The Case for the Turboprop
For about three decades, in both the General Aviation and airline markets, the jet airplane has gained market share at the expense of turboprops. The premise of this project is that market conditions are changing and the hybrid design addresses the basic reasons for this market shift. In both of these markets customers have preferred the jet because of lower cabin noise and vibration levels and the more modern image of the jet. Although the high speed of the jet is obviously a big factor for longer trips, many jet airplane trips are less than 500 miles, where the jet spends much of its flight time climbing to its best (high) operating altitude and then descending from altitude. We believe the market is changing to favor the resurgence of the turboprop for the following reasons:
- A continued increase of fuel prices is expected and the turboprop is more fuel efficient.
- There will be greater demand that all transportation modes reduce CO2 emissions
- The desire to reduce oil imports to the US, especially from unfriendly countries
- The public perception that business jets are an unjustified luxury.
The chart above was developed by a turbine engine manufacturer. The turbine engine is relatively simple in operation and has been developed to a high degree of reliability and efficiency since its introduction in WW-2. The turbine engine can be configured to produce shaft horsepower (as in helicopter or industrial operations), or it can be configured to produce thrust (forward force) for an airplane application.
The chart shows the amount of thrust produced by the same turbine, at the same fuel consumption, when it is used in different configurations for airplane propulsion. We are interested in the speed range below Mach 1.0, which is the speed of sound, which is 661 knots or 760 miles per hour at sea level. Thrust is produced by accelerating a quantity of air, either by direct flow through the turbine or by using a portion of the engine power to accelerate some of the air around the core engine with a ducted fan or a propeller. The more air that can be accelerated by the engine, the higher the thrust produced.
The thrust produced by the directing all air flow through the engine is shown on the bottom line on the chart, labeled “turbojet.” This method is least efficient in terms of thrust produced at lower speeds, but has the advantage of higher efficiency at supersonic speed. This is the type of jet engine used on supersonic military airplanes.
The next line, labeled “turbofan”, shows the thrust produced (again the same fuel flow) with part of the turbine energy directed to a “fan” mounted on the front of the core engine, with a duct surrounding the fan. This moves a greater mass of air around the engine (more thrust) and increases efficiency relative to the turbojet. This is the preferred engine for business jets and airliners which operate in the speed range of Mach 0.65 to about 0.90.
The top line on the chart, labeled “turboprop” shows the same basic turbine engine with a large portion of the energy produced absorbed by the propeller. This configuration is able to accelerate even more air than the turbo fan, and the resultant thrust is seen to be much greater that the other two configurations. However, the turboprop loses efficiency rapidly above Mach 0.75. At 30,000 feet Mach 0.75 is 508 MPH, which is faster than some of the slower fanjet aircraft. A turboprop flying at Mach 0.75 is competitive with the slower jets.
We see from the above that by using a propeller there is a lot of efficiency to be gained at speeds less than Mach 0.75. We can be competitive in the light jet market assuming we can address market demand for a comfortable cabin with jet-type comfort levels. At ranges of about 500 miles, there is very little door-to-door time difference between a high performance turboprop and a jet.
So we see that turboprops are more fuel efficient than turbojets or fan jets in the speed range we are targeting, and the following sections will show the hybrid design is more efficient than a conventional twin turboprop or a single engine turboprop.