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AD-772 935

COLD-WEATHER FLIGHT TESTS OF AN OH-58 HELICOPTER EQUIPPED WITH AN ELASTOMERIC BEARING MAIN ROTOR

C. H. Fagan

Bell Helicopter Company

Prepa red for:

Army Air Mobility Research and Development Laboratory

August 1973

DISTRIBUTED BY:

KJ National Technical Information Service U. S. DEPARTMENT OF COMMERCE 5285 Port Royal Road, Springfield Va. 22151

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THIS DOCUMENT IS BEST QUALITY AVAILABLE. THE COPY

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PAGES WHICH DO NOT

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Unclassified Security Clarification AD-77J9a-T

DOCUMENT CONTROL DATA - * & D (Security etattitication of tilt«, body o/ mbittmct and indoming atmotmtHn muet be entered when the ovmrmll report la etaeallledj

11. HCPONTICCURITY CLASSIFICATION

Unclassified ORIGINATING AC TiviTv (Corporate author)

Bell Helicopter Company Fort Worth, Texas

Ik. CROUP

3. REPORT TITLE

COLD-WEATHER FLIGHT TESTS OF AN OH-58 HELICOPTER EQUIPPED WITH AN ELASTOMERIC-BEARING MAIN ROTOR

4. DESCRIPTIVE NOTES (Typo ot report mod Incluelre dmtee)

S. AUTHORISI (Flret name, middle Initial, laat name)

C. H. Fagan

»■ REPORT DATE

August 1973 7«. TOTAL NO. OF PAGES

J6J? 7b. NO. OF REFS

JL to. CONTRACT OR GRANT NO.

DAAJ02-73-C-0024 b. PROJECT NO.

1F163209DB38

M. ORIGINATOR'S REPORT NUMTBERISI

USAAMRDL Technical Report 73-70

SB. O THER REPORT NO(S) (Any other number* that may be meelfned thle report)

BHC Report 299-099-644

10. DISTRIBUTION STATEMENT

Approved for public release; distribution unlimited.

II. SUPPLEMENTARY NOTES 12. SPONSORING MILITARY ACTIVITY Eustis Directorate U. S. Army Air Mobility R&P Laboratory Fort Eustis, Virginia

13. ABSTRACT

The report contains the results of a flight test pregram conducted to investigate the characteristics of a main rotor equipped with elastomeric bearings operating at low temperatures. The helicopter used was an OH-58A, and the main rotor tested had elastomeric bearings in the pitch-change and flapping axes. The tests were conducted at Fort Wainwright, Alaska, and operation at temperatures of 48 F, 5°F, -9°F, -13°F, -29°F, and -52°F was evaluated. Normal rotor contrc' was obtained down to temperatures of -13°F. At lower temperatures, elastomer stiffening causes increased control system loads, which limit operation.

Additional testing is needed to establish the precise minimum temperatures at which there are no control limitations, and to investigate pitch-change bearings

made from elastomers other than natural rubber. Also, consideration need^ to be given to operational procedures, or other means of "warming" the beari igs,

to minimize bearing stiffening.

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1*41% POU( 4 M T> REPLACES DO FORM 1471. I J IsJIpV , Mov ((| 4 /O OBSOLETC FOR ARMY Uli.

AN 44. WHICH IS Unclassified

Security Classlflcutton

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Unclassified Security CLt.ifiotion

KEY WORDS

Bearings Elastomers Flight tests Low temperature Main rotor OH-58 helicopter

Unclassified

/.-ft- Security Cl«i«ific«tion

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DEPARTMENT OF THE ARMY U. S ARMY AIR MOBILITY RESEARCH A DEVELOPMENT LABORATORY

EUSTIS DIRECTORATE PORT EUSTIS. VIRGINIA 23604

This report was prepared by the Bell Helicopter Company, a Division of Textron Inc., under the terms of Contract DAAJ02-73-C-0024. It documents the results of flight tests conducted to evaluate the operational flight characteristics of an all-elastomeric bearing OH-58 main rotor in extreme cold-weather conditions.

The all-elastomeric hearing OH-58 main rotor hub was previously tested under extreme cold temperatures in the Eglin AFB Climatic Hangar in June 1972. The results of that program indicated that control loads iucreased significantly as temperatures dropped due to the stiffening of the elastomer in the bearing. It was determined that a flight test effort was needed to investigate the magnitude of combined bearing stiffness loads and loads created by in-flight rotor dynamics and other phenomena concerning flight characteristics of rotors equipped with elastomeric bearings.

