23-1 Design of UAV Systems Methodology Correlationc 2002 LM Corporation Objectives Lesson objective - Methodology correlation including … F-16 RQ-4A (Global

23-1

Design of UAV Systems

Methodology Correlationc 2002 LM Corporation

Objectives

Lesson objective -

Methodology correlationincluding …

• 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

23-2



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

23-3



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

23-4



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

23-5



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!)

23-6



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

23-7



F-16 aero

• Overall model coefficients selected to approximate F-16C

- Clean aircraft Cdmin ≈ 190 ctsCfe = .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

23-8



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

23-9



Mission level comparison

• Negligible differences in gross weight (-377 lbm)

• Some differences in fuel consumption30% 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)

23-10



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

23-11



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

23-12



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

23-13



• 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

Model cont’d

23-14



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 problemAnswer – GH has a high altitude thrust problem

GH model matching

23-15



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)

23-16



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 @ 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 @ 500 nm = 8hr+- Max range = n/a- Max endurance = 12+

23-17



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

23-18



Intermission

Documents

23-1 Design of UAV Systems Methodology Correlationc 2002 LM Corporation Objectives Lesson objective - Methodology correlation including … F-16 RQ-4A (Global