6 Types of Flaps

Flaps are very useful for giving an airplane good handling characteristics at low speed. They are vitally important for giving fast planes the ability to go slow for takeoff and landing.

There are 6 types of flaps

  1. Plain Flaps
  2. Split Flaps
  3. Slotted Flaps
  4. Fowler Flaps
  5. Slots
  6. Slats

Plain Flaps

A simple hinge at the rear of the wing is used to make plain flaps. They are easy to design but they can’t provide much lift before the drag increases very significantly.

Since increasing drag is one of the purposes of a flap the performance is not too bad for landing. When taking off though, drag is not desired.

The reason this flap has so much drag is because the air from above the wing tends to separate and become turbulent when it has to flow downwards at such a steep angle. Think of it like a car on the highway having to make a sharp turn to stay on the road. This airflow separation is like that car spinning out because it can’t make the turn.

Turbulent airflow separation above the wing reduces lift.

Split Flaps

The split flap surface is actually below the wing and pushes down out of the bottom. It does provide some lift but it creates a lot of drag and is almost like a speed brake under the wing.

Slotted Flaps

This is the most common type of flap because it is relatively simple to design and build but provides a huge benefit over the plain flap. The slot created when the flap extends allows air to flow from under the wing.

This airflow provides a cushion for the air from on top of the wing to keep it from separating. It also flows along the top of the flap surface. These two benefits combine to create a lot of extra lift.

Fowler Flaps

Fowler flaps are complex but provide a lot of benefit. They act like slotted flaps opening up a channel for air, but they also slide outwards as depicted below. By sliding out from the wing they greatly increase wing area.

Put simply, they make the wing bigger. A bigger wing means lots of lift!

Most modern airliners use double or triple slotted fowler flaps. These have several flap surfaces that extend out from each other as the flaps are lowered creating a much larger wing with several slots for air to pass through.


This modification to the leading edge of the wing provides a channel of air that is pushed up over the wing and channeled towards the upper surface. At low speeds, this simple fixed device can increase the critical angle of attack (the angle at which the wing stalls). This means that the plane can fly much slower without stalling.


Slats are another type of leading edge flap. They slide down at low speeds and provide a large increase in lift like the fixed slot. The benefit of the slat is that it is retractable and won’t create extra drag at high speeds because it slides up into the wing surface.

Adverse Yaw

When an airplane makes turns it needs to change the amount of lift on either wing in order to bank. This is usually done by lowering the aileron on one side and raising the aileron on the other.

The wing with the raised aileron will have less lift than it did before, and the wing with the lowered aileron will have more lift.

The increased lift causes an increase in induced drag which will cause the airplane to yaw towards the wing with more drag.

When a bank is entered the rising wing has more drag so the aircraft will yaw away from the direction of the turn. This is adverse yaw.

Source: FAA Pilot’s Handbook of Aeronautical Knowledge

It can be counteracted easily by using the rudder while banking.

Some planes use spoilers instead of ailerons and they reduce or remove the adverse yaw effect. There is also something called a frise aileron that adds drag to the aileron that is raised to counteract adverse yaw.

Understanding Gyroscopic Precession

Airplanes use gyroscopes in many of their instruments. There are also some aerodynamic effects from the rotating movement of the propeller that require an understanding of gyroscopes. This post will focus only on precession and not on the main attributes of gyroscopes.

Gyroscopic precession affects rotating objects like a propeller or the classic example, a bicycle wheel. As an object rotates it will translate forces applied to it by 90 degrees.

If you push sideways on the top of a moving bicycle wheel, the bicycle will not fall over. Instead, it will turn because the front of the wheel will move in response to the force.

Now let’s think about the rotor of a helicopter, which is also subject to gyroscopic precession. Imagine that the blades are spinning around flat and just one of them is tilted for an instant to gather more lift. During this instant the tilted blade will have an upward force applied to it. It also already has an enormous amount of momentum and will continue spinning at a high speed.

The lift will cause it to begin accelerating up, but the momentum will very quickly move it farther around the rotor disk. Combining these two actions results in the blade rising until it reaches its peak 90 degrees from the point where it was pushed up.

Source: FAA Helicopter Flying Handbook

For example, if the blade (rotating counter-clockwise if you look at it from above) tilted momentarily while passing over the back of the helicopter then the whole rotor disk would tilt left because the right side would go up.

Confused yet? Gyroscopic precession is weird but it makes sense. If you can understand this 90 degree translation of force it will help you to understand:

  • Why airplanes have left turning tendencies
  • Why helicopter controls are mounted 90 degrees from the direction intended
  • How some airplane instruments work
  • Why your bike or motorcycle doesn’t tip over

Here are some additional resources I recommend:


Flight Instruments: Magnetic Compass

Airplane instruments and systems are usually as simple as possible. This is because simple systems will break less often. The magnetic compass is one of the simplest instruments there is.

How the magnetic compass works

A compass is made up of a housing with a lens on the front and a vertical line (called a lubber line) inside the glass representing the current heading. Inside of the housing, there is a liquid with the compass “float” suspended on a pivot. The float itself has the sensing magnet inside and markings for every heading. As the aircraft turns the float spins and indicates the planes heading along the lubber line.

