Stalling speed

Much of what has been said applies not only to level flight, but to stalls when gliding, climbing or turning; for instance, when banking on a turn the lift on the wings must be greater than the weight, and therefore the stalling speed is higher than when landing. Also at height the air density p will be less, and this means that in order to keep CL. тpV. S equal to the weight, the stalling speed V will be greater than at ground level. Fortunately the air speed indicator, which is in itself worked by the effect of the air density, will record the same speed when the aeroplane stalls as it did at ground level; in other words, the indicated stalling speeds will remain the same at all heights.

But on high airfields, such as are found in mountainous countries, the true landing speed of an aeroplane will be appreciably higher than on sea-level air­fields; and in tropical countries the air density is decreased owing to the high temperatures, and the true landing speed is consequently increased. The taking-off speed, and the run required, are also increased in both these instances, and this is perhaps an even more important consideration.

When stalling intentionally the aeroplane is pulled into a steeply climbing attitude and the air speed allowed to drop to practically nil until the nose sud­denly drops or, as frequently happens, one wing drops and the aeroplane commences to dive or spin.

Before leaving the subject of stalling it might be as well to mention that there has always been difficulty in deciding upon an exact definition of stalling or stalling speed. The stall occurs because the smooth airflow over the wing becomes separated – but this is a gradual process. At quite small angles of attack there is some turbulence near the trailing edge; as the angle increases, the turbulence spreads forward. What is even more important is that it also spreads spanwise, usually from tip to root on highly-tapered wings, and from root to tip on rectangular wings. If we define the stall as being the break up of the airflow, when did it occur? There may be buffeting of the tail plane or main planes, but this too may be slight and unimportant, or it may be violent. As a result of the change from smooth to turbulent airflow the curve of lift coeffi­cient reaches a maximum and then starts to fall. We defined the stalling angle in Chapter 3 as the angle at which the lift coefficient is a maximum. But how does the pilot know that it is at its maximum value? All the pilot knows is that if he tried to fly below a certain speed he gets into difficulties. How great the difficulties depends on the type of aircraft, and the extent to which the pilot can overcome them depends on a lot of things, but particularly on his own skill.

In fact, there are different definitions of stalling according to the point of view of the person who wishes to define it – the pilot looks at it one way, the aerodynamicist another, and so on. What is important is that each should realise that it is his own definition, and that all these things do not necessarily occur at the same time.