Turboprop Integration to Aircraft
This section is a basic description of the subject and is intended only for coursework. The discussion highlights the technical challenges but exacting details are beyond the scope of this book. Turboprop nacelle design is subjected to the same considerations as the turbofan design. A turboprop nacelle is also a multifunctional system consisting of (1) an inlet, (2) an exhaust nozzle, and (3) a noise-suppression system. Thrust-reversing can be achieved by sufficiently changing the propeller pitch angle. There are two primary types of turboprop nacelles, as shown in Figure 10.19. The scoop intake can be above or below (as a chin) the propeller spinner. It is interesting that several turboprop nacelles have integrated the undercarriage mount with storage space in the same nacelle housing, as shown in Figure 10.19a. The other type has an annular intake, as shown in Figure 10.19b. Installation losses are on the same order as those discussed for a turbofan installation.
A turboprop’s nacelle position is dictated by the propeller diameter. The key geometric parameters for a wing-mounted turboprop installation are shown in Figure 10.20.
Figure 10.20. Typical parameters for a wing – mounted turboprop installation
Typically, there can be one fourth of the propeller-diameter gap between the fuselage and the propeller tip and between other propeller tips if there are four engines. The overhang should be as far forward as the design can accommodate (like the turbofan overhang) to reduce interference drag – at least a quarter to nearly one wing-chord length is sufficient. For a high-wing aircraft, the turboprop nacelle is generally underslung, especially if it also houses the undercarriage (e. g., the Bombardier Q400). For a low-wing aircraft, the nacelle is generally over the wing to give the propeller ground clearance. The propeller slipstream assists lift and has a strong effect on static stability; flap deployment aggravates the stability changes. Depending on the extent of wing incidence relative to the fuselage, there is some angle between the wing-chord line and the thrust line – typically, from 2 to 5 deg.
A fuselage-mounted, propeller-driven system is shown in Figure 10.21. The angle between the thrust line and the wing-chord line is the same as a wing-mounted, propeller-driven nacelle. Sometimes the propeller axis has about a 1-deg downward inclination relative to the fuselage axis. These parameters assist longitudinal stability. An inclination of 1 or 2 deg in the yaw direction can counter the propeller slipstream. Otherwise, the V-tail can be inclined to counter the effect.
A piston engine nacelle on the wing follows the same logic. Older designs had a more closely coupled installation.