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Design of UAV Systems. c 2002 LM Corporation. Air vehicle parametrics. Objectives. Lesson objective - to discuss Air vehicle parametrics including … Rationale Applications Limits. Expectations - You will understand when and how to use parametric relationships. 15-1.

jermaine-sweet
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  1. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Objectives • Lesson objective - to discuss • Air vehicle parametrics • including … • Rationale • Applications • Limits Expectations - You will understand when and how to use parametric relationships 15-1

  2. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Many types of parametrics • Range/endurance related parametrics • Speed (V) • Lift-to-drag ratio (L/D) • Specific fuel consumption (SFC) • Fuel fraction (FF) • Range factor (RF) • Specific range (SR) • Example problem • Propulsion related parametrics • Internal combustion • Turboprop • Turbojet & turbofan • Afterburners • Weight related parametrics • Fuel • Payload • Structure • Systems This lesson Covered under propulsion Covered under weights 15-2

  3. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Why parametrics? • During pre-concept design we need reasonable, notdesign specific, solutions* • Good enough to support technology readiness, cost, risk and schedule estimates • Parametrics enable Pre-Concept Design studies that don’t require the user to specify a design • During conceptual design we need to systematically explore a wide range of potential concepts • Parametric design methods allow even small teams to evaluate and compare (quantitatively) a wide range of concepts and technologies • During both phases, speed and accuracy are critical • Parametric design methods can significantly reduce “design” and analysis time and produce credible results * Customers should avoid the temptation of specifying the design solution, they almost always get what you ask for and it may not be the best available 15-3

  4. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Definitions • From Webster’s New Collegiate Dictionary • Parameter – any set of physical properties whose value determine the characteristics or behavior of something • Our definition • Design parametric – fundamental design parameter whose value determines the design or performance characteristics of a design • Usually (but not always) a multi-variable relationship • e.g., wing loading (W0/Sref), Swet/Sref, etc. • Parametric design – Parametric based design approach to define, size, estimate performance and do trade offs on classes of conceptual air vehicles • Different from the traditional approach 15-4

  5. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics How is it different? • Traditional conceptual design starts with a sketch • See RayAD Chapter 3, Sizing from a Conceptual Sketch • The sketch or drawing is analyzed • Using a variety of techniques • Aerodynamics from geometry and parametrics • Weight “fraction” parametrics from historical data • Propulsion from parametrics or “cycle decks” • Performance from fuel or weight fractions and Breguet range and/or endurance equations • The concept is “sized” to meet mission requirements • - Based on results of the first analysis • A scaled drawing is made and analysis inputs generated • Higher fidelity analyses is performed • Based on actual configuration areas and features • Performance is calculated and compared to mission requirements and/or team expectations • The process is repeated until expectations are met 15-5

  6. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics What’s the problem? • The process is time and data intensive • Teams plan to evaluate a wide range of concepts but often never get beyond the first concept or sketch • Particularly for student design teams • The first concept gets most of the attention • Lots of effort expended to make it meet expectations • Teams start to fall in love with the concept • Alternatives get little attention • “Qualitative” comparisons eliminate the competition • Errors and disconnects start to surface and/or requirements problems emerge • Teams scramble to recover, fixing errors gets priority • Trade studies to improve performance are defined but seldom completed • Too much work, not enough time • Everybody hopes the reviewers won’t see the flaws • - And wish they had more time 15-6

  7. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics The alternative? • Use simple analytical geometry models instead of concept drawings to generate data for aero, weight and propulsion analysis and mission performance • - Physically capture important design variables but minimize the time and effort required to assess them • Use full mission integrated spreadsheet analysis to evaluate performance • Size for the actual mission, reduce dependence on configuration insensitive rule-of-thumb estimates • Empty weight fraction, fuel for climb, etc. • Quickly and systematically evaluate a range of concepts • Select preferred concepts and technologies based on data • Draw and analyze the preferred concept • Confirm vs. discover how it really performs 15-7

