Design and construction

[PilotDAR]

Most pilots spend a lot of effort understanding so many aspects of flying an aircraft, and leave the understanding of it's construction up to those responsible for that. An that's certainly fair, though sometimes pilots have honest questions about design and construction too. It is wise to ask! Of course the topic is thousands of books thick, and more than a hundred years old now, so can hardly be covered in any depth on such a forum, but some very small aspects can be worth a re visit from time to time.

Sometimes the craft look very frail and, well, unstrong. Most commonly, this is a result of weight saving considerations. The structure, and materials chosen will be just what is required to satisfy the design requirement, plus a factor of safety, plus what the manufaturer's legal team might ask to have added. I think of the Cessna 206, which for such a robust aircraft, still has some fuselage load bearing skins thinner than any skin found on a Cessna 150. You don't see them failing there though, so the Cessna engineers obviously did their job. I had occasion to do a simple structural analysis of the Cessna 206 outer wing structure to find that it is the wing skins which carry all of the flight loads, rather than the spar. The main purpose for the spar seems to be to hold the wing skins apart! It is amazing to see how much an aircraft wing, even the seemingly stiff Cessna wings, will flex with loading.

There are some components which really don't see much loading. It may be that they never see air loads, or are really just shields or other forms of protection, not really related to carrying any loads. These may be very light in construction, and appear frail. But, well engineered for their intended purpose, they do the job.

We do see some monumental structural failures, I think of the Hawaiian B737, whose fuselage top blew off. These are rarely a result of a primary design or construction failure, but more likely unforeseen fatigue failure. For this reason, "aging aircraft" is now it's own topic of concern, and even Cessna is getting into the business for the 100/200 series with inspection requirements, not so much for simple strength, but much more: "What has happened to this structure over the decades of service we did not design for?"

It's wise to inquire when you see a wrinkle (or certainly a crack!), but otherwise, have faith that the designers, and maintainers usually have things well figured out.

[photofly]

corollary: apply loads in an unintended direction and the otherwise strong airplane folds up like it was made of soggy paper.

EG: Keep the landings on the main gear, all is good. Contact the ground with something else first, however...

[youhavecontrol]

I love using the pop can demonstration when explaining this concept to my students. You can stand on top of an empty dent-free pop can without it crumpling, but just give the side one little flick...

[pelmet]

I had occasion to do a simple structural analysis of the Cessna 206 outer wing structure to find that it is the wing skins which carry all of the flight loads, rather than the spar.

Seems to fit the description of....semi-monocoque:
"A semi monocoque airplane’s skin supports much of the load, with some internal bracing and bulkheads in place to maintain structural integrity."

Monocoque means single shell which was an earlier aircraft design type without any of the internal bracing but it could be very heavy in order to give it the required strength, so a partial or semi-version was created.