Table of Contents
The ultimate goal of tethered gliders is to generate as much power as possible. We started by looking for a large RC plane because this would increase the tension greatly. However, we found from our simulation that a large RC plane, such as the Phoenix, would generate so much tension that our budget couldn't afford the large load cells materials that would be required to measure the tension and orientation of the plane. Because of this, we had to scale down the plane to something more manageable. Hobbyking's Bixler was an excellent choice, because it is cheap, small, and manageable.
Our simulations show that the Bixler would be capable of generating 1000 pounds of force in the tether at a wind speed of 10 meters per second. This force allows us to buy a load cell within budget.
Chosen - 30 September 2013
Stress Analysis showed that if we were to tether the bridle directly to the fuselage, the wings would be torn off due to the extreme forces that are generated in the tether. In order to prevent this from happening, the tether will be bridled directly to the wing to pull back on the deflection and reduce stress. Brute force optimization was performed to determine the optimal tether angle and tether position on the wing. In addition to this, the fuselage narrows drastically near the vertical stabilizer. The forces due to tension would cause the fuselage to fail if tethered near the tail. To prevent this, the tether will be tethered directly behind the wings where the fuselage is widest. This may cause the RC plane to become unstable because the bridle is rather small. A forth line is connected to the vertical stabilizer and the bridle. This line carries no tension and has slack. It's only purpose is to catch the plane if it ever becomes unstable.
Designed - 21 October 2013
As the tether is effectively a point load on the wing, the tether may tear through the EPO foal unless it is distributed over an area. This is done by using these plates on top of the wing. The tether is fed through holes in the wing and wraps over this plate. The plate distributes the load of an area reducing stress concentrations. The plate will be positioned over the plane's carbon fiber supports to help reduce deflection and stress. The plate is made of poly-carbonate because it has a high yield strength.
Designed - 25 November 2013
Base Station Design
The tether is attached to a load cell which measures the tension. The load cell is allowed to rotate about a shaft. The angle of this shaft is measured by a potentiometer. The angle of thhe plate is measured by another potentiometer. The two potentiometer measurements can record the orientation of the plane with a fixed tether length and the load cell can record tether tension.
Design of Experiments
Last Updated - 18 November 2013
This data acquisition (DAQ) consists of three sensors (one load cell and 2 potentiometers) that record all the required data. The load cell is a Phidgets Micro Load Cell (0-50kg), see below. The signal is measured in volts and converted to pounds. The potentiometers used are three turn potentiometers. They are used to measure the vertical and horizontal angles of the attached aircraft. The potentiometers are mounted with aluminum L-brackets and flexible couplings to avoid binding, see below.
A bridge amplifier amplifies the sensor signal so it may then be recorded by the system. The device used is a Logos Electromechanical Bridge Amplifier v2. The load cell is wired into the bridge and the resulting output is connected to the DAQ device for measuring and recording.
The data is recorded with the DAQ device, the computer, and the LabVIEW code. The DAQ device used is a NI USB-6210 module. It is a 16-bit device capable of 250 kS/s. This produces a load cell resolution of 0.008 N and a potentiometer resolution of 0.016 degrees. Each sensor has a noise range that must be taken into account when recording. All three sensors are wired to the DAQ device which is connected to the computer via USB. The computer is responsible for supplying the DAQ device with power, which then supplies +5V to each sensor.
A few other choices were made to help protect the DAQ system. The DAQ device and amplifier were placed in plastic containers for general wear and tear protection. A plastic wire clip was installed to provide a fixed connection point between the DAQ device and amplifier and the sensors themselves. This prevents the wires from ripping out of the DAQ device or amplifier in case of accidental over extension of the wires. Most of the wires on the setup were given an extra layer of shrink tubing. This helps prevents entanglement and damage due to general wear and tear.
Flight ExperimentsDue to the complexity of the experiments and the difficulty to fly an RC plane in windy conditions, multiple practice experiments were performed during the course of the design process. Originally, we flew the planes untethered in both windy and low wind days, and eventually tethered the glider during second phase of our practices. The tethered flights added a new challenge to flying the plane, because the glider would lose control and crash when the tether became taut. After attempting to enter a flight path in different methods, we discovered ways to retain stability when the tether became taut. When there was minimal wind, the glider was flown in increasingly large circles until the tether just became taut. The glider would smoothly reorient itself into the flight path. When there was large amounts of wind, it became too difficult to fly the plane smoothly into a flight path. We discovered that it was more beneficial to enter the flight path as soon as possible. We would try to gain altitude and become taut quickly. The plane would lose stability when the tether becomes taut, but with enough altitude, the pilot may regain control before crashing.
Another challenge we encountered after entering the flight path was that the glider would not gain altitude with any control surface inputs. We first thought that this would be fixed by increasing the rudder area. While this did increase the controls entering the flight path, we still did not gain altitude after the tether became taut. After viewing our flight videos, we realized with a symmetric bridle caused the lift forces to be horizontal to the ground. In order to make our glider gain altitude, we reoriented the bridle so that the lift force would be pointed towards the center of our flight radius.
Results and Discussion
Significant progress was made towards the desired end result however there is still room for improvement and refinement. The Bixler v1.1 RC plane proved to be a worthy plane to prototype. The base station design also proved to be a strong concept and provides an adequate means of collecting position and tension data from the system.
