High Lift Devices: Types Of Trailing Edge Flaps

| August 20, 2021 | By: Severe VFR
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Trailing edge flaps are the most common type of high lift device.

Flaps are found on virtually every aircraft and offer a variety of benefits to aircraft performance and capability. Flaps increase both the lift and drag produced by an aircraft. Flaps accomplish this by increasing the surface area above the chord line, resulting in a greater camber of the wings' surface.

This allows an aircraft to increase the rate of descent without gaining airspeed during landing, allows steeper approaches, and decreasing stall speed.

However, not all flaps are the same.

Different types of flaps exist, with some very simple in design, while others are much more complex. While each flap-type works under the same principles, aerospace engineers have found creative ways to get more performance out of flaps through different designs.

Plain Flap

Plain Flap Example

The plain flap is one of the most straightforward types of flaps in terms of construction and design.

Plain flaps are very similar to ailerons in their design and are hinged in such a way that allows the flap to be deflected downward.

When deployed, the plain flap increases the camber of the airfoil and increases the coefficient of lift at any given angle of attack. However, the plain flap suffers from early airflow separation from the surface of the flap. This early airflow separation results in a large wake being created behind the flap.

The large wake created causes the amount of drag acting on the aircraft to increase dramatically. The drag created is so great that the plain flap is more widely regarded as a drag maker than a lift creator.

Aircraft that incorporate this type of flap in practice will use it for its ability to slow down the aircraft, and pilots should expect very little additional lift to be generated by the wing. However, pilots can expect a nose-down pitching movement when the plain flap is deployed. The nose-down pitching movement is due to the center of pressure of the wing moving aft towards the tail.

Split Flap

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The split flap is almost designed exactly the same as the plain flap. The only difference is that the split-flap is hinged underneath the airfoil rather than behind it.

The split flap provides slightly more lift than the plain flap. However, the split-flap still suffers from a large increase in drag due to airflow separation behind the flap.

Slotted Flaps

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Slotted flaps follow the same basic designs as a plain and split flap.However, slotted flaps overcome the issue of increased drag during deployment. Additionally, the slotted flap can produce high amounts of additional lift without any additional drag.

These two properties have made the slotted flap the most popular flap used in small and large general aviation aircraft.

Slotted flaps are hinged slightly below the leading edge surface of the flap. This slight but important detail allows the flap to move slightly aft as it is deflected. The slight aft movement causes a gap between the wing and the flap to be created.

The benefit of this design is that high-pressure air under the wing can flow up and over the top of the flap. The high-pressure air can re-energize the air flowing over the top of the flap, resulting in the separation of air from the flap to be delayed.

The result is an increase in the coefficient of lift of the wing without a large increase in drag.

Additionally, the gap between the wing and the slotted flap allows the wing to be flown at much higher angles of attack without stalling. This allows even more lift to be created than typical.

Different variations of slotted flaps exist. It is not unusual to find slotted flaps with 2 or 3 gaps between the flap and wing on larger aircraft. Otherwise known as double and triple slotted flaps.

Fowler Flaps

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Fowler flaps are a type of slotted flap. However, the critical difference between a fowler and a slotted flap is that a fowler flap increases the wings area as it moves aft as well as the wings camber. Comparatively, a slotted flap increases the wings camber, but not the area.

Fowler flaps are found on complicated and more advanced aircraft such as airliners. This is mainly due to a more complicated type of construction and design being required. Pilots on aircraft with fowler flaps can expect both up and down pitching moments depending on the type of aircraft being flown.

When fowler flaps are first deployed at small angles, the flap will extend backward with very little downward rotation. This increases the wings' surface area with minimal change to the wings camber. The result is an increase in lift without an increase in drag. In this configuration, the flap is especially useful for takeoff.

At larger angles of deployment, the flap body will turn downwards as it extends aft. The wing's surface area is increased; however, the downward rotation of the flap body increases the wings camber. Lift may be increased, but drag is also increased at the same time. These larger angles of deployment are therefore used for landing. 

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