TESTING FLIGHT TIME PERFOMANCE OF AN ELASTIC-POWERED AEROPLANE
Author: Thabiso Khoza
Student number: 1849534
Course code: ELEN 1004
Group number: P2G 131
Date: Monday, 13 August 2018
The engineering project entitled ‘OPTIMISED ELASTIC-POWERED AEROPLANE’ was a group project to design, build and test an elastic-powered aircraft. In groups of two the members were assigned with the task of constructing the prototype together. Although this was a group project, the assessment was individual. A member was supposed to choose a specific task to focus on, which contributes to the total flight time of the aircraft. Hence each member was expected to present their findings in the form of a detailed individual report. Logical reasoning and team work were a major test for the members.
The objective was to construct an improved prototype from scratch that can achieve flight times greater than five seconds, and to analyse the flight of a normal rubber band powered aeroplane. The groups needed to investigate the relationship between the flight time and number of twists to come up with a conclusion as to how the two parameters influence each other. In building the prototype, members were only limited to commonly found materials and dimensions. For the prototype to produce maximum flight time two of its components must be taken into consideration namely, its gliding capabilities and active propulsion. The author’s focus was on the ‘active propulsion’ part which shall be discussed more in detail.
The prototype in figure (1) shows a similar aircraft which was the initial design. The following changes were made to this design.
(I)Balsa wood was replaced with kebab sticks as it was prohibited. Kebab sticks are lighter to maintain a low weight.
(II) Polystyrene was used for making the wings, to increase lift and decrease air resistance.
(III)Surface area of the wings was increased therefore increasing the lift and they were a bit elevated.
(IV)The wheels were removed since they added extra weight which made it impossible for the propeller to produce enough thrust.
The vertical and horizontal stabilizers were also constructed using polystyrene. For active propulsion the propeller was made using a soda can and it was twisted to keep the thrust equal along the blade 1 The prototype in figure (2) is because of the changes mentioned above and it is the actual prototype that the members constructed for this project. The aircraft flies in circles to the right to measure flight time more efficiently.
The group members tested and evaluated the aircraft inside a building on the first floor since the actual testing was going to be conducted under the same conditions. The first part of the testing was the gliding performance test. This test was to find the location of the centre of gravity (CG). The centre of gravity is a point from which the weight of a body or system may be considered to act.
The wing position and CG location that corresponds to minimum sinking speed had to be found. The sinking speed is the vertical component of the forward velocity in gliding flights so it plays a major role in helping the aircraft to glide smoothly. This contributes to the ultimate goal of maximum duration.
This test was simply conducted by supporting the stick with two fingertips and moving the plane back and forth until it balances with the stick horizontal 2. This part of the test is discussed more in detail by the other member.
The second part of the test was active propulsion, testing the efficiency of the elastic-powered propeller system. The propeller system is the most vital part of the prototype. When constructing the propeller, certain aspects were taken into consideration. Shape, pitch, length and type of material were the key aspects.
A typical energy drink can was used to create the twisted propeller blades. It was twisted at an angle so that the thrust remains equal along the blades 3. A wire was put as the propeller shaft mainly to connect the blades and make the propeller stronger preventing the blades from separating.
The testing process was simple. The members had to choose the number of windings. After winding, the plane was held in position and a gentle technique was used to launch the plane. The technique was to release the propeller, then release the plane a second after. Then observe the plane while it is powered by the rubber band. Due to the slight elevation of the wings the plane was expected to climb when released so if it stabilized quickly to a gentle climb then the wings were considered to be in the right position.
If it dived, then the number of windings had to be increased. The propeller obviously cannot convert all the power to thrust according to physics some energy is lost and, in this case, losses occur in drag. When the propeller runs out of power while the prototype is in the air the gliding part had to take over so that it ensures a smooth landing over a certain period of time and the prototype did what was expected from it.
4.RESULTS AND OBSERVATIONS
The results are clear that the aircraft did not achieve the goal during testing. The figures show that it was not able to reach the minimum time required for flight. The propeller system produced a thrust as expected but it was too weak. The prototype flew for a moment then the propeller stopped and it started to glide until it reached a stage of diving. This whole process happened under five seconds. From the graph we are able to see that the flight time increases with an increase in number of twists. This is a direct proportion. Several factors may have hindered the flight. It could be that the propeller was not large enough to create enough air flow as the spin speed of the propeller reduced rapidly. The design was not perfectly done, it is clear that there are some other external factors affecting flight time.
An additional wing would have provided more lift but without appropriate propeller power the drag would just be too much.
Flight time is directly proportional to the number of turns. The minimum flight time was not achieved due to some faults in the design. Although the plane did not reach the milestone, the recorded results were able to convince the group that indeed there is a relationship between the number of twists and the flight time.
From the observations it was clear that the prototype could fly but certain changes would have made the prototype achieve a much higher flight time. For a more efficient flight the surface area of the stabilizers could have been increased. The propeller would be able to produce more thrust if it was created in such a way that it could freewheel when not powered, this would be proof that there is no friction on the propeller system. Less glue is more. When connecting the pieces of the plane together, less glue should be used as it dries faster and less weight will be added to the plane. The angle at which the wing meets the oncoming air is crucial, the wing angle should have been increased a bit more. As the angle increases, the lift coefficient increases and this changes the amount of the induced drag resulting in more lift 4. The increasing the surface area of the wing will decrease the wing loading so the amount of weight supported by the wings will decrease.
1https://www.skybrary.aero/index.php/propeller2 www.endlesslift.com/locating-the-centre-of-gravity3 https:howthingsfly.si.edu/propulsion/propellers
4 www.grc.nasa.gov/www/k-12/airplane /inclind.html
Figure SEQ Figure * ARABIC 1 : initial design
Figure SEQ Figure * ARABIC 2: our plane