Laminar Flow Control: Back to the Future? (Invited)

In the 21 st Century, reducing the environmental impact of aviation will become an increasingly important priority for the aircraft designer. Among the various environmental impacts, emission of CO2 can be expected to emerge as the most important in the long term and reducing fuel burn to become the overriding environmental priority. Increasing fuel costs and the world’s limited oil reserves will add to the pressure to reduce fuel burn. Starting from the limitations imposed on the aircraft designer by the laws of physics – the Breguet Range Equation, the Second Law of Thermodynamics, the behaviour of real, viscous fluids – the paper discusses the technological and design options available to the designer. Improvements in propulsion and structural efficiency have valuable contributions to make but it is in drag reduction through laminar flow control that the greatest opportunity lies. The physics underlying laminar flow control is discussed and the key features and limitations of natural, hybrid and full laminar flow control are explained. Experience to date in this field is briefly reviewed, with particular attention drawn to the substantial body of work in the 1950s and 1960s that demonstrated the potential of full laminar flow control by boundary-layer suction. The case is argued for revisiting the design of an aircraft with full laminar flow control, taking into account the advances over the past half century in all aspects of aircraft engineering, notably in propulsion and materials. With approximately half the thrust provided by the boundary layer suction system, this aircraft presents a completely new challenge in airframe-propulsion integration. We understand the physics of boundary layer control, we know that an aircraft with full laminar flow is potentially much more fuel efficient than the alternatives, what is needed now is a wholehearted attack on the engineering obstacles in its path.