  8. Design of UAV Systems End cruise at W = W0 - (1-Klr)*WF 12 13 14 17 16 15 Start cruise at W = W0 - Kttoc*WF 4 9 5 6 7 8 10 11 18 Border - Loiter/Penetrate 1 0 Border - Standoff Border - Penetrate/Loiter 19 3 2 Kttoc = (taxi-takeoff-climb fuel)/Wf Klr = landing fuel reserves/Wf Wf = fuel weight Notation Terminology 0 Engine start 1 Start taxi 2 Start takeoff 3 Initial climb 4 Initial cruise 5 Start pre-strike refuel 6 End pre-strike refuel Start cruise 7 Start loiter 8 End loiter, start cruise 9 Start ingress 10 Combat 11 Weapon release 12 Turn 13 Start egress 14 End egress, start cruise 15 Start post-strike refuel 16 End post-strike refuel 17 End cruise 18 Start hold 19 End hold Standoff - Distance from loiter or combat to border (+/-) Standback - Distance from refuel to border Ingress - To target at penetration speed Egress - From target at penetration speed Range (Rge) = 2*Radius(R) c 2002 LM Corporation Air vehicle parametrics Example mission 15-8

  9. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics The UAV challenge • Parametric design requires historical data for use in preliminary sizing & analysis and reality checks • - There is a limited amount of good data available on UAVs (from public release sources) • - A lot of the stuff is marketing hype and useless for design • We will use available UAV data and fill in the gaps with manned aircraft data • - Example problems will be structured to show you how to do it 15-9

  10. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Next subject • Range/endurance related parametrics • Speed (V) • Lift-to-drag ratio (L/D) • Specific fuel consumption (SFC) • Fuel fraction (FF) • Range factor (RF) • Specific range (SR) • Example problem • Propulsion related parametrics • Internal combustion • Turboprop • Turbojet & turbofan • Afterburners • Weight related parametrics • Fuel • Payload • Structure • Systems This lesson Covered under propulsion Covered under weights 15-10

  11. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Range related parametrics • Based on factors from the Breguet range equation • - For jet aircraft (See RayAD 3.5) • R = [Vcr(L/Dcr)/TSFCcr]Ln(Wi/Wj) (15.1) • R = Cruise range • Vcr = Cruise speed • L/Dcr = Cruise lift-to-drag ratio (LoDcr) • TSFCcr = Cruise thrust specific fuel consumption • VcrL/Dcr/TSFCcr = RF (range factor) = WSR • SR = Specific Range = V/Fuel flow) • - For propeller aircraft (more about this later) • RF(nm) = 325.6p(L/Dcr)/SFCcr (15.1a) • p = propeller efficiency •  0.8 (for constant speed prop) where and where 15-11

  12. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Endurance related parametrics • Based on Breguet endurance equation factors • - For jet aircraft (RayAD 3.7) • E = [(L/Dlo)/TSFClo]Ln(Wi/Wj) (15.2) • E = Endurance (hrs) • L/Dlo = Loiter lift-to-drag ratio (LoDcr) • TSFClo = Loiter thrust specific fuel consumption • (L/Dlo)/TSFClo = EF (endurance factor) = Wbar/WdotF • Wbar = Average loiter weight (lbm) • WdotF = Fuel flow (lbm/hr) • - For propeller aircraft (more about this later also) • E = EFLn(Wi/Wj) (15.2a) • EF = 325.6p [(L/Dlo)/(VloSFClo] • Vlo = Loiter speed; p = propeller efficiency where and where 15-12

  13. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Weight fraction version • The Breguet equation is often expressed in the form of weight fractions (more in Lesson 18) where: • Empty weight fraction (EWF)  Empty weight/Gross wt. • Fuel fraction (FF)  Fuel weight/Gross wt. • Payload fraction (PF)  Payload wt./Gross wt. • Misc weight fraction (MiscF)  Misc. weight/Gross wt. • Where by defintion • Gross weight (W0)  Empty weight (We) + Fuel Weight • (Wf) + Payload weight (Wpay) + Other weight (Wmisc) • Dividing through by W0 and solving, • FF = 1 - EWF - PF - MiscF (15.3) • Maximum range and endurance occur when • (Wi/Wj)max = (1 - Kttoc*FF)/(1-(1- Klr)*FF) (15.4) • Rmax = RFln[(Wi/Wj)max] (15.5) • Emax = [(L/Dlo)/TSFClo]ln[(Wi/Wj)max] (15.6) or 15-13

