Giving F-16 Wings a Lift
An SwRI-developed tool helps crews remove and re-attach fighter’s wings quickly and precisely
By Robert Johannesson
A new U.S. Air Force program, known as Falcon Star, requires every F-16 in the fleet to undergo a series of maintenance operations. One major operation involves replacing the wing attachment fittings that are used to mount the wings to the fuselage.
The current wing removal and installation process uses a mobile crane and a three-point sling to lift and support the wing from overhead when it is detached. The sling arrangement is rigid in that the lengths of the cables are fixed, but the suspended wing swings freely and must be maneuvered into place by moving the crane.
A team of Southwest Research Institute (SwRI) engineers and Hill Air Force Base engineers and mechanics from the Ogden Air Logistics Center (OO-ALC) has developed a new, mechanized system of suspending and transporting the wings that not only eliminates the need for a crane, but also saves time and personnel during the attachment and removal procedures.
Limitations of Existing System
A mobile crane can make only relatively coarse positioning adjustments. This makes aligning the wing’s bolt holes with those on the fuselage a tedious process that can involve a great deal of time and, sometimes, the use of brute force. Damage can occur if a mechanic attempts to force the bolts into their respective holes when the holes are not properly aligned. Because the wing attachment fittings are fracture-critical parts, it is important to minimize the possibility of damage to the inside of the bolt holes during installation.
Moreover, when the leading edge flap assembly is left installed on the wing during attachment, the flap’s weight can cause the wing to tilt, contributing to misalignment of the bolt holes. Sometimes the mechanics literally hang on the wing tip to create proper alignment. Overall, using a crane and sling for wing installation is a time-consuming and potentially damage-inducing process.
Time is another critical factor because a fast-drying sealant must be applied to the wing root prior to installation. Once the sealant is applied, the wing must be installed within two hours. If that time frame is not met, the sealant must be removed and the process repeated. This occurs approximately 25 percent of the time with the current process, according to an Air Force estimate. Additionally, the cost of a crane and operator adds to the overall cost of wing installation and removal, and workflow often is impeded by the need to wait for a crane to become available.
SwRI engineers responded to a request from engineers and mechanics at Hill Air Force Base, Utah, for a process analysis study and a conceptual design for a new wing manipulation tool that would eliminate the need for a crane to remove and install F-16 wings.
Designing for Efficiency
The SwRI and Hill AFB team performed a cost analysis based on data from time studies of the existing processes. The study showed that using a wing manipulation tool would yield a 67 percent reduction in the process flow time for wing installation and removal. The data also were compiled and calculated into labor hours, then into dollars using fully burdened labor rates. The existing process was then compared to a new, hypothetical process that would use the conceptualized wing manipulation tool.
This study showed that over the life of the Falcon Star program, a new tool would save nearly $1.4 million, for a rate of return of 33.85 percent on the estimated $457,000 cost of two wing manipulation tools. The tools would pay for themselves in three years.
The above data applied only to the Falcon Star program. Additional cost savings could be realized if the tool were used for other programs requiring wing removal. Under that scenario, the projected rate of return climbed to 63 percent with a 1.4-year return on investment.
With input from Hill AFB and associated personnel, the SwRI team designed a manipulator tool that would feature electrically powered movement in two axes, fine positioning capability using hand cranks, high maneuverability and fail-safe operation. The team considered factors such as controllability, cost, manpower, operator skill level, process improvements and technical risk in the design phase.
The final design incorporated several client requirements, including six degrees of freedom, four of which are manual hand-crank driven and two of which are motorized. The motorized axes are for travel vertically and also horizontally in-and-out to make wing removal easier. When the wing is being removed, the old sealant must be removed and the wing must be pried away from the fuselage. By motorizing this axis, the fixture can put some tension on the wing, making it easier to remove the sealant. The electric motors, with variable-speed drives, operate on 120 volts, allowing the fixture to be used almost anywhere in a facility that has standard electrical plugs.
