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MIT and NASA Designing Silent Aircraft
Iddo Genuth writes "Researchers at the Massachusetts Institute of Technology Department of Aeronautics and Astronautics recently won a contract from the National Aeronautics and Space Administration to design quieter, more energy efficient, and more environmentally friendly commercial airplanes. The two-million-dollar contract from NASA is just an initial step in bringing green technologies to the sky."
"Silent" is a relative term, but the presumption is one that has noise levels approaching that of an automobile.
That simply is never going to happen. Moving air around to create thrust will always be noisy. Even if all engine noises are reduced to zero, the vibrations of the air moving at the extreme speeds we would expect will cause more than enough noise. The only way I can imagine to combat that fact would be to distribute the effect over very large areas... and even then, as the size of the air moving system approaches "too big to be practical" it would still likely be way to noisy.
Helicopter style systems would be more of the same.
They are going to go back to Roswell and Area-51 and figure out how the aliens did anti-gravity so we can have aircraft that fly with less thrust requirement.
The prop tips on these airplanes reach transonic speeds at full power
Prop designs are tailored for a specific aircraft design and engine combination. Part of the requirement for prop selection is to avoid supersonic or even transonic speeds, even while at full throttle. The reason being, efficiency significantly falls off once a prop begins to reach transonic speeds, let alone supersonic speeds. It is so important to avoid these speeds, well, you now know the origin of the scimitar shaped prop.
In short, if you are flying any Cessna 185, 206 or 207 which has a prop reaching transonic speeds, your prop needs to be replaced as it has been overhauled too many times.