01.06 Lift & Drag
Aim
At the end of this lesson, you should be able to correctly:
- Identify changes to lift and drag resulting from:
- airspeed changes;
- angle of attack changes;
- The various types of drag, including:
- parasite (zero lift), form, interference and skin friction;
- induced (lift dependent).
Lift
- Lift is an aerodynamic force
- Lift opposes weight
- Lift is a component of Total Reaction Force
- Lift acts through the Centre of Pressure
- Lift operates:
- 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
Drag
- Types of Drag
- Parasite Drag: Also known as “Zero Lift Drag”.
- Made up of:
- Skin Friction Drag (Surface Area, Roughness).
- Form Drag
- Interference Drag (mixing of airflows at airframe junctions)
- Made up of:
- 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.
Induced Drag
- 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:
- Form Drag
- Surface Friction
- Interference Drag.
- 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
Parasite Drag consists of:
- Surface Friction
- Form Drag
- Interference Drag
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
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
- 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 )
Cross-section size
- 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
- 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