# What principles of Physics do Airplanes use

## How do airplanes fly – Physics behind the navigation of aircraft

Many flying machine designs were tested because of the human’s quest to fly in the air. Finally, Wright brothers succeeded in flying, thanks to the airfoil. The wings of the aircraft were able to produce the lift force ingenious way.

Even after hundred years after Wright brothers first flight there are still debates among engineers, scientists and pilots regarding physics behind the generation of lift. There are several physical descriptions on how lift is produced. The concept of lift production is most often misinterpreted by the “Equal transit theory”.

The Wright brothers biplane wings had a curved shape, this shape pushes the air downwards resulting a reaction force from air to push the wing upwards in the equal magnitude (Newtons 3rd law). This eventually results in the lift force and aircraft will be able to fly in the air off the ground. Even today the aircrafts use the same airfoil technology to fly but with the highly aerodynamically optimised airfoil shape.

Forces acting upon an aircraft

There are four forces acting upon an aircraft. They are

Weight – acting towards the centre of the earth.

Lift – acting perpendicular to the direction of relative motion.

Thrust – acting along the direction of motion, generated by engines to move the aircraft forward.

Drag – acting opposite to the relative motion of the aircraft, generated by the air resistance.

Forces acting on an aircraft. Image source: https://www.grc.nasa.gov/www/k-12/airplane/forces.html

Thrust force makes the airplane move forward. As the airplane moves forward, the relatively flowing air over the wings will produce a lift force on the wings. Secondary control surfaces, Flaps and Slats are present on the wing trailing and leading edge are used for the lift augmentation. While taking off, these secondary control surfaces are extended downwards increasing the effective curvature and wing area thus the air is deflected more and lift produced is increased. When the lift force is greater than the weight aircraft take-off. During normal flight flaps and slats are retracted to their original position.

Axes of Aircraft Image credit:http://okigihan.blogspot.com.au/p/flight-control-surfaces-directional.html

The attitude of an aircraft can be controlled by three different control systems. Pilots use them alone or together to navigate the aircraft.

Aileron – roll control, located on the trailing edge of the wing near the wingtips.

Elevator – pitch control, mounted on the trailing edge of the tailplane.

Rudder – yaw control, mounted on the trailing edge of the vertical stabilizer.

Primary and Secondary Control Surfaces. Image Source: http://www.keyshone.com/sciences-behind-airplane-flight-mechanics/

To ascent/descent the aircraft, deflect the elevator up/down. When the elevator is deflected upwards the lift force on the tailplane decreases causing a moment to turn the aircraft upwards.

To change the path of the aircraft, one would think to use a rudder, but if a rudder alone is used there is a change in orientation of the aircraft. So, ailerons are used, when one aileron is deflected upward and other downward, there causes a difference in lift forces on different wings causing a moment for aircraft rolling motion. In this way, the aircraft changes its path but the orientation doesn’t change.