The 223 Boeing C-17 airlifters flown by the Air Mobility Command (AMC) are the biggest fuel consumers in the US Air Force. Each oversized-cargo jet burns just over 21,000 lb (9,525kg) of fuel per flight hour. Despite declining combat commitments, the AMC Globemaster III fleet logged more than 162,000 hours in the last fiscal year. Hence, Air Force leadership sought ways to reduce fleet fuel costs. Over a five-phase, 13-month flight test program, US Air Force Research Laboratory (AFRL) quantified 1-1.5% fuel savings with aerodynamic finlets and microvanes stuck temporarily to the aft fuselage of a C-17.
Savings could restore higher C-17 cruising speeds or extend range and endurance, but the original program driver was budget. “In 2012 fuel prices were much higher than now, and we had war efforts in the Middle East,” explains AFRL senior aerospace engineer Dennis Carter. “We were burning a lot of fuel. The initial interest was to reduce the fuel burn for the Air Force – you could replace the wings or engines, but we were looking for something more economical.”
The 418th Flight Test Squadron of the 412th Test Wing, based at Edwards Flight Test Center, California, flew a C-17 with finlets, microvanes and less successful wing pylon and winglet fairings in various combinations. “The finlets were made of aluminum,” continues Carter. “We drilled out existing rivets on the aircraft and replaced them with the finlets.”
Lower-profile plastic microvanes were 3D printed and then bonded to ‘speed tape’ fixed to the aircraft.
The Globemaster III is simultaneously flown by the air forces of Australia, Canada, India, the UAE and the UK. AFRL has briefed AMC and the C-17 System Program Office (SPO) on the results of the test program. With final reports from the Test Wing and contractors Boeing
and Lockheed Martin, AMC can perform a business case analysis to retrofit the C-17 fleet.
Finlets and microvanes
AFRL’s drag reduction effort for the C-17 jet began with research on the Lockheed C-130 turboprop. Tail ramps on both airlifters impose an aerodynamic penalty. “On transport aircraft you have a large upsweep of the tail to permit loading of outsized cargo,” says Carter. “When the fuselage pushes through the air, it creates a large low-pressure zone, resulting in a very large vortex that creates drag. Both the microvanes and the finlets break up that large vortex into very small vortices that are not as strong.”
C-130 finlets made by Consulting Aviation Services in Kennesaw, Georgia, USA, were first tested on a civilian Hercules at Mojave, California, in 2011. Four large finlets were arrayed on each side of the aircraft cargo ramp.
The same technology acquired by Vortex Control Technologies (VCT) flew on a later model C-130H. VCT credits its C-130H finlets with 7% fuel savings worth another 200 nautical mile range, 45 minutes of additional endurance or 3,000 lb greater payload. Microvanes developed by Lockheed Martin Aeronautics Company in Marietta, Georgia, USA, are still being tested, but the successful C-130 drag solutions have yet to win production orders.
Interest from the Royal Canadian Air Force in the Hercules drag reduction effort nevertheless led to a cooperative agreement with AFRL on the C-17. A subsequent Technical Cooperation Panel included representatives of the Australian Defence Science and Technology Group and the UK Defence Science and Technology Laboratory. AFRL and the Canadian Department of National Defence split the cost of the flight testing, while Australian and British funding helped pay for computational fluid dynamics (CFD) studies.
The wide-bodied C-17 was designed in the 1980s. “There was a crude version of CFD at the time,” Carter notes. “Boeing had evolved its capabilities over the decades since, but they hadn’t looked at these particular devices.” Boeing engineers modeled the C-17 finlets while Lockheed Martin modeled the lower-profile microvanes. Carter recalls, “All the companies did some analytic computational studies before we went anywhere.”
A 2.6% scale C-17 model was tested with and without finlets in the Canadian National Research Council low-speed wind tunnel in Ottawa. In addition the C-17 finlets were tested in the University of Maryland low-speed wind tunnel to establish correlation with the computer modeling. The microvanes and wing/winglet fairings were too small to test in the wind tunnel. “We had to rely on the CFD results,” says Carter. “They came out very well, with good correlation between the computational and experimental results. We went to flight testing after we did that testing.”
CFD modeling helped optimize the shape, quantity and locations of the C-17 microvanes. Lockheed Martin Aeronautics Company led development with engineering by the Skunk Works organization in California. Detailed design was performed by Skunk Works, and fabrication was performed by a contract vendor.
The microvanes for the flight test program were 3D printed with selective laser sintering (SLS) and made of a glass-bead filled nylon, DuraForm GF. Additive manufacturing technologies like SLS are typically used for rapid prototyping of complex shapes or small-lot production without tooling. Lockheed Martin Skunk Works design engineer Bryan Jenkins explains, “Each microvane is unique due to its location on the aircraft. Machining microvanes would be an expensive and time-consuming process. By using additive manufacturing processes we were able to quickly create the parts needed for this installation.”
