Kodiak Oil Film Interferometry & Load Cell Airfoil Testing

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=Problem Definition= This capstone project aims to quantify the impact of the rivet heads on the aerodynamic performance of the horizontal tail airfoil on a Kodiak made aircraft. Kodiak Aircraft uses universal head rivets on their horizontal stabilizes. This creates a patterned roughness on the airfoil surface. Prior research indicates that airfoils with patterned roughness experience higher drag forces, demonstrative of lower aerodynamic efficiency. This project will determine the efficiency of flush-mounted and standard rivets on the horizontal stabilizer. Two experiments were conducted in the wind tunnel facility at the University of Idaho to quantify the impact of rivet heads on lift and drag. A series of oil-film-interferometry (OFI) experiments were performed through a series of angles of attack (-4° to +4°). These experiments were executed using a Reynolds number of ~500,000. Using a calibrated silicon oil, a research-grade CCD camera, and a monochromatic light source, these experiments determined local wall-shear stress values. Additionally, a second series of experiments measured lift and drag values using a high precision load cell on the same airfoil, measuring lift and drag.

The ultimate stage of the project was designing an OFI mounting fixture on the Kodiak test aircraft itself. The flow-visualization results from the flight test were compared to the results found in the wind tunnel tests to demonstrate the large-scale applicability of the test results.

=Background= The purpose of this project is to show how different types of aircraft fasteners impact wing efficiency.

Kodiak Aircraft uses standard head rivets on horizontal wings. Load cell wind tunnel experiments are to test differences in drag and lift while Oil Film Interferometry tests are to determine the differences in surface skin friction. By constructing accurate, scalable models of the airfoil the efficiency of each can be tested via wind tunnel. The foil is fixed vertically inside the testing chamber. A load cell measures forces and torques experienced by the airfoil.

Facilities & Equipment

Principle, initial experiments were conducted in an 18-inch, Open-Circuit Wind Tunnel. Flow is conditioned with a honeycomb and screen pack through which ambient, room temperature air, is conducted through the testing sections. The air flows through a rectangular section (18x18x36-inch) of 3/4-inch thick plexiglass where the mounted NACA-0012 airfoil model can be optically observed. Max airspeed is measured at 100mph, driven by a 48-inch Axial Fan. The airfoil being tested is mounted to a Nema42 2830 stepper motor and an ATI F/T Gamma load cell sensor. The stepper motor has a **1.8 degree dimensional resolution** which is electronically turned to test different angles of attack (AOA) as would be experienced by the airfoil in a realistic scenario. Mounted between motor and airfoil interfaces the load cell; used to measure simulated lift and drag experienced by the airfoil at different angles. -Camera -Visual Data -Drag Coefficients -



Theory
The OFI (Oil Film Interference) experiments determine the Coefficient of Skin Friction of an airfoil in a wind tunnel. The airfoil is stationary and has a line of oil applied to its leading edge, which is spread by the wind. Fringes are formed on the surface of the airfoil, which are visible through a camera and light setup. By analyzing these fringes, the Coefficient of Skin Friction can be determined.

The following variables are:
 * C_d = coefficient of drag
 * rho = Fluid Density
 * L = Airfoil Length
 * v = kinematic viscosity
 * A = Airfoil Area
 * C_L = Coefficient of Lift
 * V = Velocity
 * v = kinematic viscosity
 * A = Airfoil Area
 * C_L = Coefficient of Lift
 * V = Velocity
 * C_L = Coefficient of Lift
 * V = Velocity
 * V = Velocity

Budget
=Model Creation & 3D Printing= Airfoil modeling all begins with categorizing the NACA type airfoil chord for this project, which indicates the side-profile geometry of the wing. Some wings are symmetric about the top and bottom, while others are unsymmetric; this creates different aerodynamic performance characteristics based on application. The airfoil we analyzed has a 12% thickness of the overall chord length of the wing.

The main design parameters considered were weight, loads (lift, drag), printability, cost, blockage, and how closely model aerodynamic surface conditions could be replicated (determined by Reynolds number).

Since the wind tunnel test chamber is 18’’ in diameter, a total model length was set at 17.5’’ for appropriate clearance. Another parameter to consider in the wind tunnel was blockage, which is the ratio of the airfoil area that blocks the test chamber to the original width (18’’). This blockage needs to be minimized to create predictable aerodynamic test conditions. The larger the airfoil, or angle of attack, the larger the total blockage. The chart below shows the calculated blockage, based on a simple math model, compared to a CAD driven analysis.

