(SLIDE 13) Aerodynamic and Structural Heating During the entire program, the X-15s were exposed to surface temperatures as high as 1350 degrees F and all the airplanes exhibited minor buckling of the exterior skin. During the rocket boost phase of the flight, when the aircraft was accelerating and heating rapidly, the pilot could hear this skin buckling. As Joe Walker observed, "The airplane crackled like a hot stove."
Aerodynamic and Structural Heating (Slide 13)
The leading edges of the wing were formed in several pieces and had small slots between segments to allow thermal growth (SLIDE 14) These slots tripped the boundary layer and caused high temperature and skin buckling downstream of the slots. The slots were eventually capped and an additional rivet was added behind the slot, curing the problem.
Wing Skin Buckle Following Flight (Slide 14)
On two occasions the right outside windowpane crazed (SLIDE 15) forcing the pilot to fly and land with only left-hand vision. Buckling of the window frame caused this crazing. The Inconel frame was replaced by one of titanium, which has a lesser thermal expansion and which prevented further recurrences of window crazing
Damaged Windshield Glass Following Flight. (Slide 15)
(SLIDE 16) Simulation: For planning X-15 flights and for pilot practice, extensive use was made of a fixed base analog simulator. Every flight was preceded by ten or twenty hours of simulation of normal and emergency procedures.
Simulation (Slide 16)
The simulator had to be continually updated with data gathered in flight. For example, early simulation based on wind tunnel data indicated that the X-15 would be flyable with the lower rudder turned on and the roll damper turned off. Flight test proved this not to be the case. When the roll damper was turned off, a test also revealed that the lower rudder was not required for directional stability. The lower rudder was discarded, the X-15 then became stable even with a roll damper failure, and the simulator was updated to reflect the actual aircraft characteristics.
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