Atch 2 - hdbk03-1 PWT Test Article Design.pdf

AEDC-HDBK-03-1

Test Article Design and Strength Requirements—AEDC/PWT Wind Tunnels

16T/16S/4T

Aerospace Testing Alliance Test & Facilities Support Department and

Flight Systems T & E Department

October 2003

ARNOLD ENGINEERING DEVELOPMENT CENTER

ARNOLD AIR FORCE BASE, TENNESSEE

AIR FORCE MATERIEL COMMAND

UNITED STATES AIR FORCE

Cleared for public release; distribution unlimited.

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APPROVAL STATEMENT

This report has been reviewed and approved.

Carmen Evans, 1Lt, USAF Flight Systems Test Division Test Operations Directorate

Approved for publication:

FOR THE COMMANDER

Gary Mattasits Chief, Flight Systems Test Division Test Operations Directorate

NOTICES

When U. S. Government drawings, specifications, or other data are used for any purpose other than a definitely related Government procurement operation, the Government thereby incurs no responsibil-ity nor any obligation whatsoever, and the fact that the Government may have formulated, furnished, or in any way supplied the said drawings, specifications, or other data, is not to be regarded by impli-cation or otherwise, or in any manner licensing the holder or any other person or corporation, or con-veying any rights or permission to manufacture, use, or sell any patented invention that may in any way be related thereto.

Qualified users may obtain copies of this report from the Defense Technical Information Center.

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July 2003

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Handbook

4. TITLE AND SUBTITLE

Test Article Design and Strength Requirements-AEDC/PWT Wind Tunnels 16T/16S/4T

5a. CONTRACT NUMBER N/A

6. AUTHOR(S)

Jacobs Sverdrup AEDC Group Test & Facilities Support Department and Flight Systems T & E Department

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AEDC/DOP

Arnold AFB, Tennessee

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AEDC/DOP

Arnold AFB, Tennessee

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AEDC-HNBK-03-01

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None

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Approved for public release, distribution unlimited.

14. ABSTRACT

The purpose of this handbook is to provide ground rules for acceptable model designs and stress analysis for testing in the PWT Wind Tunnels 16T/16S/4T.

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19a. NAME OF RESPONSIBLE PERSON Gary Mattasits a. REPORT

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FOREWORD

The test article design and strength requirements described herein are the result of past experiences in wind tunnel testing and are essential for the protection of the wind tunnel and for the operational compatibility of the test article. The requirements, in the form of this handbook, are issued to the users of the Propulsion Wind Tunnel (PWT) facilities to establish ground rules for acceptable model designs and structural analyses. Fulfillment of these requirements provides a satisfactory resolution of some hazards identified in the System Safety and Hazard Analysis for each project as per MIL-STD-882 and AEDC Procedures. Items that do not meet the requirements herein must be brought to the attention of the PWT staff as soon as possible for consideration of changes or countermeasures that will permit safe, satisfactory testing. Deviations from these requirements may be permitted in special cases after AEDC-PWT personnel have reviewed the design and made an assessment of the consequences of failure. Any deviations from the require-ments will be documented in the system safety hazard analysis and risk assessment.

For the convenience of the User, an index of subjects discussed is given in the back of the handbook. Decimal numbers refer to section paragraphs.

SECTION 1.0

GENERAL MODEL REQUIREMENTS

1.1 TUNNEL ENVIRONMENT

The model and its support structure must be constructed of materials suitable for the desired test conditions, i.e., levels and variations of altitude and temperature as they can be obtained in the Propulsion Wind Tunnels (16S and 16T) and the Aerodynamic Wind Tunnel (4T). In Tunnels 16S and 4T the model must not only withstand the steady-state aerodynamic loads, but also the loads resulting from starting and stopping supersonic flow. Although the tunnels are not capable of di-rectly simulating transient flight conditions, transient aerodynamic heating may occur during start-up periods in Tunnel 16S. If the model is to be tested at elevated stagnation temperatures, it must be capable of allowing a minimum rate of change of stagnation temperature of 10°F per minute.

Resulting thermal stresses and the combination with stresses from weight, steady-state, or flow-breakdown loads must be within safe limits. Tunnel operating characteristics are described in de-tail in the Test Facilities Handbook.

The testing of flight articles in the wind tunnel at flight conditions is not necessarily safe, since the structural stressing is different. In the tunnel, weight and air loads are transmitted to a few support points. In free flight, air loads are opposed by distributed weight and inertia forces. A model or flight article is exposed for longer periods to more extreme conditions in the tunnel. Rig-id tunnel mounting introduces vibrations which in turn require fatigue strength of all components and positive locking of all connectors and fasteners. Flight articles must, therefore, be reexamined and analyzed for tunnel testing.

1.2 MODEL DESIGN CRITERIA

1.2.1 Model and Tunnel Compatibility

Careful planning of the test program and the model design is required to obtain an optimum amount of data during an operating shift, considering cost of tunnel operations and plant manipulating times required to set test conditions. The cost per tunnel operating shift hour will continue regardless of whether or not data are produced. The plant manipulating time is somewhat variable and is about 20 min for Tunnel 16T, 60 min for Tunnel 16S, and 5 min for Tunnel 4T to reach test conditions from start to pumpdown. Tunnel 16S has to be stopped to set up compressor configuration changes required to go from one Mach number range to another, as described in the Test Facilities Handbook. This is accomplished while the tunnel is pumped down and requires about 30 min. In view of the above, the number of shutdowns during scheduled tunnel operation for model changes should be held to a minimum or reserved for the “air off” periods.

