Originally published March 1993 as "A Short Techie Article"
In this month's Aviation Consumer, a series of articles on homebuilts was presented. The two main topics were stability and certification. Since the Aviation Consumer generally presents impartial observations on a variety of aircraft, you should read what they have to say about Glasairs, Lancairs, and other homebuilts.
Articles about Stability and Control of homebuilts can be found elsewhere on this web site, but I'm going to highlight a couple of statements made about aircraft certification.
The simplified certification process that the EAA has sought after for the past 10 years was intended to reduce the costs of certification and hence the cost of a new airplane. Yet in the words of Frank Christensen, creator of the Christen Eagle, "...there is little wrong with [certifying an airplane under] FAR Part 23. It is simply a minimum engineering standard, and a very good one at that." The only changes to FAR Part 23 that Frank proposed were the elimination of the lightning-strike requirements and the elimination of redundant trim tab controls. Incidentally, Frank mentioned that the cost of certifying an airplane under Part 23 had little to do with the cost of the airplane. Most of the engineering that went into certifying the Husky was simply engineering that was required to design the airplane in the first place!
The EAA/SAMA simplified certification document closely resembles FAR Part 23 says Herb Andersen, chief engineer for Aviat. "The only two changes are a simplified lightning-strike criteria and the elimination of dynamic testing of seats. Both are very welcome and significant changes."
Although I don't agree entirely with Herb Andersen or the author of the article in which he is quoted about composite structures, the issue of using Part 23 as a minimum engineering standard is a good idea. Many, many years of aircraft design experience are evident in Part 23 including work done by the NACA in the earlier half of this century. Granted, most of the design criteria are conservative in nature which means that an airplane certified under Part 23 will probably be heavier than optimum. However, Part 23 offers simplified methods of computing loads on an aircraft such as gust loads and landing gear loads so you won't need advanced calculus or a Cray supercomputer to design and certify the airplane. A pocket calculator and a tablet of paper will get you through most of it. Furthermore, a good percentage of Part 23 has little to do with structural design at all. In many cases it merely puts good design practices in words. For instance, one paragraph simply states that each control system must have stops that positively limit the range of motion of each movable aerodynamic surface--an item often missed on homebuilts yet something easily complied with.
For amateur designers out there, even if you have no intention of certifying your airplane, you should still read through FAR Part 23. If nothing else, at least it'll force you to think about things that you might otherwise overlook on your design.
Now where's my calculator?