Published in the May/June 1998 issue of Woman Pilot

High Lift Devices

By Dave Esser

On a particular FAA test, the test taker was asked to select the answer that best describes the main function of flaps. The correct answer was that flaps allow an increase in approach angle without increasing airspeed. The question was valid, but the use of flaps is considerably more complex than this one application implies. The extension of flaps does increase the coefficient of drag (Cd) for the steeper approach angle, but it also affects the coefficient of lift (Cl), wing length (chord), wing curvature (camber), and boundary layer energy. This article explores the nuances of these uses of flaps and other high lift devices.

Almost all aircraft have flaps of some type. Most common is the plain flap which increases the camber of the wing section. Only the lower surface of the wing extends downward in the split flap. Split flaps are used in multi-engine aircraft where the engine nacelles are attached to the upper wing surface because it is necessary to leave the upper surface intact. The Fowler flap extends downward and aft, increasing both chord and camber. In a slotted flap an opening is created as the flap extends. This slot allows the high pressure air under the wings to mingle with and increase the boundary layer energy of the upper surface.

The boundary layer consists of the air streamlines within an inch of the surface of the aircraft. We are taught that the relative wind is equal in magnitude to and opposite in direction from the flight path. This is not completely true in the boundary layer. The molecules of air immediately adjacent to the aircraft surface are motionless. Beyond the wing surface, however, the speed of the streamlines quickly increases to that of the free stream velocity. This free stream velocity represents the speed of the relative wind when not slowed by the frictional effects of the aircraft's surface. By increasing the energy of the boundary layer the wing can sustain higher angles of attack and the maximum coefficient of lift (Clmax).

The coefficient of lift (Cl) determines at what airspeed an aircraft must be flown to produce the required lift for unaccelerated flight. The formula for this airspeed is:
V_KTAS = Square root of (295WlGreek letter sigma SCl)
Where:
v = true airspeed in knots (KTAS)
W = the weight of the aircraft
Greek letter sigma = the ratio of the ambient air density to that at standard sea level
S = the wing area in square feet
Cl = the coefficient of lift

The pilot can increase the coefficient of lift of an airfoil by increasing the angle of attack, up to its maximum coefficient of lift. The angle of attack at which an airfoil reaches its maximum coefficient of lift is called the critical, or stalling, angle of attack. The critical angle of attack for a given wing section remains the same regardless of aircraft weight or altitude. A wing section can be changed so that it can attain a higher maximum coefficient of lift and a slower minimum speed by increasing either the camber or the boundary layer energy.

The total energy of the airstream can be increased by increasing either its motion (kinetic energy) or its pressure (potential energy). There are several ways to increase the boundary layer energy. Most common is the use of slots. These openings in the wing allow the high pressure air below the wing to mingle with and increase the energy of the upper wing boundary layer.

Another method is to use vortex generators. As the air flows over the vortex generator, a tab about the size of a postage stamp, the molecules are spun up, thereby increasing their kinetic energy. To illustrate, think of how a bowler adds spin to the bowling ball to increase the "pin action." The use of vortex generators aids in high speed (transonic) flight as well as low speed flight. When approaching the speed of sound, the airstream over the wings is accelerated beyond the speed of sound. This transonic speed region involves the development of a pressure, or shock, wave. An airstream flowing through this shock wave is disturbed, causing a large region of turbulence and possible separation. By swirling up the airflow through the use of vortex generators, the kinetic energy of the air is increased, making it less susceptible to disruption.

The most elaborate system to increase boundary layer energy involves the use of small holes in the skin of the wing. Suction draws off the low energy boundary layer air adjacent to the wing and replaces it with the higher energy streamlines from above. The complexities involved in this system make it impractical for most aircraft, however.

By increasing the camber of an airfoil, the coefficient of lift for a given angle of attack is increased. Contrary to popular belief, the stalling angle of attack when flaps are extended is less than when flaps are retracted. A wing will stall when the airstream doesn't have sufficient energy to make the required change in direction to follow the camber of the wing. The greater the camber, the greater the required change of direction. The increased camber with flaps extended puts a greater demand on the airstream to change direction. The increased demand reduces the critical angle of attack. If you are thinking that you learned a wing always stalls at the same angle of attack, you should note that this is true for a given airfoil section. When flaps are extended, the airfoil section is changed as is the stalling angle of attack.

Extending the flaps also increases the angle of attack of the wing for a given pitch attitude. The angle of attack is the acute angle between the chord line of the wing and the relative wind. The chord line is the imaginary line connecting the leading edge and trailing edge of the wing. As the flaps are extended, the trailing edge of the wing is lowered, increasing the angle of the chord line, and thus the angle of attack. When practicing a no-flap landing, the pilot notices the need for a much higher pitch attitude at touchdown as compared with a full flap landing.

As the camber of an airfoil is changed, not only does the coefficient of lift increase for a given angle of attack, but so does the coefficient of drag. By dividing the coefficient of lift by the coefficient of drag, a very important ratio is attained. The lift to drag (L/D) ratio is a basic measure of an aircraft's efficiency. This ratio will determine the amount of thrust required in level flight and in the power off glide range. As the L/D ratio increases, required thrust decreases, thus increasing the glide range. As flaps are extended, the coefficient of lift is increased for a given angle of attack, but the coefficient of drag increases faster than the coefficient of lift, resulting in a lower L/D ratio. This is why we only fly with flaps extended during takeoff and landing when low speed flight is necessary.

The decreased L/D also decreases the glide ratio. Student pilots practicing emergency landings know that they shouldn't extend the flaps until they are certain that the landing field is within gliding distance. Remember the FAA test question? Yes, it is true that extending flaps does allow a steeper approach at the same airspeed.

Aeronautical engineers use a graph known as a drag polar. The drag polar plots the coefficient of lift versus the coefficient of drag. Examination of the polars of several types of flaps shows that the Fowler flap is the most efficient. Is it any wonder why this type of flap is so popular?

The trailing edge of the wing is not the only location for high lift devices. There are also leading edge flaps, slats and slots to increase camber and boundary layer energy. A slat is a leading edge device that extends forward to increase wing camber. As the slat extends forward a slot opens, allowing the high energy boundary layer energy to vent to the upper surface. Some slats are deployed by the force of springs. These slats are automatically retracted as the dynamic pressure increases with airspeed and automatically extended when airspeed is reduced. Most slats use hydraulic actuators similar to those used for flaps.

Watch these devices being used the next time you fly on a commercial jetliner. When the aircraft nears its landing, the flaps are gradually extended. As the flaps are deployed, so are the leading edge slats and slots. When on final approach the wing experiences a considerable amount of drag. The jet engines produce the thrust to overcome this higher drag. The payoff for this drag is the slower speed at which the aircraft can land. Touching down at the slower speed decreases the amount of runway needed.

There is no one wing that can perform efficiently at both slow and fast airspeeds. By using high lift devices, the wing can be altered to be efficient in the various phases of a flight. What is the function of flaps? How many ways can you now answer this question?

Return to Feature Articles