Do you need help with your Physics SL IA/Internal Assessment? In this post I will show you my IA that I submitted to IB! You can use this to see what a Physics SL IA looks like and I hope this will inspire you to create your best Physics SL IA to submit to your teachers! In this IA, I investigated the Change in Descent Speed of a Parachute with Different Suspension Line Lengths and analyzed all the possible outcomes of it. To view my Internal Assessment I submitted just open the PDF right below this! If the PDF does not show, you could read the text version right below it (not recommended since it lacks formatting & images).
Investigating the Change in Descent Speed of a Parachute with Different Suspension Line Lengths
The investigation asks the question of whether the time and the descent velocity of a parachute is affected when the lengths of the suspension lines (shroud line) of a parachute is manipulated to find out whether a longer or a shorter suspension line is better for maximum air time.
Shroud lines or suspension lines are essentially the cords that attach the canopy to the payload. Their function is also to form a net or skeleton for the canopy. As explained by Navyation, a military publication site, the lines are called the suspension lines because this skeleton absorbs most of the shock and force when the parachute opens.
As an individual, I have always been interested in how things work. Since I was introduced with the Internet, I have always used it to find the answers to my curiosities and how parachutes work is one of these questions. As I researched online, I have not seen sufficient investigations done on the optimum length for the suspension lines. There are multiple beliefs held by the parachute community and the rocketry community in how long the shroud lines should be. Compiling the different beliefs together, the results that appear the most are these two different ideas: some people believe that the length of the shroud lines should be the same as the diameter of the parachute canopy, the rest believe that the length of the suspension lines should be about 1.5 times the diameter of the canopy. Putting this debate to rest is the main motivation for this investigation. The goal of this investigation is to find the optimum ratio of the shroud lines so that future parachute makers can use this information to determine the perfect length ratio for maximum air time and overall safety of the payload.
Since the shroud lines are responsible for the structure of the canopy, in theory my hypothesis is that a longer shroud line will slow down the parachute because it allows more of the canopy’s area to open. Shorter shroud lines will mean that when the parachute drops, less area of the canopy will be exposed to the air friction because the shape of the parachute canopy will not be as round as what the longer shroud line makes. With shorter lines, the canopy will look flatter (it looks like when you drop a sheet of tissue). This is because when an object is released in midair, gravity pulls an object into the earth, however air friction (drag) slows the object down. As an object falls, the increase in speed is accompanied by an increase in air resistance which counters the force of gravity. Once the air resistance increases until it approaches the magnitude of the force of gravity, a balance of forces is attained and the parachute no longer accelerates. When this happens the parachute is said to have reached terminal velocity. The parachute creates a net force and acceleration of the falling object to go upward because of air resistance. An upward net force on an object falling down would cause an object to slow down. Essentially a parachute works by increasing air resistance of an object as it falls which is what causes parachutes to function and slow down objects.
The independent variable is the length of the suspension line measured in centimeters. The length of the suspension line is essentially the string connecting the parachute canopy to the payload or dummy mass.
The dependent variable is the descent time of the parachute and the descent speed of the parachute. The plumb line method is used to measure the rate of descent or terminal velocity of the different parachute designs.
The controlled variable includes the height at which the parachute is dropped, the mass of the dummy weight on the parachute, and the plumb line of the parachute. The experiment is also done indoors (without air conditioning) in order to prevent external factors such as unexpected wind. These variables are kept constant in order to find a more accurate result.
Materials and Equipment
|Figure 2 Camera Recording at 120 Frames Per Second
The materials needed are: plastic sheet, string, and a couple of dummy mass for creating the parachute for the experiment. The equipment are the tools needed to conduct the experiment and record the findings and data. The equipment needed for the experiment is a special camera connected to a laptop, which is used to record time, and a meter rule used to determine the length of the plumb line.
Figure 3 Photo of the parachute before launching. On the left hand side is the plumb line attached with a dummy mass (payload) and a paperclip. The paperclip acts as the connector of the plumb line, mass, and the shroud lines of the parachute.
