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150 Primary 14.3. Landing Gear Engagement Tension Arguably the most challenging aspect of the main-gear engagement active arrestor concept is "catching" the aircraft and establishing a solid connection with the landing gear. If the cable or net system misses the landing gear strut, or bounces off and is overrun by the plane, the braking devices Secondary Tension will not be able to stop the aircraft. Figure 14-3. Detail of cable tensions at 14.3.1. Deployment Path main-gear strut. Assuming that the netting or cable system starts from an initially flat configuration, flush with the runway sur- of the limiting lateral load for a strut-cable coefficient of face, it would require rapid vertical deployment to catch the friction of approximately 0.7. Therefore, on the basis of pre- strut above the top of the tires. For an aircraft with low- liminary investigation, if the maximum tension in the cable slung engines, the cable/net would have to elevate after the is limited to the critical value, lateral collapse of the landing gear engine nacelles have passed by but in time to catch the strut can be prevented. (Figure 14-9). 1.4 Drag Load Ratio 1.2 Lateral Load Ratio 1.0 Force Ratio 0.8 0.6 0.4 0.2 0.0 -0.2 0 50 100 150 200 250 300 350 Nose Gear Travel [ft] Figure 14-4. Main-gear strut loads during active system arrestment for B737-800, normalized by FAR strut limit values. 160 Primary Tension 140 Secondary Tension 120 Cable Tension (kip) 100 80 60 40 20 0 0 50 100 150 200 250 300 350 Nose Gear Travel [ft] Figure 14-5. Cable loads during active system arrestment for B737-800.
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151 80 70 Aircraft Speed Aircraft Speed (knots) 60 50 40 30 20 10 0 0 50 100 150 200 250 300 350 Nose Gear Travel [ft] Figure 14-6. Aircraft speed during active system arrestment for B737-800. 1.0 Deceleration 0.8 Deceleration (g) 0.6 0.4 0.2 0.0 -0.2 0 50 100 150 200 250 300 350 Nose Gear Travel [ft] Figure 14-7. Deceleration during active system arrestment for B737-800. Braking Coefficient = 0.25 R/T = 0 1.0 0.9 0.8 Normalized Lateral Load 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Strut-Cable Coefficient of Friction Figure 14-8. Dependence of lateral load on strut-cable coefficient of friction.
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152 Target Point: On Strut Above Tire Location of Arrestor Cable/Net at Initial Deployment By the Time of Main Strut Arrival, Cable/Net has Moved Vertically to Target Point Figure 14-9. Cable deployment concept (42). Assuming that the aircraft would be rolling forward at in deployment after the nose wheel passes the arrestor cable. 70 knots, the necessary deployment timing was calculated. The tolerance shows that the initialization of the deploy- Because the aircraft's lateral location with respect to the run- ment must take place within about 8 milliseconds of the tar- way centerline could vary, it was assumed that the cable for geted time. the system would be elevated uniformly across the entire run- Overall, the velocities and timing tolerances in this example way width. are feasible for a mechanical design. For each aircraft, these The test aircraft in this case was a B737-800. Using manufac- values would be different, which would require that a deploy- turer-published dimensions, the required cable path is given in ment system be capable of identifying the aircraft type. It Figure 14-10. The illustration gives an upper and lower bound- would then need to make a timing determination based on a ary to the cable path, as well as an optimal path. The path database of aircraft information. appears sloped because the aircraft is moving forward relative One important feature of these calculations is the upward to the cable location. However, the calculations assume that the velocity, which would be nominally 40 ft/sec (27 mph) on ini- cable moves directly upward from its location on the runway. tialization, and nearly that fast on contact with the strut. This means that the cable would still be moving upward. Depend- ing on the nature of the mechanical deployment system, slack 14.3.2. Deployment Timing in the cable combined with the upward motion could cause Table 14-2 summarizes the timing and speed requirements it to slap against the bottom of the aircraft and the landing for the deployment. The time to launch is essentially the delay gear bay doors. 50 45 Lower Bound 40 Optimal Launch Vertical Distance (in) 35 Upper Bound 30 Plane Bottom 25 20 15 10 5 0 -100 -80 -60 -40 -20 0 Horizontal Distance (in) Figure 14-10. Path calculation plot for active system B737 landing gear capture.