Lecture Leading Cadet Training Principles of Flight 3 Drag

Every part of the aircraft over which air flows produces drag. This drag directly opposes thrust. The more drag there is, the more thrust is needed to overcome it. DRAGTHRUST Drag Anyone on a bicycle knows the faster you go, the more air resistance you encounter. The force which hinders your progress is called ‘DRAG’.

Drag Drag will be least when parallel to the airflow. What causes Drag ? Drag is formed by the shape of an object. When holding a flat plate in an airflow: Drag will be greatest when the plate is at 90° The object is to reduce this Form Drag.

Drag is formed by the shape of an object. When an object passes through the air it is accompanied by a wake. Wake is the turbulent airflow immediately behind the aircraft. Wake produces drag – the bigger the wake, the bigger the drag. Drag

Surface Friction Drag Is caused by the ‘rough’ skin of an object. Interference Drag Two surfaces meeting at an angle, causes airflow drag. This is minimized by fairing and filleting to induce smooth mixing of air past the affected surfaces. Types of Drag Drag

Lift Dependent Drag Is drag caused by the lift associated with the change in the angle of attack. Most of the drag caused at take-off is lift-dependent. Wingtip Vortices Are tubes of circulating air behind a wing as it generates lift. A wingtip vortex trails from the tip of each wing. Relatively High Pressure Low Pressure VORTEX Types of Drag Drag

Zero Lift Drag Streamlined Shape 5lbs drag Form Drag (The Shape) Flat Plate 100lbs drag Smooth - 4lbs drag Surface Friction Drag (Smooth/Rough) Rough - 6lbs drag Interference Drag (Components) 5lbs drag 11lbs drag Drag

Wingtip Vortices LOW SPEED HIGH SPEED The effects of Speed on Wingtip Vortices. LARGEVORTEX SMALLVORTEX Drag

Wingtip Vortices HIGH ASPECT LOW ASPECT The effects of Aspect Ratio on Wingtip Vortices. LARGEVORTEX SMALLVORTEX CHORD CHORD Drag

How can Drag be minimised ? By not building in drag at the design stage. By eliminating as many protruding edges as possible…. By ‘streamlining’ the design. Drag

How can Drag be minimised ? Drag By designing the shape to have a fineness ratio of between 3:1 and 4:1 The fineness ratio is the ratio of the length of a body to its maximum width; "short and fat" shapes have low ratios, while "long and skinny" have high ratios.

Drag Drag and Airspeed The amount of drag varies with the square of the airspeed - Twice the airspeed = 4 times the drag. 3 times the airspeed = 9 times the drag.  AIRSPEED DRAG 

Drag and Thrust  THRUST DRAG  In un-accelerated straight and level flight: In un-accelerated straight and level flight: The aircraft will maintain speed. Thrust = Drag If thrust is greater than drag If thrust is greater than drag the aircraft will speed up. Thrust > Drag If drag is greater than thrust If drag is greater than thrust the aircraft will slow down. Drag > Thrust Drag

An Aircraft in Balance In straight and level flight, at constant speed, In straight and level flight, at constant speed, two pairs of forces act on the aircraft. The thrust opposes the drag and is equal to it, The lift equally opposes the weight. WEIGHT LIFT DRAGTHRUST Summary

Check of Understanding Which part(s) of an aircraft produce drag? Only those parts producing thrust Only those parts producing lift Every part that air flows over The fuselage but not the wings

Check of Understanding Drag directly opposes what? Thrust Pressure Lift Weight

Check of Understanding Which of these objects of equal width, will produce the least drag? Airflow D A C B

Check of Understanding How can drag best be minimised? By decreasing the airspeed By increasing the airspeed By reducing the weight By streamlining the design

Check of Understanding What type of drag is formed when two surfaces meet at a sharp angle? when two surfaces meet at a sharp angle? Interference Drag Surface Friction Drag Wingtip Vortex Drag Lift Dependant Drag

Check of Understanding When an object passes through the air, what is it accompanied by? Wake Wind Thrust Lift

Check of Understanding What type of drag is dependant upon the rate of change of velocity through the boundary layer? Wing Vortex Drag Interference Drag Surface Friction Drag Lift Dependant Drag

Check of Understanding What is Lift Dependent Drag? Drag caused by the wingtip vortex Drag caused by the shape of the object Drag caused by lift associated with the change in the angle of attack Drag caused by the ‘rough’ skin of an object

Check of Understanding A streamlined shape with an airspeed of 100 kts has a drag of 200 n. If the airspeed is increaded to 300 kts what will the drag be? 3600 n 400 n 1800 n 800 n

Principles of Flight End of Presentation Leading Cadet Training