In supersonic aerodynamics, an unstart refers to a generally violent breakdown of the supersonic airflow. The phenomenon occurs when

mass flow rate In physics and engineering, mass flow rate is the mass of a substance which passes per unit of time. Its unit is kilogram per second in SI units, and slug per second or pound per second in US customary units. The common symbol is \dot ('' ...

changes significantly within a duct. Avoiding unstarts is a key objective in the design of the engine air intakes (inlets U.S.) of

supersonic aircraft A supersonic aircraft is an aircraft capable of supersonic flight, that is, flying faster than the speed of sound (Mach number 1). Supersonic aircraft were developed in the second half of the twentieth century. Supersonic aircraft have been use ...

that cruise at speeds in excess of Mach 2.2.

Etymology

The term originated during the use of early

supersonic wind tunnel A supersonic wind tunnel is a wind tunnel that produces supersonic speeds (1.2< M<5) The Mach number and flow are determined by the

s. “Starting” the supersonic wind tunnel is the process in which the air becomes supersonic; ''unstart'' of the wind tunnel is the reverse process. The

shock wave In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a me ...

s that develop during the starting or unstart process may be visualized with

schlieren Schlieren ( ; , ) are optical inhomogeneities in transparent media that are not necessarily visible to the human eye. Schlieren physics developed out of the need to produce high-quality lenses devoid of such inhomogeneities. These inhomogeneiti ...

shadowgraph Shadowgraph is an optical method that reveals non-uniformities in transparent media like air, water, or glass. It is related to, but simpler than, the schlieren and schlieren photography methods that perform a similar function. Shadowgraph is a ...

optical techniques. In some contexts, the terms aerodynamic disturbance (AD) and unstart have been synonymous.

In aircraft engine intakes

The design of some air intakes for supersonic aircraft can be compared to that of supersonic wind tunnels, and requires careful analysis in order to avoid unstarts. At high supersonic speeds (usually between Mach 2 to 3), intakes with internal compression are designed to have supersonic flow downstream of the air intake's capture plane. If the mass flow across the intake's capture plane does not match the downstream mass flow at the engine, the intake will unstart. This can cause violent, temporary loss of control until the intake is restarted. Few aircraft, although many ramjet-powered missiles, have flown with intakes which have supersonic compression taking place inside the intake duct. These intakes, known as mixed-compression intakes, have advantages for aircraft that cruise at Mach 2.2 and higher. Most supersonic aircraft intakes compress the air externally, so do not start and hence have no unstart mode. Mixed compression intakes have the initial supersonic compression externally and the remainder inside the duct. As an example, the intakes on the

North American XB-70 Valkyrie The North American Aviation XB-70 Valkyrie was the prototype version of the planned B-70 nuclear-armed, deep-penetration supersonic strategic bomber for the United States Air Force Strategic Air Command. Designed in the late 1950s by North Ame ...

had an external compression ratio (cr) at M3 of 3.5 and internal cr about 6.6, followed by subsonic diffusion. The

Lockheed SR-71 Blackbird The Lockheed SR-71 "Blackbird" is a long-range, high-altitude, Mach 3+ strategic reconnaissance aircraft developed and manufactured by the American aerospace company Lockheed Corporation. It was operated by the United States Air Force ...

and XB-70 Valkyrie had well-publicised"Valkyrie" Jenkins & Landis 2004, Specialty Press, , pp.136-137,144 unstart behaviour. Other aircraft that have flown with internal compression include the

Vought F-8 Crusader The Vought F-8 Crusader (originally F8U) is a single-engine, supersonic, carrier-based air superiority jet aircraft built by Vought for the United States Navy and United States Marine Corps (replacing the Vought F7U Cutlass), and for the Fren ...

III, the

SSM-N-9 Regulus II The SSM-N-9 Regulus II cruise missile is a supersonic guided missile armed with a nuclear warhead, intended for launching from surface ships and submarines of the U.S. Navy (USN).Koch, Charles A"Regulus II cruise missile".''Regulus II Cruise Missi ...

cruise missile and the

B-1 Lancer The Rockwell B-1 Lancer is a supersonic variable-sweep wing, heavy bomber used by the United States Air Force. It is commonly called the "Bone" (from "B-One"). It is one of three strategic bombers serving in the U.S. Air Force fleet along with ...

. Partial internal compression was considered for the

Concorde The Aérospatiale/BAC Concorde () is a retired Franco-British supersonic airliner jointly developed and manufactured by Sud Aviation (later Aérospatiale) and the British Aircraft Corporation (BAC). Studies started in 1954, and France an ...

