When I was a student pilot, learning about flaps was like a revelation to me.
I had known they were used on landing to slow down the airplane, but I hadn’t given a whole lot of thought to other uses.
As a student flying a trainer, chances are you only use the flaps on landings. Flaps are only used on takeoff when you are practicing short- or soft-field operations, and only in accordance with the pilot’s operating handbook for the particular airplane. Some aircraft never use flaps on takeoff, and even some jets can take off without the flaps down.
Flaps do two things. First, they increase the amount of lift the wing can produce by increasing the total wing area and changing the curvature or shape of the wing. This keeps airflow in contact with the wings’ upper and lower surfaces for a greater period of time, thus increasing the coefficient of lift. This is why you sometimes need to trim the nose down when you first extend the flaps. But, even when increasing lift—as a result of increasing wing area—flaps also increase drag. As a general rule of thumb, however, as long as flap deployment is limited to less than half of the total flap extension that is available, the increase in lift will be greater than the corresponding increase in drag. It is this property that allows us to use flaps so effectively on takeoff.
As previously mentioned, flaps increase drag. Once you extend flaps past the halfway point (roughly), drag increases far more than does lift. This is why you can use your flaps to increase lift enough to allow you to fly at a slower speed near the pattern, yet use even more flaps to fly even more slowly while descending at a steeper angle on final. When the flaps are fully extended, you may need to re-trim—while adding power.
Some aircraft have flaps that truly live up to the moniker of a barn door. When Cessna was building the venerable 150 with 40 degrees of flaps, pilots had an airplane that, in a stiff headwind, could almost imitate a helicopter in its landing performance. The same holds true with jets. The Boeing 737 can land with 40 degrees of flaps, but it almost never does. For most landings it is not necessary, and using the full-flap setting creates a tremendous amount of noise because of the thrust requirement and aerodynamic noise created by the airflow over the wings. But, when it is necessary, the crew does have the flap-40 option.
Speaking of jets, the majority of jet aircraft not only have flaps on the rear of the wing, but also slats on the front of the wing. In the GA world, just about every aircraft has a straight wing. Viewed from above or below, the airplane looks like a cross. Straight wings are structurally easy to build, and aerodynamically they are simple and stable and pilot-friendly. But they also create quite a bit of aerodynamic drag. This is a huge impediment to speed.
Swept wings, like those you see on jets, reduce the amount of aerodynamic drag. Further, just about every swept wing tapers toward the tip, giving it a sort of triangular appearance. This increases speed, but does little for low-speed handling. The answer, of course, is to have flaps. Unfortunately, traditional flaps are not enough. The answer? Leading-edge slats that extend down and out from the front of the wing, essentially increasing the curvature of the wing's leading edge. The benefit is that what is normally a high-speed wing now can provide an enormous amount of lift at (relatively) low speeds. A jet that can cruise at nearly 600 miles an hour can still use a runway shorter than 10,000 feet with takeoff speeds in the 140-to-150-knot range, and can approach at speeds as low as 120 to 130 knots when lightly loaded. In the airliner world, the Boeing 757 is one of the best short-field aircraft going. The secret is the overall design of the wing, especially the flaps and slats (slats extend as a part of the normal flap extension process; there is not a separate switch).
Not all jets have or need slats. Straight-wing jets don’t have them, and some business jets and small airliners like the Canadair RJ lines don’t have them. Such wings are called “hard wings.” The decision to leave them out may be driven by the mission (anticipated airport usage); by space (the wing may simply not be able to accommodate the hardware without sacrificing fuel capacity or taking on extra weight); or by economics, if not all of the above.
Flaps, though, are mechanical, and like anything mechanical, they can break. When a flap motor burned out on a Cessna that I was flying, I learned that I needed to be able to confidently land without flaps. So, I started practicing landing in various configurations. Starting with full flaps, I did a number of landings in each configuration, and then I would slowly decrease the amount of flaps—going from 40 to 30 to 20 to 10 degrees, and eventually none at all. Going from full flaps to half did relatively little to affect the behavior of the airplane, but the change in pitch and speed was noticeable. It became noticable when the flaps were in the 10- to 20-degree range. Lift was a much greater part of the equation than drag, and the approach profile needed to be changed as well. One of the biggest differences was the need to close the throttle much earlier on final to allow for a longer float to the runway.
The starkest difference came with a zero-flap landing, which requires quite a bit of advanced planning. You also need to be aware of the zero-flap stall speed of your bird. My home airport was 2,900 feet with a displaced threshold, and the change in landing distance was considerable. If you are considering practicing these exercises—and you should—you should start with a fairly long runway. With zero flaps, plan on using a slip to lose altitude quickly and safely without gaining too much speed.
Flaps, like any other system, often have limitations. Cessna has long advised against performing slips with flaps extended all the way in several models; check your airplane’s POH to see if it advises against this practice. The reason is that the airflow can cause harmful vibrations to the tail assembly of the airplane. It also became widely recognized that conducting a go-around with flaps at 40 degrees in a Cessna 150 could be dangerous if not done properly. In any airplane, proper energy management during a go-around is critical, and the flaps play an integral role in that process.
Most limitations on flaps are related to speed. There are extension speed limits (to prevent undue aerodynamic pressure on the drive motors or jackscrews; the CRJ 50 became notorious for flap-fail events that led to numerous modifications to the extension speeds), and on larger aircraft, there usually are limits on minimum speeds based on the weight and associated stall speed of the airplane.
It is easy to take flaps for granted, but they are a wonder of aerodynamic flexibility, and a lack of understanding can lead to less-than-desirable consequences. Know the limitations of your bird, and use them properly—you will find it to be an uplifting experience.
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