Turboprop Engine: Formulation
Turbo props are described in Section 10.4.4. They are very similar to turbojets and turbofans except that the high energy of the exhaust jet is utilized to drive a propeller by incorporating additional low-pressure turbine stages, as shown in Figure 10.7. Thrust developed by the propellers is the propulsive force for the aircraft. A small amount of residual thrust could be left at the nozzle exit plane, which should be added to the propeller thrust. The relationship between the thrust power (TP) and the gas turbine SHP is related to propeller efficiency, nprop, as:
TP = SHP X Vop + F X 1% (10.20)
ESHP is a convenient way to define the combination of shaft and jet power, as follows:
ESHP = TP/Vop = SHP + (F x VoVnprop (10.21)
Aircraft at a static condition have an ESHP = SHP because the small thrust at the exit nozzle is not utilized. As speed increases, ESHP > SHP, as there is some thrust at the nozzle. SFC and specific power are expressed in terms of ESHP.
The formulae provide good reasoning for the gas turbine domain of application, as shown in Figure 10.2. Turboprops provide the best economy for a design flight speed at and below Mach 0.5 and are well suited for shorter ranges of operation. At higher speeds, up to Mach 0.98, turbofans with a high BPR provide better efficiencies (see the comments following Equation 10.5). At supersonic speeds, the BPR is reduced and, in most cases, uses an AB. Smaller aircraft have piston engines up to a certain size (i. e., « <500 HP). Above 500 HP, turboprops prove better than piston engines.
Low pressure High pressure
Compressor Compressor (shaded)
Figure 10.14. Installation effects
10.2 Engine Integration with an Aircraft: Installation Effects
Engine manufacturers typically supply bare engines to aircraft manufacturers, which install them to integrate with an aircraft. The same type of engines can be used by different aircraft manufacturers; each has its own integration requirements. Installing an engine in an aircraft is a specialized technology with which aircraft designers must be knowledgeable. Engine integration is accomplished by aircraft manufacturers in consultation with engine manufacturers.
A bare engine at the test stand performs differently than an installed engine on an aircraft. The installation effects of an engine result from having a nacelle – that is, the losses of intake and exhaust plus off-takes of power (e. g., driving motors and generators) and air-bleeds (e. g., anti-icing and environmental control). The total loss of thrust at takeoff could be as high as 8 to 10% of what is generated by a bare engine at the test bed; at cruise, the loss can be reduced to less than 5%. Figure 10.14 shows typical off-takes that are required due to various installation effects. Designers conduct analytical and empirical studies to establish key parameters in order to arrive at a design that produces satisfactory thrust to meet aircraft – performance requirements.
A nacelle is the housing for the engine and it interfaces with an aircraft; typically, it is pod/pylon-mounted in civil aircraft designs. A nacelle on an aircraft with more than one engine is pod/pylon-mounted on the wing and/or the fuselage. Propeller-driven engine nacelles are also similar to podded nacelles, modified by the presence of a propeller (see Section 10.7.2). An aircraft with one engine is aligned in the plane of the aircraft symmetry; engines with propellers can have a small lateral inclination of 1 or 2 deg about the aircraft centerline to counter the slipstream and gyroscopic effects from a rotating propeller. As discussed previously, wing-mounted nacelles are best for relieving wing-bending in the flight load. The engines on military aircraft are buried in the fuselage and therefore do not have a nacelle unless the designer chooses to have pods (e. g., some older designs). Military – aircraft designers must consider intake design as described in Section 10.8.2. The
Figure 10.15. Installed turbofan housed in a nacelle pod under slung below aircraft wing (Courtesy of Bombardier-Aerospace Shorts)
position of the nacelle relating to the aircraft and the shaping to reduce drag are important considerations (see Section 9.8).