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I have seen small RC aircraft built from just about everything: cardboard, sytrofoam, fiberglass, aluminum, coroplast, etc.

Most these designs are at least partial monocoque designs. Is there any reason why sheet steel cannot be used? In comparison to aluminum, thin sheet steel is very cheap or even free. Most appliances in the US are built from approx. 1/32 inch sheet steel. It can be worked using standard tools available in a hardware store.

Are there any requirements that sheet steel would not be able to meet? I am aware that the weight-to-lift ratio of the aircraft is likely to suffer, but as long as it is capable of flight I don't consider this as much of a problem.

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    $\begingroup$ With a large enough engine anything will fly. Keep in mind that a steel flying machine with a large engine will probably have to fly fast to stay in the air (compared to a non-steel version of whatever you build). If you end up building this please find a very remote location for test flights. I'm picturing what amounts to an armored lawn dart filled with nitro fuel and a 1/2 horsepower blender attached to the nose. $\endgroup$ – acpilot Aug 14 '16 at 13:45
  • $\begingroup$ I think your point is with a high enough performance engine, it could fly. I can't just put a J58 turbojet on my plane and expect flight. $\endgroup$ – Eric Urban Aug 14 '16 at 13:49
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    $\begingroup$ My point is that a balsa/foam version of what you're describing may be over-powered with a .4 but a steel version will weigh considerably more and need a much more powerful engine. I'm not saying you SHOULDN'T do it...but if you do please put a video on youtube! $\endgroup$ – acpilot Aug 14 '16 at 13:57
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Yes, it can be done, but to do it well is a lot of work. You will also need to use a thinner gauge than 1/32" to get the weight down.

Note that one of the very first all-metal aircraft was made of 0.2 mm (1/128") sheet steel and flew well. However, building it required a corrugated substructure and a welded steel tube frame for strength. Still, it was heavier than necessary so its builder switched to aluminium for later designs.

Generally, the tension length of steel (strength relative to density) is roughly equal with that of aluminium or titanium; however, you will need to use high-grade steel to be comparable with aircraft-grade aluminium, so some of the price advantage will be lost.

But there is a distinct disadvantage with higher-density materials. While their strength scales linearly with wall thickness, their buckling strength scales with the square of their thickness. You will end up with a very delicate structure which will require careful handling to avoid local dimples and a stiff substructure to avoid early failure in compression. Also, introducing loads locally (say, for a landing gear or an engine mount) will require local reinforcements which should ideally be multi-layered and taper out in several steps.

If you want to scale an existing structure down, make sure you place the stiffeners closer than what a scale construction would require. But I am sure you will prefer to bond the sheet steel to a styrofoam core. Use a higher-density styrofoam with better compression strength and be generous with local reinforcements, and the model should turn out fine.

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  • $\begingroup$ Note that the 1/128" sheet aircraft is a full-sized aircraft. For a "small" RC plane (by RC standards, "small" is typically 1m or less wingspan or fuselage length) you'd need steel sheets thinner than tissue paper - so thin that you can probably weld it with a lighter and it would most likely be impossible to touch (kind of like planes built with nitrate microfilms which melt on touch) $\endgroup$ – slebetman Aug 15 '16 at 1:50
  • $\begingroup$ @slebetman Based on the feedback in answers, perhaps the real answer is a steel fuselage with a more traditional wing structure. $\endgroup$ – Eric Urban Aug 15 '16 at 2:25

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