Basically, this study aimed to design a two-component system to measure simultaneously the force components acting on an object, as in the case of aerodynamic forces on a body. As in the example of the force on a wing seen in Fig. 1, by measuring the horizontal (drag) and vertical (lift) forces on the wing, one can obtain both the magnitude (F) and the direction (θ) of the force.
The measurement system consists of an arm to which the object is attached, a five-bar linkage, and load cells containing strain-gages (Fig. 2). This mechanism is made of thin aluminum rectangular profiles with 5*20 mm cross-section and bearings are used at the connection points to reduce friction. Therefore, the rods can rotate freely around the joints. In addition, buttresses are used to fix the load cells and stationary joints of the five-bar mechanism to the stand. The buttresses were made from aluminum sigma profile with 25mm size. The five-bar mechanism is integrated into the system for force transmission in the x and y directions. For absolute balance, double elements are used in the horizontal bars, and also balance weight was positioned on top of one of these horizontal bars.
Bolts are attached to the load cells to transmit the load at an angle of 90 degrees. Thanks to the connections with ball bearings, the forces in the x and y directions are properly transmitted to the load cells independently of each other. As can be seen, while the system is operating, due to the lift force on the body in the vertical direction, the bar-1 which the body is attached, moves in the y-direction and forces the bar-3 to rotate about the fixed joint. The bar-3 creates a strain on the load cell and thus the lift force is measured. In the case of force on the body in the x-direction, the force is transmitted from the number bar 1 to the bar-4 through the bar-2. Thus, the force in the x-direction in the bar-4 is transmitted to the other load cell that measures the drag force. All system components are designed and manufactured. The system with supports and movable bars is mounted on a stand made of aluminum sigma profiles to maintain stability. It has been also ensured that the loads on the load cells are only lift and drag forces.
The system is suitable for accurately measuring the vertical and horizontal forces exerted on bodies such as bluff bodies, airfoils, symmetric or asymmetric, bionic wing structures. This design always transfers the forces coming on it to the load cells.
The electronic equipment including HX711amplifier were used to amplify the signals from the load cells and transfer them to the computer. 2 load cells are used in the system which aims to measure the force with strain gauges. Arduino Uno is used as the controller in the system. Required electrical power is supplied by a 5 V, 2A power supply. Other electronic parts integrated into the system are PCB circuit board, 220 ohm resistor, button, and wires.
10 Hz data is obtained separately from the load cells. Two programs have been written to process this data. The first of these enables the measured signal to be converted into a force unit of newton. For this purpose, a calibration study was carried out and multiplier factors were determined for horizontal and vertical forces. The software that performs this process is given in Appendix-A.
The second program is the main software, and the recorded data can be processed as desired and statistical information can be obtained by this software. The multiplier factors determined separately for lift and drag forces were also inserted in the main software. The prepared software is given in Appendix-B. This software averages the data collected at 10 Hz frequency over a short period of 2 minutes for both lift and drag forces. The unit of the output data is mN.
These forces are made dimensionless by dividing them by the dynamic force, as in Eqs. 1 and 2. That is the lift and drag coefficients can be written as
respectively. Where, ρ is the density of air, u is the velocity of air, and A is the projection area of the model. Thus, the calculation was made by taking the average of approximately 1200 data in total for each force measurement given as a result.