Month: July 2022

New Project: Standard Part Libraries

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New Project: Standard Part Libraries

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

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

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

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

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

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

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

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

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

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

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

Update 27th July 2022:

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

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

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

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

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

Technote: P-39 Airacobra Update Horiz Stab.

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Technote: P-39 Airacobra Update Horiz Stab.

In a previous post, I covered the significant new model for P-39 Airacobra. This model is fully inclusive of all aspects of the aircraft. Within this post, I mentioned the extensive study involved in determining the layout for the Horizontal Stabiliser; the dimensions of which were unclear in the available blueprints

https://hughtechnotes.wordpress.com/2022/05/18/technote-bell-p-39-airacobra-updated-model/

I was particularly keen to establish verification for the leading edge angle and though I had written to a number of organisations that have the P-39; surprisingly none of them took the time to either acknowledge or indeed reply…which of course was disappointing. From my experience, the industry is normally very supportive with regard to technical inquiries.

I revisited the documentation I do have and established that relevant information was included in the NACA Wartime Report L-602 which gives the chord length at Sta 49.25. It turns out; from my initial assessment; that the dimension at “2” was barely 2mm out and the Leading Edge angle is now 16.7796 degrees.

I mentioned in my last post that this latest study is available now which also includes the original model; which was more of a 3D modelling exercise than a dimensional study.

The P-39 Airacobra new CAD/Ordinate study is an impressive project.

All inquiries as usual to; hughtechnotes@gmail.com

Technote: Divide a Line in Inventor

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Technote: Divide a Line in Inventor:

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.

Drop me a line at hughtechnotes@gmail.com