Last summer Boeing acknowledged it was seriously examining hybrid laminar flow control (HLFC) as a way of clawing back performance of the stretched 787-9. It was one of several innovations being examined in the wake of the decision to stick with the shorter 197.3-ft wingspan of the -8 instead of the 207.9-ft span originally planned. Now, new details of the how the HLFC system will work appear to have emerged in a recent U.S. patent awarded to a Boeing employee. Although originally filed back in 2007, the patent was not granted until January 2011.
U.S.Patent Office - Areas of potential HLFC application
Boeing has revealed very little so far about the laminar flow system, which would be the first application of its type on any commercial airliner. The laminar flow system reduces drag by sucking the turbulent boundary layer in through tiny holes in the skin to a plenum, or hollow chamber, inside the leading edge of the fin or stabilizer. Unlike experimental large-scale systems demonstrated in the past by NASA amongst others, the HLFC device under study for the 787 is essentially passive. This is important because passive systems are less complex, and lighter. Active systems, by contrast, require a turbocompressor, or other mechanical device, to suck the air into the wing. Assuming the patent forms the basis for the 787 system, the flow through the system is generated by passageways which vent the air to local areas of low pressure. These areas are located in areas such as the tips of the fin and stabilizers, and created by the motion of the aircraft itself. The drawings appear to indicate control of the flow via vent doors, similar to those seen on current environmental control system outlet ducts.
US Patent Office - Image showing how the system vents through the fin tip
Boeing says the plan is to use the system on the 787’s empennage, which suggests the horizontal as well as vertical tail. Redesign work on the horizontal stabilizer, which for the 787-9 will apparently be assembled in the U.S. rather than in Italy where the -8 units are made, is already underway. As with other advanced Boeing structures and systems work including the initial composite fuselage barrels for the 787 and the wing for the F/A-22, this seems to be focused on Boeing’s large and secretive Developmental Center – a building originally erected for the long-abandoned 2707 supersonic program.
US Patent Office - A cross-section through the leading edge showing plenum