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DocLightning
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How Much Thrust Do T/R's Make?

Fri May 04, 2012 5:08 am

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.
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tdscanuck
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RE: How Much Thrust Do T/R's Make?

Fri May 04, 2012 7:16 am

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.).

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RE: How Much Thrust Do T/R's Make?

Fri May 04, 2012 7:23 am

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.
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RE: How Much Thrust Do T/R's Make?

Fri May 04, 2012 8:37 am

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?
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RE: How Much Thrust Do T/R's Make?

Fri May 04, 2012 5:47 pm

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.
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RE: How Much Thrust Do T/R's Make?

Fri May 04, 2012 6:44 pm

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.
 
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RE: How Much Thrust Do T/R's Make?

Sat May 05, 2012 1:48 am

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.
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26point2
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RE: How Much Thrust Do T/R's Make?

Sat May 05, 2012 1:59 am

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.
 
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RE: How Much Thrust Do T/R's Make?

Sat May 05, 2012 2:05 am

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...

But yes, it's a value...   Thx!
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CCA
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RE: How Much Thrust Do T/R's Make?

Sat May 05, 2012 2:50 am

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.

[Edited 2012-05-04 19:52:33]

[Edited 2012-05-04 19:53:43]
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N243NW
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RE: How Much Thrust Do T/R's Make?

Sat May 05, 2012 3:28 am

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.
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RE: How Much Thrust Do T/R's Make?

Sat May 05, 2012 4:39 am

Quoting DocLightning (Reply 6):
On the A320, only the IAE's do.

Slightly incorrect: The CFM56's also have this design:

http://en.wikipedia.org/wiki/File:Easyjet_thrust_reversers_arp.jpg

I say slightly because the engine can support both cascade and pivot
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RE: How Much Thrust Do T/R's Make?

Sat May 05, 2012 5:06 am

Quoting airportugal310 (Reply 11):

Slightly incorrect: The CFM56's also have this design:

So they do. I wonder why I remember them sliding...
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RE: How Much Thrust Do T/R's Make?

Sat May 05, 2012 4:39 pm

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.


http://en.wikipedia.org/wiki/CFM_International_CFM56#Reverse_thrust

Your memory is still ok  
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RE: How Much Thrust Do T/R's Make?

Sat May 05, 2012 6:34 pm

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.

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RE: How Much Thrust Do T/R's Make?

Sat May 05, 2012 7:40 pm

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.
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RE: How Much Thrust Do T/R's Make?

Sun May 06, 2012 4:03 am

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?).

I was trippin'.
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RE: How Much Thrust Do T/R's Make?

Sun May 06, 2012 2:52 pm

Quoting Larshjort (Reply 14):
Quoting CCA (Reply 9):
Using a 100,000lbs of thrust

Limited to 70% in reverse = 70,000lbs

Thrust isn't linear with RPM.


Was this not clear?

Quoting CCA (Reply 9):
Just a stab in the dark here, very rough figures. Change the limit or fan thrust as you see fit.
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RE: How Much Thrust Do T/R's Make?

Sun May 06, 2012 11:04 pm

Regarding clamshell/bucket reversers, those terms seem to be used interchangeably, but they are technically distinct types:
http://engineering.purdue.edu/~propu.../basics/bigpic/thrustreverse1.html
http://www.boeing-727.com/Data/systems/infothrustreverse.html

Some old threads:
Types Of Thrust Reversers On Engines (by CcrlR Oct 14 2005 in Tech Ops)
Thrust Reverse Types (by Baw2198 Feb 15 2004 in Tech Ops)

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.

Lastly this may be off topic, but here is some thrust reverser safety humour for anyone who hasn't seen it  http://www.youtube.com/watch?v=yz3AC93DvDo

[Edited 2012-05-06 16:13:48]

[Edited 2012-05-06 16:15:17]

[Edited 2012-05-06 16:23:28]

[Edited 2012-05-06 16:25:07]

[Edited 2012-05-06 16:25:56]
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RE: How Much Thrust Do T/R's Make?

Mon May 07, 2012 12:22 am

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)

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RE: How Much Thrust Do T/R's Make?

Mon May 07, 2012 1:02 am

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).

Tom.
 
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RE: How Much Thrust Do T/R's Make?

Mon May 07, 2012 1:44 am

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!
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RE: How Much Thrust Do T/R's Make?

Mon May 07, 2012 2:48 am

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.
 
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RE: How Much Thrust Do T/R's Make?

Mon May 07, 2012 3:55 am

Quoting PGNCS (Reply 22):
That may be a more restrictive carrier procedure, but it is not an aircraft limitation.

Got it - thanks for the input  
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RE: How Much Thrust Do T/R's Make?

Mon May 07, 2012 12:34 pm

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.
 
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RE: How Much Thrust Do T/R's Make?

Mon May 07, 2012 3:24 pm

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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