Propellers: Propellers accelerate a large amount of fluid (large ) a small amount (small Ve - V0) in order to generate thrust. A small Ve - V0 gives good propulsive efficiency (extends range and endurance), but limits the top speed of the vehicle.

Jets: Using nozzles instead of propeller blades, turbojet engines are able to accelerate the flow to much higher velocities, giving large thrust and high top speeds, but propulsive efficiency suffers, especially at lower speeds.

These two propulsion approaches indicate a clear tradeoff between speed and propulsive efficiency. A wide variety of mixed or hybrid variations of the two have been introduced to span the range of possibilities in between and provide the right solution for a given application. The general tradeoff one may expect between speed and propulsive efficiency for different engine types is illustrated in the scale below.

Thermal efficiency: In addition to propulsive efficiency, thermal efficiency also plays an important role in vehicle range and endurance because it determines how efficiently the engine converts fuel stores into mechanical output used to generate thrust. For propeller-based propulsion systems, thermal efficiency is determined entirely by the engine driving the propeller (see Principles section). For internal combustion engines, thermal efficiency is controlled primarily by the engine compression ratio as indicated in the description of propeller propulsion. Typically gasoline engines can achieve thermal efficiencies in the range of 25-30% and diesel engines can obtain thermal efficiencies in the range of 35-40%. Because of their higher thermal efficiency (leading to increased range and endurance) and because they can run on the same fuel as turbojet engines, diesel engines are making inroads into the gasoline-dominated piston-powered aircraft market.

For turbojet engines and their derivatives, thermal efficiency is determined primarily by the compression ratio of the compressor. The higher this ratio, the more the air is compressed and the higher the temperature that can be achieved in the burner. A key limiting factor, however, is the maximum temperature that can be sustained by the turbine blades. Although higher temperature will increase thermal efficiency, it cannot be so high as to melt the turbine blades. Research into advanced turbine blade materials and cooling techniques is an ongoing effort in order to improve turbojet/fan/prop efficiencies. Typical turbojet/fan thermal efficiencies are in the range of 30-50%.

Example Problem: Mechanical Propulsion Analysis

Further Reading

See How it Flies (by John S. Denker), Chapter 3, “Airfoils and Airflow.”

NASA Beginners Guide to Propulsion (BGP) website, “Propulsion Systems,” “Turbine Engines,” and “Internal Combustion Engines” sections.

Pratt & Whitney "How Engines Work" website, "Turbofan Engines" and "Turboprop Engines" sections.

GE "Jet Engines 101" website, "How Jet Engines Work" section.


Cengel, Y. A., and M. A. Boles, Thermodynamics, An Engineering Approach, 2nd Ed., McGraw-Hill, New York, NY (1994).

Glauert, H. (1935) “Airplane propellers” in Aerodynamic Theory, vol 4, Ed. W. F. Durand, Springer, Berlin.

Hill, P. G., and C. R. Peterson, Mechanics and Thermodynamics of Propulsion, 2nd Ed., Addison-Wesley, Reading, MA (1992).

Munson, B. R., D. F. Young, and T. H. Okiishi, Fundamentals of Fluid Mechanics, 5th Ed., Wiley, Hoboken, NJ (2006).

Newman, J. N., Marine Hydrodynamics, MIT Press, Cambridge, MA (1977).