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An aircraft is designed to produce enough lift to take off and stay in the air. However, it needs extra lift at lower speeds such as during takeoff and landing.
However, many high-lift devices allow an aircraft to produce extra lift during the landing and takeoff phases, and the Fowler flap is just one of them.
Fowler flaps are high-lift devices situated at the trailing edge of the aircraft wings. When deployed, they slide backward and downwards, increasing the wing's surface area and camber, producing a large amount of lift. It’s like having the power to change the shape of the wing!
Although Fowler flaps produce a lot of lift at lower speeds, they also generate a lot of drag, due to which pilots deploy them only in certain flight phases. As a result, the aircraft has to carry the extra weight. This article will discuss how Fowler flaps produce lift and how they compare with the other types of flaps out there.
Here, you’ll learn about Fowler flaps in detail from the perspective of a pilot and aviation enthusiast with years of experience working with fowler flaps.
What Does a Fowler Flap Do?
Fowler flaps are like wing extensions present at the trailing edge of the wing. They extend backward to increase the surface area of the wing. This allows the wing to generate more lift. Along with that, you can see them moving downwards once deployed, increasing the wing's camber as well. Camber defines how curved the wing is. Greater the camber, the greater the lift.
Fowler flaps are deployed during landing to increase the lift of an aircraft so it can land at lower speeds without covering too much landing distance on the runway. This allows for a steep approach to the runway, and the aircraft can easily avoid any obstacles in its path. Some aircraft also deploy Fowler flaps during takeoff if they don’t have a high enough lift coefficient.
A pilot only has to use Fowler flaps in a small amount if they become necessary during takeoff. This is because when a pilot engages the Fowler flap, it moves backward and then sticks downwards against the airflow, generating lift and drag. Drag during takeoff is harmful, so pilots don't use Fowler flaps or any other type of flaps during that phase.
How Do Fowler Flaps Increase Lift?
Although we know that Fowler flaps increase the lift on an aircraft in two ways: increasing the wing’s surface area and camber, why do they help to increase lift in the first place?
You should know that two factors affect how much lift a wing can generate. These are the speed of the incoming air or the aircraft and the angle of attack.
The angle of attack or AOA is an important concept in aviation and aerospace to remember. To understand this, you should know some common terms about the wing or airfoil (note that airfoil is the cross-section of an actual 3D wing):
- Leading Edge: This is the front section of the wing that comes in contact with the airflow first.
- Trailing Edge: This is the rear edge of the wing.
- Chord: The width of the wing from the leading edge to the trailing edge.
Angle of attack is the angle the chord makes with the incoming airflow. The greater the angle of attack, the greater the lift on the wing.
When Fowler flaps are deployed, they move downwards, increasing the wing's camber and changing its trailing edge position. This alters the chord line position. In conclusion, Fowler flaps increase the wing's angle of attack and thus the lift on the wing.
However, before Fowler flaps move downwards, they first extend backward, increasing the wing's surface area. A greater surface area means more air travels onto the surface and thus greater lift. We can explain this better using the lift equation:
Lift= 1/2 * V2 * Cl * S
p = Density
V = Velocity
Cl = Coefficient of lift
S = Surface area of the wing
We can see here that lift is directly proportional to the surface area. Thus, the greater the wing's surface area, the greater the lift.
What is The Difference Between Fowler Flaps and Split Flaps?
Because Fowler flaps are the only flaps to produce large amounts of lift, unlike other flaps, they are present on larger aircrafts such as jets and commercial airlines. Another reason why Fowler flaps are widely used is that the top and bottom surfaces of the flap are not entirely sealed.
This results in a gap allowing high-pressure air from underneath the wing to move through the gap to the wing's top surface. This high-pressure air stabilizes and modifies the boundary layer on the wing's upper surface. As a result, this air forms a low-pressure region on the leading edge, due to which the flow over the top of the wing remains attached even for very high flap deflections.
On the other hand, a Split flap doesn't do this. This flap's top and bottom surfaces are sealed, and it can't move backward either, but only downwards. For this reason, the Split flap increases the wing's camber but not the surface area. Compared to Fowler flaps, Split flaps don't produce as much lift but a lot of drag. This is why only a few aircraft use them and they aren't widely popular.
