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Design of UAV Systems. c 2002 LM Corporation . Parametric weights. Lesson objective - to discuss Parametric weight methods ….the minimum level of fidelity required to predict air vehicle weights for pre-concept and conceptual design assessments of subsonic UAVs.

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  1. Design of UAV Systems c 2002 LM Corporation Parametric weights Lesson objective - to discuss Parametric weight methods ….the minimum level of fidelity required to predict air vehicle weights for pre-concept and conceptual design assessments of subsonic UAVs Expectations - You will understand how to apply the basics and to avoid unnecessary detail 19-1

  2. Design of UAV Systems c 2002 LM Corporation Parametric weights Importance These are the fundamental weight relationships needed to define an air vehicle for a conceptual UAV system 19-2

  3. Design of UAV Systems c 2002 LM Corporation Parametric weights Editorial comment • The first part of this lesson will be a fairly conventional, albeit UAV slanted, discussion of weight fractions (empty weight, fuel and payload) • Assumed weight fractions are traditionally used as a starting point for air vehicle sizing • The advantage is simplicity, the weaknesses is high sensitivity and the inability to capture important configuration or technology features • Therefore, assuming weight fractions is not my favorite way of sizing air vehicles • - At the end of the lesson we will discuss another method that is almost as simple and does a better job of capturing design and technology features Nonetheless, it is important that you understand the weight fraction method and how it is applied 19-3

  4. Design of UAV Systems c 2002 LM Corporation Parametric weights Discussion subjects • Parametric weights • Weight categories • Weight fractions • Empty weight • Fuel • Payload • Miscellaneous • Performance • Bottoms-up weights • Geometry based weights • Conceptual design weights 19-4

  5. Design of UAV Systems • Weights are typically defined in categories such as • W0 = We + Wpay + Wf + Wcr+ Wmisc (19.1) • W0 = Gross weight ≈ Takeoff weight • We = Empty weight • Wpay = Payload weight • Wf = Fuel weight • Wcr = Crew weight (for UAV = 0) • Wmisc = Other weights (trapped fuel, oil, pylons, special mission, equipment, etc.) where c 2002 LM Corporation Parametric weights Weight definitions - review* * For additional information see RayAD 3.2-3.5 & 6.2 and RosAD.1 2.0-2.4 19-5

  6. Design of UAV Systems c 2002 LM Corporation Parametric weights Empty weight • Empty weight is also defined in categories such as: • We = Waf + Wlg + Weng + Wfe + Wos (19.2) • Waf = Airframe (structure) weight • Wlg = Landing gear weight • Weng = Propulsion system weight • Wfe = Fixed equipment weight (avionics, etc) • Wos = Other systems • These categories are useful for concept design • Their weights are typically driven by different design issues. For example: • - Airframe weight often scales with wetted area • - Landing gear weight scales with takeoff weight • - Fixed equipment weight is constant, etc. • Later we will use equations 19.1 and 19.2 to do what we will call a “bottoms-up” weight estimate where Later we will combine both of these into one We category –systems+avionics or Wspa 19-6

  7. Design of UAV Systems c 2002 LM Corporation Parametric weights Weight fractions - review • Another commonly used form of weight parametric. • From Equation 19.1 • We/W0 + Wpay/W0 + Wf/W0 + Wmisc/W0 = 1 (19.3) • where by definition • We/W0 = Empty weight fraction (EWF) • Wpay/W0 = Payload fraction (PF) • Wf/W0 = Fuel Fraction (FF) • Wmisc/W0 = Misc. weight fraction (MWF) • There is a similar form of Equation 19.2 • EWF = Waf/W0 + Wlg/W0 + Weng/W0 + Wspa/W0 + Wfe/W0 (19.4) • RosAD.5 Appendix A tabulates these weight fractions for a wide range of manned aircraft 19-7

  8. Design of UAV Systems c 2002 LM Corporation Parametric weights Weight fractions - review • Empty weight fraction and fuel fraction are key design parametrics • - They vary widely with design mission and vehicle class • - There are physical constraints on what they can be • Range and/or endurance, speed, maneuver, payload and technology level are primary drivers. Typical value 19-8

