The modern fan engine has a propeller-like component whose aerodynamic performance differs from that of a propeller at the blade level but serves the same function. The blading aerodynamics differs from that of a compressor in notable ways.
- Inlet guide vane
- Bypass ratio
11.1 The Fan on a Turbofan Engine Has No IGV
Before leaving the subject of velocity diagrams in the compression parts of the engine, the reader may be curious about the flow through a fan that, in practice with high bypass engines, has no inlet guide vanes. The flow geometry is quite similar to that through a propeller as described in Chap. 8. The caption of Fig. 11.1 describes the diagram with flow that exits the stage in the axial direction imposed by the exit guide vane. A sketch of the flow through a fan of a modern turbofan engine is shown the figure. The comments about the helical nature of the flow through an “axial” flow compressor also apply to the fan, even with no inlet guide vane. The sketch shows the angular component of the rotor outflow in the laboratory reference frame (displayed as L2). The angular displacement of the stream tubes is much smaller than in a compressor because there is only one stage.
A couple of things about the engine in Fig. 11.2 are worth observing. First, the engine has some resemblance to a propeller driven system with the engine behind the fan as the power source. This and all high bypass engines have done away with the inlet guide vane and located the rotor to face the stream as the first blading element. Strong differences that set this engine type apart from the propeller with its ICE, however, are apparent. Among these is that the power source is the gas turbine in contrast to the ICE of yesteryear. That allowed power levels more than fifty times larger than what the largest reciprocating ICEs ever could. The fan is shrouded so that it can operate in an airstream that has been slowed by an inlet (not shown here) and there is an exit guide vane. Efficiency levels of this engine type are such that non-stop airliners can operate over distances covering half the globe. The post war ICE powered airliner was challenged to cross the Atlantic!
11.2 Bypass Ratio
The two fan jet engines pictured in Figs. 10.2 (1960) and 11.2 (1993) are embodiments of the goal to ever better propulsion efficiency by the reduction of the (average) jet velocity in favor of increased air flow rate. The amount of air bypassing the prime mover engine is expressed as a multiple of the air flow rate processed by the engine, i.e., the compressor, the combustor, and the turbine. Early in the age of turbofans the bypass ratio was around 1 (meaning fan and engine flow rates were about equal) and later 2. The widebody jets introduced bypass ratio of 5 and current airplanes may sport this ratio around 10. These increases in bypass ratio have very large impacts on fuel usage because of the improved propulsive efficiency.
The large fan in high bypass engines makes it imperative to consider its design for aerodynamic performance and, importantly, for weight. The early engines of this type, like the Pratt & Whitney JT9D powering the Boeing 747, had a large number (46) of high aspect ratio blades, meaning they had a relatively short chord. These blades had “clappers” between the blades to stiffen them and prevent harmonic vibrations from causing damage or failure. The blades were made of titanium alloys. Attempts to reduce the weight of the fan include the first design involving composite blades made with a metal leading edge to minimize foreign object damage by such events as bird strikes. These blades had a wider chord and were fewer in number. The General Electric GE90-115B has 22 such blades with composite materials (carbon fiber) at the rear of the blades (see Fig. 11.2). Another approach to weight reduction is the use of hollow all-metal, wide chord, blades. This was the approach taken by P&W in its upgrade of the JT9D (renumbered as PW 4000). The challenge of building a light-weight fan did not always go smoothly or without serious consequences, but it was met and serves well in modern fanjet engines. These blades are a tribute to the art and capabilities of modern aerodynamicists and engineers more generally.
11.3 Turboprops and Turboshaft Engines
The idea of the bypass turbofan is now fully implemented with fan engines used everywhere for commercial transport. An extension of the turbofan idea is the turboprop where nearly ‘all’ of the power is removed from the primary gas turbine engine flow to drive a gear box and with it a propeller. The effective bypass ratio is very large (larger than that of the ducted fan) but, as with an ICE powered airplane, the propeller sets an airplane speed limit.
Naturally, with an engine delivering its power by means of a rotating shaft (a turboshaft engine), all kinds of other applications can be imagined. Among them are helicopters, ships, etc. The success of the turboprop engines was very much due to their greater power output capability than were the piston ICEs. They also provided the user with engines that were much easier to maintain in service and could operate much longer between needs for service.
A few words about differences between gas turbine engines and the ICE will clarify the former’s great advantage in aviation. The gas turbine is lighter as measured by power or thrust per pound of engine. Typically, at sea level conditions, 5–6 lbs of thrust or horsepower are produced by each pound of a gas turbine engine weight. This is quite an improvement over the ICE at 1 hp/lb where even that level is achieved only for fairly demanding circumstances such as those set for military aircraft.
The gas turbine uses a lower cost “jet fuel”, similar to kerosene, rather than aviation gasoline required by the ICE. The lower volatility of the kerosene-like jet fuel also makes it safer.
Finally, we can say that the gas turbine with very few moving parts, and none of them reciprocating, does not require the mechanical maintenance of the ICE. To wit, the P&W R-4360 engine used in the Boeing Model 377 Stratocruiser airliner was fortunate if it lasted longer that 500 h between major overhauls. Allied bombers in WW II routinely replaced entire engines every 100 flight hours. These overhauls were costly in terms of labor and facilities and demanded an inventory of ready-to-go engines in case of an engine failure on an airplane that just came in. In commercial service, engine shutdowns were not uncommon, especially in the summer months. By contrast, the need for extensive overhauls of modern turbofan engines is in excess of 20 or 30 thousand (!) hours.
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Decher, R. (2022). Bypass and Other Engines. In: The Vortex and The Jet. Springer, Singapore. https://doi.org/10.1007/978-981-16-8028-1_11
Publisher Name: Springer, Singapore
Print ISBN: 978-981-16-8027-4
Online ISBN: 978-981-16-8028-1