Invisible Strength: Forgings' Hidden Value in Military Aircraft
Often hidden beneath the skins of military aircraft, forgings were nonetheless widely used in the engines, ordnance casings and landing-gear assemblies of historic military aircraft, just as they are at the present time.
The National Museum of the United States Air Force in Dayton, Ohio, offers a unique opportunity to see the “invisible strength” (and durability and toughness) of forgings in this historic fleet of aircraft. Other than landing-gear components, forged components are typically shrouded behind the smooth, slick skins of high-performance aircraft. Despite their lack of visibility, forgings provide a unique combination of mechanical properties, typically unrivaled by other manufacturing processes. During a recent walking tour of the museum’s impressive and extensive collection of aircraft and subsystems in the “Birthplace of Aviation,” one can readily see this invisible strength.
The early days of aviation reveal little of forgings in aircraft designed and built with wood, wire and canvas. Forgings of the era are typically found in engines as crankshafts, gears and connecting rods and perhaps fasteners. The Curtis JN 4 (nicknamed the Jenny) included many forgings (per the history of Wyman Gordon), especially in its 90-horsepower engine.
It wasn’t until the early 1920s that forgings became more visible as aluminum or steel propellers, which replaced wooden blades. As the size of aircraft and propulsion requirements increased, wood propellers could no longer withstand the service requirements, pushing engine builders and aircraft designers to forged blades. Admittedly, since the author did not take a picture of this class of structures, he jumped forward in time from the “19 Teens and 19 Twenties” to World War II, from the United States Army Air Corps to the German Luftwaffe.
Messerschmidt Bf 109G-10
Interestingly, the limits of the Treaty of Versailles resulted in forging advances in pre-World War II Germany. As part of that treaty, Germany retained its hammer-forging capability while having access to plentiful sources of magnesium. With the limited slip planes of magnesium, hammering magnesium into shape resulted in cracked components. To overcome this technical issue, German forgers turned to hydraulic forging presses to fabricate parts with slower deformation rates to eliminate part cracking.
With this technological innovation, aircraft designers called out forged magnesium in many components, which clearly reduced weight for a variety of aircraft. The design community also cleverly used common part design, which resulted in part families seen across these aircraft. The engine bearer shown on this Messerschmidt epitomized the application of forged magnesium in German aircraft.
As World War II unfolded and Allied forces engaged the Luftwaffe, downed aircraft were examined to determine enabling design features. The Allies noted the extensive use of magnesium, particularly forgings. Further intelligence indicated the focus of magnesium research in Bitterfeld, Germany. At the war’s conclusion, American and Russian manufacturing engineers raced to Bitterfeld to assess and acquire this technology. Upon arrival at the site, the Allies discovered several large forging presses.
Recognizing that Bitterfeld, on the east side of Germany, was closer to Russia, American analysts were second to arrive. Consequently, a significant percentage of the capability was acquired as “war prizes,” dismantled and shipped east to Russia. Details are not fully known, but the Russians acquired some presses and parts and presumably designs for a 33,000-ton press. The Americans acquired at least two 16,000-ton presses that apparently landed in Adrian, Mich.
Understanding that larger forging presses could produce bigger forgings, competition unfolded between the Americans and Russians, namely the race to build the world’s largest hydraulic presses. These presses enabled aircraft designers to conceive and build large, weight-efficient, monolithic structures to further enhance aircraft performance. The early entrants to this race included presses commissioned by the United States Air Force in what is now Arconic in Cleveland, Ohio, and PCC Wyman Gordon in North Grafton, Mass. It is interesting to note that competition exists today because large presses are still being designed, built and commissioned in the U.S., Japan and China.
In retrospect, the race for larger and larger forged components was started by magnesium forgings hidden under the engine cowling of a German fighter.
B-52 Landing Gear
Fortunately, not all forgings are hidden from view, as illustrated by landing-gear systems. Although in plain sight, the landing gear is probably the most underappreciated but overly burdened substructure of aircraft. Imagine having to operate 24 hours a day in cyclic conditions. For example, loads range from static loads bearing the complete weight of the aircraft while awaiting the next mission; then dynamic loads of taxiing and take-off, which are relieved upon retraction into the wheel wells only to be exposed to extreme cold at altitudes; and then the resumption of the dynamic loads upon landing and taxiing only to repeat the cycle again and again and again.
To compound matters, expose the subsystem to the elements, especially precipitation and even worse, a salt fog. And of course, weight and cost parameters are always on the buyer’s mind, while the designer needs to fold and unfold landing gear repeatedly and reliably. To meet this combination of requirements, landing-gear designers depend on forgings.
Landing gear – nose or main – are complex systems that require lots of forged parts. The language of landing gear is replete with unique names: main fitting, bogie beam, trailing arm, axle, torque links, drag and side braces, etc. Despite the names, many of these components are forged to exploit the combination of strength, toughness and durability of forgings in one of the most demanding aircraft subsystems.