Adverse weather conditions, ice-fog, and aircraft equipment problems precluded the acquisition >f sufficient data to fully investigate the flight characteristics of the elastomeric bearing or to establish a limiting operating temperature. However, the limited data acquired is deemed to be of sufficient value to tae aviation comnunity to be reported.

The conclusions contained herein are concurred in by this Directorate.

The technical monitor for this contract was Mr. John W. Sobczak of the Military Operations Technology Divission.

i.cs

Project 1F163209DB38 Contract DAAJ02-73-C-0024

USAAMRDL Technical Report 73-70 August 1973

COLD-WEATHER FLIGHT TESTS OF AN OH-58 HELICOPTER EQUIPPED WITH AN ELASTOMERIC-BEARING MAIN ROTOR

BelL Helicopter Report 299-099-644

By

C. H. Fagan

Prepared by

Bell Helicopter Company Fort Worth, Texas

for

EUSTIS DIRECTORATE U. S. ARMY

AIR MOBILITY RESEARCH AND DEVELOPMENT LABORATORY FORT EUSTIS, VIRGINIA

Approved for public release; distribution unlimited,

SUMMARY

Presented in this report are the results of a program designed to test the low-temperature flight characteristics of a helicopter main rotor equipped with elastomeric bearings. Prior to these tests, no flying with these bearings had been accomplished at temperatures below 0°F. The main rotor (designated Bell Helicopter Company (BHC) Model 640), which uses conic elastomeric bearings in the pitch-change axis and radial elastomeric bearings in the flapping axis, was flown on an OH-58 helicopter at Fort Wainwright, Alaska. Data are presented for operation at temperatures of 48°F, 5°F, -9°F, -13°F, -29°F, and -52°F.

Results of low-temperature tests of an identical main rotor im the Climatic Laboratory at Eglin Air Force Base, Florida, indicated that the main rotor control loads would increase with reductions in temperature below 0°F, but would remain normally operable to about -35°F. These conclusions were nnudt fully substantiated in Alaska. Because of engine problems and adverse atmospheric conditions, insufficient data were acquired to establish a limiting cold-temperature level.

Hydraulic boost-off operation was round to be norm.il for the -13°F temperature flight. However, control system loads approached the endurance limit for the cyclic boost links. At some temperature between -13°F and -52°F, elastomer stiffs ening in the pitch-change bearings will increase the control system leads to an unacceptable level unless operational procedures can be def i led to "warm" the bearing or reduce bearing stiffness. Additional cold-weather flight-test date, are Deeded to determine the specific minimum temperature for flight operations with this particular configuration and to continue the development of elastomeric bearings for rotor application.

in

FOREWORD

This report was prepared under Contract DAAJ02-73-C-0024 (Project IF163209DB38), "Cold-Weather Flight Evaluation of an OH-58 Helicopter Equipped with an Elastomeric-Bearing Main Rotor," with the Eustis Directorate, U. S. Army Air Mobility Research and Development Laboratory (USAAMRDL). Contract work was initiated on 1 December 1972, and the helicopter and test equipment were shipped to Fort Wainwright, Alaska, on 9 January 1973. The elastomeric-bearing main rotor flight tests were conducted on a noninterference basis on the OH-58 helicopter that was undergoing elastomeric-bearing tail rotor tests under USAAVSCOM Contract DAAJ01-72-A-0015 (P2E), D.O. 0001.

This program was under the technical cognizance of Messrs. L. Bartone, J. Daniel, and J. Sobczak of the Military Opera- tions Technology Division of the Eustis Directorate. Principal Bell personnel associated with this program were Messrs. L. Arrick, W. Cresap, and T. Gardner.

The writer expresses his appreciation and that of all BHC personnel for the cooperation and assistance of the Fort Wainwright personnel.

Preceding page blank

TABLE OF CONTENTS

Page

SUMMARY

FOREWORD

LIST OF ILLUSTRATIONS . .

LIST OF TABLES

INTRODUCTION

DESCRIPTION

Test Vehicle .... Test Rotor

DISCUSSION OF TESTS . . .

Operating Procedures Engine Hot Start . . Pilot Evaluation . . Tests Conducted . . Data Acquisition . .

DISCUSSION OF RESULTS . .

CONCLUSIONS

LITERATURE CITED ....

