the top and lower ordinates recorded from each rib at each chord plane and compiled the resulting data into a table in Inventor which was then exported to MS Excel for analysis. The analysis confirms that the wing tip profile is accurately drawn and the ordinates on the drawing profile are correct. I shall also do a similar exercise to check the dimensions of the main beam at the flap and ailerons.
Drop me a line for further information at hughtechnotes@gmail.com
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Following on from my previous posting regarding the Excel Transpose function; wherein I mentioned the updates to the Grumman F4F/FM2 Cad/ordinate dataset; I thought I would share a few screenshots of progress so far.
As you can see the aircraft is partially 3D modeled…there is actually a good reason for this other than the fact I enjoy the 3D modeling! I have found that on the main assembly layout drawings, the dimensions are often shown to one side of the spar whereas the actual connecting part is defined to the other side. To ensure I get this stuff right I would model the main spar to correct material thickness and check alignments. Admittedly I did get a bit carried away with modelling some of the ribs.
The wing is probably the most complex assembly to do due to the main ribs being in 3 parts…the leading edge, mid-section, and the trailing edge. Each profile will be recorded separately; as per the Grumman drawings and then compiled to provide full rib profiles at each station. The wing also has 5 datum lines that are occasionally misidentified in the part drawings which can be really frustrating alongside incorrectly placed dimensions…generally wrong vertical dimensions are associated with the wrong rib station, more common than I would like.
Still some work to do to finish these main areas as well as the cockpit canopy, fuselage, and front cowl. I haven’t looked at the undercarriage as yet… development of that will be dependent on available information…we will see!
It is not my intention to fully 3D model this aircraft but where it helps check associativity between parts then I will. The project will fully develop all key profiles for ribs and frames which will be fully documented on Excel spreadsheets as a permanent dimensional record. I plan to have this update completed by the end of September.
The aim of these cad/ordinate datasets is to produce the most accurate dimensional records available anywhere for the various aircraft…nothing is assumed or taken for granted.
If you can help me with the spiraling costs of these projects please consider making a small donation. As usual for all enquires please get in touch at hughtechnotes@gmail.com
I am currently updating the F4F (FM-2) Wildcat Ordinate dataset which required transposing Excel Rows to Columns so I figured I should write a quick Technote on the process involved.
Before I get into the detail it is necessary to appreciate that I could have saved myself a bunch of work if I simply created the table in the first place with the columns and rows reoriented to better suit the required end goal. When I develop these tables it is important that the layout is the same as the original data so that ongoing cross-referencing and updating are much easier to achieve. As you can see in the screenshot of the original drawing the tabulated information is not very clear, in fact, some of it is completely illegible… which incidentally is the primary reason why I do this in the first place…initially, I develop the coordinates as best I can and then create the profiles whereupon any variations can be visualized and therefore corrected…essentially working from what we know to determine what we don’t know.
Getting back on track. What I need to do is to create a live link to the rows (highlighted) but in a columnar format to list the required X, Y coordinates for each profile. You could of course just simply copy the rows and use the Paste Special function to transpose the values to a column…however, the copied data is not linked so any changes will not be apparent in the column values. The best way I found is to use the INDEX function.
With the INDEX function, you first need to establish a range of values to be indexed…in this case, it is the values from the table shown in the red border… which give us the range from L64 to P90 (press F4 to lock that in).
The value A1 after the Column and Row values is related to the first entry in the range…it does not relate in any manner or form to the actual cell A1. I have shown alphabetically in the first image above how this A1 would change according to the values selected. So you would write this formula at the top of the column where you want the values transposed, select this cell, and use the + sign at the bottom right to pull the values down. For each column you would have a different starting point…for example, in the very first column (X-Coord) the Formula would be written as follows:
It is alphabetically the 3rd row from the first selected cell in the specified range and numerically in the first column. For each group of values you need you would adjust the starting point of the selection to the first value in the row required. When you get the CAD/Ordinate dataset for the F4F Wildcat the spreadsheet is fully editable and you will see for yourself how this was done.
As usual for further details get in touch hughtechnotes@gmail.com
Fastener Library Update: AN/MS Standards (Updated Jan 2024)
Over the years I have been further developing my AN (Army/Navy) or MS (Military Standard) parts library and only this morning did I eventually get around to uploading all the new files.
This is the list of Standard Fastener Parts now currently in the library…over 300 parts.
I have decided to make these files available as the original Inventor iParts. I was getting requests for different conversions to STP, Parasolid etc, and also at different scales…doing all that on request takes a lot of time. Don’t be put off by the fact that they are Inventor files as Inventor is readily available as a 30-day trial product which gives you several options for working with these parts. You can even install a Read Only version of Inventor
It is really simple to work with these files…let me show you. For a start, an iPart is actually a normal IPT part file inclusive of a table of parameters so you can generate multiple variations of the part in one file.
Part Conversion: I would assume that many people who don’t use Inventor will wish to convert to a file format more suited to their application.
You can tell you have an iPart when the icon next to the part name in the model browser is shown as a table. To convert the file you simply expand the table folder; select the part or multiple parts and select generate files which will create a single IPT part file for each variant. This is placed in a subfolder named the same as the iPart filename. From there you can open this part file and Export whatever model format you want. Alternatively, if you would like to build your own version in a different CAD system it is useful to use the underlying sketch which can be Exported from this model; as shown in the second image which you can link separately to the Excel spreadsheet.
Table Editing: As I mentioned the part has an internal parameters table a bit like the format used by Excel which is fully editable. For the majority of the Library Parts, I also include the Excel table as a separate file.
Accessing the Table is as simple as right-clicking on the “Table” text and selecting what editing option you want…either “Edit Table” (which opens the part table itself) or “Edit via Spreadsheet” which will open this same table in Excel. When you save the table in Excel it will revert to the Cad Part file and update the model with any changes. Making changes is much easier in Excel where you can add new variants of the part or amend existing ones. The dimensions are all in inches but if you bring this part into an mm metric part it will automatically adapt the inch dimensions to mm…so you can be assured that the part will be correct regardless of which units you use.
These part libraries include the most commonly used sizes so you can add to this as you desire. A copy of the original specifications is also included for reference. If you are looking for Aviation-related specifications then check out this free site: http://everyspec.com/library.php.
This library is included in all the CAD/Ordinate datasets and is now also available as a separate package. See this page for more details: https://hughtechnotes.com/resources/
Manufacturers Standard Drawings:
Included in the many blueprint archives are manufacturers’ Standard drawings, some of which are commonly shared specifications between various aircraft by the same manufacturer. I have a spreadsheet listing those standards for both Grumman and North American Aviation. This is available free at this link:
In the top right-hand corner of each worksheet is a link to a separate download area where all those standard drawing files are stored. As usual, the spreadsheet is fully editable so you can add to the data record as you find more information. I am sure you will find this is a beneficial resource by having all these important standards in one location. If you find these useful please consider a small donation to help support my work.
Aviation CAD TechNotes 