Jury Struts

[Jim Heffelfinger]

At what point do main wing struts need jury struts? Or the reverse - when can they be eliminated in a very small (600# gross) air frame
Reason for asking; cons: added complexity, added weight, added drag, additional wing structure to support struts.
jim

[dougbush]

I suppose when you make the main struts stronger to resist bending and buckling under compression.

[Dana]

Jury struts are needed when the design load of the strut (in compression) times the appropriate safety factor is higher than the Euler (buckling) load.

Not much extra structure is required; the actual load on the jury struts is very low. You use them when the added weight and drag is less than the added weight and drag of a main strut sufficiently larger to not need them.

[WLIU]

A Luscombe is an example of a light aircraft that has dual struts with juries on the earlier models and a single strut on later models. On the earlier models the two smaller wing struts on each side have a small strut between them and then two jury struts up to the bottom of the wing. This design controls flex and vibration of the two wing struts and also allows some rigging of the washout of the wing. Someone must have done some more calculations for the later models where one larger sized strut is used without any jury strut supports. I believe that the later models are a little faster.

Best of luck,

Wes
N78PS

[martymayes]

A Luscombe is an example of a light aircraft that has dual struts with juries on the earlier models and a single strut on later models. On the earlier models the two smaller wing struts on each side have a small strut between them and then two jury struts up to the bottom of the wing. This design controls flex and vibration of the two wing struts and also allows some rigging of the washout of the wing. Someone must have done some more calculations for the later models where one larger sized strut is used without any jury strut supports. I believe that the later models are a little faster.

There's more to it than that. The Luscombes with 2 struts are rag wings, the single strut Luscombes have wings skinned in aluminum. The rag wings don't have any torsional strength so they need a strut for the front spar and one for the rear spar. The 2 strut planes have stuts made from steel, the single strut is aluminum.

[Dana]

Early Luscombes had fabric covered wings so two struts were no doubt needed to react torsional loads in the wing, like most other fabric winged planes. Later models had metal skinned wings, which would be much stiffer torsionally, so only a single strut would be needed. The [larger] single strut would be stronger in buckling loads, so the jury struts weren't needed.