  9. Design of UAV Systems c 2002 LM Corporation Parametric weights EWF variation Within a given aircraft class, EWF will also vary - widely Data source - Roskam, (RosAD.1) 19-9

  10. Design of UAV Systems c 2002 LM Corporation Parametric weights Fuel Fraction variation • Ditto for fuel fraction (FF). Design is about choices. Fuel and EW fractions reflect these choices Data source - RosAD.1, Table 2.4 19-10

  11. Design of UAV Systems c 2002 LM Corporation Parametric weights UAV weight fractions • Current UAVs are designed primarily for endurance. Empty weight and fuel fractions correlate accordingly Global Hawk Global Hawk Data sources - Janes UAVs, Shepard UAVs, AUVSI 19-11

  12. Design of UAV Systems c 2002 LM Corporation Parametric weights Payload fraction • Payload fraction (PF) is another fundamental design driver • -Most aircraft designs are driven primarily by payload requirements • Payload definitions • - Internal/external stores and removable mission equipment are considered payload • - For manned aircraft, passengers are defined as payload, crew members and their equipment are not • - In order to correlate manned and unmanned aircraft our payload fraction will include crew weight, crew equipment and payload in a single equivalent “ payload” parametric 19-12

  13. Design of UAV Systems c 2002 LM Corporation Parametric weights PF comparisons 19-13

  14. Design of UAV Systems c 2002 LM Corporation Parametric weights Miscellaneous weight fraction • Miscellaneous weights can be initially estimated as a gross weight fraction • A typical value would be 1% • - A small number but one we should not ignore • It is better to guess at a number than to leave it out • - We might forget to put it back in! • ….or as a percentage of useful load • - Useful load is defined as gross weight minus empty weight • - A typical value would be 2%, 19-14

  15. Design of UAV Systems c 2002 LM Corporation Parametric weights Typical application • How do the example TBProp and TBFan UAV empty weights compare to manned aircraft? • - Based on Predator B and C, we assumed empty weight fractions of 0.44 and 0.39 • The assumptions do not fit manned aircraft data • Are Predator B/C designed that much differently from their manned TBProp and TBFan counterparts? 19-15

  16. Design of UAV Systems c 2002 LM Corporation Parametric weights Application cont’d • The calculated TBProp and TBFan fuel fractions (FFs) were 0.175 and 0.354. • - Both fit manned aircraft parametric data • What happened? • By definition, FF is not affected by assumed EWF 19-16

  17. Design of UAV Systems c 2002 LM Corporation Parametric weights One more application • How do TBProp and TBFan UAV “payload” fractions PFs (0.375 and 0.25) compare to manned aircraft? • They don’t • Payload fraction is a design choice but…. • They also don’t fit UAV payload fraction parametrics either (chart 19-13) • Another indication of questionablesizing results 19-17

  18. Design of UAV Systems c 2002 LM Corporation Parametric weights Wrap up - weight fractions • Within any vehicle class, weight fractions can vary widely • - Yet most conceptual sizing procedures start with assumed empty weight, fuel or payload weight fractions • Often the result is a significant difference between initial size estimates and subsequent ones based on higher fidelity methods • Lots of effort is spent analyzing the wrong size concept • Therefore, we will use another sizing approach • Nonetheless, weight fraction are still useful for parametric comparison • We can use them to test the validity of our calculated weight estimates • If they don’t fit within the data range, we need to make sure we understand why 19-18

  19. Design of UAV Systems c 2002 LM Corporation Parametric weights One last subject • Performance weight fractions • Raymer and Roskam also use gross weight fractions to make preliminary fuel consumption estimates for some mission segments • - Examples from RayAD Table 3.2 • Warmup and takeoff = 0.97 • Climb = 0.985 • Landing = 0.995 • Notional values are really not required • - Physically relevant mission segment calculations can replace notional values with little additional work • We will will address this further in lesson 21 19-19

