Thrust Reversal Principals & Differences
There are two types of thrust reversal systems utilized, depending on the engine design. For engines like the JT8-D where the fan by-pass air is channeled around the engine inside the nacelle and re-mixed with the hot gas flow while still inside the nacelle, a clamshell deflector system is generally employed. This system was used starting on 727s, 737s and DC9s. Upon landing and throttle retardation to idle, the thrust reversal is initiated by clamshell doors blocking the axial exhaust flow of the entire engine and two deflectors deploying that redirect the airflow in a forward angle straight upward and straight downward. Once deployed, the reverse thrust power is governed by the amount of thrust power selected. It is effective throughout the complete roll-out distance and in combination with the brakes, very effective regardless of speed, hence the power back capability departing a gate.
When this system was applied to the underwing engine mounted 737 directly from the 727 tail mounted engines, it didn't work. The bottom deflector caused lift on the wheels resulting in ineffective braking. The system was replaced with a new cylindrical "target" deflector system skewed 35° from vertical, but the principle remained the same.
In the development and utilization of high by-pass engines, the thrust reverser system changed to fan reversal only. After touchdown, thrust reversal is initiated, blocker doors close shutting off axial fan by-pass flow while a cowing slides back exposing a series of cast aluminum cascade panels. Each panel is custom designed for a particular circumferential location (there are some common ones), for each aircraft/engine combination, usually by the engine designer/manufacturer. The turning vanes in these panels redirect the air radially forward, 40 - 50° from axial. There are left hand and right hand panels depending on which wing the engine is to be mounted on. During engine removal, the panels stay with the aircraft. Reversal of the fan stream affects the core nozzle in that the fan stream influence is removed. The core engine thrust is more than overcome by the fan and core flow ram drag. Ram drag is the largest of the forces in the pure engine. This becomes more complicated on wing of the aircraft as the reverse flow shrouds a part of the aircraft and changes aircraft drag. The reversers are used down to 60 knots at a minimum to prevent air re-ingestion into the fan, e,g, FOD potential. On Boeing aircraft, during a few seconds after thrust reversal, the hot core exhaust pattern is disturbed heating the pylon fairing under the wing (1000℉ for a couple of seconds). Over time, aluminum is not suitable and fairings are protected by shaped castings welded together, originally 17-4PH SS, but now Ti6-4 to save hundreds of pound of weight per aircraft.
Net reverse thrust is defined as fan reverser air, minus forward thrust from the the engine core, plus form drag.
Hope this is informative.
Turbine D