The three factors above should produce two clear relationships and one which is more uncertain.
* Payload weight ? descent speed
* Surface area ? 1
* The no of lines which the parachute uses to suspend the payload weight will affect the descent speed, but it is difficult to predict how the speed will vary.
In order to get a range of speeds which were easy enough to measure (i.e. not too fast) a number of parachutes were constructed to find an optimum range between which more parachutes could be built. The results of these preliminary experiments showed that the proposed drop height (standing on a desk, around 2.5m) was too small, as the parachutes did not have time to unfurl. The school stairwell was chosen as its height was more than satisfactory.
EXPERIMENT 1 – PAYLOAD WEIGHT
The experiment was carried out using the middle – sized parachute of 17.5cm radius. The school stairwell’s height was measured using a plumb line constructed from string and Blu-Tac. The height from the top of the banister to the floor was found to be 4.87m.
Six string suspension lines were used to hang the mass from the canopy.
The parachutes were dropped by hand, with a stopclock on hand to measure the time for descent. The mass was made from Blu-Tac, due to its flexibility it could be moulded around the base of the parachute easily. It was increased from 25g to 125g in 25g steps. The initial intention was to drop the parachute with no payload too, however with no weight the parachute lost all structural integrity and would not float properly.
Overleaf is the table of results followed by its corresponding graph.
Notice how the line of best fit is not continued to the y-axis. This intercept should give the speed of the parachute with no mass attached, but as previously discussed the randomness of the parachute’s fall meant that no real value was suitable for its speed. It was decided that the trend line should only be drawn in the range of measurements taken.
It is clear however that as the payload mass increases, so does the decent speed so the first experiment was a success. The physics behind this result are that the downward force on the parachute will increase as the mass is increased, making the upward drag force of the parachute canopy less and less effective due to its constancy. The downward acceleration will therefore increase.
EXPERIMENT 2 – PARACHUTE RADIUS
Five separate parachutes were constructed, with a range of radii from 10cm up to 25cm. The mass was kept at a constant 50g and the height again was 4.87m. Six suspension lines were used. Each ‘chute was dropped three times and an average descent time calculated. Below are the results and graph.
Again, the graph shows the expected trend. It is essentially the opposite of the first graph, with an inverse proportionality to the line of best fit. The physics backs up this result, as instead of the parachutes drag force being constant this time it is the weight force, the opposite situation to the first experiment. This means that the drag of the parachute increases, and has more effect on slowing the acceleration of the parachute while the weight force become (relatively) less effective.
EXPERIMENT 3 – NUMBER OF SUSPENSION LINES
The expected results for this experiment were not initially known, but after careful consideration it was decided that as the number of suspension lines increase, the descent speed of the parachutes would also increase. The suspension lines would drag the canopy edges inwards, effectively decreasing its surface area and increasing its speed (as found from the pervious experiment).
Again the weight was kept at a constant 50g, using the 20cm radius parachute. The height was once more 4.87m. The results and graph are below.
The graph provided an interesting, if not expected shape. However after analysis the physics behind it can be explained. Initially, as the number of lines increases, the speed decreases – this is the opposite to what was predicted. It could be however, that due to the small number of lines the parachute did not have the required stability. Air could rush through the canopy easily where there where no lines to support its shape causing the ‘chute to fall faster. More lines would make the parachute more stable which would allow for a slower descent speed. However as was first predicted too many lines would restrict the parachute opening out fully reducing its surface area and therefore increasing its speed.
The investigation was on the whole a successful one. The experiments had few problems and the results gained seemed to fit the original hypotheses. However there was room for improvement in some areas.
* The parachutes tended to hit obstacles on their descents, meaning that those drops needed to be repeated, wasting time. A drop zone without these obstacles would be an improvement.
* The lines tended to become tangled after a drop, an time was wasted untangling them. Some way of keeping the lines separate would certainly improve the investigation.
* The process of collecting the parachutes from the bottom of the stairwell was tedious and tiring. A recovery mechanism would have been a good idea – possibly some lightweight string attached to the top of the parachute, enabling it to be pulled back up the stairwell.
With more time available, it would have been beneficial to construct more parachutes of differing radii, to see if there was an optimum radius for a particular mass, requiring the least material. This would be relevant to a real world situation, where parachutes need to be safe but also use a small enough amount of material in order to be economically viable to produce.
Another interesting experiment would have been to see if the payload’s angle of suspension made a difference to the descent speed. E.g. “Would a payload close to the canopy fall faster or slower than one which was further away?”