1.2.2 Model Support Equipment

The support equipment existing in each tunnel, types of tests that have been conducted, and available utilities are briefly described in the Test Facilities Handbook. A considerable amount of special equipment, struts and adaptors, is available for a potential user. The PWT staff should be consulted to determine suitable combinations for a particular test installation.

1.2.3 Taper Seats and Coupling Threads

Stings and adaptors furnished by customers that match with AEDC equipment will be finish-machined at AEDC if special gages are required, such as tapers, coupling threads, or hole patterns.

Machining allowances should be discussed during the planning conference. All holes and slots should be added after finish grinding of taper. Dimensional taper standards will be furnished by PWT.

1.2.4 Model Balance Installation

A series of internal balances is available at PWT as listed in the Test Facilities Handbook.

Model balance mounting configuration drawings and sketches describing balance orientation, mounting tolerances, machining finishes, pin sizes, etc. should be obtained at the planning conference. A typical model-to-balance adaptor design and installation is shown in Fig. 1.

Section A - AA A

A A

Balance Sting

"Push-Off" Screws Between Balance and Sting

Key Pin Between Balance and Sting

Pin Must Be Drilled and Tapped to Provide Means for Pulling Pin

Pin Between Model and Model to Balance Adaptor

Taper in Model to Balance Adaptor is Normally Finish Machined at AEDC

Model to Balance Adaptor Must Be Threaded for "Push-Off" Screw (Thread Size Must Be Established by AEDC)

Flat Socket Head Cap Screw or Socket Head Cap Screw Over- Lapping Pin to Retain Pin in Model (Screw Should Be "Nylock" Type If Possible)

Flat Washer

"Pull-On" Screw Between Balance and Model to Balance Adaptor (Thread Size Must Be Established by

AEDC)

Key Pin Between Balance and Model or Balance Adaptor (Hole & Key Pin Are Normally Machined at AEDC)

Model to Balance Adaptor Should Have a Snug Slip Fit with the Model and Have Sufficient Length to Transmit Pitching Moment from Model to Adaptor

"Pull -On" Screws Between Balance and Sting

Balance Force Center

Model Body or Fuselage

Figure 1. Typical Model-to-Balance Adaptor Design and Installation

1.2.5 Angle-of-Attack Indicators

Angle-of-attack indicators that can be mounted inside of the model are shown in Fig. 2.

Bubble packs need to be furnished by the User. Bubble packs that give signals at distinct angles are very useful for reference.

1.3 MODEL DESIGN COORDINATION AND SCHEDULING

1.3.1 Review of Preliminary Design

Close coordination with PWT personnel should be maintained in order to ensure compatibility of the test article with the required test conditions. It is of major importance that preliminary model loads, stress analysis, and drawings be submitted to PWT before or at an early stage of the model fabrication so that possible changes required by PWT can be incorporated in the design. Otherwise the User runs the risk of having to change furnished components or of having restrictions imposed on his test programs.

1.3.2 Model Delivery

The model should be delivered to PWT at least three weeks prior to the scheduled starting date of the test. When special instrumentation is necessary or when calibration and/or installation procedures are extensive, additional lead time may be required. Special handling, installation, and calibration equipment is to be furnished by the User with the model. Shipping instructions are giv-en in the Test Facilities Handbook.

Clearance Hole for #8-32UNC Cap Screw 2 Places

Base Plate Supplied by AEDC

Schaevitz® LSRP

1.780

2.130

1.6001.850

Figure 2. Angular Position Indicator

1.3.3 Delivery of Drawings and Analysis

At least six weeks prior to the delivery of the test article to AEDC, the User will furnish the following final information to PWT:

a. three complete sets of drawings, one set for the Projects Team and two sets for the Test Support & System Maintenance Team;

b. three sets of schematic diagrams for electrical, hydraulic, and pneumatic services;

c. two copies each of model load criteria, stress analysis, and material verification documents;

d. if a balance is supplied by the User, a complete set of detailed balance drawings, all available balance history, and a stress analysis of the balance and its attachment; and

e. the User will perform a complete calibration of all User-furnished instrumentation, submit the data to PWT, and supply all of the calibration equipment necessary for final or check calibration at PWT.

SECTION 2.0

MODEL DESIGN REQUIREMENTS

2.1 MATERIAL SELECTION

2.1.1 Selection of the Best Material

The selection of the best material for the job depends on many considerations such as test requirements, type of model, strength, weight, dimensional stability, corrosion resistance, availability, shop capabilities, etc. Some properties are more important than others. Based on past experience the following properties are of foremost importance.

2.1.2 Endurance

Fatigue strength is an absolute requirement for structural members such as stings, struts, wings, flexures, etc. Dependent on the application, a material with the best balance of ultimate, yield, and fatigue strength should be selected. The design properties must be based on reliable and substantiated test results. Consideration must be given to actual material soundness, surface finish, notch sensitivity, and stress concentrations. A modified Goodman or another representative fa-tigue-strength diagram can be constructed and the allowable combination of steady-state and oscil-lating stress levels determined (Fig. 3).