DISTRIBUTION

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2 2

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5 5 5 7 9

11

27

28

29

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■■

LIST OF ILLUSTRATIONS

Figure Page

1

2

3

k

5

6

7

10

u 12

13

14

15

All-Elastomeric-Bearing Hub

Main Rotor Pitch-Change Bearing . . .

Main Rotor Flapping Bearing

Helicopter Preflight

Arctic Clothing

Helicopter Cold Soak

Main Rotor Hub and Controls Instrumentation

Collective Boost Link Load Versus Airspeed

Left-Hand Cyclic-Boost Link Load Versus Airspeed

Right-Hand Cyclic-Boost Link Load Versus Airspeed

Red Pitch-Link Load Versus Airspeed .

White Pitch-Link Load Versus Airspeed

In-Flight and Climatic Laboratory Main Rotor Pitch-Link Loads Versus Temperature

In-Flight and Climatic Laboratory Cyclic-Boost Link Loads Versus Temperature

In-Flight and Climatic Laboratory Collective Boost Lick Loads Versus Temperature

3

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10

13

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16

17

18

19

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LIST OF TABLES

Table Page

I

II

III

IV

V

VI

VII

VIII

Flight Tests Conducted 8

Main Rotor ControLs Instrumentation ... 9

Main Rotor Control Loads for Flights Nos. ^1 and k2 21

Main Rotor Control Loads for Flight No. kk 22

Main Rotor Control Loads for Flight No. hS 23

Main Rotor Control Loads for Flight No. k9 2k

Main Rotor Control Loads for Flight No. 50 25

Main Rotor Control Loads for Flight No. 52 26

IX

INTRODUCTION

Elastomeric bearings (E.B.'s) have been investigated for application to heLicopter rotors at Bell Helicopter Company (BHC) for the past eight years. During that time, several main and tail rotor configurations, which used E.B.'s in the flapping and pitch-change axes, have been flight evaluated (References 1 and 2).

E.B.'s installed in the rotor system show advantages over conventional bearings, because they need no lubrication. They can be visually inspected without hub disassembly; they can provide longer service lives; and the gradual deterioration of the elastomer provides early warning to enhance the flight safety of the aircraft. However, low-temperature stiffening of the elastomer in the pitch-change bearing causes increased loads in the control system, and is a factor which should be considered during the initial design phases for new applications.

An all-elastomeric-bearing main rotor, BHC Model 640, was flight tested on an 0H-^8A helicopter in 1971 at normal Texas climatic conditions. In addition, low-temperature tests were conducted on an 0H-58A equipped with the Model 640 rotor in the Climatic Laboratory at Eglin Air Force Base, Florida, in June 1972. Rotor and control loads are reported in Refer- ence 3 for helicopter ground-run operations at temperatures of 70°F, 0°F, -25°F, -35°F, -45°F, -55°F, and -65°F. Control system load data shows that the loads increase with reduction in temperature for operation below 0°F. The higher loads were caused by an increase in the pitch-change bearings torsional spring rate as the temperature was reduced.

Prior to the tests discussed in this report, no flying had been accomplished with E.B. rotors at temperatures V'low 0°F. With this program, the Model 640 rotor, which uses -onic E.B.'s in the pitch-change axis and radial E.B.'s in the flapping axis, was tested during forward flight in ambient temperatures of 48°F, 5°F, -9°F, and -U°F and during hover maneuvers at -29°F and -52°F. The low-temperature flight tests were conducted at Fort Wainwright, Alaska.

DESCRIPTION

TEST VEHICLE

A production OH-58A helicopter equipped with experimental main and tail rotors was used as the test vehicle. The two-bladed main rotor was equipped with elastomeric bearings in the flapping and pitch-change axes. The tail rotor tested was the same as a production tail rotor except for elastomeric bearings in the flapping axis. Additional equipment consisted of a main rotor brake and an experimental winterization kit.

TEST ROTOR

A Bell Helicopter Company Model 640 main rotor was evaluated. Blades for this rotor are the same as production OH-58A blades except for an inboard modification to pick up two hub bolts, giving a rotor diameter of 35.3 feet. The flexbeam type hub is equipped with two conic E.B.'s in each grip to carry all blade loads and to allow blade pitch motions. Also, the rotor is equipped with two radial E.B.'s in the flapping axis to carry the rotor thrust and drive loads and to accom- modate flapping motions. Details of the flexbeam hub with one grip assembled are shown in Figure 1. The conic pitch- change and flapping bearings, both with a quarter section removed, are shown in Figures 2 and 3, respectively.

