Methods of reverse thrust
Gas Turbine, Subsystems II
In WW-II and shortly thereafter, piston powered aircraft peaked in power, performance and complexity wise. Power went up to over 4000 bhp for large multi-row radial engines. Only to be defeated by the jet, which was developed by (among others) Germany's Dr. Hans von Ohain and separately in the UK by Sir Frank Whittle. Its principles are based on the "Aeolipile" of the ancient Greek scientist Hero and other great thinkers like Leonardo da Vinci and the laws of Isaac Newton.
Compared to a piston, the gas turbine has less parts and the moving parts rotate in only one direction without stopping and accelerating as the pistons normally do in a engine. Thus, a running gas turbine is basically free of the vibrations normally found in piston models, which translates in much longer service life (TBO) and higher reliability.
Afterburning, to increase power, and thrust reversers are related subjects, therefore we will discuss them here on one page.
Principle of a Gas Turbine Engine
Since some GTEs still produce forward thrust, even when the power levers are at idle, it really helps to be able to reverse the thrust so that stopping a jet airliner on a slippery surface or short runways is possible. Or even to control forward speed during taxi. Landing distance is also greatly reduced when thrust reversers are used to augment the normal wheel brakes during landing. Turboprop aircraft are able to change the propeller blade angle to reverse thrust.
The perfect thrust reverser works by bending the thrust 180° but this is mechanically not really doable. The maximum angle is about 135°, which contributes to the reverse thrust. Bypass engines reverse only the cold bypass air by moving the translating cowl aft and blocking doors reverse the cold air through cascade vanes forward.
After selecting reverse thrust the pilot must wait a second or two to let the reserve thrust system reposition itself before applying higher thrust. Especially in multi engine aircraft where not all components move in sync this could introduce some unwanted yaw.
Propeller reverse thrust
On turboprop aircraft selecting reserve thrust moves the blades in a negative pitch angle. During blade change, fuel supply to the engine must be regulated/controlled so that when the blades move through the zero thrust angle the engine does not overspeed.
This is a very effective way of increasing the thrust without installing a heavier model with a possibly larger frontal area and dealing with higher operational cost to run that engine, only this method is not very often used in commercial airliners. The Concorde had engines with afterburners, for the rest they are mainly used in military aircraft.
Afterburning is done by using the large amount of air in the exhaust to burn extra fuel after the turbine in the tailpipe. A series of concentric fuel spray bars inject fuel in the exhaust and this will be ignited by a catalytic (platinum based) or electric igniter or hot streak of flame from a combustor. The J-79 used in the F-104 Starfighter had a noticable flam when looking in the engine tailpipe for exactly this purpose.
The hot exhaust alone is not enough to ignite the fuel. Exhaust in the burner cans is cooled by secondary air to keep the temperature of the turbine blades within limits, which is too low to auto-ignite JET fuel.
Fuel consumption is increased as is thrust and speed, which could result in the aircraft being flown to far from base on afterburner and not being able to fly back (or to any airport) on the fuel remaining.
Ignition of the afterburner must be realised under different circumstances as varying airspeeds, mixture strengths, gas flow rates and altitudes. Cooling of the inside of the tailpipe is done by a layer of cooler air to protect it against flame temperatures of over 1500°C.