During construction how is a spar tested to ensure it can adequately cope with bending and twisting forces of the completed structure, or is this performed after complete?Piper cub kit from wicks

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    $\begingroup$ Are you asking as the builder, or the kit manufacturer? $\endgroup$
    – Ron Beyer
    Dec 27, 2017 at 5:08
  • $\begingroup$ As a builder. How does one know what they built is up to snuff $\endgroup$
    – Jim
    Dec 27, 2017 at 9:50

1 Answer 1


For a homebuilt a static load test is required. Whereas a full certification would require a loading to 1.5 the maximum load, this factor is reduced for experimental aircraft to 1.0. But performed it must be nonetheless.

For the test the aircraft is propped up upside down and sandbags (or water bottles) are placed on the wing until the calculated load is reached.

Structural test of the Electra One

Structural test of the Electra One (picture source)

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    $\begingroup$ "But performed it must be..." yoda would approve of this :-) $\endgroup$
    – Fattie
    Dec 27, 2017 at 12:41
  • $\begingroup$ I don't know if the economics are still sound, but when I helped a friend do the static load test on his Q2 we used bags filled with Portland cement. The cement was cheaper than sand and being much more dense, the bags (of equivalent weight) were smaller and easier to arrange on the wing. $\endgroup$
    – Gerry
    Mar 31, 2018 at 15:55
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    $\begingroup$ @Gerry: Cement bags are well suited (they will conform to the local surface just as well as sand), but sand bags can be individually filled to fine-tune the load. The fine dust produced by handling cement should be avoided near the airframe, or all holes should be taped shut. $\endgroup$ Mar 31, 2018 at 17:25
  • $\begingroup$ @PeterKämpf Agreed. It was a long time ago, so I don't remember all the details. The cement bags were very heavy (~60 lbs) and unwieldy. We filled (outside) heavy duty garbage bags to 25 lbs each. Took care of the dust, made them easier to handle, and the standard weight simplified tracking and placement. $\endgroup$
    – Gerry
    Mar 31, 2018 at 18:55
  • $\begingroup$ "Maximum load"...... You mean gross weight? @Peter Kämpf $\endgroup$ Jul 26, 2018 at 17:04

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