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OCR for page 148
148 CHAPTER 14 Main-Gear Engagement Active System Concept The main-gear engagement active arresting system con- in similar longitudinal positions on the aircraft. Conse- cept was evaluated in two major respects. First, a loading quently, any device that would propel the arresting cable model was developed to determine the decelerations possi- up to engage the main landing gear would have to be timed ble and the load effects on the landing gear. This model to activate after the nose gear and engine nacelles had assumed that a cable-based arrestor system had successfully passed by. engaged the main-gear struts of the aircraft. The second part of the evaluation assessed the performance require- 14.2. Prediction of Arresting Loads ments to successfully engage the landing gear of a fast-moving aircraft. 14.2.1. Predictive Tool An analytical spreadsheet model was developed to calcu- 14.1. Overview of Active late a time-marching aircraft deceleration. The mechanical System Deployment input variables and output responses included in the model are shown in Table 14-1. Three issues complicate the deployment of a cable-based The predictive tool assumed the geometry shown in Fig- arrestor: aircraft identification, overrun event detection, ure 14-1. Figure 14-2 is a detail of the main landing gear and timing of deployment. First, the type of aircraft must engaged by the cable with tension. As shown, the initial decel- be determined prior to the arrest. Civil aircraft in service eration tension in the cables, which the brake units create, at U.S. airports have a variety of main landing gear ge- produces a secondary tension between the main-gear struts. ometries. Therefore, the type of aircraft would have to be The primary and secondary cable tensions produce a rear- known prior to deployment. This issue could be resolved ward deceleration load on the struts and an inward lateral with a dedicated transponder or a video-based recognition load. The lateral load depends on the coefficient of friction system. between the cable and the strut. As the plane rolls further past Second, unlike an EMAS, which passively arrests aircraft, the arresting engines, the angle decreases, which increases the net or cable that engages the main gear must be acti- the lateral load on the struts. vated to be erected in the event of an overrun. Under nor- The limiting lateral and longitudinal loads were determined mal circumstances, the net or cable will be retracted and from FAR part 25 for a Boeing 737-800. level with the runway. Activation depends on detection of an overrun. This issue could be addressed by locating sen- sors at or near the threshold of a runway. When the aircraft 14.2.2. Arrestment Simulations crosses the threshold, the system could automatically arm Arrestment simulations were made for a B737-800 that was itself. With the correct detection sensors, the need for a initially travelling at 70 knots. The aircraft was assumed to ground operator to manually activate the system could be have a 0.25 braking coefficient during the arrest. eliminated. During the simulation, the cable tension was set to maxi- The third issue is the timing of deployment. Erecting an mize the deceleration without exceeding the main-gear limit engagement device to capture the main gear must be done loads. The total load on the strut due to the cable force and while avoiding contact with the aircraft engines. As shown the braking force summed to equal the longitudinal limit in Figure 14-1, the main landing gear and the engines are load. Due to the angle change for the cable, the tension was

OCR for page 148
149 Table 14-1. Cable-based arrestor model parameters. Input Variables Output Responses Geometry Stopping distance Distance between brake Max lateral load units Max longitudinal load Figure 14-1. B737-800 main landing gear and aircraft Slack in cable engine (42). Maximum tension in cable Coefficient of friction between strut and cable continually adjusted to maintain the total overall load. This Aircraft type (B737-800) was done dynamically during an arrest to simulate an active Limiting lateral load feedback control for the arrestor system. Because the deceleration was designed to maximize the Limiting longitudinal load drag loading on the main gear, a fairly low coefficient of Aircraft braking condition friction ( = 0.3) was used between the main strut and the Braking cable. With this configuration, the strut loads were calcu- Free-rolling lated to be as shown in Figure 14-4. The struts are loaded maximally in the longitudinal (drag) direction, and the lat- eral strut loads climb steadily until the end of the arrest- Figure 14-7 shows, the system concept is capable of providing ment. If the friction between the strut and the cable is ideal constant decelerations. higher, the lateral load becomes the limiting factor, and the cable tension must be reduced, lengthening the stopping 14.2.3. Additional Friction Study process. For the B737-800, the cable tensions are given by Figure 14-5. Simulations were conducted to determine whether, for For the 140 kip tension shown, the cable would need to be a typical arrest case, the lateral load on the landing gear composed of high-strength steel with a diameter of 1.4 in. or would reach a limiting value. Preliminary simulations more, depending on strength. This cable size presents compli- showed that if the maximum tension in the cable were kept cations, especially when dealing with large and small aircraft. below a critical value, the lateral load on the landing gear Strong decelerations of a large aircraft will require a thick, remained below the limiting lateral load implied by FAR heavy cable. When a small aircraft is arrested by such a system, Section 25.485. the weight of the cable itself could be sufficient to cause dam- For a braking coefficient of 0.25 and no reverse thrust, sim- age to the landing gear. ulations were run for a B737-800 in which the strut-cable The aircraft speed and deceleration are shown in Figure 14-6 coefficient of friction was varied and the maximum lateral and Figure 14-7, respectively. This deceleration essentially load on the landing gear was recorded. These maximum lateral represents a best-case solution for the aircraft, assuming the loads were normalized by the limiting lateral load. As shown in longitudinal FAR limit load criterion for the main gear. As Figure 14-8, the maximum lateral load nearly reached 90% Braking Unit Primary Landing Gear Tension Primary Tension Braking Unit Figure 14-2. Cable-based aircraft arrestment.