Wind Sails
Wind sailing is an ancient form of atmospheric propulsion. The first use of sailing boats dates back to approximately 6000 years ago, even before humans learned how to ride horses. It is clear why - anybody can build a wind sail and it is probably easier then to train a horse. Just stretch a piece of fabric between two sticks and you can now harness wind power. Besides simplicity, wind sails are also efficient as they utilize the energy stored in the atmosphere - no need to carry your own energy. Both lighter-than-air vehicles and marine vehicles can benefit from wind-sails for propulsion. Unlike photon sails (as covered in a previous edition) atmospheric wind imparts significantly more momentum per unit area, so even small sails can provide great thrust given sufficient wind speed. Wind sails can generate thrust by air moving directly into them (drag) or air moving parallel to them (lift) - this allows a sail to generate thrust at various angles.
Early humans used a very rudimentary type of sail. But for a sail to be truly practical it must possess a certain number of features, which add to its complexity (though not by a lot). The first feature is retraction - the sail must be able to fold so that if the winds are too strong, thrust can be reduced. Also, by altering the total surface area that is retracted we can vary the thrust output, similar to a "throttle setting". The second feature is control - obviously winds change direction and speed all the time. The angle of the sail must too be adjusted in a timely manner to maintain vehicle direction. It is easier to have multiple sails, a big one for propulsion and a smaller one for direction control. Just look at those old pirate ships - quite a lot of sails on there and the crew was always busy maintaining and adjusting them.
In general, wind sails are divided into two categories - rigid and tethered. A rigid sail can be found on most sail-boats. It is called rigid because the sail connects directly with the vehicle structure through a mast and so the thrust transfers directly from wind to vessel. A tethered sail, on the other hand, is also known as a "kite" and works much like a parachute. Once in the air, the kite inflates and the thrust is transferred via tethers to the vehicle. Kites are mostly used in kite-surfing but there are companies out there such as Beyond the Sea which offer extra-large kites "SkySails" for use on big boats. A container ship could save a good amount of fuel during an ocean-transit by deploying one or two of such tethered sails. A tethered sail offers several advantages over the rigid sail - it can harness a more powerful and more consistent wind layer at a higher altitude and is also compact and easier to store onboard. However, a tether sail also has some disadvantages - it cannot be controlled as easily as a rigid sail - so changing throttle and adjusting angles becomes more difficult.
领英推荐
My favorite aspect of the wind sail is that it is not limited for use in Earth's atmosphere. A sail could be used in any atmosphere, on boats as well as airships. It does not care about the make-up of the atmosphere as long as the gases are moving. In 2011, NASA has considered the use of a robotic ship (Titan Mare Explorer) with a retractable sail to explore the vast oceans of liquid methane on Saturn's moon Titan. Similarly, hot-air balloons or blimps can be used to explore gas giants. Lighter-than-air vehicles don't need large sails, just small ones to allow them to endlessly explore vast atmospheres. I find it quite incredible that such an ancient technology is by no means phased out, and will certainly be used in the future as we explore other planets.
Head of Aerodynamics at Bound 4 Blue
1 年If you like these, I suggest you look into suction sails like bound4blue’s eSAIL. Using active flow control, CL values as high as 8 can be achieved!