The first Split flap was invented by Orville, one of the Wright Brothers, in 1920. They generate more lift than the plain flaps, but shortly after, Slotted and Fowler flaps were introduced that were way better and more efficient than Split flaps, ruling them out of use. The Douglas DC-1 is one of the few aircraft to use Split flaps.
What is The Difference Between Fowler Flaps and Slotted Flaps?
Slotted flaps are similar to Fowler flaps, but like Split flaps, they only move downwards and not rearwards when deployed. Slotted flaps have slots or gaps, which like Fowler flaps, allow high-pressure air to move from the bottom surface of the wing to the top, stabilizing the boundary layer at the top and thus delaying airflow separation.
This generates additional lift. Also, Slotted flaps look like airfoils, which means air can travel at the bottom and top surfaces, creating a pressure difference and thus generating lift.
There are three types of Slotted flaps: single-slotted, double-slotted, and triple-slotted. The double-slotted flaps have the same function as the single-slotted flaps but have two slots or gaps, which means they produce more thrust than the former. Triple-slotted flaps are like Fowler flaps.
While Fowler flaps are used in large aircraft, Slotted flaps are used in passenger, training, and cargo aircraft. The difference between the two types of flaps is that Slotted flaps can't move backward while Fowler flaps can. This way, Slotted flaps don't benefit from the increase in surface area and thus the additional lift.
Who Invented Fowler Flaps?
Harlan Davey Fowler invented the Fowler flap. He was born in 1895 and was into aviation from a young age. After building model airplanes as a teenager, he grew up to join the U.S Aviation Experimental Bureau during World War I. His love for aviation quickly made him second in command in engineering design at Edwards Air Force base. In 1921, he created the Fowler flap, then known as Variable Area Wing.
However, it wasn't until 1927 that he built many prototypes of the Fowler flap and tested them in the National Advisory Committee for Aeronautics wind tunnels. At that time, Fred Weick, who had designed Ercoupe and Piper Pawnee, looked after the wind tunnels,
He gave Fowler the go-ahead, concluding from the gathered data that Fowler's flap improved climb and reduced landing distance and speed. In 1935, the Fowler flap was used on the Martin 146 bomber prototype for the first time. Later, it was also present on the Lockheed 14 Super Electra, Boeing 247, and the Douglas DC-2.
Fowler Flap Settings for Landing and Takeoff
Different types of aircraft have different settings pertaining to flaps. Some aircraft, like the Cessna 172, let the pilot choose different Fowler flap angles, such as 0 degrees, 10 degrees, 20 degrees, etc. Other aircraft have options like approach and landing corresponding to specific angles already fed into the system.
To know what flap setting to use for a specific maneuver, you should refer to the aircraft's pilot's operating handbook (POH), also known as the airplane flight manual (AFM).
Usually, pilots should fully extend the Fowler flaps during landing to slow down the aircraft and produce greater lift so it can land quickly. However, pilots should never completely deploy Fowler flaps during takeoff.
Takeoff is not just a moment in time when the aircraft is airborne. It is the entire journey from the point when the aircraft starts moving on the runway to when the aircraft takes off into the air and is around 35 ft to 50 ft above the ground. When the aircraft is moving on the runway, we call it ground roll. When it has taken off, we call it the airborne segment.
If the pilot deploys the Fowler flaps during takeoff and increases their angles, it will have the following effects on the two stages of takeoff:
- An increased Fowler flap deflection angle during the ground roll means the aircraft can achieve greater lift at lower speeds and quickly take off into the air.
- The drag increases, which makes it difficult for the aircraft to climb further and accelerate. This increases the overall takeoff distance.
In conclusion, Fowler flap deflection is not effective if an aircraft has to travel on a shorter runway and needs to avoid a tree line at the end.
Are Fowler Flaps or Other Flaps Necessary During Landing?
They are necessary because they are efficient during landing. However, aircraft can land without the help of Fowler flaps or any other flaps.
In a case when flaps are out of order or when an aircraft doesn't have any, a pilot can land the aircraft safely. A pilot might need to fly faster while approaching the runway in a no-flap situation. The aircraft would also need a longer runway due to a longer ground roll.
Landing without flaps is not an emergency situation, as pilots are prepared for such a scenario. With that said, Fowler Flaps increase the camber and the wing's surface area, which greatly increases the lift.
An increased lift allows the aircraft to land at lower speeds with reduced landing distance on the runway. So, while aircraft can do without flaps, their presence can benefit them hugely.