| dc.description.abstract | The laminar-flying-wing aircraft appears to be an attractive long-term prospect
for reducing the environmental impact of commercial aviation. In assessing its
potential, a relatively straightforward initial step is the conceptual design of a
version with restricted sweep angle. Such a design is the topic of this thesis.
In addition to boundary layer laminarisation (utilising distributed suction) and
limited sweep, a standing-height passenger cabin and subcritical aerofoil flow are
imposed as requirements. Subject to these constraints, this research aims to:
provide insight into the parameters affecting practical laminar-flow-control suction power requirements; identify a viable basic design specification; and, on the basis of this, an assessment of the fuel efficiency through a detailed conceptual design study.
It is shown that there is a minimum power requirement independent of the suction
system design, associated with the stagnation pressure loss in the boundary
layer. This requirement increases with aerofoil section thickness, but depends
only weakly on Mach number and (for a thick, lightly-loaded laminar flying wing)
lift coefficient. Deviation from the optimal suction distribution, due to a practical
chamber-based architecture, is found to have very little effect on the overall
suction coefficient. In the spanwise direction, through suitable choice of chamber
depth, the pressure drop due to frictional and inertial effects may be rendered
negligible. Finally, it is found that the pressure drop from the aerofoil surface to
the pump collector ducts determines the power penalty; suggesting there is little
benefit in trying to maintain an optimal suction distribution through increased
subsurface-chamber complexity. For representative parameter values, the minimum
power associated with boundary-layer losses alone contributes some 80% - 90% of the total power requirement.
To identify the viable basic design specification, a high-level exploration of the laminar-flying-wing design space is performed, with an emphasis above all on
aerodynamic efficiency. The characteristics of the design are assessed as a function
of three parameters: thickness-to-chord ratio, wingspan, and unit Reynolds
number. A feasible specification, with 20% thickness-to-chord, 80 m span and a
unit Reynolds number of 8 x 10[superscript 6] m[superscript -1], is identified; it corresponds to a 187 tonne aircraft which cruises at Mach 0.67 and altitude 22,500 ft, with lift coefficient 0.14. The benefit of laminarisation is manifested in a high lift-to-drag ratio, but the wing loading is low, and the structural efficiency and gust response are thus likely to be relatively poor.
On the basis of this specification, a detailed conceptual design is undertaken. A
220-passenger laminar-flying-wing concept, propelled by three turboprop engines,
with a cruise range of 9000 km is developed. The estimated fuel burn is 13.9
g/pax.km. For comparison, a conventional aircraft, propelled by four turboprop
engines, with a high-mounted, unswept, wing is designed for the same mission
specification and propulsion characteristics, and is shown to have a fuel burn of 15.0 g/pax.km. Despite significant aerodynamic efficiency gains, the fuel burn
of the laminar flying wing is only marginally better as it suffers from a poor
cruise engine efficiency, due to extreme differences between takeoff and cruising
requirements, and is much heavier.
The laminar flying wing proposed in this thesis falls short of the performance improvements expected of the concept, and is not worth the development effort. It is
therefore proposed that research efforts either be focussed on improving the engine
efficiency, or switching to a low aspect ratio, high sweep, design configuration. | |
