Moderators: richierich, ua900, PanAm_DC10, hOMSaR

 
nema
Topic Author
Posts: 483
Joined: Fri Feb 24, 2006 3:18 am

The Centre Engine On A 3 Holer

Fri Oct 23, 2015 12:21 pm

I know that sadly the heydays of the L1011's, DC10's, MD11's, 727's and Tristars for example are pretty much behind us now.

However, its often said that an airliner can, under certain conditions, still fly on one engine. I have often wondered would that be the case on a 3 holer where just the centre engine was working?

I ask in part because of its location atop of the aircraft plus i wonder if it had the same thrust levels of the wing mounted engines?
There isnt really a dark side to the moon, as a matter of fact its all dark!
 
sccutler
Posts: 5839
Joined: Thu Jan 27, 2000 12:16 pm

RE: The Centre Engine On A 3 Holer

Fri Oct 23, 2015 1:24 pm

Absolutely, yes, on any of the three engines, for most of the three-engined fleet.
...three miles from BRONS, clear for the ILS one five approach...
 
User avatar
glen
Posts: 354
Joined: Thu Jun 30, 2005 4:43 pm

RE: The Centre Engine On A 3 Holer

Fri Oct 23, 2015 1:29 pm

I can only speak for the MD-11. 2-engines out operation is possible, however you are very, very performance limited (for example a go-around after gear-down or below 1500ft AGL is not possible).

There are no additional restrictions whether it's a wing engine or engine number 2 (tail engine) which is still running. Handling characteristics are different in these cases due to different thrust lines, however all engines have the same thrust available.
"The horizon of many people is a circle with zero radius which they call their point of view." - Albert Einstein
 
wilco737
Posts: 7275
Joined: Sun Jun 20, 2004 12:21 am

RE: The Centre Engine On A 3 Holer

Fri Oct 23, 2015 1:31 pm

Quoting nema (Thread starter):
have often wondered would that be the case on a 3 holer where just the centre engine was working?

I was a pilot on the MD11 for 3,5 years and in the simlator it was done many times. It is mandatory training for the MD11.
You have an engine failure, fly an approach, go around with one engine out and then the 2nd engine quits. Then you fly another approach and land.

Under certain conditions even a 2 engine out go around is possible. Did that as well in the sim, quite impressive.

Now I am fling 744/ 748 and we do 2 engine out approaches in the sim all the time. No big deal. Even once we tried a 3 engine approach at Max landing weight. It was still possible, but just barely  

wilco737
  
 
User avatar
Horstroad
Posts: 524
Joined: Thu Apr 08, 2010 8:19 pm

RE: The Centre Engine On A 3 Holer

Fri Oct 23, 2015 2:32 pm

Quoting wilco737 (Reply 3):
Even once we tried a 3 engine approach

Three engine out you mean? Landing on three engines shouldn't be that much of a deal 
 
BravoOne
Posts: 4094
Joined: Fri Apr 12, 2013 2:27 pm

RE: The Centre Engine On A 3 Holer

Fri Oct 23, 2015 2:52 pm

Interesting factoid was that on the DC10-10 and the L1011-1 if you lost two engines at your HNL-Mainland ETP you would have to dump some fuel to reduce your gross weight so as to keep out of the water. As the weight was reduced you would start a drift up to somewhere around 10,000 for the remainder of the flight. Never tried it so you'll just have to take my word for it 
 
User avatar
ptrjong
Posts: 4123
Joined: Wed Mar 02, 2005 9:38 am

RE: The Centre Engine On A 3 Holer

Fri Oct 23, 2015 3:22 pm

The centre engine should be theoretically be the best for flying on one engine because of preventing assymetic thrust, right?

The three engines always have the same thrust rating but is there in practice any thrust loss, however minimal, if there's an S duct in the tail?

Peter 
The only difference between me and a madman is that I am not mad (Salvador Dali)
 
wilco737
Posts: 7275
Joined: Sun Jun 20, 2004 12:21 am

RE: The Centre Engine On A 3 Holer

Fri Oct 23, 2015 4:18 pm

Quoting horstroad (Reply 4):

Haha, of course. A 3 engine approach in the 744 is quite easy. You're right, I forgot to say a 3 engine out approach...

wilco737
  
 
MrBuzzcut
Posts: 134
Joined: Thu Jan 31, 2008 8:25 am

RE: The Centre Engine On A 3 Holer

Fri Oct 23, 2015 5:29 pm

Quoting wilco737 (Reply 3):
I was a pilot on the MD11 for 3,5 years and in the simlator it was done many times. It is mandatory training for the MD11.

