Understanding Wing Flaps

They seem to come in as many varieties as hotdogs-and with about as many interesting names. You know what I mean: Chicago Style, New York Style, Half-Smoke, Coneys, Hot Links and so forth. Ordering without confessing unfamiliarity with variations in flavors, spices and condiments can prove painful.

Its the same with flaps, though the consequences of inadequate knowledge are different. For example, you need to know what kind of flaps youre flying and how theyre used. This knowledge, in turn, enables their management for takeoffs, landings and other operations. Knowledge of your flaps and their peculiarities can be particularly powerful should

you find yourself in unusual circumstances, and need to maximize their benefits. Lets take a look.

Yes; some airplanes dont even employ them. No, its not true you can take from that knowledge the idea of flaps as optional. In general, airplanes lacking flaps also lack any real need to further reduce their stalling, takeoff or landing speeds, and therefore required-runway distances.

But the truth is that most airplanes come with some sort of wing-flap system and do so because their designer recognized benefits to pilots that outweigh the added costs, complexity and weight from incorporating flaps into a wing. As weve touched on, those benefits mainly are realized on or close to a runway, and involve shorter landing or takeoff distances.

But flaps also are used to enhance the performance of certain flight operations, mainly involving low airspeeds. Almost by definition, for example, the last time you demonstrated slow-flight technique for your instructor, wing flaps were deployed. They helped you slow down, and once you got slow, they also enhanced stability while lowering the airplanes stall speed.

When properly designed and deployed, flaps accomplish two seemingly opposite effects on basic aerodynamics. First, they increase a wings drag coefficient, with the “why” and “how much” varying with the type. Second, deployed flaps increase the coefficient of lift of the wing and make it more efficient. See the dichotomy?

We commonly see four basic flap designs in general aviation aircraft. The plain flap uses a simple hinge point, allowing the wings trailing edge to droop as a single unit. The split flap is a cousin to the plain flap, except the only part hinged downward is a panel under the wings trailing edge; the upper portion of the trailing edge remains in place-only the bottom pivots down, much like a plain flap.

The Fowler flap common to Cessna singles actually moves aft and downward at the same time, effectively increasing the wings chord and physically increasing lifting area. In addition, a path between the wing trailing edge and the leading edge of the flaps is created for moving air. The added high-pressure airflow from beneath the wing flows over the flap to increase lift and help improve low-speed handling.

The slotted flap also pivots downward without moving aft, opening a slot between the flap leading edge and the wing through which air flows to help maintain airflow over the top of the wing, further improving lift at lower speeds.

Flaps work by making two basic alterations to the design of the wing airfoil. First, they change the airfoils camber; to some degree all flaps increase the wings effective camber, making it thicker.

The other thing flaps do is increase the wings effective area, helping reduce stall speed. According to the general formula for calculating lift, a given amount of it can be achieved at a lower airspeed by increasing the coefficient of lift and/or with an increase in lifting area. By increasing the wings ability to generate lift, reducing its stall speed and increasing overall drag, flaps make it possible to lift off and land at speeds lower than possible without their use. In the bargain, the airplanes cruising speed is higher, since the flap-equipped wing doesnt generate as much drag as it would if designed for identical runway performance without flaps. These benefits also extend to increased flexibility for using shorter runways and less wear-and-tear on the airplane. However, each individual flap design does this differently-and how it increases the wings effective

area is what makes a specific flap design preferable to others.

For example, slotted and Fowler flaps add to that effective outcome by actually enlarging the lifting area of a wing as they deploy. One of the trade-offs is a heavier, more-complicated deployment mechanism, along with higher manufacturing cost. Plain and split flaps create an effective increase in wing area by increasing chord, are reasonably easy to manufacture and are lighter than other designs, but perhaps not as effective.

We generally use settings called “takeoff flaps” and “landing flaps” to match the phase of flight. But if a little flap is a big help, why not use maximum flap all the time and get more help? Well…it doesnt work that way.

If you think of a conventional lift vs. drag curve, theres something similar going on with flap deployment, i.e., a point beyond which the curves depicting increased lift and increased drag diverge as the flaps are deployed. That point generally is somewhere between 10 and 15 degrees of deployment. With this small amount of flap, the wing generates more lift than it gains drag. Also, most of the stall-speed reduction is realized here. The normal takeoff flap setting-which rarely is greater than 10 degrees in personal airplanes-generates only slightly more drag, not nearly as much as added lift. Beyond about 15 degrees, however, the drag contribution grows rapidly so, at maximum flaps-40 degrees on many popular designs-the added drag serves only to slow the airplane further, requiring more power to maintain level flight at approach speed.

For getting into a small fields, maximum flaps help by allowing a steeper final approach without a resulting speed increase, actually using engine power to pull the airplane forward against the drag.

Flaps tend to work best when the airplane is flown between 1.3 and 1.5 of its landing-configuration stall speed (V_S0). Flaps also change the load-carrying ability of the structure, dropping the Normal Categorys +3.8/-1.5 load limits to +2/-1.1 with flaps deployed. The lesson here is to avoid deploying flaps in rough air or when maneuvering into higher loads than normal. In rough air, youll likely want to fly your approach a little faster than on a smooth day, anyway, so using less than maximum or even no flap is a good idea.

