Air University Review, July-August 1978
William G. Holder
The cost of new aircraft today is astronomical. Thus, it is not surprising that a dramatic modernization of the Air Force inventory is under way to utilize fully the potential of existing aircraft. Several aircraft have been structurally modified to extend their already lengthy careers. Others have been modified to carry new weapons on new weapon mounts.
There is also an interesting proposal being considered: the possibility of stretching the fuselage of an existing transport, greatly increasing its hauling capability. Along the same lines, there has been some consideration given to modifying commercial transports so they can be converted to military cargo hauling in an emergency. Basically, the evolving inventory can be characterized by three distinct changes: in the wings, weapons, and fuselages. The changes and modifications are as different from one another as night and day, but they all have the same intent--get as much as possible for as long as possible.
Wings do many things. Their first purpose, of course, is to provide aerodynamic lift to get the aircraft off the ground; but they also carry heavy engines, bombs, and fuel pods. Still others carry undersigned-for external ordnance. One other common thing these all important wings do is get tired and fatigued and many times they develop dangerous cracks. Sometimes it happens much sooner than predicted. The Air Force has examples of each type. The first is the venerable 15- to 20-year-old B-52 and, surprisingly the relatively new C-5A.
The oldest of the giant eight-engined bombers still flying, the B-52D, was one of the most numerous of the giant Stratofortresses built. Extensively used in Vietnam, many of these late 1950s versions were sporting well over10,000 flight hours although they were originally designed for only half that number. In 1971, the planned phase-out year for the D-birds, the Air Force decided that the aircraft would be needed much longer. But something had to be done--and in a hurry, since the aircraft were showing their age, large structural cracks appearing with increasing regularity. So it was decided that a portion of the B-52D fleet would get a sizable face lift.
Starting in the mid-1970s, 80 of the D-birds were extensively modified by Boeing, the last modifications completed in February of 1977. These modifications entailed the scrapping of about 15 tons of parts from each aircraft. Then new leading edges and stiffeners were provided. wings were partially reskinned, wing pylons modified, and the fuselage partly reskinned at the wing root. The program, called Pacer Plank, resulted in a price tag of $2.6 million per copy--a large percent of the original cost of the aircraft when it was built two decades earlier.
In the modification process, Boeing removed the wings from the fuselage and worked on the fuselage and wings separately. Following modification, wings and fuselage were rejoined, tested, and sent on their way.
A surprising phenomenon resulted from the Pacer Plank modifications. The new wing skin is much cleaner aerodynamically than the skin it replaced, resulting in considerably less drag. Even though the modified plane weighs about 3400 pounds more, its cruise range has been increased by three percent. After three decades, this aircraft is getting older, but she is also getting better.
The later G and H models are also undergoing structural changes, although not as extensive as those on the B-52D* Not to be forgotten are the 615 KC-135 tankers that are also undergoing wing modifications to stretch their operational lifetime into the 1990s.
* "USAF is considering the use of winglets on its Boeing B-52Gs. The modification would not only enhance aerodynamic performance of the bomber, earmarked as the air-launched cruise missile carrier, but also would provide distinctive markings…of those B-52G models used for cruise missiles and thus avoid having all B-52s counted as potential carriers of this weapon." (Aviation Week & Space Technology, March 6,1978, p. 9).
But wing problems certainly have not been restricted to 20-year-old airplanes. The relatively new C-5 transport fleet is also having its problems. The first indication of C-5 wing structural deficiencies occurred in July1969, when the wing root failed in static test at 126 percent of design limit load. Then, in January1970, cracks in the rear beam cap of the third C-5A built were discovered.
The Air Force Scientific Advisory Board (SAB) was convened in February 1970, to review the C-5 structural failures and plan corrective actions. However, by this time, the fortieth airplane was in final assembly, and major wing parts were already machined for the sixtieth article. It was clearly impractical to incorporate in production a major redesign for improved fatigue life.
So, in 1975, the Air Force awarded a $28,454,000 contract to Lockheed for design of a modification to the C-5A wing. The wing modification involved redesign of the center, inner, and outer wing boxes. The existing leading and trailing edge structures, as well as all wing subsystems, were retained. It was hoped that these changes, coupled with the use of an improved wing material with better fracture toughness characteristics, would ensure a long life wing. The design contract awarded in 1975 was the first phase in a four-phase program and covered the necessary engineering analyses and tests to define the modification details.
The second phase of the modification program called for fabrication of two kits of new wing sections and associated hardware. A C-5 fuselage (ground test article) outfitted with one "kit" would receive extensive fatigue testing. The second wing kit will be installed on a selected C-5A aircraft and flight tested at the Air Force Flight Test Center in the fall of 1980. If approved Phase III, the production of wing modification kits, would be initiated in early 1980. Installation of the wing modification on the remaining C-5 force (76 aircraft) would begin in early 1982 and would constitute Phase IV of the program. The aircraft would be modified at a rate of 1.5 per month, with a downtime of eight months.
As part of Phase IV, an Active Lift Distribution Control System will be reinstalled on the C-5 wing as a fuel conservation measure. It is estimated that this system, which automatically reduces wing loads in flight will save six million gallons of fuel per aircraft over a period of 30 years, based on current usage rates.
Also, currently being proposed is a modification to the A-7 D wing. The Vought Corporation (a subsidiary of the LTV Corporation) has a two-aircraft test program- to test the feasibility of the concept which, unlike the previous examples, is not motivated by structural deficiencies. The concept consists of a maneuvering flap that has been tested as part of a continuing program to improve the close air support and search and rescue capabilities of the A-7D. It offers an effective means of increasing turn performance and high medium speeds.
