AERO470 Aircraft Design Laboratory Lecture 2 (Fall 2023) PDF

AERO470: Aircraft Design Laboratory Lecture 2: Sizing from Conceptual Sketch Lecturer: Dr Rafic Ajaj Office: C01026G, Abu Dhabi Campus Email: [email protected] Fall 2023 Sizing from a conceptual sketch Sizing is the estimation of the required total weight and fuel weight to perform the design...

AERO470: Aircraft Design Laboratory Lecture 2: Sizing from Conceptual Sketch Lecturer: Dr Rafic Ajaj Office: C01026G, Abu Dhabi Campus Email: [email protected] Fall 2023 Sizing from a conceptual sketch Sizing is the estimation of the required total weight and fuel weight to perform the design mission. A conceptual sketch might be necessary for initial sizing is the design is significantly different from previous aircraft. The sketch is used to estimate aerodynamics and weight fractions by comparison to previous designs. A conceptual sketch is Given on the right. 2 Sizing from a conceptual sketch A “first order” sizing is needed to generate an initial design layout: A three-view drawing with internal design elements that are large enough to affect the shape of the aircraft (landing gear, payload/passenger compartments, etc.). Several cross-sections may need to be included to show that everything fits. 3 Sizing from a conceptual sketch After analysis of the initial design layout, aerodynamics, weights and propulsion characteristics are analyzed to do a detailed sizing calculation. Considering that, and other design requirements, the lightest or lowest-cost aircraft design is found by optimization. 4 Initial design layout 5 Sizing procedure • Weight of an aircraft is the first parameter to be estimated in design • Earlier the estimations (in conceptual design) , are always cruder than later estimations ( in preliminary design etc.). The actual weight of the aircraft is found after the design in complete. That could take lots of years, people and resources. • First approximations are based on historical data. • If designing a conventional fighter jet, consider an F-15 with take-off gross weight of 44,500 lbs. • If designing a mid-range 300pax airliner, consider Boeing 777 with take-off gross weight of 545,000 lbs • A more accurate estimate involves dividing the total weight into major weight groups. It is still crude, but quick and simple. 6 Design take-off gross weight People necessary to perform the mission (specified in design requirements) W0  Wcrew  Wpayload  Wfuel  Wempty goods and or passengers (specified in design requirements) Structures, engines, landing gear, avionics, etc. 7 Rearranging terms Express Wfuel and Wempty as fractions; divide them by W0 Wf We W0  Wcrew  Wpayload  W0  W0 W0 W0 Solve forW0 Wf We W0  W0  W0  Wcrew  Wpayload W0 W0 Wcrew  W payload W0  W f We 1  W0 W0 Fuel weight fraction 8 Empty weight fraction Estimating empty-weight fraction Use historical trends. See the table below and the graph on next slide. For advanced composite material aircraft, multiply the statistical value by 0.95 9 10 Estimating Fuel weight fraction • The fuel weight is highly dependent on the type of aircraft, engine, and mission requirements • CAN NOT use historical trends • an aircraft does not use all its fuel during the mission. The fuel supply is the sum of • Mission fuel: necessary for performing the mission • Reserve fuel: for emergencies and/or allowing for engine degradation • Trapped fuel: in fuel pipes etc. that cannot be pumped out • Fuel used is somewhat proportional to the total weight, so fuel fraction W f is assumed to be independent of gross weight at this W0 time 11 Typical Mission Profiles Numbered mission segments 12 Weight at mission segments Vehicle weight at each segment is also numbered: Segment 0: mission start vehicle weight = W0 Segment 1: warm up and take off vehicle weight = W1 Segment 2 etc. vehicle weight = W2 etc. Mission segment weight fraction = total weight at the end of a segment divided by total weight at the beginning of that segment (Wi Wi 1 ) where i is the mission segment number. If a mission has total x segments, the weight fraction at the end of the mission, i.e. end of segment x: Wx Wx W2 W1 W0 13  Wx1    W1 W0 Cruise segment weight fraction V L Wi 1 R ln C D Wi Range Wi 1  e Wi   RC     V (L D)  where V is velocity, L/D is lift to drag ratio, C is specific fuel consumption (also known as SFC): rate of fuel burned divided by resulting thrust. For jet engines, in British units, its unit is pounds of fuel per hour per pound of thrust; TSFC  C  14 lbs hr lbs In metric units; kg TSFC  hr N g or s  mg Ns kN For propeller engines , SFC is fuel burn rate to produce unit power (one Watt, or one break horsepower, bhp=550 ft lb/s) at the propeller shaft ; lbs Cbhp mg hr  , C power  bhp Ws V V C  Cbhp  C power 550 p p Where ηp is the propeller efficiency. It can be assumed to be 0.8. 15 Typical SFC values for jet and propeller engines Mach number 16 Loiter segment weight fraction Similar to the cruise segment, but using Endurance equation: L D Wi 1 E ln C Wi Wi 1  EC   exp Wi LD L/D depends on the aircraft configuration. In level flight, Lift = Weight (known). Drag can be broken into two components: Induced drag and Parasite drag. Induced drag is caused by generation of Lift, function of wing span Parasite drag is caused by skin friction, function of surface of the aircraft exposed (wetted) to the air. 17 Wing efficiency is expressed using aspect ratio: AR  b 2 S ref b is wing span, Sref is the wing reference area. For a rectangular wing, where c is chord length AR  b 2 bc b c Wings with higher AR may have less drag 18 Same L/D with different aspect ratios Why same L/D with huge AR difference? 19 Wetted area ratio S wet S ref The designer has more control over Sref than Swet. He chooses AR and configures the aircraft accordingly (draws a conceptual sketch). He then estimates Swet using following figure as a guide. He uses that to find L/D using historical trends 20 21 Max. L/D trends 22 L/D for cruise and loiter Drag depends on altitude and velocity. L/Dmax occurs at different speeds for different altitudes. • For a jet aircraft, most efficient loiter speed is the speed for L/Dmax, but most efficient cruise speed is the speed for 0.866 L/Dmax • For a propeller aircraft, most efficient loiter speed is the speed for 0.866 L/Dmax , but most efficient cruise speed is the speed for L/Dmax 23 Estimating Fuel fractions For a mission with x segments, total mission weight fraction is Wx W0 This simplified sizing method does not work for mission segments involving payload drops (bombs, etc.) . Therefore all the weight loss should be due to fuel usage. Since all the weight lost is W0  Wx , mission fuel fraction must be Wx 1 W0 Assume 6% fuel is reserve and trapped fuel: W f  1.061  Wx    W0 24  W0  Iterative take-off gross weight calculation 1. Calculate fuel fraction as above 2. Assume a value for W0 using historical data, then calculate empty weight fraction using statistical data (use Table 3.1) 3. Substituting given crew and payload weights in to the equation below, calculate W0 Wcrew  W payload W0  W f We 1  W0 W0 4. If the result is different from initial guess, use a value between the two for next iteration. The process should converge in a few iterations. 25