There is also a second adjustable magnet in the bottom of the unit to correct for errors.

Finally, most magnetic compasses will have a light mounted above the lens so it can be viewed at night.

Source: FAA

Magnetic Compass Errors

The construction of the compass causes a few problems when reading it during turns and changes in speed. There are 3 basic types of errors.

Oscillation Errors

This is the simplest type of error. In turbulence, the indicator may bounce around because it is floating. If the compass is moving around continuously don’t expect to get a precise heading from it.

To determine your heading during turbulence, look for the midpoint or average of the oscillations.

Dip Errors – Turning

When banking the compass will turn to follow the vertical component of the earth’s lines of magnetic flux. In other words, the compass is drawn down towards the earth.

So if you are heading North and you start a turn the compass will try to point down towards the low wing. As the magnet is drawn down it will turn the indicated magnetic heading indicating a turn in the wrong direction.

Conversely, if you begin a turn while heading South the compass will indicate a turn in the right direction but it will turn more than your actual heading.

During turns, the compass lags when you are heading North and leads when you are heading South.

Remember OSUN or UNOS.

  • Undershoot North
  • Overshoot South

Dip Errors – Accelerating

When the aircraft accelerates the inertia of the heavy magnet causes the compass to rotate. It pulls towards the Northerly heading. Conversely, when slowing down the magnet pulls the card towards a southerly heading as it is moved forwards. This effect is most prominent when heading East or West and doesn’t have any effect when heading North or South.

Remember ANDS.

  • Accelerate North
  • Decelerate South


An aircraft is full of magnetic parts and flowing electrical currents that can interrupt the magnetic compasses ability to sense magnetic North. The adjustable compensator is set by a technician to account for these errors.

However, the compensator can’t fix this completely so a compass card is included with deviations corrections. When flying a magnetic heading read the heading you want under “For” and then turn to the indicated heading under “Steer”. Don’t forget about this when setting your directional gyro to match your compass.

Accident Study: Helicopter in the Water

I have been watching this particular accident for more information since the day I first saw it in the news. In short, a helicopter was giving a routine tour of New York City  when it was forced to make an emergency landing in the Hudson river. The pilot survived but all 5 passengers drowned.

Source: NTSB

What Happened?

Liberty Helicopters gives tours of New York with the doors removed and passengers riding in harnesses that keep them tethered to the helicopter. This allows them to freely take photographs without falling out. These harnesses played a critical role in the accident.

The problems began when a passenger’s tether(some sources report that it was passenger’s bag) slipped beneath the fuel shutoff lever and pulled on it. The pilot began seeking a place to land as the engine died and did not realize that the shutoff had been pulled.

He considered landing in central park but decided there would be too many people around. Instead, he made his way to the river and inflated the floats designed to allow the helicopter to land on the water.

As he prepared for the landing he realized that the fuel shutoff had been activated and began a restart. The timing was wrong though and the engine would not restart fast enough. He followed the procedure and shut the fuel off again just before impact.

I say “impact” because you can see in the videos of the crash that the helicopter did not land gently at all. This could be because of a poor autorotation, or just because there was not enough energy available to begin with.

After landing the helicopter began rolling to the right and was quickly upside down. This is one of the parts that upset me the most because these floats are designed to keep this from happening. However, a malfunction caused the right side floats to not inflate properly!

As it hung under water the skids were the only thing visible. The pilot was unable to free anybody else and he was picked up by a boat responding to the emergency. A big part of this tragedy was the harness system that held the passengers trapped underwater. The tour company supposedly instructed passengers that they were to use a knife attached to the harness to cut themselves free in an emergency. This obviously was not realistic as nobody was able to do it, including a firefighter who was among those lost.

A Chain of Questions

An accident like this produces more questions than answers because there are so many things that went wrong, and if just one of them had gone right these people would still be alive today.

  • Why was the tether able to get around the fuel shutoff?
  • Why didn’t the pilot realize this had happened?
  • Why didn’t the pilot choose a landing on solid ground?
  • Why was the water landing so rough?
  • Why did the pilot not allow the engine to continue its restart?
  • Why didn’t the floats inflate properly?
  • Why were the passengers unable to free themselves?
  • Why was the pilot unable to free anybody?


There are a number of things that could have changed the outcome. In this case, the biggest part of the accident chain was built into the company’s operation with the tethered harnesses. But if the floats had functioned this would not even be considered.

It is important to reflect on these accidents even if it may be difficult to stomach. As pilots, we have a grave responsibility for the safety of others. This is why everything must be done with seriousness and absolute professionalism.

I pray for all those involved in this terrible tragedy.


IMPORTANT: Please read my disclaimer below about accident studies


This study and all accident studies are not meant to judge anyone, their actions, or their skills as a pilot. I do not claim to know what the pilot did or what he/she was thinking. The purpose of these accident studies is to better understand what causes accidents and how to avoid them. Comments and other points of view are always welcome as long as they are respectful towards everyone involved.