EAA Sport Aviation - February 2009

estingly, it was an aileron. The aircraft is for short takeoff and landing (STOL), and the ailerons are drooped with the flaps to maximize lift. The ailerons are double-slotted whereas the flaps are single-slotted. This presumably keeps the flow attached to the ailerons below the stall speed of the flaps and allows full aileron control down through stall.

One more trailing edge flap worth covering in detail is a flap originally invented by Orville Wright in 1922. The split flap is a simple hinged surface on the bottom of the wing. The top surface of the wing remains stationary, and the bottom surface swings down, creating a large gap in the trailing edge. This increases the camber of the wing and makes the air behave as though the wing has a larger area by creating stable vortexes in the space behind the wing. At small deflections, these flaps have less drag and more lift than a plain flap, but as they increase in deflection, they have as much as three times the drag of a plain flap of equal area. These characteristics allow small deflections to be used for increased take-off performance and large deflections for use in making steep approaches for landing. These flaps saw a lot of use on World War II aircraft, notably the B- 17, P- 38, P- 40, and the C- 47. Their popularity during this period, when the National Advisory Committee for Aeronautics (NACA) was developing most of its airfoils, is likely why most NACA airfoil data includes lift curves for split flaps.

The Zap flap is a hybrid of the Fowler flap and the split flap. It increases the wing area by moving rearward as the trailing edge droops but prevents flow between the flap and the wing. It creates more lift than a simple split flap, but its mechanical complexity and marginal benefit have prevented widespread usage.

Setback hinge balance

Cruise flaps deflect upward instead of, or in addition to, downward. These are used on some aircraft to improve efficiency at cruise speeds where the airfoil has been optimized for lift, either at lower speeds or higher altitudes. This allows the wing to have minimum drag at the design point without creating too much drag at higher speeds. They may also be used to counteract pitching moments for high-speed flight, which reduces the drag from tail surface lift. These are commonly seen on high-performance sailplanes and some powered aircraft designed for high-altitude flight, both of which are designed for minimum drag at high lift coefficients but need to occasionally fly fast and still minimize drag.

Moving to the front edge of the wing, there are some options here as well. Leading edge flaps are effective and have some beneficial characteristics in certain situations. There are generally two types of leading edge flaps: a slotted flap and droop flap. These two devices work in very different ways but achieve the same result, which is to extend the range of useful lift to a higher angle of attack. This allows the wing to stall at a significantly increased angle; more angle equals more lift. The big advantage of leading-edge flaps is that they do not affect pitching moments. Take notice on some commercial flights; leading-edge flaps will be deployed for landing with no trim change because these flaps do not shift the center of lift or increase the lift at normal lift angles. See chart on page 56.

The slotted flap was developed very early in the history of aircraft design as a means of stabilizing the flow over the wing, preventing it from stall separation until higher angles of attack are achieved. Simple implementations have the slots fixed, but versions that are more complex were made retractable or even automatic on early jet fighters, such as the German Me-262 Swallow. Since the air stays attached at higher angles of attack, the lift continues to increase beyond the normal stall. The droop flap works by increasing the camber of the wing at the leading edge but does not move the center of lift as trailing edge flaps do. More camber equals more potential for lift.

Both types of leading edge flaps can be used with trailing edge flaps with added benefit in maximum lift coefficient. The disadvantage is they achieve the increased lift at increased angles relative to the original pitch angle, so forward visibility is reduced when these are used to achieve high lift. The increase in angle of attack to stall can be significant, as much as 8 degrees for thick airfoils and perhaps double that for thin airfoils as on fighter aircraft. Nearly all modern fighters and commercial jet aircraft have some sort of leading edge flap to obtain higher coefficients of lift. I often wonder where they hide the fuel in a complex wing when it seems nearly all of it moves or contains mechanisms.

I always look at the control surfaces and flaps to see which types have been chosen, and then I ponder the choices. A few simple observations can tell a lot about the airplane and what forces will be used to control and land it using its various flaps.

Todd S. Parker is president of EAA Chapter 58 in Ogden, Utah, and a member of Chapter 23 in Salt Lake City. He works as an engineer for the U.S. Air Force.