1. High Aspect Ratio and Aeroelastic Flexibility
In aerodynamic theory, induced drag—drag generated as an unavoidable byproduct of creating lift—is inversely proportional to wing aspect ratio (the ratio of wingspan squared to wing reference area). The 787 features an aspect ratio of approximately 9.6, significantly higher than earlier widebody transport airframes that generally hovered between 7.0 and 8.2.
Constructing such a slender, high-span wing from aluminum would have resulted in excessive weight penalties due to the required thickness of internal spars needed to resist bending moments. By fabricating the wing skins and spars from carbon fiber composite laminates, engineers achieved high structural stiffness at lower overall mass.
Moreover, the flexibility of the composite laminate allows intentional upward aeroelastic deflection under aerodynamic flight loads. In steady-state 1G cruise flight, the 787 wingtips flex upward by approximately 3 to 3.5 meters (10 to 12 feet) relative to their resting static position on the ground tarmac. This engineered upward dihedral shift naturally enhances roll stability and absorbs gust energy without transferring high bending shocks to the fuselage wing box.
2. Supercritical Aerofoil Optimization
Commercial aircraft cruise in the transonic flight regime (Mach 0.80 to Mach 0.86), where airflow accelerating over the curved upper surface of the wing reaches local supersonic speeds even though the aircraft itself travels subsonically. On standard conventional airfoils, this local supersonic pocket terminates in a harsh recompression shock wave, separating the boundary layer and triggering substantial wave drag.
The 787 employs refined supercritical aerofoil contours:
- Flattened Upper Profile: Weakens the intensity of the supersonic shock wave, delaying the critical Mach drag rise and allowing efficient cruise at Mach 0.85.
- Substantial Aft Camber: Generates significant lift toward the trailing edge through a downward-curved cusp, compensating for lift reductions over the flattened forward upper surface.
- Reduced Leading Edge Radius: Promotes stable laminar flow development while maintaining predictable stall characteristics at high angles of attack.
3. Raked Wingtips Versus Traditional Blended Winglets
Rather than mounting vertical blended winglets or split-scimitar endplates—common retrofits on transport aircraft—Boeing aerodynamicists chose raked wingtips for the 787.
A raked wingtip consists of an integral span extension where the leading edge sweeps back more sharply than the main wing panel. Because wingtip vortices form as high-pressure air beneath the wing curls upward around the tip into low-pressure air on top, the raked geometry shifts these vortices further outboard and aft:
"Raked wingtips achieve equivalent or superior lift-induced drag reductions compared to vertical winglets without concentrating bending stresses into localized vertical junctions, saving weight and preserving gate limits."
Computational fluid dynamic (CFD) optimizations indicated that for long-haul profiles where climb and cruise dominate flight duration, raked tips deliver superior fuel efficiency over vertical winglets while fitting within standard ICAO Code E 60-meter gate spacing envelopes.
4. Trailing-Edge Variable Camber (TEVC)
Throughout an intercontinental flight, an aircraft burns tens of metric tonnes of kerosene, becoming steadily lighter. An airfoil optimized for maximum takeoff weight at the beginning of a sector will be sub-optimal during mid-cruise or descent.
The Boeing 787 employs Trailing-Edge Variable Camber (TEVC) controlled dynamically by the primary flight control computers (PFCCs). During cruise, electric flap actuators automatically adjust the trailing-edge flaps up or down by fractions of a degree:
- As fuel burns and aircraft gross weight drops, the trailing flaps subtly deflect to maintain the optimum lift-to-drag (L/D) ratio for the instantaneous aircraft weight and altitude.
- The system also modulates spanwise lift distribution to avoid concentrating load at the wingtips, minimizing structural wing-root bending moments and further trimming total cruise fuel consumption.