An interplane strut is an aircraft airframe component designed to transmit lift and landing loads between wing (mainplane) panels on biplanes and other aircraft with multi-wing designs. They also maintain the correct angle of incidence for the connected wing panels and are often braced with wires. Very early aircraft used interplane struts made from bamboo, while later designs employed streamlined struts made either from spruce or ash; woods chosen for their strength and light weight. More modern or higher performance aerobatic biplanes often feature metal interplane struts.
The most common configuration is for two struts to be placed in parallel, one behind the other. These struts will usually be braced by "incidence wires" running diagonally between them, and more than one of these strut pairs may be placed along the span of the wing, creating multiple "bays" (see below). Other common arrangements include N-struts, where the diagonal bracing wires are replaced by an extra wooden or metal member running diagonally from the top of one strut to the bottom of the other in a pair; V-struts, where the struts from the upper wing converge to a single point on the lower wing (commonly used when the lower wing has a considerably smaller chord than the upper wing); and the I-strut, where the pair of struts is replaced by a single, thicker strut. Some biplane wings are braced with struts forming a Warren truss.
The space enclosed by a set of interplane struts is called a bay, hence a biplane or triplane with one set of such struts connecting the wings on each side of the aircraft is a single-bay biplane. This provided sufficient strength for smaller aircraft such as the First World War-era Fokker D.VII fighter and the Second World War de Havilland Tiger Moth basic trainer. The larger two-seat Curtiss JN-4 Jenny is a two bay biplane, the extra bay being necessary as overly long bays are prone to flexing and can fail. The SPAD S.XIII fighter, while appearing to be a two bay biplane, has only one bay, but has the midpoints of the rigging braced with additional struts, however these are not structurally contiguous from top to bottom wing. The Sopwith 1½ Strutter has a W shape cabane, however as it doesn't connect the wings to each other, it doesn't add to the number of bays. Large transport and bombing biplanes often needed still more bays to provide sufficient strength. These are sometimes referred to as multi-bay biplanes. A small number of biplanes, such as the Zeppelin-Lindau D.I have no interplane struts and are often referred to as being strutless.
Zeppelin-Lindau D.I strutless biplane
Nieuport 23 single-bay sesquiplane
SPAD S.XIII single-bay biplane with auxiliary struts
Curtiss JN-4 two-bay biplane
Handley Page V/1500 multi-bay biplane
V-strut on a Nieuport 10
N-strut on a Boeing-Stearman Model 75
I-strut on an Antonov An-2
Warren truss struts on a Fiat C.R.42
- Taylor, 1990. p. 71.
- Taylor, John W.R. The Lore of Flight, London: Universal Books Ltd., 1990. ISBN 0-9509620-1-5.