AirplaneDesign.aero

[Gary James]

During the development of a high performance sport plane for a customer, I uncovered a problem with the mechanization of the polyconic surfaces in AeroPack.

Let's take the example of a conventional bubble canopy sport plane wherein the fuselage must be described by several different polyconic surfaces and that the length of the upper and lower surfaces may not be the same length. Example, the upper surface extends from the spinner base to the seat back bulkhead and then a separate upper surface forms the turtledeck from seat back bulkhead to the tailcone. The lower surface may consist of one or more surfaces but that the beginning and ending of the surfaces may not coincide with those of the upper surface. This is a VERY common configuration. Most sport planes can be described this way.

In this case, section cuts through the upper and lower surfaces will not match up at the half-breath mold line. This is apparently because the surfaces are handled internally to the program as a parametric with a length value of unity. Interpolation between defined points are done with straight lines. When cuts are taken through the upper and lower surfaces, they are not taken at the same "proportional" position because the surfaces are of different length. And thus the cuts don't match at the MHB. This is most noticeable in the case of a typical cowling for a horizontally opposed engine.

Because the surface cuts do not match at the MHB this makes the tool almost useless for surface development of typical general aviation airplanes.

If there is ever an update to AeroPack, please contact me first so that I can get together with Tim and the Punch team and address the serious bugs that I have found.
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* Gary James
* Mini-IMP Aircraft Co.
* PO Box 2011
* Weatherford, TX. 76086
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* Kits, Plans and support for the Mini-IMP