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
New Ordinate/CAD Project: The Lockheed P-38 Lightning is an American single-seated, twin piston-engined fighter aircraft that was used during World War II. Developed for the United States Army Air Corps by the Lockheed Corporation, the P-38 incorporated a distinctive twin-boom design with a central nacelle containing the cockpit and armament.
This project will dissect the complexity of the aircraft dimensions with fully developed spreadsheets, CAD models and drawings. I have drifted back and forth on this project over the last few months, studying the blueprints in detail to determine the best way of presenting the data in a usable format.
Surprisingly the wings are probably one the most complex parts of this study. The complexity comes about as a consequence of how the dimensional data has been recorded. For example, the wing chord is at a dihedral angle of over 5 degrees with the wing ribs actually perpendicular to the ground plane.
When we define the wing ribs we are actually working on a vertical plane angled to the wing chord line with the main beam and rear shear beams perpendicular to the chord on section. We also have the dimensions for the basic wing airfoil profile. Initially, I will record the rib dimensional information and generate the correct array of points at each Station. Then I shall calculate the airfoil profile at each station based on the given formulae Yu = YuT+(YuL-YuT)A. This should give us a means of verifying the tabulated data, for example; the table values for the Main Beam on 35% chord should match with the calculated airfoil values.
The plan is to record the dimensions as noted, vertical, horizontal and chord aligned in inches and millimetres exactly as defined on the original blueprints. Then I will extrapolate the X, Y, Z, coordinates for each point taking into account the chord angle of 2 degrees so that we can simply transpose these points directly into CAD at the correct positions relative to the origin point where the Nose Ref Line intersects with the Fuselage Ref Line.
The other caveat to all this is the 0% chord line is actually set back from the leading edge. There is yet another table of dimensions that relates the curvature of the leading edge to the 0% chord line. Ultimately to define the wing ordinates will involve a lot of work and then checking to ensure accuracy and correct alignment with the airfoil claculated profiles. At the end of the day, it is about making sense of all this fragmented information into a workable solution that makes it easier to interpret and use in any CAD system.
This is essentially how I work with all these Ordinate/CAD datasets. It is not just about recording information but also to check that the information works and that the end-user can transpose this into whatever system they are using. It is quite common for the information on the blueprints to be obscured, missing or simply illegible which usually requires a fair amount of time searching for answers. To complete this project I estimate something in the region of 300 manhours.
Update: 26th April 2022:
I have not yet decided on how best to present the Wing Ordinate dataset. I am looking at establishing check tables that will effectively compare the noted tabulated dimensions on the Blueprints with the calculated values. Also, we need to derive locational information directly from the wing plan CAD drawing for the Rear Shear beam and do a calculated check. Just to give you some idea of where I am going with this see screenshot below. As I mentioned above, the information on the drawings is fragmented so it is important that the excel spreadsheet data is presented in a clear and legible manner. Just now it is a bit of a muddle.
A quick update: Have rearranged the spreadsheet now with calculated values in lieu of listed values so the CAD model will be considerably more accurate. Calculated values are in blue text.
The rest of the Rib station tables will be added with similar calculated values and then I shall create a second worksheet with the airfoils for each corresponding station. The final sequence will be the extrapolation of 3D Ordinate points from a single datum so it will be possible to build an entire wing just from one collection of X, Y, and Z coordinates in one step. At least up to STA 254…still need to figure out the intricacies of the wingtip geometry.
Ordinates for each wing STA profile are calculated and recorded as shown. The highlighted rows at the 35% chord, are checked with those corresponding values listed in the tables above from the Lockheed original drawings. By the way, the drawing on the right is the Basic Layout Engine Mounts…there are 2 variations on this; both of which will be developed.
In the above screenshot, I have highlighted 2 minor corrections to the wing rib locations. They should be the decimal value for 85 11/16″ and 106 5/16″.
