Principles of Flight-4

Principles of Flight-4

Stalling This diagram has been shown before. It is the graph of co-efficient of lift and angle of attack. From it you can determine CL at any particular angle of attack. Notice the line starts straight, showing CL increases proportionally with angle of attack, until it curves and bends down again, which means that beyond a certain point any increase in angle of attackno longer increases lift—this is called the stalling angle. Principles of Flight-4 CL CL max 1.0 5° 0 ° 5 ° 10 ° 15 ° 20 ° Stalling angle

The stalling angle is that angle of attack at which the lift of an aerofoil is at a maximum and beyond which will begin to decrease owing to the air becoming turbulent instead of streamlined. The diagram illustrates the breakdown of the airflow over the upper surface of the wing. Notice that not only is the airflow turbulent but even tends to reverse in places. Stalling occurs when the airflow breaks down. Principles of Flight--4 The stalling angle

Characteristics Stalling characteristics may vary. The wing with gentle stalling characteristics may have a CL curve like the upper diagram, if the stall is more pronounced, then the reduction in lift as the stall has been reached would be more pronounced as in the lower diagram. The wing section chosen will to some extent determine the stalling characteristics. One important point is the nose drops despite the pilots attempts to restrain it. Principles of Flight--4 CL CL

With changes in angle of attack there are pressure distribution changes. We have already seen how the centre of pressure moves, but only for angles of attack below the stalling angle. In fact beyond the stalling angle the C of P moves aft again. These two diagrams show the pressure distribution before and after the stall. This aft movement of the C of P is the reason for the nose drop. Principles of Flight--4 Distribution changes CP 15° CP 20°

Here we have seen the forces acting on the glider. Not the simple case where TR and W act at the same place . Because they don’t there must be a lifting force on the tail plane to keep the forces in equilibrium. Principles of Flight--4 Why the nose drops

When the glider stalls, the TR moves aft and reduces in value. The lower diagram illustrates the movement and if you think about it, the nose must pitch down. The stalling characteristics of a wing can be modified by introducing a twist in the wing. Principles of Flight--4

This twist is called washout, and is illustrated in the upper diagram in which the relationship of the wing root and the wing tip sections are compared. For any given angle of airflow, the wing tip will meet it at a smaller angle of attack than the wing root. The graph should clarify this: with the wing root at an angle of attack of 12° the tip is only at an angle of 6°, assuming that is, that the wing has 6° of washout. CL root tip Principles of Flight--4 Root section Tip section

The significance of that is if we increase the angle of attack to the wing root exceeds the stalling angle (upper diagram) , then the wing tip will not be stalled, and not only will the stall be more gentle but there will be less tendency to drop a wing. At high speed (lower diagram), washout has a different effect. The wing tip generates less lift than the wing root and if the angle of attack is increased sufficiently, the tip will cease to produce lift at all. A glider may, when it stalls, drop a wing . If it does, the situation will be made worse if ailerons are used to correct the wing drop. Principles of Flight--4 tip CL root CL root tip

This diagram shows three wing sections with ailerons up, neutral and down. Consider the lower diagram. If this wing is dropping in the stall, then the use of ailerons to pick that wing up will only increase the angle of attack still further and the wing will be even more stalled. On the upgoing wing, the aileron will help to either unstall or make the wing less stalled. Principles of Flight--4 Ailerons at the stall Up-going wing Angle of attack decreased Down-going wing Angle of attack increased

Principles of Flight--4

Principles of Flight-4