The output of a jet engine declines, broadly speaking, in proportion to the density of the air in which it operates. You can’t turbocharge a jet engine because the engine itself is fundamentally a self-propelled turbocharger. On the other hand, at high speeds, a jet profits from ram effect, which its compressor compounds, while lower ambient density reduces resistance to the outflow from the exhaust nozzle. Jets therefore operate more efficiently—that is, require less fuel per unit of thrust—at high speed and altitude than they do when flying low and slow. //

The situation of the propeller is different. The engine driving the propeller can be turbocharged and may maintain sea-level power up to a high altitude. But as the density of the air diminishes, the propeller blades have less to work with. Once they are at their optimal angle of attack, the only way to maintain thrust against increasingly rarefied air is to increase the area, number or speed of the propeller blades.

At the same time, as the airplane takes advantage of thinner air by increasing its true airspeed—which is the main reason for wanting to fly high in the first place—the pitch angle of the propeller blades, governed by both their own rotational velocity and the forward speed of the airplane, must increase. As blade pitch increases, the lift vector of each blade points farther and farther away from the thrust axis. This difficulty might be alleviated if it were actually possible to spin the propeller faster, but in fact, the faster the airplane goes, the slower the propeller must turn to keep its tip speeds away from the transonic drag rise. The efficiency of a given propeller therefore decreases as speed and altitude increase.