  14. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Typical weight fractions Typical value Caution • - Within any vehicle class, weight fractions can vary widely • - Nonetheless, most initial concept design procedures start with an assumed empty weight fraction (EWF) • - This can cause problems, as we will see later • - Later we will introduce an alternative approach 15-14

  15. Design of UAV Systems SE-Prop ME-Prop Biz Jet Reg. Turbo Jet Transp. Mil. Trainers Fighters Mil PBC FW UAV GW Min 1817 2183 4550 5732 44000 2238 5291 50706 4 Mean 2750 6625 20000 14550 220000 4188 29975 158730 1245 Max 11574 10325 68200 57250 775000 11100 91500 769000 25600 EWF Min 0.437 0.555 0.479 0.517 0.428 0.461 0.381 0.327 0.359 Mean 0.595 0.623 0.548 0.577 0.536 0.697 0.472 0.491 0.605 Max 0.791 0.689 0.622 0.662 0.610 0.789 0.639 0.732 0.870 FF Min 0.059 0.115 0.291 0.149 0.199 0.101 0.101 0.208 0.111 Mean 0.125 0.178 0.359 0.230 0.326 0.213 0.212 0.405 0.266 Max 0.283 0.298 0.417 0.334 0.536 0.381 0.387 0.674 0.566 c 2002 LM Corporation Air vehicle parametrics Database variation examples 15-15

  16. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Next • Range/endurance related parametrics • Speed (V) • Lift-to-drag ratio (L/D) • Specific fuel consumption (SFC) • Fuel fraction (FF) • Range factor (RF) • Specific range (SR) • Example problem • Propulsion related parametrics • Internal combustion • Turboprop • Turbojet & turbofan • Afterburners • Weight related parametrics • Fuel • Payload • Structure • Systems Covered under propulsion Covered under weights 15-16

  17. Design of UAV Systems • Typically determined by propulsion system type • Internal combustion(IC)* …..……. • Turboprop (TBP)* ……………….. • High BPR turbofan (TBF)……….. • Low BPR TBF without AB……….. • Low BPR TBF with AB…………... • Turbojet (TBJ) without AB……….. • Turbojet with after burner (AB)….. • Turbo Ramjet (TRJ)……………… • Ramjet (RJ)……………………….. • Scramjet (SRJ)……………………. • Rocket……………………………… 50 - 300 Kts 200 - 350 Kts 350 - 500 Kts 350Kts - M1.5 400Kts - M2.5 350Kts - M1.0 400Kts - M3.0+ M2.0 - M3.5+ M2.0 - M5.0+ M5.0 - M12+ M1.0 - M25+ c 2002 LM Corporation Air vehicle parametrics Cruise speed ranges * Typical operating regime - higher speeds have been demonstrated 15-17

  18. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Variation with altitude 15-18

  19. Design of UAV Systems • Condensed from RosAD.1,Table 2.2* • Single & twin engine - prop …………... • STOL …………………………………… • Business jets ………………………….. • Regional turboprop……………………. • Jet transports………………………….. • Military trainers………………………... • Fighters………………………………… • Supersonic cruise…………………….. • Average Cruise 8 - 10 5 - 7 10 - 12 11 - 13 13 - 15 8 - 10 4 - 7 4 - 6 8.9 Loiter 10 - 12 8 - 10 12 - 14 14 - 16 14 - 18 10 - 14 6 - 9 7 - 9 11.4 (+25%) Global Hawk** …………………………. 33 - 34 * Also see RayAD Fig 3.6 ** Flight International, UAVs, page 28,5 /1/01 c 2002 LM Corporation Air vehicle parametrics Typical L/D ranges 15-19

