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
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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Developing this wing tip turned out to be more complex than I originally thought it would be. Because the model required a few interesting techniques I figured it is worthy of a quick technote that hopefully will assist others.
First, off the bat, you will probably have noticed the center partition which came about as a consequence of the development process. I will try to explain how this transpired…read on for more details.
What we have is essentially one main rib profile at Station 289 and 2 others towards the tip which you would normally just loft to achieve the finished surface assuming that the required outline guide rails were included in the initial data set. Actually in this case we didn’t have those curved outlines as a 3d profile only a 2d outline on the plan view. Even with the guide rails in place just lofting the full rib profiles did not work due to the continuity of the rails in a circular manner that prevented a successful loft.
By the way, the circular guide rails at “A” and “B” were generated as intersection curves using a side profile (top right in the background) and the plan profile to derive the resulting intersection lines. I initially wanted to extrude the 2d plan profile and build a 3d curve on the face of the surface but I was unable to apply a tangent constraint to align with the Leading and Trailing edges…so my only option was a 3d intersection curve.
Realizing that a full rib profile loft was not achievable I decided to fill each rib profile with a patch surface and then split the surface at the main beam intersection, which incidentally is perpendicular to the ribs. So this gave me a patchwork of surfaces fore and aft that I used as surface profiles and lofted each section as shown using the guide rails at “A” and “B” and the center rail at “C”…this created the partition I mentioned in the beginning.
Once the main fore and aft sections were modeled I then proceeded with the extreme tip which was simply a case of again adding a surface patch to the small projecting profile in the center and lofting the surfaces separately as before. Occasionally when you have problems with lofting it often helps to break it down into more manageable chunks.
Accuracy is extremely important to ensure a good surface finish with no small deviations or folds. So I checked the coordinates of each profile mathematically and adjusted the dimensions accordingly for the top surface.
The rib profile at 1,2 and 3 was adjusted to the new coordinates for the top line only but making sure that the LE and TE were tangential to the mathematically generated curves shown in red. These end ribs are actually modified profiles according to the tabulated information on the Lockheed drawings…apparently, the profile at the wing tips is based on a NACA 4412 airfoil but when I generated a 4412 it did not match…I am not sure why but it is something that warrants further research. As I did not have the mathematical formulas or guidance on hand to check the lower profiles I accepted what information was contained in the tables…mind you I could have generated a line equation from this information in Excel. Incidentally, all the wing ribs were checked mathematically with the resulting dimensions used to generate the profiles throughout.
The first image shows a sample of the modified values at Rib station 289, highlighted in green alongside the normal profile on the left. The second image shows the explanation of how the main wing rib profiles were generated. All this information is included in the CAD/ordinate dataset. Also on the second image, you can see a typical rib profile extracted from the Lockheed drawings which shows the 0% chord is actually set back from the Leading Edge, which is most unusual. This created a few problems because now I had to determine from the CAD model the Actual Leading Edge before I could define the curved guide rails for generating the wing tip lofts.
This all may seem overkill and a lot more work than one would expect just to build a wing tip but the Inventor Loft command requires absolute precision when lofting with guide rails so it pays dividends to mathematically check everything where possible to ensure successful lofting. I shall update the CAD/Ordinate dataset over the next few days to include this new data.
Following a recent inquiry about the P-39 Airacobra, I was asked if I had a model for the P-39 Carburettor Air Scoop. At the time I didn’t, though I did have some preliminary outlines that were done as part of the dimensional ordinate study. So I decided to get stuck in on this new project and see what can be accomplished…by the way did I mention this will be used on a real aircraft. The template moulds will be 3d printed and used to form the aluminium plates.
When you look at a photograph of the Carb Scoop it looks deceivingly uncomplicated however it turns out this part is surprisingly complex. The main body part itself is challenging with the curved profiles and filleted interfaces, the curvature of the fuselage; which by the way is not actually documented anywhere and the transitions from one frame to another to achieve smooth curvature. The internal duct comprises many varying profiles…the profiles tend to be rectangular with different corner radii throughout culminating in what can be described as a slot profile for the Air Scoop inlet.
