The following tasks summarize the challenges of supersonic aerodynamic design:
• Provide aerodynamic data suited for interdisciplinary’ design optimisation. These data need not result from best achievable accuracy limits, but must be reliable within specified accuracy conditions for a wide range of configurations.
• Maximize aerodynamic performance (Lift/Drag * L/D) for given geometrical constraints: improve quality of aerodynamic tools to better reflect flow physics, balance wave drag, induced drag, friction drag (including lammansation concepts) for minimum overall drag.
• Determine the limits of special flow phenomena, like suction force, etc.
5.3 Low Speed Flight Regime
53.1 Dominating flow phenomena
At take-off and landing, low speed of the aircraft generates only small dynamic pressures. Generation of aerodynamic forces, therefore, requires high specific aerodynamic loading, up to the separation limits.
Low speed lift generation:
To improve lift, span can be increased (variable geometry aircraft like the OFW). camber can be increased by flaps and wing area by Fowler flaps Angle of attack can be increased (especially for highly swept wings) and engine air can be used directly for lift generation or for support of flap efficiency. Most SCT configurations have only limited possibilities to use flaps: the symmetric (Concorde-like) configurations have large wing areas, where flaps can contribute only marginally. For an arrow wing, inner wing flaps can be efficient, because they generate lift near to the center of gravity which reduces trim losses. For the OFW the pitching moments connected with camber or fowler flaps limit their application.
Of special importance arc leading edge flaps for symmetric SCT configurations which do not generate lift but reduce lift dependent drag Concorde generates additional lift using the lifting vortices generated by leading edge separation on highly swept wings Those vortices, on the other hand, produce high drag. For a new SCT it is intended to use those vortices • if used at all – only at lift-off and perhaps flare During climb, no vortices should separate at the leading edges to allow for lower climb drag. There are proposals to use droop flaps (Figure 31), or – proposed by Boeing – suction at the leading edge to delay leading edge vortex separation.
Figure 31 Drooped Leading Edge Separations:
At high aerodynamic loadings separation may occur. For landing, separation with separation drag is welcome, but separation must alway s be controlled; it must not suddenly alter the flight handling Leading edge separation must therefore he confined to highly swept leading edges, where the individual separation vortices arc til to the large lifting vortex. Trailing edge separation has to occur smoothly and at selected pans, like for subsonic aircraft Especially the OFW needs much drag for landing to inactivate its superior aerodynamics. Drag producing devices arc then requested which introduce only minor pitching moments
Due to the small dynamic pressure, control surfaces become less efficient. In addition, separation on control surfaces limits the achievable forces, separation on wing, fuselage and nacelles introduces additional disturbances.
Especially for highly swept trailing edges – like for arrow wings and OFW – the effcc- tivity of control surfaces is not >et completely undcrxood. Some additional research is needed here
In order to exploit the wing’s lift performance w ith droop leading edges and cambered or even fowlcrcd trailing edges, it is necessary to balance the aircraft by an additional control surface like a horizontal tail or canard
At take-off. main emphasis is on good L/D to reduce thrust and noise. For landing, though, high drag is necessary to allow step descent and slowdow n, when the engines still run at flight idle with not too low thrust levels. On the other hand, enabling flare or allowing for go- around, drag must stay below some limits or must rapidly be reduced