  20. Design of UAV Systems • Determined by propulsion and fuel • IC** …………………………………. • TBP*** ……………………………… • High BPR TBF ……………………. • Low BPR TBF (without AB)………. • Low BPR TBF (with AB)………….. • TJ (without AB)……………………. • TJ (with AB)………………………... • TRJ………………………………….. • M4 RJ (Hydrogen/Hydrocarbon)…. • M8 SRJ (Hydrogen/Hydrocarbon).. • Rocket (Hydrogen/Hydrocarbon)… Cruise 0.4 0.5 0.5 0.8 2+ 0.8 2+ 2+ 0.9/2 1.5/3.5 8/10 Loiter 0.5 0.6 0.4 0.7 - 0.7 - - - - - * Data from Roskam, Raymer and others ** IC SFC = Fuel Flow/HP; Turbine SFC = Fuel Flow/Thrust *** Turboprops use both forms - SFC(hp) and TSFC (Lbf) c 2002 LM Corporation Air vehicle parametrics Typical SFC ranges* 15-20

  21. Design of UAV Systems Cruise 0.4? 0.5 0.5? 0.8? Loiter 0.5 0.6? 0.4? 0.7? IC** ………... TBP*** …….. HBPR TBF .. LBPR TBF… c 2002 LM Corporation Air vehicle parametrics Real cruise SFCs From previous chart Notation 0 = Sea level static cr = Typical cruise altitude & speed 15-21

  22. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Range factor (RF) For typical fighters (@ subsonic cruise*) - we will derive range factors for UAVs during the course F-100 4920NM F-101 4530NM F-102 5390NM F-104 4500NM F-105 5200NM F-106 5400NM F-111 6450NM F3D 3750NM F3H 4480NM F4D 3820NM F-4 4200NM F-86 4870NM F-89 3970NM * From RAND N-2283/2-AF, Dec 1987, approved for public release • From 15.1 and 15.1a, Range factor (RF) • For jets (nm)  KTAS*(L/Dcr)/TSFCcr • For prop (nm)  325.6*p*(L/Dcr)/SFCcr 15-22

  23. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Specific range (SR) • A simple performance parametric used in many flight manuals • R = SR*W (15.7) • where SR = V/Wfdot (NM/Lbm-fuel) • and Wfdot = Fuel flow (lbm/hr) • - Typically used for optimum (constant Mach) cruise above 36 Kft • - Or high-q dash performance 15-23

  24. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Next • Range related parametrics • Speed (V) • Lift-to-drag ratio (L/D) • Specific fuel consumption (SFC) • Fuel fraction (FF) • Range factor (RF) • Specific range (SR) • Example problem • Propulsion related parametrics • Internal combustion • Turboprop • Turbojet & turbofan • Afterburners • Weight related parametrics • Fuel • Payload • Structure • Systems Covered under propulsion Covered under weights 15-24

  25. Design of UAV Systems 27.4 Kft 212 nm 27.4 Kft 10 Kft 158 nm 27.4 Kft 100 nm 200 nm x 200 nm c 2002 LM Corporation Air vehicle parametrics Example problem - review • Five medium UAVs, four provide wide area search (two are comm. relay), fifth does positive target ID • WAS range required (95km) not a challenge • No need to switch roles, simplifies ConOps • No need for frequent climbs and descents • Base communications and relay distances reasonable • 158nm & 212 nm • Reasonable dash speed (282kts) • WAS and ID operating altitude • differences reasonable • But…………. • What kinds of air • vehicles? • What propulsion? • How big will they be? • How will they perform? • What will they cost? 15-25

  26. Design of UAV Systems Atmospheric conditions (customer defined) Cloud ceiling/visibility Clear day, unrestricted 10Kft ceiling, 10 nm 5Kft ceiling, 5 nm 1Kft ceiling, 1nm • Percent occurrence • 50% • 30% • 15% • 05% c 2002 LM Corporation Air vehicle parametrics Positive ID - review • We have a threshold requirement for positive (visual image) target identification (ID) 80% of the time • To design our baseline for the threshold requirement • We have to be able to operate at or below 10 Kft for 30% of the target identifications • 50% of the time we can stay at altitude and 20% of the time we won’t see a target (unless we image at <= 5 Kft) • This places 10Kft efficient cruise, loiter and climb and descent rate requirements on the air vehicle 15-26

