Aircraft Aerodynamics Airflow types

Steady Streamline Flow

The flow parameters (eg speed, direction, pressure etc) may vary from point to point in the flow but, at any point, are constant with respect to time.  This flow can be represented by streamlines and is the type of flow which it is hoped will be found over the various components of an aircraft.  Steady streamline flow may be divided into two types:

Classical Linear Flow.  The flow found over a conventional aerofoil at low incidence in which the streamlines all more or less follow the contour of the body and there is no separation of the flow from the surface.

Controlled Separated Flow or Leading Edge Vortex Flow.  This is a half-way stage between steady streamline flow and unsteady flow described later.  Due to boundary layer effects, generally at a sharp leading edge, the flow separates from the surface;  the flow does not then break down into a turbulent chaotic condition but, instead, forms a strong vortex which, because of its stability and predictability, can be controlled and made to give a useful lift force.  Such flows are found in swept and delta planforms particularly at the higher incidences.

Unsteady Flow

 In this type of flow the flow parameters vary with time and the flow cannot be represented by streamlines.

Two-Dimensional Flow

If a wing is of infinite span, or, if it completely spans a wind tunnel from wall to wall, then each section of the wing will have exactly the same flow pattern round it except near the tunnel walls.  This type of flow is called two-dimensional flow since the motion is confined to a plane parallel to the free stream direction.

As the air flows round the aircraft its speed changes.  In subsonic flow a reduction in the velocity of the streamline flow is indicated by an increased spacing of the streamlines whilst increasing velocity is indicated by decreased spacing of the streamlines.  Associated with the velocity changes there will be corresponding pressure changes.

As the air flows towards an aerofoil it will be turned towards the low pressure (partial vacuum) at the upper surface;  this is termed ‘upwash’.  After passing over the aerofoil the airflow returns to its original position and state;  this is termed ‘downwash’ as shown.  The reason for the pressure and velocity changes around an aerofoil is explained in later paragraphs.  The differences in pressure between the upper and lower surfaces of an aerofoil are usually expressed as relative pressures by ‘-‘ and ‘+’.  However, the pressure above is usually a lot lower than ambient pressure and the pressure below is usually slightly lower than ambient pressure (except at high angles of attack), ie. both negative.

Three-Dimensional Flow

The wing on an aircraft has a finite length (ie a wing tip) and, therefore, whenever it is producing lift the pressure differential tries to equalise around the wing tip.  This induces a span-wise drift of the air flowing over the wing, inwards on the upper surface and outwards on the lower surface, producing a three-dimensional flow.

Because the effect of the spilling at the wing tip is progressively less pronounced from tip to root, then the amount of transverse flow reduces towards the fuselage.  As the upper and lower airflows meet at the trailing edge they form vortices, small at the wing root and larger towards the tip.  These form one large vortex in the vicinity of the wing tip, rotating clockwise on the port wing and anti-clockwise on the starboard wing;  viewed from the rear.  Tip spillage means that an aircraft wing can never produce the same amount of lift as an infinite span wing.  If the wing has a constant section and angle of incidence from root to tip then the lift per unit span of the wing may be considered to be virtually constant until about 1.2 chord distance of the wing tip.

The overall size of the vortex at the trailing edge will depend on the amount of the transverse flow.  Therefore, the greater the force (pressure difference) the larger it will be.  The familiar pictures of wing-tip vortices showing them as thin white streaks, only show the low pressure central core and it should be appreciated that the influence on the airflow behind the trailing edge is considerable.