Aircraft

Tweak My Ride

Cable TV is filled with programs highlighting specialty firms with colorful personalities who take ordinary cars, motorcycles, and other vehicles and turn them into overpriced show pieces that aren’t really practical. The emphasis is on bling, not on better.

But in the real world, a few straightforward exterior tweaks will make larger military aircraft, like C-130 and C-5 transports and P-3 maritime patrol aircraft, markedly better.

According to a 2011 Department of Defense report, the US armed forces consumed nearly five billion gallons of fuel of all types in military operations in 2010. Those gallons cost $13.2 billion, a 255 percent increase over the fuel bill in 1997. “Saving even one percent of those five billion gallons is a huge amount of fuel and a big reduction in cost,” said Chuck Hybart, who headed the fuel efficiency studies program for the Lockheed Martin Skunk Works, the company’s advanced technology development organization.

Air Mobility Command, which runs the US Air Force’s global network of cargo transports and aerial tankers, is the single largest consumer of fuel in the Department of Defense. Recognizing the need for ways to reduce its fuel usage, AMC opened its Fuel Efficiency Office in 2010. Lockheed Martin began its own internal studies eighteen months earlier. “In the fall of 2008, when gas was $4 a gallon, we started looking at ways of making large aircraft more fuel efficient. We figured the Air Force would be interested in saving some money,” noted Hybart

“We started by looking through the big end of the funnel. Anything that was remotely possible was considered,” continued Hybart, who works out of the Skunk Works operation at the company’s Marietta, Georgia, facility. “We then narrowed the field for more serious study to those options that appeared to offer the best return on investment.”

Using mostly company research and development dollars, the Skunk Works researchers found that what will be the most practical answer—an elusive intersection of cost, difficulty to engineer and implement, improvement in aircraft efficiency, and length of time for payback on investment—ended up being slightly different for the C-130, the C-5, and the P-3.

Microvanes
With aircraft, drag is a bad thing. Lift and thrust have to overcome gravity and drag for an aircraft to fly. The easiest, least expensive way to improve C-130 fuel efficiency seems to be counterintuitive: installing eighteen small, lightweight, strake-like devices on each side of the aircraft’s aft fuselage near the cargo ramp door and horizontal tail. Called microvanes, these roughly ten-inch-long vanes, which are only lightly loaded aerodynamically, create minimal localized drag. However, working as a group, the microvanes slow the natural, much larger drag-creating vortex that forms as airflow goes over and under the wing and swirls around the aft end of the aircraft.

Research into mounting large, fixed strakes mounted under the horizontal tail of the Hercules at the upper part of the cargo ramp door took place in the late 1970s and early 1980s. While these strakes did reduce drag significantly, they interfered with airdrops and cargo loading, and also had an adverse effect on aircraft structure, all of which made them operationally unacceptable.

Developed by a team led by Dr. Brian Smith, the microvane design was patented in August 2011. “Microvanes, which are relatively inexpensive to produce, are bonded to the aircraft,” noted Hybart. “The net result is a fifteen-count reduction in drag at long range cruise speeds, which equates to about a twenty-five gallon per hour saving.”

Testing was carried out in August 2011 using a new Canadian CC-130J Super Hercules just off the Marietta production line. The aircraft, instrumented to gather data, was flown on three test flights that showed computational fluid dynamics, or CFD, predictions and reality matched. CFD involves using a computer to calculate boundary conditions where airflow meets a solid surface. Realizing that the blade vanes could possibly impede paratroop or airdrop parachute lines, a second type of microvane was also developed and tested. This microvane, called a bump vane, is a rounded, snag-free version.

“The bump vanes are placed lower on the fuselage near the cargo ramp and paratroop door and the blade vanes are higher up the tail,” noted Hybart. “We predict there will be no impact on airdrops, but we will need to test them.” Working with the Air Force Research Laboratory, airdrop testing using both types of vanes is likely to occur by late 2012. Several different thermoplastic materials that can be injection molded are being considered for production microvanes.

