Wright Field 5 Foot Wind Tunnel

 

Designated the 114th National Historical

Mechanical Engineering Landmark

by the

American Society of Mechanical Engineers

March 22, 1995

Introduction

   The Wright Field Five-Foot Wind Tunnel is an outstanding example of an early aerodynamic testing facility that remains active today.  The 5 foot (1.52m) diameter refers to the diameter of the test section where the model to be tested is located.  The tunnel was well known from the early 1920s to the late 1950s for its aerodynamic testing, contributions to aeronautical research and the development of nearly every major aircraft and associated hardware used by the U.S. Air Force and its predecessor, the Army Air Service.  It was conceived, designed, and built when very little aerodynamic theory or test data was available that could be used as a baseline for its design.  The wind tunnel is a remarkable wood working job and was considered an important money saving device guiding the design and construction of early aircraft.  It is a unique specimen of a highly sophisticated wind tunnel and is one that remains today.  In a sense, it represents an extension of the Wright brothers' principle of applying the wind tunnel testing as a technique to develop aerodynamic parameters and as a vital step in the airplane development cycle.  Early wind tunnels showed the way for the first successful powered flights.  They enabled the establishment of aeronautics as an exact science by allowing the measurement of lift and drag coefficients.  Airplane development would have been more costly, dangerous, and slow if the five-foot wind tunnel had not been available for use prior to flight testing.  The Air Force Institute of Technology today maintains and uses this facility as a teaching and research tool, and it remains a part of aviation history.

 

Facility History & Significance

   The five-foot wind tunnel was constructed at McCook Field in Dayton Ohio during the 1921-1922 time period and was housed in a standard steel hanger.  It was moved to its current site, Area B at Wright-Patterson Air Force Base during the 1928-1929 time period.

   The five-foot wind tunnel was conceived by Air Service Engineering Division personnel at McCook Field base partially upon contracts they had with National Advisory Committee for Aeronautics (NACA) personnel in the early 1920s, their earlier constructed high speed 14 inch (35.6cm) tunnel and their general knowledge of other facilities.  The 14 inch tunnel had been designed using the tunnel in the laboratory of Orville Wright as a basis for size and other characteristics.  Prior to initiating the design of the five foot tunnel, they had numerous contracts with the Massachusetts Institute of Technology (MIT) for testing models of early aircraft designs in the MIT 4 foot tunnel.  Engineers at McCook Field originally designed an 8 foot (2.44m) diameter tunnel but compromised to a 5 foot one due to air flow requirements and characteristics, speed range, cost, and portability requirements.  It was projected during design that the tunnel would move to a permanent location at Wright Field in the near future.

   The Design and technical specifications leading to the construction of the tunnel were prepared by personnel of the Engineering Division, Air Service.  The tunnel components were planned and fabricated by McCook Field workmen.  It was completed in 1922 and the final inspection team included Orville Wright.

   The tunnel is the oldest operating wind tunnel in the country and represents a significant part of aviation history.  It is an early example of a modern wind tunnel as well as a rare artifact that remains from McCook Field which was the predecessor of Wright Field.  The tunnel is now located in Building 19, Area B, Wright Field which is a historically significant structure due to its age and that the building has not changed since it was built.  The tunnel was planned as a research tool and also for engineering development.  The tunnel was also used for various types of testing in the 1920s, 1930s, through World War II, and beyond.  In 1958, the management of the facility was turned over to the air Force Institute of Technology (AFIT) where it is still used as a teaching and research tool.

   Currently the Wright Field five foot wind tunnel has been dismantled and is in storage while Bldg 19, where it has been housed, is being renovated.

 

Technical Background

   The five foot wind tunnel can test models with wing spans up to 40 inches (1.02m).  It is a semi open-circuit type tunnel:  room air is mixed with discharge air drawn through the air straightener into the rounded 10 foot (3.05m) intake bell.  The cutaway view of the tunnel shows the air straightener pulled back from the tunnel air inlet, for illustration only, to show the tunnel inlet shape.  The tunnel then tapers in diameter to the 5 foot test section.  After the test section, the diffuser gradually increases in diameter to its maximum of 12 feet (3.66m).  At this point, the air enters the first fan, then passes through the counter-rotating second fan before being discharged into the room.  The two-fan design was developed for economic considerations.  The tandem fan arrangement was a compromise between power, fan diameter, and fan speed available.  Four Spraque dynamometers, rewired as electric motors, were available as surplus from another program.  Conserving electric power and low initial costs were considered important design goals.

