Reverse Thrusts in real life

Hey guy,

It seemed like this section of the threads were popular so I was hoping someone could answer my question.

I have recently been wondering, why do most airplanes or airlines these days not really use reverse thrust at landings? I noticed when, my flight from Manchester (UK) to zLahore (PK), I flew in a Boeing 777-300er and when landing, it seemed very very quiet like no engine reverse was used. I know there have been many forums and posts about this before but I haven't been able to find out clearly.

Is it some sort of policy or do planes just stop using them because they don't need them, could someone please explain, thank you.

Actually, the thrust reversers are used most every flight by jet aircraft, and reverse thrust, or Beta, is used by the turboprops. Thing about the reversers is that, like the engines themselves, they have gotten much quieter and more efficient than they once were. You may not notice them operating, but watch the big boys landing at the major airports sometime. they will touch, and you will see the reverser doors on the sides of the engines pop open fairly quickly, about the same time as the spoilers. And, of course, close again once the bird slows below a certain speed.

It's not like the onld days of reverser buckets slamming shut behind the engines, with the hot exhaust being redirected, both effectively and noisily foward to slow the plane. Now the only portion of the thrust being redirected is from the ducted fan portion forward of the relatively small actual jet it's self. The hot exhaust still is permitted out the back, and that's where a lot of the noise comes from. Remember that a fair percentage of the thrust of the engines these days comes from the ducted fan air blowing around the jet portion, and out the back. Yes the jet provides a lot of thrust, but the ducted fan portion (The big fan at the entrance of the engines, in the large cowling at the engine's front) provides a lot, if not most, of the thrust. I don't know the exact percentages involved. I DO know the fan portion is MUCH quieter than the jet portion. edirecting this thrust forward, thus is much quieter also.

As the planes get bigger and heavier, the amount of thrust they produce, relatively, also becomes larger, but the brakes are not a heck of a lot more effective, so reverse thrust is just as important if not more so, than it ever was, but also, as engines become more efficient and quieter, reversers follow suit. Just because you don't notice them from inside, doesn't mean they're not being used.

Hope this answers your questions adequately.

Pat☺

Actually I believe also that with some engines (Airbus 319/320 from Cebu Pacific comes to mind) the thrust reverser doors only open on the side of the engine pod that is away from the fuselage to reduce the noise in the aircraft.

Jorgen

Logically, many runways have been extended to cope with newer, larger aircraft such has the A380 or simply to allow the airport to handle a wider range of international traffic.

If the runway is long enough, the amount of reverse thrust that is needed is reduced. So it's quieter.

I found an article on another subject that gave the actual numbers of fan vs engine percentages of thrust:

For the PW 4168, the maximum rated takeoff thrust, at sea level with an ambient of 30°C is 68,000 lbs. Of the volume of air sucked by the huge fan at the front, 17% of the air is forced into the Low Pressure (LP) compressor. 83% of the air simply exits the engine without entering the compressor, and this is the bypass air. It is this 83% of air that is responsible for most of the thrust developed by the engine. The remaining air that entered the compressor only helps keep the engine cycle running.

Granted, that is for one specific engine, but I believe it's representative of most of the turbofan engines out there. As you can see, most (83%) of the engine total thrust is from the fan portion, with, thus, only 17% being from the actual jet portion.

All this helps me to explain why they use the fan portion for reverse thrust: very very quiet, and a large percentage of the total engine output. So the reversers are thus also much quieter, and very difficult to detect from inside the aircraft.

the thrust reverser doors only open on the side of the engine pod that is away from the fuselage

I knew there was a reason I suggested he watch the big boys land fro the outside to see the reverser doors pop :)

Pat☺

Hey guy,

It seemed like this section of the threads were popular so I was hoping someone could answer my question.

 

I have recently been wondering, why do most airplanes or airlines these days not really use reverse thrust at landings? I noticed when, my flight from Manchester (UK) to zLahore (PK), I flew in a Boeing 777-300er and when landing, it seemed very very quiet like no engine reverse was used. I know there have been many forums and posts about this before but I haven't been able to find out clearly.

 

Is it some sort of policy or do planes just stop using them because they don't need them, could someone please explain, thank you.

It all depends on runway length versus what is the required on the landing roll. Most of the time I don't use reverse thrust because the brakes are carbon and work better at higher temps. Also you can use idle reverse which IRL does provide a little bit of deceleration without going deep into full reverse. When operating reversers, it does add up maintenance costs. As for the 777, I don't really know. Wasn't flying it and I don't know the landing performance.