The wing manipulation tool is mounted on an aluminum transport frame with shock-absorbing castors, forklift slots, a tow bar, an electrical cable reel for power outlets and a pneumatic hose reel for serving air-powered tools. The tow bar locks into two upright positions for manual positioning. The frame also features two crossbars that support the weight of the wing, about 1,100 pounds with flaps intact or 900 pounds with the flaps removed, when it is lowered for towing.
The frame also includes retractable outriggers that stabilize it during lifting operations. The outriggers have sensors to indicate when they are bearing weight, and indicator lights on the main control enclosure indicate the outrigger status. The sensors are connected to an interlock so that the wing can be raised only if the outriggers are deployed.
The frame supports a custom electrical skillet-lift that is often used in automotive assembly lines to lift the vehicle chassis during the assembly process. The lift is operated with a three-speed control pendant that allows coarse and fine positioning. The lift supports a rotation stage that has hand cranks for fine positioning and a release lever that allows the wing to be freely rotated 90 degrees left or right between the towing position and the wing installation and removal position. The rotation stage supports an X-Y translation stage that allows manual positioning of the wing fore and aft along the fuselage, and powered positioning inboard and outboard from the fuselage. Mounted to the top of the translation stage is a “wrist” that allows the wing to be pitched and rolled. The wrist assembly incorporates a unique crankshaft design and two 3-ton screwjacks to allow mechanics to easily position the wing with fine motion control. Once removed, a wing can be secured by straps built into the frame and then towed to where the replacement wing attachment fittings are installed and machined.
The wing manipulation tool is designed to accommodate both left and right wings through the use of an innovative locking ball-and-socket joint based on a NASA design. The ball portion of the mechanism resembles a trailer hitch ball. Three balls are mounted on a T-shaped bar that can be attached to the existing ordnance hard-points on the wing bottom. The sockets are of a custom design, mounted to a Y-shaped structure that lines up with the hitch balls on the T-bars.
When the tool is driven upward and into contact with the hitch balls, the sockets lock around them automatically. Load cells mounted beneath each socket measure the socket load, and an integrated meter displays individual socket loads as well as the total load distributed across the three sockets. The meter also has relay interlocks to prevent scissor-lift operation if a socket is overloaded. This prevents the lift from putting undue stress on the wing while still attached to the fuselage.
Even with the overload protection features in place, the wing tool was designed to handle a load of two people working on top of the wing within four feet from where the wing attaches to the fuselage. Finite element analysis verified that key components could function fully under extreme loading conditions and SolidWorks™ software executed the mechanical design.
At Hill AFB, the tool successfully removed and installed several wings in a fraction of the time needed with the crane and sling. The new tool supports the wing so accurately that when the final bolt is removed from the fuselage and the sealant is removed, the wing stays perfectly stationary. With the sling method, the crane operator had no feedback to indicate how much of the wing’s weight was being supported during removal, and when the last bolt was removed, the wing would often pinch the bolt in the hole, causing potential damage to fracture-critical parts.
For wing installation, the tool allowed alignment of the wing to the fuselage so accurately that the attachment bolts could be pushed through the holes with very little effort. In fact, the wing may now be dry-fitted and aligned to the fuselage prior to applying sealant. This eliminates another common problem of the sling method, involving the creation of fuel leaks due to smearing of freshly applied sealant during wing installation. In addition to the above process improvements, quality and safety have been improved by removing the crane from the process.
Early indications are that the SwRI-developed wing installation and removal tool has provided improvements in time, precision, labor and safety compared to the existing method. Once the evaluation of the initial tool has been completed, additional units can be ordered from independent manufacturers using design documents supplied by SwRI under terms of the project. With minor modifications, the system also could be applied to aircraft other than the F-16, increasing the utility of the new manipulation tool for both military and civilian applications.
Published in the Summer 2005 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.