Figure 4 Plumb Line Method diagram by www.seekdl.org
Before conducting the experiment, the height of the parachute’s drop and the length of the plumb line were determined and were set the same for each parachute designs. The plumb line method was used to calculate the descent time and the descent speed of the parachute. This method utilizes a small weight (light enough not to affect the descent speed of the parachute) to keep the plumb line falling straight during the descent of the parachute. The plumb line method works by suspending a mass with a string of a constant known length (the plumb line) connected to the bottom of the dummy payload. After the parachute is dropped, timing (T1) is recorded when the mass of the plumb line touches the ground. Next, when the dummy payload touches the ground, the timing is recorded as T2.
The descent speed is found by dividing the plumb line length to the difference of time taken for the dummy payload to hit the ground (T2) to the time taken for the suspended mass to hit the ground (T1).
The experiment was done with plastic parachutes built from a garbage bag cut into congruent circles with a diameter of 33.5cm. Then, strings with different shroud line lengths of 33.5cm, 36.5cm, 39.5cm, 42.5cm, 45.5cm, 48.5cm, and 51.5cm were glued on to the each of the measured edges of the different parachutes by using hot glue at 0°, 45°, 90°, 135°, 180°, 235°, 270°, 315° from the center of the circle. This is done to make sure that the parachute canopy shape is round when dropped. Then, free ends of the strings were attached to a paperclip with a metal weight attached that acts as the dummy payload. Next, the 47.5cm plumb line added with a small weight was hanged onto the dummy payload. Having been completed, one of the parachutes was picked and dropped at a constant height, and the time of the plumb line was measured. The process was repeated five times to get five trials, then the same process was repeated with the other parachutes containing different shroud lines.
Uncertainties: Issues of Resolution, Precision and Accuracy
The parachutes were positioned in a constant height and spot, namely from the ceiling of a room, to eliminate inaccuracies and inconsistencies. To measure the time, a human is not able to accurately and precisely monitor a parachute when it falls and lands. Therefore, a calibrated camera recording at 120 frames per second was used to record and determine the time from plumb line’s first impact until when the parachute’s dummy mass hit the ground. By recording at 120 frames per second, it allowed a slow motion and detailed analysis of the fall as accurate as possible and to find the drop to the closest millisecond. The recorded footage the camera provided can also help with determining other possible factors that might affect the end results, so that issues of resolution, such as errors and uncertainties, can be determined and what happened during the experiment can be analyzed.
|Figure 5 Software that allows the footage to be reviewed frame by frame|
The data-logging process recorded the raw data of time in seconds by using a special software that allows the exact time to be found by analyzing the 120 frames per second footage frame by frame. After manually recording the data, to find the time taken for the plumb line, the equation was used in excel to automatically create the table and graph. The raw data of each drop out of the five trials were shown in seconds in a table in the Appendix.
|Figure 6 Processing the raw data in Excel
The averages of the trials were calculated by dividing the total trials of all the plumb lines by five since there are five total trials. Then, each of the average time of the different plumb lines was recorded into a table.
|Shroud Line Length (cm)||Average Time [T2-T1] (seconds)|
Table 1 showing the shroud line length and the average time it took using the plumb line method.
|Table 2 showing shroud line length, average time, terminal velocity after calculating the speed of descent
To find the descent speed of the parachute, the previous equation was used for each of the different shroud line lengths and the results were recorded into a table.
|Shroud Line Length (cm)||Average Time [T2-T1] (seconds)||Terminal Velocity (cm/s)|
The scatter graph of average time of plumb line drop against shroud line length is shown below.
However, to properly analyze the data, take note of the outlier shown in the previous graph since the point will be ignored for now. Next, a graph for the average time was created, completed with the line of best fit and the error bars. Note that the error bars are there, but not visible on the graph because the values on the bars are too small as demonstrated by the example below. This happens because I used a 120frames per second camera which has more accuracy and less errors overall. Below is the method used to calculate the error bars.
|Example: Calculating error bars of shroud line 33.5cm with average time of 0.282 seconds.||Shroud Line Error Bar: 0.001492537
Average Time Error Bar: 0.014775414
Terminal Velocity Error Bar: 2.740169056
|Calculating Error Bars|
m or gradient = 0.0225, b or y-intercept = 0.4606, R2 or relationship to line of best fit= 0.8959
Using the graph above, it is possible to graph out the graph of average velocity of the parachute against shroud line length.