During this program the main rotor pitch-change bearings were indexed for no torsional load at 12 degrees up from minimum collective. This is four more degrees up collective than the previous indexing of the 640 rotor. The change in indexing reduces the steady loads at the pilot's collective lever by about 20 pounds for any flight condition except autorotation.

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Figure L. ALL-ELastomeric-Bearing Hub

Figure 2. Main Rotor Pitch-Change Bearing,

Figure 3. Main Rotor Flapping Bearing

DISCUSSION OF TESTS

OPERATING PROCEDURES

During cold soak, the helicopter was left in an open, unpro- tected area of the flight ramp. Two 100-watt electric light lamps were placed in the wooden enclosure which housed the instrumentation package. These lights were used to provide heat during cold-soak periods to minimize warm-up time for the oscillographs. Also, an auxiliary power unit, shewn in Figure 4, waz used to furnish electrical power for a small heater to warm the fuel controls and to provide power for starting the helicopter engine. The combustion cabin heater was operated before engine start to accelerate warm-up time for the oscillographs and to provide heat for the helicopter cabin. Special arctic clothing, as shown in Figures 5 and 6, was worn by the test crew during operation outside the hangar. In addition to the clothing shown in Figure 5, cloth face masks were available and used at extreme low temperatures (below about -30°F). The added clothing increased the time necessary to perform minor tasks, such as the removal and replacement of a bolt. In fact, the collective control boost tube was disconnected, to remove any preload introduced by the pitch-change E.B.'s, during recording of the no-load instrumentation record. This operation was re- quired before and after each flight.

After the helicopter was started and operating speed was reached, main rotor and throttle controls were exercised for 3 to 5 minutes to investigate control motion limits prior to flight. An effort was made by the pilot to conduct all maneuvers at the same rate to provide comparative load data.

ENGINE HOT START

During an attempted engine start on February 1, 1973, after a cold-soak period of 24 hours (Figure 6) at a temperature cf -36°F, a severe engine overheat condition was experienced. Although the pilot's throttle was closed, sufficient fuel had accumulated in the engine to cause the damaging overheat. Subsequent investigations revealed that fuel would leak past the engine fuel control and accumulate in the engine when the fuel pump was operated to provide fuel for the cabin heater.

PILOT EVALUATION

The pilot investigated cyclic and collective control stick loads prior to each flight during ground run operation. Con- trol inputs were executed to determine the effects of low- temperature stiffening of the pitch-change E.B.'s on limiting

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DATA ACQUISITION

Loads for the instrumentation channels as listed in Table II were recorded on photosensitive paper by an oscillograph. Data were recorded at time intervals selected by the pilot and flight test engineer to obtain records of specipic flight operations. After each flight the paper was removed from the oscillograph and developed for permanent records. Locations of the instrumentation gages are given in Figure 7.

TABLE II. MAIN ROTOR CONTROLS INSTRUMENTATION

Item Parameter Units*

Collective Link L.H. Cyclic Link R.H. Cyclic Link Pitch Link (Red) Pitch Link (White)

axial load axial load axial load axial load axial load

lb lb lb lb lb

*Link in compression for minus values. -■

I^HB^B* BSBH^BP

R. H. Cyclic Boost Link Gage

Collective Boost Link. Gage

■White Pitch Link Gage

L. H. Cyclic Boost Link Gage

Figure 7. Main Rotor Hub and Controls Instrumentation.

10

DISCUSSION OF RESULTS

Listed in Figures 8 through 12 are loads data presented graphically for the main rotor control links. These data are the recorded maximum values for stabilized level flight at 354 main rotor rpm and at ambient temperatures of 48 °F, 5°F, -9°F, and -13°F. Additional data are presented in Tables III through VIII for maneuvers conducted in ambient temperatures from 48°F to F. Low of -52 °F. An effort was made by the pilot to execute all control inputs at a constant rate.

Collective boost link loads are compared in Figure 8 for temperatures ranging from 48°F to -13°F. The comparison shows an increase of about 20 percent in both steady and oscillatory loads for the lower temperature. Collective link loads were not considered to De excessively high even for operation at -52°F, as shown in Table V and Figure 15 (approximately 50- percent increase over loais for operation at 70°F). The data at 70°F were obtained from the IR&D Test Program.