Of course, since flaps function to reduce our touchdown speed and vertical velocity, a no-flap landing will necessarily involve flying the approach a little faster and accepting a higher touchdown speed. There being no free lunch, flatter, faster approaches and landings mean lengthier runway requirements and longer roll-outs. Combine a windy, gusty day, a short field and obstacles at both ends, and…you may want to go somewhere else. Remove the short runway and the obstacles, however, and you may be able to tell your mechanic which wheels are out of balance after a no-flap landing.

Flaps for departure are, as noted, generally the first setting; full flaps will serve only to slow your takeoff roll and slow your climb. An exception involves short or soft fields and your airplane manufacturers recommendations. Generally, if a notch of flaps is recommended for normal takeoffs, two notches are used for soft fields. If “normal” is no flaps for takeoff, the

recommendation may be for only one notch. Check your POH. In either instance, the idea is to accept the increased drag in exchange for the greater lift the additional extended flap setting generates. Once off the soft runway, reduce the angle of attack, accelerate to a safe speed, retract a notch of flaps, then climb out normally.

Depending on the aircraft, that second notch of flaps on a short field may also provide just enough further reduction in stall speed to allow an earlier liftoff at lower speed-without adding drag to the point of hindering acceleration. Be prepared for reduced climb rates, but you may be able to use the steeper climb angle to your benefit.

Learning what setting does what to your airplanes stall and speed settings should be part of your transition experience, whether undertaken during formal training or out in the “wild” as an exercise in self-enlightenment. It occasionally paid to know, for example, that my Comanche could safely rotate at 53 knots with one notch of flaps, and at 50 with two. With full flaps, 50 knots remained the lower limit-but it took the airplane an extra 300 feet to accelerate to 50.

The fastest acceleration time to the earliest rotation, as measured in an afternoon of touch-and-go flying, came with the first notch of flaps; the plane reached 53 knots faster with one notch than it reached 50 with two or three.

Conversely, with three notches it was possible to raise the nose slightly and keep power up to reach the touchdown zone at a steeper angle after delaying descent for obstacle clearance; the tactic cut my runway needs by about 300 to 400 feet compared to a normal glidepath angle initiated after passing the obstacle.

In many high wings, that last notch of flaps can steepen the descent angle with the nose down-again, using a little power to keep the airplane moving forward on your preferred descent angle.

When to stow flaps and what to expect when you do varies largely with the flap design. Fowler and slotted flaps, for example, can produce a considerable performance change when retracted during a steep climb; the culprit is the progressive reduction in lifting area through the return to normal camber and chord-the amount of lift available suddenly decreases. Pilots need to know and allow for this trait, lest they start to stow flaps while crossing over any obstacle closer than a couple hundred feet below.

Plain and split flaps can, to varying degrees, cause an airplane to pitch down when deployed, a factor of how they increase drag aft of the center of lift. So, when theyre retracted-you guessed it-the airplane will want to pitch up. Again, this may not be something you want to have happen at low speeds and altitudes.

In some installations, such as the Cessna 182RG, stowing flaps and gear together could, if properly set up for climb, generate no pitch change, just acceleration and increased climb rate. The gear and flap changes cancel each other out. Whether its advisable to retract both at the same time is a judgment call, as long as both changes remain within the airplanes speed

limitations.

Of course, when extending flaps, you always want to respect those same limitations. As we all should know, the airspeed indicators white arc is the range within which full flaps may be deployed. To do otherwise risks damaging the flaps themselves and/or the deployment mechanism. The catch is some airplanes allow an approach-flap setting-nominally 10 degrees-to be deployed at speeds above the white arc. Read your POH for the details, if any. Regardless, youre always safe waiting until within the white arc, the top of which marks the maximum flap-extended speed (V_FE) while the lower end is the stall speed with all the “stuff out” (V_S0)-gear down, landing flaps deployed.

There are occasions when operational necessity requires out-of-the-ordinary flap use. An example might be a two-notch takeoff from a short field with an obstruction. Depending on the manufacturers recommendations, you want to retract the flaps quickly for a maximum-angle climb right after liftoff.

Inbound to a runway with a displaced threshold or an obstacle-clearance issue may require full flaps for maximum drag, steepening your descent without unduly upping the airspeed. The need to S-turn in traffic to increase separation is another non-standard time to consider two notches of flap, maybe even three, to get the most out of slowing and maneuvering while using extra engine power to hold altitude against the higher drag; just dont forget to stow a notch on final or else youll be driving to the runway when youve no need.

Stowing flaps rapidly on a friction-challenged runway can help increase the weight on the tires and improve braking performance.

Meanwhile, many manufacturers advise against slips with flaps deployed, since the downwash can impact the horizontal stabilizer and disrupt pitch control.

A final tip is to refrain from extending flaps in a turn, as when turning from downwind to base. The main reason is to enable rapid recognition of a split-flap condition, where one deploys and the other doesnt. If the flap deployment mechanism breaks in a bank, the failure mode could force a much steeper bank than you wanted, possibly increasing the roll rate beyond the ailerons ability to correct at a low speed. Rather than deploy flaps in a turn, do it when flying straight so any rolling moment created by a split-flap condition is immediately recognized and a bank in the wrong direction doesnt worsen a bad situation.

Knowing your airplanes flaps system and resulting performance is just one small part of mastering the airplane. You cant achieve the latter without the former.