References:

1. Spotts, M. F. Design of Machine Elements, Second Edition, New York: Prentice-Hall, 1953

2. Shigley, J. E. Mechanical Engineering Design, New York: McGraw-Hill, 1963

3. Petersen, R. E. Stress Concentration Design Factors, Second Edition, New York: John Wiley & Sons, 1959

4. Lipson, C., Noll, G. C., and Clock, L.S. Stress and Strength of Manufactured Parts, New York: McGraw-Hill, 1950

Gross Allowable Oscillating Stress

Mean Stress

M in

S tr es s

Mea n S tre ss

Max Stress

Parallel

45 deg

Stress σu (Ultimate) σy (Yield) σy σu σe σe (Endurance)

Figure 3. Design of a Modified Goodman Diagram

2.1.3 Dimensional Stability, Corrosion Resistance, and Machinability

Dimensional stability, corrosion resistance, and machinability are important for balances, balance sleeves, wings, stabilizers, and models for long and/or repeated testing. Materials such as 17-4 PH,13-8 PH, 15-5 PH, AISI 4130 and 4340, as well as 7075, 6061, and 2024 aluminum have been used successfully. Precipitation-hardening steels are not to be used in the solution-heat-treated (A) condition. Model leading edges are subject to erosion or sandblasting and should not be made of soft materials. Stainless steels of the 300 series are less dimensionally stable and therefore should not be used in fabrications requiring close fits. They also tend to gall easily. Composite structures with wood and fiber-reinforced plastics are less dimensionally stable but are satisfactory for some models.

2.1.4 Weldability

For structural welds the selection of a suitable alloy together with close control of the weld-ing technique is necessary to obtain sound welds. The majority of materials requires unique weld techniques using specific established methods, i.e., pre- and post-heat treatments, filler materials, shielding, etc. It is recommended that welding techniques be discussed with PWT. Some materials used successfully are mild steel, precipitation-hardening steels, Carrilloy T-1, and 6061 aluminum.

2.1.5 Nonmetallic Material

Nonmetallic materials have been successfully used for composite construction. But, consideration should be given to the attachment to the backup structure such as aluminum or steel.

Bonding alone is not totally reliable, and additional mechanical fastening is required for the prima-ry load. Wood should be kiln-dried, sealed, bonded and mechanically fastened to the backup structure. The sealing is required also over hidden interior surfaces. Best results are obtained with one or two plies of fiberglass together with a final gel coat over the whole assembly. Metal inserts are required in the nonmetallic material for fasteners that frequently have to be removed for configuration changes or model maintenance.

2.1.6 Thermal Properties

Thermal properties must be considered for operation at elevated temperatures with regard to change in strength values and differential expansion.

2.2 STRUCTURAL JOINTS

2.2.1 Bolted Structural Joints

In bolted structural joints, the bolt preload has to be such that oscillating stresses in the threads are avoided to prevent fatigue failure.(see paragraph 3.4.7 and Figs. 4 and 5). Thread en-gagements with nuts and/or tapped holes must be sufficient to develop a strength equal to the bolt.

The length of engagement required varies with the type of thread (coarse or fine) and the material of engagement. Coarse threads should be used in soft materials such as magnesium, light metal casings, and aluminum. Shear loads should be transmitted by keys, pins, pilots, and /or shoulders to ensure that bolts are only used in tension. Shear bolts are acceptable where close tolerance slip fits are not required, but the probability of galling or fretting should be carefully considered. Alloy steel socket head cap screws of forged and roll threaded type are recommended for all bolted joints that are critical. Countersunk screws are not acceptable in major structural joints because of the difficulties in determining shear areas, grip length, and bearing areas, and because of the inability of obtaining sufficient thread preload to avoid fatigue failure. Countersunk, flat head, or button head screws may have only 85 percent or less of the strength of socket head or other full head screws. A successful pin joint is shown in Fig. 6. To avoid galling in joints, the use of dissimilar materials and/or materials of different hardness is strongly recommended, provided the materials do not cause any galvanic action.

Preload (P)

Allowable Moment (M) Contact Area (A)

Preload P = number of bolts times preload per bolt (torqued to 1/2 yield is recommended)

= section modulus of the contact area with respect to the neutral axis

The allowable moment

M = 1 C1.25 I

A P

C I

Figure 4. Allowable Moment for a Bolted Joint

Figure 5. Allowable Moment for a Statically Loaded Bolted Joint

M

P1

P1 or P5 = or in general Pi =

P2

P4

P5

M h2 h1

2(h1)2 + 4(h2)2

Mh1 n Σ o

(hi)2i =

Mh1

2.2.2 Welding of Structural Joints

Welding of structural joints must be done with caution and established procedures to pro-duce sound welds with good fatigue properties. Cracks, voids, impurities, lack of fusion, partial penetration welds, and overheated welds will lead to early fatigue failure. The most troublesome joint seems to be the one on a simple strut between the base and strut blade. Continual vibrations in the lateral direction are the cause of fatigue failure. Degrees of improvements are shown in Fig. 7.

P1 or P5 =

P1 or P5

P1 or P5 P1 or P5

Tensile Stress of Tear Out

Tensile Tear Out Stress ≤

Shear Stress in Pin ≤

Bearing Stress <

Nylock Screw

Tear Out Area

Skin, compressive yield stress of skin.

or ≤ σUlt. Skin σy Skin τUlt. Pin σ yc

P1 or P5

Tear Out Area

M h2 h1

2(h1)2 + 4(h2)2

M(h1)

Figure 6. Allowable Moment for a Statically Loaded Pinned Joint

Simple thru-welds are easier to make and to inspect. User’s welding processes and subsequent heat-treat procedures should be submitted for review and approval (see also paragraph 2.6.2).

2.2.3 Socket Joints

Socket joints, whether they be tapered or cylindrical fit, must include a key pin(s) for posi-tive roll orientation and for resisting roll moment.

In cylindrical socket joints, key pins should be capable of resisting the applied roll moment at the joint while maintaining a factor of safety of 4.0 on the shear strength of the key pin material.

