Cadets aim to cut KC-135 fuel costs

  • Published
  • By John Van Winkle
  • U.S. Air Force Academy Public Affairs
KC-135R Stratotanker wings could look a little different in the future and use less fuel because of a Department of Aeronautics research project taking place here.

The Air Mobility Command and the Air Force Research Laboratory co-funded a cadet research project to evaluate and design wingtip modifications for the KC-135R Stratotanker, including wingspan extensions, raked wingtips and winglets.

A winglet is a short extension of the wing, which curves upward sharply from the wingtip, and is currently in use on the C-17 Globemaster III. A raked wingtip is a wingspan extension with additional sweep at the wingtip. Raked wingtips are currently in use on the Navy's P-8 Poseidon anti-submarine warfare aircraft and several versions of the Boeing 767 and 777 aircraft.

A research team comprising Cadet 1st Class Daniel Prescott, Cadet 1st Class Joel Halpert, Ken Ostasiewski, Tech Sgt. Sam Church-O'Brien, Jeff Falkenstine, Michael Arndt, and led by Dr. Tom Yechout, have been at work since the 2008 spring semester to investigate and define the optimal configuration of each of the wingtip devices, determine the improvement each offers in terms of increased range and endurance, and fuel savings.

This isn't the first time someone has tried to stretch the Stratotanker's fuel efficiency. NASA and Boeing each investigated winglet designs for the Stratotanker in the 1970s and 1980s. So the team dusted off the NASA and Boeing reports as they started their investigations.

"We took their winglet idea first and then modified it in several ways, to see if there was a more optimal configuration," said Cadet Halpert.

Boeing and NASA each predicted approximately a 7-percent reduction in total drag based on their winglet design. The Air Force evaluated these design improvements at the time but committed its finite budget dollars to obtaining new engines for the KC-135 instead .

But recent financial pain from spiking oil prices refocused national attention on fuel costs, and on methods to stretch one's gas dollars. So AMC tapped into the Air Force Academy's Department of Aeronautics research prowess to find ways of stretching the command's jet fuel dollars by looking at improvements to the KC-135, which is projected to be in the Air Force's inventory for many years to come.

After initial wind tunnel testing in the Spring 2008 semester, Cadet Prescott worked with AMC mission planners at Scott AFB, Ill., as part of the Cadet Summer Research Program to define three representative KC-135 missions and then modified AMC mission planning software to project increases in fuel savings, range and endurance with the optimized winglet modification.

At the AFRL Flight Vehicles Directorate, Wright-Patterson AFB, Ohio, Cadet Halpert modified their Flight Optimization System Performance Prediction Program to analyze the same KC-135 missions and configurations. Their combined work independently predicted an 8-percent improvement in overall performance for the aircraft with the optimized winglet implemented. 

The research continued during this academic year at the Academy, as the team tested numerous design variations of not only winglets but also raked wingtips and a wingspan extension. As part of the evaluation and optimization, various angles and surface areas were changed for each wingtip device to find the exact aeronautical sweet spot which offered the best benefits.

The Academy team complemented their experimental work with extensive data-crunching in computational fluid dynamics via the Academy's Modeling and Simulation Research Center here.

The secret to fuel savings comes in the invisible yet tangible changes these modifications make in the airflow at the wingtips.

"Wingtip devices are all about reducing the strength of wingtip vortices. The wingtip vortices are created by the high pressure air under the wing migrating to the low pressure area above the wing, at the wingtips. This creates a tornado-like circular flow, and that takes energy out of the aircraft in the form of induced drag," said Doctor Yechout, the Department of Aeronautics professor here who is one of the Air Force's research gurus on drag reduction.

After a year of computational and experimental research, the team's findings offer much potential for fuel savings via drag reduction. In fact, more than $50 million in annual fuel savings is projected for the KC-135 fleet.

"The results show that winglets, raked wingtips, and a wingspan extension do improve the aircraft's performance capability," said Cadet Prescott. An optimized winglet may be the best choice based on less weight addition and less additional stress to the wing, along with a minimal span increase, he added.

The cadets' research predict up to 8 percent fuel savings for optimized winglets and raked wingtips, and lower fuel savings for wingspan extensions.

"The final decision about whether or not to install wingtip devices on the KC-135 will include a cost-benefit analysis that will take into account many different variables including the KC-135's remaining mission life, cost for the upgrade, wing structural loading, etc.," said Cadet Halpert.

"The next step in this process would be a structural assessment of the feasibility of each modification and then a cost-analysis," he added.

The Academy team presented this research at the American Institute of Aeronautics and Astronautics Region V Student Competition held at the University of Minnesota last month and won third place. They will also present their findings to AMC later this month.