# Fuselage Group – Civil Aircraft

A fuselage is essentially a hollow shell designed to accommodate a payload. The drivers for the fuselage group mass are its length, L(t); diameter, Dave (t); shell area and volume, (t); maximum permissible aircraft velocity, V(t); pressurization, (t); aircraft load factor, n(t); and mass increases with engine and undercarriage installation. The maximum permissible aircraft velocity is the dive speed explained in the V-n diagram in Chapter 5. For a noncircular fuselage, it is the average diameter obtained by taking half the sum of the width and depth of the fuselage; for a

rectangular cross-section (invariably unpressurized), it is obtained using the same method. Length and diameter give the fuselage shell area: the larger the area, the greater is the weight. A higher velocity and limit load n require more material for structural integrity. The installation of engines and/or the undercarriage on the fuselage requires additional reinforcement mass. Pressurization of the cabin increases the fuselage-shell hoop stress that requires reinforcement, and a rear-mounting cargo door is also a large increase in mass. (The nonstructural items in the fuselage – e. g., the furnishings and systems – are computed separately.)

Following are several sets of semi-empirically derived relations by various authors for the transport aircraft category (nomenclature is rewritten according to the approach of this book). The equations are for all-metal (i. e., aluminum) aircraft.

By Niu [6] in FPS:

WFcivil = k1k2 |
2,446.4 |
ncrw, w л Л, 1.5ДP^°’5 ”| 0.5(Wflight-gross-weight + ‘Wlanding_weight) x ( 1 + 4 ) x |

lSnet. fus. wetted. area X [0.5 (W + D)]0’5 X L0’6 X 10-4 – 678 J |

(8.11) |

where k1 = 1.05 for a fuselage-mounted undercarriage

= 1.0 for a wing-mounted undercarriage where k2 = 1.1 for a fuselage-mounted engine

= 1.0 for a wing-mounted engine Snet-fus-wettedjarea = fuselage-shell gross area less cutouts

Two of Roskam’s suggestions are as follows [5]:

1. The General Dynamic method:

WPcivU = 10.43 (Kiniet)1A2 (qB/100)0’283 (MTOW/1,000)0’95 (L/D)071 (8.12)

where Kiniet = 1.25 for inlets in or on the fuselage; otherwise, 1.0 qD = dive dynamic pressure in psf L = fuselage length D = fuselage depth

2. The Torenbeek method:

Wpcivii = 0.021 Kf {VdLht/ (W + D)}° ‘ 5 (f s_gross_area )12 (8.13)

where Kf = 1.08 for a pressurized fuselage

= 1.07 for the main undercarriage attached to the fuselage = 1.1 for a cargo aircraft with a rear door Vd = design dive speed in knots equivalent air speed (KEAS)

LH_tail = tail arm of the H-tail Sfus. gross. area = fuselage-shell gross area

By Jenkinson (from Howe) [7] in SI:

Mfcmi = 0.039 X (2 X L x Dave x VD’5)1’5 (8.14)

The author does not compare the equations here. As mentioned previously, the best method depends on the type – weight equations show inconsistency. Toren – beek’s equation has been used for a long time, and Equation 8.14 is the simplest one.

The author suggests using Equation 8.14 for coursework. The worked-out example appears to have yielded satisfactory results, capturing more details of the technology level.

MFcivil = cfus X ke X kp X kuc X kdoor X (MTOM X HuttY X (2 X L X Dave X v°’5)y,

(8.15)

where cjUs is a generalized constant to fit the regression, as follows:

cfus = 0.038 for small unpressurized aircraft (leaving the engine bulkhead forward)

= 0.041 for a small transport aircraft (<19 passengers)

= 0.04 for 20 to 100 passengers = 0.039 for a midsized aircraft = 0.0385 for a large aircraft = 0.04 for a double-decked fuselage = 0.037 for an unpressurized, rectangular-section fuselage

All k-values are 1 unless otherwise specified for the configuration, as follows:

ke = for fuselage-mounted engines = 1.05 to 1.07 kp = for pressurization = 1.08 up to 40,000-ft operational altitude = 1.09 above 40,000-ft operational altitude kuc = 1.04 for a fixed undercarriage on the fuselage = 1.06 for wheels in the fuselage recess = 1.08 for a fuselage-mounted undercarriage without a bulge = 1.1 for a fuselage-mounted undercarriage with a bulge kVD = 1.0 for low-speed aircraft below Mach 0.3 = 1.02 for aircraft speed 0.3 < Mach < 0.6 = 1.03 to 1.05 for all other high-subsonic aircraft kdoor = 1.1 for a rear-loading door

The value of index x depends on the aircraft size: 0 for aircraft with an ultimate load (nutt) < 5 and between 0.001 and 0.002 for ultimate loads of (nult) >5 (i. e., lower values for heavier aircraft). In general, x = 0 for civil aircraft; therefore, (MTOM x nult)x = 1. The value of index y is very sensitive. Typically, y is 1.5, but it can be as low as 1.45. It is best to fine-tune with a known result in the aircraft class and then use it for the new design.

Then, for civil aircraft (nult <5), Equation 8.15 can be simplified to:

MFcivil = cfus X ke X kp X kuc X kdoor X (2 X L X Dave X VD5)!-5 (8.16)

For the club-flying-type small aircraft, the fuselage weight with a fixed undercarriage can be written as:

MFsmalla/c = 0-038 X 1.07 X kuc X (2 X L X Dave X У£5)!-5 (8.17)

If new materials are used, then the mass changes by the factor of usage. For example, x% mass is new material that is y% lighter; the component mass is as follows:

MFcivil _new-material — MFcivil x/y X MFcivil + x X MFcivil (8.18)

In a simpler form, if there is reduction in mass due to lighter material, then it is reduced by that factor. For example, if there is 10% mass saving, then:

MFcivil — °-9 X MFciviLall metal

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