(the Supersonic Transport Aircraft Committee, in 1959, had recommended an SST to cruise at Mach 2.2) but an "external configuration was chosen for the inherent stability of its shock system, it had no unstart mode". Even though there was some internal compression terminated by a normal shock local to the ramp boundary layer bleed slot inside the intake, the intake was aerodynamically self-compensating with no trace of any unstart problem. Early in the development of the

its mixed external/internal intake was changed to an external one, technically safer but with a small compromise in cruise speed. It subsequently had fixed intakes to reduce complexity, weight and cost. Work in the 1940s, for example by Oswatitsch, showed that supersonic compression within a duct, known as a supersonic diffuser, becomes necessary at M2-3 to increase the pressure recovery over that obtainable with external compression. As flight speed increases supersonically the shock system is initially external. For the SR-71 this was until about M1.6 to M1.8 and M2 for the XB-70. The intake is said to be unstarted. Further increase in speed produces supersonic speeds inside the duct with a plane shock near the throat. The intake is said to be started. Upstream or downstream disturbances, such as gusts/atmospheric temperature gradients and engine airflow changes, both intentional and unintentional(from surging), tend to cause the shock to be expelled almost instantaneously. Expulsion of the shock, known as an unstart, causes all the supersonic compression to take place externally through a single plane shock. The intake has changed in a split second from its most efficient configuration with most of its supersonic compression taking place inside the duct to the least efficient as shown by the large loss in pressure recovery, from about 80% to about 20% at M3 flight speeds. There is a large drop in intake pressure and loss in thrust together with temporary loss of control of the aircraft. Not to be confused with an unstart, with its large loss in duct pressure, is the duct over-pressure resulting from a hammershock. At speeds below the intake starting speed, or on aircraft with external compression intakes, engine surge or

compressor stall A compressor stall is a local disruption of the airflow in the compressor of a gas turbine or turbocharger. A stall that results in the complete disruption of the airflow through the compressor is referred to as a compressor surge. The severity of ...

can cause a hammershock. Above the intake starting speed, unstarts can cause stalls depending on the intake systems design complexity. Hammershocks have caused damage to intakes. For example, the North American F-107 during flight at high speed experienced an engine surge which bent the intake ramps. The

, during development flight testing, experienced significant damage to one nacelle after both engines surged.

Intentional

When an unstart occurred on the SR-71, a very large amount of drag from the unstarted nacelle caused extreme rolling/yawing. The aircraft had an automatic restart procedure which balanced the drag by unstarting the other intake. This intake had its own tremendous amount of drag, with the spike fully forward to capture the shock wave in front of the intake.

Avoidance

Decelerating from M3 required a reduction of thrust which could unstart an intake with the reduced engine airflow. The SR-71 descent procedure used bypass flows to give unstart margin as the engine flow was reduced. Thrust reduction on the XB-70 was achieved by keeping the engine flow stable at 100% rpm even with idle selected with the throttle. This was known as "rpm lock-up" and thrust was reduced by increasing the nozzle area. The compressor speed was maintained until the aircraft had slowed to M1.5.

Theoretical basis

Using a more theoretical definition, unstart is the

supersonic Supersonic speed is the speed of an object that exceeds the speed of sound ( Mach 1). For objects traveling in dry air of a temperature of 20 °C (68 °F) at sea level, this speed is approximately . Speeds greater than five times ...

choking phenomenon that occurs in ducts with an upstream mass flow greater than the downstream mass flow. Unsteady flow results as the mismatch in massflow can not gradually propagate upstream in contrast to subsonic flow. Instead, in supersonic flow, the mismatch is carried forward behind a 'normal' or terminal shock wave that abruptly causes the gas flow to become subsonic. The resulting normal shock wave then propagates upstream at an effective acoustic velocity until the flow mismatch reaches equilibrium. There are other ways of conceptualizing unstart which can be helpful. Unstart can be alternatively thought of in terms of a decreasing

stagnation pressure In fluid dynamics, stagnation pressure is the static pressure at a stagnation point in a fluid flow.Clancy, L.J., ''Aerodynamics'', Section 3.5 At a stagnation point the fluid velocity is zero. In an incompressible flow, stagnation pressure is equ ...

inside of a supersonic duct; whereby the upstream stagnation pressure is greater than the downstream stagnation pressure. Unstart is also the result of a decreasing throat size in supersonic ducts. That is the entrance throat is larger than the diffusing throat. This change in throat size gives rise to the decreasing mass flow which defines unstart. The choking reaction of unstart results in the formation of a

inside of the duct.

Shock instability or buzz

Under certain conditions, the shock wave in front or within a duct may be unstable, and oscillate upstream and downstream. This phenomenon is known as buzz. Stronger shock waves interacting with low momentum fluid or boundary layer tend to be unsteady and cause buzz. Buzz conditions can cause

structural dynamics Structural dynamics is a type of structural analysis which covers the behavior of a structure subjected to dynamic (actions having high acceleration) loading. Dynamic loads include people, wind, waves, traffic, earthquakes, and blasts. Any structur ...

-induced failure if adequate margins are not incorporated into design.

References

{{reflist, 30em Aviation risks