Flight at transonic speeds – the pilot’s point of view

We have so far discussed the problems of approaching the speed of sound very much from the point of view of the designer – but what about the pilot? Well, to find out what is going on, or what is likely to happen, the first thing needed is an instrument to measure at what Mach Number an aircraft is flying. Various types of machmeter are already in existence, and no doubt they will be improved in accuracy and reliability. For a machmeter to give a reliable indication of the Mach Number it must measure, in effect, the true speed of the aircraft and the true speed of sound for the actual temperature of the air. The first is usually done via the indicated speed, which can be corrected for air density by a compensating device within the instrument, but which still includes position error. A temperature compensating device can be used to give the true speed of sound, but in modem instruments this has been eliminated, and a combination of aneroid barometer and air speed indicator gives all the correction required – except that of position error. The term ‘Indicated Mach Number’ is sometimes used for the reading of the machmeter, but it is an unsatisfactory term since it differs from indicated air speed in that the main correction, that of density, has already been made in the instrument itself.

In the absence of a machmeter the pilot will find that the air speed indicator is apt to give very misleading ideas – even more so than usual. This assumes, of course, that the pilot is not one of those who have already discovered that what the air speed indicator reads is not an air speed at all! Without such knowledge, a pilot may reason that if the speed of sound is around 340 m/s it is impossible to run into trouble with say 103 m/s on the clock; a shock stall at this speed might come as a real shock.

Yet such a shock is possible because –

In the first place, the speed of sound is a real speed, a true speed.

Secondly, it decreases with fall of temperature, down to 295 m/s, or less, in the stratosphere.

Thirdly, the speed as indicated is not the true speed, and the error is more than 100 per cent in the stratosphere, so that at a real speed of 340 m/s at say 40 000 ft, the indicator will read less than 154 m/s.

Fourthly, trouble begins not at the speed of sound but at the critical Mach Number, and if this is 0.7 (a low value but by no means unknown), a shock stall may occur at 0.7 X 154, i. e. 108 m/s on the clock.

Fifthly – and a point not so far mentioned – a shock stall occurs at an even lower critical Mach Number during manoeuvres, so that in a turn the 108 might be reduced to less than 103.

And there we are!

The figures are all possible, they might even be worse.

There is one compensation; the pitot head is one of the first parts to experience the effects of compressibility, which may cause the air speed indicator to over-read at very high speeds – but this is usually allowed for in calibration.

Table 11.4 Air speed indicator readings and truespeed Height True speed True speed at which Reading of ASI of sound shock stall will occur at this speed _____________ assuming M 0.7_________________________________ feet metres knots m/s knots m/s 0 0 661 340 463 463 238 10 000 3048 640 329 448 385 198 20 000 6096 614 316 430 315 162 30 000 9144 589 303 412 253 130 40 000 12192 573 295 401 200 103 50 000 15 240 573 295 401 156 80

Table 11.4 may be a help to a pilot in realising what is going on; the figures are calculated on the assumption of the International Standard Atmosphere and will vary to some extent according to how much actual conditions differ from this.

The figures in the last column are the readings of the air speed indicator at which a shock stall may occur (it may even occur at lower indicated speeds because the figures given do not allow for manoeuvres), and they are apt to be rather alarming. There is, however, another way of looking at it – and one that is much more heartening.

Suppose one dives from 50 000 ft at a constant true speed of, say, 450 knots; that is at a rapidly increasing speed on the clock of –

176 knots at 50 000 ft,

225 knots at 40 000 ft,

275 knots at 30 000 ft,

328 knots at 20 000 ft,

387 knots at 10 000 ft,

and 450 knots at sea-level,

strange as it may seem, the actual Mach Numbers would be decreasing as follows –

450/570 or 0.77 at 50 000 ft,

450/570 or 0.77 at 40 000 ft,

450/590 or 0.76 at 30 000 ft,

450/616 or 0.73 at 20 000 ft,

450/640 or 0.70 at 10 000 ft,

450/666 or 0.68 at sea-level.

This means that if the critical Mach Number were 0.7 an aircraft that was shocked stalled at 50 000 ft would become unstalled at a height of about 10 000 ft.

Even if the true speed were to increase during the dive, as would probably happen in practice, there might still be a drop in Mach Number.

This consoling feature of the problem is based on the assumption of rise in temperature with loss of height – if the temperature does not rise, that is to say, if there is an inversion, well the reader – and the pilot – can calculate what will happen!