INITIAL SIZING Estimation of Design Gross Weight Prof. Rajkumar S. - PowerPoint PPT Presentation

INITIAL SIZING Estimation of Design Gross Weight Prof. Rajkumar S. Pant Aerospace Engineering Department IIT Bombay

What is Initial Sizing ?  Estimation of its design take-off gross weight W o  Weight at the start of the design mission profile  Mission Profile specified by the user  Additional Requirements by Regulatory Bodies  Objectives  Identify requirements that are likely to drive the design  First estimate of the size of the aircraft, through W o

Vary with the purpose of the aircraft MISSION PROFILE AE-332M / 714 Aircraft Design Capsule-3

Mission Profiles  Mission profile  purpose of the aircraft  General Aviation Aircraft  Simple Cruise + Hold  Commercial Transport Aircraft  Main Profile + Missed Approach + Diversion + Hold

Mission Profile: Simple Cruise Cruise 3 4 5 Loiter 5 Approach 1 2 6 7 Warm up, Taxi-out, Landing, Taxi-in Take Off AE-332M / 714 Aircraft Design Capsule-3

Mission Profile: Air Superiority Aircraft Cruise 2 7 Cruise 1 6 4 3 Combat Loiter 5 5 Approach Loiter 8 9 1 2 Weapon Drop Landing, Taxi-in Warm up, Taxi-out, Take Off AE-332M / 714 Aircraft Design Capsule-3

Mission Profile: Ground Attack Fighter Cruise 2 7 Loiter 6 Cruise 1 4 3 Loiter Combat Approach 1 2 5 5 8 9 Warm up, Taxi-out, Landing, Weapon Drop Take Off Taxi-in AE-332M / 714 Aircraft Design Capsule-3

Mission Profile: Strategic Bomber Cruise 3 10 Loiter 9 Cruise 1 4 3 6 5 Combat Approach 7 8 1 2 11 12 Warm up, Taxi-out, Landing, Weapon Drop Take Off Taxi-in * R: Re-Fuelling AE-332M / 714 Aircraft Design Capsule-3

Mission Profile: UAV Predator (Tier II) Mission Profile AE-332M / 714 Aircraft Design Capsule-3

Mission Profile: UAV Predator (Tier II) Mission Profile AE-332M / 714 Aircraft Design Capsule-3

Issues in Initial Sizing  Very little known about a/c configuration  Most methods are deeply rooted in past  Statistical inference of parameters  Similar aircraft designed earlier  Most procedures empirical / semi-empirical  Various methodologies / approaches, e.g.,  Loftin’s method  Raymer’s approach (explained here)

Typical Take-off weight break-up Empty weight Payload Usable Fuel Trapped Fuel 25 20 25 5 50

Take-off weight build-up  W o = W crew + W pay + W fuel + W empty W empty   Weight of structure, engines, landing gear, fixed equipment, avionics, etc.  W crew and W pay are both known  User-specified requirements  W fuel & W empty are unknowns to be determined

Equation for Initial Sizing = + + + W W W W W o crew pay fuel empty + W W = crew pay W   o W W − + empty fuel   1   W W o o + W W = crew pay { } W − + o ˆ ˆ 1 w w e f ˆ ˆ w & w are the two unknowns to be determined e f

Estimation of empty weight fraction ώ e C * K vs ώ e = A W o  Where “A” and “C” are constants  Their values for various aircraft types are obtained from statistical curve-fits  K vs is a factor depending on the a/c sweep  K vs = 1.00 for conventional, fixed-wing  K vs = 1.04 for wing with variable sweep

“A” and “C” for various a/c types A/C type A C  Sailplane (unpowered) 0.83 -0.05  Sailplane (powered) 0.88 -0.05  Homebuilt-metal/wood 1.11 -0.09  Home-built composite 1.07 -0.09  General Aviation-1 Engine 2.05 -0.18  General Aviation-2 Engine 1.40 -0.10  Agricultural a/c 0.72 -0.03  Twin turboprop 0.92 -0.05  Flying Boat 1.05 -0.05  Jet trainer 1.47 -0.10  Jet fighter 2.11 -0.13  Military cargo 0.88 -0.07  Jet transport 0.97 -0.06 Note: W o in kg

