Lecture Leading Cadet Training Principles of Flight 2 Lift and Thrust

Lift Free Stream Flow Leading Edge Trailing Edge Chord Line Angle of Attack α In most flight conditions, the wing is inclined to the airflow at a slight positive angle (The Angle of Attack) This is the angle between the Chord Line, and the oncoming air or Free Stream Flow. and the oncoming air or Free Stream Flow.

Resulting in an upward force which adds to the lift needed to hold up the aircraft in flight. This deflects some of the lower airflow downwards. Lift

How Lift Varies We know that pressure difference between the air above and below the wing, tends to lift the wing up. But lift is not evenly distributed over the wing.

How Lift Varies All lift forces act at 90 o to the airflow, but the top of the wing generates more lift than the bottom. The greatest amount occurs where the surface is most curved, towards the leading edge. Oncoming Air

How Lift VariesAirspeed Altering the airspeed will vary the amount of lift. Lift increases at the square of the speed. Double the airspeed gives four times the lift; Treble the airspeed gives nine times the lift. Airspeed Lift

How Lift Varies Lift Air Density If the air becomes ‘thinner’ or less dense, the amount of lift is reduced. Consequently the higher you go, the less the lift. Air Density

+ + _ + + _ + + _ How Lift Varies Angle of Attack and the Pressure Envelope Free Stream Flow Angle of Attack α 0o0o 5o5o 10 o 15 o + + _ The Angle of Attack alters the Pressure Envelope. As the Angle of Attack increases, the pressure above decreases, and the wing produces more LIFT.

How Lift Varies Wing Shape General PurposeHigh Lift High Speed

How Lift Varies Wing Shape and Area Low aspect ratio 2.4:1 High aspect ratio 16:1 ASPECT RATIO The ratio of : wing span mean chord or wing span 2 wing area

When all the ‘variables’ are put together we can derive a formula for lift: Lift = CL ½ρ V 2 S CL = Coefficient of Lift (the ratio between lift and dynamic pressure). ρ = Density (rho) V = True Airspeed S = Surface Area Lift Formula Coefficient of Lift CL Angle of Attack α o -5 o 0 o 5 o 10 o 15 o 20 o 25 o 30 o Maximum CL Stalling Angle

Besides the forces of lift and weight, which act on an aircraft in flight, there are other forces to consider. Initially, how to propel the aircraft, and for this we need – Thrust Thrust Thrust

To produce lift, a wing needs airflow over the top of it ….. This airflow is increased by the engine This airflow is increased by the engine ‘thrusting’ the aircraft forwards by “pushing” the air backwards Thrust

The aircraft engine thrusts air backwards, by either: by either: A propeller - which ‘screws’ the air backwards … or A jet engine – expelling air backwards Thrust

In both cases, throwing the air backwards, thrusts the aircraft forward. Thrust

Check of Understanding Where is the greatest amount of lift normally generated on an aerofoil? The Trailing Edge The Top Surface The Leading Edge The Bottom Surface

Check of Understanding If the air density in an airflow is reduced, and all other factors are unchanged, What happens to the lift generated by the wing in that airflow? The lift becomes unpredictable The lift is reduced The lift is increased The lift stays the same

Check of Understanding If the airspeed is increased, what happens to the lift? It fluctuates It decreases It increases It remains constant

Check of Understanding If you double the airspeed, the lift would increase by a factor of what? Eight (8) Two (2) Six (6) Four (4)

Check of Understanding Which of these wing sections is for general purpose? D A C B

Check of Understanding What is the force called that drives an aircraft forwards? Drag Thrust Weight Lift

What name is given to the term if the wing is inclined to the airflow at a slight positive angle? The Angle of Attack The Free Stream Flow Angle The Chord Line Angle The Angle of Pressure Check of Understanding

What is meant by the term Angle of Attack? The angle the wing is swept-back The angle between the chord line of the wing and the oncoming air The angle at which the wing is fixed to the fuselage The angle between the chord line of the wing and the leading edge Check of Understanding

Principles of Flight End of Presentation Leading Cadet Training