“This technology applies across the C-130 board,” noted Hybart. “Both legacy and C-130J operators can benefit because the shape of the back end of the aircraft hasn’t changed. Microvanes can be installed on the production line or as an easy retrofit. These vanes were easy to model, are relatively inexpensive, and offer a good payback. We haven’t seen a downside.”

Lift Distribution Control System
To prevent overstressing the C-130’s wings during a mission, fuel is carried in the outer wings. This fuel, called wing-relieving fuel, keeps the outer wings from flexing up too far, particularly when carrying heavy payload weights. This fuel, while necessary, is essentially dead weight and can’t be used during the mission.

Using what’s called a lift distribution control system, or LDCS, both ailerons are deflected up—as opposed to one deflected up and one deflected down while the aircraft banks—which shifts the aerodynamic loading inboard toward the center wing and fuselage.

Computer modeling predicts that shifting the loads inboard reduces wing bending loads by ten percent on a long-fuselage C-130J with its more powerful and more efficient engines and by twenty-one percent on a legacy C-130H. Shifting the aerodynamic loads also increases the range of the C-130, as the fuel in the outer wings can be used for the mission. Using an LDCS increases range on a C-130J carrying 42,000 pounds of payload by 140 percent and by more than 300 percent for the C-130H.

A real-world example helps illustrate how effective an LDCS would be for a C-130J. A squadron commander is tasked to transport 42,000 pounds of cargo 1,500 miles. His current options are to take all 42,000 pounds on one aircraft or split the load and use two C-130Js. The first option requires two legs to get to the destination. The crew will have to take off, climb, cruise, and land with required fuel reserves at the 750-mile mark. After refueling, the crew will have to repeat the same flight profile to the destination. The second option, assuming the payload isn’t a vehicle and can be divided, allows the cargo to be transported nonstop, but it requires twice the aircraft and crew. With an LDCS, one C-130J crew could move the entire 42,000 pound payload in one flight, nonstop, with required fuel reserves on landing.

“Uprigging the ailerons forces the aircraft to fly at a slightly higher angle of attack,” observed Hybart. “Flying slightly nose-high has the effect of reducing drag in the back of the aircraft, which we didn’t expect. So, while having both ailerons up does create a very small amount of drag over the wings, the net effect is reduced drag on the aircraft.”

The now mostly retired L-1011 airliner and the C-5 both have an automatic LDCS. The aircraft’s computer determines aileron position to relieve wing loading. What is being looked at for the C-130 would be a manual system controlled from the flight deck that could be used at the crew’s discretion, depending on mission and payload.

Winglets
Winglets are one promising option that turned up on the C-130, C-5, and P-3. Winglets are the upturned wingtip devices that improve the efficiency of fixed-wing aircraft by reducing drag through partial recovery of the vortex energy created by the airstream as it goes over the wingtip. These devices also increase the effective aspect ratio—that is, wing length-to-chord—without materially increasing wingspan.

That winglets work can be seen in the fact that commercial airlines—a notoriously penny-pinching bunch—have been buying new aircraft designed with winglets for the past decade. The Deltas and Americans of the world have also recently started retrofitting winglets to many of their older aircraft.

A combination of CFD studies and actual wind tunnel testing was conducted for both the C-130 and P-3. CFD was also used on the C-5. However, a large, high speed wind tunnel is required to determine winglet effectiveness accurately for that very large transport.

More than 400 variations of C-130 winglets were evaluated through CFD, and the most promising models were then tested extensively in the wind tunnel. The design was then optimized to minimize induced wing loading and to maximize aerodynamic benefits. The data collected during the wind tunnel testing verified the CFD predictions.

The optimal C-130 winglet design stands about five feet tall and adds about five feet to the 132-foot wingspan of a Herk. Predicted results show either about a four percent increase in range for a C-130J on a long-range cruise mission with a 17,250 pound payload or about a twenty-one gallon per hour reduction in fuel consumption on a typical 2,500 nautical mile mission with a 20,000 pound payload.

A set of trial winglets, made of composite material and aluminum, will be ready for flight test in late 2012. The Process Development Center, a Lockheed Martin rapid prototyping center at its Fort Worth, Texas, facility, built the prototypes.

