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A few biplanes like the Stearman have a strut parallel to the fuselage, halfway along the rigging's landing wires and flying wires, not fastened to the airframe, not bearing any obvious load. One on each side. What are they for? What are they called?

I'd guess that because some of the Stearman's wires are doubled (also unusual?), the strut prevents them from banging into each other and chafing. But that seems unlikely because their tension is 800 to 1400 pounds, according to page 24 of the Dec. 2007 issue of Vintage Airplane (p. 26 of the pdf). Even if a maneuver temporarily halved that tension, deflecting one of those by two inches to hit its twin would take tons of force.

Boeing Stearman
(image source: Wikimedia)

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    $\begingroup$ One might imagine the strut helps control fluttering of the cables at certain airspeeds. The "buzz" would be (at least) very annoying, and could possibly compromise any attached fittings (due to vibration). $\endgroup$ – Robert DiGiovanni Oct 4 '20 at 16:52
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    $\begingroup$ It's a handle for the wing-walker. $\endgroup$ – Hot Licks Oct 5 '20 at 19:00
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    $\begingroup$ I think it's called a javelin. $\endgroup$ – Flynn Oct 5 '20 at 20:08
  • $\begingroup$ @Flynn That's descriptive, and likely. There's a 1963 EAA reference to "javelin struts" taped(!) to the flying and landing wires of a Waco 9 biplane to reduce "flutter." Worth an answer in itself. $\endgroup$ – Camille Goudeseune Oct 5 '20 at 20:22
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    $\begingroup$ hah. If you played the 'cello you'd know in an instant :-) . We have similar gadgets called "wolf eliminators" which do the same job: kill off unwanted resonances. $\endgroup$ – Carl Witthoft Oct 6 '20 at 11:51
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It's just a stick set between the flying and landing wires (the "wires" are actually solid stainless steel rods with an almond cross section for streamlining - they are VERY expensive). Normally they are made from wooden dowel with notches for the wires to fit, retained in the notches by wraps of lockwire, or on fancier biplanes they may be made from aluminum tube with a bullet nose and pointed tail.

They are usually called "wire stays". Their main function is to stop the wires from vibrating in the airflow (they're more or less metal vocal cords), which can create a lot of noise (that's the traditional diving airplane sound effect from the movies, as you can hear hear here, at 1:50:30, in Hell's Angels, one of the earliest sound movies with airplanes). The noise is from the "Wind in the Wires" (the title of a book by WWI pilot Duncan Grinnell-Milne). More importantly, the vibration can also induce fatigue cracking of the streamlined rods over the long term. Linking them together changes the natural frequency of the wires to something much higher than an individual wire, and stops them from vibrating in the airplane's speed range.

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  • $\begingroup$ Yes, I see how such a stick raises the resonant frequency. (1) Am I just inattentive to not see it on other biplanes, with similar rigging? Why not on those as well? (2) I found a few pre-WWI references to wire stays on airplanes, but it wasn't clear that the term meant these sticks. Any references for this term? (fyi "wire stay" in common sailing parlance means a forestay or backstay made of wire instead of solid rod, hemp, etc.; that's what it sounded like in what I found, the "flying wires" were themselves called "wire stays.") $\endgroup$ – Camille Goudeseune Oct 4 '20 at 21:22
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    $\begingroup$ Edited to remove the parenthetical about the diving airplane's sound effect being from vibrating cables. That particular sound is actually from the Junkers Ju-87 "Stuka" dive bomber, which used a siren to try and scare civilians. Every other time a diving airplane has made that sound, it's because some Foley artist thought that diving planes should make that sound, so they added it in post-production. $\endgroup$ – HiddenWindshield Oct 5 '20 at 13:20
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    $\begingroup$ @HiddenWindshield Your edit is wrong. The Stuka's siren is quite different from wire howling and diving airplanes making a howl due to the wires was part of the public consciousness in the 30s as soon as sound film involving airplanes came out. I've restored my edit with a link to an early movie, Hell's Angels, that predates the Stuka's entry into the public consciousness by about 6 or 7 years at least. I've restored the part you took out with a link to the movie. $\endgroup$ – John K Oct 5 '20 at 15:53
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    $\begingroup$ @JohnK Do you have a source for that? I've heard plenty of biplanes, and have never heard them make any sound that's anywhere close to a "Stuka Scream". I also can't find any reference to the sirens being developed because of existing sounds that biplanes made. $\endgroup$ – HiddenWindshield Oct 5 '20 at 17:38
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    $\begingroup$ Something I read many years ago in a magazine, Don't have a reference. In any case,some foley artists may have used Stuka recordings for their effects, but as I showed, it predates the Stuka, like Hell's Angels", where they were adding the noise long before the Stuka was around. For the real deal, watch this clip of Dawn Patrol at 1:36 youtube.com/watch?v=7eSvpeIaMRI The film recorded the aircraft flying by and you can actually hear the howl from the wires. Read accounts of WWI combat pilots, like the author I linked to, and references to screaming wires is common. $\endgroup$ – John K Oct 5 '20 at 18:51

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