Flow Complexities of Slender Wing Rock

Abstract

Introduction M ILITARY design trends toward the use of unmanned, Ž nless aircraft have renewed interest in the unsteady aerodynamics of slender wing rock. One important aspect of the problem of wing rock control is the potential impact of typical conŽ gurational details on delta-wing aircraft, such as the presence of a centerbody. Early experimental results for a sharp-edged 80-deg delta wing indicated that breakdownof the leading-edgevorticesplayed a role2,3 (Fig. 1). It could be shown that rather than being the cause of the wing rock problem, the breakdownphenomenon limited the growth of the limit-cycle amplitude. By the consideration of the effect of the roll-inducedsideslip on the effective leading-edge sweep of the windward (dipping) wing-half, an upper limit for the limit-cycle amplitude could be determined. The measured start of wing rock (Fig. 1a) is in good agreement with the prediction that, for zero bearing friction, loss of roll damping should occur at a 20 deg (Fig. 2). Bearing friction caused the delay to a = 25 deg of the loss of roll damping in the test of a pure delta-wingmodel (Fig. 1a). It will be shown that the earlier start of wing rock for the othermodel3 was caused by the presence of a centerbody or fuselage. As even unmannedcombat aircraftare likely to have a fuselageor centerbodyof some kind, it is important to know that the effect of a centerbodyondelta-wingaerodynamicscanbe large.8 Experimental results for a 69.33-degdelta-wing–bodyconŽ gurationdemonstrated that the centerbody promoted vortex breakdown.9 This could be explained by the body-induced camber effect, which according to experimental results for the effect of static camber,11 would have promoted breakdown, in agreement with the experimental results. Figure 3 shows conŽ gurational details of the models giving the results in Fig. 1. Whereas the Langley model behaves essentially as a pure,sharp-edgeddeltawing, theothermodel3 hasa centerbody. Becauseof its bluntness, it behavesas a cylindricalcenterbody, promoting vortex breakdown, thereby causing a reduction of the maximum wing rock amplitude. The earlier loss of roll damping, before the predictedvalue7 a 1⁄4 20 deg (Fig. 2), could also havebeen causedby the presenceof the centerbody.By limiting thewing area, the centerbody acts to increase the effective leading-edge sweep, thereby causing earlier loss of roll damping. The experimental results13 in Fig. 4 show that when a pointed ogive-cylinder centerbody, similar to the one used in Ref. 9, is moved aft to start behind the wing apex, as in the case of the extensively tested 65-degdelta-wing–body conŽ guration, insteadof promotingbreakdown, the body delayedvortex breakdownto occur 30% or more aft of the measured position for a pure 65-deg deltawing.14 It is described in Ref. 5 how this is the expected resultwhen the pointed ogive-cylinder body is moved aft, as shown in Fig. 4,