Maximum and minimum speeds of horizontal flight

From the combination of the two curves (Fig. 7.2) some interesting deductions can be made. Wherever the power available curve is above the power required curve, level flight is possible, whereas both to the left and right of the two intersections it becomes impossible for the rather obvious reason that we would require more power than we have available! Therefore the intersection A shows the least possible speed (51 m/s, as we had discovered before for other reasons), and the intersection В the greatest possible speed (115 m/s), at which level flight can be maintained. Between the points A and В the difference between the power available and the power required at any particular speed,

i. e. the distance between the two curves, represents the amount of extra power which can be used for climbing purposes at that speed, and where the distance between the two curves is greatest, i. e. at CD, the rate of climb will be a maximum, while the corresponding point E shows that the best speed for climbing is 77 m/s. From the weight of the aeroplane 50 kN, and the extra power CD (680-320, i. e. 360 kW) available for climbing, we can deduce the vertical rate of climb, for if this is xm/s, then the work done per second in lifting 50 kN is 50 000 x watts,

So 50 000 x = 360 000

and x = 360 000/50 000 = 7.2 m/s

This represents the best rate of climb for this particular aeroplane, but it will only be attained if the pilot maintains the right speed of 77 m/s. As in gliding, there is a natural tendency to try to get a better climb by holding the nose up higher but, as will be seen from the curves, if the speed is reduced to 62 m/s only about 250 kN will be available for climbing, and this will reduce the rate of climb to 250 000/50 000, i. e. 5 m/s. Similarly, at speeds above 77 m/s the rate of climb will decrease, although it will be noticed that between certain speeds the curves run roughly parallel to each other and there is very little change in the rate of climb between 72 and 88 m/s; obviously at 51 m/s and again at 115 m/s, the rate of climb is reduced to nil, while below 51 and above 115 m/s the aeroplane will lose height.

As a matter of interest, the speeds for maximum endurance (F), 64 m/s, and maximum range (G), 82m/s, have also been marked. As was explained in Chapter 5, these are the best speeds from the point of view of the aeroplane, but they may have to be modified to suit engine conditions. Note that the speed for maximum endurance (F) could be deduced from the curve, since it is the lowest point on the curve, i. e. the point of minimum power required for level flight. The speed for maximum range (G), however, must be obtained from Table 5.2, which showed the drag at various speeds.