High lift aerofoils

What, then, has been done, and what can yet be done to decrease minimum speed? If У is to be small, CL must be as large as possible. In other words, we must have a larger lift coefficient. So the aerofoils which give the largest maximum lift coefficient will give the lowest minimum speeds. Unfortunately, however, these aerofoils are usually those with a large drag, and so they seriously affect the high speed end of the range. Therefore we must turn to some device by which the shape of the aerofoil can be altered during flight, and so we naturally think of flaps and slots.

In an earlier chapter (Fig. 3.32) we noticed the effect of various kinds of flaps and slots on maximum lift and speed range. The idea of variable camber is an old one, but it is only in recent years, when maximum speeds have increased so much, that the problem has become really urgent and these devices have come into their own. In this respect necessity has proved to be the mother of invention, and many and varied have been the devices which have been tried. It is not easy to compare the respective merits of all these types of slots and flaps, or combinations of slot and flap, because so many conflicting qualities are required. If a low speed was our only aim, the problem would be comparatively simple, the device giving the highest maximum lift coefficient being the most suitable. But what we really need is a low minimum speed and a high maximum speed, i. e. a good speed range. This condition means that the device must be such that it can be altered, or will alter automatically, from the position giving maximum lift (e. g. slot open or flap down), to the position of minimum drag (e. g. slot closed or flap neutral). Even that is not the end of our requirements for, having landed as slowly as possible, we must pull up quickly after landing. The former (slow landing) needs high lift, the latter (quick pull – up) needs much drag, lift being of no consequence at all. For a quick pull-up we really need a definite air brake which will assist the wheel brakes. Notice, however, that an air brake cannot reduce actual landing speed, it can only improve the pull-up after landing. Once we are on the ground we want to get rid of lift as quickly as possible to achieve minimum wheel brake effectiveness; hence the use of lift dumpers after touchdown. These are usually devices which disrupt the flow over the top of the wing, increasing drag, and decreasing lift.

Yet another aspect of the problem, so far as landing is concerned, is the ques­tion of attitude, and in this respect some of the otherwise most effective types of slots and flaps are at a disadvantage, for they attain their maximum lift coefficient at a greater angle of attack than the ordinary aerofoil; this means that in order to make full use of them the angle of attack when landing may need to be 25°, or even more. But when an aeroplane with a tail-wheel type of undercarriage rests with its main and tail wheels on the ground, the angle of inclination of the wings is only about 15°. With a nose-wheel type of undercarriage the problem is if any­thing, worse – as the reader will no doubt realise – for, in order to land at an angle of 25°, we are faced with four possibilities all of which have serious drawbacks – [6] [7] [8] [9]

(d) Large riggers’ angle of incidence

Fig 6.7 Difficulties of landing at large angles of attack

maximum lift and still maintain a reasonable attitude for landing. Improvements in design on these lines have resulted in a real, and by no means negligible, reduction in landing speed (or perhaps, more correctly, has halted further increases in landing speed) but at the expense of ever more sophistication – and ever more complication.