Circuit Descriptoin:

Test Section Characteristics

Test Section Doors

In the 40 by 80 foot test section the doors are 40 feet wide and 49 feet long. There is one door on each side of the tunnel center line on top of the test section. When fully open, a clear opening of 78.5 feet by 49 feet is provided.

Main Hoist and Gantry Crane

In the 40x80 test section a 35-ton hoist and a 5-ton hoist, mounted on a common bridge at the top of the test chamber, are available to hoist model into the test section from the floor of the High Bay. The bottom of the hoist hook is twenty-seven feet above the test section door when they are fully closed; a clearance figure of importance when planning the model lift-in.

Back to Top Menu  

Model Support Systems

Primary Model Support System

The primary model support system in the 40-by 80-foot test section consists of two mechanical systems operated from the control room during a test; these systems change either the angle-of-yaw, by rotating the turntable systems, or the angle-of-attack, by telescoping the nose/tail strut.

* Click on image to enlarge

Description

The model support system has two major elements. The first, the metric subsystem, supports the model and consists of three model struts, a lower turntable, the floating frame, and the balance system. The three moveable model struts are mounted on the lower turntable called the "T Frame", which is mounted on a floating frame that is in turn connected to the balance system.

The second major model support element, the non-metric subsystem, consists of the upper-floor turntable and strut fairings. Each model strut is shielded from the air stream by a fairing that is independently mounted to the upper floor turntable. The two turntables, upper and lower, are separately supported to isolate fairing aerodynamic loads from model aerodynamic loads. This isolation not only permits separation of model loads from fairing aerodynamic loads, it also provides a system for detecting a "foul" during a test run. If during a test, any part of the non-metric system contacts the metric system, a warning light flashes in the control room indicating that the measured areodynamic loads for the model may be in error or "fouled".

The model is mounted in the tunnel on two main struts and the single telescoping nose-tail strut. The struts can be moved to accomodate models of different treads (widths) and different tail or nose lengths. The drawing below presents the range of tread and tail lengths available in the 40-by 80-foot test section.

Back to Top Menu  

The model may be mounted in the tunnel at variable heights through a system of different main struts and tip extensions. The main strut configuration can vary from a minimum of 2' 6.5" to a maximum of 21' 7.5" above the tunnel floor. The aerodynamic fairings, which shield the struts, also vary in height. In addition, other arrangements may be fabricated for special tests.

Back to Top Menu  

The telescoping nose/tail strut uses a ball screw and tip extension to achieve variable heights. Nose/tail strut configurations can vary from a minimum of 3' 0" to a maximum of 25' 6" above the tunnel floor. The ball screw has an extension length of 11' 6" from the fully retracted to the fully extended length. The maximum tilt of the gimbal for the nose / tail strut is ±15°. The table below presents the available tail strut travel for different tip and fairing arrangements.

Back to Top Menu  

Allowable Strut Loads

The two main struts are "fixed" at their base and carry model dead weight, lift, slide, and drag forces, and all rolling, yawing, and pitching moments. In contrast, the nose/tail strut is mounted in a gimbal and carries only pure axial loads from model dead weight and lift forces.   

The different load combinations for anticipated aerodynamic forces and model dead weight must be developed for the various test configurations for the angle-of-attack and the angle-of-yaw. These load combinations must be examined to calculate the resultant loads on the supporting struts. The horizontal resultant force acting on the two main struts include the side forces, the drag forces, and the yawing moments. The vertical resultant forces acting on the two main struts include model dead weight, lift forces, pitching moments, and rolling moments.

Back to Top Menu  

Model Support Connections

The model connections to the main struts and nose / tail strut are made with ball socket mounting assemblies. These joints are a potential single point failure, and therefore, are critical to structural safety and to model support. Through-bolting for these joints is desirable whenever possible, however, a drilled and tapped configuration is also acceptable. To obtain the maximum load capacity of the system with a drilled and tapped configuration, the mounting pad from the ball socket to the model interface must be a 1 - 1/4" thick, 4130/4340 RC10 steel plate, welded to the model's structural frame.

