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Home | Basket and experiments |
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Study of the 3rd try |
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The number of sensors on board was maximum: - 2 ambient temperature sensors - 2 inside temperature sensors o 1 at the bottom o 1 at the upper surface of the balloon - 2 horizontal-directed lighting sensors - 2 pressure gages Unfortunately, as we noticed when we got the balloon back, the inside temperature sensor which was installed 30 cm from the upper surface fell few seconds after take-off and the measured inside temperature was very close to the ambient one. Take-off Parameters at take-off: ambient temperature: 18°C; inside temperature (center of the balloon): 34°C; atmospheric pressure: 1018 hPa. So, the inside air cubic mass is 1.15kg.m-3, and the ambient air cubic mass 1.22 kg.m-3. At take-off, the minimum difference between the inside and ambient airs is about 70 g.m-3. As our balloon needed at least a difference of 53 g.m-3 to take-off, we concluded we had a minimum thrust of 17 g.m-3, about 13N with a volume of 76 m3 (those who do not feel confident with Newton data, 13N is a force equal to the force of a 1.3 kg mass...). Climbing
On the above chart, the curve represents the balloon altitude according to the data sent by the pressure gage during the flight. We noticed that the climbing occurred at a constant speed, about 93 m.min-1, i.e. roughly 1.5 m.s-1. In the same manner, the descending speed is 116 m.min-1, about 1.9 m.s-1. On the document below, altitude and lighting are recorded. We notice that the balloon started to slow down after 80 minutes of flight, as the lighting began to decrease quickly as the sun set as well, while the balloon kept climbing
Thanks to these data (ambient temperature, center balloon temperature and pressure), we processed the difference of cubic mass between the inside and ambient airs in the center of the balloon, during the climbing and descending phases. But we noticed, as shown on the following curve, that the difference is not sufficient to allow the balloon to climb during the whole flight. The explanation is similar to our studies on the ground: the inside temperature of the balloon is not homogenous and so its average value is far more important than the one at the center of the balloon.
Nevertheless, we are able to estimate the minimum average temperature inside the balloon, while it is climbing evenly and vertically. In these conditions, it means that buoyancy is more important than or equal to friction forces and weight altogether. In the following chart, we estimate the average temperature inside the balloon so as to buoyancy compensates for the weight of the system. This temperature is always a minimum because friction forces shall be considered. We noticed that the higher the system climbs, the more important the difference of temperature must be. As the pressure decreases – at a more significant rate than the ambient temperature, the cubic mass of the air decreases too, according to the formulae given in the 1st part. After 80 minutes of flight, the temperature gradient must be above 25°C while the balloon is flying at 8,000 m.
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