Objectives

1
Objectives

• Lesson objective -
• Methodology correlation
• including
• F-16
• RQ-4A (Global Hawk)
• DarkStar

Expectations You will have a better
appreciation for the validity of the integrated
design and analysis spreadsheet methods
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Importance

• It is important that we understand how well (or
  poorly) the simplified methods reflect reality
• - We know the methods are approximate
• - But are they good enough for concept design?
• We will first compare against a manned aircraft
  (F-16 ferry mission)
• - Available database (geometry, aero, weight,
  propulsion and performance)
• Then we will do UAV comparisons
• - Global Hawk and DarkStar are reasonably well
  documented
• Turboprop and piston powered aircraft comparisons
  are still in work
• To date correlations have focused on propulsion
• Addressed in Lesson 18

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Overall F16 comparison

• Parametric model calibrated to F-16C
• - Overall geometry (span, tail ratios, etc)
• - Basic unit weights and fractions (structure,
  gear, propulsion, etc) based on ferry GTOW
• - Overall aero coefficients (Cfe and e)
• - Sea level static propulsion (T0, TSFC0, BPR,
  etc)
• Model estimates compared with actuals
• - Wetted area
• - Cruise and climb aero
• - Cruise and climb propulsion
• - Overall weight history and range/endurance

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Comparison mission

• Spec F-16 ferry mission with external tanks
• - (2) 370-gallon wing tanks
• - (1) 300-gallon centerline tank
• - Wing tip mounted missiles included
• - 480 knot cruise speed
• Mission profile assumes cruise climb
• - We will define initial and final cruise
  altitudes

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F-16 geometry

• Overall geometry parametrics were matched
• - AR 3 ? .2275 Sht Svt 0.21Sref etc.
• Sref - defined by wing loading
• Fuselage diameter - estimated from fuselage
  maximum cross sectional area
• - Df 2sqrt(2600/?) 4.8 ft
• Other fuselage geometry defined in relative terms
• - Lf/Df 9
• - Nominal nose (0.2) and aft (.1) body length
  fractions
• Nacelle Swet defined as 50 of a constant radius
  cylinder
• - Dnac f(engine size), Ln/Dn 4
• Resulting geometry model came out very close
• - Swet predicted within 4 sqft (accuracy
  coincidental!)

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F-16 weights

• Model defined to match F-16C ferry weights
• - Initial fuel fraction with full internal fuel
  (2) 370g (1) 300g 0.394
• - Overall airframe weight/Sref 26.72
• - Engine installation factor 1.2
• - Other fractions to match F-16C
• - Payload external tanksAIM-9schaff 1700
  lbm
• - Misc weight fraction pilot provisions
  fluids unusable fuel/W0 0.009
• By definition the individual weight fractions
  matched
• - But overall weights had to converge on their own

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F-16 aero

• Overall model coefficients selected to
  approximate F-16C
• - Clean aircraft Cdmin 190 cts
• ?Cfe .019300/1404 .004
• - Cdmin with tanks 1.4clean aircraft
• Other parameters selected at nominal values
• - e 0.8, etc.
• Induced drag, lift coefficient and L/D calculated
  using Lesson 17 methodology

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F-16 propulsion

• Model constructed to fit published F-100-229
  values from the Mattingly engine design website
• - Military power thrust (SLS) 17800 lbf
• - Military power SFC0 (SLS) 0.74
• - Military power WdotA (SLS) 248 pps
• - Fsp-fn was selected to match Fsp0 at BPR 0.4
  with Fspgg 90
• - Fuel-to-air ratio was calculated from fuel flow
  assuming WdotAgg 177.1 pps (248pps/1.4) or
• f/a .0218
• - SFC was increased 5 per spec mission rules
• Thrust, air flow and fuel flow at speed and
  altitude were fall outs of the model

www.aircraftenginedesign.com
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Mission level comparison

• Negligible differences in gross weight (-377 lbm)
• Some differences in fuel consumption
• 30 underestimate of start-taxi-takeoff fuel
  (-218 lbm)
• 2 overestimate of fuel to climb (26 lbm)
• 2 underestimate of cruise fuel (-253 lbm)
• 8 underestimate of loiter/landing reserves (-138
  lbm)
• Negligible difference in landing weight (205
  lbm)
• Negligible difference in overall cruise range
  (6nm)
• 27 underestimate of time to climb (-3.1 min.)
• 36 underestimate of distance to climb (-31 nm)
• 3 overestimate of cruise range (46nm)

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Overall assessment - F16

• Predicted size, weights and performance are
  within concept design accuracy requirements
• Time and distance to climb not an issue for this
  design phase
• Gross weight, empty weight and radius are the key
  parameters of interest

