Aircraft Flaps: Types and Uses

The operation of flaps forms a major part of a pilot’s skillset. In this article, we take a look at the aerodynamic effect of aircraft flaps, the types of flaps and give a few practical tips on how to ensure pilots are using basic references to increase flap selection practice effectiveness.

Contents hide

1 Introduction

2 A brief history of flap design requirements

3 Flaps, and their effect: A do-it-yourself matrix assessment next time you fly

4 Types of Flaps

5 Drawbacks from Flaps

6 Conclusion

Introduction

Early on in aviation aviators had the challenge of slowing the aircraft down during an approach to landing. Referencing a simple diagram below, one can clearly see that without some drag-increasing device, the aircraft (or any mass) will struggle to reduce speed whilst on descent.

A brief history of flap design requirements

The need for a device to increase drag during descent was thus evident. Increasing drag in total would not have worked – i.e having a massive flat plate welded to the undercarriage would have made climbing impossible and cruise flight slow. The drag device thus needs to be “selectable” or able to cycle on/off. Visualisation is a great tool for understanding flaps and this three-minute¹ video explains what 20 pages would struggle to cover.

The result was the addition of movable sections of the wing that could be lowered into the airflow during descent and landing, thus increasing drag and allowing the aircraft to slow down. This movable section of the wing was linked to a control in the cockpit which allowed the pilot to select the surface as required. The flight testing confirmed the theory and flaps were soon incorporated into aircraft. One thing that designers did not foresee was the increase in lift by lowering the flaps. For an in-depth explanation of wing flap and aerodynamics, you can click here².

“As flaps are deflected, the aircraft may pitch nose up, nose down or have minimal changes in pitch attitude. Pitching moment is caused by the rearward movement of the wing’s center of pressure; however, that pitching behavior depends on several variables including flap type, wing position, downwash behavior, and horizontal tail location.”
flight-study.com/2019/09/transition-to-complex-airplanes²

aircraft flaps, aircraft flap types — Making effective use of flaps is an important skill that pilots should possess. This skill will be far easier to master if the effect of the flap selection is understood and anticipated.

Flaps, and their effect: A do-it-yourself matrix assessment next time you fly

Flaps are pilot-controllable, movable sections of a wing, that result in an increase in lift and drag when selected. Making effective use of flaps is an important skill that pilots should possess. This skill will be far easier to master if the effect of the flap selection is understood and anticipated. Unfortunately for pilots, there are many types of flaps, and each aircraft reacts slightly differently in timing and severity during flap selection. Some aircraft gain significantly more lift, resulting in the whole aircraft rapidly moving up, which is uncomfortable for passengers. To get pilots acquainted with the characteristics of the aircraft they fly, flaps selection exercises are done. The objective of this exercise is to be able to extend and retract the flap while maintaining the aircraft’s flight path.

The best way for a pilot to judge a constant flight path:

1) Look in the distance and choose an aim point on the horizon (something prominent like a mountain peak)

2) Imagine a line between the point and yourself. Then choose another closer reference point on that line to be able to maintain a constant track. (a lake/town/prominent feature)

By keeping those points aligned, a pilot can ensure the aircraft is maintaining a constant heading. However, the heading is just one component – the second is the descent angle. To achieve this do the following: To maintain a constant descent angle will also require two reference points

3) The first reference point will be the horizon, and

4) The other will be the extended flight path of the aircraft (i.e, where will the aircraft impact the ground if continuing on a constant angle)

If the two points remain equidistant in your field of vision, the angle of descent is constant. If the two points get closer, you’re under-shooting the aim point, and if they get further away, you’re overshooting.

aircraft flap uses — There are 4 main types of flaps – plain, split, slotted and Fowler.

Pilots may find this difficult if they aren’t relaxed and they’re focusing too much on one point.

Practice extending the flap in straight and level flight until the effect of the flap is well understood and its effects are anticipated and counteracted to prevent any alteration to the flight path. Practice speeds should vary from max flap extension speed down to minimum stall speed so the full range of flap effect is appreciated, and counteracted effectively.

