Abstract
Sideslip-induced asymmetrical vortices are critical to the performance of canard-configuration aircraft. In this paper, a canard-configuration model was found to roll around a nonzero trim angle in a free-to-roll experiment, indicating a loss of lateral static stability. This loss was also evidenced by the positive slope of the rolling-moment versus sideslip curve. To reveal the underlying mechanism of this phenomenon, we focused on sideslip effects, taking surface-pressure measurements and conducting PIV (particle image velocimetry) tests. At the attack angle of 16°, sideslip causes inward extension of the windward low-pressure area on the main wings, which leads to a decrease in rolling moment. The inferred cause is the inward movement of the windward reattachment point. At the attack angle of 35°, complex multi-vortex structures were analyzed using PIV for three forms of the model: with all components; without canard; with neither canard nor strake. The effects of sideslip on vortex flow at high attack angle were then studied. As sideslip angle changed from -1° to 1°, sideslip promotes the breakdown of windward vortices, causing a sharp decrease in windward suction. Subsequently, as sideslip angle changed from 1° to 4°, the backward movement of the breakdown point and an increase in strength of vortices are observed on the leeward side, which coincides with an increase in both leeward suction and the rolling moment. As the sideslip angle continues to increase, the inboard vortex on the leeward side rises and becomes weak, which leads to a decrease in leeward suction, and hence, the recovery of lateral static stability. Moreover, the coiling of leeward vortices weakens under sideslip.
Highlights
The close-coupled canard configuration whose canard wings and the main wings are placed closely, utilizes the downwash produced by the canard to delay stall and increase maximum lift, thereby providing a higher controllable attack angle (α), which is defined as the angle between the projection of the aircraft's velocity on the aircraft’s symmetrical plane and the body axis
The development of and interaction between vortices over a yawed delta wing with leadingedge extension (LEX) were analyzed by flow visualization, and the results show that with increased α, coiling of the wing and LEX vortices was intensified, under sideslip, the coiling, the merging, and the diffusion of the wing and LEX vortices increased on the windward side, whereas they became delayed on the leeward side
The lateral static instability of a canardconfiguration model at a high angle of attack was made evident by wing-body rock around a nonzero trim angle and a positive slope in the rolling-moment versus sideslip curve
Summary
The close-coupled canard configuration whose canard wings and the main wings are placed closely, utilizes the downwash produced by the canard to delay stall and increase maximum lift, thereby providing a higher controllable attack angle (α), which is defined as the angle between the projection of the aircraft's velocity on the aircraft’s symmetrical plane and the body axis (see Figure 1). Despite its advantages in aerodynamics, the complexity of the structure presents difficulties in understanding the flow mechanism. Verhaagen and Jobe conducted wind-tunnel tests to examine the effect of sideslip angle (β) on the flow over a flat-plate 65○ swept delta wing at α=30○ Their results showed that sideslip promoted vortex breakdown on the windward side and delayed it on the leeward side, the model was statically stable. We conducted experiments in a low-speed wind tunnel to reveal the underlying mechanism of the nonzero trim roll angle of a canard-configuration aircraft’s wing-body rock by studying the effects of sideslip. The results of static force measurements indicated that the model lost lateral static stability at small β which was the reason for the nonzero trim roll angle To study this phenomenon, surfacepressure measurements and PIV experiments were performed
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