In tapered socket joints, key pins should be capable of resisting the applied roll moment at the joint while maintaining a factor of safety of 2.0 on the shear strength of the key pin material.

Best

Better

Poor t t

C

C

A

B

B

This joint is most reliable.

It is taken from a rolled or extruded structural shape.

It is not always available, and almost nonexisting in heat treatable steels.

This joint is located in an area where the temperature during welding can be held uniform and where the stress is more uni-axial. It requires slightly more machining preparations, but is much more reliable than fillet welding.

This joint is difficult to make without defects. The smaller heat dissipation in direction A over B often leads to over-heating and undercutting at C.

Change in metal properties due to excessive heat together with stress concentrations at under-cuts in Zone C result in early fatigue cracking.

Figure 7. Comparison of Structural Joints

The lower factor of safety for the tapered socket joint recognizes that friction in a properly seated tapered joint resists the roll moment; however, due to the variables of surface condition, lubrica-tion, degree of fit, and the dynamic operation environment which tends to unseat the taper, the key pin(s) alone should be capable of resisting the applied roll moment with a nominal factor of safety.

For socket joints the following formula has given good results; the tensile stress in the fe-male portion is

Figure 8. Socket Joints Formula where M is the moment at the center of the socket, l is the length of engagement, and t is the wall thickness. This equation is also used for taper connections. Contact PWT Stress Engineer if explanation of the above formula is desired.

2.3 STINGS AND STRUTS

2.3.1 Sting Design

To keep aerodynamic interference to a minimum, size, length, and attachment of stings and/ or struts to the model play an important role. For sting-supported models this involves making the best compromise between balance choice, balance location, distance between balance center and tail, tail clearance, sting length, sting stiffness, and sting material. From a safety standpoint, sting stiffness and flexibility must be kept within safe limits (see paragraph 3.4). The tail clearance must be large enough to allow for deflection caused by steady-state and dynamic loads as well as a foul-ing strip of about 0.010-in. thickness (see paragraph 2.3.4). The allowance for dynamic loads should be about 25 percent of that required for steady-state loads. Several computer programs tailored for different types of balances and sting shapes are available at PWT to obtain answers with respect to sting stress, differential deflection between sting and model tail, and model-sting stability. To use the programs for a balance-supported model, the following specific inputs are re-quired:

M

4M t

T = l 2t σ

a. the balance deflection constants C1 and C2 for the angular rotation of the model with respect to the sting end gage diameter of the balance given by the equation

∆θ = C1N + C2M the balance deflection constants C3 and C4 for the translation of the model at the balance center with respect to the gage diameter given by the equation

∆Z = C3N + C4M where N = normal force (lb), M = pitching moment (in. x lb);

b. in the case of a “Task”-type balance, the distance between balance center and the gage diameter. In the case of a beam-type balance, the distance from the balance center to the sting end gage diameter.

c. the worst-case model loads and locations;

d. the balance capacity; and

e. the sting and/or strut configuration.

2.3.2 Model Dynamics

Mechanical vibrations transmitted from machinery through the tunnel ducting and/or foundations and aerodynamic forcing caused by flow separations, stalls, and other flow instabili-ties exist. Should the vibration or forcing frequency be in tune with the natural frequency of the system, destruction is imminent and immediate abortion of the conditions is called for, especially when a balance is involved. Chances for the occurrence of that situation are greatly minimized when model and support structure are designed in accordance with the requirements of paragraph 3.4., when model weight is kept low and the support structure is stiff, and when model center of gravity, center of pressure, and balance center closely coincide. If the model is to be tested in Tun-nel 16S, it is desirable that the balance center also be located at the flow breakdown force center (see paragraph 3.2.3). Buzzing in model flow through inlet ducts may be in resonance with the model balance system and could lead to early fatigue failure of the axial-force element. Provision should be made to detune the buzzing frequency or to avoid it.

2.3.3 Sting, Strut, and Balance Materials

It is desirable to find a material with a high ultimate strength, a high endurance limit, and a high modulus of elasticity. The properties for some materials are shown in Table 1 for comparison but not for final selection. The more sophisticated a material is, the more skill and precaution is required in welding, machining, and heat treatment, and the more exact and stringent the inspection will have to be. Experience has shown that the use of ultra-high-strength (>220 ksi) maraging steels requires additional precautions in the wind tunnel environment. These materials by their very nature are very low in toughness, low in modulus, low in fatigue strength, and are highly suscepti-ble to brittle failure in the presence of extremely small flaws (Ref. Table 2). Because of the diffi-culty of detecting critical flaws it is required to qualify maraging steels for structural members by providing inspection criteria, results, and analyses of sufficient scope to assure that system safety is maintained (see paragraph 2.6.4). If there is no alternative, PWT prefers raising the allowable stresses for 13-8 PH and Nationalloy steels instead of using ultra-high-strength maraging steels.

When an ultra-high-strength steel (>220 ksi) is used in a critical fatigue application, it is necessary that the steel be a vacuum-remelted material. The attainment of a high tensile strength is not as im-portant as high toughness, low crack growth rates, cleanliness, and inspectability. Any component whose failure would result in a catastrophic loss of hardware should be designed using a safe-life concept, i.e., those components must remain crack-free while in service (>107 cycles). A comprehensive fatigue analysis should be provided along with material data and inspection results.

If a critical structure is not fully inspected, analyzed for fatigue, or adequately characterized, the assumption will be made that a flaw or crack exists in a critical area and stress limits will be estab-lished low enough to preclude additional crack growth.

