Effect of height

Table 5.1 was worked out for ground level conditions. What will be the effect of height on the relationship between air speed and angle of attack? The answer, once it is understood, is simple – but very important.

Whatever the height, the air speed indicator reading is determined by the pressure ipV1. In this expression, У is the true air speed. As has already been explained in Chapter 2, when the air speed indicator reads 200 knots at 3000 m, it simply means that the difference in pressure between pitot and static tubes (i. e. IpV2) is the same as when the air speed was 200 knots at ground level. Now, it is not only the pitot pressure tha^depends on ipV2; so do the lift and the drag. Therefore, at the same value of IpV2, i. e. at the same indicated speed, the lift and drag will be the same as at ground level, other things (such as Cj) being equal. Therefore the table remains equally true at all heights, pro­vided the air speed referred to is the indicated speed, and not the true speed. Thus the angle of attack, or the attitude of the aeroplane to the air, is the same in level flight at all heights, provided the indicated air speed remains the same.