I have been working on this for a few days now studying various modelling methods to achieve the most accurate and consistent results. So far I have the main scoop body and the fuselage skin modelled. The internal ductwork is set out on a sketch, though I will still have to define a number of intermittent profiles to ensure I get that right as well. Overall there are 12 individual parts for this assembly all detailed on one drawing…so some interpolation of design intent and cross-referencing with a few external drawings is essential.
At the Scoop inlet, there is a small lip that I have yet to model. The drawing has very little information on this so I decided to model the scoop without the Lip and then I will have to sculpt the curved form from extrapolated model information. That Lip at the end of the day will probably look inconsequential but the development work for such a small item cannot be understated. I shall update this article as work progresses.
An overview of the underlying geometry sketches for the CAD model. Point “1” is the fuselage skin…as mentioned the ordinates for this profile do not exist on available Bell drawings. So I work with what I know, namely the fuselage frames fore and aft and below. These were surface lofted and then the profile at “1” was patched to align with curvatures of known surfaces. using tangency and G2 on selected edges.
Point “2” is the scoop outline sketches, in which I made continuous elliptical profiles with an additional circular profile at the very tip below the fuselage plate surface.
The blue lines at “3” are the lofting guidelines, absolutely essential to getting this right. I initially skimped on this, instead, I attempted to just loft and use G2 or Tangency adjustments…it did not work well…so if you are doing this don’t skimp on these guidelines. Once the scoop body was lofted there was trimming to do with the flange plate (it was the offset from the fuselage surface) and then the 2 items were stitched. This provided an edge to which a variable fillet was applied. Cautionary note on the variable fillet…when initially applied don’t try and create adjustment points all at once…take your time, just create one pair at a time making micro adjustments and let the model regen and repeat.
The other key consideration is that the Bell drawing dimensions are generally only accurate to 1/32″ and 1/16″ (0.8mm and 1.6mm respectively). This will invariably impact the eventual quality of the end product when using CAD so it is important to understand where and how you need to compensate.
Update: Internal Duct:
I have now modelled the internal duct which has a partial concave curve on both sides to allow clearance to the main scoop housing. That was a real pain to model and to be honest, to achieve continuity with the duct curvature I simplified it slightly.
The sides of the duct are shown on the Bell drawings as being flat from the base level almost to the scoop inlet itself; merging to a point just past the horizontal breaker bar. I tried various methods of doing this but failed to achieve a good result…even using freeform curves…mathematically it is not to be! I settled for a smooth loft of the various sections to ensure that at the very least I could still achieve the partial concave surface and a smooth shell.
The front curved edge of the scoop inlet has a weld seam which is shown in the centre of the edge on the Bell drawings. I decided to move that joint further inward because an extended flange may reduce the installation clearance when installing the duct.
The curved plate you see will be cut back to finalise the flange for the scoop, which I shall leave until the main parts are all modelled. More updates to follow.
Duct Vane and Scoop Ring Stiffeners:
All inquiries as usual to HughTechnotes@gmail.com
Update 11th Sept 2022: Almost There:
Looking good and is deadly accurate. Trimmed the flange for the scoop main body and added the fuselage frame stiffeners. I still have a small panel door, forward lip and a few miscellaneous items to finish. One more day should do it.
The Main Body Sorted: Just the Lip to model and add to the finished model.
If you require a professional design and draughting service for your projects then please do not hesitate to drop me a line. Providing professional engineering, draughting (time served, old school) and modelling services in CAD since 1985. Fully conversant with Geometrical Tolerancing, Geometric Dimensioning and Tolerancing (GD&T), ISO Geometrical Product Specification (ISO GPS), BS 8888 and mechanical specification.
Email: hughtechnotes@gmail.com
Update 12th Dec 2022:
I have just received word that the P-39 Carb Scoop has just come off the 3d printer after 4 days. They printed this in 2 parts as you can see. Still a lot of finishing work to do before they have a chance to fit it onto the P-39.
