DocLightning From United States of America, joined Nov 2005, 16943 posts, RR: 57 Posted (1 year 1 month 2 weeks 3 days 4 hours ago) and read 3658 times:
I did a search and couldn't find the answer, so if I missed a thread, please point me to it.
My question is:
What % of the maximum forward engine thrust can a thrust reverser produce? I'm assuming it depends on T/R type, of course, but give me some examples.
tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80 Reply 1, posted (1 year 1 month 2 weeks 3 days 2 hours ago) and read 3608 times:
Quoting DocLightning (Thread starter): What % of the maximum forward engine thrust can a thrust reverser produce? I'm assuming it depends on T/R type, of course, but give me some examples.
I've heard rules of thumb that you get 50-60% of rated thrust in full reverse but I've never seen that documented anywhere.
Sliding-sleeve (aka cascade) reversers are the best at generating reverse thrust. These are the ones you see on all current production Boeings. Petal reversers (most Airbii) aren't as good at reverse thrust but, for smaller engines, are lighter and simpler. Clamshell (aka bucket) reversers are very good at neutralizing forward thrust and simple but not that great at generating a lot of reverse thrust. You see these mostly on old JT-9 style nacelles (737-100/200, DC-9, etc.).
BMI727 From United States of America, joined Feb 2009, 14419 posts, RR: 26 Reply 2, posted (1 year 1 month 2 weeks 3 days 1 hour ago) and read 3605 times:
Quoting tdscanuck (Reply 1): Clamshell (aka bucket) reversers are very good at neutralizing forward thrust and simple but not that great at generating a lot of reverse thrust. You see these mostly on old JT-9 style nacelles (737-100/200, DC-9, etc.).
If you used either of the other types on those engines it wouldn't do a lot of good since they only affect the bypass stream, which on a JT8D isn't a whole lot compared to modern engines.
Why do Aerospace Engineering students have to turn things in on time?
bueb0g From United Kingdom, joined Jul 2010, 547 posts, RR: 0 Reply 3, posted (1 year 1 month 2 weeks 3 days ago) and read 3565 times:
Quoting BMI727 (Reply 2): If you used either of the other types on those engines it wouldn't do a lot of good since they only affect the bypass stream, which on a JT8D isn't a whole lot compared to modern engines.
That's actually a very good point. So I guess as engines become more efficient in the future and bypass ratios increase, so will the amount of thrust delivered by T/R's?
BMI727 From United States of America, joined Feb 2009, 14419 posts, RR: 26 Reply 4, posted (1 year 1 month 2 weeks 2 days 15 hours ago) and read 3405 times:
Quoting bueb0g (Reply 3): So I guess as engines become more efficient in the future and bypass ratios increase, so will the amount of thrust delivered by T/R's?
Depends on the thrust ratio of the engine and the design of the thrust reverser. Using a petal or cascade type reverser that only reverses the bypass stream on an engine with a bypass ratio of about 1 isn't going to help you much.
Why do Aerospace Engineering students have to turn things in on time?
BoeingGuy From United States of America, joined Dec 2010, 2387 posts, RR: 7 Reply 5, posted (1 year 1 month 2 weeks 2 days 14 hours ago) and read 3339 times:
Quoting BMI727 (Reply 4): Using a petal or cascade type reverser that only reverses the bypass stream on an engine with a bypass ratio of about 1 isn't going to help you much.
Yeah, but the older engines that were around 1:1 used the bucket type reversers that reversed bypass and hot air.
The newest engines are getting up to around 12:1 bypass, so the cascade reverser does reverse most of the thrust.
DocLightning From United States of America, joined Nov 2005, 16943 posts, RR: 57 Reply 6, posted (1 year 1 month 2 weeks 2 days 7 hours ago) and read 3197 times:
Quoting tdscanuck (Reply 1): Petal reversers (most Airbii) aren't as good at reverse thrust but, for smaller engines, are lighter and simpler
Actually, it seems to be as much of an engine thing as an airframer thing. OK, admittedly only Airbii have petal-type, but the petal-type reverser seems to be unique only to specific engine options in each aircraft. On the A330 only RR has petals. On the A320, only the IAE's do.
26point2 From United States of America, joined Mar 2010, 688 posts, RR: 0 Reply 7, posted (1 year 1 month 2 weeks 2 days 7 hours ago) and read 3192 times:
BR 710 as used on GLEX and GV types has a maximum rated reverse thrust of 70% N1. Speed controlled by engine FADEC. At least it's a value? This engine uses the bucket style T/R.
DocLightning From United States of America, joined Nov 2005, 16943 posts, RR: 57 Reply 8, posted (1 year 1 month 2 weeks 2 days 7 hours ago) and read 3189 times:
Quoting 26point2 (Reply 7): BR 710 as used on GLEX and GV types has a maximum rated reverse thrust of 70% N1. Speed controlled by engine FADEC. At least it's a value? This engine uses the bucket style T/R.
Says that the fan runs at 70% N1 but that doesn't translate into how much thrust it makes...
