At the end of this lesson, you should be able to correctly:
Identify changes to lift and drag resulting from:
angle of attack changes;
The various types of drag, including:
parasite (zero lift), form, interference and skin friction;
induced (lift dependent).
Lift is an aerodynamic force
Lift opposes weight
Lift is a component of Total Reaction Force
Lift acts through the Centre of Pressure
perpendicular (90°) to Relative Air Flow
perpendicular (90°) to lateral axis
What do we have influence over – what don’t we have influence over?
The Coefficient of Lift (CL) is a dimensionless coefficient that relates the lift generated by a ’lifting body’ (read wing) to the fluid density (read air) around the body, the fluid velocity (read relative air speed) and an associated reference area (read wing area).
CL is a function of the angle of the body to the flow, its Reynold number and its Mach number. The lift coefficient cl refers to the dynamic lift characteristics of a two-dimensional foil section, with the reference area replaced by the foil chord.
CL Vs Angle of Attack
Total Reaction Force
Centre of Pressure
The centre of pressure is the point where the total sum of a pressure field acts on a body. In aerospace, this is the point on the aerofoil (or wing) where the resultant vector (of lift and drag) acts.
Span-wise Lift Distribution
Types of Drag
Parasite Drag: Also known as “Zero Lift Drag”.
Made up of:
Skin Friction Drag (Surface Area, Roughness).
Interference Drag (mixing of airflows at airframe junctions)
Induced Drag: Also known as “Lift Dependent Drag”
Drag as a result of the production of Lift
The combination of these 2 types of drag is called TOTAL DRAG.
CD is essentially a function of Lift production. It is defined as the ratio of Drag Pressure to dynamic pressure and S.
It is directly related to Angle of Attack. Drag is directly proportional to α, V² and S.
created by the production of lift by a wing of finite span
visible through wing-tip vortices
Wing Tip Vortices are the resultant of the production of Lift. ie – Induced drag
The Higher the Angle of Attack, the greater the Induced Drag, the larger the Vortices.
Induced Drag also consists of increments of:
These are less significant at low Angles of Attack, and more significant at High Angles of Attack
Induced drag varies directly with pressure differential
More pressure differential = more induced drag
Less pressure differential = less drag
Pressure differential varies directly with angle of attack
Larger angle of attack = more pressure differential
Less angle of attack = less pressure differential
Induced Drag – effect of weight
An aircraft with less mass (weight) will require the wing to produce less lift
If less lift is to be produced, a smaller angle of attack is needed
smaller angle of attack = less pressure differential
less pressure differential = less induced drag
For the opposite reasons, a heavier aircraft will generate more induced drag
Parasite Drag consists of:
A body at rest generates no parasite drag. Parasite drag increases as velocity increases.
Effects of Drag
Skin Friction Drag
Results from the frictional forces existing between an object and the air through which it is moving.
Depends on the following:
Surface Area of the object
Surface Roughness of the object
Velocity at which the object is moving through the air ( V² )
Form Drag is caused when airflow separates from the surface, forming eddies which disturb the Streamline Flow.
Form Drag Depends on the following:
Frontal size and Shape of an object.
Streamlining – which minimizes turbulent flow, which delays separation
Velocity at which the object is moving through the air (V²)
Interference Drag is caused by the mixing, or, “interference” of converging airflow at the junctions of various surfaces. (e.g. Wings to Fuselage)
It is minimized by Blending; Shaping; Fairing (which maintains Laminar Flow) and Streamlining.
It is proportional to the Velocity at which the object is moving through the air ( V² )
Both aircraft flying at 120 kts in S&L flight
Which aircraft has greater cross section?
Which aircraft has the greatest parasite drag?
Total Drag = Induced Drag + Parasite Drag
Vmin drag is speed where least total drag is generated
More induced drag = increased Vmin drag
More parasite drag = increased Vmin drag
Thrust – Drag Couple
The Lift / Drag Ratio
The Ratio of Lift to Drag at any particular Angle of Attack is called the Lift / Drag Ratio
i.e. Lift / Drag
There is a particular Angle of Attack which produces the maximum amount of Lift, for the minimum amount of Drag – The Best Lift / Drag Ratio
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