Update 3rd May 2022:
Have made good progress on the datasets for the Wing, Boom and Engine Mounts. Whilst working on this project I thought it may be prudent to compile an assembly list for each aircraft type for the basic dimension layouts as shown below. I plan to do a Technote shortly updating work methods using the ordinate dataset from Excel spreadsheets and include information on Sketch coordinate systems; manipulating the X, Y, Z-axis locally…so look out for that.
The updated revision C version for the rear fuselage and tailfin is now available in the P-51 Mustang CAD/Ordinate package as both a DWG and DXF format.
Incorporates additional curve data, dimensions and general revision.
As usual, all inquiries to hughtechnotes@gmail.com.
I have run this blog for almost a decade now in order to pass on and share my research and technical know-how related to the amazing designs of historical aircraft.
The blog is a journal to document my experiences on this journey with articles on the various aircraft designs interspersed with a bunch of Technotes. There’s a lot of stuff so I thought it may be prudent to compile a list of the more popular technotes to date:
Technote Sopwith Pup: Spar Clip Technote: An article discussing some of the design vagaries for the Sopwith Camel as well as in-depth use of sheet metal development.
Other Blog: Pioneering modular workflow and design solutions for use of 3D CAD in Substation design for the Power Industry that defied “expert” optinion. Prior to my work on this subject there was no viable solution for this industry. This was a long time ago but much of the subject matter is still pertinent to the industry today.
Theres a bundle of other interesting stuff and discussions on CAD, design as well as a number of articles on Excel.
I came across a site that provided a link to this blog describing it as a “guy that does CAD from aviation blueprints”. Actually they could not be more wrong. Sure there is a stack of CAD related stuff but the serious work is researching and compiling accurate dimensional data. Did you know for example that the top of the rear fuselage for the P-51D Mustang has only 4 verifiable ordinate points…prior to my documenting this no one actually knew this. The ordinates came from blueprints, reports, manuals and letter correspondence..the latter consumed a lot of time. I am probably only one of a handful of people who has actually studied every single drawing in the P-51 Mustang archive. This is serious research not just CAD!
Just started a new project to determine the structure ordinates for the JRF Goose. Typically for the Grumman drawings, this will require resources from a combination of tabled ordinate data and extrapolated dimensions from the individual part drawings.
With the NAA drawings for the B25 Mitchell I was spoiled as these guys tend to love ordinate tables and it is much easier to develop the data spreadsheets whereas the Grumman guys tend to fragment the information over several drawings. The wing ribs, for example, are actually in 3 separate drawings; the nose, intermediate and tail-end.
Why Ordinate datasets are important;
I spend a lot of time developing these datasets as a record of my research that can be utilised for various purposes including development of CAD 2D and 3D models. As an engineer, I know from experience that when the skeletal framework of an aircraft is correct then everything else will fall into place. I often see modellers dive headfirst into creating 3d part models and end up encountering problems with alignment and fits.
It is therefore prudent to first check the geometry prior to committing to 3d modelling…it will save you a lot of time, frustration and work in the long run.
The datasets already completed for the P-51 Mustang and the B25 Mitchell have been used by restoration companies, researchers, modellers and RC enthusiasts. The criteria for each group vary so it makes sense to provide the correct geometry in formats that can be translated to any engineering systems in a manner that can be used according to their specific needs.
The Goose Dataset:
The above cross floor drawing is an example where the ordinates are first compiled in a spreadsheet in both inch and millimetre formats. The core data is then extrapolated to determine the workable X, Y, Z coordinates. This is an interesting aspect of the aircraft design as the cross-floor profiles share similarities with the sister aircraft, the J2F Duck. Where I have cross-references between similar aircraft this information will also be included on the spreadsheet as a record of data resources.
The wings; as mentioned; are compiled from 3 different sections for the nose, intermediate and tail-end which require 3 sets of tables for each rib and then consolidated.
The ribs once integrated into the CAD assembly are then checked at each ordinate point to verify alignment with the neighbouring profiles to ensure accurate alignment. Occasionally the originating data is unclear so it is absolutely essential to continually check neighbouring associations to achieve accuracy.
The wing tip float: as well as the float profiles; depicted in the image above; I will also be studying the support structure and relationship with the wing.