There's even a You Tube video out there from when AA was flying the MD11. Pretty impressive even for a simulator:

Here's a link for anybody that hasn't seen it already: http://www.youtube.com/watch?v=uRux78eK4rk
 
Tristarsteve
Posts: 3667
Joined: Tue Nov 22, 2005 11:04 pm

RE: The Centre Engine On A 3 Holer

Fri Oct 23, 2015 5:37 pm

I worked as a technician on the Tristar for Gulf Air in BAH.
Twice I have been on board when we performed two engine ferry back to BAH.
For a L1011-1 with 22B engine the range was about an hour, the 524B engine would fly farther.
We unloaded all the catering, empty holds, empty water tank etc, minimum fuel. Boroscoped both the engines.
Even then during the first couple of minutes after take off there was no safety speed. If the second engine failed then the aircraft would descend. Bit thought provoking when we were taking off from KHI about midnight with just dark everywhere.
 
User avatar
zeke
Posts: 15112
Joined: Thu Dec 14, 2006 1:42 pm

RE: The Centre Engine On A 3 Holer

Fri Oct 23, 2015 5:42 pm

Quoting nema (Thread starter):
However, its often said that an airliner can, under certain conditions, still fly on one engine. I have often wondered would that be the case on a 3 holer where just the centre engine was working?

3 holders can also takeoff with one engine inop for a ferry, same with quads. The in engine inop takeoff will normally allow some performance even with the failure of another engine.

Quoting wilco737 (Reply 3):
Even once we tried a 3 engine approach at Max landing weight. It was still possible, but just barely

You can maintain altitude on the A340 with 3 engines out (around 5000 ft), and you can start the APU in flight. As you said it is no big deal, once you have gear and flap out for landing, you are normally committed. One of our aircraft had to do it years ago in Rome after discovering that 3 engines do not produce the same amount of thrust after ingesting a flock of birds.
Human rights lawyers are "ambulance chasers of the very worst kind.'" - Sky News
 
nema
Topic Author
Posts: 483
Joined: Fri Feb 24, 2006 3:18 am

RE: The Centre Engine On A 3 Holer

Fri Oct 23, 2015 5:51 pm

Some very interesting replies guys, thanks.
There isnt really a dark side to the moon, as a matter of fact its all dark!
 
User avatar
7BOEING7
Posts: 3039
Joined: Fri Oct 12, 2012 5:28 pm

RE: The Centre Engine On A 3 Holer

Fri Oct 23, 2015 5:51 pm

Quoting ptrjong (Reply 6):
The centre engine should be theoretically be the best for flying on one engine because of preventing assymetic thrust, right?

The three engines always have the same thrust rating but is there in practice any thrust loss, however minimal, if there's an S duct in the tail?

Relative to the 727 the thrust asymmetry was not a factor. As for the S duct on the 727 you wanted your most stable engine in the center due to possible surge issues.
 
User avatar
ptrjong
Posts: 4123
Joined: Wed Mar 02, 2005 9:38 am

RE: The Centre Engine On A 3 Holer

Fri Oct 23, 2015 6:11 pm

Quoting 7BOEING7 (Reply 12):

Relative to the 727 the thrust asymmetry was not a factor

I can see that, but I was thinking of the DC-10/MD-11/L-1011 with the #1 and #3 engines farther out.

Quoting 7BOEING7 (Reply 12):
s for the S duct on the 727 you wanted your most stable engine in the center due to possible surge issues.

As in, when you had three engines coming out of maintenance, you'd put the 'best' one in the middle?
The only difference between me and a madman is that I am not mad (Salvador Dali)
 
User avatar
Jetlagged
Posts: 2564
Joined: Sun Jan 23, 2005 3:00 pm

RE: The Centre Engine On A 3 Holer

Fri Oct 23, 2015 6:18 pm

Quoting ptrjong (Reply 6):
The three engines always have the same thrust rating but is there in practice any thrust loss, however minimal, if there's an S duct in the tail?

An S duct will introduce small losses in the form of slightly lower intake pressure at the engine. So at the same EPR as a pod engine exhaust pressure is lower and therefore the thrust will be slightly less. But relative to the other engines the difference is small.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
Euclid
Posts: 324
Joined: Fri Apr 22, 2005 3:42 am

RE: The Centre Engine On A 3 Holer

Fri Oct 23, 2015 6:53 pm

In 1982 an Eastern Airlines TriStar flying from Miami to Nassau started losing oil pressure in the center engine, which was then shut down. Due to worsening weather conditions at Nassau, the captain elected to return to Miami.

