Last night the Blogbrother came over and we continued to work on the right wing of the RV-12. When we last left you, the bottom of the wing was skinned*, but what we didn’t tell you is that these skins weren’t fully attached, because they wrap around the leading edge to the top surface and go back about a foot or so, where they’ll form (we think, without reading ahead in the plans in detail) a lap joint with the skins that run aft from that point. The aft upper skins have a lip that hooks into the wing ribs and also serves to stiffen the skin (and therefore the wing, when all is assembled) considerably.
The wing is designed so that the aerodynamic forces of flight, which can be calculated straightforwardly with algebra and trigonometry, are transferred to the spars, which are the backbones of the wing, and then to the fuselage. Many pilots know useful rules of thumb here. The wing must support the whole weight of the plane and its passengers and contents, which are limited by regulation to 1,320 lbs. So each wing bears 660 lbs of force in unaccelerated flight (If you suspect the numbers are metric figures expressed in Imperial units, you’re quite right — the Light Sport Aircraft standard is a 600-KG standard, vis-a-vis the parallel Euro regulation which limits the planes to 450-Kg gross weight). This is not an aerobatic category airplane, which limits the maneuvers it is designed for (again by regulation, one is not to exceed 60º of nose-up, -down, or bank). At 60º of bank, some of the force vector of the wings (“lift”) goes to oppose gravity and keep the plane in the air, and some goes to keep the plane turning in a tight circle…conveniently enough, that doubles the load-bearing of the wing, so each wing (left and right) is independently bearing 1,320 lbs.
So the designer must make each wing twice as strong as the bare minimum to lift the plane off in a straight line… but wait! We haven’t accounted for any safety margins. In practice, most light planes are designed for +3.8 and -1 G, unless they are destined for hard work (“utility” category) or aerial athleticism (“aerobatic” category), and they are designed for that load plus a safety margin, which is usually a multiple of 1.5.
Some airplanes are built much stronger (one well-known aerobatic plane is good for 12G in all axes, an acceleration which would conk most of us right out if applied quickly. And big transports are designed for lower maximum air loads, and are flown within narrower parameters. (The -1 manual for the C-130, for example, restricts pilots to +1.5, -0 G at Maximum Take-Off Weight). So how does the AC-130 fly in up to a 60º bank? Ah, those restrictions are at MTOW, the equivalent of max gross weight. So the limit is 1.5 x the plane’s full gross weight, loaded. If the airplane is many tons lighter, the pilots can horse it around quite a bit more without worrying about it reverting to kit form inflight.
Light airplanes may be certificated in multiple categories at different gross weights, also. So you might be Normal at 1670 lbs, Utility at 1400 lbs, and Aerobatic at 1200 lbs., with different maneuvers permitted at each level. It all comes back to the engineer’s original calculation of the load bearing capability of the structure, and of the air loads imposed on the wing. Of course, the design should not only be substantiated by engineering calculations, but also proven by flight test. (Even engineers make mistakes). The FAA requires homebuilt experimentals to undergo a period of flight testing before they’re used to carry passengers or fly over congested areas: airplanes falling from the sky in a rain of aluminum pieces are generally bad for the occupants, anyone underneath, and the reputation of aviation as a whole.
Every once in a while, some national socialist gets the idea that there’s way too much freedom loose in the country when people can build and fly their own airplanes. Not surprisingly, these are often the same national socialists who are alarmed at the idea that you might manufacture your own firearm. So far, we’ve retained this liberty (from some time in the New Deal until 1952, building your own airplane was verboten in the USA).
So, what has all this airplane stuff got to do with “scratching?” Last night, the mosquitos had their way with us as we worked in an open garage. Hence today’s pruritis. Hence, scratching.
Administrative note: later today we may have a movie review up (slotted in yesterday) for the first time in a while. We have quite a backlog of films to review, but the one scheduled for yesterday’s review is only half watched!
* Isn’t “skinned” a funny word? If you’ve got an airplane wing skeleton and a pile of aluminum sheets, it’s “skinned” when you put the skins on. If you’re Hannibal Lecter, on the other hand….