The maneuvering flap concept involves programming leading and trailing edge flaps to extend or retract automatically to configure the wing for best aerodynamic performance during widely varying maneuver conditions. Mechanizing the maneuvering flap on the A-7D is rather easy since it already has the essential element--a well-designed flap system. Installing a control box activated by the flap position handle to automatically position the flaps in the maneuver mode completes the basic package.
Development tests to date indicate that the maneuvering flap will substantially improve maneuvering performance of the A-7D at low and intermediate speeds. Also, the airplane can be flown to a five-degree-higher angle of attack without stall departure. Sustained turns with maneuvering flaps are buffet-free up to maximum g. In fact, sustained turn capability with the flap is greater than the instantaneous turn capability of this basic airplane over most of the flap flight envelope. The concept will be incorporated by the Air -Force in their A-7s.
Vought is also examining the effects of a new outerA-70 wing constructed of graphite and boron. The advantages of these composites, depending on the structure to be replaced, are lighter weight and potentially lower costs as compared to the riveted aluminum structures that almost all the world's aircraft rely on today. The lighter weight can provide longer range and bigger payloads with less fuel consumption.
After additional flights by company test pilots prove the wing to be fully flightworthy, the contract calls for 12 other such wings to be delivered to the Air Force. The Air Force expects to give the wings service testing by substituting one composite wing on each of 12 different A-7D aircraft. The opposite wing structure on each aircraft will be of conventional aluminum construction.
The B-52G and H models, originally designed as high-attitude nuclear bombers, have had their mission horizons broadened considerably. These late model B-52s, as we have seen, have already received a number of modifications.
Quick-start packages were added that enable all eight engines to be started simultaneously. The so-called electro-optical viewing system (EVS) was a much more extensive change in that it involved structural changes and more costly hardware. The system consists of a television system and a forward-looking infrared imaging system. The sensors are housed in two chin blisters, which required modification to the aircraft's nose. The system is tailored to counteract certain types tracking radars.
But the biggest change to the G/H birds is their ability to carry the short-range attack missile (SRAM) and, eventually, the air-launched cruise missile (ALCM). The missiles are launched from external pylons and a rotary launcher mounted on hardpoints in the aft part of the bomb bay. The eight-missile launcher can handle both of the radically different missiles.
The total additional weight for the missile-launching aircraft modification is well over five tons. A full B-52 load of SRAM missiles--20 in all--weighs an additional 44,000 pounds. The old birds just ain't what they used to be!
Stretching a fuselage for greater payload volume is not a new idea. The DC-8 commercial airliner is a good example of the technique, and that was done a number of years ago. But the DC-8 modification was accomplished as a new aircraft. The Air Force is planning a stretch, but it is going to be done to an already operational aircraft, the C-141 StarLifter. The modification will greatly enhance the aircraft's carrying capability and can be done at a fraction of the cost of a new transport. -The prototype flew in March1977.
In addition to the 23.3 -foot stretch in the C-141 fuselage (accomplished by a 160-inch plug forward of the wing and a 120-inch plug aft of the wing), aerial refueling capabilities have been installed, and improved wing fairings have been added. Aerial refueling will greatly improve the range capabilities of the new model plane and add to the operational command options; among other things it will enable mission completion without landing on foreign bases, should this be desirable.
New, improved wing fairings result in two benefits: (1) they decrease the aerodynamic drag, enabling higher speeds and lower fuel consumption; and (2) they change the wing lift distribution so that the aircraft weight may be increased, with greater payloads, without affecting the wing's fatigue life.
The stretched fuselage means 233 square feet of added cargo floor space and more than 2100 cubic feet of added volume (meaning the ability to carry three additional 463L pallets for a total of 13), giving a total of 8630 cubic feet of clear cargo compartment volume.
Spelled out, the stretch program will increase the productivity of each StarLifter by 33 to 45 percent--a tremendous saving of taxpayer dollars.
A recent program could produce the strangest looking update yet to the existing fleet. It is strictly a program to fight the same energy problem that we are all fighting with our homes and cars--namely, the energy problem. With the spiraling costs of jet fuel, any attempt at fuel conservation receives immediate attention. With that thought in mind, USAF and NASA recently undertook a joint program to demonstrate a set of winglets on a KC-135, which hopefully wilt effect an overall drag reduction of about eight percent. The winglets will be attached to the wingtips of the test KC-135, be made of aluminum, and weigh almost 300 pounds apiece. The KC-135 was chosen as the test aircraft because its wings can be easily modified, and its test results can be applied to other transport-class aircraft. However, should the results confirms the engineering predictions, there just might be a fleet of dog-eared KC-135s in the future inventory.
The contract has now been let to Lockheed-Georgia for the C-141 StarLifter stretch program and actual work to begin in September. The modification program should be completed by July 1982.
William G. Holder (B.S., Purdue University) has been an Aerospace Engineer for the last 12 years at Hq Foreign Technology Division, Air Force Systems Command, Wright-Patterson AFB, Ohio. He has previously worked for the Boeing Company on the Bomarc and Saturn V programs and served as an air defense guided missile officer in the U. S. Army. Mr. Holder is an active free-lance writer, with five books and numerous articles to his credit, and a regular contributor to Air University Review.
The conclusions and opinions expressed in this document are those of the author cultivated in the freedom of expression, academic environment of Air University. They do not reflect the official position of the U.S. Government, Department of Defense, the United States Air Force or the Air University.
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