The Airplane - Jay Spenser [54]
A key benefit of high wing loadings is that the wing is less affected by turbulence in the air through which it is passing, so passengers enjoy a smoother ride. On the minus side, high-wing-loading airplanes need longer runways because they take off and land at higher speeds. This is the reason for wing flaps, as described below.
Boeing designed the Model 247 with a wing loading only slightly higher than that of the airplanes it replaced, so it needed no wing flaps. In contrast, Douglas audaciously jumped to a wing loading half again higher than the 247’s, giving the DC-1 and its successors superior performance. However, this design choice also meant that these Douglas airplanes required flaps to keep their takeoff and landing speeds sufficiently low.
Wing flaps are movable parts of a wing that angle down to give it greater camber, increasing both its lift and aerodynamic drag. Flap extension converts a high-speed wing into one suited to a lower operating-speed range. With flaps fully down, an airplane cannot fly very fast, but it can land more slowly and use smaller airports with shorter runways.
There are many flap configurations, the most significant being the plain flap, split flap, and Fowler flap. A plain flap is a hinged portion of the wing trailing edge located between the ailerons and wing roots. When actuated, a plain flap rotates down to give the wing greater camber. A split flap resembles a plain flap except that it is tucked beneath a fixed trailing edge. When actuated, this underside flap diverges from the wing’s top like an opening book cover.
The most aerodynamically efficient system is the Fowler flap, which is significantly more sophisticated than hinged flaps. Fowler flaps extend aft and downward, not just downward, to increase the wing’s total area, not just its camber. Fowler flaps have been around since the late 1930s. The Messerschmitt Bf 109 had them in the Spanish Civil War and World War II. The Lockheed P-38 Lightning and Northrop P-61 Black Widow also had them in the latter conflict. In general, however, Fowler flaps were not widely used until the jet age.
Many civil and military jet transports have the slotted flap, which is the Fowler flap with an opening between the flap and the wing. This allows some of the high-energy air beneath the wing to flow through the slot and stream down over the top of the flap. This blast reduces drag and lowers the airplane’s stall speed by keeping the airflow attached to the flap longer (aerodynamicists call this “reenergizing the boundary layer”).2
Jetliners have Fowler flaps with one, two, or even three segments (these are termed single-, double-, or triple-slotted flaps). While such flap systems are aerodynamically very efficient, they are also very expensive to build and maintain. In recent times, designers have learned how to give jets equivalent efficiency with greater simplicity.
Modern jet transports also have leading-edge devices that further convert their high-speed wings into low-speed wings. Slats are the most common leading-edge devices. A slat is a small leading-edge flap that slides forward and downward to further increase the wing’s total area and camber (in addition to Fowler flaps, the Messerschmitt Bf 109’s advanced wing also had slats that deployed automatically at low speed).
Some jetliners have an alternative leading-edge device called the Krueger flap. A Krueger flap is a hinged leading edge that rotates forward and downward into the slipstream to achieve the same aerodynamic benefits in a different way. When retracted, slats and Krueger flaps are the leading edge for the portion of the wing that they cover.
On your next commercial flight, look out the window at your jet’s wing as it begins letting down to a landing. Depending on whether you’re sitting ahead of the wing or behind it in the cabin, you’ll get to see either the leading-edge devices or the trailing-edge flaps in action.
If your view is from behind, you will notice the flaps initially extending just a short distance aft when your airplane