Wind Tunnel Testing on Start/Unstart Characteristics of Finite Supersonic Biplane Wing

This study describes the start/unstart characteristics of a finite and rectangular supersonic biplane wing. Two wing models were tested in wind tunnels with aspect ratios of 0.75 (model A) and 2.5 (model B). The models were composed of a Busemann biplane section. The tests were carried out using supersonic and transonic wind tunnels over a Mach number range of with angles of attack of 0°, 2°, and 4°. The Schlieren system was used to observe the flow characteristics around the models. The experimental results showed that these models had start/unstart characteristics that differed from those of the Busemann biplane (two dimensional) owing to three-dimensional effects. Models A and B started at lower Mach numbers than the Busemann biplane. The characteristics also varied with aspect ratio: model A ( ) started at a lower Mach number than model B ( ) owing to the lower aspect ratio. Model B was located in the double solution domain for the start/unstart characteristics at , and model B was in either the start or unstart state at . Once the state was determined, either state was stable. 1. Introduction A sonic boom is caused by shock waves and expansion waves generated by a supersonic aircraft. As the sonic boom generates an impulsive noise at the ground, it produces undesirable effects on not only people but also animals and architecture. Sonic boom mitigation is thus required for the development of supersonic commercial aircraft [1, 2], and extensive studies have been carried out regarding this [3, 4]. Recently, Kusunose et al. proposed the supersonic biplane theory [5–8] as a method of sonic boom mitigation. This theory enables significant reduction if not complete elimination of shock waves and expansion waves by the wave reduction and wave cancellation effects of a biplane configuration. The concept of a Busemann biplane, which was first proposed by Busemann in 1935 [9, 10], forms the basis of supersonic biplane theory. Figure 1 shows the Busemann biplane (two dimensional) in a supersonic flow; this biplane consists of two half-diamond airfoils facing each other. Figure 1(a) shows the start state: compression (shock) waves generated from the leading edge of the elements are canceled by an expansion wave at the shoulder; the wave drag due to thickness is reduced significantly by the mutual cancellation of waves. Thus, the waves propagating outside the elements can be eliminated. Figure 1(b) shows the unstart state: a curved bow shock forms in front of the elements owing to the choked-flow phenomenon; the wave drag increases greatly. Naturally, a strong