Take a look out the window the next time you’re on an airliner, and there’s a very good chance you’ll see winglets – the typically upward-facing wingtip devices on the ends of the wings that reduce efficiency by reducing them to improve air resistance. In addition to being common in new designs, winglets have been retrofitted on a variety of older aircraft such as the ubiquitous Boeing 737 series to improve performance and reduce fuel costs. One of the guys that not have them is the US Air Force’s hardworking fleet of KC-135 Stratotanker aerial tankers and their many derivatives. But the reality is that winglets became tested on the KC-135. In fact, the type played an important role in developing the concept. The results have been overwhelming. So it’s crazy to understand that the KC-135 never got winglets decades ago.
Much of the propulsion that resulted in a KC-135 flying with winglets – as early as 1979, long before they became commonplace – was the energy crisis of the same decade. The effects of the Yom Kippur War resulted in disruptions in exports of oil from the Middle East and western countries began to feel the crisis, with a further deterioration in the situation in 1979 triggered by the Iranian Revolution and the events that followed. As the crisis drove up fuel prices, the effects were felt strongly by the gas-hungry Air Force.
At NASA’s Langley Research Center, Dr. Richard T. Whitcomb, meanwhile, developed the winglet concept, noting that its addition to a wing tip reduced drag while saving fuel. Interestingly, in the 1950s, Whitcomb was also responsible for developing the area-dominated hull of the “Coke Bottle” before adopting the supercritical wing Today, this is another efficiency-enhancing feature of many jet transports. As such, his name certainly deserves to be better known.
After the successful wind tunnel tests of the Whitcom winglets in Langley, a Memorandum of Understanding was signed in 1976 for a joint development program between NASA and the Air Force. In June of the following year, Boeing won a $ 3 million contract to develop, install, and then test a set of winglets on a KC-135. At this point, the wind tunnel tests showed that the modification should reduce the total drag by 8 percent while cruising. Because of this improvement, the Air Force should reduce annual fuel requirements by an incredible amount 37 million gallons across the entire fleet.
Work on the winglets continued for most of the first half of 1978, and that same year the Air Force’s NKC-135A test stand, serial number 55-3129, had been handed over to NASA at Edwards Air Force Base, California. for the modifications.
By May 1979, it was time to install the winglets by converting the wing tips of another KC-135A, serial number 55-3126, a former one VIP transport with Speckled Trout. Each winglet weighed about 150 pounds, was about eight feet tall, and could be adjusted to various angles of incidence to collect a variety of test data. A long boom of air data meters was attached to the nose, completing the new look of the 55-3129 and providing real-time flight test data presented on a CRT display in the cockpit.
Between July 1979 and January 1981, the winglet-equipped 55-3129 completed 39 flights and over 170 hours of flight in this configuration, resulting in a fuel saving of 6.5 percent during normal flight operations of the aircraft, representing nearly 45 million gallons per year across the aircraft corresponds to the fleet. At the same time, the newly configured aircraft also showed improved take-off performance and fuel discharge capability.
Despite the obvious promise of the winglets, the Air Force was not interested in the modification and instead opted for an alternative route to save fuel. This was the Fuel Savings Advisory / Cockpit Avionics System (FSA / CAS), a flight management system based on a new computer.
Compared to the winglets, the benefits of FSA / CAS were disappointing – only 2 percent fuel savings, and this was only achieved by the KC-135 flying a specially tailored airliner-style flight profile that little with the Stratotanker’s normal missions. On the positive side, the fleet-wide installed FSA / CAS update brought with it the MIL STD-1553 data bus, which facilitates the introduction of future cockpit upgrades.
55-3129, the aircraft had lost its distinctive winglets by the end of 1981 and was returned to the Air Force for various other test tasks.
Robert S. Hopkins III, who flew 17 different variants of KC-135 tankers, EC-135 air command posts and RC-135 reconnaissance aircraft during his Air Force career, and now writes in detail about this and other aircraft, called back on The war zone that NASA, Air Force Logistics Command (AFLC), and Air Force Systems Command (AFSC) were “all for the winglets”. They consistently reduced air resistance across the entire flight spectrum and thus saved fuel from take-off to landing. “
“Strategic Air Command (SAC), however, was less interested in fuel savings per se than in the future computerized expandability of the KC-135 and chose the FSA / CAS,” said Hopkins. “Unfortunately, this was built on software that mimicked an airliner step-ascent profile, which was completely irrelevant to the KC-135 mission profile of take-off, in-flight refueling, navigation section (all at constant altitude), and landing exercises. The fuel savings were actually less than the 6% promised, an average of 2%, and the damn thing never worked. “
“Programming during preflight was a nightmare,” says Hopkins of the FSA / CAS. “It refused to accept the SAC flight profile as ‘invalid’. So it was like being told ‘If your internet service is down, please email us’. It couldn’t be turned off, so I turned the brightness to full dimming so I wouldn’t look at the damn thing. “
If the Strategic Air Command had known in 1981 that 40 years later the KC-135 fleet would still be the most important part of the Air Force’s tanker fleet, it would be impossible to believe that the savings achieved by a winglet fleet of aircraft the Stratotanker family would not have found broader support.
In this case, the Stratotanker family underwent various other important modifications, notably the replacement of the older aircraft’s jet engines, four Pratt & Whitney J57s, with an equal number of more powerful and efficient CFM56s.
Other upgrades since then have included Pacer Crag, which replaced various parts of the cockpit and fuel management system between 1997 and 2001.
With the Air Force now trying to get at least a portion of its fleet of nearly 400 KC-135Rs to fly by fiscal 2040 and possibly a decade after that, the fuel cost savings after the winglets were introduced many decades ago would have been absolutely massive, to say the least.
Also, the fleet of RC-135 intelligence reconnaissance aircraft is still busy monitoring potential hotspots around the globe and would have similarly benefited from the added efficiency that winglet modifications provide. However, not all of these aircraft may have been able to receive the winglets because sensors and antennas are placed around the airframe.
In addition to Block 45’s cockpit upgrades, potentially designed to keep the KC-135R operational through 2050, the Air Force is also exploring other capabilities for the venerable Stratotanker, including serving as a communications hub for other platforms. With tankers and other means of support to enemy air defense in view, the Air Force is also increasingly looking for ways to protect their KC-135s as well, including one Laser defense system as well as loyal wingman drones.
Meanwhile, the almost-antique C-135 family is as busy as ever, and the introduction of the new KC-46A Pegasus to the air-refueling community has been painful to say the least. With this in mind, it seems that the Air Force missed a huge opportunity when it got the chance to equip these aircraft with winglets in the early 1980s. The modifications would have paid for themselves many times over, and then a lot more.
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