The vertical tail of an aircraft is a highly definitive feature, cutting a iconic shape that makes an outline instantly recognizable as an airplane. It wouldn’t be unreasonable to assume the tail to be an absolutely essential part to an aircraft’s operation, seeing as many commercial craft, such as the Boeing 747, have large and heavy tails.
The surprising truth, however, is that the vertical tail of an airplane actually plays a very small role in normal flight. Planes can takeoff, fly, and land just fine without a tail. At the high speeds of cruising flight, the rapidly moving air allows even a massive plane to be moved by a fairly small rudder, with the tail structure itself becoming mostly irrelevant. So why would engineers saddle their craft with massive tails, thereby reducing the craft’s fuel economy?
The true purpose of a large vertical tail is for one primary function: allowing the pilots to control the craft in the event of an engine failure during takeoff or landing. Should one of an airplane’s engines fail, the craft experiences thrust asymmetry and starts to rotate. A large tail is essentially insurance against losing an engine, a tail has to be large enough that it can counteract this lack of symmetrical thrust and keep the plane flying straight.
“But this means planes have a tail that’s too big 99 percent of the time.” says Emilio Graff. “Imagine if the only way you could have airbags…was to tow them in a big trailer behind your car, just in case there was an accident. It ends up sucking up a lot of fuel.” Graff is the leader of a Caltech project to reduce the size of the tail, and thus improve the fuel economy, by using devices known as sweeping jet actuators.
Airplane tail designers do not need to worry about losing an engine while the craft is at cruising speeds, since the rapid airspeed allows even a small tail to compensate for thrust asymmetry. At the relatively slower speeds of landing and takeoff, however, the lack of fast moving air becomes a real problem. Running compressed air over the tail rudder to simulate high winds would solve this issue, but covering the tail of an airplane is air nozzles might be even more inefficient than an overlarge tail. This is where the sweeping jet actuator comes into play.
A nifty piece of technology that has been around for more than half a century, the SJA has mostly found a home on showerheads and irrigation systems. With no moving parts and able to be built to work with either water or air, the the device is designed to take a constant stream of fluid and force it to oscillate left and right on a two dimensional axis. Some variants are able to cycle at incredibly fast speeds, such as the Caltech SJA above with a period of about 2 milliseconds.
Sweeping the air, rather than using straight jets, allows the entire area of the rudder to be covered by fewer nozzles, making the actuators highly useful in this scenario. Emilio Graff and his team’s initial tests with the system, dubbed Active Flow Control, were on a scaled down airplane tail contained in the 5×6 foot Lucas Wind Tunnel at Caltech. This reduced scale test yielded promising results, with a 20% increase in rudder effectiveness. Emboldened by their success with the Active Flow Control system, the next logical step was be to test the system on a full size airplane tail.
Acquiring a Boeing 757 tail from a salvage yard, the team turned to the National Full-Scale Aerodynamics Complex at Moffett Field, California. Home of the two largest wind tunnels in the world, the complex, located within the Ames Research Center, proved ideal for Graff to test out the idea on a 1:1 scale. Working with engineers from NASA and Boeing, the team used the smaller of the two cavernous tunnels to test their idea. It was shown during these full scale tests that, using this system, an airplane’s tail could be reduced in size by about 17%. This translates into a 0.5% reduction in fuel consumption due to both the reduced mass and drag from the smaller size. This might not seem like much, but this could significantly lessen the environmental impact of air travel in the long run.
The idea is still moving forward, as the sweeping air system was placed onto Boeing’s ecoDemonstrator, a 757 the company uses for experimental testing purposes. The craft performed a variety of maneuvers designed to measure the amount of side force generated by the system, as well as simulate engine failure. While Graff doesn’t expect the system to start appearing on planes anytime soon, since aircraft take years to design, he is hopeful that his research will one day make flying both a little cheaper and a little cleaner.