Friday is here, and the 'Weekend Science Bite' corner is back — number 100 🎉
This time: the fascinating physics of propellers, from drones on Mars to laptops. Plus a few words from me in honor of installment #100 (in the first comment on the post).
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Propellers propel ships and lift helicopters by pushing the medium through which they spin — water or air.
A propeller works efficiently when the medium it's pushing is dense enough. The thinner the medium, the larger the blades must be, or the faster they must spin, to push enough molecules for forward motion.
Another key factor in propeller efficiency is the speed of sound in the medium.
As long as a blade moves slower than the local speed of sound, it pushes the gas or liquid ahead of it, which flows smoothly around it (aside from tip vortices). Once the blade exceeds the speed of sound, the pressure wave it generates can no longer outrun it, and turbulence forms that resists forward motion.
In helicopters and drones, the speed of sound is a more significant constraint than in aircraft, because the blade tip moves far faster than the section near the hub — making the load distribution along the blade inherently uneven.
The speed of sound is affected by the composition and temperature of the air. The heavier and colder the molecules in the air, the more energy is required to propagate a sound wave through it, and so the speed of sound decreases.
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The best way to travel across Mars is by drone, which can easily hop over the surface's rough terrain. The problem is that the Martian atmosphere is anything but friendly to drones.
Unlike Earth's atmosphere, which is composed of nitrogen and oxygen, Mars's atmosphere is 95% heavy carbon dioxide, and surface temperatures are low. As a result, the speed of sound on Mars is only 869 km/h, compared to 1,223 km/h on Earth. This means that increasing blade speed quickly brings the blades to the speed of sound, rendering them inefficient.
Atmospheric pressure on Mars is only 1% of Earth's, making it even harder for a drone to generate lift and requiring exceptionally high rotational speeds. NASA is already operating the first drones on Mars, but is looking for ways to improve their efficiency and payload capacity.
In the experiment shown in the video, NASA scientists test a carbon-fiber propeller inside a sealed, controlled chamber that simulates Martian surface conditions. They managed to spin it at an astonishing 3,750 RPM, and with the addition of an external airflow boost, it reached Mach 1.08 on Mars's speed-of-sound scale — without breaking.
The success of this experiment will allow next-generation Mars drones to carry up to 30% more payload per flight.
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Another interesting aspect of propellers is noise.
In the past, propellers were built with equal spacing between blades to ensure stability. But this means the frequency of the noise they produce is constant, and the combined noise from all the blades creates a loud drone at a frequency the human ear is highly sensitive to.
To solve this problem, the helicopter industry developed a propeller with unequal spacing between blades, so the noise generated is spread across different frequencies and is far less bothersome to people.
Apple once filed a patent for a similar technology in the MacBook Pro's cooling system. The fan blades were designed with uneven spacing, so the noise produced by the spinning fan is nearly inaudible to the human ear, keeping the computer quiet.
Shabbat Shalom 😊
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Video credit: NASA/JPL-Caltech
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