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Does the approach below seem reasonable for a first-order estimate of the bending stress on a fuselage? I want to specifically confirm the end result, which shows that the maximum bending moment, and therefore maximum stress occurs at the nose.

Approach/Assumptions:

  • Model fuselage as a beam in bending
  • Beam is fixed at the CG, with lift from the wing and tail acting perpendicular to the beam
  • Weights of each component - battery, wing, payload, motors are combined into a singe load that acts thru the CG of the airplane
  • Thrust and drag forces act horizontally thru the fuselage centerline

Calculations: FBD and Reaction Forces Section Partitioning Section I-I Section II-II Section III-III Shear and Bending Moment

Update After following sophit's suggestion to split up the weight of each component and fix the beam at the wing, I'm now getting that the max bending moment occurs near the wing, which makes more sense.

Updated Shear and Bending Moment

Seth
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    I'd fix the fuselage at the wing (point A) instead at the CG; and I'd put the weight of each component where each component is instead of concentrating everything in the CG. Max bending moment at the nose looks strange to me... maybe max deflection? – sophit Oct 18 '22 at 20:04
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    One problem with you current iteration of calculation is, that the lift of the wing is ahead of the CG. This causes the problem with a moment existing at the nose of your aircraft since the whole systems moment isnt balanced. – user33651 Oct 19 '22 at 12:24
  • Thanks for the input sophit. I ended up splitting up the weights of each component and fixing the fuselage at the wing. I'm now getting a more reasonable result with the max bending moment at the wing. See updated shear and bending plots in the post. – Seth Oct 19 '22 at 20:03
  • Thanks for the input hph304j. That is a good point - I'm estimating the CG to be fwd of the wing, this was just an error in the diagram. Since I ended up splitting the weights of each component, the precise CG location was not used for the calcs – Seth Oct 19 '22 at 20:05
  • I'm glad I could help :-) Now it definitely looks more realistic – sophit Oct 19 '22 at 20:28

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