Table 1. Comparison of Some Material Properties

Note: Above values except for 13–8 were taken from the “Aerospace Structural Metals Handbook” AFM-TR-68-115.

Values for 18–8 are manufacturer’s data. The endurance limits are based on standard rotating beam tests with approximately 10 × 106 cycles. The values in parentheses ( ) for maraging steel 300 are from axial fatigue tests.

Mechanical Properties

18% Ni Maraging Steels for Ultimate Strength

AISI 4340

Heat Treatment

Precipitation Hardening Steels ksi ksi ksi ksi

17–4 H900

13–8 H1000

Ultimate Tensile, ksi 275 250 210 210 190–200 210

Endurance Limit, at Stress Concentration, Kt ksi 130 1.0

100–105 1.0

100–120 1.0

1.0

1.0

100–110 1.0

Endurance Limit, at Stress Concentration, Kt ksi 57(26) 2.2(3)

50–55 2.2

49–50 7.5

40–45

Modulus of Elasticity, 10-6 psi 26 25 26.2 29.5 28–29 28–29

Table 2. Plane Strain Fracture Toughness of Selected Alloys

*NOTE:Critical flaw size for a through-thickness center notch imbedded in are infinitely large sheet sufficiently thick to develop plane strain conditions at the tip (Hertzberg, 1976).

2.3.4 Sting Model Fouling

At PWT it is recommended to install a fouling strip around the sting at the model tail or any other close spaces to indicate interference at certain load conditions. Fouling strip material consisting of brass film insulated on one side is available at PWT. After bonding to a sting it has a thickness between 0.008 and 0.012 in. This strip acts as a switch to close an electrical circuit when contacted by a conducting part of the model.

2.4 PRESSURE VESSELS

2.4.1 General Requirements

Pressure vessels must be designed, fabricated, inspected, tested, and installed in accordance with the ASME Boiler and Pressure Vessel Code.

2.4.2 Definition

All shells, test chambers, and tanks designed for internal and external pressures greater than 15 psig and not made from standard pipe and fittings are deemed to be pressure vessels. Pipes with outside diameters greater than 48 in. are also classified as pressure vessels.

Material Yield

Strength, ksi

KIC,

ksi .

Applied Stress, ksi

*Critical Flaw Size, in.

2014–T651 66 33 33.0 0.141 2024–T3 50 40 25.0 0.815

7075–T651 72 22 36.0 0.119 4340 125 90 62.5 0.660 4340 220 55 110.0 0.080

17–7PH 208 70 104.8 0.144

13–8H950 210 70 105.0 0.141

13–8H1000 205 95 68.3 0.682 13–8H1050 190 150 95.0 0.794

250 Maraging 225 50 112.5 0.063 300 Maraging 280 50 93.3 0.132 350 Maraging 225 35 162.5 0.015

52100 300 13 150.0 0.002 in

2.4.3 Applicable Code Section

Pressure vessels shall be designed in accordance with the latest edition of the ASME Boiler and Pressure Vessel Code, Section VIII or Section III. Either the requirements of Section VIII or Section III must be satisfied. Selection of various rules from both sections is not permitted. For welding qual-ifications refer to Section IX, and for welding inspection refer to Section V of the above code.

2.4.4 Definition of Ultra-High-Pressure Vessels

Pressure vessels with a maximum difference in pressure between the inside and outside or between any two adjacent chambers greater than 10 percent of the tensile strength at room temperature shall be designed in accordance with the design philosophy put forth in ASME Boiler Code Section III.

2.5 PRESSURE PIPING

2.5.1 General Requirements

All piping and any other components conveying a pressurized fluid shall be designed, fabricated, inspected, tested, and installed in accordance with the latest edition of the ANSI Code for Pressure Piping, ANSI B31. Table 3 serves as a guide to the use of the Piping Code.

Table 3. Guide for the Use of the Piping Code

Medium Type of System Code Secion

Water Water supply other than plumbing Hydrostatic testing facility

Chilled water or brine Steam electric generating station

Hot water supply for central or district heating systems

B31.3 B31.3 B31.3 B31.1 B31.1

Air Air supply or exhaust systems Pneumatic testing

Steam electric generating system Central or district heating system

Vacuum systems

B31.3 B31.3 B31.3 B31.1 B31.1

Steam All systems B31.1

Oil Hydraulic actuator system Transmission line

Steam electric generating plant Central or district heating systems

B31.1 B31.4 B31.1 B31.1

Domestic or Industrial Fuel Gas

(Natural Gas, etc.)

Storage areas, transmission and distribution to buildings In buildings and between buildings Steam electric generating stations

Central and district heating systems

B31.8 B31.2 B31.1 B31.1

Liquified Petroleum

Gases

Storage areas, transmission and distribution to buildings Refrigeration

Other systems in buildings and between buildings

B31.4 B31.5 B31.2

Table 3. Concluded

*When published, Code Section BN31.10 will cover all systems oprating at temperatures below 120 K (-244°F).

Code Section 31.5 will cover refrigeration systems operating at temperatures above 120K.