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
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
Dividing a sketch line in Autocad is very straightforward and the question is often asked how this can be done in Inventor. There are a number of options to do this which I will explore and then I will discuss an application where the solution is not so obvious.
Where you have a known length and you wish to locate a point at 20% of the LENGTH it is simply a matter of applying the formula “LENGTH*0.2” for the dimension value. Another option is when you want to divide the line into 5 equal portions then you can use the RECTANGLE Pattern command. You first set the number of points, expand the dialogue and select FITTED; you will then need to select the line dimensions or measure as I have done here for the value.
Another way of doing this is to draw five line segments in succession and apply an equal constraint to each one. For the above; the length is a required parameter, so what do you do when you don’t actually know the length?
The following example is the P-38 Lightning Horizontal Stabiliser tip for which I wanted to document the ordinate points for the ribs. The ribs perpendicular to the stabiliser axis are known dimensions based on the standard profile however I also needed to record the profile dimensions of the ribs set at an angle to the main axis. Admittedly the Lockheed archive does contain a number of ordinate profiles for the canted ribs where unfortunately the majority of dimensions are illegible.
I like to record numbers so it should come as no surprise to those that visit this blog regularly that I was keen to tabulate the ordinate profiles for these canted ribs. The above image shows a number of magenta profiles which are the rib templates illustrating how the surface converges towards the tip extents. Incidentally, the diagonal lines on the main rib profile actually have a purpose…as you view the stab tip on the elevation you will notice that the ordinate points (projected) align with those diagonals.
Getting back to the main subject. The wing rib and horizontal stabiliser ribs follow industry-standard percentage increments for defining the ordinates as shown in the following image. Now we are getting to the main topic…where I wanted to transfer the ordinate locations for the perpendicular ribs to define the ordinate profiles for the canted ribs.
The Horizontal stabiliser ribs are based on the NACA 0010 airfoil profile which is listed as per the Lockheed drawings in the table on the left. The column on the immediate right is the calculated values to improve accuracy which also verifies the recorded data. The table on the right is the transposed calculated values for the main perpendicular Horizontal stabiliser rib with a chord length of 45″.
The above image is the plan view for the Stabiliser tip which shows the centres for the canted ribs and over to the right a number of red vertical dotted lines indicating the position of the reference perpendicular rib profiles. Between those ribs is a blue dotted line with a small circle indicator which is actually the main subject of this article.
The easiest way of defining the canted ribs is simply to loft the known perpendicular profiles and cut along the axis of the canted ribs…it definitely is the quickest way of doing this. However, that leaves a lot of miscellaneous activities in the cad model which just adds clutter.
Transposing the location of percentage increments from the rib table ordinate table to the canted ribs is done like this.
The perpendicular profile chord is the blue dotted line and the canted rib is the red centre line. The LENGTH is the chord length and the dimension A is the percentage increment on that line that we need to find the comparative intersection for on the cant rib. At this point, we do not know the LENGTH as this is dependent on the line position relative to the cant rib at whatever percentage increment we chose.
As mentioned at the start of this article for say a 20% chord dimension we could simply draw 5 lines in succession and apply an equal constraint and so on for the equal divisible portions…but that is not very practical.
So what we do is to locate the template rib line at any arbitrary point on the cant rib and then dimension the length…it does not matter at this stage what the dimension is. Now, this is the key thing we must do…select the LENGTH dimension and change it to a Driven Dimension. Now define the percentage increment (multiplied by Length) you wish to interrogate from the NACA table above and the template rib line will automatically relocate to a position where the Dim A is actually the percentage dimension you define of the total chord length. The software calculates the correct length according to the parameters specified.
An example would be where you specify 15%: you would write “0.15*D20” where D20 is the Driven Dimension.
I have included in the ordinate spreadsheets a table that will calculate the ordinate rib offsets depending on the chord length derived from the above exercise.
You then simply transfer those ordinate offsets to the intersection point of the cant rib. It really is quite clever when you think about it…you are asking the software to define the length of a line based on a percentage value relative to another canted line within boundaries specified by the arc.
