Project AVIATION IRIS
I like to give a cool name to all my projects, personally because it increases the cool factor of my work even more and motivates me to put in even more effort. I particularly enjoy reviewing all my project records and scanning through a list of projects with interesting codenames. For the development of wind tunnel testing capabilities for the Cornell University Design-Build-Fly team, of which I am a subteam lead of, I have codenamed the proejct as AVIATION IRIS because it gives the team a better look at the aerodynamics at play in our aircraft designs.
Fig. 1 The Multi-Axis Wind Tunnel Rig developed by Cornell DBF
As the lead engineer for wind tunnel testing, I feel extremely lucky to have the opportunity to perform aerodynamic effects testing at such an early segment of my undergraduate education.
To read my full report on our progress in Spring 2020, click here.
The Cornell University High Voltage Laboratory Wind Tunnel
Cornell University's Sibley School of Mechanical and Aerospace Engineering owns a massive wind tunnel located a few minutes off campus. It has a working area of 4 x 3 x 6 feet, and can reach top speeds of approximately 34 m/s (76 mph).
Fig. 2 The Large Working Area of the HVL Wind Tunnel
Using the massive working area of the wind tunnel, DBF performs aerodynamic tests on prototype aircraft banners and prototype wings, for use in the AIAA Design-Build-Fly competition in 2020.
The Initial Challenge
One of the challenges of developing wind tunnel testing capabilities for my project team is the scarcity of specialists to ask for reference and advice. Many professors at Cornell's College of Engineering do not specialize in wind tunnel testing, and those that do are usually performing different experiments that have completely different setups. I initially found the research and development process to be challenging, having no former team members to rely upon for prior experience. I developed my own set of references and advisors, delving into reference material such as "Low Speed Wind Tunnel Testing" by Barlow, Rae and Pope, seeking out my high school teacher who performed wind tunnel testing for his PhD. in Aerospace Engineering, and the professors who advise the DBF project team.
The Banner Tests
One of the biggest unknowns of this year's competition is the amount of drag that the banner causes when towed by the aircraft. The drag force determines the cruise speed of the aircraft, and can adversely affect the number of laps our aircraft can achieve in a limited time. The wind tunnel tests provide a controlled environment where two kinds of tests can be performed on prototype banners:
A quantitative experiment of drag force vs. flight speed for each prototype banner of varying sizes and materials.
A qualitative experiment of banner deployment that ensures the banner may deploy from the aircraft and release from the aircraft safely without suffering damage.
For the quantitative experiments, a threaded rod acts as a sting for mounting the banners to a set of load cells installed underneath the bottom board of the working area. Below the working section, I sit in my comfortable little hutch and record the data using an Arduino system I constructed that sends the values for force to a MATLAB program I wrote that saves the data and analyzes it. The wind speeds are recorded using an anemometer, later to be upgraded to a pitot tube for better accuracy and precision.
Fig. 3 The Banner Test Configuration (and me, in my comfort zone)
Fig. 4 Test of a Prototype Banner made of Garbage Bag Material
The Data Analysis
The data is processed by an Arduino system linked to the load cells using the Sparkfun HX711 Load Cell Amplifier. The Arduino in turn communicates with the MATLAB script I wrote using serial port communication, which saves the data for processing later. Skipping the intricate details of the program, the script takes the time average and standard deviation of force values at known airspeeds, subtracts the drag force due to the sting in order to provide and accurate plot of drag force versus airspeed of each prototype banner. A sample data set has been shown below.
Fig. 5 Raw Data of Drag Force from Load Cells
Fig. 6 "Null Dataset", Drag Force due to only the Threaded Rod Sting
Fig. 7 Force versus Velocity Graph for a Prototype Banner
Notice that the error bars are greater at low airspeeds than they are at high airspeeds. This is because the prototype banner undergoes large amplitude, low frequency oscillations at slow speeds and undergoes low amplitude, high frequency oscillations at higher wind speeds.
The Wing Tests - Work in Progress!
Our wing testing capabilities are still in development at this time. Check back at a later date to review our progress on analyzing 2D wing effects and 3D wing effects.
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