Angle-of Attack Range

The angle-of-attack range for the model support system is limited by the ball socket design to ±23°. However, specific requirements for each model vary and may limit the angle-of -attack by any one of three conditions:

1) The range of the nose / tail strut tilt and height

2) Interference of the strut fairings with the model

3) The capacity of the model support and balance system for reacting aerodynamic loads

The insertion of wedges at the model attachment pads can offset the strut ball and socket angular limits by any amount up to ±20°

Angle-of-Yaw Range

The total angle-of-yaw range is 290° and is adjusted by rotating the turntable in a clockwise or counterclockwise direction. The angle-of-yaw range is limited by the travel of the "stem" of the T Frame: the area between +14° and +84° in the upper right quadrant of the turntable circle. When the stem is oriented with the tail strut in the down stream position, the angle-of-yaw is +194 in the clockwise direction and -96 in the counterclockwise direction.

Balance System

The balance system supports the test frame model, the model struts, the lower turntable "T Frame", and the floating frame. The rectangular floating frame is supported by independent balance scales at seven separate locations. The balance system is designed to measure the forces for the six degrees of freedom transmitted from the model to the floating frame. These forces include the dead weight of the model, lift, roll, side and drag forces, and rolling, pitching, and yawing moments.

Each of the four corners of the floating frame is supported by a lift post which carries pre axial load. The front and rear center locations of the floating frame are each supported by independent front and rear side force scales. Additionally, the center of the floating frame is supported by a drag link connected to the drag scale and is directly under the center of rotation of the lower turntable "T Frame". The front roll and rear roll scales are lever arms that are interconnected to the two front lift scales and the two rear lift scales. These resolve roll measurements directly from these lift scales.

This balance system with its digitizers and computer interface is referred to as the Static Force System for recording instrumented test data.

Back to Top Menu  

Balance System Capacities

The different load combinations used for model strut capacities must be examined to calculate the expected loads that will be transmitted into the balance system. The calculations must include the moments that are due to the drag and side forces acting above the drag and side force links. The drag moments must be included in the calculations for the front and rear roll scales.

Moments are summed about the front and center of the floating frame. The NFAC also has the equations for equilibrium developed in a computer software program. This program may also be used to evaluate scale requirements for a known test configuration and test envelope.

Model geometry and model support configuration both have a significant effect on balance system capacities. Care should be taken to evaluate both of these parameters when evaluating balance system requirements. For example, the side force capacity is ±8000 pounds for each of the front and rear side force scales. If the total side force is applied at the center of rotation of the T Frame, the total side force capacity of the balance system may be ±16,000 pounds.

The capacities of these scales may limit the envelope of some tests. If required, the balance system can be locked out to use the ligher allowable loads of the model struts. Note that the higher loads on the two main struts will result in larger deflections at the tip of the struts.

 

Balance System Capacities * Click on images to enlarge

The scale accuracy figures quoted in the table below are based on data collected under controlled, static loading conditions. The figures were obtained from directly loading the individual scale load cells, and thus should be considered as best-case values. A wide variety of factors influence scale accuracy and repeatability, including: model load range, model induced dynamic loads, data collection frequency and duration, data filtering, settling time between points and aerodynamic and frictional hysteresis. Individual scale calibrations will be performed prior to testing over the expected load range of the model. In addition, it is recommended that occasional repeat runs that contain the same angle of attack and angle of yaw combinations be planned into the run schedule. Consultation with the NFAC staff is recommended for all tests using the wind tunnel scale system as the primary source of force and moment data.

Back to Top Menu  

Alternate Support Systems

Two alternate support systems exist, and others can be fabricated for use on specific tests. Where the primary system described above does not appear suitable, contact the NFAC staff regarding the available alternative systems. The existing alternate systems are:

• A floor-mounted turntable for semi-span models.

• A sting mount with high pressure air capability.

Back to Top Menu  

Operating Envelope

The operating envelope for the 40-by 80-foot test section pressure or velocity is controlled in two ways: by changing the speed of the six tunnel drive motors or by changing the pitch angle of the tunnel drive fan blades.