The Bixler's price, durability, and simplicity of design led to a rapid flying learning curve, short down time in between flights for repairs, and ease of modification. The EPO foam from which the glider is made was sturdy, resilient, and withstood a significant number of hard crashes as we were learning how to fly the glider both in normal flight and in looping flight. In total 3 Bixler gliders were purchased. The first was lost due to lack of flying experience and high winds. The second was retired due to damage. The third is operational as of this writing. Foam glue was great for assembling the glider but we quickly learned that extra structural reinforcement was necessary in critical areas. The Bixler's wings were not held to the fuselage well and could not withstand the harsh crashes. In hard nose-first impacts, they would pull out of the fuselage and showed an inclination to rip apart from leading edge towards the trailing edge near the wing root. To overcome this we glued the wings to the fuselage and applied tape to the wing near the root of the wing. This worked until the glue holding the wings and fuselage together failed. We chose to just use duct tape to hold the wings together. This wasn't the best solution because the wings could wiggle slightly but it was effective for a quick repair. Other failure points were the control surface joints tearing and the narrow portion of the fuselage would break. We used duct tape to fix these however it severely limited the range of motion of the control surfaces. The tail (vertical and horizontal stabilizers) was also a weak point as it was a major joint on the Bixler. Duct tape and glue were constantly applied to maintain a solid connection. A critical note for the duct tape reinforcements is the amount of weight it adds to the glider. The balance between rigidity and weight is delicate as proven by our second glider. The second glider was forced to be retired because it became too heavy to fly at about 750 g, just under 100 g of addition weight in tape.
In place of the hollow foam and plastic cockpit that is included with the glider, we created a similarly shaped solid piece of EPP foam. We had a portion of the foam fit snug inside the glider, and was held in place with duct tape. This addition was needed to protect the nose of the glider from direct impacts. The foam is thin and vulnerable here so the added EPP foam helped to significantly minimize structural damage to the glider. Based on the impacts the nose of the third glider withstood compared to the second glider, it is clear that this addition was valuable.
The Bixler v1.1 was a good choice for a first RC glider but an EPP foam glider would have been a better one. It would have been more resilient and less duct tape would have been needed for reinforcement.
Dyneema was a great choice for the tether. It is flexible, bright orange, and strong. We were able to easily use it as a bridle and adjust it as necessary. Initially marking off the tether at 5 meter increments helped us keep a consistent tether length across the tests.
Base Station Construction
The base station's design is robust yet still compact. The most vulnerable part of it is the exposed load cell and vertical potentiometer however any impact damage to these components would remain localized. The rest of the base station is made of steel, aluminum, or half-inch plywood. The ball bearings used for the vertical potentiometers provide a smooth range of motion. The tapered roller bearings provide smooth range of motion for the horizontal potentiometer while dissipating the vertical loads that are applied. Preliminary testing shows that tracking occurs by the base station with approximately 2 lbs of tension on a 1 m tether.
While the base station works well, it was difficult to manufacture for inexperienced machinists. After consulting with professional machinists, the parts were re-machined to a higher degree of precision. This greatly improved the bearing alignment resulting in minimal turning resistance. Installing the base station in a field was very easy. The auger installation is simple and is secure when in the ground. A rod is used to attach the auger to two hooks on the underside of the base station. The three adjustable legs are use to level the base station with the aid of a 2D bubble level. The base station, fully constructed, can be seen below.
The Coupled System
When flying with the glider attached to the base station it is recommended to videotape both the glider's flight and the base station. While not a requirement for the data acquisition, we found it valuable for identifying whether the base station was acquiring accurate data throughout the test. One loop of looping flight was captured via the data acquisition system. The loop had minimal tension (the load cell was not operational at the time of data capture) and captured data featured a lot of noise. A possible source of the noise was the slow movement of the loop allowed the analog noise of the potentiometers to become significant in the plot. With only one data sample to analyze, a credible conclusion can't be made. More data must be collected via either successful looping flights or by designing a controlled experiment to test the potentiometer's performance in similar conditions to what is predicted.
Recommendations for Future Designs
The Bixler v1.1 was a good choice for a first RC glider but an EPP foam glider would have been a better one. It would have been more resilient and require less duct tape for reinforcement. Changing the tether's small markers to larger streamers might also be useful. When testing, it is sometimes difficult to see the tether. On video, it is nearly impossible. The streamers can help identify the location of the tether and indicate any slack in the line. A light material will be needed to avoid adding more tether drag.
Future work should include a quick disconnect to the amplifier (Research: Logos Electromechanical Bridge Amplifier Screw Clamp Connector Kit). Also, labeling should be added to the wires and plastic boxes for ease of identification. If noise becomes an issue, wiring shielding or other forms of noise canceling setups should be investigated. Finally, time should be put into improving the prevention of wire binding at extreme cases of rotation.
Recommendations for Future Testing
Our tests showed that the glider was losing altitude while performing loops even when there was significant wind. We believe that this is caused by the bridle setup; for example, the bridle caused the glider to be oriented so that the lift force component was downward when at the top of the flight path. This can be fixed by changing the bridle to a two-point bridle where the tether connects to the fuselage at two points. The ailerons can control the roll of the glider while in the flight path so that the lift force component always is in the positive vertical direction. This may be too difficult to be human controlled so a future project may need to automate the process. A new glider may be need to be designed for this specific purpose in mind.
Last Updated - 15 May 2014