Aviation CAD TechNotes

Technote: P-38 Lightning Tailfin Rudder Calcs

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

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

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

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

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

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

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

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

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

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

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

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

Update 26th May 2022:

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

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

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

Update 10th July 2022:

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

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