1.2 GENERATION OF AERODYNAMIC REACTIONS

  GENERATION OF AERODYNAMIC REACTIONS

1.2.1 Aerodynamic reaction due to airflow: Flight forces are the result of aerodynamic reactions. Aerodynamic reaction is the effect of airflow over the aerofoil. In theory of flight, aerodynamics is mainly concerned with three distinct things:

• Aerofoil
• Relative Air Velocity Or Relative Wind
• Atmosphere

When an aircraft flies, air of the atmosphere flows over its wings. Wings are the aero foils. Air velocity over it is the relative wind. Result of the aerodynamic reaction is the force called Total Air Reaction (TAR) is a force. Lift and Drag are two components of TAR. Generations of these forces are related to the aerodynamic characteristic of the aerofoil in relation to the relative airflow over and under it.

Before explaining production of Lift and Drag forces due to aerodynamic reactions on an aerofoil, effect airflow over a simple flat plate will be explained first. Then, a curbed plate will be taken for better effect and finally aerofoil will be taken as a practical aerodynamic surface.

1.2.2 Effect of airflow Flow over a flat plate: When a body is moved through the air, or any fluid that has viscosity, such as water, there is a resistance produced which tends to oppose the body. For example, if you are driving in an open top car, there is a resistance from the air acting in the opposite direction to the motion of the car. This air resistance can be felt on your face or hands as you travel. In the aeronautical world, this air resistance is known as drag. It is undesirable for obvious reasons. For example, aircraft engine power is required to overcome this air resistance and unwanted heat is generated by friction as the air flows over the aircraft hull during flight.

We consider the effect of air resistance by studying the behaviour of airflow over a flat plate. If a flat plate is placed edge on to the relative airflow (Figure 1.1), then there is little or no alteration to the smooth passage of air over it. On the other hand, if the plate is offered into the airflow at some angle of inclination to it angle of attack (AOA), it will experience a reaction that tends to both lift it and drag it back. This is the same effect that you can feel on your hand when placed into the airflow as you are travelling, e.g. in the open topped car mentioned earlier. The amount of reaction depends upon the speed and AOA between the flat plate and relative airflow.

Figure 1.1: Airflow over a flat plate 

As can be seen in Figure 1.1, when the flat plate is inclined at some AOA to the relative airflow, the streamlines are disturbed. An upwash is created at the front edge of the plate causing the air to flow through a more constricted area, in a similar manner to flow through the throat of a Venturimeter. The net result is that as the airflows through this restricted area, it speeds up. This in turn causes a drop in static pressure above the plate (as explained in the Bernoulli’s principle) when compared with the static pressure beneath it resulting in a net upward reaction. After passing the plate, there is a resulting downwash of the air stream.

The total reaction on the plate caused by it disturbing the relative airflow has two vector components as shown in Figure 1.2. One at right angles to the relative airflow known as lift and the other parallel to the relative airflow, opposing the motion, known as drag.

The above drag force is the same as that mentioned earlier, which caused a resistance to the flow of the air stream, over your hand.

Figure 1.2: Nature of reaction of relative airflow on flat plate

The effect may be summarized as follows: if a flat plate is inclined in a moving stream of air, the air flowing over the upper surface decreases in pressure. This creates a depression over the upper surface which produces a sucking effect on the plate. At the same time, the higher pressure on the underside of the plate produces an upward force.

Lift is the force which overcomes the weight of the aircraft which acts vertically downwards. We are interested in how much of the lifting force is acting vertically upwards. This is done by splitting the total reaction into two component forces, one vertical and one horizontal. The horizontal force adds to the total drag on the aircraft and is referred to as lift induced drag.

1.2.3 Production of lift on a curbed plate:

An improvement on the flat plate is one with a curved front. This tends to produce a smoother flow of air over the upper surface and produces a total reaction, which is nearer to the vertical. This gives higher lift and lower drag. See Figure 1.3.

The airflow across a curved plate remains smooth at higher angle attack than the flat plate.

1.2.4 Lift on an aerofoil: Flat and curved plates have little strength and resistance to bending and torsion and are not suitable for aircraft wings. The modern aerofoil required depth of section to resist bending and must form a box structure to resist torsion whilst still retaining the basic curved plate shape.

Flow maintains the shape of the body over which it is flowing over the aerofoil set at an AOA much greater than that could be set for a flat of curbed plate. As a result, a greater lift force may be created keeping drag to a minimum value. As is illustrated in Figure 1.4, stream line flow is maintained over the surface of an aerofoil where it can be seen that the successive cross sections are represented by lines that run parallel to one another hugging the shape of the body around which the fluid is flowing. Aerofoil is further discussed in subsequent sections.