Mounted on pedestals at the museum, visitors can view the main landing gear of the Cold War Era B-52, seeing all of the forged components of the extended landing-gear system. The visitor can imagine the complexity of the system when it is retracted and folded compactly into the landing-gear well upon take-off. Elsewhere, in the Museum’s gallery, the visitor sees another Cold War aircraft – the B-1B.
During the production of the B-1B in the early 1980s, Bruce Zelus of Rockwell International Corporation’s North American Aircraft Operations reported on the “Application of Precision Forgings and Cost Effectiveness” on this supersonic bomber. During the research and development phase of the B-1A (four aircraft), most parts were machined from bar, billet and plate. With a firm fixed-price contract to produce 100 B-1Bs, the economics pushed forgings into a favorable and competitive position.
After extensive trade studies involving 8,300 drawings, the company converted over 1,430 machined components to precision forgings and another 280 components to conventional forgings. Forty-two percent of the aircraft was comprised of aluminum, and, of the wrought alloys selected, 7075-T73 represented many of the conversions. Where added strength was required, 7075/7175-T736 was used. In few instances, where high fracture toughness was required, a variation of 7075/7175-T736 with a high fracture toughness (HFT) was designated. A few high-temperature applications required 2024-T6 as well.
In the end, the aerospace forging industry rallied to address these rapid, cost-effective requirements. In addition, the Air Force sponsored a Technology Modernization Program to increase the plan-view area of precision forgings to 600 square inches. This increased the manufacturing capability of the U.S., which was also required to supply parts for F-16s and F-15s.
Looking back on the B-1B program it is clear to see that the economics of forging played a significant role in this critical leg of the U.S. nuclear triad, which includes aging B-52s, ground-based missiles and submarine-based missiles. As an added benefit, the forgings provided the metallurgy of grain flow, fatigue resistance and fracture toughness, which is truly appreciated for an aircraft that is now 35 years old and serving the nation around the world.
In the end, the metallurgy of forging, offering higher design flexibilty, came along for the ride, which was financed by the cost savings of large economic production quantities. Proving our point, these forgings remain invisible behind the sleek features of the B-1B.
Bomb Cases and Bomb Lugs
Although clearly visible while hanging off a wing of an attack aircraft or less visible in the bomb bay of a B-52, B-1 or B-2, the metallurgical beauty of the Mk80 series of bombs lies in the cases wrought from steel tube to encase explosives and the forged-steel lugs that attach the weapon securely and safely to a plane. The steel bomb cases can be “tricked out” to function for a variety of delivery methods, ranging from a “high drag” (parachute attached) option to a “slick” (sans-parachute) option.
Still, the general-purpose bomb can be supplemented with other features for guided and even precision-guided targeting, which reduces collateral damage. In recent years, in an attempt to create an alternate, competing supply chain and to augment the wrought industrial base of bomb cases, cast ductile iron surfaced as an alternate process and material. Searching recent procurements by the federal government, however, wrought steel appears to be the current process and material of choice.
Since a bomb without an attachment lug is relatively useless, parallel procurements of forged-steel lugs are also required. Simple in design but demanding in application, the bomb suspension lug is engineered for safety. Currently designated as “MS3314 and MK3-0 Suspension Lugs, Aircraft Ordnance,” these components are deemed safety-critical items. Failure of these forged 4340 steel lugs can result in damage or destruction of aircraft and injury or death of aircrews in flight or innocent people on the ground.
These relatively small components, which can fit in the pilot’s hand, are produced by the thousands in any given year to match the parallel procurement of wrought bomb cases. At one time,
the business of forging bomb lugs was so attractive even a domestic manufacturer of motorcycles supplied lugs! Regardless of the manufacturer, the small suspension lug plays a vital role by offering strength and toughness in attaching ordnance to a plane until it is ready for delivery to a target.
Clearly, the forging community is better equipped to illustrate the power of forged products despite the lack of visibility of forgings. Competition to forgings – casting, machining, composites, additive manufacturing – increases every day, but forgings provide an unseen and unappreciated combination of geometry, size and mechanical properties, hence acquiring the tagline of “invisible strength.” Forgers are encouraged to reveal these attributes of forging to their customers as they design and build future generations of high-performance aircraft.
Readers of FORGE are encouraged to visit the National Museum of the United States Air Force in Dayton, Ohio, to see the invisible strength of forgings through the decades of aviation history. The author is grateful to the Defense Logistics Agency for supporting his work as the Procurement Readiness Optimization – Forging Advanced Systems and Technologies (PRO-FAST) Program Manager from 2001 until 2017. In June 2017, the author led a small group of technologists on a tour of the museum.