Originally Posted by
retired guy
That's it Skyjob
Crank it up to full reverse especially at high speed say >100 its and it really kicks in. Much more effective at high airspeeds hence the need to select right after touchdown. the 737-200 which could reverse almost the entire jet flux being low by pass, using giant buckets, and could stop on reverse alone in about 1800 metres. Landing in Jersey on a wet runway at 1650 metres it was a godsend.
Many of the later High By Pass engines seem to just be big noise machines but even so you are probably reversing 40% of the max thrust - just an estimate. Review any good video of reverse in operation on a wet runway during testing and you can see it is still a big player.
Few reversers do better than about 25% effectiveness - but as noted getting them out early is a big benefit due to the ram drag of all that air going in the inlet at higher speeds. Very early high bypass engines (JT9D/747) had core exhaust 'spoilers' - they didn't really reverse the core exhaust but pretty much eliminated the forward thrust component. However keeping the actuators and such working in the hot exhaust stream was difficult and expensive, and the benefit was minor, so they went away early on.
Most high bypass reverse systems limit N1 to around 80-85% - above that the forward core thrust increases faster than the reverse fan thrust, so there is no benefit to going higher.
The theoretical benefit of not using the reversers is it saves the wear and tear of the deployment actuators and related mechanism (MTBF of a reverser is in the 5000 cycle ballpark). That being said, back during the initial 777 development, I'd just been involved in the Lauda crash investigation (one of the most difficult, painful things I did during my career), and I semi-seriously asked the question 'why don't we just get rid of the reversers' - it was a lot of weight and complexity, they were potentially dangerous, and we didn't get any direct cert credit for them. I was told that on a 777 sized aircraft, using reversers saved ~$100/landing in brake wear and maintenance...