2.5.2 Applicable Dimensional Standards

Pressure vessels fabricated from standard pipe and fittings with an outside diameter of 48 in.

or less are deemed to be pressure piping; they are defined as those covered by the following dimensional standards:

Pipe: ANSI B36.10 and ANSI B36.19

Fittings: ANSI B16.9

Flanges: ANSI B16.5

Medium Type of System Code Secion

Gasoline and Kerosene All Systems B31.4

Hydrocarbon Jet Propulsion (JP) Fuels

All Systems B31.4

Hydrocarbon Rocket Propulsion (RP) Fuels

All Systems B31.4

Refrigerants All temperatures to -320°F All temperatures below -320°F

B31.5* B31.3*

Cryogenic All Systems B31.3

Chemicals All Systems B31.3

Fluidized Solids All Systems B31.3

Water Containing Solids in Suspension All Systems B31.3

Radioactive Fluids All Systems B31.7

Gaseous Helium, Hydrogen, Nitrogen, and Oxygen

All Systems B31.3

Hazardous Liquid-Propellant Fluels and Oxidizer:

Alcohols Alkyl boranes

Anhydrous ammonia Aniline

Chlorine trifluoride Ethylene oxide

Fluorine Hydrazine

Hydrogen peroxide Monomethylhydrazine

Fuming nitric acids Nitrogen tetroxide

Penaborane Perchloryl fluoride

Normal propyl nitrate Unsymmetrical-Dimethylhydrazine

Oxygen difluoride

All Systems B31.3–1976 and Addenda 31.3a–1978

(Ref.Air Force) Manual AFM 160–39 and

T.O.-25–223)

2.5.3 Welder Qualifications

All welders, welding operators, and welding procedures shall be qualified with Section IX of the ASME Boiler and Pressure Vessel Code except as modified by the applicable section of the piping Code.

2.5.4 Allowances

Allowances for pipe threading, for corrosion, and for thinning of wall attributable to pipe bending shall be made as required and/or recommended by the Piping Code ANSI B31.

2.5.5 Pipe Joint Requirements

Threaded joints, flange joints, and seal welding of threaded joints shall be in accordance with the requirements and recommendations of the Piping Code ANSI B31.

2.5.6 Miter Bends

Miter bends and branch connections shall be in accordance with ANSI B31.

2.5.7 Tube Fittings

In the wind tunnel environment, flared tube fittings* have proven to be superior and are strongly recommended. Steel or stainless-steel AN fittings through 1 in. can be used to 3000 psi and through 2 in. to 1500 psi. Pressures for other materials should be adjusted to reflect differences in code allowable stress. Other pressure limits may be established when actual burst pressure data is available as 1/4 of burst pressure. Tube fittings larger than 2 in. may not be used.

Flareless fittings of the swage type or bite type can only be used with special approval ex-cept for “instrument” lines smaller than 1/8 in.

For plastic or rubber tubing 3/8 in. or smaller, used in low pressure service (less than 150 psi), any connectors with an established history of satisfactory service are suitable. Other sizes and pressures require special consideration.

2.5.8 Metal Tags

For reasons of safety, installation, checkout, maintenance, and operation, it is required that all service lines are properly identified with respect to working pressure, flow direction (in or out),

* Ref. MIL-F-18866B Fittings, Flared Tube, 37 Degree, Steel and fluid or gas carried. The User must at least identify all loose lines, including spares, shipped separately from the model as well as the model interfaces where User's lines and/or lines furnished by others need to be connected. Metal tags, supported by wire which is compatible with the piping and/or hose material, are preferred at PWT. Lines longer than 1 ft should be identified near each end. For large models, the User should identify the internal lines according to MIL-STD-1247 or by his own method.

2.5.9 Corrugated Hose

Corrugated metal hose having annular or helical corrugations and wire braid cover shall not be used for gaseous service at pressures above 150 psig unless it is absolutely necessary. Their use, allowable pressure, and installation requirements and relevant information shall be coordinated with AEDC/PWT prior to incorporation.

2.5.10 Rubber and Thermoplastic-Type Hoses

Rubber and thermoplastic-type hoses having a rubber or thermoplastic inner tube and cover and reinforced with wire or non-metallic fiber braid shall be used only where the use of rigid pipe, tubing, or swivel joints are not practical or feasible. Their use, allowable pressure, and installation requirements and relevant information shall be coordinated with AEDC/PWT prior to incorporation.

2.5.11 Exception to the Safety Standards

Although every effort must be made to comply with codes and standards, there will be occasions in wind tunnel testing where exceptions are required. However, when such deviations are necessary, rational engineering analyses shall be used to demonstrate that adequate safety has been provided for personnel and equipment (AEDC-ENGR-STD-T-2). In the case of jet simula-tion testing using high-pressure air, it would be impractical to design the model to source pressure level. Therefore, a lower design pressure can be justified if the probability of flow blockage by for-eign material is negligible. This low probability of blockage can be obtained by:

a. blowing out of the supply lines before their hookup to the model,

b. using filters in the supply line, and

c. sizing choke plate holes as large as practical.

2.5.12 Pressure and Leak Testing

All model pressure components as well as the supply lines that will be used during the test will have been pressure tested as individual components or as subassemblies after fabrication as required by the Piping Code ANSI B31. A verification of the tests is to be included in the stress reports. The Code requires a hydrostatic test to 1.5 times the maximum operating or design pressure. For high-pressure air systems, a final leak check and pressure test will be done pneumatically when the model is installed in the tunnel utilizing the plant high-pressure air supply system. A procedure for such a test will be provided by PWT for each individual model and test using high-pressure air. Visual leak checking of most high-pressure air systems will be limited to 300 psi due to a requirement prohibiting personnel in the immediate vicinity of some PWT test peculiar piping at pressures greater than 300 psi. The User must furnish the following information in order to size supply lines and flow meters:

a. pressure and mass flow range at the model interface and

b. the required model plenum pressures and internal pressure drops.

The User is to furnish all plugs and flanges for pressure testing for all items supplied by him.

