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 wind – tunnel 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):

D1 — highlight diameter; the forwardmost point of the nacelle. If the keel

cut is not in the vertical plane with the crown cut, then its projection at the vertical line can be used.

Dth — throat diameter; the minimum cross-sectional area of the intake

geometry

D Tip — Dfan — the tip of the fan (supplied by the engine ^nanufacturer)

DHub — rotor-hub diameter (supplied by the engine manufacturer)

Dmax — maximum external nacelle diameter Ldiff — diffuser length, from throat to fan face

bps — nacelle forebody length; the distance from the highlight to the maxi­mum diameter, DMAX

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

в — 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):

Ai — highlighted area — n (Rl)2 ATH — throat area — n(RTH)2 ATO — 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 ma as the intake airmass at the maxi­mum cruise gives:

Al = ma / (рю Цю)

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 Di 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 internal – contour geometry are (1) the LCR R1/RTH (i. e., Ai/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 ATO < 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 ATO = A1, there is no additive drag, but it would need longer Ldiffusi0n 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.

At the throat, if the Mach number is high (e. g., reaches the local sonic speed), there is more loss and a longer diffuser is needed to decrease air velocity to around Mach 0.5 at the fan face. It is best to keep the average Mach number at the throat just below 1.0. Care must be taken that at yaw and/or high angles of attack, the fan-face flow distortion is minimized.

Finally, to make a proper divergent part of the subsonic intake acting as the diffuser, the internal contour shows an inflection point (at around 0.5 to 0.75 Ldiff). At that point, to avoid separation, the maximum wall angle в should not exceed 8 or 9 deg.

Typically, the nacelle length, LN, is 1.5 to 1.8 times the bare engine length, LE. The maximum diameter is positioned around 0.25 to 0.40 of the nacelle length (LFB) from the front end. The nacelle external cross-section is not purely circular but rather has a “pregnant-belly” shape at the keel cut to house engine accessories. Use the maximum radius at the crown cut as 1.1 to 1.4 times the engine fan-face radius and at the keel cut as 1.2 to 1.6 times the engine fan-face radius. The side cuts are faired between the two.

For the worked-out Bizjet example, use the following values (see Section 10.10.3):

Given fan-face diameter, Dfan = 0.716 m (2.35 ft)

R1 = 0.9 X Rfan

At maximum cruise, MFR = 1 (ATO = AH)

At cruise, MFR = 0.7 (ATO < AH)

LCR = 1.12

Dmax = 1.5 x 0.716 = 1.074 m (3.52 ft)

Lower-lip fineness ratio (a/b) = 3

Upper-lip fineness ratio (c/d) = 5

Ldiff = °.65 Dfan

(b + d)/Ldiff = 0.18

Lfb = 1.4 times Dfan

Engine manufacturers supply the data for bare engines. A bare engine may come with an exhaust duct as a nozzle that fits within the nacelle exhaust.

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