Of course, I did not have to do this for all the cant rib offsets just the ones that were missing from the Lockheed drawings.
The P38 Lightning project is now finished. Only known dimensional data is included in this study. The engine Nacelle and Carb intake are omitted due to lack of dimensional information…however the creative among you will find it straightforward to interpolate fairly accurate profiles from the known information incorporated in this model and accompanying spreadsheet dataset.
These are the basic profiles for the P-38 Cockpit Canopy glass panels derived from the XP-38 drawings. Knowing that there were differences between the prototype XP-38 and the production models I was initially reluctant to accept the XP-38 dimensions for developing the cockpit canopy. The production drawings do not contain any useful information to develop these profiles nor indeed was there any drawing stating the inclination angle of the windshield. There was also not enough information from the Lockheed ordinate drawings for the fuselage frames which left me with the only option to use the XP-38 information.
It transpires the dimensions on the XP-38 drawings are indeed pertinent to the production models. There are exceptions which relate to the side windows.
The drawing on the left is the P-38H side glass frame and you can see this is dimensioned as a radius value which differs from the XP-38, which is defined by ordinate dimensions. There is also a slight variation in the overall length, so I naturally presumed that there may be other variables that conflicted with the prototype model. The only way to know for sure was to build the model based on the XP-38 and cross-check against known information with the production models.
So after 3 days of frustrating intensive work, I now have the base model for the XP-38 glass profiles and I have concluded that the profiles for the front, top and rear panels dimensionally are compatible with production variants. The only area that has marginally changed is the side panels, although changing from ordinate to radial dimensions still retains alignment with the known fuselage frames.
Also worth noting is that Lockheed uses a 3-inch grid system for aligning all the fuselage components which are useful when you are trying to locate these panels where no location is noted…you just have to align the 3-inch offsets to the grid. Each of the 3-inch offsets on this drawing section for example can be matched with the full-size grid to locate the correct elevation for the top glass panel and so on.
It is actually a really clever idea and helps obviate any doubt about where an item should be located.
One further tip when working with these Lockheed drawings is that for plan views and elevation views there may not be enough dimensions to fully locate a 3d point for determining a complex curved line.
For the windshield, there was sufficient information in the vertical plane and the horizontal plane but as they were not related I could not derive specific 3d points from this data alone.
So I resolved to replicate this on 2 sketches and extrude a surface profile for each sketch. The intersection of the surfaces gave me the requisite 3d glass mold line.
The final check; that ensures this is correct; is to view the final glass panel along its axis to check that the curvature matches exactly with the top of the ordinate fuselage profile at STA 126…which it does.
For some reason, the ordinate dimensions are on STA 123 instead of STA 126 which means the end result will need to be projected to get the full glass panel model…I haven’t done that here. These are primarily dimensional studies and I tend to only include 3d models where this benefits the purpose of confirming data integrity. Oh by the way the inclination angle for the windshield is 27 degrees…don’t be sidetracked by the frame connectors that show 26.5 degrees…the reason for the 0.5-degree variance relates to the interface with the rubber sealing. Hopefully, you will find this useful.
I wrote an article on using the Ordinate datasets many moons ago, which is now rather dated so I figured it was time to write an update with a better explanation.
First of all the reason why? It’s like every other construction project where you first start with a skeletal framework and then develop the project’s envelope. Whether it be a building with a steel frame, a boat, even the human body relies on having in place the skeleton on which to build the construction elements.
Aircraft projects are no different and to this end, many manufacturers provide this information in the form of ordinate dimensions. This information occasionally is listed in tables or included on the individual part blueprint drawings. I firmly believe that once you have the basic framework dimensionally accurate then everything else falls into place…so it is incredibly important.
Basic Ordinate Overview:
Let’s take an example from the Bell P-39 Airacobra.
