Design Aspects of Long Range Supersonic LFC Airplanes with Highly Swept Wings 881397
Supersonic Laminar Flow Control (LFC) airplanes with externally braced highly swept LFC wings of high structural aspect ratio (with the sweep increasing towards the wing root) offer particularly high supersonic cruise (L/D)'s with low sonic boom overpressures. At the design cruise condition the flow in the direction normal to the upper surface isobars is transonic (with embedded supersonic zones) and practically shockfree over most of the span. 3-body type supersonic LFC airplanes with a central fuselage and two smaller outboard bodies (alleviating wing bending and torsion) enable further increased spans and aspect ratios to reduce accordingly the lift induced wave- plus vortex drag as well as the volume induced wave drag (L/D)Cruise thus increases further.
Full span cruise flaps increase the low drag CL -range and (together with the active control surfaces of the outboard bodies) may be used to actively reduce wing loads and aeroelastic deformations, suppress flutter and augment aerodynamic damping and stability to further increase the wing span and (L/D)cruise. In addition, the low CLopt. of suction laminarized LFC airplanes (due to its very low CDO with laminar flow) raises Mdesign ⊥ the airfoil to allow a reduction of the wing sweep angle φ, which in turn increases the span, reduces CDi . and raises (L/D)Cruise accordingly.
Variable wing sweep is desirable to reduce CDi . and raise L/D in low speed flight. The structural weight-and aerodynamic performance penalties involved can be particularly low with highly swept strut-braced high aspect ratio LFC wings.
(L/D)cruise -values of 19 (27) and 16 (22), respectively, appear feasible at M = 2 and 2.5 with reasonably extensive (practically all) laminar flow over the airplane exposed surfaces.
With φ = 67° and 72° at M = 2 and 2.524, respectively, and the high Rec 's of supersonic LFC airplanes instability of the front wing attachment line- and cross-flow boundary layer in the leading edge- and rear pressure rise zone become critical. Relatively sharp leading edges alleviate these problems. Boundary layer suction at the attachment line decreases Reθa.l. and thus alleviates attachment line boundary layer instability further.
Boundary layer crossflow compensation, using a suitable flow overexpansion in the leading edge region, further alleviates the crossflow stability problems in the leading edge zone and practically eliminates boundary layer crossflow in the flat rooftop area of the upper surface. To minimize the suction power involved suction in the front acceleration zone of the upper surface should be tailored such that the boundary layer crossflow in the suction area remains about neutrally stable.
With boundary layer crossflow thus practically absent the upper surface rooftop boundary layer must be stabilized against Tollmien-Schlichting (TS)-disturbances by weak suction in spanwise suction strips. This is relatively easy due to the stabilizing influence of compressibility on TS-instability.
Boundary layer crossflow stabilization in the rear pressure rise area of the upper surface requires relatively strong local suction for 100% laminar flow. The necessary suction power decreases by decelerating the flow in this area over a short distance and tailoring suction such as to maintain a neutrally stable crossflow in the upstream part of the pressure rise. The suction rates and -power involved are surprisingly modest, enabling extremely low equivalent CD∞ 'S for the upper surface with all laminar flow.