This ordinate set will comprise the dimensional data as spreadsheets and as 2d DWG cad profiles for every frame and rib. For the main fuselage, the drawings will contain the key dimensional information in lieu of the usual spreadsheets due to the complexity of the frames. All other areas; wings, cross floor, nacelle and empennage will have both spreadsheet data and drawings.
These datasets are designed to help you get a heads up on your own aviation projects and as a resource for research. I do this work and research so you don’t have to…so please consider supporting my efforts. Thank you.
Update 3rd June 2020:
Have been quite busy figuring out the vagaries of working with the Grumman drawings. They are generally quite good but to be honest the inclusion of a few more ref dimensions would not go amiss! The development of the tail fin and rudder required referencing 3 separate drawings in order to ascertain the correct relationships between the fuselage, tail fin and rudder.
I also noticed a number of incorrect dimensions during the development of the fuselage and tail. When this happens it is imperative to cross-reference various associated drawings and sometimes even the Structural manual to determine correctness. This is actually where a lot of time is consumed in sorting these issues.
For the wing the ordinates are being checked as the profiles are developed. Part of this process involves developing key structural components as 3d models to ensure that the profile ordinates align correctly. In the following image it shows that the ordinates points align as expected with the red points (intermediate wing section) on the aft of the front beam web and the yellow wing nose points fall on the forward face.
I am not planning to fully model this aircraft only where necessary to investigate alignments.
TechTip: It can be frustrating working with Grumman drawings…take nothing for granted. The wing ribs as mentioned comprise 3 drawings, the Nose, Box Section and Intermediate. For the sake of complicity I shall refer to them as Nose, Mid and Rear.
One would assume a certain degree of consistency particularly when the detail drawings relate to Station locations. For example: you would expect the STA 37.5 would be a location that would be consistent for the mid and rear sections…but it is not. For the Rear section it refers to the back face of the rear beam and for the Mid section it refers to the front face of the rear beam. So when aligning the various actions it is imperative that the connecting line is either of the chord stations on either side of STA 37.5 (ie STA 40) and not STA 37.5. It is easier for the Nose and Mid Sections as they both have ref dimensions to the common STA 25, however the rear section does not reference chord STA 25.
Seriously a few additional reference dimensions consistently applied would make working with these drawings a lot easier.
I carried out a dimensional study on the spreadsheet data to check the relationships between chord STA, 30, 37.5 and 40. It revealed a number of inconsistencies in the STA dimensions but we did have consistency with the offsets at STA 30 and 37.5 (highlight red).
I would expect that the dimensions from STA 30 and 40 would be consistent with no variation as noted on the Mid and the Rear rib profiles…however that is not always the case. Out of all the ribs only 4 were what I would envisage as being correct. This requires further in-depth analysis to determine the best solution.
This will be a lot of work but a clear example why it is important to record the data in spreadsheets so an analysis like this can be done.
Update 14th June 2020:
Fuselage Frames, Tail Fin and Rudder now complete. Horizontal Stabiliser, Stringers, Flaps and Ailerons, Nacelle and revised wings still to do.
This will be the full package, spreadsheets and drawings. The latter will be all the frames and ribs at 1:1 in Autocad DWG format as well as the full 3d model.
I seriously think this will make a great foundation for an RC model at whatever scale you desire.
Update 2oth June 2020:
With reference to the Techtip above I have revised the wing layout to correct identified anomalies with the Grumman wing rib drawings.
I first established 5 ribs that are deemed to be correct, setup a work plane at Chord STA 40 and checked the relationship with the established ribs. For reference I initiated 4 axis selected from 4 known points on the ribs. I then placed the Rib at STA 271 to act as a check. The ordinate points on the profile for this rib is within 0.04mm of the projected axis and the dimensional offset from the work plane is only 0.025mm.
Having now established correct alignments I will introduce each of the remaining ribs, then check dimensions for each one with the work plane and each of the 4 axis. The end result will be a dimensionally accurate wing.