  27. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Derived requirements - review • Derived requirements (from our assumptions or studies) • System element • Maintain continuous WAS/GMTI coverage at all times • One target recognition assignment at a time • Assume uniform area distribution of targets • Communications LOS range to airborne relay = 158 nm • LOS range from relay to surveillance UAV = 212 nm • Air vehicle element • Day/night/all weather operations, 100% availability • Takeoff and land from 3000 ft paved runway • Cruise/loiter altitudes = 10 – 27.4Kft • Loiter location = 158 nm (min) – 255 nm (max) • Loiter pattern – 2 minute turn • Dash performance =141 nm @ 282 kts @10 Kft • Payload weight and volume = 720 lbm @ 26.55 cuft • Payload power required = 4700 W 15-27

  28. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics How do we start ? • Analyze the problem • What does the air vehicle have to do? • Is any information missing? • Look at some potential solutions • What are the overall design drivers? • Payload weight and volume • Range and endurance • Speed and propulsion type • Pick a starting baseline • Analyze it • Size/weight; range/endurance; cost and support • Define and analyze the other approaches • Compare results and select preferred baseline • Define/trade preferred overall system • Reasonable balance of cost, risk and effectiveness • Document results 15-28

  29. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics What kind of air vehicle? • One that operates from a 3000 ft paved runway • One that provides WAS over an area of interest • At h = 27.4 Kft, 158nm - 255 nm from base, • Fly circular pattern, 2 minute turns • Maximum coverage area = 50nm x 50 nm each • One that can ID targets at 141 nm in 30 minutes • Based on analysis of WAS sensor information • Based on other information • One that can image targets from 10 Kft • Once per hour (at maximum fly out distance) • But how long must it loiter? • 6 hours, 12 hours, 24 hours or even longer? • …and what is the definition of “all weather”? • Typhoons included? 15-29

  30. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Getting answers • Confer with team and/or ask the Systems Engineer • And insist on definitive (quantifiable) answers • Some typical responses – • Loiter time: • What the team wants to say - “interesting question, what are the trades?” • What the team needs to say - “lets baseline a 12 hour loiter and do a trade study on the effects of from 6 to 24 hours?” • All weather definition : “Statistics indicate terrible (unflyable) weather 10% of the time • Note: this conflicts with our 100% availability requirement 15-30

  31. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Other resources • Lessons to follow – the basic understanding, analysis methods, models and parametric data for preliminary sizing and estimating overall mission performance • Lesson 16: (Standard atmosphere) : Simple models that describe atmospheric properties as functions of altitude and speed • Lesson 17: (Aerodynamics) : first-order aerodynamic prediction methods that capture key configuration features • Lesson 18: (Parametric propulsion) : simplified engine models applicable across the performance envelope • Review 19: (Parametric weights) simplified weight models that capture key configuration features • Lesson 20 : (Parametric geometry) : simplified geometry models required to generate aerodynamic and weight inputs • Lesson 21: (Flight mechanics) simplified physics based relationships used to predict flight performance by mission segment • Lesson 22: (Integrated performance) : Spreadsheet models to perform initial sizing and calculate overall mission performance • Plus parametric data from real air vehicles needed to test and validate simplified model predictions 15-31

  32. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Our first decision • It is a very important one • What is the best propulsion cycle for the mission? • Internal combustion (ICprop), turboprop (TBProp) and turbo fan (TBFan) engines can all meet the baseline speed (280 kt) and altitude (10-27Kft) requirements • We bring our team together for the decision • Speed is at the upper end of IC capability and high availability required will be a real challenge for IC engines • TBProp is a good cycle for low-medium altitude operations • TBFan is best at altitudes > 30 Kft and has best reliability • We select a … • TBProp for our starting baseline and agree to evaluate a TBFan as the primary alternative • IC alternative decision will be based on size required • We start with conventional wing-body-tail configurations • - We can evaluate more innovative concepts during conceptual design 15-32

  33. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Conflicting requirements • System analysis thus far assumed 100% air vehicle availability and now weather limits availability to 90% • This will affect SAR sizing (primarily) • We assumed SAR operation 100% of the time, therefore, the SAR only needed 80% area coverage • At 90% availability, the SAR would need to provide 89% area coverage (range increase to 102km) to achieve overall 80% (threshold) target coverage • What should the we do, leave the baseline alone or resize the payload and start over? • Answer – leave it alone! • During any design cycle, there will always be design and requirement disconnects • If we change baselines every time we find a disconnect, we would never complete even one analysis cycle • Orderly changes occur at the end of an analysis cycle 15-33

