Flaps are now fitted to most aircraft because:

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Flaps are high lift devices which, in effect, increase the camber of the wing and, in some cases, as with the Fowler Flap, also increase the effective wing area. Their use gives better take-off performance and permits steeper approach angles and lower approach and landing speeds.

When deflected, flaps increase the upper camber of the wing, increasing the negative pressure on the top of the wing. At the same time, they allow a build up of pressure below the wing. During take-off, flap settings of 10 degrees to 20 degrees are used to give better take-off performance and a better angle of climb, especially valuable when climbing out over obstacles.

However, not all airplane manufacturers recommend the use of flaps during take-off. They can be used only on those airplanes, which have sufficient take-off power to overcome the extra drag that extended flaps produce. The recommendations of the manufacturer should, therefore, always be followed.

Flaps do indeed increase drag. The greater the flap deflection. the greater the drag. At a point of about half of their full travel, the increased drag surpasses the increased lift and the flaps become air brakes. Most flaps can be extended to 40 degrees from the chord of the wing. At settings between 20 degrees and 40 degrees, the essential function of the flaps is to improve the landing capabilities, by steepening the glide without increasing the glide speed. In an approach over obstacles, the use of flaps permits the pilot to touch down much nearer the threshold of the runway. Flaps also permit a slower landing speed and act as air brakes when the airplane is rolling to a stop after landing, thus reducing the need for excessive braking action. As a result, there is less wear on the undercarriage, wheels and tires. Lower landing speeds also reduce the possibility of ground looping during the landing roll.

Plain and split flaps increase the lift of a wing, but at the same time, they greatly increase the drag. For all practical purposes, they are of value only in approach and landing. They should not normally be employed for take-off because the extra drag reduces acceleration.

Slotted flaps, on the other hand, including such types as Fowler and Zap, produce lift in excess of drag and their partial use is therefore recommended for take-off.

From the standpoint of aerodynamic efficiency, the Fowler Flap is generally considered to offer the most advantages and the fewest disadvantages, especially on larger airplanes, while double slotted flaps have won wide approval for smaller types.

On STOL airplanes, a combination of double slotted flaps and leading edge slats are common.

Changes in flap setting affect the trim of an airplane. As flaps are lowered, the centre of pressure moves rearward creating a nose down, pitching moment. However, in some airplanes, the change in airflow over the tailplane as flaps are lowered, is such that the total moment created is nose up and it becomes necessary to trim the airplane "nose down".

The airplane is apt to lose considerable height when the flaps are raised. At low altitudes, therefore, the flaps should be raised cautiously.

Most airplanes are placarded to show a maximum speed above which the flaps must not be lowered. The flaps are not designed to withstand the loads imposed by high speeds. Structural failure may result from severe strain if the flaps are selected "down" at higher than the specified speed.

When the flaps have been lowered for a landing, they should not ordinarily be raised until the airplane is on the ground. If a landing has been missed, the flaps should not be raised until the power has been applied and the airplane has regained normal climbing speed. It is then advisable to raise the flaps in stages.

How much flap should be used in landing? Generally speaking, an airplane should be landed as slowly as is consistent with safety. This usually calls for the use of full flaps. The use of flaps affects the wing airfoil in two ways. Both lift and drag are increased. The Increased lift results in a lower stalling speed and permits a lower touchdown speed. The increased drag permits a steeper approach angle without increasing airspeed. The extra drag of full flaps results in a shorter landing roll.

An airplane that lands at 50 knots with full flaps selected may have a landing speed as fast as 70 knots with flaps up. If a swerve occurs during the landing roll, the centrifugal force unleashed at 70 knots is twice what it would be at 50 knots, since centrifugal force increases as the square of the speed. It follows then, that a slower landing speed reduces the potential for loss of control during the landing roll. It also means less strain on the tires, brakes and landing gear and reduces fatigue on the airframe structure.

There are, of course, factors, which at times call for variance from the procedure of using full flaps on landing. These factors would include the airplane's all-up-weight, the position of the C.G., the approach path to landing, the desired rate of descent and any unfavourable wind conditions, such as a strong cross wind component, gusty winds and extreme turbulence. With experience, a

pilot learns to assess these various factors as a guide to flap selection.

In some airplanes, in a crosswind condition, the use of full flap may be inadvisable. Flaps present a greater surface for the wind to act upon when the airplane is rolling on the ground. The wing on the side from which the wind is blowing will tend to rise. In addition, cross wind acting on full flaps increases the weather vaning tendencies, although in an airplane with very effective rudder control even at slow speeds, the problem is not so severe. However, in many airplanes, the selection of full flaps deflects the airflow from passing over the empennage, making the elevator and rudder surfaces ineffective. Positive control of the airplane on the ground is greatly hampered. Since maintaining control of the airplane throughout the landing roll is of utmost importance, it may be advisable to use less flaps in cross wind conditions. In any case, it is very important to maintain the crosswind correction throughout the landing roll.

A trim tab is a small, adjustable hinged surface on the trailing edge of the aileron, rudder, or elevator control surfaces. Trim tabs are labour saving devices that enable the pilot to release manual pressure on the primary controls.

Some airplanes have trim tabs on all three control surfaces that are adjustable from the cockpit; others have them only on the elevator and rudder; and some have them only on the elevator. Some trim tabs are the ground-adjustable type only.

The tab is moved in the direction opposite that of the primary control surface, to relieve pressure on the control wheel or rudder control. For example, consider the situation in which we wish to adjust the elevator trim for level flight. ("Level flight" is the attitude of the airplane that will maintain a constant altitude.) Assume that back pressure is required on the control wheel to maintain level flight and that we wish to adjust the elevator trim tab to relieve this pressure. Since we are holding back pressure, the elevator will be in the "up" position. The trim tab must then be adjusted downward so that the airflow striking the tab will hold the elevators in the desired position. Conversely, if forward pressure is being held, the elevators will be in the down position, so the tab must be moved upward to relieve this pressure. In this example, we are talking about the tab itself and not the cockpit control.

Rudder and aileron trim tabs operate on the same principle as the elevator trim tab to relieve pressure on the rudder pedals and sideward pressure on the control wheel, respectively.

The tabs are usually controlled by a wheel which is often situated on the floor between the two front seats. Some aircraft have the trim controlled by a small rocker switch on the control column. The aircraft should be trimmed after every change in attitude or power setting. It takes a little practice to  trim an aircraft, but in the end it is done unconsciously.