‘Twisteron’ concept may reduce drag

 - March 15, 2007, 7:51 AM

A new control surface could reduce induced drag on commercial and business aircraft by up to 14 percent, resulting in fuel savings of more than $400 million per year across the entire U.S. air transport fleet. So claims Utah State University aeronautical engineering professor Warren Phillips, who recently introduced the devices, which he calls “twisterons.”

According to Phillips, who recently published a textbook titled Mechanics of Flight, the induced drag directly caused by a wing’s generation of lift generally accounts for 50 percent of total drag on an aircraft during normal flight conditions, and can be as high as 90 percent during landing.

Phillips’ technology twists the trailing-edge portion of the wing, using spanwise circulation control to incur the least possible induced drag during all phases of flight, thereby reducing the amount of fuel needed to overcome the induced drag. The twisterons span the entire length of each wing and can act as flaps and ailerons as well as provide the required twist for changing flight conditions.

A New Way To Reduce Drag
“Back in the 1920s, people developed one solution for minimizing induced drag,” Phillips said during an interview with AIN. “That solution was an elliptical wing, which was used on Britain’s Supermarine Spitfire…But that shape is complex and expensive to manufacture, so few airplanes have been built with elliptical wings.”

Washout, the fixed twist found on most transport and business jet wings, is another solution to minimizing induced drag, but this optimizes the wing for only one flight condition and is usually geared toward preventing wingtip stall rather than specifically minimizing induced drag.

Wingtips also provide some relief against induced drag, but their fixed nature means they too are effective only in certain flight conditions. Since induced drag varies with the coefficient of lift, which changes throughout the course of a flight as the aircraft’s weight and speed and the ambient air density change, minimizing induced drag in all phases of flight requires nearly constant changes in the airfoil shape to respond to the varying coefficient of lift throughout the flight.

Phillips’ design uses electrical signals to drive individual control mechanisms along the twisteron trailing edge as a flight computer calculates the specific degree of twist needed to achieve minimum induced drag based on various flight factors.

Phillips says there are a number of ways to achieve the twist mechanically, although the key is using a flexible material for the twisteron control surface. Composites are a possibility, and Phillips is also talking to the inventor of a machining process that allows aluminum aircraft skin to maintain stiffness in one direction and flexibility in another.

Since the twisterons can also deflect individually to work as ailerons, extend together to work as flaps, or deflect and extend like flaperons, current aircraft could be retrofitted with twisterons replacing these other wing control surfaces. Phillips says his design is “totally size independent” and can be scaled for aircraft of nearly any size.

“The solution that I developed predicts the total amount of twist you have to have in a wing of any planform shape and with any amount of fixed twist in it already,” said Phillips. “The advantage of having [twisterons installed on] just a trailing-edge portion of the wing is that you can make the structure of the wing support the [load] forces and require only the twisteron to be flexible.”

The university has applied for a patent and is looking for industry partners to research the costs of commercializing Phillips’ twisteron technology. According to Steven Kubisen, vice president of USU’s technology commercialization office, the university has received “two or three” inquiries from major aircraft manufacturers regarding the use of twisteron technology.

“University technology is fairly early on,” Kubisen said. “That’s why we’re in the stage of looking for partners that can help us identify costs and benefits [associated with] a commercial application.”

Minor Modifications
Kubisen said that one reason the university thinks the twisteron technology is commercially viable is that manufacturers need to make few changes to the aircraft to implement the technology. He cited winglets as an example of a similar technology that required some modification of the wing structure to implement but still made it to market.

“[With twisterons] we’re not talking about modifying whole wing surfaces, just the ailerons,” Kubisen said. “There may be some beefing up of the wing required because of the modification of the ailerons. We don’t believe it will be required, but it’s too early to tell.”

The twisteron technology has so far been implemented only on a 10-foot-span radio-controlled model, but Phillips believes twisterons will soon be seen on unmanned aerial vehicles (UAVs). He is already working with Lockheed Martin on a proposal to outfit a newly designed UAV with twisterons for the U.S. Air Force.
“[The UAV] is designed for long endurance,” Phillips said. “It weighs 50,000 pounds dry and 150,000 pounds with full fuel, and it needs to be able to fly from Mach .6 to as slow as possible. So the lift coefficient will vary widely over its flight envelope, and the amount [of] twist needed to minimize induced drag at one flight condition is greatly different from the amount of twist needed at another flight condition.”

Although twisterons are implemented using electric servos, control rods and flexible surface materials, the whole theory behind them comes down to a few relatively simple equations. Phillips said that he emphasizes to his students how really understanding mathematics is “our window into the future of technology.”