Empty Weight Fraction Trends

Empty Weight Fraction Trends

Weight Trend Data - Single Aisle Jet Transport From The Elements of Airplane Design, Schaufele. 140000 Bae 146-100 DC-9-10 130000 BAC-111 120000 BAE 146-200 y = 0.5598x W empty - Empty Weight (lbs) F100 110000 BAE 146-300 100000 DC-9-30 737-200 90000 DC-9-40 DC-9-50 80000 717-200 70000 737-300 737-400 60000 MD-81 50000 737-600 737-700 40000 80000 100000 120000 140000 160000 180000 200000 220000 240000 WTO - Maximum Takeoff Weight (lbs) AE-332M / 714 Aircraft Design Capsule-3

Estimation of mission fuel fraction ώ f  W fuel = W mission fuel + W reserve fuel  W mission fuel depends on  Type of mission  Aircraft aerodynamics  Engine SFC  W reserve is required for  Missed Approach, Diversion & Hold  Navigational errors and Route weather effects  Trapped Fuel (nearly 0.5% to 1 % of total fuel)  Assumption  Fuel used in each mission segment is proportional to a/c weight during mission segment  Hence ώ f is independent of the aircraft weight

Estimation of Mission Segment Weights  Various segments or legs are numbered, with ‘0’ denoting the mission start  Mission segment weight fraction for i th segment = W i /W i-1  Total fuel weight fraction (W 6 /W 0 ) obtained by multiplying the weight fractions of each mission segments

Estimation of Mission Segment Weights  The warm-up, take-off, and landing weight fraction estimated by historical trends  Fuel consumed (and distance traveled) during all descent segments ignored

Weight fractions in Climb and Acceleration

Effect of using historical data Mission Profile W W W W W W W = ⋅ ⋅ ⋅ ⋅ ⋅ 6 6 5 4 3 2 1 W W W W W W W 0 5 4 3 2 1 0 W W W = ⋅ ⋅ ⋅ ⋅ ⋅ 6 5 3 0 . 995 1 . 0 0 . 985 0 . 97 W W W 0 4 2 W W W = ⋅ ⋅ 6 5 3 0 . 95067 W W W 0 4 2

Using mission profile and historical data for engines ! ESTIMATION OF FUEL WEIGHT FRACTION AE-332M / 714 Aircraft Design Capsule-3

Breguet Range Equation = − × × dW tsfc T dt Fuel Consumption: V dW = = − ∞ ds V dt ( ) T Range for dW fuel ∞ tsfc = = T D , W L During Cruise Drag changes due to changing lift: assume L/D is constant,     V L dW   = − ∞   ds   Hence:     tsfc D W Assuming L/D , tsfc and V ∞ (= aM) are constant: AE-332M / 714 Aircraft Design Capsule-3

Breguet Range Equation   a L W =   initial R M ln   tsfc D W final Engine efficiency Aerodynamic Structural (fuel consumption) efficiency efficiency a is sound speed W initia l = MTOW (Maximum Takeoff Weight) W final = OEW + Pax + reserve fuel OEW = Operational Empty Weight = Empty Weight + Crew + trapped fuel & Oil Source: Jet Sense; The Philosophy and the Art of Aircraft Design , Zarir D. Pastakia AE-332M / 714 Aircraft Design Capsule-3

Fuel Fraction in Cruise segment  Cruise segment mission weight fraction can be estimated using the Breguet Range Equation     V L W = ⋅ ⋅ −   cruise i 1 R ln       c D W cruise cruise i R = Cruise Range (m) c cruise = Specific Fuel consumption in cruise (per sec) V cruise = Cruise Velocity (m/s) [L/D] cruise = Optimum lift to drag ratio during cruise = [L/D] max for Propeller driven a/c = 0.866*[L/D] max for Jet engined a/c

Fuel Fraction in Loiter segment  Loiter segment mission weight fraction can be estimated using the Breguet Endurance Equation     1 L W = ⋅ ⋅ −   i 1 ln E       c D W loiter loiter i E = Endurance (sec) c loiter = Specific Fuel consumption in Loiter (per sec) [L/D] loiter = Optimum lift to drag ratio during loiter = 0.866 [L/D] max for Propeller driven a/c = [L/D] max for Jet engined a/c

Mostly using historical data ! ESTIMATION OF MAX L/D AE-332M / 714 Aircraft Design Capsule-3