Figure 10.19. Typical wing-mounted turboprop installation

from the engine after deducting the losses is sufficient for the full flight envelope as specified. In hot and high-altitude conditions, it becomes critical at takeoff if the runway is not sufficiently long and/or there is an obstruction to clear. In that case, an aircraft may take off with lighter weight. Airworthiness requirements require that an aircraft maintain a minimum gradient (see Chapter 11) at takeoff with its critical engine inoperative (customer requirements may demand more than the minimum).

Following are the obligations of designers when installing an engine and integrating it with an aircraft:

Generate the external and internal contours of the nacelle. Multiengines are either wing-mounted (i.e., larger aircraft) or fuselage-mounted (i.e., smaller aircraft).

Compute the compressor air-bleed for the ECS (e.g., cabin air-conditioning and pressurization, de-icing and anti-icing, and other purposes).

Compute power off-takes from the engine shaft to drive the electric generator, accessories, and so forth.

Substantiate for the certifying agencies that the thrust available from the installed engine is sufficient for the full flight envelope.

Current developments involve laminar flow control over the external surface of the intake duct and technologies for noise and emission reduction.

10.7.2 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 wingmounted 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.

10.7.3 Combat Aircraft Engine Installation

Combat aircraft have engines that are integral to the fuselage, mostly buried inside; however, in cases with two engines, they can bulge out to the sides. Therefore, pods are not featured unless having more than two engines on a large aircraft is required.

Figure 10.21. Typical flight parameters for a fuselage-mounted turboprop installation