The tunnel fan drive motors can be operated in two modes. The first operating mode, the Induction Frequency Control (IFC) system, is used to start the six tunnel motors and to synchronize them at a starting speed of 36 revolutions per minute (rpm). The IFC mode can also be used to control the tunnel drive motors at a lower rpm for special tests that require low background noise. The fan blade pitch may be varied in this mode to control the operating envelope.

Back to Top Menu  

Performance

Back to Top Menu  

Utilities

 Fan Drive System

The 40-by 80-foot and the 80-by 120-foot test sections share the fan drive system. Adjusting the position of Vane Sets three and four directs the aiflow through one test section into the other. When the 40-by 80-foot test section is in operation, work may simultaneously continue in the 80-by 120-foot test section. However, due to the pressurized downstream airflow, work may not continue in the 40-by 80-foot test section when the 80-by 120-foot test section is in operation: a point of consideration in test scheduling.

Emergency Stop Circuit

The tunnel drive system is equipped with an emergency stop circuit. While the drive system itself cannot be damaged by making an emergency stop, there is the possibility of doing significant damage to models during such a stop. This possibility, though, depends on the type of model being tested. Specific procedures for emergency stops must be developed for each test and reviewed at the Test Readiness Review. These approved procedures must be diligently followed during testing.

Electric Line Power

Electric line power is available for the 40 by 80 test section:

• 120-volt, 60-hertz, single phase

• 208-volt, 60-hertz, single and three phase

• 480-volt, 60-hertz, single and three phase

Jet Engine Starting

• Electric - Intermittent use 28-32 volt DC, 1500 amps normal with up to 1800 amps for 3 minutes.

• Air Start - High Pressure Air

Shop Air
Standard Aircraft Portable Start Units ("Huffers")

Variable Frequency Power Supplies

These power supplies are available in both the 40-by 80 and 80-by 120 test sections.

• There are two 0- to 150-hertz sets available: called the 40 x 80 set and the 14-foot set.

See figure below for the operating limits on this equipment.

 

* Click on image to enlarge

• The 40 by 80 set maximum continuous limits are any one of the following conditions:

- 2300-hp @ 150-Hz
- 1711-kW @ 150-Hz
- 1600-A, M.G. Set Loop Current
- 90° C (194° F) M.P. Set Stator Temperature

• The 14-foot, 150-Hz set continuous limits are any one of the following conditions:

- 2600 hp @ 150-Hz
- 1927-kW @ 150-Hz
- 1800-A, MG Set Loop Current
- 90° C (194° F) Stator Temperature

• There are two 0- to 400-hertz, 706 kVA sets available.

See figure below for the operating limits on this equipment.

 

* Click on image to enlarge

*Arrangements for use of these variable frequency power supplies
must be made early in the test planning process

Back to Top Menu  

Treated Cooling Water

Cooling water systems supply water to the models in the test section and also on the ground floor of the High Bay. The water is filtered and treated with a corrosion inhibitor. The water pressure on the ground floor is 60 to 70-psi with a 50 to 60 gallons per minute flow rate.

• In the 40 by 80 test section at the model, the pressure is 15 to 20-psi with a 20-gpm flow rate.

Fuel Supply

JP-5 fuel can be supplied in both test sections for the operation of turbojet or turbo fan engines. While the upper limits of the flow rate are 50-gallons per minute at a pressure of 10-psi, system controls can create pressure and flow rate reduction. A booster pump that will increase fuel and pressure is also available, but such arrangements for any fuel requirements should be made at the time the test request is made. Please note that Customer or Sponsoring Agency must bear the cost of any fuel used in a test.

The use of on-board aircraft or model fuel tanks is not allowed; direct hook-up to the NFAC fuel system is required. All aircraft fuel tanks must be purged and pressurized with an inert gas.

Compressed Air

• Shop Air - 125-psi, sufficient for hand tools

• Instrumentation Air - 125-psi, 50-cfm, clean, dry air.

•High pressure air

0 to 3000-psi at temperatures from ambient to 204° C (400°F)
Unheated flow rates to 40-lbs/sec
Heated to 400° F flow rates to 12-lbs/sec

Hydraulic Fluid

In the 40 by 80, hydraulic fluid is available at 23 gallons per minute maximum output at regulated pressures of up to 3000-psi continuous use.

Back to Top Menu