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I´ve seen some UAV models without internal structure (stringers, longerons and frame). I guess the drone´s shell is the frame itself. So what is the best approach to calculate the shell thickness to avoid using a more complex internal structure?

Data:

  • Wingspan: 2.4m
  • Length: 0.9m
  • AR: 11
  • Speed: 16m/s
  • Estimated weight: 3.2 kg
  • Fuselage and wings materials: carbon fiber
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  • $\begingroup$ Is the shell made out of steel, or paper mache? $\endgroup$
    – Ron Beyer
    May 20 '20 at 18:03
  • $\begingroup$ Carbon/ glass fiber $\endgroup$
    – VicenteC
    May 20 '20 at 18:27
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    $\begingroup$ Welcome! You've taken the tour, now browse the help center to see how to best ask a question to get a good answer. "What's the best approach" questions are very broad and open ended and tend to receive very opinionated answers and are usually closed because of that. If you can provide some details of the design you're working on with a specific, answerable question, someone (or 6) will comment on it. Also, please edit the additional info into the question. Not everyone reads all the comments and comments can get deleted at any time. $\endgroup$
    – FreeMan
    May 20 '20 at 18:28
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    $\begingroup$ The terms you are lacking are "monocoque" and "semi-monocoque", two different ways to build a structure. The UAV you describe that didn't have visible longerons, etc was a either a monocoque structure, or what you were seeing was just a fairing and the load bearing elements were elsewhere. We cannot answer your shell thickness question without a lot more data, but maybe you meant to ask: "what is the upper practical MTOW limit of monocoque airplanes designs?" or something similar. In that case please reword it a bit to make this clear. $\endgroup$ May 20 '20 at 19:07
  • $\begingroup$ If you don’t get a good answer here, you might also try asking on the new drones.SE site. $\endgroup$
    – dalearn
    May 27 '20 at 3:26
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I very much doubt that you will hit your mass target (kg is a unit of mass, btw) if you build those wings without a proper spar. The shell is for carrying shear loads to provide torsional stiffness, but it cannot give much bending stiffness, especially if there is no internal structure that keeps the upper shell apart from the lower one under bending loads.

So build a light shell and add spar caps, a web and a couple of ribs where loads are locally introduced or where cutouts (say, for ailerons) start.

Regarding the fuselage: If there are many point loads inside from the payload and systems (camera gimbal, engine mounts) it makes sense to have local reinforcements. Otherwise, a shell should be fine given the higher curvature of a typical fuselage. On the other hand, everywhere you have large radii or flat panels, a local stiffener will be advisable.

As for calculating the shell thickness: At that size I expect that handling loads and inertial loads from point masses will determine the minimum size. There is no simple formula and you need to balance ease of operation with performance. Depending on your priorities, the shell will become thicker for a more rugged device or thinner for a lighter but more delicate one.

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