  20. Design of UAV Systems c 2002 LM Corporation Parametric weights Next subject • Parametric weights • Weight categories • Weight fractions • Empty weight • Fuel • Payload • Miscellaneous • Performance • Bottoms-up weights • Geometry based weights • Conceptual design weights 19-20

  21. Design of UAV Systems c 2002 LM Corporation Parametric weights Bottoms-up weights • A bottoms-up estimate is a process for estimating weights in categories, each of which is influenced by similar design drivers as discussed earlier, e.g. • Payload weights are defined by mission requirements • Fuel fraction is determined by mission requirements and aero-propulsion performance • - Airframe weight is influenced by wing-body-tail Swet, etc. • Landing gear is driven by maximum vehicle weight (W0) • Engine weight is driven required air vehicle thrust-to-weight (TO/W0), etc. • Our initial bottoms-up UAV estimate categories will be defined by combining equations 19.1-19.4 or • W0 = [Wpay+Wfe]+[(Waf/Sref)Sref]+ [FF+(Wlg/W0) • +(Weng/T0)(T0/W0)+Wos/W0]W0 • +[Wmisc/(W0-We)](W0-We) (19.5) 19-21

  22. Design of UAV Systems c 2002 LM Corporation Parametric weights Bottoms-up weight inputs • A variety of sources will provide parametric data for bottoms-up weight estimates • Payload weight and fuel fraction will be input as variables • Airframe weight (initially) will be estimated from parametric data • We will use an airframe weight parametric (Waf/Sref) • A similar parameter (We/Sref) will also be used for parametric empty weight comparisons • Later we will use airframe unit weights (e.g. RayAD Table 15.2) and geometry to refine the estimates • RayAD Table 15.2 weight fractions are used for landing gear and systems plus avionics (aka, “all else empty”) • Lesson 18 propulsion parametrics will provide engine thrust-to-weight or power-to-weight inputs • A nominal 2% useful load will be used to account for miscellaneous weights 19-22

  23. Design of UAV Systems c 2002 LM Corporation Parametric weights UAV weights • Little detailed UAV weight data is available • - We will assume that selected and/or adjusted manned aircraft weight data can be used until more UAV data becomes available • Parametric comparisons indicate that manned and unmanned aircraft weights are comparable (exc. GH) Expanded scale Global Hawk Global Hawk TR-1 19-23

  24. Design of UAV Systems Airframe Weight Comparisons - (data from Roskam and Janes) 25 Biz Jet SE Piston Prop 20 ME Piston Prop Reg Turbo Jet Trans 15 Jet fighters Mil Train 10 5 TR-1 0 0 25 50 75 GTOW/Sref (psf) c 2002 LM Corporation Parametric weights UAV weights – cont’d There are reasons why manned aircraft weight data should be applicable - Landing gear weight will be 3-6% W0 whether manned or unmanned - Engine T0/W0 and Bhp0/W0 will be no different for UAVs - Most system weights should scale with empty or gross weight whether manned or not - Payload avionics, however, will be UAV unique And for now we will assume that airframe weights are comparable and correlate like EW/Sref Note - Roskam definition includes landing gear in airframe weights, we do not 19-24

  25. Design of UAV Systems c 2002 LM Corporation Parametric weights Typical application • Example TBProp UAV bottoms-up weight estimate • For W0/Sref = 30, we calculated Bhp0/W0 = 0.092 to meet our 1500 ft ground roll requirement (chart 18-22) • - From our Breguet range analysis (chart 15-40) we estimated W0 = 1918 lbm and can calculate Sref = 1918/30 = 63.9 sqft • - From chart 19-24 we can estimate Waf/Sref = 9 psf and calculate Waf = 575.4 lbm • - From Shp0/W0 we know BHp0 = 176.5 • - Chart 18-13 shows that a TBP of this size produces about 2.25 Shp/lb so that Weng (uninstalled) = 78.4 lbm • - Using RayAD Table 15.2 installation factor (Kinst) = 1.3 we calculate installed engine weight = 101.9 lbm each 19-25