The idea is to ensure the pilot can control the aircraft smoothly and confidently while extending or retracting the flap at any speed, and instinctively knows what’s required to maintain a constant flight path and safe speed. The numbers indicated on the instruments aren’t important! What’s important is knowing when an aircraft is about to stall and how to control it so it doesn’t. After practicing flap extension and retraction while maintaining straight and level, a little additional practice should include extending and retracting flap in climb and descent configurations.

This may take more time than you’d think is warranted, but it should be realized that you’ll revert to first learned skills under pressure, so, if they’re learned well, these skills will save potential issues later on and enable far greater development.

Types of Flaps

An understanding of the types of flaps is also important and I will discuss the four main types:

Types of flaps – 4 types of flaps³

A – Plain Flap

A plain flap is a part of the trailing edge of a wing encompassing the whole camber that lowers into the airflow. This is the most basic type of flap and is usually found on light aircraft, a piper Cherokee a common example. The operation is usually simple and can be actuated either manually, electrically or hydraulic.

The selection of flap simply results in a part of the wing into the airflow. As with most flap designs, the flaps are usually situated on the wing roots side of the aileron.

Both wing flaps also work in conjunction – i.e the left and right flaps are either both lowered or both retracted.

B – Split Flap

Split flaps do not differ majorly from plain flaps, the distinguishing factor is that only a part of the total camber of the wing moves. I.e if you look at the wing from the top, there will be no change, as only the bottom half of the wing is actually part of the flap.

Split flaps are not commonly found.

types of flaps — Flap selection effect on flight path can easily be mitigated by well-trained pilots and failures are rare.

C – Slotted flap

Slotted flaps are similar to plain flaps, but have a significant gap between the flap and the wing when selected. This airflow re-energises the boundary layer, thus increasing lift.

A C208 Caravan is a good example.

D – Fowler flap

Fowler flaps were designed in 1920 by Harlan Fowler and has quite an interesting history that can be read here⁴.

Harlan Fowler wanted to increase lift, rather than necessarily increasing drag. Fowler reckoned that by having the flaps move aft before deflecting, the upper surface of the wing and the wing camber increase, thus increasing lift. And if one movable flap is good, why not add two or three?

Fowler flaps are common on Boeing aircraft.

“Flaps help your wing adapt to your current phase of flight. Are you taking off or landing? Extending your flaps increases lift, and allows you to fly at slower speeds. Are you cruising at altitude? Retracting flaps reduces lift, and in turn, decreases drag.”
boldmethod.com/learn-to-fly/aircraft-systems/how-the-four-types-of-aircraft-flaps-work³

Drawbacks from Flaps

Airplane Flaps are an amazing design solution to common problems encountered during landing and take-off, and the main effect is to increase the applicability/usability of aircraft in different situations. Flaps are not without their problems though.

Flaps are subject to major aerodynamic forces and like all movable parts on machines, are subject to failure. Designers thus limit the speeds flaps can be deployed at. This is known as the Vfe – flap extend speed limit.

Flaps can also sometimes fail on one wing, thus creating a major imbalance in lift and drag between the wings which can create uncontrollable roll.

On airlines, flaps often create too much lift after touchdown, needing airlines to add spoilers – devices that “dump lift” to create effective contact between tyres and ground for breaking.

The last danger is forgetting to set flaps for Take-off and Landing which dramatically changes the flight characteristics, often with disastrous consequences, as can be seen here⁵.

aircraft flaps — Airplane Flaps are an amazing design solution to common problems encountered during landing and take-off, and the main effect is to increase the applicability/usability of aircraft in different situations.

Conclusion

Pilots love flaps, and the advantages they provide on takeoff and landing are major. Flap selection effect on flight path can easily be mitigated by well-trained pilots and failures are rare. The understanding of the aerodynamic effect, increasing lift and drag, forms the basis of this anticipation, whether flying a major airline with Fowler flaps, or the simple trainer with plain flaps. The fact that the basic operation of flaps has remained constant for close to 100 years is a testament to its effectiveness.