Bell P-39 Airacobra: Fuselage
This is an update on the P-39 project. I have actually been drifting between this and the P-51 Mustang as a number of inquiries have come in regarding the ordinates and various questions on the Oil Cooler model and landing gear mechanisms; which has been an interesting diversion.
Getting back on topic, I thought it may be prudent to write a quick update on what I am doing with the P-39 Airacobra and where I hope the journey will take me.
I have of course continued working on the ordinate data spreadsheet which is derived from the part drawings themselves. This serves as a check whilst I am developing the structure. The 3D models are being developed in context, i.e the individual part models are located to the 3D spatial ordinates relative to a single datum so when I plug these into the assembly they will import to the correct 3D location thus negating the requirement for constraints.
This is the first time I have worked this way as I usually just model the part and then constrain to the corresponding items in the assembly, but this is usually dependent on the quality of the assembly scans to clearly identify and ensure correct alignment of the parts. As we all probably know these scanned files are the most likely to have problems with legibility. In many respects having the part files modelled relative to ordinates in 3D space ensures that the parts line up correctly and I don’t have to worry too much about the quality of the assembly scans.
The P-39 main assembly drawings are actually not too bad as the image above shows. This is a scrap view of the fuselage Longitudinal, comprising many small parts all riveted together to form the assembly. The area in red is where I am working at the moment; which is a major node; just aft of the engine bay; where the many struts and braces overlap on both sides of the stiffener plate. The following image gives you some idea of the detail to which this is being developed.
The pilot holes for the rivets are unique to each individual part and just like the real process of construction these holes will be match drilled to all the other corresponding parts in assembly.
Modelling the complex parts and locating all those holes takes a lot of time but I believe the end result will be worthwhile. With this degree of accuracy you could just about build one of these aircraft from scratch!.
Quick Technote: This is the lower level fuselage cross member that has a built in twist to align with the connecting frames at both ends. The model consists of 3 profiles with the 2 outer ones containing a small angular deviation in the centre at point A. Normally I would loft the profiles to create the finished surface but this projects the deviation throughout the length giving us 2 surfaces; which does not look good.
I therefore deleted the resulting 2 base surfaces and simply replaced them with a boundary surface. I’m sure you will agree the result is a much smoother gradation of curvature; that matches expectations.