Lecture Leading Cadet Training Principles of Flight 1 Introduction

The Leading Edge Is generally the part of the wing that first contacts the air. The Trailing Edge The rear of the wing, where the airflow rejoins after passing over and under the wing. The Chord Line The line passing through the Centres of Curvature of the Leading and Trailing Edges. Definitions Leading Edge Trailing Edge THE AEROFOIL The shape of a wing as seen in cross-section. Chord Line

Free Stream Flow The Airflow remote from the Aircraft, and unaffected by its presence. sometimes called the Relative Airflow. The Angle of Attack Symbol α (alpha) The Angle between the Chord Line and the Free Stream Flow. Definitions Leading Edge Trailing Edge Chord Line Free Stream Flow α

Definitions The Mean Camber Line The line drawn equidistant from the upper and lower surfaces of the wing. If the mean camber line lies above the chord line (as in the illustration) the aerofoil has a ‘positive camber’; it is a CAMBERED AEROFOIL Leading Edge Trailing Edge Chord Line Free Stream Flow α Mean Camber Line

Definitions If the mean camber line is co-incident (or identical to) the chord line it is a SYMMETRICAL AEROFOIL Leading Edge Trailing Edge Chord Line Free Stream Flow α Mean Camber Line

Air is a Substance The greater the speed of air, the greater the force it exerts on its surroundings. but what happens if the air is still and an object moves through it ? Imagine cycling along a flat road on a calm day. As your speed increases, you feel the air against your face. The faster you go, the greater the force of wind.

How is this lifting force to be given to an aircraft? Hold 2 sheets of paper about 1 finger width apart. Blow between the papers. You would think the papers would be blown apart. But the opposite happens ! The harder you blow, the more firmly the papers are drawn together. Pressure

In a wind tunnel, air speed and pressure are measured. Air enters the tunnel at ‘A’ A B C and escapes at ‘C’ The amount of air leaving, must equal that entering. The air speeds up to pass through the narrow point ‘B’. As speed increases, pressure drops & vice versa

PressureA B C Bernoulli's Principle : As airspeed increases, air pressure decreases. As airspeed decreases, air pressure increases. + + _ _

Pressure Back to our sheets of paper … The speed of the air between the paper increases. So, the air pressure between the paper decreases. Therefore, the air pressure outside is greater, pushing the paper together.

Pressure A wing works in the same way as the Wind-Tunnel. Because the air is faster over the top surface, the pressure is decreased. And the wing produces LIFT. ____

The Pressure Envelope The area of static pressure in the surrounding area. Pressure ++ _ TR Total Reaction (TR) The consequence or ‘resultant’ of all the aerodynamic forces, usually on the aerofoil only. The Centre of Pressure (CP) The point at which the Total Reaction is said to act. Where all the forces balance and all lift is said to act. CP o

The Pressure Envelope Pressure ++ _ TR CP o Oncoming Air LIFT Flight Path LIFT - The component of the Total Reaction which is ‘perpendicular’ to the oncoming air and the flight path. DRAG DRAG - The component of the Total Reaction which is ‘parallel’ to the oncoming air and the flight path.

Newton’s 3 rd Law “TO EVERY ACTION, THERE IS AN EQUAL AND OPPOSITE REACTION” If a car weighing 1 tonne is parked on the road. If a car weighing 1 tonne is parked on the road. Its weight presses down on the road with a force of 1 tonne Newton’s Law states that to support the car, the road must press up with a force of 1 tonne.

Newton’s 3 rd Law Similarly a boat is supported by an upward force (from the sea) otherwise it would sink !

The same must apply to an aircraft in flight. But how can an aircraft be supported by such a flimsy substance as air? An aircraft can only stay airborne as long as it moves. Newton’s 3 rd Law

Weight If you hold a brick at arm’s length, you will feel the effort needed to balance the downward force of gravity acting on the weight of the brick. Less effort from your muscles (less lift) makes the brick sink. More effort from your muscles (more lift) and the brick will rise. GRAVITY EFFORT

Lift and Weight Lift Greater than Weight Ascending Flight Lift Equal to Weight Straight and Level Flight Weight Greater than Lift Descending Flight

Lift Hold a sheet of paper so the end between your fingers is horizontal, and allow the rest to hang down. Blow along the top of the paper. and watch the paper lift. The top surface of an aircraft wing behaves in a similar manner.

Air flowing past a stationary object has the same effect as if the object were moving through the air. But in real life, the airflow rarely approaches a wing as shown here. Lift

The top of an aircraft wing is curved, which forces the air into a smaller space. The air flows over the wing at an increased speed - And therefore at a reduced pressure, compared with the surrounding atmosphere.

Lift The resulting pressure difference between the air above and below the wing, tends to lift it up.

Check of Understanding On an aerofoil the straight line passing through the Centres of Curvature of the Leading and Trailing Edges is called... The Centre of Pressure The Chord Line The Angle of Attack The Mean Camber Line

Check of Understanding If the mean camber line lies above the chord line, what type of camber does the aerofoil have? An ovoid camber A positive camber A symmetrical camber A negative camber

Check of Understanding In the diagram below, what has happened to the air pressure at point ‘B’? It is lower than ‘A’ and higher than ‘C’ It is equal to that at ‘A’ and ‘C’ It is lower than at ‘A’ and ‘C’ It is higher than at ‘A’ and ‘C’

Check of Understanding The Centre of Pressure on an aerofoil is... The consequence or ‘resultant’ of all the aerodynamic forces Where all the forces balance and all lift is said to act. Half way along the chord line Where all the forces balance and all weight is said to act.

Check of Understanding In normal flight, what happens to the speed of the air passing over the top surface of a wing? The speed fluctuates The speed remains constant The speed decreases The speed increases

Check of Understanding When air flows over the wing at an increased speed, what happens to the air pressure? The air pressure fluctuates The air pressure increases The air pressure decreases The air pressure remains constant

Principles of Flight End of Presentation Leading Cadet Training