Landing speeds and the future

So far as landing speed is concerned, we are reaching an interesting stage in the history of aviation. Wing loadings are still going up; they went up slowly but surely for the first thirty years of flight, and rather less slowly, but more surely, during the Second World War – and since. There is at the moment no sign of any halt in this progress – for progress it certainly is. We must assume, therefore, that wing loadings will go still farther.

During this time, slots and flaps, and then better flaps, have been invented, and the maximum value of CL has gone up from just over 1 to about 3, or even 4, for a good modern aerofoil section with slotted flaps and slots extending along 60 per cent of the wing span. When the maximum CL was 1.22 (RAF 15), a wing loading of 500 N/m2 was considered high; but with a CL max of 3, even 5000 N/m2 has already been exceeded. Now a CL max of 1.22 and W/S of 500 gives a landing speed of about 50 knots (93 km/h) whereas a CL max of 3 and W/S of 5000 gives a landing speed of about 102 knots (188 km/h).

So we have accepted a considerably higher landing speed – but how long can this go on? The increase in wing loading has already had a greater effect than the increase in maximum lift coefficient, but so far we have discovered better and better flaps. Now, however, it would seem that there is not much hope of any further great improvement in flaps – so what of the future? The first thing we must do is clear enough – flaps, and slots too, must extend along the whole span of the wing, perhaps also under the fuselage; this has already been done in some types of aircraft, sometimes by arranging that the ailerons act also as flaps, therefore known as ‘flaperons’ or by dispensing with the ailerons altogether and adopting an alternative form of lateral control, such as spoilers – this will be discussed later, or by differential movement of the tail surfaces, thus known as ‘tailerons’.

Such methods might give us another 40 per cent increase in maximum lift coefficient and a landing speed of about 86 knots (158 km/h) for a wing loading of 5000 N/m2. So what are the prospects for the future? Well, there has been no shortage of ideas, and the patent offices of the world contain hundreds of examples, ranging from the good to the impractical or ill-conceived. Amongst the good ideas is that of using high pressure air bled from the engine compressor to help induce attached flow over the flaps (blown flaps) at large deflections, or even to produce a downward curtain of air which can turn the main airflow through extremely large angles: the so-called jet flap. Such devices, though effective, are however complex and heavy, and have generally only been used for specialised aircraft, usually military. An example of an apparently promising idea that never went into general use is the Custer channel wing shown in Fig. 6.8. The idea was quite simple; the propellers drew air over the curved wing sections, thereby allowing the flow to remain attached at high angles of attack. However, there are two main problems with most of such ideas. Firstly the lifting surface also needs to have a good lift to drag ratio at high speed, and secondly, as the minimum speed of the aircraft decreases, it becomes more and more difficult to provide adequate stability and control by conventional means. Other approaches to providing lift at low or even zero speed are dealt with below.

Fig 6.8 The idea of the ‘Custer’ channel wing

The engines – with pusher propellers – were suspended in the channels with the ailerons just outboard of the channels.

Short and vertical landing and take-off

The idea of low landing speeds or even vertical landing was a much sought – after goal even in the earliest days of flying. In fact, until the twentieth century, the only manned flights were made in vertical take-off devices: balloons and airships. Over the years, there has been considerable research into both vertical and short take-off aircraft, and one firm outcome has been the coining of two acronyms, VTOL for Vertical Take-off and handing, and STOL for Short Take-off and handing. To this has been added some hybrids like STOVL, Short Take-off and Vertical handing. In the following sections, some of the more successful or promising types are described.