Cobra maneuver unsteady aerodynamic considerations

Abstract

Introduction T HE COBRA maneuver, performed by Pougachev at 1989 Paris air show with Soviet Su-27 Flanker, ' and later performed also with a MiG-29 at Canadian National Air Show in Ottawa, requires lateral high-alpha stability superior to what until then had been demonstrated by other aircraft. One conclusion that has been drawn is that somehow symmetric flow separation has to be assured through alpha range 0 50 deg. The separation asymmetry on a slender forebody is controlled by so-called microasymmetries, as has been demonstrated by effect of roll angle on a pointed ogive. The flow separation can be forced to be symmetric by use of body strakes, trips, and by other means. It is noted in Ref. 3 that the Su-27 radome does have small chines located at apex of nose. These chines are small but probably have a very positive effect on high AOA stability. That chines may not always be successful in eliminating or even alleviating forebody flow asymmetry at zero side slip has been demonstrated in low-speed tests with conic noses. In these tests a nose boom was found to reduce maximum side force | CY max more than lateral strakes located near apex. However, this favorable nose-boom effect appears not to be present at higher Reynolds numbers. H Thus, not too much of an effect should be expected from presence of nose booms on Su-27 and MiG-29. The vortex wake from very slender nose boom embeds apex of forebody, possibly having an alleviating effect similar to that of nose bluntness. Consequently, experimental results for a blunted conecylinder geometry in rapid pitching motion (Fig. 1) should be of some relevance.