I’ve been reading quite a lot about aircraft design and structures. Interesting contrast with rocketry. I’ve also been toying around with a few possible home designs and builds. I might write a more proper history / introduction of that someday.
The following is a very much simplified history of aircraft structures. By no means 100% correct, but should give you the general picture.
Basically, aircraft started with a wood frame, wood ribs and fabric covering. Later during the first world war, there was at times some plywood covering at places, later there were often steel tube frames (in the fuselage), aluminium ribs and skin and sometimes corrugated stiff skin (Junkers). Then, between the wars the planes got ever faster, and the fabric couldn’t hold up anymore. Duraluminium panels were used instead (still with wood interiors), then aluminium structures and stringers and finally just before the second world war, a semi-monocoque structure where the aluminium skin held a significant portion of the loads (besides providing the aerodynamic shape), resulting in a good weight reduction. (Boats had used the semimonocoque structure.) DC-3 and Spitfire are good examples. There were some oddities in the late war, like De Havilland who had the Mosquito and Vampire which used plywood skin with balsa core as a composite. Also, Vickers made bombers had geodesic grid construction all around.
Then, a long time passed until again in the late sixties and early seventies people started experimenting with something new, fiberglass. Again from the boat building industry. There are a few alternatives on exact fiberglass construction: to use a mold, or then use a foam core on which to laminate the fibers that then stayed inside the final aircraft part. Ken Rand built one of the first composite planes in foam core and Burt Rutan later got famous with it.
Sailplanes went totally composite decades ago as the performance is vastly superior to aluminium. From there the technology has been slowly seeping to the rest of the aviation.
Nowadays all big commercial aircraft (over 6 passengers or so) are aluminium semi-monocoque construction. The skins and stringers are joined with rivets. The technology is about 70 years old. There is an increase in composite parts, where light weight and stiffness are required. For example a Finnish factory manufactures some spoilers for Airbus. Glass fiber or carbon fiber in an epoxy matrix, all made in a mold in a high pressure and temperature autoclave.
There is more variation at the very light end. The cheapest hang gliders are usually aluminium tubes for structure and dacron fabric for the airfoil. More expensive ones have carbon fiber parts and sometimes some foam, still fabric covered. Ultralight trikes have an aluminium or steel tube chassis attached to a big hang glider. Sometimes with some fiberglass fairings to shield the pilot from wind.
Ultralight aircraft beyond trikes (the requirements for an ultralight are weight under some 450 kg and a low stall speed below about 70 km/h over here) are a various bunch. Ranging from tube-fabric to wood-fabric, fiber-fabric to totally fiber made (with foam core or molded). A few completely Al craft also exist.
Light aviation is mostly either totally aluminium (Cessna, Piper etc), or totally molded composite (Diamond, Grob etc). There are homebuilt experimentals that don’t fit the ultralight category since they are too big or fast. Some of them use foam core, like Rutan’s design Long-Ez, and its derivatives like Cozy.
There are reasons why different structures in homebuilt aircraft exist.
Glass fiber is good for making arbitrary smooth and stiff shapes, lending itself very well to highly efficient aerodynamic shapes – that’s why it’s used in sailplanes and efficient small travel planes, where small drag is important. Some kits have lots of premolded skin parts. Some only have drawings, and you cut your own foam, and laminate by hand on top of it. Laminating is messy by hand unless you use the modern vacuum bag technique. Complex composite structures might also develop cracks that are not visible on the surface, making them dangerous. Examples: KR-2, Long-Ez, Cozy.
Aluminium on the other hand is lasting, rugged, easily inspected and can take the weather. It’s used for some bush planes which have STOL capability and where absolute aerodynamic cruise efficiency is not so important. Unfortunately, there are a huge amount of cut, bent and riveted parts in even a small ultralight aircraft, and build times are many thousands of hours taking many years from start to first flight. Examples: Zenair CH701 STOL and its numerous copies.
Tube and fabric is especially good when you want to dismantle your craft for transportation. That is true for hang gliders and many ultralights. It is also possibly quite easy to construct. There is a French bolted aluminium tube design (wrapped with dacron fabric) currently manufactured in Ukraine which requires no welding at all and should be very quick to build, the Skyranger. Rans Coyote is one too.
Welded steel tube is something that requires a builder who is a good welder, and the complicated structure needs lots of cuts and welds, which makes it time consuming.
Wood – it is a mixed bag. It has good strength to weight ratio, comparable to aluminium or steel, but there are moisture problems and the issue that the raw material is by its nature always somewhat uneven. Solutions like plywood try to get around this: if one ply has a weaker spot, it doesn’t weaken the whole structure much. The material could be potentially cheap, though it depends on where you are. Wooden rib construction is quite complicated and time consuming, requires lots of sawing, cutting and gluing and the number of ribs and reinforcements is huge. Examples: Mag-01, Junqua Ibis.
My Own Ideas
I’ve dropped a few hints on the way as to what could be my preferred approach, if I were to build my own airplane. But I’ll handle more of that in a later post, as I’m now in such a hurry and gotta go. You can learn a surprising amount when researching the history – airplanes are such a common subject of dreams and experimentation that a great many things have been tried.
p.s. Changing the blog title picture now.