  34. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Next decision • How many engines? • Generally determined by available engine size • The smallest number of engines will generally be the lightest and lowest drag • How big will they be? • Engine size is determined by thrust or horsepower-to-weight required to meet performance requirements • One sizing consideration is takeoff; others are speed, acceleration and maneuver • Initially we size for takeoff (see RayAD, page 99) • We assume a 3000 ft takeoff balanced field length • Balanced field length means the air vehicle can accelerate to takeoff speed, have an engine failure and brake to a safe stop within the specified length • We assume half the distance to reach takeoff speed • Later we will calculate performance over the entire mission and ensure that all requirements can be met 15-34

  35. Design of UAV Systems 200 nm x 200 nm c 2002 LM Corporation Air vehicle parametrics Next decision • Which mission do we size for? • WAS with maximum cruise out = 255nm at 27.4Kft • Baseline operational endurance is 12 hr, with trade study options for 6 hr and 24 hr endurance • ID mission with cruise out = 200 nm @ TBD Kft • Maximum ID distance constant at 141 nm, 282 kts • WAS missions are performed at best loiter speed (max L/D) and SFC • ID missions are at max. speed • (out and back), L/D will be lower • and SFC will be higher • Both will have the same take • off and landing requirements • Answer…. 100 nm 255 nm 158 nm simpler to design for WAS and calculate fallout ID mission performance 141 nm 15-35

  36. Design of UAV Systems 12 13 14 17 16 15 4 9 5 6 7 8 10 11 18 1 0 19 3 2 Border - Penetrate/Loiter Border - Loiter/Penetrate Border - Standoff Notation Terminology 0 Engine start 1 Start taxi 2 Start takeoff 3 Initial climb 4 Initial cruise 5 Start pre-strike refuel 6 End pre-strike refuel Start cruise 7 Start loiter 8 End loiter, start cruise 9 Start ingress 10 Combat 11 Weapon release 12 Turn 13 Start egress 14 End egress, start cruise 15 Start post-strike refuel 16 End post-strike refuel 17 End cruise 18 Start hold 19 End hold Standoff - Distance from loiter or combat to border (+/-) Standback - Distance from refuel to border Ingress - To target at penetration speed Egress - From target at penetration speed Range (Rge) = 2*Radius(R) c 2002 LM Corporation Air vehicle parametrics Mission notation 15-36

  37. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Defined & derived requirements • Defined • Remain airborne 24/7  90% of the time • Derived – payload, distances and altitudes • Payload : Wpay = 720 lbm • Cruise/loiter altidude: Hcr = Hlo = 27.4 Kft • Operating radius: D3-4+ D4-7 = D17-14 = 255 nm • Ingress/Egress: D8-14 = 0 • Assumptions – typical values (design independent) • Landing fuel reserves; Klr = 5%; MiscF = 1% • Propeller efficiency: p = 80% • First cut estimates – refine later (design dependent) • Taxi/takeoff/climb fuel: Kttoc =10% • Average rate of climb: ROCavg = 1500 fpm • Average climb speed: Vcl = 0.8 Vcr (more about this later) • Parametric estimates – Next chart (design dependent) • Unknowns – Gross weight (W0); Fuel fraction (FF) 15-37

  38. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Parametric estimates • Chart 15-14 shows nominal empty weight fractions for manned TBProps (EWF = 0.58) and UAVs (EWF = 0.6) • - Predator B/Altair shows EWF = 0.44; probably more representative of our concept • Chart 15-18 shows typical economic cruise/loiter speeds at 27Kft to be in range of 180-300 kts • We select lower value (180 kts) to maximize performance (for both cruise and loiter) • Chart 15-19 shows typical TBProp cruise/loiter LoDs • Regional TBProp: LoDcr = 11-13, LoDlo = 14-16 • Global Hawk LoDlo much higher (33-34) @ AR = 25 • We will select intermediate values @ 23 and 25 • Charts 15-20 (table) and 15-21 (plot) TBProp cruise & loiter SFCs conflict (not unusual for parametric data) • The plot is from our engine database (real TBProps) so we use it and estimate SFCcr = SFClo = 0.4 15-38