N243NW From United States of America, joined Jul 2003, 1502 posts, RR: 21 Reply 10, posted (1 year 1 month 2 weeks 2 days 5 hours ago) and read 3139 times:
Quoting DocLightning (Reply 8): Says that the fan runs at 70% N1 but that doesn't translate into how much thrust it makes...
Another (pretty much worthless) number: the maximum reverse EPR used on the MD-80 is usually 1.6, since any amount greater than that puts the pilot in danger of blanking out the vertical stabilizer and effectively making the rudder worthless. How much thrust this EPR actually results in I can only guess.
airportugal310 From United States of America, joined Apr 2004, 3109 posts, RR: 2 Reply 11, posted (1 year 1 month 2 weeks 2 days 4 hours ago) and read 3110 times:
DocLightning From United States of America, joined Nov 2005, 16943 posts, RR: 57 Reply 12, posted (1 year 1 month 2 weeks 2 days 4 hours ago) and read 3094 times:
Quoting airportugal310 (Reply 11):
Slightly incorrect: The CFM56's also have this design:
So they do. I wonder why I remember them sliding...
airportugal310 From United States of America, joined Apr 2004, 3109 posts, RR: 2 Reply 13, posted (1 year 1 month 2 weeks 1 day 16 hours ago) and read 2924 times:
Quoting DocLightning (Reply 12): So they do. I wonder why I remember them sliding...
You very well might. The engine supports both types of R/T:
The CFM56 is designed to support several reverse thrust systems which help slow and stop the aircraft after landing. The variants built for the Boeing 737, the CFM56-3 and the CFM56-7, use a cascade type of thrust reverser. This type of thrust reverse consists of sleeves that slide back to expose mesh-like cascades and blocker doors that block the bypass air flow. The blocked bypass air is forced through the cascades, reducing the thrust of the engine and slowing the aircraft down.[44][45]
The CFM56 also supports pivoting-door type thrust reversers. This type is used on the CFM56-5 engines that power many Airbus aircraft. They work by actuating a door that pivots down into the bypass duct, both blocking the bypass air and deflecting the flow outward, creating the reverse thrust.
Larshjort From Denmark, joined Dec 2007, 1221 posts, RR: 0 Reply 14, posted (1 year 1 month 2 weeks 1 day 14 hours ago) and read 2865 times:
Quoting CCA (Reply 9): Using a 100,000lbs of thrust
Limited to 70% in reverse = 70,000lbs
Thrust isn't linear with RPM.
These numbers are for the CF34-3A1/3B1 used on the CRJ100/200
Normal TO thrust 8729 lbs with N1/fan speed 100% = 7400 RPM
APR delivers 169 RPM extra which is about 2,3% N1. This provides ~500 lbs of extra thrust, that is an increase of ~5,5%
APR is Automatic Performance Reserve which automatically kick in if one engine drops below 67% N1 during takeoff or first 5 minutes climb.
Jetlagged From United Kingdom, joined Jan 2005, 2462 posts, RR: 17 Reply 15, posted (1 year 1 month 2 weeks 1 day 13 hours ago) and read 2841 times:
Quoting CCA (Reply 9): Just a stab in the dark here, very rough figures. Change the limit or fan thrust as you see fit.
Using a 100,000lbs of thrust
Limited to 70% in reverse = 70,000lbs
Say 80% of thrust from the fan = 56,000lbs
Therefore 20% core forward thrust = 14,000lbs
Result 56,000 - 14,000 = 42,000lbs.
If limited to 90% reverse thrust = 54,000lbs
This is also assuming all the fan force is directed forward when it's actually directed forward and outwards.
So I'd say any engine would be lucky to develop half its thrust in reverse.
But any retarding force is better than none when you need it.
This is true with the aircraft static. At landing speed the situation is different because of the effect of intake momentum drag (ram drag). This always acts to slow the aircraft down.
Fn = Fg - Fram
When static Fram = 0, therefore Fn = Fg.
Normally gross thrust acts forwards so net thrust is gross thrust minus ram drag. In reverse thrust the gross thrust acts in the rearward direction, so net thrust is the sum of ram drag and gross thrust.
Using your 90% figures, and assuming gross thrust is the same at 150 knots as it is static (which is a reasonable):
Fg = -56,000
Fn = -56,000 - Fram
Assume Fram at 90% N1 is 10,000 lb at 150 knots so net thrust in reverse at touchdown is:
Fn = -56,000 - 10,000 = -66,000
Ram drag is of course always present with forward speed, reverse thrust or not, but if the engine was at idle it would be much less than it would be at full reverse.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
DocLightning From United States of America, joined Nov 2005, 16943 posts, RR: 57 Reply 16, posted (1 year 1 month 2 weeks 1 day 5 hours ago) and read 2710 times:
Quoting airportugal310 (Reply 13): You very well might. The engine supports both types of R/T:
The engine does, but the specific engine used on the A320 family does not. 737's slide. A320's petal (blossom?).
Does anybody know which are the aircraft that used to, or still do self-pushback on reverse thrust? I don't know if it's done at full power or not, but theoretically if you wanted to calculate it you could probably infer the reverse thrust being created based on the rearward acceleration of the aircraft at full power if you knew its weight?