  26. Design of UAV Systems c 2002 LM Corporation Parametric weights Application cont’d • - From RayAD Table 15.2 we assume Wlg/W0 = 0.05 and calculate Wlg = 95.9 lbm • - We use the RayAD Table 15.2 Wspa or “all else empty” factor of 12% to estimate system and avionics weights • - In doing this we are assuming that the additional systems and avionics needed for manned aircraft are offset by the systems and avionics unique to a UAV (may not be valid) • We assume Wmisc = 2% of useful load • - Payload weight is given & FF = 0.175 TBP weight calculation (lbm) Waf 575.4 Wpay 720 Weng (instl) 101.9 WF 375.7 Wlg 95.9 Wmisc 18.3 Wspa 230.2 W0 2117.4 We 1003.4 Note that W0 differs from our initial value (1918 lbm) 19-26

  27. Design of UAV Systems c 2002 LM Corporation Parametric weights Weight iteration • One characteristic of a bottoms-up weight estimate is a requirement to iterate the solution to convergence • We do this by brute force using spreadsheet analysis methods and after a number of iterations (17) the following bottoms-up weight estimate results • Copy bottoms up weight equations “n” times, update W0 each iteration (See ASE261.BUWeights.xls) Converged TBP weights (lbm) Waf 739 Wpay 720 Weng (instl) 131 WF 431 Wlg 123 Wmisc 23 Wspa 296 W0 2463 We 1288 EWF = 0.52 19-27

  28. Design of UAV Systems c 2002 LM Corporation Parametric weights Next subject • Parametric weights • Weight categories • Weight fractions • Empty weight • Fuel • Payload • Miscellaneous • Mission segment • Bottoms-up weights • Geometry related weights • Conceptual design weights 19-28

  29. Design of UAV Systems c 2002 LM Corporation Parametric weights Geometry related weights • RayAD Table 15.2 lists airframe component unit weights (weight per unit area) for three vehicle types • - Unit weight factors can be used to do an airframe component weight build-up when areas are known: • - Fuselage (Wfuse) = SwetFus*Uwf (19.6) • - Wing weight (Wwing) = SrefExp*Uww (19.7) • - Horizontal tail (Wht) = Sht*Uwht (19.8) • - Vertical tail (Wht) = Svt*Uwvt (19.9) • - Uwf = Fuselage weight /Swet-fus • - Uww = Wing weight / SrefExp • SrefExp = Exposed wing area • Uwht = Tail weight /Horizontal tail area (Sht) • - Uwvt = Tail weight /Vertical tail area (Svt) • - Where for simplicity we assume fuselage weights include engine nacelles or • - Uwfpn = Fuselage+nacelle/Swetfpn = Uwfpn where 19-29

  30. Design of UAV Systems • Therefore, by definition, airframe weight is given by • Waf = Wfuse + Wwing + Wht + Wvt = • Uwfpn*Swetfpn + Uww*Sref*(Srefexp/Sref) + Uwht*Kht*Sref + Uwvt*Kvt*Sref • Waf/Sref = Uww*(Srefexp/Sref) + Kht*Uwht • + Kvt*Uwvt + Uwfpn*Swetfpn/Sref (19.10) • Combining equations 19.3, 19.4 and 19.10: • W0/Sref = Waf/Sref /((1 - FF - PF)/(1 - Kwmisc) - Kwprop - Kwlg - Kwspa) (19.11) • Kwmisc = Misc. wt /(useful load  W0 - We) • Kwprop = Installed propulsion weight fraction = Kint*(T0/W0)/(Neng*T0/Weng) • Kwpint = Propulsion installation weight factor (≈ 1.3) • Kwlg = Wlg/W0 (≈ 0.3 - 0.6) • Kwspa = (Wsystems+Wavionics)/W0 (≈ 0.10 - 0.17) or where c 2002 LM Corporation Parametric weights Airframe weight 19-30