m or gradient = -5.5995, b or y-intercept = 339.86, R2 or relationship to line of best fit= 0.8682
As expected, by looking at the line of best fit and the R2 value, it can be seen that there is a relationship between the length of the shroud line and the average time and velocity of the parachute. The closer R2 value is to 1.0, the better it is for the scattered points to reach the line of best fit. Although the difference in time is just by milliseconds, since this is a small-scale experiment, the graph especially the trend line confirms that a longer shroud line results in a longer air time. In fact, based on the footage analysis provided by the slow motion camera, the shroud lines are able to extend the duration of the fall and the velocity of the parachute by altering the area of the parachute’s canopy. A shorter shroud line causes lesser canopy area to be opened and trapped by the air friction when dropped. Therefore, this causes the canopy to not look as round as in the visualization but flatter and this allows air friction to spill out of the sides of the canopy. A longer shroud line allows more of the canopy to be opened and exposed to air friction, which, therefore, causes the canopy to look rounded. A rounder canopy allows more air friction to be trapped in the canopy and prevent too much air spilling out of the canopy, which in turns will slow down the parachute. However, from the analysis on the previous scattered graph, there are peculiar entities happening when the shroud line is 51.5cm long. From the analysis of the footage frame by frame, it shows how the 51.1cm long shroud line is so heavy that instead of slowing down the fall, the added mass of the extended string causes the parachute to fall down faster although the canopy is fully deployed. Just like a spring, parachute also has its limits.
Based on the data collected, when the ratio of shroud line is compared to the diameter of the canopy in order to get longer air time, the optimum ratio of the perfect shroud line is around the value between the ratio of 48.5cm long shroud line and the ratio of 51.5cm long shroud line. Therefore to calculate the optimum ratio:
|Calculating the Optimum Shroud Line Ratio|
Therefore, the optimum shroud line ratio is about 1.5 times the diameter of a parachute’s canopy.
Conclusion and Evaluation
From the figures and the analysis, it is apparent that there is a correlation between the shroud line, the descent time, and velocity of the parachute to a certain extent. Often times, the longer the shroud line, the parachute will experience more air time. This is caused by the area of the parachute’s canopy interacting with more air friction and lesser air spilling out of the chute. Yet, apparently after a specific length, instead of creating more air time, the longer shroud line causes the parachute to descent faster because of the added mass from the extra string used. Apparently the previous theory and hypothesis of how long the shroud line length should be is correct. The ratio for the optimum shroud line length is 1.5 times the diameter of the parachute’s canopy.
The limitation of the experiment is the uncontrollable direction of where the parachute fell. This is caused by the design of my parachute, which allows air to spill out to one side of the parachute’s canopy and pushes the canopy into a direction, or sometimes it is too unstable for the parachute to not deploy properly. The uncontrollable direction of the parachute causes the distance travelled to never be constant and time keeping to be difficult. Although this problem was eliminated by using the plumb line method, in real life this will not really be a problem since a real parachute’s design is more complicated and already accounts for the spilling of air, resulting in a more stable parachute.
Another limitation would be the location of where the experiment was done. Since the experiment was done indoors to remove external factors such as wind to affect the result of the parachute, I had to adapt to the environment by creating a smaller scale parachute. This is why the difference in time between each sets of trials were only milliseconds. Perhaps, by having a greater experiment area with a higher drop height I would be able to create a bigger parachute with a longer plumb line that would show a wider difference in time. This will allow a better analysis of the effects of the shroud line and make the results more accurate and precise.
Finally, to further improve the accuracy of this investigation, it would be better if an accelerometer and a data acquisition system is attached into the parachute.
Ahmed, Mahboob, Surendra M. Jain, Deepak Kumar, and Alok Jain. A Novel Method for Measurement of Terminal Velocity of Aerodynamic Decelerators. Institute of Research Engineers and Doctors, 2013. Web. 14 Feb. 2016. <http://seekdl.org/nm.php?id=1305>.
Parachute. Digital image. Parachute. StratoCat, 14 May 2014. Web. 10 Feb. 2016. <http://stratocat.com.ar/stratopedia/119.htm>.
“Suspension Lines – Navyation.” Suspension Lines – 14014_303. Integrated Publishing – Your Source for Military Specifications and Educational Publications. Web. 21 Feb. 2016. <http://navyaviation.tpub.com/14014/css/14014_303.htm>.