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Thread: Figuring out strut replacements

  1. #1
    DaleB's Avatar
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    Figuring out strut replacements

    I'm no engineer, but I do understand that there is a pretty significant drag penalty associated with round tubing hanging out in the air flow. So let's assume for a moment that there is a biplane design that uses round aluminum tubing for cabane and interplane (N) struts. And let's assume that you wonder if streamlined struts might be a better idea, to reduce drag to some degree.

    1. How would one go about calculating the specs for either streamlined aluminum tubing, or wood, that would replace the round tubing with equivalent or better strength?
    2. How would one calculate the difference in drag force between round and streamlined sections, to decide if it's even worthwhile to make the change?
    Last edited by DaleB; 01-11-2017 at 06:50 PM.
    Measure twice, cut once...
    scratch head, shrug, shim to fit.

  2. #2

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    As a start, you can compare the E X I of the original to the replacement strut cross section. E is the elastic modulus of the strut material and I, the area moment of inertia. The EI products of both struts should be close. If both original and replacement materials are the same, an equivalent I should be a good approximation for bending loads. For pure tension/compression loads, the cross sectional areas should be close.

  3. #3
    DaleB's Avatar
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    Thanks! That is a very good start. Which is to say, after an hour or so of looking at how I is calculated, I think I understand how to compare (for example) round aluminum and an elliptical wood shape. Onward.
    Measure twice, cut once...
    scratch head, shrug, shim to fit.

  4. #4

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    To do the analysis correctly, there are buckling properties to analyze for each material/shape and allowable tension/compression stresses so you don't want to make a direct substitution based on EI alone.
    A more through analysis requires knowing loads and material properties and combined stresses acting at same time. It's been many years since I did those calculations.

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