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Parametric weight methods
Design of UAV Systems Parametric weights c LM Corporation 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
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Design of UAV Systems Importance
Parametric weights c LM Corporation Importance These are the fundamental weight relationships needed to define an air vehicle for a conceptual UAV system 19-2
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Editorial comment Design of UAV Systems
Parametric weights c LM Corporation 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
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Geometry based weights Conceptual design weights
Design of UAV Systems Parametric weights c LM Corporation 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
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Weight definitions - review*
Design of UAV Systems Parametric weights c LM Corporation Weight definitions - review* 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 * For additional information see RayAD & 6.2 and RosAD 19-5
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Empty weight is also defined in categories such as:
Design of UAV Systems Parametric weights c LM Corporation 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
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Weight fractions - review
Design of UAV Systems Parametric weights c LM Corporation Weight fractions - review Another commonly used form of weight parametric. From Equation 19.1 We/W0 + Wpay/W0 + Wf/W0 + Wmisc/W0 = (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/W (19.4) RosAD.5 Appendix A tabulates these weight fractions for a wide range of manned aircraft 19-7
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Weight fractions - review
Design of UAV Systems Parametric weights c LM Corporation 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
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Data source - Roskam, (RosAD.1)
Design of UAV Systems Parametric weights c LM Corporation EWF variation Within a given aircraft class, EWF will also vary - widely Data source - Roskam, (RosAD.1) 19-9
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Fuel Fraction variation
Design of UAV Systems Parametric weights c LM Corporation 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
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Data sources - Janes UAVs, Shepard UAVs, AUVSI
Design of UAV Systems Parametric weights c LM Corporation 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
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Payload fraction (PF) is another fundamental design driver
Design of UAV Systems Parametric weights c LM Corporation 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
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Design of UAV Systems PF comparisons Parametric weights
c LM Corporation PF comparisons 19-13
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- Useful load is defined as gross weight minus empty weight
Design of UAV Systems Parametric weights c LM Corporation 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
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The assumptions do not fit manned aircraft data
Design of UAV Systems Parametric weights c LM Corporation 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
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- Both fit manned aircraft parametric data What happened?
Design of UAV Systems Parametric weights c LM Corporation Application cont’d The calculated TBProp and TBFan fuel fractions (FFs) were and - Both fit manned aircraft parametric data What happened? By definition, FF is not affected by assumed EWF 19-16
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Payload fraction is a design choice but….
Design of UAV Systems Parametric weights c LM Corporation 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 questionable sizing results 19-17
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Wrap up - weight fractions
Design of UAV Systems Parametric weights c LM Corporation 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
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Performance weight fractions
Design of UAV Systems Parametric weights c LM Corporation 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
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Geometry based weights Conceptual design weights
Design of UAV Systems Parametric weights c LM Corporation 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
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Payload weights are defined by mission requirements
Design of UAV Systems Parametric weights c LM Corporation 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 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
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Bottoms-up weight inputs
Design of UAV Systems Parametric weights c LM Corporation 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
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Little detailed UAV weight data is available
Design of UAV Systems Parametric weights c LM Corporation 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
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There are reasons why manned aircraft weight data should be applicable
Design of UAV Systems Parametric weights c LM Corporation 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 Airframe Weight Comparisons - (data from Roskam and Janes) 5 10 15 20 25 50 75 GTOW/Sref (psf) Biz Jet SE Piston Prop ME Piston Prop Reg Turbo Jet Trans Jet fighters Mil Train TR-1 Note - Roskam definition includes landing gear in airframe weights, we do not 19-24
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Example TBProp UAV bottoms-up weight estimate
Design of UAV Systems Parametric weights c LM Corporation Typical application Example TBProp UAV bottoms-up weight estimate For W0/Sref = 30, we calculated Bhp0/W0 = 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 we can estimate Waf/Sref = 9 psf and calculate Waf = lbm - From Shp0/W0 we know BHp0 = 176.5 - Chart 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 = lbm each 19-25
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TBP weight calculation (lbm)
Design of UAV Systems Parametric weights c LM Corporation 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 Wpay 720 Weng (instl) WF Wlg Wmisc Wspa W We Note that W0 differs from our initial value (1918 lbm) 19-26
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Converged TBP weights (lbm)
Design of UAV Systems Parametric weights c LM Corporation 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 Wpay Weng (instl) WF Wlg Wmisc Wspa W We EWF = 0.52 19-27
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Geometry related weights Conceptual design weights
Design of UAV Systems Parametric weights c LM Corporation 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
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Geometry related weights
Design of UAV Systems Parametric weights c LM Corporation 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
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Therefore, by definition, airframe weight is given by
Design of UAV Systems Parametric weights c LM Corporation Airframe weight 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 (≈ ) Kwspa = (Wsystems+Wavionics)/W0 (≈ ) or where 19-30
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Fighters Bombers & transports General aviation
Design of UAV Systems Parametric weights c LM Corporation Application We can use Eq 19.9 to check the Chart airframe weight parametric using typical area ratios for the 3 aircraft types in RayAD Table 15.2 Fighters Bombers & transports General aviation Uww (psf) Uwht (psf) Uwvt (psf) Uwfpn(psf) Overall* Comparison with Chart 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
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Geometry based weights Conceptual design weights
Design of UAV Systems Parametric weights c LM Corporation 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
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Conceptual design weights
Design of UAV Systems Parametric weights c LM Corporation 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 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
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You should now understand Basic weights and weight parametrics
Design of UAV Systems Parametric weights c LM Corporation 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
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Other expectations - ABET
Design of UAV Systems Parametric weights c LM Corporation 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
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Design of UAV Systems Other expectations ABET (Continued)
Parametric weights c LM Corporation 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
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Identify any errors in my example problems
Design of UAV Systems Parametric weights c LM Corporation 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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