   Considerable effort was expended in conducting model tests to achieve smooth, efficient air flow.  The diffuser allows an interchange of potential and kinetic energy of the fluid flow.  One goal was to develop a diffuser design where the total energy of air moving at the downstream end of the diffuser end was within ten percent of its value at the inlet end.  No thorough analysis of the aerodynamic characteristics of this interchange of energy had been previously perfected, and the design of the five foot tunnel was based upon model tests.  When built, it was considered the most efficient wind tunnel in the world.  Other tunnel components such as the intake bell and air straightener were designed in a similar manner to produce smooth air flow.  The tunnel centerline was elevated above the floor such that the volume above and below the tunnel centerline were in the optimum relationship to maximize return air.

   A honeycomb type structure was inserted into the throat of the intake bell for low speed tests.  The air-flow straightener is on rails to facilitate the installation or removal of this honeycomb.  It was also necessary to generate uniform air flow in the test section to add an aerodynamic fairing in front of the fan hub.  A protective screen and air division cone were also placed before the first fan.  Air diversion fins were located radially around a cylindrical core corresponding to the hub size and extending between the fans.  The air passage way in the annular space has a constantly increasing area and air flow reaches its minimum velocity just before entering the first fan.

   The intake bell, cylindrical tube, and diffuser sections of the tunnel were constructed from wood.  Outer rings were built up by gluing together a number of segments to a thickness of about four inches and a depth of about six inches.  To make the individually curved segments, large square boards were glued together and the segments cut out to the proper curvature.  Narrow staves of seasoned Port Arthur cedar were cut in a four sided molder with tongue and grove joints.  These were then placed inside the circumferential rings and glued and screwed together.  ach section of the tunnel was therefore a rigid unit.  These units were then bolted together at the flanges to form the tunnel walls.  Sections were supported from the floor by cradles under alternate rings.

   Models were supported in the test section by wires connected to the measuring balances.  Two force balances were provided for the tunnel.  For low speed work, a standard National Physics Laboratory (NPL) type balance was used to compare data with earlier tests.  For high speed work, a balance of the Wright type, which still exists today, was used.  This balance was a new feature, that eliminated velocity fluctuations and allowed the operator to directly read the lift to drag ratio.  Today, a full six component strain gage balance measures aerodynamic forces, and the model is mounted at the front of a beam (sting) which is supported downstream from the tunnel walls or floor to reduce or eliminate air turbulence on the model.  Models were installed or removed from the tunnel through two small doors located in the top of the tunnel test section.

 

Facility Contribution

   The Wright Field five foot wind tunnel made significant contributions to the development of early aviation in the 1920s and continues today to provide students with a basic understanding of wind tunnel testing procedures.  It retains its ability to test complete scale models of an actual airplane and to conduct tests in support of research on the general principals of aeronautics.

   The tunnel was considered advanced for its time because of its power, speed range, and combined features which were not available in any other single facility.  These features were the air straightening vanes intake bell, and decelerating cone.  At McCook Field, two interchangeable test sections were also available.

   The five foot wind tunnel contributed to the development of methods to measure aerodynamic performance factors including lift, drag, and stability on a model and convert these data to the full scale airplane.  In its early existence, over a five year period, approximately 50 tests were conducted on projects valued at several million dollars.  A 1/70 scale model of the XNBL-1 Barling Bomber was one of the airplanes tested in the facility during the early 1920s.  The tunnel provided critical data on the performance and stability of this 20 ton airplane which had six engines and was the largest built at the time.  It contained many novel and untried features.  The stabilizer setting needed for stability was predicted by wind tunnel tests and was found to be correct during its initial flight.  Other five foot wind tunnel tests conducted on scale models during the same time period were on the Curtis P-1 Pursuit airplane and the Army Night Observation airplane.  Testing was also conducted to determine the general effectiveness of ailerons on airplane control.  In the 1930s and 1940s, the five foot wind tunnel contributed data important to the solution of flutter problems that could lead to catastrophic failures.  After World War II, aircraft that have benefited from tests in the five foot wind tunnel include the F-15, F-4C, C-130, EC-135 and many missile systems.  The facility has also been used recently for flutter testing research.

 

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