j0rdan From Canada, joined Feb 2010, 127 posts, RR: 0 Posted (4 years 11 months 3 weeks 1 day 11 hours ago) and read 8184 times:
Forgive me if this has been discussed before but i cannot seem to find it anywheres, this is also my first post so feedback is appreciated.
OK, to the question(s),
I was wondering if anyone could explain generally how reverse thrust works, concepts of it etc..
I know there is a few different type such as 'clamshell' on 732, DC9; One where 4 'flaps' pop out on some A32X aircraft; and one where the engine 'slides' open such as on the 767, 777 etc. But i was wondering more specifically what is happening inside the engine when reverse thrust is deployed more particularly the 'sliding' style I mentioned. Also if there are proper names for these different styles.
What kind of reverse thrust do the 748 and 787 use?
The translating part of the cowl is mounted on slider tracks or rails, and is pushed back either by jackscrews or pneumatic / hydraulic jacks. Inside the fan duct are blocker doors that are attached to pull rods on their aft edge, and to the translating cowl on their forward edge. As the translating cowl moves back, the geometry of the pull rod and blocker door is such, that the blocker door moves to block off the fan duct.
Simultaneously, as the translating cowl moves back, it uncover a series of cascade assemblies. The cascade assemblies have angled vanes to redirect the fan air forward. Thus, the blocker doors block the normal, rearward path of the fan discharge air, which is then redirected partially forward by the cascades to achieve reverse thrust.
These videos show the cascade vanes and blocker doors.
Now, I can't speak knowledgeably about how exactly the thrust reversers work (i.e. the actual components that get shifted around) as I'm not familiar with aircraft engines. But here's the concept(s):
On clamshell reversers, the whole airflow (core and bypass) gets redirected forward and outward. So you basically have zero thrust pushing the airplane forward, and some percentage of your total thrust pushing the airplane backward (i.e. braking).
On petal and cascade reversers, only the bypass airflow is reversed. However, on modern turbofan engines, the vast majority of the thrust on the ground comes from the bypass air. So most of your thrust has been redirected forward and outward, and is braking the airplane. The core flow is still going out the back end of the engine, and is therefore still pushing the airplane forward.
Note that I said "forward and outward" for the reversed thrust. That means that you can never get 100% of your max thrust directed solely forward. Also, engines typically can't be used up to max thrust when you're in reverse. So even at max reverse, you'll only be getting some percentage of your max thrust, now directed forward.
Also worth noting that reverse is typically only used from touchdown down to some specified speed (i.e. 60 or 80 knots). Below that, you increase the danger of FOD (foreign object damage - the engine sucking in a rock or something), and re-ingestion of exhaust (the engine re-inhaling the exhaust from the reversers).
I'm sure I missed some stuff, so anyone please feel free to correct me!
Welcome! There is a lot of knowledge available here -- that's why you joined, right? Be sure to try out the search feature. It can take a few tries to get the results you're looking for, but many questions like this one have been answered several times over. Here are links to a few more threads on reverse thrust:
tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 4, posted (4 years 11 months 3 weeks 15 hours ago) and read 7988 times:
Quoting vikkyvik (Reply 2): On petal and cascade reversers, only the bypass airflow is reversed.
Although this is common in practice, there's no inherent reason you can't reverse core flow with a petal or cascade reverser. The C-17 has full (bypass and core) cascade reversers. I believe early L-1011's did as well.
They did...however, the core T/Rs were clamshell, IIRC. I remember seeing a picture of it on the #2 engine of an early L-1011 (TWA?) before they were deemed too heavy for their own good and removed from existing airframes.
vikkyvik From United States of America, joined Jul 2003, 10459 posts, RR: 26
Reply 6, posted (4 years 11 months 3 weeks 10 hours ago) and read 7926 times:
Quoting tdscanuck (Reply 4): Although this is common in practice, there's no inherent reason you can't reverse core flow with a petal or cascade reverser. The C-17 has full (bypass and core) cascade reversers. I believe early L-1011's did as well.
You know, I knew that and I remember intending to include it in my post....but I didn't.