i am a bit confused about reverser actions. i understand that it redirects engine thrust fforwards. now the engine sucks air and redirects the same air forwards ...so where does the net braking action come from ..? taking air from the front and returning it to the front ....
The 'reversed' air does not exit out the front of the engine. IOW the air coming in the inlet is still compressed, mixed and ignited. It is the exhaust thrust which gets redirected, either by a simple blind (clamshell-type), angled out the top (727 etc.) or out the sides (high-bypass engines). This does two things - reduces the forward thrust and adds some reverse thrust. Both slow the airplane's forward momentum.
The air isn't coming out completely sideways, its angled forwards by vanes within the engine so it is actually coming out with a forwards component. Plus any air going forward isn't going out at the back and providing unwanted forward thrust.
Think of the residual unwanted thrust as a bit like an automatic car; It will always wear the brakes down quicker than a manual transission car (driven the same way) because of the 'creep' even when the throttle is shut. On the jet engine there is still residual thrust pushing the a/c forward even when the thrust levers are closed (someone more technical than me will post the %age) and this is counteracted by selecting Idle Reverse on landing, i.e. the engines are still at idle with the exhaust gases being pushed forwards as already described. Selecting more reverse thrust then spools the engines back up whilst still directing the exhaust gases forwards.
Some interesting points for those not that familiar with reverse thrust are;
Certainly on the 757 (anyone elaborate on other types?) that I fly application of reverse thrust does not stop you quicker as many would think. You set an autobrake selection from 1 to 4 or 'max auto' and then use reverse 'as required' and what this does is the more reverse thrust you take the less wheel braking is applied thus still stopping you in the same distance. As brakes are cheaper than engines our company policy is to use idle reverse under normal conditions. If conditions were slippery it may be more desireable to take more engine reverse than wheel braking.
The reverser levers can only be raised when the forward levers are in the idle position, and when pulled aft to the interlock the autothrottle is disengaged(if not already) and the auto speedbrakes deploy.
During rollout the PNF will call '80kts' and '60kts'. The reversers will be put back to idle by 60kts and stowed before taxi speed is reached to prevent debris being blown forwards of the nacelle where it could be ingested into the engine.