“Transonic Flight”, 1957, Shell Oil Co., 20 minutes. Shows the critical transonic range, a challenging phase where airflow introduces flight instability. Learn about the complexities and solutions as aircraft designers tackle the obstacles of achieving supersonic speeds.

Summary: 0:52 The film begins by explaining how changes in air density around an aircraft model in a high-speed wind tunnel are made visible: red indicates denser, slowed-down air, while blue indicates less dense, speeding-up air.1:16 It then introduces the concept of taking the model through the “transonic speed range,” a particularly unstable condition.1:29 To understand the transonic range, the film examines airflow around an aerofoil at a small angle of incidence, showing that after the critical Mach number, a shock wave forms on the upper surface, creating a supersonic region (lighter green) within the otherwise subsonic flow.1:57 As airspeed increases, a lower shock appears, and both shocks move back. The upper shock then stops due to “flow separation” (turbulent boundary layer breaking away from the surface).2:13 When the lower shock reaches the trailing edge, the upper one moves back. Near Mach 1, both shocks meet at the trailing edge, and flow separation is significantly reduced.2:41 As the flight Mach number passes Mach 1, the free airstream becomes supersonic, and a “bow wave” appears in front of the wing, similar to a boat’s bow wave on water.3:08 The bow wave is formed by the piling up of pressure waves from points near the leading edge within the subsonic region. As speed increases, the bow wave quickly moves towards the leading edge, but a subsonic region remains between it and the wing.3:30 The film differentiates between an “oblique shock” (supersonic flow on both sides) and a “normal shock” (perpendicular to airflow, always with subsonic flow behind it).3:46 As speed continues to increase, shocks bend further back. At a high enough Mach number, with a sharp leading edge, the bow wave attaches to it, making the flow everywhere supersonic.4:13 The film categorizes flying speeds into three ranges: “subsonic” (flow everywhere is subsonic), “transonic” (mixed subsonic and supersonic flow, from critical Mach to about Mach 1.3), and “supersonic” (main flow everywhere is supersonic).5:00 The film explains that “bangs” are shock waves from transonic aircraft. When an aircraft passes Mach 1, a bow wave appears, and even if the aircraft slows down, these waves travel on and strike the ground, causing sonic booms.5:45 All aircraft must pass through the transonic range to reach supersonic speeds, where shock wave troubles are worst. Slight speed or attitude changes cause violent shock movement.6:00 “Flow separation” causes most problems in the early transonic range, leading to the “shock stall.”6:15 Shock stall causes a “shock stall,” whose effects in flight resemble a normal low-speed stall (also caused by separation).6:30 The first shock stall trouble is a sharp rise in drag, known as “wave drag,” caused by energy dissipated as heat by the shock wave. As shocks move back and strengthen, wave drag increases.7:53 Changes in drag are seen in the “drag coefficient,” which is constant at low Mach numbers but rises sharply above critical Mach, peaking around Mach 1 (the “sound barrier”).8:27 At shock stall, lift drops suddenly. The “lift coefficient” is constant at low speeds, increases slightly below critical Mach (due to less dense air over the wing), but plunges suddenly above critical Mach due to flow separation.9:06 Shock stall also causes instability, such as “fore and aft trim changes” (nose-down tendency), “wing dropping,” “porpoising,” “snaking,” and “Dutch roll.” The center of lift always shifts rearward in the transonic range, causing a final nose-down trim change.10:58 “Buffeting” is caused by turbulent separated airflow striking the tailplane. More separation can directly affect other parts like the wing-root or cockpit.11:13 Controls may lose effectiveness because shock waves prevent control surface changes from affecting air ahead, and the control surface operates within aerodynamically “dead” separated airflow. This turbulent wake can also cause violent oscillations of the control surface itself.12:56 To counter these troubles, design features like thin wings and sweepback raise the critical Mach number and lessen shock stall effects.13:25 Flow separation can be reduced by “vortex generators” (small metal veins that stir fast-moving air into the sluggish boundary layer), or potentially by blowing air into the boundary layer.14:28 Buffeting is reduced by positioning the tailplane out of line with the wings. For greater trim control, “all-moving tailplanes” (trimming tailplanes with separate elevators, geared elevators, or slab tails) are used, requiring smaller deflections.15:34 “Powered controls” are almost essential to overcome the huge forces on control surfaces at transonic speeds.16:01 To reduce wave drag, a new design principle called “area rule” is used, which involves smoothing out the cross-sectional area changes along the aircraft’s axis, often by giving the fuselage a “waist” where the wings are.17:11 Above Mach 1, bow wave drag can be reduced by having sharp rather than rounded leading edges.17:31 The film concludes by stating that these design improvements are solving the problems of transonic flight, making aircraft safe at both high and low speeds. Many aircraft have already mastered the shock stall, and with greater thrust from afterburning jets, ramjets, and rocket motors, future aircraft will easily pass through the transonic range into fully supersonic flight.