Design of UAV Systems
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
c 2002 LM Corporation
Methodology Correlation
23-1
Design of UAV Systems
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
c 2002 LM Corporation
Methodology Correlation
23-2
Design of UAV Systems
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
c 2002 LM Corporation
Methodology Correlation
23-3
Design of UAV Systems
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
c 2002 LM Corporation
Methodology Correlation
23-4
Design of UAV Systems
F-16 geometry
• Overall geometry parametrics were matched
- AR = 3;  = .2275; Sht = Svt = 0.21*Sref etc.
• Sref - defined by wing loading
• Fuselage diameter - estimated from fuselage
maximum cross sectional area
- Df = 2*sqrt(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!)
;
c 2002 LM Corporation
Methodology Correlation
23-5
Design of UAV Systems
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 tanks+AIM-9s+chaff = 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
c 2002 LM Corporation
Methodology Correlation
23-6
Design of UAV Systems
F-16 aero
• Overall model coefficients selected to approximate
F-16C
- Clean aircraft Cdmin ≈ 190 cts
Cfe = .019*300/1404 = .004
- Cdmin with tanks = 1.4*clean aircraft
• Other parameters selected at nominal values
- e = 0.8, etc.
• Induced drag, lift coefficient and L/D calculated
using Lesson 17 methodology
c 2002 LM Corporation
Methodology Correlation
23-7
Design of UAV Systems
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
* www.aircraftenginedesign.com
were fall outs of the model
c 2002 LM Corporation
Methodology Correlation
23-8
Design of UAV Systems
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)
c 2002 LM Corporation
Methodology Correlation
23-9
Design of UAV Systems
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
c 2002 LM Corporation
Methodology Correlation
23-10
Design of UAV Systems
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
c 2002 LM Corporation
Methodology Correlation
23-11
Design of UAV Systems
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
c 2002 LM Corporation
Methodology Correlation
23-12
Design of UAV Systems
Model cont’d
• 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
c 2002 LM Corporation
Methodology Correlation
23-13
Design of UAV Systems
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 < 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
c 2002 LM Corporation
Methodology Correlation
23-14
Design of UAV Systems
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 @ 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)
c 2002 LM Corporation
Methodology Correlation
23-15
Design of UAV Systems
Result
• DS Model
• DS (DARO FY1996)
- Lfus = 14.9ft
- Lfus = 15 ft
- Wfus = 12 ft
- Wfus = 12 ft
- Hfus = 3.5 ft
- Hfus = 3.5 ft
- LoDavg = 30.2
- LoDavg = n/a
- W0 = 8759
- W0 = 8600 lbm
- We = 4466
- We = 4360
- Sref = 305
- Sref = 300
- Swet = 921
- Swet = n/a
- Hdot3 (SL) = 2104 fpm
- Hdot3 (SL) = 2000 fpm
- Hdot4 (42 Kft) = 56 fpm
- Hdot7 (45 Kft) = n/a
- End @ 500 nm = 12.5 hr - End @ 500 nm = 8hr+
- Max range = 4068 nm
- Max range = n/a
- Max endurance = 16 hr
- Max endurance = 12+
c 2002 LM Corporation
Methodology Correlation
23-16
Design of UAV Systems
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
c 2002 LM Corporation
Methodology Correlation
23-17
Design of UAV Systems
c 2002 LM Corporation
Methodology Correlation
Intermission
23-18