Another way would be to compare the deceleration curves for an aircraft rolling out with and without thrust reverser - the difference would give you the braking force or reverse thrust magnitude.
okie From United States of America, joined Jul 2003, 2410 posts, RR: 3 Reply 19, posted (1 year 1 month 2 weeks 8 hours ago) and read 2390 times:
Quoting CCA (Reply 9): This is also assuming all the fan force is directed forward when it's actually directed forward and outwards.
So I'd say any engine would be lucky to develop half its thrust in reverse.
No one has come up with the vector of the diverted thrust.
I am thinking 50% would be generous given the vector of the diverted thrust and that the core is still producing forward thrust with today's high bypass designs.
Quoting wingscrubber (Reply 18): Does anybody know which are the aircraft that used to, or still do self-pushback on reverse thrust? I don't know if it's done at full power or not, but theoretically if you wanted to calculate it you could probably infer the reverse thrust being created based on the rearward acceleration of the aircraft at full power if you knew its weight?
I have never been on a DC-9/MD-80 series that has powered back although I have observed them.
I will say that I have been on a multitude of AA 727 power backs at DFW when they used to operate them and noise wise in the cabin it was quite loud for at least 5 to 7 seconds just to get the plane to creep back from the gate. Indicating to me that there is just not an over abundance of actual thrust being created from two engines (they normally just used 1 & 3)
tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80 Reply 20, posted (1 year 1 month 2 weeks 8 hours ago) and read 2372 times:
Quoting wingscrubber (Reply 18): Another way would be to compare the deceleration curves for an aircraft rolling out with and without thrust reverser - the difference would give you the braking force or reverse thrust magnitude.
With most autobrake systems today, this method doesn't work. The autobrake setting defines a decelleration, not a braking force. So, if you pick autobrake 1, you'll get the same deceleration curve whether you use just brakes or brakes + T/R (up to the point that the T/R is doing all the work).
DocLightning From United States of America, joined Nov 2005, 16943 posts, RR: 57 Reply 21, posted (1 year 1 month 2 weeks 7 hours ago) and read 2361 times:
Quoting wingscrubber (Reply 18): Does anybody know which are the aircraft that used to, or still do self-pushback on reverse thrust
DC-9 series and 727 for sure. I have been aboard both types (NW) when they powered back. It's noisier than a pushback, of course, but the acceleration wasn't spectacular. That said, it was noisy but it didn't sound like they pushed to anything like TO power. After all, one does not set 80% N1 on a power-back!
PGNCS From United States of America, joined Apr 2007, 2519 posts, RR: 44 Reply 22, posted (1 year 1 month 2 weeks 6 hours ago) and read 2334 times:
Quoting N243NW (Reply 10): Quoting DocLightning (Reply 8):
Says that the fan runs at 70% N1 but that doesn't translate into how much thrust it makes...
Another (pretty much worthless) number: the maximum reverse EPR used on the MD-80 is usually 1.6, since any amount greater than that puts the pilot in danger of blanking out the vertical stabilizer and effectively making the rudder worthless. How much thrust this EPR actually results in I can only guess.
That may be a more restrictive carrier procedure, but it is not an aircraft limitation.
DashTrash From United States of America, joined Aug 2006, 1321 posts, RR: 4 Reply 24, posted (1 year 1 month 1 week 6 days 20 hours ago) and read 2214 times:
Bucket types also provide drag when they're open. They are fairly low profile on the JT8Ds, but with the AE3007s on the Citation X they are very noticeable when you open them even before spooling the engines.
BoeingGuy From United States of America, joined Dec 2010, 2387 posts, RR: 7 Reply 25, posted (1 year 1 month 1 week 6 days 17 hours ago) and read 2166 times:
Quoting DocLightning (Reply 21): Quoting wingscrubber (Reply 18):
Does anybody know which are the aircraft that used to, or still do self-pushback on reverse thrust
DC-9 series and 727 for sure. I have been aboard both types (NW) when they powered back. It's noisier than a pushback, of course, but the acceleration wasn't spectacular. That said, it was noisy but it didn't sound like they pushed to anything like TO power. After all, one does not set 80% N1 on a power-back!
This is an oft discussed topic, probably because Powerbacks were fun to watch on be a passenger on. It was mostly done on the tail mounted models - DC-9/MD-80/717, 727 and F100. However, Eastern used to do it on their 757s and a few customers used to do with their 737-200s. AA tried it with their 757s and quickly decided that it wasn't a good idea due to great risk of FOD and greater potential for a problem if an engine failed (with a higher lever arm with wing mounted engines it would be much greater asymmetry in a tight gate area).
Boeing has an AFM limitation for most models that prohibits backing them with reverse thrust - 747, 767, 777 and 787. The 737 and 757 do not prohibit this, although I'm sure Boeing wouldn't recommend it.
NW was the last airline that I know of to regularly do Powerbacks, with their DC-9s until about 2006. FL and AA stopped it in 2005. From what I understand, AA just stopped it as a regular practice, but do not totally prohibit it if the need arises.
CO discontinued it around 15 years ago. DL and AS never did it AFAIK.
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