One way to measure the total power is to look at the rate of fuel consumption. Figure 16 shows the fuel consumption versus gross weight for a large transport airplane traveling at a constant speed (obtained from actual data). Since the speed is constant the change in fuel consumption is due to the change in induced power. The data are fitted by a constant (parasite power) and a term that goes as the load². This second term is just what was predicted in our Newtonian discussion of the effect of load on induced power.

Figure 16. Fuel consumption as a function of weight for large jet at a costant speed

The increase in the angle of attack with increased load has a downside other than just the need for more power. As shown in Figure 12 a wing will eventually stall when the air can no longer follow the upper surface. That is, when the critical angle is reached. Figure 17 shows the angle of attack as a function of airspeed for a fixed load and for a 2-g turn. The angle of attack at which the plane stalls is constant and is not a function of wing loading. The angle of attack increases as the load and the stall speed increases as the square root of the load. Thus, increasing the load in a 2-g turn increases the speed at which the wing will stall by 40%. An increase in altitude will further increase the angle of attack in a 2-g turn. This is why pilots practice "accelerated stalls" which illustrates that an airplane can stall at any speed, since for any speed there is a load that will induce a stall.

Figure 17. Angle of attack versus speed for straight and level flight and for a 2-g turn

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