  31. Design of UAV Systems c 2002 LM Corporation Parametric weights Application • We can use Eq 19.9 to check the Chart 19-24 airframe weight parametric using typical area ratios for the 3 aircraft types in RayAD Table 15.2 • FightersBombers & transports General aviation • Uww (psf) 9.0 10.0 2.5 • Uwht (psf) 4.0 5.5 2.0 • Uwvt (psf) 5.3 5.5 2.0 • Uwfpn(psf) 4.8 5.0 1.4 • Overall* 19.6 35.2 6.9 • Comparison with Chart 19-23 shows good agreement for fighters at Swet/Sref = 4, bombers at Swet/Sref = 6.5 and general aviation aircraft at Swet/Sref = 4.5 • Raymer’s unit weights look good! * Landing gear included 19-31

  32. Design of UAV Systems c 2002 LM Corporation Parametric weights Next subject • Parametric weights • Weight categories • Weight fractions • Empty weight • Fuel • Payload • Miscellaneous • Mission segment • Bottoms-up weights • Geometry based weights • Conceptual design weights 19-32

  33. Design of UAV Systems c 2002 LM Corporation Parametric weights Conceptual design weights • Conceptual design weight estimates are typically based on statistical weight methods (see RayAD 15.3) • - Component aircraft weights are compiled and statistically analyzed • RayAD Equations 15.1-15.59 are examples available from US government public release documents • Individual companies typically have their own proprietary weight equations that reflect actual internal design and manufacturing capabilities • For student design projects, Raymer’s equations are more than adequate • Even though our spreadsheet analysis methods are most applicable for pre-concept design studies, they can also be used for concept studies and trades • during the early phases of conceptual design • However, statistical weight equations should be used to generate multipliers to correct the pre-concept design weight estimates for the baseline design 19-33

  34. Design of UAV Systems c 2002 LM Corporation Parametric weights Expectations • You should now understand • Basic weights and weight parametrics • Where they come from • How they are used • The limits of their applicability 19-34

  35. Design of UAV Systems c 2002 LM Corporation Parametric weights Other expectations - ABET • Each final report should contain a section (one paragraph or longer) addressing each of the following points. Note that not all of these issues may be relevant to your project, but you should think about them before concluding that they are irrelevant and justify your decision. These sections should be included in your index and should be mentioned in your executive summary. • Economic Issues – How will your project, if done, affect the economy of the US and perhaps the world. Does your project require resources that are difficult to obtain? • Environmental Issues – How will your project, if done, affect the environment of the earth? Discuss any positive and/or negative factors. • Sustainability Issues – Are there sustainability issues with your design. Is it meant to be one shot or the backbone for later work. • Manufacturability Issues – Will your spacecraft I facility be manufactured on earth, in space, on Mars, or where. Will it be assembled and then flown or flown in parts and assembled later. 19-35

  36. Design of UAV Systems c 2002 LM Corporation Parametric weights Other expectations • ABET (Continued) • Ethics Issues – Are there ethical issues associated with your project? If so, identify and discuss them. • Political Issues – Are there political issues associated with your project? If so, identify and discuss them.  • Health and Safety Issues – Are there health and safety issues associated with your project? If so, identify and discuss them. • Social Issues – Are there social issues associated with your project? If so, identify and discuss them. • Global Impact – What is the global impact of your project? Discuss it. 19-36

  37. Design of UAV Systems c 2002 LM Corporation Parametric weights Homework • Write a spreadsheet program to calculate bottoms up weights with W0/Sref, Wpay, FF, Kmisc, BHp0/W0, Bhp0/Weng, Kinst, Wlg/W0, Waf/Sref and Wspa/W0 as inputs - (team grade) • 2. Run your spreadsheet for the example problems (charts 19-25/27) and compare results (team grade) • Identify any errors in my example problems • 3. Use the team spreadsheets to calculate bottoms up weights for your proposed air vehicle (individual grade) • 4. Compare your spreadsheet results to ASE261.BUWeights.xls and identify differences (individual grades) • 5. Discuss ABET issues #1 and #2 and document your conclusions (one paragraph each– team grade) 2nd week 19-37

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