2.6 MATERIAL INSPECTION

2.6.1 Inspection by Nondestructive Testing

All structural or load-carrying parts of the model and model support system, such as stings, struts, brackets, and castings must be inspected utilizing the best nondestructive testing technique (NDT) to find cracks, inclusions, voids, stringers, and/or other defects detrimental to the end use of the material. Some recognized techniques are magnetic particle, fluorescent penetrant, X-ray, and ultrasonics. For machined parts the magnetic particle fluorescent wet method is far superior to the dry particle method. Any indications or flaws are to be brought to the attention of and discussed with PWT. Surface cracks are not permissible.

2.6.2 Weld Inspection

User’s welding process and inspection specifications are usually acceptable; however, they must be submitted to PWT for approval. For areas where it is impossible to inspect for degree of penetration by X-ray and/or ultrasonics, the following procedures may be used:

a. In welds with stresses larger than one-half (1/2) of the allowable, NDT inspection for cracks should be performed after each pass.

b. In welds with stresses less than one-half (1/2) of the allowable, NDT inspection for cracks of the completed weld is acceptable.

2.6.3 Inspection Reports

Inspection reports covering these items must accompany the final stress report. Special agreements need to be made for materials that cannot be inspected by the above methods.

2.7 FASTENERS

All structural bolted or screwed connections must be provided with positive mechanical locks, such as Kelox inserts, locking-type Helicoils, metal Flex-Lock nuts, and safety wiring. Within their rated temperature, elastic stop nuts, Nylock screws, and screw-lock-treated threads are satisfactory for some components of the test article. Liquid or semi-liquid compounds such as “Vibra Tite” and “Locktite” applied to threads at final assembly may be acceptable provided curing time is allowed, and that their use was coordinated with PWT. Drilled heads must be provided if safety wiring is to be used. Pins and keys must be retained by some means also. Nichrome stripping is acceptable. The strength and quality of fasteners as well as the torque values for tightening of screws and nuts should be listed on drawings for all structural connections (see also paragraphs 2.2.1 and 3.4.7).

2.8 PRESSURE INSTRUMENTATION

2.8.1 Electronically Scanned Pressure (ESP)

NOTE: The information included in Section 2.8.1 is general information for using ESP’s (11/94).

As the result of an ESP Working Group a new manual of detailed recommendations for use of ESP’s in the PWT Tunnels is being developed and available (6/97). Contact PWT for a copy of ESP Manual and most recent recommendations.

The most recently developed method of obtaining pressure readings from wind tunnel mod-els is by the use of “Electronically Scanned Pressures” (ESP). Each PWT module (Fig. 9) has 64 pressure ports plus a calibrate and reference port. Two additional ports are used for 100 psi air required for the mechanical movement of a shuttle block inside the module.

0.125" (3.2 mm)

0.10" (2.5 mm)

1.300" (33.0 mm)

3.15" (80.0 mm)

1.13" (28.7 mm)

1.38" (35 mm)

1.00" (25.4 mm)

Figure 9. Individual ESP Unit ~ 64 Port

The ESP unit is mounted inside the model close to the source being measured. This greatly reduces the lag time usually encountered when using the “High Precision Pressure Transducers” (ref. paragraph 2.8.2). Pressure measurements are transmitted to the data system via electrical cables. Each cable is capable of supporting 8 ESP units. As many as 42 ESP units have been used in PWT/16T at one time inside a single model.

Because the units are mounted inside the model, some services are required to be supplied to them.

1. Electrical cable that supplies power to the module as well as returns the signals to the data system (3/8-in.-diam cables supplied by PWT) must pass through sting.

2. Five pressure tubes (1/8-in.-diam copper) must pass through sting. (1/16-in.-diam. may be used for short distances if clearance is small.)

a. Two are used for shuttle pressures designated as C1 and C2 (100 psi each).

b. Reference

c. Calibrate

d. Verification

ESP modules presently available at PWT are 15 psi or 100 psi units. If both sizes are used within a model, then 7 lines (1/8-in.-diam each) must be run to the model and then manifold dis-tributed to the individual units.

C1 and C2 ∼ common to both 15 psi and 100 psi

Reference ∼ common to both 15 psi and 100 psi

Calibrate ∼ 15 psi

Calibrate ∼ 100 ps

Verification ∼ 15 psi

Verification ∼ 100 psi

Ports 1 and 64 are normally used for verification ports on all modules.

The modules are placed within the model in an area that is protected from airflow and rapid changes in temperature. In 16T the modules may be placed inside an environmental chamber (Fig.

10) that is heated to a temperature greater than tunnel temperature. This technique keeps the units at a more constant temperature, thereby decreasing the frequency of calibration. Likewise in 16S a similar technique is used to maintain a constant temperature except the units are surrounded by a wa-ter-cooled cavity rather than a heated cavity. The space requirements of each technique are identical so that a model may be tested in 16T and 16S without modifying the space required for the units.

NOTE: The 64 port ESPs are temperature compensated from 0 to 80°C; therefore, environmental chambers are generally no longer used in 16T.

2.8.2 High Precision Pressure Transducers

The size of orifices and pressure tubes must be carefully selected for a minimum lag time so that an optimum use of allocated test time can be realized. A computer program is available at PWT for quick evaluation of lag times considering orifice and tube diameter, as well as tube length, absolute pressure level, and pressure level changes. The program will also optimize the tube diameter. Inasmush as the lag time is inversely proportional to the absolute pressure level, or-ifices sensing static pressure, especially in Tunnel 16S, must be carefully checked. The high-preci-sion pressure transducers are permanently installed in the test section plenum chamber and are connected to pressure tubing disconnect panels. For detailed calculations of lag times, the sizes

1.700 in.