Cyclic-boost link loads versus airspeed are presented in Fig- ures 9 and 10. They show an increase of approximately 25 percent in oscillatory loads for the temperature range from 48°F to -13°F. However, the fact that the load increase was less at 100 knots than at 80 knots indicates that the pitch- change E.B.'s experienced some beneficial warm-up during the higher speed (increased cyclic motion) conditions. Mean loads in the right-hand eye Lie-boost link increased almost 100 percent over the temperature range, causing high peak loads. Infinite life endurance limits for the lower cyclic-boost link, ±265 pounds (±238 pounds measured in the instrumented upper link) were exceeded during a 120-knot level flight at a temperature of +5°F. Maneuver load limits, calculated to be ±595 pounds in the instrumented link, were never reached. However, boost locking capability in the r'.ght cyclic cylinder was exceeded during a recovery from rigl t sideward flight at -52°F temperature (Table V record number 675). For that maneuver, a load of -335 ±454 pounds was rfcorded, resulting in a compression peak load in the link r£ 789 pounds. A load on this link greater than 735 pounds w?LI exceed the locking capability of the boost system and allow loads to feed back to the cyclic stick. Due to the mechanical linkage, loads at the pilot's cyclic stick will be 25 percent of the loads in excess of the boost locking capability, or in this instance, 14 pounds.

Red and white pitch-link load data versus airspeed are presented graphically in Figures 11 and 12. In Figures 13 through 15 the pitch-link and boost link data obtained during this program are compared with the data from ground-run operation in

11

the Climatic Laboratory as reported in Reference 3. The Climatic Laboratory data (scatter band) were taken with a 6- degree cyclic pitch input in an attempt to simulate cruise level flight. In-flight data for 70CF temperature were taken from the results of a previous Model 640 rotor test program. Although oily a small temperature range was covered during the Alaska flight tests, the data tend to validate the results from the Climatic Laboratory.

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26

CONCLUSIONS

Satisfactory operation of an OH-58A helicopter equipped with a Model 640 alL-eLastomeric-bearing main rotor was demonstrated at low temperatures to -13GF. Also, some flight testing was conducted down to -52°F.

No problems were encountered with the flapping elastomeric bearings, and it appears that they are satisfactory for operation at low temperatures to -52'F. However, elastomer stiffening with reductions in temperature increased the torsional stiffness of the pitch-change bearings and caused problems with the rotor control system. At some low temperature, Lhe control system loads will increase to a point where they cause fatigue damage, rate-limit control inputs, and generate feedback loads at the pilot's controls. Due to time constraints and adverse climatic conditions, a thorough investigation of the in-flight characteristics of the blade feathering elastomeric bearings was not accomplished. No attempts were made to determine operational procedures that might be used to reduce the bearing stiffness and accompanying high control loads to an acceptable operational level. Addi- tional tests are needed to establish the minimum temperature at which there are no control limitations, and to investigate the cold-weather operation of pitch-change bearings made from elastomers other than natural rubber.

For future helicopter rotcr systems using elastomeric bearings, stiffening of the elastomer at cold temperatures should be a design consideration for the bearing, for the rotor hub (configuration), and for the control system.

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LITERATURE CITED

1. Fagan, C. H., IMPROVED CONCEPT OF ELASTOMERIC BEARINGS FOR UH-1 TAIL ROTOR ASSEMBLY, Bell Helicopter Company; USAAVLABS Technical Report 68-84, U.S. Army Aviation Material Laboratories, Fort Eustis, Virginia, October 1968, AD 681244.

2. Fagan, C. H., FLIGHT EVALUATION OF ELASTOMERIC BEARINGS IN AN AH-1 HELICOPTER MAIN ROTOR, Bell Helicopter Company; USAAVLABS Technical Report 71-16, Eustis Directorate, U.S. Army Air Mobility Research and Development Laboratory, Fort Eustis, Virginia, March 1971, AD 724192.

3. Fagan, C. H., LOW-TEMPERATURE TESTS OF ELASTOMERIC BEARING ROTORS ON AN OH-58 HELICOPTER IN THE CLIMATIC LABORATORY AT EGLIN AFB, Bell Helicopter Company; USAAMRDL Technical Report 73-9, Eustis Directorate, U.S. Army Air Mobility Research and Development Laboratory, Fort Eustis, Virginia, January 1973.

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