Consider a jet and a propeller driven airplane side by side on the tarmac. The engines will of course look very different. The jets wings will be swept back much farther and it will look sleeker and faster. But most key differences will not be apparent.
Airplanes driven by propellers are ideal for applications where efficiency and cost outweigh speed requirements. Small planes, regional aircraft and cargo planes are good examples. Don’t be mistaken into believing propeller aircraft are slow and aerodynamically crude machines. On the contrary, some are very high tech, pressurized and fast.
Propellers provide an efficient means to pull an airplane through the air. Unlike a screw type ship propeller, these are basically spinning airfoils. They are fuel efficient and inexpensive. Propeller equipped aircraft can generally fly slower and can use shorter runways. Gravel runways pose little danger to the engines. Noise can be a disadvantage, however, and supersonic flight is quite out of the question. Altitude is limited by the propellers need for thicker atmospheric air to “bite into”. Piston engines will never be as safe and reliable as jet engines. Turboprop’s do away with the piston engine by replacing it with a gas turbine. Although expensive, these are far safer and more powerful.
Some aircraft require much more power or speed than a propeller could ever provide. Jet engines power large aircraft, fighters and anything else that needs to go fast or high at the expense of cost. The largest GE jet engine produces 128,000 pounds of thrust. Unfortunately, fuel consumption is sometimes measured in gallons/second – a truly horrific expense. Jets, being faster, require the aerodynamics of a swept wing. The wing, in turn, requires a faster landing speed than propeller aircraft. Jet engines are also incredibly expensive. The same Roll Royce engine costs $21 million. Still, if you need to power a 747 or an F-22, there’s really no other way.
Jet engines provide enormous “kick in the pants” thrust by combusting jet fuel under very high pressure and temperature. The turbine visible on the front of a jet engine is the first stage of the compression process. Vast amounts of air (and anything else in the way) are ingested into the engine and hurled back into the next compressor section. The constantly combusting, super heated gas exiting the combustion chamber is forced through the rear turbofans at high speed. This movement turns the rear sets of turbines to perpetuate the process. While much of the engines thrust is achieved by the exiting gas, even more is derived from the spinning turbo fans. The thrust of exiting gas is greatly expanded when afterburner sections are used on military aircraft.
As complex and precise as they are, jet engines have few moving parts. A jet engine can run for thousands of trouble free hours, making them much safer than piston driven propeller aircraft.
Both types are evolving into what we need them for. Smaller, lighter, cheaper jet engines continue to be developed. Propellers and piston engines are getting quieter and even more efficient. Regardless, some facts remain written in stone. We will never see a propeller driven aircraft fly as fast or fly as high as a jet. Dirt strip bush planes will never see a jet engine. But until a better invention comes along, we will always need both jet and propeller driven aircraft