0.150 in.

Typ.

1.950 in.

4.200 in.

Figure 10. Environmental Enclosure and lengths of tubing permanently installed between disconnect panels and transducers are as fol-lows:

a. in Tunnel 16T, 240 tubes with 0.063-in. I.D. and an average length of 13 ft,

b. in Tunnel 16S, 100 tubes with 0.063-in. I.D. and 140 tubes with 0.090-in. I.D., with an average length of 8 ft for both sizes, and

The average length of tubes required in the tunnels between a sting-supported model and the disconnect panels is 75 ft for Tunnel 16T, and 50 ft for 16S.

2.8.3 Typical Pressure Tube System

Under typical conditions of aerodynamic testing in Tunnels 16S and 16T, orifices 0.045 in.

in diameter with 0.063-in. I.D. tubing connected to the disconnect panels will be satisfactory.

Stainless steel or Monel tubing with 0.063-in. O.D. and 0.047-in. I.D. may also be acceptable for a length of 10 ft downstream of the orifice; however, this is dependent on pressure levels. Final lead length and size should be determined after project initiation. Sizes, materials, and temperature ranges of plastic tubing as used in the past are listed in Table 4.

Table 4. Sizes and Ranges of Plastic Tubing

2.8.4 Pressure Tubing

In fabricating model pressure instrumentation parts, rakes, disconnects, etc., it is desirable to use hard soldering (brazing), oven brazing, or heliarc welding in preference to soft soldering. For hard soldering (brazing), chemically clean surfaces and using the proper silver solder and flux are

Material and Size Type of Tube Connection

Temperature Range, °F

Range of Pressure

Differential, psi

Polyethylene or Stripatube

3/16 O.D., 3/32 I.D.

7/64 O.D., 3/64 I.D.

Slip-On

1/8 O.D.

1/16 O.D.

Up to 150

Up to 150 Up to 150

Tygon 7/32 O.D., 3/32 I.D. 1/8 O.D. Up to 130 15

Viton‚ B 0.031 I.D Up to 165 15

Butyl 3/16 O.D., 3/32 I.D.

7/64 O.D., 3/64 I.D.

1/8 O.D.

1/16 O.D.

Up to 250 Up to 250

Silicone Rubber 7/32 O.D., 3/32 I.D. 1/8 O.D. Up to 350 15 the prerequisites for a strong joint. Silver solder composed of 50 percent silver, 15 percent copper, 16 percent zinc, and 18 percent cadmium and with a melting point of 1175°F (Easy-Flo brand braz-ing filler metal) is recommended. When used with the proper flux the solder will flow easily and make a good air-tight connection. All joints must be cleaned of flux by dipping in hot water and scrubbing.

2.8.5 Pressure Tube Bonding

Drill all holes (d + 0.005)-in. diameter, countersink 90 deg by (3/2 d)-in. diameter both sides if possible. For illustration see Fig. 11. When routing tubes in surface grooves, careful consider-ation should be given to mechanically anchoring tubes and particularly tube groups, using epoxy solely as a filler.

Surface preparation: clean all parts by sandblasting or, preferably, by chemical etching.

Several bonding agents are usable, but need to be applied and cured as specified by manufacturer's instructions. Adhesives should be identified and property information supplied.

After curing is complete, flush off excess cement and tubing while flowing plant air through tubing. Excess cement on the inside of the model wall serves to prevent cracking of tubing and bond during handling.

2.8.6 Pressure Tube Identification

For proper hookup and leak checking, all pressure tubing needs to be identified in accor-dance with pressure schematics furnished by the User. This identification is usually done by oil-resisting markers either at the User's plant or at PWT. Mylar-shielded markers, such as Brady Company Stock No. PS-34-66 (B-600 Tape Release CTG RC-16), have given satisfactory service.

d

Figure 11. Pressure Tube Installation by Bonding

2.9 MODEL REMOTE CONTROLS

2.9.1 General Requirements

Remotely controlled model components, including its drive system, shall be designed and fabricated with due consideration to basic strength, as required by Section 3, and tight linkage joints.

Analysis is to show that bearing pressures are chosen properly to keep wear and backlash to a minimum. Actuators or drive motors should have at least 50-percent power reserve for maximum calculated power requirements.

2.9.2 Position Sensors

The position sensor should be directly connected to the control surface to show actual posi-tion regardless of a backlash in the drive system. Backlash is a problem in any closed-loop control system and must be reduced to an absolute minimum. If feedback control is used in presence of backlash, it is mandatory to use an additional sensor directly connected to the electrical or hydrau-lic actuator for control of its position. If backlash is not a problem, the same sensor could be used for position indication and for feedback control. However, the use of dual sensors is recommended for backup purposes. Good results have been obtained with plastic-type high-resolution potenti-ometers in the range of 5000 Ω or less for position indication and feedback control. User-furnished controllers will be located in the control room. Real-time display of the actual position of a model component can be provided at the User's controller if the position sensor is provided to the data systems. Six permanently installed on-off controllers and three hydraulic controllers are available in Tunnels 16S and 16T for the control of model support and model component systems. A porta-ble rack with 12 additional hydraulic controllers is available. Standard hookup has been estab-lished for all hydraulic systems and should be followed for User-furnished components.

2.9.3 Safety of Operations

All controlled model and model support components must have positive stops at the ends of their planned travel for which model loads were estimated and the model and model support structure were analyzed. Electrical actuators must be provided with limit switches and/or slip clutches. Brakes are required if the actuator drive is not self-locking.