For this aircraft, the ordinate dimensions are noted on the actual part blueprints so I have developed a series of tables listing this information in excel spreadsheets as shown. They list the Station Location from the aircraft Zero plane (this is usually identified by the manufacturer). The Station number is actually the station dimensions from this plane which defines the Z component. The next column on the table is the Vertical Y-component or the dimension to the Waterline and finally, we have the Horizontal X-Dim which lists either the Buttock Line position or Half Breadth dimension.
Commonly the Horizontal axis on the aircraft is known as the Fuselage Reference Line (FRL) or occasionally the Thrust Line. The Vertical Line is simply known as the Centre of the Ship to the Aircraft.
Waterline (WL): Horizontal Axis, Buttock Line (BL): Vertical Axis. An example of this is where we commonly have a designation like WL4…which means the Waterline at 4″ above or below the Centre of the Fuselage. So when it is not specifically dimensioned you would know from the designation where it is located.
Once I have the tables of known dimensions I would occasionally extrapolate this data to list the actual X,Y,Z dimensions in separate tables to make it easier to copy and paste into any CAD system.
As you can see from the above image, the dimensions are initially listed in 3 columns, X,Y,Z and next to that is the same data listed with comma delimiters. The reason for this is because Mechanical design packages like Inventor and Solidworks will recognise separate columns of data in the requisite order as stated whereas Autocad will require combined data for Mulitple Point input as comma-delimited.
The way I do this is to have a separate excel spreadsheet which I keep on my desktop which I call Scrap.xlsx. The format is common as shown in the image on the left though I should note the top 2 rows are optional. If there are no units specified it will default to the CAD template units. I usually don’t bother with the top 2 lines. Once the points are imported into CAD I tend to delete the values in the spreadsheet Scrap.Xlsx and start again.
The comma-delimited column data in the above image can also be copied onto a Notepad Text file and used in Autocad. Worth noting is that if you try to import X, Y, Z coordinates onto a 2D sketch it will only import the first 2 lines and ignore the third…so make sure the columns are in X, Y, and Z-order.
An important consideration is that not everyone uses Inventor or Solidworks or even Autocad which is why the spreadsheets are critical because then everyone can use the data to build their own models.
Actually building the model can be done in several ways. You can build a part file with multiple workplanes on which to sketch the profiles from the input ordinate data or individually in separate part files. You can model the parts in context, i.e. taking into consideration the Station (Z-axis) dimensions so when input into the assembly they locate correctly in 3d space. Or just the X, Y, ordinates in the part file and locate to the Z-axis offset in the assembly.
Dealing with problem data:
This is perhaps one of the main driving initiatives behind the development of Ordinate datasets with regards to the legibility of the original manufacturer’s blueprints.
This example is actually quite reasonable whilst others are quite illegible. As most of these datasets are listed in Inches; which are normally factions; it is easy to confuse whether a fraction is 3/16, 5/16 or 9/16 when all you have is a blob of dark matter.
What I tend to do in these circumstances is develop what I do know and develop the profile using splines to connect the points and then apply the curvature to help determine the missing point location or check that a point is correct.
Occasionally points you need to complete a profile just don’t exist on the blueprints or are completely illegible which will then require more extensive research. Sometimes this information is included in the maintenance or Repair manuals or in the case of the P-51 Mustang a missing point was actually found in correspondence. Either way compiling this data and building the profiles is very time-consuming.
Another fairly common problem is wrong dimensions. Every aircraft project I have worked on from this era has this problem, not because they are bad draughtsman (very much to the contrary) it is because many of the drawings are only records of the Template Lofts and occasionally the dimension is recorded incorrectly. The skill is identifying that the dimension is wrong; it is unwise to assume that because something does not look quite right that it is actually a mistake. So you have to check with associated parts and layouts to be sure.
The image above is the Horizontal Stabiliser leading edge. The rib in blue (1) was obviously wrong because of a distinct kink in the curved edge, which when corrected aligns well with its neighbours. The one in red (2) also appears to be wrong even though the curvature looks fine the forward edge does not match with the projected alignment (I tend to use an Axis feature to check this). Before I apply any corrections I will check the part drawing and then the assemblies to determine if there is an error or if it is actually a design feature.
