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I'd like to perform a static structural load test of a wing for a UAV. The wing is a rectangular planform of constant thickness throughout the wingspan with no twist, dihedral, or sweep.

During steady-level flight, the lift distribution will be a uniformly distributed load. In order to simplify the test, I'd like to load the wing with a concentrated force at the tip as opposed to a distributed load along the span. This allows me to use a single weight as opposed to various sandbags.

Are there any issues in applying a concentrated load at the wing tip that would result in an equivalent maximum bending moment at the root to verify its structural integrity?

Actual Load:: Actual Load where M_max = FL/2

Approximated Load: Approximated Load where M_max = FL

Where F_approximated = F_actual/2

Seth
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  • Moment and shear are the same in both cases at the root, but obviously their distributions along the wingspan is different. – sophit Nov 07 '22 at 21:36
  • For sure, but if anything the fixed point load case should be over-conservative compared to distributed load case. So I should be covered if the fixed load test is successful I would think. – Seth Nov 07 '22 at 21:42
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    In your approximation the moment is a linear function of distance, in the actual load case it is quadratic function. For this reason your approximation is a conservative approximation. I see no problem, but I am no structural engineer – U_flow Nov 07 '22 at 21:51
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    Yes, it is actually conservative... or your wing is oversized and heavier :-) – sophit Nov 07 '22 at 21:58
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    If your wing is rectangular and has no twist or change in airfoil, then your lift will not be uniformly distributed (I'm assuming you mean the same lift per span), but rather somewhere between rectangular and elliptical, see Schrenk's approximatino method. – Raketenolli Nov 07 '22 at 22:36
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    I'm with @sophit with this one. If your wing can handle the standard load but loaded entirely at a point at the tip, then the wing panels and spar are significantly stronger-- and heavier-- than needed. Maybe you're fine with that, in which case go for it. – Kenn Sebesta Nov 07 '22 at 23:34
  • Thanks all for the feedback - agree with your points. For ease of manufacturing, I went with a constant cross-section airfoil along the span, which was sized based on the max bending stress at the root. Understood that it is heavier than ideal, but am trading that for simplified manufacturing. – Seth Nov 07 '22 at 23:50
  • So to sum up, no one put up an answer but OP ist happy anyways :D – U_flow Nov 08 '22 at 11:33
  • @U_flow: that's the scope of this SE, make people happy :-D – sophit Nov 10 '22 at 19:05

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