The two wing mounted engines then also lost oil pressure and both eventually flamed out. The center engine was then restarted and the aircraft made it back to Miami on that one engine.

The cause was oil loss from all engines due to the incorrect O-ring being used as a seal after some engine sensors were replaced.

Wikipedia article: https://en.wikipedia.org/wiki/Eastern_Air_Lines_Flight_855
 
PGNCS
Posts: 2260
Joined: Sun Apr 15, 2007 5:07 am

RE: The Centre Engine On A 3 Holer

Sat Oct 24, 2015 2:00 am

Quoting 7BOEING7 (Reply 12):
As for the S duct on the 727 you wanted your most stable engine in the center due to possible surge issues.

Are you saying that airlines would cherry pick the engine with the best surge/stall margins and put it in the center consciously? I flew the 727 for a while and am aware of the issues involved, but I had never heard airlines would do something like this, but I'm a pilot nor a mechanic so that's entirely possible. Could you elaborate on this a bit? Thanks!

Quoting ptrjong (Reply 13):
Quoting 7BOEING7 (Reply 12):
Relative to the 727 the thrust asymmetry was not a factor
I can see that, but I was thinking of the DC-10/MD-11/L-1011 with the #1 and #3 engines farther out.

I can't address the DC-10/MD-11, but can certainly address the L-1011: Have you seen the size of that rudder? It is HUGE. It's what allowed the engines to go so much further out the wing compared to the competition and there is MORE than enough rudder authority to deal with any realistic scenario in an L-1011.
 
User avatar
DL_Mech
Posts: 2495
Joined: Fri Feb 18, 2000 7:48 am

RE: The Centre Engine On A 3 Holer

Sat Oct 24, 2015 4:34 am

A Delta L-1011 made a successful landing in 1977 after an elevator jammed in the full nose up position prior to takeoff. The Captain firewalled the #2 engine and reduced #1 and #3 thrust to get the nose to pitch down. Not a single engine flight, but it shows the importance of the #2 engine in this case.

From a NASA narrative:

L-1011, San Diego, California

On April 12, 1977, an L-1011, Delta Airlines flight 1080, had an undetected failure in which the left stabilizer jammed in the full trailing-edge-up position before takeoff from San Diego. This failure resulted in a large noseup and rolling moment that almost exceeded the capability of the flight controls. The airplane was just about to stall in the clouds when the captain, with
unusual insight, reduced power on the wing engines and began using the throttles to supplement the remaining
flight controls, using differential and collective engine thrust. The crew of this airplane did an exceptional job,
learned rapidly, and completed a safe landing. A less capable crew would not likely have been able to save this airplane.

http://www.nasa.gov/centers/dryden/pdf/88414main_H-2048.pdf

http://www.airlinepilotforums.com/archive/index.php/t-86527.html

EDIT: See page 190+ for more about DL 1080:

http://citeseerx.ist.psu.edu/viewdoc...=10.1.1.135.2157&rep=rep1&type=pdf

[Edited 2015-10-23 21:55:21]
This plane is built to withstand anything... except a bad pilot.
 
Tristarsteve
Posts: 3667
Joined: Tue Nov 22, 2005 11:04 pm

RE: The Centre Engine On A 3 Holer

Sat Oct 24, 2015 8:47 am

Quoting Jetlagged (Reply 14):
An S duct will introduce small losses in the form of slightly lower intake pressure at the engine.

The L1011-1 had the RB211-22B with 42000lbs of thrust.
Four years later the L1011-200 was produced with the RB211-524B with 52000 lbs of thrust. The engines were the same size, and looked the same. but different inside. This resulted in 30pc more airflow in the S duct to nbr 2 engine.
It was impossible to run Nbr 2 engine up to take off power without fwd speed. It was laid out in the manuals, but above about 90pc power on the brakes and the engine would rumble. We had to tow the aircraft around the airfield to get it into wind to do high power runs.

Quoting 7BOEING7 (Reply 12):
As for the S duct on the 727 you wanted your most stable engine in the center due to possible surge issues.

Maybe, but we never swapped engines for this. In the early days we were changing engines all the time anyway. The original cast HPTB blades had a life of 1500 cycles, which they rarely achieved, and we operated many cycles a day on short hops around the Gulf.
 
