• What are flaps on a plane?

    How to fly slow and low?

You have probably heard that both take off and landing are the most delicate phases of a flight. Well, it ‘s true.

Take off and landing are the phases in which it is most necessary to take the maximum precautions as the aeroplane is moving at low speed and very close to terrain. We have recently talked about the importance of speed in aviation.

Aeroplanes are designed to fly quickly and efficiently from one place to another but what happens if they have to fly at a low speed? This is when flaps and slats come into play.

What are flaps and slats on a plane?

Flaps and slats are hyper-sustentation devices designed for aircraft to fly at low speed in a safe manner. That is, they modify the aircraft wing anatomy and its aerodynamics so as to increase sustentation.

In order for this to occur, various techniques are used:

  • Increasing the effective wing area
  • Increasing the wing curvature
  • Creating a slot that energises the boundary layer

Some types of flaps and slats only use one of these techniques, whereas others are a combination of various types.

And what are they used for?

Surrounding the wings, there is a layer of air that slides sideways alongside the wings’ surface known as the boundary layer. This surface is essential in producing Bernoulli’s effect, which is indispensable to generate sustentation allowing planes to fly.

When aeroplanes’ speed decreases, or if the angle at which attack increases, the layer will eventually come off completely from the wing, generated sustentation will be practically null and the aeroplane will go into loss.

To prevent this, aeroplanes are equipped with flaps and slats, which we tell you further about here.

Types of flaps on a plane

Flaps are hypersustentators on the edge of wings. There are various types:

Plain Flap

This is the most simple flap that exists. Its mechanism is based on a part of the wing which in some cases can reach 30% of the string and deflects downwards.

These flaps increase sustentation increasing the curvature of the wings’ edge and can deflect up to 40 or 50 degrees.

As well as increasing sustentation, they also increase generated resistance, forming a great area of vortexes or turbulence wakes.

Slotted Flap

These have a design which is similar to plain flaps but once extended, have a hole or slot between wing and leading edge.

This hole sends a small air current upwards, which circulates alongside the lower part of the wing, energising the boundary layer.

Split Flap

As with plain flaps, split flaps have a wing trailing edge that deflects downwards, but only on the underside or intrados whereas the extrados or upper part of the wing keeps its shape.

Split flaps generate the same increase in lift as plain flaps, the difference being that generated resistance is much lower here.

Fowler Flap

These are the most complex flaps that exist. They not only increase sustentation but also the effective surface wing area.

In the same way as slotted flaps, it creates a hole between the wing and the leading edge of the flap to energise the boundary layer.
The fowler flap produces an increase of the maximum sustentation coefficient with a light increase of generated resistance.

Types of slats

Flaps are on the leading edge of a wing and the hyper sustentator devices on the trailing edge of a wing are slats.

In the same way as flaps, slats also increase the maximum coefficient of sustentation that a wing produces, allowing the aeroplane to fly in a safe way at a lower speed.

Slats are always extended earlier and are placed back afterwards, as they are much more efficient than flaps.

Following are different types of slats:

Ventilated Flap

It’s a type of slat that generates a slot between the leading edge and the slat itself so that the boundary layer is energised.

Sealed Flap

It’s a type of slat that increases the sustentation coefficient, increasing wing curvature. In contrast to ventilated slats, no slots are formed here.

Droop nose

Even though it isn’t a slat, it is a hyper-sustentation device of the trailing edge. This design is based on increasing curvature of the trailing edge to increase the effective angle of attack, which also increases the sustentation coefficient the wing can generate.

Krueger Flap

Yes, it’s a flap on the trailing edge, so it’s also a slat.

We can describe it as a mixture of split and fowler flap because it increases the maximum sustentation coefficient, increasing the effective area of the wing, its curvature and creating a slot to energise the boundary layer.

Now you know what flaps are and why they are so important

Now you know flaps and slats are used to change the aerodynamic properties of an aeroplane’s wing, perhaps you’d like to continue learning about the anatomy of aeroplanes with our post on fuselage, landing gear or even the types of fuel used in aviation.

Also, remember if you’re thinking about training to become a commercial pilot, in our blog, you’ll find much more information to help you on your way.

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