Thrust and momentum

The propeller, the jet, and indeed all conventional aircraft propulsion systems involve changes in momentum of the air. When a change of momentum occurs, there must be a corresponding force, but it should not be thought that thrust is caused directly by the change of momentum, with no other mechanism being involved. As we have seen in the above examples, the force is produced and transmitted to the structure by pressure differences acting across the various sur­faces of the device. It is perhaps best not to think of rate of momentum change and force as cause and effect, but as two consequences of one process. In mak­ing practical measurements, or even theoretical estimates, we normally have to consider a combination of pressure-related forces and momentum changes.

Comparison between jet and propeller for thrust production

Figure 6.3 shows a jet aircraft and a propeller-driven one producing equal amounts of thrust at zero forward speed. In the case illustrated, the jet engine is transferring energy to the slipstream or jet five times as fast as the propeller. Since this energy must ultimately have come from the fuel, it indicates that the propeller-driven aircraft is producing the thrust more economically.

When the aircraft are in motion, the jet engine will still transfer energy to the air at a faster rate than the propeller at any given thrust and forward speed, but the difference in energy transfer rate becomes less marked as the speed increases.