Max Q
Posts: 8452
Joined: Wed May 09, 2001 12:40 pm

RE: The Centre Engine On A 3 Holer

Sat Oct 24, 2015 10:15 am

With only one engine operative on the B727 performance was very critical.


One of our simulator exercises was a two engine approach followed by a go around during which you'd lose another engine.


With just one remaining the immediate drill was to lower the nose to get 200 knots and retract the gear, flaps and slats, this had to be managed very carefully, you had to get the nose down assertively to get the speed up and be able to get clean, however, if you pushed over too much you would hit the ground.


Once clean (the FE would have started dumping fuel automatically with the loss of the second engine down to the 'standpipes' around 10k pounds) you were able to climb at around 2-300 fpm, in other words barely and this depended
on good technique as described.


You would then maneuver for a Flaps 5 landing at around 170 knots, this approach was initially flown at around 200 knots slowly decelerating down the glideslope, keeping as much speed in hand as possible due to the lack of performance.


Once you lowered the gear there was no possibility of going around, you were committed.



Never did it for real but I did do three real one engine inop landings in the B727, that was very uneventful, not even considered an emergency.
The best contribution to safety is a competent Pilot.


GGg
 
User avatar
Jetlagged
Posts: 2564
Joined: Sun Jan 23, 2005 3:00 pm

RE: The Centre Engine On A 3 Holer

Sat Oct 24, 2015 4:05 pm

Quoting Tristarsteve (Reply 18):
Four years later the L1011-200 was produced with the RB211-524B with 52000 lbs of thrust. The engines were the same size, and looked the same. but different inside. This resulted in 30pc more airflow in the S duct to nbr 2 engine.
It was impossible to run Nbr 2 engine up to take off power without fwd speed. It was laid out in the manuals, but above about 90pc power on the brakes and the engine would rumble. We had to tow the aircraft around the airfield to get it into wind to do high power runs.

Interesting stuff. One question though. How did the No.2 engine generate more airflow through the S-duct? Or was the duct itself modified?
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
Tristarsteve
Posts: 3667
Joined: Tue Nov 22, 2005 11:04 pm

RE: The Centre Engine On A 3 Holer

Sat Oct 24, 2015 7:12 pm

Quoting Jetlagged (Reply 20):
Or was the duct itself modified?

No the duct was the same. It just worked  
 
KC135Hydraulics
Posts: 443
Joined: Sat Nov 03, 2012 2:05 am

RE: The Centre Engine On A 3 Holer

Sat Oct 24, 2015 7:19 pm

Quoting Jetlagged (Reply 20):
Interesting stuff. One question though. How did the No.2 engine generate more airflow through the S-duct? Or was the duct itself modified?

I think what he meant was that there was a 30% increase in the flow of air being sucked into the engine as a result of the increase in engine thrust. Without some forward momentum or a headwind, the engine could not consume enough air to reach full power because of the size of the duct.
MSgt, USAF
KC-135R / C-17A Pneudraulic Systems Mechanic Supervisor
 
User avatar
tb727
Posts: 2222
Joined: Thu Jun 30, 2005 1:40 pm

RE: The Centre Engine On A 3 Holer

Sat Oct 24, 2015 8:47 pm

Quoting Jetlagged (Reply 14):
An S duct will introduce small losses in the form of slightly lower intake pressure at the engine. So at the same EPR as a pod engine exhaust pressure is lower and therefore the thrust will be slightly less. But relative to the other engines the difference is small.

Add .02 to #2! If the Engine Anti-ice is on, they are all the same.

Quoting Max Q (Reply 19):
One of our simulator exercises was a two engine approach followed by a go around during which you'd lose another engine.

Yep, on the type and PC's the Feds had been wanting the 2nd engine failure at 400' on the 2 engine go-around, it was a handful but I kinda knew it was coming so I maybe got her climbing a little quicker to get a couple hundred more feet for the pattern!

Quoting Max Q (Reply 19):
Never did it for real but I did do three real one engine inop landings in the B727, that was very uneventful, not even considered an emergency.

When I went into training on the Airbus they kept waiting for me to declare an emergency when we lost an engine. I had to get out of the mindset that an engine out was an abnormal in the 727, not an emergency!

Quoting PGNCS (Reply 16):
Are you saying that airlines would cherry pick the engine with the best surge/stall margins and put it in the center consciously? I flew the 727 for a while and am aware of the issues involved, but I had never heard airlines would do something like this, but I'm a pilot nor a mechanic so that's entirely possible. Could you elaborate on this a bit? Thanks!

The only compressor stalls I've had on that airplane it was on #2. