I2 Motor Used to Test Critical KC-46 Component

January 14, 2014

Early in 2013 engineers of Federal Industries Inc., El Segundo, CA were tasked with developing a test protocol and supporting hardware for a complex planetary gear box that was at the heart of the centerline hose and drogue inflight refueling (IFR) system chosen for the new USAF tanker aircraft. As a subcontractor to Sargent Fletcher Inc., the primary contractor for the IFR system, Federal Industries had teamed with FO Engineering of Valencia, CA to design and build an actuation system that brings major improvements in safety, reliability and maintainability to the difficult and dangerous task of refueling aircraft in flight. The gear box, shown in figure 1 is the key link between the radically improved variable displacement hydraulic actuation system and the hose reel that carries the inflight refueling hardware.

Figure 1: Planetary Gear Box

The gear design is extremely robust and precise. It must have minimal hysteresis and backlash in order to accurately control drogue position while withstanding high forces from hose extension and retraction and high amplitude hose oscillations from multiple aircraft engagements with the drogue. Run in and testing of the gear box after initial assembly requires a complex profile to properly seat and mesh the gears. After looking at various alternatives, project engineers decided the optimum solution for running this profile was the I2 Motor made by Specialty Motors of Valencia, CA. The compact, integrated architecture of the I2 Motor greatly simplified the task, reduced design time and cut development cost. The I2 Motor is a completely integrated servo system that incorporates a brushless DC motor, power supply, amplifier, programmable inputs and outputs and drives in one compact package. This meant that no other components, connections or wires were required to control or run the test. Integrating the test stand motion control system only required mounting the motor on the test stand and plugging it in. This supported the design of the simple, low cost test stand shown in figure 2.

Figure 2. Gear Box Test Stand

The run-in and test program required 19 different commands with a unique speed, direction, acceleration, deceleration and run time for each. It was programmed in just a few minutes with a standard PC using the indexing feature of the I2 “Wizard” host software . The total profile, including all commands is shown in figure 3;

Figure 3. Programming the I2 Motor

Once the set up was completed, the profile was downloaded to the motor controller and the PC was no longer required to run the test, although it could be used if desired. Control of the I2 Motor at the test stand is accomplished with the switches on the Integration Board shown in figure 2. This eliminates the need to take a computer into the shop environment.

The importance of the gear box is illustrated in the following block diagram.

Figure 4. Variable Displacement Drive System

The gear box, shown as item 31 in figure 4, directly controls extension, retraction and relative position of the IFR hose (item 22) and drogue (item 24). The drive system incorporates numerous advantages over current IFR control systems that rely on 40 year old technology based on mechanical linkages and microswitches. Solid state electronics replace old style components and result in a system that is lighter, more reliable and easier to maintain.

Perhaps the biggest improvement with the new system is in safety. Flying an aircraft to successfully engage the drogue trailing behind the tanker, as shown in figure 5, is a difficult maneuver that requires very precise control.

Figure 5. USMC F-18 Engaging drogue

The IFR probe of the aircraft receiving fuel must hit the drogue with enough force to engage latches inside the “basket” that allow fuel to flow but not so hard that it causes slack in the hose. With the tanker and receiving aircraft flying at 180 to 320 knots (nautical miles per hour) closure speed with the drogue must be maintained within a 2 to 4 knot window. The margin for error is often less than 1%. If closure speed is too fast and the IFR probe hits the drogue too hard, it can set up an oscillating sine wave that causes the drogue to flail and may result in significant damage if it hits the refueling aircraft as shown in figure 6.

Figure 6. Navy Aircraft Damaged During Inflight Refueling

Using the latest motion control technology the Federal Industries / FO Engineering team introduced major improvements in position sensing of the reel as well as hose reel acceleration and take-up speed to correct what could otherwise be a potentially dangerous situation.

According to Del Boardman, project engineer for Federal Industries, the I2 Motor was a perfect fit for the requirements of this project. It saved time and money and was extremely user friendly and easy to use.