The North American B-25 Mitchell is a medium bomber that was introduced in 1941 and named in honour of Major General William “Billy” Mitchell, a pioneer of U.S. military aviation. Used by many Allied air forces, the B-25 served in every theatre of World War II, and after the war ended, many remained in service, operating across four decades. Produced in numerous variants, nearly 10,000 B-25s were built. These included a few limited models such as the F-10 reconnaissance aircraft, the AT-24 crew trainers, and the United States Marine Corps’ PBJ-1 patrol bomber.
This project will be another research and study effort to develop the ordinate datasets similar to the P-51 Mustang project. The ordinate data is compiled from drawings, reports, manuals, documentation and correspondence so it does take a long time to do.
For example. the above spreadsheets show the work process, starting with recording the ordinates exactly as set out on the NAA drawings. In this case, the original ordinates are in inches so a second table is created to convert this data to millimetres. The third table is the transposed version; retaining original formula cells; which is then used to extrapolate the actual X,Y,Z coordinates for input into a CAD system (the first 10 frames are shown).
This table is the stringer ordinates which follows the same convention of recording the first table exactly as per NAA drawings then converting this to millimetres. The third step is slightly different; transposing the table data in 4 sections to align the data according to stringer number.
This last table is for the wing center section. The process is similar to the previous tables with the main difference being the extrapolated X,Y,Z coordinates originate from the 30% chord. The actual location of intersection between the wing chord line and the wing reference line is calculated at 33%.
This is a lot of work just to get to this point I have spent in excess of 48 hours and I still have a long way to go. Once the frame X,Y,Z coordinates are listed they are then transferred to individual frames in the CAD system whereby they will be checked for accuracy.
There are a few ordinates that are illegible on the original drawings which will require further intensive research to determine.
To fully complete all the known ordinate spreadsheets for the B25 Mitchell I estimate will consume almost 300 hours of work. The P-51 Mustang set; created in a similar manner; was almost 3 times the number of manhours.
The end result is a comprehensive list of known coordinates that will generate the requisite fuselage, wing and empennage profiles within seconds in all major CAD systems…so it definitely is worth doing.
Fuselage total X,Y,Z points 2x 1043 = 2086
Wing total X,Y,Z points 2x 870 = 1740
Update 7th May 2020:
Continuing the development of the B25 Ordinate dataset I now have the majority of the wing rib profiles recorded. Some reconstructive work was necessary on the outboard ribs to obviate the poor quality of the original NAA drawings.
Every legible point is added to the spreadsheets and then meticulously created in the CAD system. Where information is unclear the cad extrapolated values are closely checked against the appropriate entry on the original NAA drawing to identify matching numericals or part thereof. Once I have consistency with the graphic output and the NAA drawing information this is then entered into the ordinate spreadsheet.
The attention to detail is typical of my approach to building these ordinate sets. Nothing is taken for granted and the primary reason why these datasets take so long to develop.
Update 12th May 2020: Project Status:
Fuselage: Frame Ordinates and CAD Profile 100%
Fuselage Stringers: Ordinates and CAD Profile 30%
Inner Wing: Ordinates and CAD Profile 100%
Outer Wing: Ordnates and CAD Profile 100%
Rudder: Ordinates and CAD Profile 100%
Vertical Stab: Ordinates and CAD Profile 100%
Horiz Stab: Work in Progress.
Update 16th May 2020: Empennage:
Update 19th May 2020: Rear Fuselage:
Often it is necessary to pull together several resource documents into one drawing to better understand key datum relationships as I have done here with the rear fuselage.
Update 21st May 2020: All Done:
This is a good example of what the ordinate datasets are all about.
Making sense of this:
To develop this:
The complete list of known ordinate points for the B-25 B,C,D Fuselage, Wings and Empennage are now recorded in a set of excel spreadsheets. A few additional drawings (PDF and DWG) have been created to further clarify the main datum points for aligning the main assemblies and a 3d Autocad drawing of full assembly profiles.
I seriously think this will make a great foundation for an RC model at whatever scale you desire.
Aviation CAD TechNotes 