A scale model of the airfoil was calculated by inserting a canvas into CAD software package. A canvas is a document that can be uploaded to the work space that serves as a blueprint for design work; the engineering drawings provided by Kodiak aircraft worked well for this purpose. Once inserted into the workspace, selectable points were enabled on the surface of the canvas, which allowed for measuring and scaling. Since the provided drawing labeled the original aircraft chord length as 40’’, the canvas was scaled symmetrically until the real-time rendered dimension was true to the aircraft. From there, each dimension was measured and recorded for project learning purposes about the original aircraft.

The next step was shrinking the canvas symmetrically by scaling down by .225, which resulted in the desired model chord length of 9’’. Each dimension was measured again, such as the distance between ribs and rivet strings. Most dimensions were measured from the leading edge of the airfoil. Then all of the dimensions were exported into a separate assembly file where the rivets and blank airfoil would soon join.

The density of rivets, measured in rivets per inch, was kept to scale between the airplane and the model. This consistency maintains aerodynamic similitude on the surfaces. Over 90% of the rivets used to fasten the skin onto the airplane are type 4, which means it has an affective shank diameter of 4/32 of an inch. The rivets were patterned onto the airfoil surface at correct scale, while considering tangent error.

The modeled airfoil was estimated to weigh ~1175 grams, which exerts a force in the Z direction on the loadcell of ~11.5 Newtons. This is well within the operating parameters of the testing apparatus. Other considerations made to accommodate testing from a design standpoint include capping the top of the airfoil so the metal rod will not slide through, and creating a hole for the airfoil and rod to be pinned together. This design prohibits the airfoil from rotating around the rod while experiencing torque due to aerodynamic loads.

(NEED LINK TO stratasys and DATA Sheet Provided)

Final Prints
=Testing Method=

XFOIL The program 'XFOIL' is used to acquire theoretical data for the specific airfoil. XFOIL is a program used to design and analyze subsonic isolated airfoils. The program has stored data of the coordinates of airfoils. This project utilized a NACA-0012 airfoil. The angle of attack is specified in the code to generate a plot that looks like Figure 1. The figure provides information on Reynolds number (Re), Angle of Attack (α), Coefficient of Lift, Momentum, Drag (CL, CM, CD), Lift-Drag Ratio (L/D), and Free Flow Turbulence (Ncr).

Figure 1: Flow distribution at 300,000 Reynolds number at 1.80º angle of attack

During the validation stage of the setup in the wind tunnel, experiments were run at a Reynolds number of 300,000 from to ∼ 14.5º. Experimental data was compared to the theoretical data gathered from XFOIL. Figure 2 was generated in MATLAB, where it shows the red solid line as the theoretical data from XFOIL and the blue circles as the experimental data from the airfoil.

=Wind Tunnel Experiments= There are two types of experiments conducted: Load Cell Experiments, and Oil-Film Interferometry Experiments (OFI). Load Cell Experiments are conducted in a wind tunnel to determine the Coefficient of Drag and Lift, while OFI experiments are done to determine the Coefficient of Skin Friction. While both experiments are performed in a wind tunnel, they require different setups and equipment to perform.

Load Cell Experiments The purpose of the Load Cell Experiments is to determine the Coefficient of Drag, and the Coefficient of Lift on the airfoil. This will take place at various Angles of Attack (AOA). The airfoil rotates between +4° and -4° relative to the direction of airflow in the wind tunnel. A stepper motor is used to control this rotation. To determine the forces acting on the airfoil, am ATI Gamma F/T sensor is used. The airfoil, load sensor, and stepper motor are interfaced through several custom-machined aluminum adapters. The airfoil, mounted inside the wind tunnel test section, is attached to the load cell with a steel tube extending beyond the boundaries of the wind tunnel, through the wall. The load cell is mounted on top of the stepper motor. This setup allows for the axis of measurement employed by the load cell to rotate with the angle of attack. This is for simplicity in data recording, and works preventatively against overloading the load cell.

A series of runs are conducted; the first being a 'zero-run' where the wind tunnel operates at '0' velocity. The velocity is slowly increased in consecutive runs, aiming for a targeted Reynolds number increasingly higher (ie. 200,000 300,000 400,000 500,000). The data is collected through a Labview System DAQ and plotted in LabView.

Oil-Film Interferometry Experiments OFI experiments are used to determine the Coefficient of Skin Friction of the airfoil. The airfoil is fixed mounted inside the wind tunnel, there is no load cell or stepper motor. A small line of oil is applied to the surface, on the leading edge of the airfoil. The wind causes the oil film to spread across the surface. Small fringes (a pattern of discrepancies in height) are formed along the surface. These are viewed using a camera and light setup. The oil film acts as a thin, transparent layer that changes the path length of the light that passes through it. These fringes are visible on the camera. By analyzing the fringes, the Coefficient of Skin Friction is determined.

Data Acquisition
=Coding=

=Results & Conclusion=