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Global Hawk comparison

• Maximum range/endurance mission from 1999 Global
  Hawk Public Release International Presentation
• - Maximum internal fuel
• - 350 knot cruise speed
• - 50 to 65 Kft cruise, 65 Kft loiter
• - 13,500 nm maximum range
• - 38 hour maximum endurance
• - 24 hour endurance at 3200 nm operational radius

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Model development

• Geometry model calibrated to match known or
  estimated GH data
• - Overall aero surface geometry known
  (span,areas)
• - Overall Swet estimated from published L/Dmax
  and span assuming state-of-the art Cfe .0035, e
  0.75
• Fuselage areas unknown - estimated from fuselage
  length and diameter
• Weight model developed from various sources
• - Payload, gross and empty weight from NG data
• - RR AE3007H weight from Janes, installed at 120
• - Other fractions (gear and systems) estimated
• - Fuselage, wing and tail unit weights estimated
  at nominal values and iterated to match published
  EW
• - Resulting Airframe Wt/Sref 6.42 psf

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Model contd

• Propulsion model calibrated to match published
  data
• - T0 8290 lbf, TSFC0 0.33, BPR 5
• - Fuel-to-air ratio adjusted to fit TSFC0
• - Assumed Fspgg 90 Fspfn 30
• - 10 installation loss assumed
• - Airflow scaled to match SLS thrust
• Performance model inputs from published data
• - 25 minute ground idle, 5 minute full power
  takeoff
• - 50 Kft initial and final cruise altitudes,
  loiter at 65 Kft
• - 350 kt cruise and loiter speed
• - 200 nm distance to climb to 50 Kft
• - Outbound leg 3000 nm inbound 3200 nm
• - 60 minute landing loiter, assume 5 landing
  reserve
• - Range and mid-mission operational loiter a
  fallout

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GH model matching

• Model as constructed approximated published
  performance
• - Operational loiter 23.1 hrs vs. 24 hrs at
  3200 nm
• - Max range 14026 nm vs 13500 nm
• - Max endurance 41.2 hr vs 38 hr
• - L/Dmax - 34.8 vs 33-34
• - Multipliers could be applied make the numbers
  match published data
• But there were disconnects in thrust available
• - 50 Kft model data was OK (Ta ? D)
• - 65 Kft thrust was not (Ta lt D)
• - At final cruise and initial loiter weights
• - Thrust available multipliers required 2.1
• Either model is off or GH has a high altitude
  thrust available problem
• Answer GH has a high altitude thrust problem

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Another example

• All wing UAV (DarkStar type)
• - Wpay 1100 lbm (inc. comms) , Vcr 250 kt at
  45 Kft
• - W0/Sref 28.7 psf AR 14.1 FF 0.33 T0/W0
  0.22
• What we change (from GH)
• - t/c 16 (est.) Cfe .003 (RayAD Table 12.3)
• - e 0.8 (chart 17-6)
• - Dfus-equiv 6.5 ft (estimated from sketch)
• Lfus/Dequiv-fus 2.3 Wfus/Hfus 3.4
• See chart 20-19, Eq 20.8 for Deq and fuselage
  Swet methodology
• - Neng 1, BPR 3.2, T0/Weng 4.25 lbm/lbf
  (FJ-44)
• - 5 propulsion installation loss (estimate)
• - L/Dnac 4, Swet-nac _at_ 0 (buried engine)
• - U-2 airframe, DS system weights (7.5 psf and
  18)
• - Landing gear from RayAD Table 15.2
• - Non-payload/fuel misc items (2 useful load)

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Result

• DS Model
• - Lfus 14.9ft
• - Wfus 12 ft
• - Hfus 3.5 ft
• - LoDavg 30.2
• - W0 8759
• - We 4466
• - Sref 305
• - Swet 921
• - Hdot3 (SL) 2104 fpm
• - Hdot4 (42 Kft) 56 fpm
• - End _at_ 500 nm 12.5 hr
• - Max range 4068 nm
• - Max endurance 16 hr

• DS (DARO FY1996)
• - Lfus 15 ft
• - Wfus 12 ft
• - Hfus 3.5 ft
• - LoDavg n/a
• - W0 8600 lbm
• - We 4360
• - Sref 300
• - Swet n/a
• - Hdot3 (SL) 2000 fpm
• - Hdot7 (45 Kft) n/a
• - End _at_ 500 nm 8hr
• - Max range n/a
• - Max endurance 12

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Conclusion

• Hopefully these comparisons help convince you
  that simplified performance and geometry models
  do a reasonable job of predicting real aircraft
  trends
• - Once you get confidence in the approach and
  learn how to adjust models using multipliers, you
  can approach configuration design, configuration
  trades and technology trades from a whole new
  perspective
• - Develop an analysis model first, use it to help
  you define a better initial configuration
• - Then draw and analyze the configuration
• - Recalibrate the model to match the new analysis
• - Use the new model to guide trade study planning
  to reduce the size of the matrix and to predict
  trends
• - Define a new configuration and repeat to
  convergence

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Intermission
23-18