Aerodynamic Surface Cooling for Laminar Flow Control for Hydrogen-Fueled Subsonic Aircraft

Cryogenic-fueled aircraft offer an excellent potential for laminar flow control by use of the sensible heat of the stored fuel to cool aerodynamic surfaces. Cooling the boundary layer below adiabatic wall conditions delays transition to the turbulent region and reduces frictional drag. For the liquid hydrogen fuel case, preliminary design calculations, using gaseous nitrogen as the coolant, will result in a 20-percent reduction in fuel requirement (wings plus nacelles cooling) to 28-percent reduction (addition of fuselage cooling) for a M = 0.85 cruise wide body-type aircraft on a 12,000-km flight. The added costs associated with liquefying hydrogen for use as a fuel can be offset by the improved flight performance (and lower operating costs). Implementation of surface cooling increases aircraft dry weight through addition of heat exchangers, pumps, control subsystems, etc. These must be factored into aircraft performance to realistically evaluate fuel savings. Also, assessment of the cool down cycle is a nontrivial problem because of the large variability in local heating rates and temperatures which must be evaluated in conjunction with structural and operational constraints. Results of a preliminary study of a conceptual cooling system for the M = 0.85, 12,000-km range aircraft are presented and potential problem areas discussed. Cooling of wing and nacelle surfaces appears feasible and within the current state of technology.