  39. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Solution approach • Calculate cruise ranges and range factors (15.1a) • Time to climb = 27.4 Kft/1500 fpm = 0.30 hr • Climb speed  0.8180 = 144 kts • Climb distance = 43.8 nm • R4-7 = 255 - 43.8 = 211.2 nm • R14-17 = 255 nm • RFcr = 325.6p(L/Dcr)/SFCcr = 14977.6 nm • Calculate outbound initial/final cruise weight ratios (15.1) • R4-7 = 14977.6 nmLn(W4/W7) or…. W4 = 1.014W7 • Calculate inbound initial/final cruise weight ratios (15.1) • R14-17 = 14977.6Ln(W14/W17) or…W14 = 1.017 W17 • Calculate initial/final loiter weight ratios (15.2 and 2a) • EFlo = 325.6p[(L/Dlo)/(VloSFClo] = 90.4 hr • E7-8 = 12hrs = EFloLn(W7/W8) or…W7 = 1.142 W14 • - Note: W8  W14 (no ingress/egress) 15-39

  40. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Size estimate • By definition the initial and final cruise weights (W4 and W17) are given by (see chart 15.8) • W4 = W0[1-FFKttoc] where Kttoc = 0.1 • W17 = W0[1-FF(1-Klr)] where Ktlr = 0.05 • Therefore: • W4 = W0[1- 0.1FF] = 1.014W7 = 1.0141.142 W14 • = 1.0141.1421.017 W17 • = 1.0141.1421.017W0[1 - 0.95FF] • FF = 0.175 • Then from 15.3 • FF = 1 -EWF -MiscF - PF = 1 -0.44 -0.01 -720lbm/W0 • or…..W0 = 1918 Lbm • And maximum range and endurance (from Eq 15.5-6) are • Rmax = 2453 nm and Emax = 14.8 hrs or 15-40

  41. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Parametric comparison • Whenever we calculate a performance parameter or size a vehicle, we should always ask ourselves if the calculation makes sense • - In this case, the sizing results should make sense since we used parametric data from similar aircraft as inputs • Nonetheless, we should still make a reality check using our UAV data spreadsheet ASE261.UAV data.xls • Which shows that we have a problem • Compared to other TBProp UAVs, our calculated FF is low for Emax = 14.8 hrs • Other TBProp UAVs require higher FFs for this level of performance • The data shows our inputs must be optimistic 15-41

  42. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Issue resolution • There are many possible explanations for why our estimated fuel fraction is low • LoDs and SFcs were estimated, not calculated • Ditto for empty weight fractions, speeds, etc. • What should we do? • Press on with a higher value of fuel fraction? • Stop and try to resolve the issues • Proceed with the knowledge that our performance estimates are optimistic • We can press on and sort it out later • Our spread sheet design and analysis methods are designed to handle uncertainties and disconnects • Corrections can be made with a few input changes or multipliers on performance parameters • However, if we were using traditional design methods, we would need to resolve the issue or risk a major down stream redesign or disconnect 15-42

  43. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics TBFan alternative • Defined • Remain airborne 24/7  90% of the time • Derived – payload, distances and altitudes • Payload : Wpay = 720 lbm • Cruise/loiter altidude: Hcr = Hlo = 27.4 Kft • Operating radius: D3-4+ D4-7 = D17-14 = 255 nm • Ingress/Egress: D8-14 = 0 • Assumptions – typical values (design independent) • Landing fuel reserves; Klr = 5%; MiscF = 1% • Propeller efficiency: p = 80% • First cut estimates – refine later (design dependent) • Taxi/takeoff/climb fuel: Kttoc =10% • Average rate of climb: ROCavg = 1500 fpm • Average climb speed: Vcl = 0.8 Vcr (more about this later) • Parametric estimates – Next chart (design dependent) • Unknowns – Gross weight (W0); Fuel fraction (FF) Same assumptions as TBProp 15-43

