## What Are Aircraft Slats ? How Do They Work?

What are aircraft slats - Wikimedia

The main function of the slats of the aircraft are to increase the critical angle of attack of the aircraft and prevent the aircraft from early stalling.
The critical angle of attack of an aircraft is the angle of attack at which an aircraft starts stalling. Stalling of an aircraft is the condition when an aircraft looses sufficient lift force to continue flight. Technically speaking stalling is the condition when the boundary layer of the air flow separates from the wing surface i.e air flow separates from wing surface and hence they cannot produce and lift. As the air flow separates, the wing surface looses its lift. So, what slats do is, they keep the air flow in contact with the wing surface and delay the onset of stalling.
Aircraft Stalling flow separation - Wikimedia

Now, lets see how slats do this!

### How Slats work?

Actually slats are an extended part of the wing. When slats are extended they move a little ahead of the wing and leave little space between them and the wing leading edge.

Condition I - No slats
As the air flow approaches the wing it gets divided into two paths. One above the wing and one below it. The point at which the air flow gets divided into two paths is known as the stagnation point. As angle of attack of an aircraft is increased, its stagnation point moves toward the bottom of the wing. And energy of the air flowing over the top of the wing reduces. And hence the air flow separates from the top of the wing. This condition is known as boundary layer separation. As air flow separates aircraft looses it's lift.

Now the aircraft will stall at certain angle of attack known as critical angle of attack. Stalling of an aircraft can be delayed by increasing critical angle of attack of the aircraft. To make this happen the air flow should remain attached to the wing surface for a longer time. This is what slats do

Condition II - Slats Extended

Extended Slats - Wikimedia

When the slats are extended and angle of attack is increased, air from the bottom part of the wing enters to the top part of the wing. This speeds up the air flowing over the upper surface of the wing and increases its kinetic energy. This keeps the air in contact with the wing surface for a longer time and hence results in delaying the onset of stalling.

Flow over extended slat Airfoil - Wikimedia

In such a way stalling of the aircraft can be delayed with the help of slats.

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## What do numbers on the runway mean?

Image from : - Flickr

Have you ever noticed big numbers on the runway? If not, then just open google maps and turn on satellite mode and search for the airport near your city and check out the ends of the runway, you will notice those big numbers on them. Those numbers have significant meaning. Let's find out the meaning of these numbers.

These numbers on the runways are used to name them, so that, if multiple runways are available on same the airport then ATC (Air Traffic Control Tower) can mention runway name and tell the pilot  which runway to land on.

Numbers on the runways are based on the magnetic azimuth in which they are heading. North heading is taken as reference. Those numbers are known as QFU codes. So East is 090°, South is 180°, West is 270° and North is 360°. While naming runways, first two digits of the angle of inclination of the runway with magnetic north are taken. So a runway numbered 09 points east (090°), a runway numbered 18 is south (180°), a runway numbered 27 points west (270°) and a runway numbered 36 points to the north (360° rather than 0°).

Compass - Wikimedia

Now if a runway is heading 150° to north then it will be numbered as 15. Similarly if a runway is heading 310° to the north then, it will be numbered as 31.

As each runway has two ends, it has two headings. Every runway has two numbers. The two headings of the runway differ by 180°. So if one end of the runway has a heading of 150° to the north then that end will be numbered as 15 as mentioned earlier. Heading of the other runway will differ by 180°. So heading of that runway will be 150+180 = 330°. Hence, the number on other end of the runway will be 33.

Now suppose the runway has a heading of any random degrees say 144°. Then such number will be rounded off to nearest ten. So in this case for 144° runway will be rounded off to 140° and numbered as 14. Now suppose a runway has an heading of 316°. Guess what will be the number on the runway?
You guessed it right! It will be 32 as 316° will be rounded off to 320°.

Now there is one more possibility, what if there are multiple runways parallel to each other. As runways are parallel to each other their heading will be same. Now how will you number them. In this case location of the runway on that airport is considered. Suppose if there are two runways then the runway located at the right side of the heading will be numbered with the prefix 'R'. R here stands for right. Such as 27R, 32R, 18R, etc. Similarly runway located at left side of the aircraft heading will be numbered with L as a prefix, which stands for left. Such as 27L, 32L, 18L, etc.

John F. Kennedy International Airport Left Runway - Google Maps

John F. Kennedy International Airport Right Runway - Google Maps

Now suppose there are three parallel runways then runways will be located as right, center and left. Right and left runways will be numbered as mentioned above and for the runway in between these two, it will be prefixed with (you guessed it) 'C', which stands for right. Such as 27C, 32C, 18C, etc.

Chicago O'Hare International Airport Center Runway - Google Maps

Hope now you understood those mysterious big number that you've noticed while travelling on an airplane.

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## How Flaps Work?

How flaps work - wikimedia

All of the aircraft including small Cessna 172 to giant Antonov An225 have flaps. But what is the purpose of these flaps? And How do they work?  Let's Check it out!

Antonov An225 Flaps - Wikimedia

How flaps work?

First of all main purpose of flap is to produce more lift to the aircraft. This is done by increasing the camber and the surface area of the aircraft wing. As we know the lift formula is

Here S is Surface area of the wing. So from the formula above we know lift is directly proportional to surface area of the wing. So as Surface area of the wing increases lift to the wing increases with it.

Now let me explain about lift in some details

For lift on the wing there are two theories. One is proposed by Bernoulli and other by Newton. And combination of these two theories is reason for production of lift on the wing. Here for explanation of working of flaps we need to know about newton's lift theory.

Flow over Aerofoil - Wikimedia

Air flowing over the wing remains attached to the surface of the wing. This is due to coanda effect. When airflow leaves the wing surface, wing applies force on airflow and deviates the direction (due to coanda effect) of the flow of airflow. As we know newton's third law of motion: Every action has an equal and opposite reaction. Airflow exerts reaction force on the wing of equal and opposite magnitude of force which wing exerted on the airflow. This reaction force is known as Lift.[Learn more about lift here]

Now what flaps do is they increase the camber of the aircraft wing. Camber is just the curve of the wing. As curve of the wing is increased, wing exerts more force on the airflow (Coanda effect), So as reaction to this force airflow exerts more force on the wing so more lift.

Purpose of the Flaps

Flaps Increase the drag on the aircraft

As flaps increase lift on the wing, they also increase lift induced drag with it. Extending flaps creates wake behind the wing. This wake creates more drag. This increased drag helps aircraft to slow down.

Flaps reduces the stall speed

As extending the flaps increase the lift. Hence Aircraft can fly with even slow speed without stalling.

Flaps Settings

While Take-off only few degrees of flaps are extended as extending flaps create more drag and its not good idea to create more drag while speeding up for take-off.

Boeing 747 Take off - Wikimedia

While Landing pilot can use maximum settings for flaps. As Flaps increase lift and drag. This will allow aircraft to touchdown slowly.

Boeing 747 Landing - Wikimedia