Drag and Lift Elliott

Basics This aeroplane is flying at a constant speed at a constant height There are two pairs of opposing forces: Weight due to gravity opposed by the lift from the wings; Thrust from the engine opposed by the drag from the air.

Drag Drag is a result of collisions of air molecules on the body of the aeroplane.  The faster the aeroplane flies, the more frequent the collisions are, and the greater the change of momentum. 

How can drag be reduced? (2 marks) Check Your Progress How can drag be reduced? (2 marks)

Answer By streamlining, to avoid large flat areas  perpendicular to the direction of travel.

You don’t need to know this equation Fluids Air is a fluid, which is a material that can adopt the shape of a container.  In other words it is any liquid or gas. Drag depends on these factors: The cross-sectional area facing the direction of movement; The density of the fluid; The square of the speed; The drag coefficient. You don’t need to know this equation

Drag Coefficient The drag coefficient (CD) depends on the shape of the object and a factor called the Reynolds number. 

Fluid Flow and Aeroplanes The collision of molecules on surfaces in the right place gives rise to lift.  Wings on aircraft are specially designed to use the Bernoulli effect.  Air moves fast around the top of the wing and more slowly around the bottom.  This causes a pressure difference.

Calculating Wing Pressure Atmospheric pressure is measured in bar (b). 1 bar = 1000 mb = 1.013 × 105 N m-2

Check Your Progress Calculate the pressure exerted on the wings of a light aircraft of mass 1100 kg if the wings have an area of 16 m2.  How does this compare with atmospheric pressure which is 1.013 × 105 Pa?

Answer

Increasing Lift Lift can be increased by increasing the angle of the wing, or by altering the shape of the wing by lowering a flap.  Aircraft coming in to land have their flaps lowered: It increases the lift at low speeds; It also increases the drag, helping to slow the plane down.

Take Off - Stalling Aircraft taking off need to rotate, which means that they are at an angle to the ground.  This gives a higher angle of attack, leading to more lift.   However, if the angle of attack is too high, or the speed is too low, the airflow breaks up, leading to a stall.  The plane will no longer fly and falls.  At height, this can be easily recovered (and is a standard exercise with novice pilots).  Near the ground it can be disastrous.

Check Your Progress How has the pilot increased the drag on this plane? Use your understanding of kinetic energy to explain why.

Answer The pilot has deployed the air-brake… …and lowered the flaps. Kinetic energy µ speed2 Double the landing speed, you have to dissipate 4 times the energy.

Power and Speed

Check Your Progress By considering the two relationships above, suggest how the power is related to the speed in an aircraft.

Answer Power = force × speed Drag force is proportional to speed2.  Therefore power is proportional to speed3.