  44. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics TBFan alternative • Chart 15-14 shows nominal empty weight fractions for manned TBFans (EWF = 0.55) and UAVs (EWF = 0.6) • - Predator C shows EWF = 0.39; probably more representative of our concept • Chart 15-18 shows jet aircraft economic cruise/loiter speeds at 27Kft to be in range of 250-525 kts • We select a lower value (300 kts) for both cruise and loiter (but not the lowest since RF  Vcr) • Chart 15-19 shows typical TBFan cruise/loiter LoDs • BizJet TBFan: LoDcr = 10-12, LoDlo = 12-14 • Global Hawk LoDlo much higher (33-34) @ AR = 25 • We will select intermediate values @ 22.5 and 23.5 • Charts 15-20 (table) and 15-21 (plot) TBFan cruise & loiter SFCs conflict (not unusual for parametric data) • The plot is from our engine database (real TBFans) so we use it and estimate TSFCcr = TSFClo = 0.65 15-44

  45. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics TBFan cont’d • Calculate cruise ranges and range factors (15.1a) • Time to climb = 27.4 Kft/1500 fpm = 0.30 hr • Climb speed  0.8300 = 240 kts • Climb distance = 72 nm • R4-7 = 255 - 72 = 183 nm • R14-17 = 255 nm • RFcr = Vcr(L/Dcr)/TSFCcr = 10385 nm • Calculate outbound initial/final cruise weight ratios (15.1) • R4-7 = 10385 nmLn(W4/W7) or…. W4 = 1.018W7 • Calculate inbound initial/final cruise weight ratios (15.1) • R14-17 = 10385 Ln(W14/W17) or…W14 = 1.025 W17 • Calculate initial/final loiter weight ratios (15.2 and 2a) • EFlo = (L/Dlo)/SFClo = 36.2 hr • E7-8 = 12hrs = EFloLn(W7/W8) or…W7 = 1.394W14 • - Note: W8  W14 (no ingress/egress) 15-45

  46. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics TBFan cont’d • By definition the initial and final cruise weights (W4 and W17) are given by (see chart 15.8) • W4 = W0[1-FFKttoc] where Kttoc = 0.1 • W17 = W0[1-FF(1-Klr)] where Ktlr = 0.05 • Therefore: • W4 = W0[1- 0.1FF] = 1.021W7 = 1.0181.394 W14 • = 1.0181.3941.025W17 • = 1.0181.3941.025W0[1 - 0.95FF] • FF = 0.354 • Then from 15.3 • FF = 1 -EWF -MiscF - PF = 1 -0.39 -0.01 -720lbm/W0 • or…..W0 = 2914 Lbm • And maximum range and endurance (from Eq 15.5-6) are • Rmax = 3885 nm and Emax = 13.5 hrs 15-46

  47. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Expectations • You should understand • (1) How to analyze requirements to meet mission altitude, speed, operating distance and loiter time requirements • What is defined • What to assume • What to estimate and later refine • What to solve for • (2) How to calculate fuel fraction and gross weight • To meet operating distance and loiter time requirements • (3) How to use parametric data • To assess/select inputs • To check results 15-47

  48. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Second half approach • 40 individual design projects  10 team projects • Every team member responsible for team product • Team grade shared (1/3) • Everybody responsible for one element (1/3 credit) • System engineering or system, payload or support elements • Every team member responsible for at least one configuration concept (1/3 credit) • Each team will carry multiple configuration concepts through logical configuration evaluation/comparison • Configuration down selects must be based on quantitative vs. qualitative assessment • Top 10 first half (1H Top 10) projects will be revealed • Teams should form around each of the 10 projects • Team leads (System Engineers) = 1H Top 10 15-48

  49. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Homework • Establish your project design teams. List names (team grade) • 2. Document your project plan (team grade) • Schedule your project activities • Who is responsible for what element/task • 3. Select 4 air vehicle concepts (one per team member) to be evaluated during the 2nd half of the semester (team grade) • 4. Size your configuration concept (individual grades) • Calculate FF and W0 • Compare your calculations to parametric data and assess the results 15-49

  50. Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics Intermission 15-50

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