Potential fuel savings due to hybrid laminar flow control under operational conditions

Hybrid laminar flow control (HLFC) is an active drag reduction technique that permits extended laminar flow on an aircraft surface and thus offers the potential for significant fuel savings. This is at the expense of an increase in system weight and specific fuel consumption. An overview of HLFC system failure types and consequences is presented as an introduction to this study, which investigated the impact of a potential loss of laminar flow due to flight in cirrus clouds. At typical cruising altitudes, the ice crystals are of a sufficient size and may result in sufficient particle flux to cause a temporary transition of the boundary layer. A computer performance model of a twin engine aircraft in the class of the Boeing 757 has been used to study the impact of alternative fuel planning scenarios on the fuel consumed by a HLFC aircraft, taking into account a model of probable cloud encounter. Based on the models, the study showed that if the fuel planning assumed 25% time-in-cloud (TIC) during the cruise, then in the extreme case of 55% TIC during the cruise, the contingency fuel (taken as 3% of the trip fuel), would be sufficient for the aircraft to complete the mission (including the alternate leg and hold).