An extract from A View from the Hover

The following is a short extract from John Farley's new book, A View from the Hover. In it John begins to explain the theory behind stalling, often a contentious issue for pilots. However, as you'll see, John manages to make the subject easy to read and understandable on many levels.

In A View from the Hover, John also explains spinning and the theory behind aerodynamics, as well as telling about his many years of involvement in test flying and offering advice and observations for private pilots

I think all pilots have definite ideas about stalling. The extent and form of these ideas will naturally depend on their experience and background.

So far as a pre-solo PPL student is concerned, these ideas may well be limited to a determination to stay faster than something called the 'stalling speed' unless told by their instructor to do otherwise and only then at a safe height and after some special cockpit checks that they have yet to commit to memory.

On the other hand, an experienced pilot going out to do a C of A flight test on somebody else's aeroplane and, noting some hangar rash on the leading edge of one of the wings, may well take a mental punt that when the aircraft stalls the damaged wing may drop first.

If I were to have a face to face chat with either of these pilots and the subject of stalling came up, I would naturally say quite different things to each of them and, even more importantly, would further adjust how I banged on about my own ideas depending on how I saw they were being received. Sadly, such interaction is not possible here. Therefore, please be patient in reading this because it is extremely unlikely that I shall pitch it just right for you!

One really needs to have a satisfactory grasp of the basics of lift to appreciate what is going on with stalling and spinning. While I admit I said in Chapter 1 that all the chapters in this book were intended to be stand alone, there is necessarily a degree of overlap with some topics. To avoid repeating myself, please may I ask those who may not be totally happy about the relationship between the lift coefficient (CL) and the angle of attack (alpha) to take a look at the Lift part of Chapter 10. For convenience though I will repeat the CL/alpha curve here to save you having to turn back.

How do we define 'the stall'? This is not actually an easy question to answer. Indeed it is one that had BAe and McDonnell Douglas (as they were then) rushing to their lawyers in the late 80s and early 90s to decide who was to blame for a possible contractual liability regarding the stall of the T-45 Goshawk trainer for the USN. Clearly in some circumstances stalling can become a very complex issue indeed. That is the bad news.

The good news is that it is not in the least a complex matter for a pilot to fly around avoiding accidental stalls and also to thoroughly enjoy the experience of deliberate stalls. That is what I want to write about here.

The normal actions to control lift no longer work when an aircraft stalls so pilots lose control of the flight path and have to take stall recovery action.

This raises the question of just how do pilots normally control lift? The answer of course is that they do it by pulling and pushing on the controls which varies the lift coefficient of the wing by varying the alpha - and nothing else. If you are not happy with this statement may I once again refer you to Chapter 10.

As the illustration shows, there is a straight line relationship between CL and alpha for most of the curve. This is where we are when flying clear of the stall and enjoying a linear response. That is to say moving the stick twice as far produces twice the response. Such a linear relationship is something humans prefer for any control task.

I have deliberately not put any values at the end of each axis of the illustration as they are very type dependent. Many aircraft would be pushed to achieve a maximum lift coefficient of 2 while a few might reach 3 with full high lift devices deployed. Equally some wings will stop their linear increase of lift by 12 degrees alpha, while others may hang in there to 20 degrees or more. It all depends on the wing design, both the aerofoil section and the planform shape. The essence of what Fig 1 shows is that, if you pull back and keep increasing the alpha to the right of where the line stops going up, you have stalled, although not every company lawyer might agree!

All of which is a very long-winded way of getting to the vital point that aeroplanes don't stall because of the speed at which they are flying but because of the alpha that the pilot is using. Believing and living your life by that statement is the secret of success when deliberately stalling as well as enjoying the experience as we shall see later when discussing some airborne techniques.

To read the rest of this fascinating book, click here