Downwash and its importance
The trailing vortices are not just a mildly interesting by-product of wing lift. Their influence on the flow extends well beyond their central core, modifying the whole flow pattern. In particular, they alter the flow direction and speed in the vicinity of the wing and tail surfaces. The trailing vortices thus have a strong influence on the lift, drag and handling properties of the aircraft.
Referring to Fig. 2.7, we see that the air behind the wing is drawn downwards. This effect, which is known as downwash, is apparent not only behind the wing, but also influences the approaching air, and the flow over the wing itself. Figure 2.8 shows that the downwash causes the air to be deflected downwards as it flows past the wing.
There are several important consequences of this deflection. Firstly, as we can see from the diagrams, the angle of attack relative to the modified local airstream direction, is reduced. This reduction in effective angle of attack means that less lift will be generated, unless we tilt the wing at a greater angle to compensate.
The second, and more important consequence may be explained by further reference to Fig. 2.8. It will be seen that, since the air flow direction in the
vicinity of the wing is changed, what was previously the lift force vector, is now tilted backwards relative to the flight direction. There is therefore a rearward drag component of this force.
This type of drag force was at one time called induced drag, but the more descriptive term trailing vortex drag is now usually preferred. We shall deal with drag forces in more detail in Chapter 4.
Another consequence of downwash is that the air flow approaching the tailplane is deflected downwards, so that the effective angle of attack of the tailplane is reduced. The downwash depends on the wing circulation and therefore varies with flight conditions.
It is often thought that the downwash is entirely responsible for the lift, by the principle of momentum change. This is not so. What is invariably forgotten is that the trailing vortices also produce a large upwash outboard of the wing tips. The upward momentum change thus produced cancels out the downward momentum change of the downwash. If we sandwich a wing between the walls of a wind-tunnel, so that there are no trailing vortices, air particles behind the wing will return roughly to their original height, and yet the lift is greater than when downwash is present. In calculating lift, it is always necessary to consider forces due to pressure as well as momentum. A detailed discussion of the concepts involved is however beyond the scope of this book.