Way back when I first joined QF, I did some research on reverse thrust. The upshot was that the big turbo fan engines produce far less of it than people expect. On a large engine, the vast majority of the thrust is produced by the fan, with the core of the engine (the bit that burns the fuel) producing only about 20%, but of course ultimately producing all of the power, by driving the fan.
When you go into ‘reverse’, nothing actually starts going the other way, nor does the blade pitch change to a negative angle. All that happens is that the air flow is redirected, by various forms of blocker doors. This flow from the fan is largely blown sideways, with a small component in a forward direction. At the same time, the core is being spooled up, to drive this flow to the maximum. But, as the core exhaust is not blocked at all, you end up with the fan pushing one way, and the core pushing the other. As such they cancel each other out. I don’t recall the exact numbers that we came up with now, but I think the overall effect was reverse being worth about 10,000 lbs of thrust on a 747’s P&W engines. Drag from the wheels alone (without any braking) was of a similar amount.
So, reverse does very little reversing. But, it does offer other benefits. As sadly proven by QF1, it blasts water off the runway, giving the tyres a chance to grip. It disrupts the airflow over the wings, reducing lift, and helping the aircraft sit all of its weight down. And the disrupted airflows themselves produce drag. Interestingly though, Airbus actually planned to produce the A380 without reverse at all, and in the end only did so on the inner engines.
Older jet aircraft that are capable of these ‘powerbacks’ have a reverse style that also blocks the core of the engine. Newer aircraft that can do it have been designed with it in mind (for instance the C17 and perhaps the Viggen). Given that most people can’t back a car, it doesn’t seem like all that good an idea.
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