Airplanes with propellers were invented a long time ago. After that, jet engines came into existence. My question is: why do we still have propeller engines?
The reasons I can think of are:
Besides these, are there any other reasons general aviation airplanes built nowadays don't have jet engines?
Jet engines are only efficient at high altitudes and high speed (25,000+ feet or so and 300+ knots or so true airspeed). The cost of making and operating an airplane capable of flying there is very high, and most general aviation flights have no need of such performance (or expense) hence, propeller airplanes.
Reciprocating engine propeller airplanes are relatively efficient at low altitudes and airspeeds, they are more economical for the average types of general aviation flights. They are also much cheaper to build and operate than a turbine powered airplane.
Turboprops fill the efficiency gap between reciprocating and pure jet engines, being most efficient at mid altitudes and airspeeds. Still more expense and performance than most general aviation flights need though.
It is all mission related.
Propeller engines - and I am including turboprops - are much more efficient at lower speeds and altitudes. Example: the C-130 Hercules cargo aircraft. It can get in and out of fields much shorter than a turbojet or turbofan can, while trucking a large load, because it has those huge propellers clawing at the air. Jets, as stated before, have the advantage high and fast. But that is not the mission of the Hercules.
As mentioned they are efficient at lower altitudes (and service the GA mission quite nicely) but in many ways it simply boils down to cost. There are companies out there designing and working on smaller jets like Cirrus with the SF50. Then there are a few jets out there like the Cessna Citation Mustang that are single pilot certified and can be flown with appropriate ratings on a PPL. But the fact remains that it is not cheap nor easy to bring a new airplane to market. Cirrus first flew their jet in 2008 and its still not on the market officially. That puts them at nearly 8 years from prototype to roll out and Im sure the first lines for it were drawn well before 2008.
On the other hand companies like Piper and Cessna have approved designs that are still fast and easy to maintain even by todays standards. Very much the "If it ain't broke don't fix it" issue. It is no mystery that GA sales are down and some of the companies could be doing better.
There is a bit of a safety issue. Propeller planes are nice and easy to fly with fast throttle response and lower speeds they make great trainers. Some may argue that jets are just to much plane to handle for a new pilot.
For what its worth there are some very fast piston singles out there. The Glasair claims a cruise of 252Ktas (although its experimental) and the full production Mooney Acclaim Type S pulls in at 242Ktas both of these planes approach turbo prop speeds to the piston engine while an old design can still deliver performance (and high speeds).
There is also some cross over with automotive piston engines. Through out history there have been a few auto engines certified for aircraft. Once upon a time Ford had some motors that found their way into planes. Mooney has attempted to use car motors at least twice starting with the Crosley engine that was found in early Mites and culminating with the epic (if you are a Porsche fan like me) PFM3200 that featured the Porsche 3.2 engine out of the late 80's Carrera. While the PFM3200 was riddled with issues and Porsche eventually dropped the FAA certification for the motor it was an interesting display of engineering. Parts for the 3.2 were arguably easier to come buy and there were plenty of people who knew how to work on them. Currently Diamond offers a diesel DA42 that runs on modified car motors. Again using a motor that is tried and true on the road. So the costs are not just because the engines are cheaper to make but made in much higher quantities as well as being able to offset some of the engineering costs with this bulk.
Piston-driven propeller powerplants are:
Cheaper. Because the engine is a proven and well-understood design, they're cheaper to design, to make, and to maintain, all of which lower expenses for the pilot. You can pick up an older Piper Cherokee for about \$40k nicely equipped with plenty of hours before its next overhaul. The absolute cheapest Piper turboprop I can find with a quick search is a twin-engine Cheyenne for \$275,000, and it probably needs work as they average closer to \$400k. Other turboprop airframes approach $2 million a pop.
Smaller. A four-cylinder air-cooled naturally-aspirated piston engine is not much bigger than the V-twin on a motorcycle. Smaller tends to equal lighter, and it also allows for more cabin room in a fuselage of the same total volume, and less variance of the CG with cabin weight (which is a big deal in small planes).
Simpler. There's not as much to go wrong with a fixed-pitch air-cooled piston-engine plane. This again reduces maintenance costs but also increases safety by lengthening "mean time between failures" that would require emergency procedures. Constant-speed props are a little more mechanically complex but not too bad. Turboprops a little more complex still.
Better-performing at low altitudes. Most turbofans are designed for operation above 10,000 feet, while most propeller-driven craft are designed to fly at 8,000 or below because their cabin is unpressurized. At lower altitudes where the air is thicker, piston engines run more efficiently and cooler than turbofans and much better than pure jets.
Ultra-light jets were all the craze a decade ago, and never took off: they were too fuel inefficient. Propellers do a much better job at converting fuel chemical energy into thrust that propels the aeroplane forward, than jets do.
Aerodynamic thrust can be written as $T = \dot{m} \cdot \Delta V$, with $\dot{m}$ the mass stream of air in kg/s and $\Delta V$ the speed increase of the propelled air mass in m/s. To get the highest propulsive efficiency, accelerate the largest amount of air to the smallest $\Delta V$. Propellers have a much higher effective disk area than pure jet engines do, and deliver way more thrust per installed engine power than pure jets do.
The turbofan is much more fuel efficient than a pure jet because the large bypass fan works kind of like a propeller does. However, the turbofan is still less efficient than a turboprop, simply because of the larger blade radius.
The issue of piston engine vs turbine engine for general aviation is a separate one altogether. Piston engines are heavier but have better fuel economy. Weight-wise they scale up very poorly, while this is where the turbines fully come into their own: nothing beats a large turbine in power/weight ratio. Smaller turbines are not very efficient due to lower pressure ratios from blade/shroud gaps.
It's very simple: Prop planes are manufactured because most airports around the world have runways too short for jets.
