Lift Augmentation Lesson

Lift augmentation devices enhance lift, mainly through trailing edge and leading edge flaps.

Flaps fitted to aircraft can do a number of things to make take-offs and landings a lot safer and more comfortable. To obtain a high cruise speed and/or a high maximum speed, a thin high-speed wing section must be used. The use of a high-speed aerofoil helps the designer achieve higher airspeeds by reducing drag. This gain is offset by low maximum lift coefficients.

Most of the aircraft utilising this type of aerofoil also have a high wing loading and are, therefore, subject to high landing and stall speeds. The length of the runway required to operate these aircraft sets a limit on their usefulness.

However, with the use of flaps and slots, we can increase the amount of lift that an aerofoil can produce. This additional lift reduces the stall speed, which in turn allows the aircraft to be flown at a slower and safer speed during take-offs and landings.

Advantages:

  • Increased camber
  • Increased surface area

Disadvantages:

  • Increased drag

Use of Flap

Take-off: Lift coefficient is increased, allowing us to become airborne at a slower speed. Drag coefficient is also increased, meaning the aircraft will take longer to reach a certain airspeed. Different aircraft have different recommended settings for the use of flap at take-off. At the recommended setting, the lift-to-drag ratio is such that maximum advantage is obtained for the minimum drag penalty. The runway surface also plays a role in determining the recommended flap setting. If the runway surface is soft, the use of flap will increase the lift coefficient and, therefore, decrease the drag effect of the aircraft sinking into the grass. Over the first 10–20 degrees of flap, there is a considerable increase in lift compared to the increase in drag. Beyond that, the drag is far more considerable than the increase in lift.

After take-off: Never raise the flaps all at once, and always retract the flaps in stages. The sudden loss in lift at a low airspeed will cause the aircraft to sink before it can accelerate fast enough to produce the same amount of lift. Flaps lowered will reduce the rate of climb.

Approach: The high drag of a fully lowered flap on approach allows the aircraft to have a steeper descent angle without the speed increasing; it has the effect of an airbrake. This is an advantage when there are obstacles on approach. An extending flap has the added advantage of lowering the nose, thereby giving better forward visibility.

Landing: Flaps were primarily designed for the landing phase of flight. The increased lift coefficient decreases the stall speed; therefore, we can approach at a slower speed. A lower touchdown speed, therefore, means a shorter ground roll. Another advantage is that the higher drag causes a rapid deceleration during the period of float after rounding out and before touchdown.

Go-around: When a go-around (overshoot) is initiated, the flaps should be retracted in stages and only after a positive rate of climb has been established.

Many types of flaps are available in the aviation industry. Here is a good selection. You should have a good knowledge of the following.

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Effect of Flap on the Lift-to-Drag Ratio

Lowering the flaps produces an increase in the lift coefficient at a given airspeed but at the same time, there is a disproportionate increase in total drag. For a Cessna 172, the first 10° of flap produce a steady rise in coefficient of lift and a small increase in coefficient of drag. Flap settings of 20° or 30° produce an increase in lift at a reduced rate but the coefficient of drag rapidly increases.

Remember: Whenever flap is lowered, the lift-to-drag ratio is always reduced.

Effect of Flap on Movement of Centre of Pressure

Trailing edge flaps: Due to more lift being produced around the trailing edge of the wing, the centre of pressure moves rearwards. This creates a nose down pitching moment.

Leading edge flaps: Due to more lift being produced around the leading edge of the wing, the centre of pressure moves forwards. This creates a nose up pitching moment.

Effect of Flap on the Stalling Angle

When trailing edge flaps are lowered, the result is to decrease the stalling angle of attack. When leading edge flaps are lowered, the result is to increase the stalling angle of attack.

Note that the pitching moments specified above are generic. Each aircraft will respond differently in pitch when flap is lowered. Whether the aircraft is high or low-wing has a large part to play here.

Slats: A slat is a small auxiliary aerofoil of a highly cambered surface fixed to the leading edge of the wing (often along its complete span). This creates a slot between the two, through which air can pass. The main function of a slat is to ‘prolong the lift curve’ by delaying the stall until a higher angle of attack.

Automatic Slats: When not in use, the slat lies flush against the leading edge of the wing. Automatic slats make use of the forward and upward suction effect of the airflow near the leading edge of the aerofoil. The force on the slat lifts it upwards and forwards as the angle of attack is increased. In this open position, a slot is formed between the two surfaces. The opening of the slat to form a slot occurs at a pre-determined angle, usually near the stalling angle. No separate control is required to operate the slats.

Slots: A slot is the gap formed between the main aerofoil and the leading edge slat. At a high angle of attack on a normal unslotted aerofoil, the airflow is unable to maintain a laminar flow. It is unable to adhere to the surface, and the aerofoil would have reached its stalling angle of attack. With the addition of a slot, some of the airflow will flow through it from underneath the wing. The boundary layer over the wing is assisted as the air flowing through the slot is speeded up by the venturi effect. It can be said that the slot re-energises the boundary layer, by sustaining laminar flow over the surface of the main aerofoil up to higher angles of attack. Slots and slats can increase the maximum coefficient of lift by 60%, causing the stalling angle to increase up to as much as 15° to 22° and, in doing so, decreasing the stalling speed.

Note that a slot also exists between the trailing edge of a wing and a Fowler flap. The principles of operation are much the same.

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Spoilers: A spoiler is a device on an aerofoil that disrupts the airflow and, therefore, disrupts lift. When not in use, the spoiler lies flush against the upper surface of the wing. When deployed, a spoiler will protrude into the airflow. This disrupts lift and assists slowing the aircraft down, or if used differentially helps roll the aircraft.

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Image by Richardgm (Used with permission)

Discuss the following questions in class now.

  1. What is the main purpose of Boundary Layer Control?
  2. Why do some aircraft use flap for takeoff?
  3. What are the disadvantages of using flap for takeoff?
  4. Draw a:
    1. Zap Flap
    2. Slotted Fowler Flap
    3. Split Flap
    4. Kruegar Flap
  5. Draw a diagram to explain how a Slot works.
  6. How do Slots and Slats affect the Critical Angle?
  7. What do Slots and Slats do to the Coefficient of Lift?
  8. Draw the Lift to Drag (L:D) curve (label the important points)
  9. What is the most efficient Angle of Attack?
  10. Why does an Aerofoil require a High Max CL
  11. List three benefits of a good Lift to Drag ratio on an Ideal Aerofoil.

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