Testing the "Choo-Choo" - My First Wind Tunnel Test

By Tom Hegland, Project Engineer for Sub-scale Wright Flyer Tests

The 1/8th scale steel model
The 1/8th scale steel model in the wind tunnel. The bracing wires and struts had to be modeled slightly larger than true scale due to strength requirements for high Reynolds number testing.

During the summer of 1981, I received a degree in Aeronautical engineering from Cal Poly San Luis Obispo. At this time I was extremely eager to start my career as a wind tunnel engineer with a major airframe company. Being part of a wind tunnel test group involved in testing the "next generation" supersonic fighter aircraft, bombers and cruise missiles was very exciting. After months of helping other experienced engineers with their test programs, I was deemed ready to be the engineer in charge of my own wind tunnel test. Would I be assigned to a cruise missile, advanced fighter or a bomber program? To my surprise, I was assigned instead to coordinate wind tunnel activities for the upcoming AIAA Wright Flyer test scheduled to be conducted in our low speed wind tunnel. Well, I thought, I won't have to worry about compressibility effects, spillage drag, or transonic drag rise relating to model fidelity. Not exactly what I was expecting, but what a great learning and fun experience it was.

The model design, model fabrication, and testing were performed by "rookie" engineers and technicians motivated to learn and gain experience in a new profession. The model design presented various challenges including strength requirements to withstand loads produced by 150psf dynamic pressure (see note #1), the flexibility to change canard and rudder configurations, and the requirement to minimize sting/balance block interference on the aerodynamic data. The model strength requirement dictated that stainless steel be used in the fabrication of the wings, struts, and guide wires of the Wright Flyer model. The fabrication took several months and tested the patience of both the "rookie" and senior model builders. Each upper and lower wing consisted of three contoured airfoil sections (left hand side, right hand side, and center section) welded together at the correct dihedral angle. Next, the stainless steel wings, struts, and guide wires were aligned in "rig" and then silver soldered. This process proved extremely tedious as the wings tended to change shape at each silver soldered joint. This experience resulted in the following phase heard for many years in our model shop, "Welding is for trailer hitches, NOT wind tunnel models".

The tenacity and attention to detail exhibited by the individuals who fabricated this model is clearly evident as the appearance and fidelity of this model are excellent. This model can be seen on display at the Wright Flyer Project area every Saturday from 9AM to noon.

The subsequent wind tunnel test was conducted entirely by volunteers and was not without its own excitement. During the first pitch polar up to 20 degrees angle of attack, significant post stall dynamics were encountered - the model seemed to want to shake itself loose from the supporting sting! With all that wing area it wasn't surprising that "dynamically" we pegged the rolling moment gauge at its limit several times.

The high Reynolds number runs were also interesting (see note #2). As we increased the dynamic pressure in our test, from 80psf (pounds per square foot) to 100psf, the wing guide wires (1/8 inch stainless steel rods) started to "sing" (exhibit aerodynamic buzz). At each subsequent dynamic pressure, 120psf and 150psf, a distinct frequency or tone was heard. The Wright Flyer model was singing to us! I was glad to complete that high dynamic pressure series and still have a model left for post test display.

Now over 20 years later, I'm still in the wind tunnel business and will never forget my first wind tunnel test.

Tom Hegland
Wind Tunnel Engineer
NASA Ames Research Center


Note #1: Dynamic pressure can be thought of as the "strength of the wind". Air moving at 100 mph will exert less force at an altitude of 30,000 ft than it does at sea level; this is due to the lower density of air at altitude. Early engineers found that they could reduce the amount of data required from wind tunnel testing if they divided all forces by the dynamic pressure in the wind tunnel and a reference area of the model to arrive at a non-dimensional coefficient. They can then use the projected dynamic pressure and areas of the real airplane to predict the full scale loads.

Note #2: The Reynolds number can be thought of as the ratio of the influence of the invicid flow to the boundary layer effects - the lower the Reynolds number, the more the boundary layer affects the aerodynamic properties. And as any radio controlled model fan can tell you, the behavior of an airplane flying at very low Reynolds numbers can be substantially different than the behavior of a full scale airplane. Since it is directly proportional to both speed and geometry, when you reduce the scale of a model you can maintain the same full-scale Reynolds number by increasing the speed of the test. This is not a perfect representation as the additional effects of compressibility (Mach number), loads, and strength must be considered, however testing in this manner can still yield acceptable results with suitable analysis of the data.

The 1/8th scale steel model, aft view An aft view of the 1/8th scale model. The large tube underneath the model is the supporting sting and six component balance to measure forces and moments felt by the model.

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