determination of flight loads for the hh-60g pave hawk ... · loads calculation overview •use...

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

Steven Lamb

US Air Force

Robins AFB, GA 31098

Slide 1

2014 Aircraft Structural Integrity Conference

December 2-4, 2014

San Antonio, TX

Robert McGinty, Gregory Wood, Jeff Brenna

Mercer Engineering Research Center

Warner Robins, GA 31088

2014 ASIP Conference December 2-4, 2014

Presented at the 2014 ASIP Conference, San Antonio, TX, December 2-4, 2014.

Copyright © 2014 by the ASIP Conference. All rights reserved.

• USAF tasked MERC to implement several ASIP initiatives to further improve management of the HH-60G fleet

– Loads / Environment Spectra Survey (L/ESS)

– Global Finite Element Model (FEM) Development

– Flight Strain Survey

– Durability and Damage Tolerance Analyses (DADTA)

• Goal

– Develop an EFH tracking capability, at multiple structural locations, for the fleet

Background

Slide 2 December 2-4, 2014 2014 ASIP Conference

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

- Refueling probe addition

• Applicability of original design loads questionable

- Auxiliary fuel tanks added

- Horizontal stab modifications


Loads Calculation Overview

• Use data from instrumented flight tests to compute flight loads for HH-60G aircraft to support structural analyses

Experimental Data

Flight Loads

Stresses Throughout Aircraft

Structural strength, fatigue, damage tolerance, and service life analyses

MAIN Rotor Forces & Moments

Tail Rotor Forces & Moments

Horizontal Stab Forces & Moments

Aerodynamic Forces

Aerodynamic Forces


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)

• 63 strain gages • 1,000 Hz sample rate

Flight Strain Survey


Strain Gage Photos


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


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 𝐴𝑖𝑗 values

where 𝐹𝑗 are force and moment values

3. Insert experimental 𝜖𝑖 values

and perform multivariable

linear regression to solve

for forces and moments

j Name

1

2

3

4

5

6

𝐹𝑧𝑀𝑎𝑖𝑛

𝑀𝑥𝑀𝑎𝑖𝑛

𝐹𝑦𝑇𝑎𝑖𝑙

𝐹𝑧𝐻𝑜𝑟𝑖𝑧

𝑀𝑦𝑀𝑎𝑖𝑛

𝑀𝑧𝑀𝑎𝑖𝑛

Significant Forces / Moments 1 equation per gage 𝐴𝑖𝑗 𝐹𝑗 =

𝑁

𝑗=1

𝜖𝑖𝑚𝑒𝑎𝑠


Loads Calculation (cont)

1. Issue #1 – Gages are applied while aircraft is subject to 1G gravity

Resolution – Modify equation to account for additional strain

where 𝜖𝑖1𝐺 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 – 𝑀𝑧𝑀𝑎𝑖𝑛 + 𝑟𝐶𝐺 × 𝐹𝑦

𝑇𝑎𝑖𝑙 = 0

1 equation per gage 𝐴𝑖𝑗 𝐹𝑗 =

𝑁

𝑗=1

𝜖𝑖𝑚𝑒𝑎𝑠 + 𝜖𝑖

1𝐺


Fz

Fz

Mz

Mx My

Fy

Fz

Loads Calculation (cont)

• Additional complexity – Weight and CG change during flight (fuel is burned)

– This affects the sensitivity coefficients, 𝐴𝑖𝑗, and in turn

influences the calculated force/moment values

– The effect is accounted for in calculations


-1500

-1000

-500

0

500

1000

-1500 -1000 -500 0 500 1000

Me

asu

red

Str

ain

, me

Predicted Strain, me

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 (R2 = 0.94)


Strain Correlations

• Alternative view of Sym Pull Up correlation (R2 = 0.94)

• Gage locations indicated on structure

• Blue is predicted Red is measured


0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

0.88 0.89 0.90 0.91 0.92 0.93 0.94 0.95 0.96

R2

Correlation Coefficients

Histogram of R2 values for 30k PITS

Average R2 is 0.934 and all values are above 0.88


Correlation Coefficients

Histogram of R2 values for 30k PITS

Average R2 is 0.934 and all values are above 0.88

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

0.75 0.78 0.80 0.83 0.85 0.88 0.90 0.93 0.95

R2

𝝐𝒊𝟏𝑮 INCORPORATED

(same as previous slide)

𝝐𝒊𝟏𝑮 NEGLECTED

Effect of 1G gravity loads at time of strain gage application ignored


Loads Assessment

• R2 values reflect excellent capability to obtain loads producing

predicted strains that are closely correlated to measured

values

- Average R2 value is 0.934

• 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, 𝐹𝑧𝑀𝑎𝑖𝑛


• Distribution of computed main rotor lift force

• HH-60G Weight: 15,800 lb -- 21,800 lb

Main Rotor Lift

0

1,000

2,000

3,000

4,000

5,000

16,000 18,000 20,000 22,000 24,000 26,000 28,000 30,000

Main Rotor Z-Force (lbf)


0%

5%

10%

15%

20%

25%

30%

35%

40%

0 4 8 12 16 20 24 28 32

% o

f R

egim

e O

ccu

ran

ces

Magnitude of Fz (thousands of lbs)

• Higher forces for pull-ups than push-overs

Push Over Level Flight

Pull Up

Main Rotor Lift


• Z-force increases with weight

• Force is higher for hi-G (Nz) maneuvers

Main Rotor Lift


• Excellent 1-to-1 correlation to Nz * Weight

Main Rotor Lift vs Nz * Weight


• Gross weight found to be incorrect on Flight 04 (blue points)



• Main Rotor Z-Force correlates very well after gross weight correction



Main Rotor Mz Correlation

• Surface plot of Mz versus %Engine Torque and Weight


• Correlation between Main Z-Moment and Torque * (1 – Weight / 30,830)

Main Rotor Mz Correlation


• Main Z-Moment varies significantly for MaxContPwrAscent regime

• But with knowledge of x-axis value, Mz can be determined much more precisely

Mz - MaxContPowerAccent

Variation of Mz within regime

Variation of Mz for given x-axis value


Comparison to Design Loads

33k 54k -7k

0 Design Loads (1970’s)

Flight Loads (today) Main Fz

3.4k 3.8k -1.5k Tail Fy’

-0.5k

1.0M 600k -600k

Main My -1.2M

-3.6k 4.6k -3k Horiz Fz

3k

100k 300k -400k

0

Main Mx -70k

200k 300k -700k

Main Mz -1.3M


• Structure is color-coded by load case producing highest stresses/strains in each area

- Load Case 1 - Load Case 2 – High Tail Rotor y-Force - Load Case 3 – High Horizontal z-Force - Load Case 4

Loads Map


Summary

• Forces and moments computed for 30,000 points-in-the-sky (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


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


determination of flight loads for the hh-60g pave hawk ... · loads calculation overview •use...

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