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The Blackbird
The SR-71, unofficially known as the “Blackbird,” is a long-range, advanced, strategic reconnaissance aircraft developed from the Lockheed A-12 and YF-12A aircraft. The first flight of an SR-71 took place on Dec. 22, 1964, and the first SR-71 to enter service was delivered to the 4200th (later 9th) Strategic Reconnaissance Wing at Beale Air Force Base, Calif., in January 1966. The US Air Force retired its fleet of SR-71s on Jan. 26, 1990.
Throughout its nearly 24-year career, the SR-71 remained the world’s fastest and highest-flying operational aircraft. From 80,000 feet, it could survey 100,000 square miles of Earth’s surface per hour.
The SR-71 used the same powerplant as the A-12 and YF-12. It consists of three main parts: inlet, J58 engine and its nacelle, and ejector nozzle.
SR-71 Inlet Duct Unstart
A phenomenon known as an “inlet unstart” could happen when without proper scheduling, disturbances inside the inlet could result in the shock wave being expelled forward.
But what caused an Inlet Duct Unstart in the SR-71 and how Blackbird drivers dealt with it?
USAF SR-71 Blackbird pilot Stormy Boudreaux explains on Quora;
‘First of all this is the most misunderstood term in all of Blackbird lore. The proper and correct name for this condition is “Inlet Duct Unstart”. The Blackbird pilots shorthand of just “UNSTART” has led to most of the uninitiated to falsely believe that since they start a car’s engine or start an airplane engine to begin a flight that an “Unstart” must mean that the engine has stopped. It is even erroneously implied in this question: What caused engine “unstarting”?
‘It is the inlet duct that is experiencing an “upstart” NOT the engine!
‘Throughout an SR-71 Inlet duct upstart the supersonic flow condition established inside the inlet is disrupted. The internal shock wave is expelled and a “normal” shock wave forms ahead of the inlet cowl. All flow within the inlet becomes subsonic and pressure in the inlet is decreased. When an inlet alternatively starts and upstarts rapidly, the change in inlet pressure which occurs results in severe airframe roughness. Unstarts are generally recognized by airframe roughness, a loud “banging: noises, aircraft yawing and rolling and a decrease of compressor inlet pressure toward 4 psi from the normal 17–18 psi. The afterburner may blow out.
Thrust asymmetry
‘A distinct increase in drag and loss of thrust because of the air spillage around the inlet and reduced airflow through the engine can cause the EGT to rise – the closer the aircraft is to limit Mach number and ceiling altitudes the faster and more serious the temperature rise. The “started” inlet provides at Mach 3 and greater, 80% of the thrust from that engine/inlet combination. The sudden (nearly instantaneous loss of 80% thrust on one side of widely displaced engine nacelles violently moves the nose of the airplane laterally. It has been described as nearly the same as getting T-boned in a car by being hit by another car at high speed crossing an intersection!
‘The aircraft yaws suddenly toward the unstated inlet, this yaw causes a roll in the same direction of the yaw. If the upstart occurs during a turn, the roll could exceed the pilot’s ability to counter the roll since aileron effectiveness is reduced at high altitudes and high angles of attack. Roll control becomes critical if the upstart occurs on the inboard inlet during a bank. At altitudes above 75,000 feet, aileron control may be ineffective in controlling roll during an upstart unless the angle of attack is immediately reduced.
‘There are many reasons and a multitude of corrective actions that the pilot must perform correctly. During an upstart, primary emphasis must be placed upon maintaining pitch control in order to prevent nose up pitch rates and angles of attack. Thrust asymmetry should be reduced as soon as possible.’
Flight Manual checklist items about SR-71 Inlet Duct Unstart
Boudreaux concludes;
‘Flight Manual checklist items are:
- ALPHA WITHIN LIMITS
- SIMULTANEOUSLY DISENGAGE AUTOPILOT AND SELECT BOTH RESTART SWITCHES ON
- BOTH THROTTLES – MILITARY
- MAINTAIN ATTITUDE CONTROL – OPTIMIZE PITCH AND ROLL
- AIRSPEED – ADJUST TOWARD 350 KEAS. DO NOT EXCEED 3.1 MACH if roughness does clear after 10 seconds:
- AFT BYPASS switch – OPEN, then when roughness clears:
- AFT BYPASS switch – Normal schedule
- Restart Switches – FWD/BYPASS OPEN (individually)
- Restart switches – OFF (individually), then when inlet starts:
- Fuel derichment arming switch – Recycle below 790 degree C EGT if derrick activated
- Throttles – as required
- If upstarts repeat or inlet roughness does not clear:
- Engine and inlet instrument and hydraulic pressure – check
- Repeat procedure.
Attempting inlet restart
- If unseats persist:
- Attempt inlet restart and operation using manual inlet controls. Observing manual inlet limitations. (Not above 80,000 feet, transfer sufficient fuel forward to obtain at least 0 pitch trim. Max bank angle now limited to 30 degrees bank at speeds up to Mach 3.0. when a small heading change is needed using a smaller bank angle will reduce the possibility of an upstart. If 20 degrees of bank must be exceeded then the FWD BYPASS door should be adjusted one position lower that specified in the manual schedule, the spike should be adjusted 0.1 Mach number position less than that indicated by TDI.
- The flight manual continues with another 1.5 pages of technical info for manual inlet operation and limitations.
Be sure to check out Linda Sheffield Miller (Col Richard (Butch) Sheffield’s daughter, Col. Sheffield was an SR-71 Reconnaissance Systems Officer) Twitter X Page Habubrats SR-71, Instagram Page SR71Habubrats and Facebook Page Born into the Wilde Blue Yonder Habubrats for awesome Blackbird’s photos and stories.
Photo credit: Lockheed Martin, U.S. Air Force / Jim Goodall Collection, NASA and Linda Sheffield Miller