Locating Sketch Datum Points:
Creating workplanes for sketches as offsets from the primary X, Y or Z planes tends to copy the originating plane datum point which is not always where we need it to be when importing a series of points. The best option is to use the Parallel To Plane Through Point when creating a workplane as this allows you to select the point which will be the datum point on that sketch plane for locating the point data.
Some of the datasets are setout specifically to make it easier to input the data from the spreadsheet. For example, the extrapolated X, Y, and Z, coordinates for the P-51 Mustang wing have been compiled and calculated so they will input at the location of the 25% wing chord. This is assumed to be the logical setout point from the CAD World Coordinate system which saves you a lot of hassle.
If however, you have to create a workplane on an incline this option may not be available in which case you need to adapt the local sketch coordinate system to suit the required datum point.
In Inventor, you would right-click the Sketch in the model browser and select the Edit Coordinate System option which initiates an adjustable Coordinate icon on the sketch.
Suffice to say this icon can be manipulated, moved and rotated to any point on the sketch to suit your requirements. I will do a more comprehensive article on this shortly.
Other Excel Ordinate Examples:
The actual layout of the Ordinate spreadsheets depends entirely on the form from which the data is developed. Where the original blueprint data are listed in tables I will generate the excel spreadsheet in exactly the same format…which helps when checking the data input. If there are no tables but data from the part drawings then I will generate tables according to how the dimensions are noted.
All the dimensions are listed in Inches and Millimetres. I normally extrapolate the X, Y, and Z coordinates to millimetres as this is easier for me to work with…but it is easy to change that to inches if required. All the spreadsheets are fully editable and not restricted in any way.
Finally a quick Excel tip:
If you work with percentages a lot you will find this useful. When entering the value in the cell just add the % sign after the numbers and Excel will automatically format the cell as a percentage value.
Ordinate Data set Availability.
The NAA P-51 Mustang (probably the most comprehensive study) is available as a separate package from the Blueprints archive. The B-25 Mitchell is also a separate package and the Grumman Goose. The F6F and F4F are currently included in the Blueprint archive as they are not so well organised (work in progress) for now.
The Bell P-39 Airacobra is currently included with the blueprints but as I am now working on a new update this will shortly only be available as a separate package.
The P-38 Lightning is brand new and will not be available until June.
Final Note: All the Ordinate packages include the 3D cad model as developed in Inventor. This should not be an obstacle to anyone wanting to interrogate the model as a 30-day evaluation of the Autodesk Inventor is readily available for download. You can even extract sketches from the model as DWG files if required.
Many of the Ordinate packages include fully dimensioned Autocad 2D drawings and PDFs. These are mainly layout drawings and critical location information where it is essential to better understand relationships between wings, fuselage and empennage. Again all these are fully editable.
For all inquiries and feedback please get in touch: hughtechnotes@gmail.com
While I source new information for the P-38 Lightning I decided to revisit and update the Grumman F6F Hellcat and the Bell P-39 Airacobra Cad/Ordinate datasets. This work relates to the empennage for which I have decided to make the Autocad DWG and a PDF copy of these documents available for download.
These drawings are preliminary Basic Layouts for early release. The 3D CAD model updates will only be available to those that have previously purchased a copy from me directly. Though these won’t be finalised until nearer the end of May 2022. I shall contact the buyers directly in that respect.
I am also in the process of tidying up the Ordinate datasheets to make them easier to read. The datasheets list the ordinates for each frame/station profile in both Inches and Millimetres with a second sheet that extrapolates this data and compiles the data as X, Y, and Z coordinates for input into any CAD system. These X, Y and Z coordinates have initially 3 columns for each ordinate which is ideal for Mechanical systems like Autodesk Inventor plus an additional concatenate column which combines all coordinates comma-delimited for Multiple Point input into Autocad.
Other CAD/Ordinate Datasets:
These CAD/Ordinate packages are designed to help you kickstart your own projects. All the dimensional research has been done for you, which will save you weeks of work.
For more information drop me a line at: hughtechnotes@gmail.com
Aviation CAD TechNotes 