“The microvanes, the lift distribution control system, and the winglets all provide benefits in different parts of the flight regime, but they offer significant net savings. Combined, they could result in saving fifty gallons of fuel per hour,” noted Hybart. “In terms of seeing these improvements on a C-130, I think the microvanes could be near-term. LDCS and winglets are probably a little farther out.”

P-3 And C-5 Improvements
Many of the same winglet designs tested on the C-130 were also evaluated for installation in the P-3 as the wing designs of the two aircraft are similar. The optimal design for the Orion winglet stands about three feet tall and adds about four feet to the aircraft’s wingspan. Preliminary CFD results indicate measurable drag reduction and a two to three percent increase in cruise performance and a roughly four to six percent increase in time on station or reduced fuel usage on a standard mission.

“The P-3 has some unique issues,” noted Hybart. “Many P-3s have a number of antennae and electronic support measure equipment in the wingtips. All of that gear would have to be fitted in a winglet. The winglet itself would also have to be made of material that the sensor apertures could see through.”

Two other improvements being looked at for the P-3 are the incorporation of a new propeller and an updated engine. The proposed propeller is the very aerodynamically efficient, eight-bladed, composite NP2000 prop now being fitted to the US Navy’s  E-2C/D Hawkeye airborne early warning and control aircraft.

The Rolls-Royce T56 Series 3.5 is an upgrade to existing engines, not a new engine. The Series 3.5 offers increased turbine life and better engine performance for hot day and high altitude operations. Most importantly, the upgrade has demonstrated up to nine percent savings in specific fuel consumption. The Series 3.5 engine enhancements are incorporated as part of a traditional engine overhaul and does not require any aircraft or engine control system interface modifications.

CFD results indicate that winglets would offer about a three percent increase in specific range for a C-5M Super Galaxy. Every one percent of fuel efficiency in a C-5M equates to about 1,750 gallons of fuel saved during a nonstop, unrefueled mission from Dover AFB, Delaware, to Incirlik AB, Turkey.

Winglets are estimated to be the single best way to improve C-5 fuel efficiency. However, winglets are the most expensive option. Engineering and building ten-foot-tall winglets without increasing aircraft wingspan—so the C-5 will still fit in many hangars—would be challenging. “With such a small fleet, the Air Force initially didn’t have a serious interest in winglets for the C-5. The perception was the payback would take too long,” observed Hybart.

However, the Air Force Research Laboratory will now sponsor wind tunnel testing of the C-5 winglets in the sixteen-foot transonic wind tunnel at Arnold Engineering Development Center at Arnold AFB, Tennessee. The Air Force is providing the test facility and Lockheed Martin is funding the update to the wind tunnel model. Testing is expected to begin later this year.

A combination of improvements in nine other areas on the C-5 offers nearly as much fuel savings as installing winglets. Some of the ideas are simple, such as not flying with the un-aerodynamic ground jack pads installed—something Air Force crews routinely do—or putting simple fairings around the pads to reduce drag.

The upper end of the cost spectrum would involve redesigning areas of the C-5 airframe prone to slow pressurization leaks. The engines have to work harder to compensate—increasing fuel usage—to keep the aircraft fully pressurized. Other areas of investigation include using the automatic LDCS to reduce drag—similar to what is proposed for the C-130, designing and installing new aerodynamic navigation lights; and designing and installing fairings that don’t interfere with the operation of the drag-inducing Large Aircraft Infrared Countermeasures system turrets and flare boxes.

“Payback on any of these fuel efficiency improvements is a complicated equation,” concluded Hybart. “It takes time to modify the aircraft, and, while being modified, it’s out of service. Fleet size is a big factor. How many flight hours an operator puts on those aircraft; what part of the flight regime those aircraft are flown in; and how much fuel costs all need to be taken into account. Higher-cost improvements obviously take longer to pay back. But the cost of buying a new aircraft is small compared to operations and support costs over that aircraft’s lifetime. Some tweaks to the aircraft will save a few percent in fuel use. Over time, that will have a big impact on life cycle costs.”

Jeff Rhodes is the associate editor of Code One.
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