Determination of Flight Loads for the HH-60G Pave Hawk Helicopter

Transcription

1 Determination of Flight Loads for the HH-60G Pave Hawk Helicopter Robert McGinty, Gregory Wood, Jeff Brenna Mercer Engineering Research Center Warner Robins, GA Steven Lamb US Air Force Robins AFB, GA Aircraft Structural Integrity Conference San Antonio, TX Presented at the, San Antonio, TX,. Copyright 2014 by the ASIP Conference. All rights reserved. Slide 1

2 Background USAF tasked MERC to implement several ASIP initiatives to further improve management of the HH-60G fleet Goal Loads / Environment Spectra Survey (L/ESS) Global Finite Element Model (FEM) Development Flight Strain Survey Durability and Damage Tolerance Analyses (DADTA) Develop an EFH tracking capability, at multiple structural locations, for the fleet Slide 2

3 Motivation ASIP tasks (DADTA, EFH, etc) require correct aircraft loads Structural modifications and mission changes make current HH-60G operations different from original Black Hawk design specifications - Gross weight increase - Engine upgrades - Auxiliary fuel tanks added - Horizontal stab modifications - Refueling probe addition Applicability of original design loads questionable Slide 3

4 Loads Calculation Overview Use data from instrumented flight tests to compute flight loads for HH-60G aircraft to support structural analyses Flight Loads Tail Rotor Forces & Moments Experimental Data MAIN Rotor Forces & Moments Aerodynamic Forces Horizontal Stab Forces & Moments Aerodynamic Forces Stresses Throughout Aircraft Structural strength, fatigue, damage tolerance, and service life analyses Slide 4

5 Flight Strain Survey 63 strain gages 1,000 Hz sample rate Sample Flight Strain Data 30 hours (5,000 regimes) of prescribed maneuvers across operational envelope Operational data also collected, bringing total to 160 hours (30,000 regimes) Slide 5

6 Strain Gage Photos Slide 6

7 Finite Element Model HH-60G Finite Element Model - 1,430,000 nodes - 1,050,000 elements - Primarily shell elements - Nominal 1 x 1 element size Slide 7

8 Loads Calculation 1. Apply unit loads to all force / moment locations (one at a time) 2. Execute Nastran inertia relief runs for each unit load to determine its influence on strain at each gage location gives A ij values N j=1 A ij F j = ε i meas where F j are force and moment values 3. Insert experimental ε i values and perform multivariable linear regression to solve for forces and moments 1 equation per gage Significant Forces / Moments j Name FMain z MMain x Main M y Main M z Tail F y FHoriz z Slide 8

9 Loads Calculation (cont) 1. Issue #1 Gages are applied while aircraft is subject to 1G gravity Resolution Modify equation to account for additional strain N j=1 A ij F j = ε i meas + ε i 1G 1 equation per gage where ε i 1G is strain due to 1G gravity (predicted by FE model) 2. Issue #2 Certain forces and moments are closely correlated Resolution Add equations enforcing equilibrium Example M z Main + r CG F y Tail = 0 Slide 9

10 Loads Calculation (cont) Additional complexity Weight and CG change during flight (fuel is burned) This affects the sensitivity coefficients, A ij, and in turn influences the calculated force/moment values The effect is accounted for in calculations F y F z M z F z M y M x F z Slide 10

11 Measured Strain, me 1000 Strain Correlations Accuracy of computed loads is assessed by how well they lead to predicted strains matching measured data Chart for Sym Pull Up shows typical level of correlation (R 2 = 0.94) Predicted Strain, me Slide 11

12 Strain Correlations Alternative view of Sym Pull Up correlation (R 2 = 0.94) Gage locations indicated on structure Blue is predicted Red is measured Slide 12

13 Correlation Coefficients 8,000 7,000 6,000 5,000 Histogram of R 2 values for 30k PITS Average R 2 is and all values are above ,000 3,000 2,000 1, R 2 Slide 13

14 Correlation Coefficients 8,000 7,000 6,000 5,000 4,000 3,000 2,000 Histogram of R 2 values for 30k PITS Average R 2 is and all values are above 0.88 ε i 1G NEGLECTED Effect of 1G gravity loads at time of strain gage application ignored ε i 1G INCORPORATED (same as previous slide) 1, R 2 Slide 14

15 Loads Assessment R 2 values reflect excellent capability to obtain loads producing predicted strains that are closely correlated to measured values - Average R 2 value is But this does not guarantee accurate computed loads - For example, if the entire FE model were too stiff by a factor of 2, then all computed loads would be too high by same factor of 2 Following slides will compare computed loads to flight parameters - Begin with main z-force, F z Main Slide 15

16 Main Rotor Lift Distribution of computed main rotor lift force HH-60G Weight: 15,800 lb -- 21,800 lb 5,000 4,000 3,000 2,000 1, ,000 18,000 20,000 22,000 24,000 26,000 28,000 30,000 Main Rotor Z-Force (lbf) Slide 16

17 % of Regime Occurances Main Rotor Lift Higher forces for pull-ups than push-overs 40% 35% 30% 25% Push Over Level Flight 20% 15% Pull Up 10% 5% 0% Magnitude of Fz (thousands of lbs) Slide 17

18 Main Rotor Lift Z-force increases with weight Force is higher for hi-g (Nz) maneuvers Slide 18

19 Main Rotor Lift vs Nz * Weight Excellent 1-to-1 correlation to Nz * Weight Slide 19

20 Main Rotor Lift vs Nz * Weight Gross weight found to be incorrect on Flight 04 (blue points) Slide 20

21 Main Rotor Lift vs Nz * Weight Main Rotor Z-Force correlates very well after gross weight correction Slide 21

22 Main Rotor Mz Correlation Surface plot of Mz versus %Engine Torque and Weight Slide 22

23 Main Rotor Mz Correlation Correlation between Main Z-Moment and Torque * (1 Weight / 30,830) Slide 23

24 Mz - MaxContPowerAccent Main Z-Moment varies significantly for MaxContPwrAscent regime But with knowledge of x-axis value, Mz can be determined much more precisely Variation of Mz for given x-axis value Variation of Mz within regime Slide 24

25 Comparison to Design Loads Main Fz Tail Fy Horiz Fz -7k -1.5k -0.5k -3k -3.6k 0 Design Loads (1970 s) 54k 33k Flight Loads (today) 3.8k 3.4k 4.6k 3k -400k Main Mx -70k 0 300k 100k Main My -1.2M -600k 600k 1.0M Main Mz -1.3M -700k 300k 200k Slide 25

26 Loads Map - Load Case 1 - Load Case 2 High Tail Rotor y-force - Load Case 3 High Horizontal z-force - Load Case 4 Structure is color-coded by load case producing highest stresses/strains in each area Slide 26

27 Summary Forces and moments computed for 30,000 points-in-thesky (over 200 unique regimes) Forces/Moments can now be used for structural analyses, DADTAs, EFH estimates, etc Excellent correlations of predicted and measured strains Excellent correlations of forces and moments to aircraft Torque, Nz, & Weight Forces and moments can be determined much more precisely given Torque, Nz, & Weight than regime ID alone Current My and Mz moments exceed design values Slide 27

28 Future Work Will use a similar process, based on Fourier Transforms and Modal Analyses, to determine dynamic (i.e. vibration) components of loads Addition of dynamic components to average force values presented here will permit Determination of peak maneuver loads Development of load spectra for damage tolerance analyses and probabilistic risk analyses Slide 28