Plane & Pilot
Tuesday, April 9, 2013

The Logic Of Flaps

Flaps make pilots’ lives easier—for those aviators who know how to use them

Flaps come in a variety of shapes and sizes, so many that it would take most of this magazine to analyze them all. Here are the four basic types.

1 From a manufacturing point of view, the easiest flap to produce is the plain, hinged flap. This typically mounts at three of four points along the aft wing and often extends to half span. It's usually mounted with hinges that allow it to deploy 20 to 40 degrees below the trailing edge. Plain flaps are popular with homebuilt aircraft because they're easy to construct and the operating mechanism can be as simple as a manual cable device. Piper Cherokees and Beech Bonanzas employ plain flaps.

2 A variation on the plain flap is the split flap that leaves an upper surface extending to the original trailing edge. The flaps deploy beneath the wing and don't allow air to escape through a slot. In this way, airflow across the top of the wing is unaffected. Cessna 300 and 400 series twins and DC3s use the split-flap concept. Split flaps were popular on WWII-era fighters. They're hinged several inches forward on the wing's trailing edge and allow the top wing surface to maintain its original shape during deployment.

3 Conversely, a slotted flap is specifically designed to create a slot at the flap leading edge thru which air can flow up and across the top-aft surface, allowing airflow to remain attached and thereby improve lift. This process effectively "recharges" the air flowing across the top of the flap, often helping to preserve some minor laminar flow and prevent a stall. Most high-wing Cessnas utilize slotted flap.

4 The Fowler flap is a more complex surface that slides aft on tracks as it translates down, actually increasing the wing chord as it deflects. Fowlers are more expensive to build and are therefore most common on high-performance aircraft of all types, from modern airliners to advanced technology general aviation aircraft. STOL machines, such as the Robertson conversions of the '70s and '80s, also used Fowler flaps to provide maximum short-field performance.

Some engineers use combinations of different types of flaps to achieve specific effects. The double-slotted Fowler flap creates two gaps as the flaps deploy, allowing more air to leak through two slots to the top surface and increase lift. The flaperon is a long, narrow flap that spans the entire wing trailing edge. In normal straight and level flight, flaperons act exactly like large ailerons, deflecting up or down to provide roll control. In takeoff or landing mode, flaperons droop symmetrically to help reduce landing speed and improve visibility while also providing effective roll control. The homebuilt Kitfox used flaperons.

Slats aren't flaps in the strictest sense, but they help accomplish the same mission. They're built into the leading edge of the wing and droop down and forward, again to increase the camber of the wing and reduce the stall speed.

Don't confuse spoilers with flaps, however. Spoilers provide nothing but drag, whereas flaps contribute some lift while adding drag. Unlike flaps, spoilers typically have no operating limits and may be deployed all the way to redline, whereas that's not the case with flaps.


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