A Lockheed Martin test lab near Marietta performed coupon testing on the lightly loaded microvanes and bonded 3D-printed test samples to aluminum panels. The lab had tested
both epoxy and sealant bonding techniques during the C-130 microvane program. According to Jenkins, “The sealants performed better than the more brittle epoxy, and we selected
a sealant that is currently used on the F-22 canopy installation.”
Pull-off tests were conducted at various temperatures and included intentional bonding flaws. “The end results showed that we had a very high margin of safety compared with the actual loading expected during the flight envelope.”
Accurate placement of the finlets, microvanes and fairings was also essential to achieve the drag reductions modeled with CFD. “With the finlets, they could just go to the skin breaks and rivets and drill the holes out. That was very simple,” says Carter. “For the microvanes, there was a definite position reference on the aircraft. They used laser positioning to set the location and the angle for the microvanes and the fairings on the aircraft. They’re looking at alternative ways to do this if they go into fleet service.”
Constant altitude AND speed
AFRL and the 412th Test Wing planned the C-17 flight test series to characterize drag reduction combinations at two cruise speeds. The best cruise speed for the C-17 was originally Mach 0.77 at 35,000ft, but economic considerations led the Air Force to cut the prescribed speed fleet-wide to Mach 0.74 at 30,000ft. “Once we found out what the drag reduction was going to be,” says Carter, “we didn’t know if the service would be back up to Mach 0.77 or stay with the fuel savings at 0.74.”
The C-17 test aircraft was ballasted to its maximum peacetime take-off gross weight (585,000 lb/265,352kg) and flew constant speed and altitude profiles. “That’s how we calculated the drag reduction – by measuring fuel burn at a given speed and altitude with a given lift coefficient,” says Carter. “We got the accuracy required out of the baseline C-17 instrumentation.” Fuel flow was logged over three-minute periods throughout long flights. “That gave us the different center of lift for each hour. For each center of lift, that gave us our drag reduction through the lift range.”
Data collection flights launched from Edwards were routed over the Pacific. “We had discovered during the C-130 program that when flying over Edwards you got
a lot of thermals from the desert floor that put great variation in our results,” explains Carter.
Even with its drag reduction modifications, the C-17 test aircraft remained essentially uninstrumented. “We did put some accelerometers on the devices – the finlets and microvanes – to ensure they weren’t adding any untoward vibration into the fuselage structure that would reduce the fatigue life of the aircraft,” notes Carter. Pilots also looked for changes in handling qualities. “We kept the same pilots in the same aircraft for the entire test program. They were comfortable flying the C-17. When we made a change, they flew the same flight spectrum at slow speed, approach to stall, flaps up and down, to make sure there were no surprises in the handling qualities. They did that away from our normal test range areas. There were no problems. It flew just a like a C-17.”
The standard C-17 has one big strake on each side of its cargo ramp to disrupt drag-inducing flow. The flight test program took baseline measurements with and without the strakes, and then flew drag reduction devices in various combinations. “We tested two configurations of finlet,” says Carter. “First, we kept the standard strake and added three finlets above the strake on the tail of aircraft. We got over 1% drag reduction. We were not displeased with that.” The second configuration dispensed with the standard strake and used eight finlets per side to achieve a 1.5% drag reduction. Smaller microvanes were arrayed six per side and provided a 1% drag reduction. Fairings on the wing/engine pylon junctures were credited with just 0.1% drag reduction.
The flight test program included drops of cargo pallets and parachute mannequins. “We did a lot of cargo drops off the ramp,” says Carter. “We also dropped heavy and light mannequins out of the side doors to simulate the spectrum of jumper weights with their equipment. We found no problems.”
In total the C-17 drag reduction flight test program included 10 flights and 35 hours with six finlets per side, nine sorties and 34 hours with eight finlets per side, and eight sorties and 31 hours with microvanes. Another eight sorties and 18 flight hours tested the combined fairings. A business case analysis would have to balance fuel savings with modification costs.
Carter concludes, “The highest drag reduction would be with eight finlets per side. They gave an additional 50% improvement over the microvanes and the three finlets per side. They’re going to cost a little more, but getting about 1.5% drag reduction over the course of the life of the aircraft is undoubtedly going to pay it back.
It should be straightforward to put them on, and they should last forever.”
Frank Colucci is the author of an early book on the AH-64 Apache. Frank has written in depth about the rotorcraft industry and other aerospace and defense topics for more than 30 years.