Figure 10.23. Airflow demand at various conditions (civil and military aircraft intakes)

10.8 Intake and Nozzle Design

Engine mass-flow demand varies significantly, as shown in Figure 10.23. To size the intake area, the reference cross-section of the incoming airmass-stream tube is taken at the maximum cruise condition, as shown in Figure 10.23a, when it has a cross-sectional area almost equal to that of the highlighted area (i.e., A∞ = A1; see Section 10.8.1 for nomenclature). The ratio of mass flow rates (MFR) relative to the reference condition (i.e., airmass flow at maximum cruise) is a measure of the spread the intake would encounter. At the maximum cruise condition, MFR = 1 as a result of A∞ = A1. At the typical cruise condition, the intake-airmass demand is lower (MFR < 1, as shown in Figure 10.23b). At the maximum takeoff rating (MFR > 1), the intake airmass-flow demand is high; the streamlined patterns are shown in Figure 10.23c. Variations in the intake airmass-flow demand are significant.

If the takeoff airmass flow demand is high enough, then a blow-in door – which closes automatically when demand drops off – can be provided. Figure 10.23d shows a typical flow pattern at incidence at high demand, when an automatic blow-in door may be necessary. At idle, the engine continues running with little thrust generation (MFR 1). At an inoperative condition, the rotor continues windmilling to minimize drag. If a rotor seizes due to mechanical failure, there is a considerable drag increase.

Currently, the engine (i.e., fan) face should not exceed Mach 0.5 to avoid degradation due to compressibility effects. At a fan-face Mach number above 0.5, the relative velocity at the fan-tip region approaches sonic speed due to the high bladerotational speed.

The purpose of the intake is to provide engine airmass-flow demand as smoothly as possible – there should be no flow distortion at the compressor face due to separation and/or flow asymmetry. The nacelle intake-lip cross-section is designed using logic similar to the aerofoil LE cross-section – that is, the flow should not separate within the flight envelope.

10.8.1 Civil Aircraft Intake Design: Inlet Sizing

This section describes an empirical approach for developing the intake contour of a podded nacelle that is sufficient for the conceptual design stage. This would

Figure 10.24. Schematic diagram of nacelle forebody section (crown cut)

generally be followed by proper refinement using extensive CFD analysis and windtunnel testing for substantiation. The nacelle external contour is influenced by interaction with the aircraft flow field. The simplified procedure is suitable for this coursework.

Figure 10.24 provides definitions of the design parameters required for the design of a pod-mounted subsonic civil aircraft intake. The nacelle section is similar to the aerofoil shape. The throat area is the minimum area of the intake duct and acts as a diffuser. The associated nomenclature follows (for the radius, replace D with R and the subscripts remain unchanged):

a= semi-major axis of the internal lip

b= semi-minor axis of the internal lip

c= semi-major axis of the external lip

d= semi-minor axis of the external lip

θ = internal contour wall angle (below 10 deg; better at 6 deg)

Associated areas are as follows (radius R is half of diameter D in the nomenclature):

Al = highlighted area = π (Rl )2

ATH = throat area = π (RTH)2

A∞ = free-stream cross-sectional area

To size the intake, the first parameter considered is establishing the throat area. The proper method is to obtain the maximum airmass-flow demand at takeoff and the maximum-cruise demand. If the takeoff demand requires a much larger size, then blow-in doors (which close automatically when demand drops – mostly applicable to military designs) are provided. Using m˙ a as the intake airmass at the maximum cruise gives:

Al = m˙ a/(ρ∞ V∞)

The throat area is sized from the lip contraction ratio (LCR) = A1/ ATH (typically, from 1.05 to 1.20). LCR = 1.0 represents a sharp lip and 1.2 represents a well-rounded lip.

The highlighted diameter D1 is typically 0.9 to 0.95 times the fan-face diameter. Keep the Ldiffuser = 0.6 at 1 time Dfan and LFB = 1 to 2 times Dfan (it must conform with the lip contour).

The next task is to establish the lip contour before developing a suitable aerofoil section for the intake cowl. As for the wing aerofoil, NACA developed nacelle forebody aerofoil contours. NACA 1 is a good design guideline for the external contour (i.e., the upper lip is nearly elliptical). In general, the lower lip (i.e., elliptical) contour is developed by the engine manufacturer and matches the upper lip.

In Figure 10.24, the nacelle lip is in the shape of a quarter-ellipse with semimajor axis a and semiminor axis b. The parameters that define the inlet-lip internalcontour geometry are (1) the LCR R1/RTH (i.e., A1/ ATH), and (2) the fineness ratio (a/b).

At the crown cut:

internal-lip fineness ratio, (a/b) = from 2 to 5 (typically 1.5 to 3.0) external-lip fineness ratio, (c/d) = from 3 to 6 (typically 3 to 5)

At the crown cut, the lip-thickness ratio of (b + d)/Ldiff is around 15 to 20% (the lip-thickness ratio is not like the aerofoil t/c ratio because the cowl length extends beyond the fan face when the ratio decreases substantially). Typically, b is 1.5 to 2 times d.

At the keel cut, if it houses accessories, the thickness ratio is (b + d)/Ldiffuser by about 20 to 30%.

The side cuts of the nacelle result from the merging of the crown cut and the keel cut. If ground clearance is a problem, the accessories are distributed around the keel and the contours are merged accordingly. This book keeps the design simple by using crown-cut geometry all around, with the understanding of actual problems.

The throat Mach number and the airmass-flow demand at maximum cruise determine the DTH. The throat Mach number must be maximized to the point to maintain the fan-face Mach number below 0.5 at the maximum cruise condition. At A∞ < A1, there is precompression when associated spillage generates additive drag (see Figure 9.7). Then, long Ldiffusion is not required for internal diffusion because external diffusion has partially achieved it. At A∞ = A1, there is no additive drag, but it would need longer Ldiffusion for internal diffusion. Figure 9.6 indicates that additive drag decreases as the MFR increases. At cruise (i.e., MFR above Mach 0.6), additive drag is minor.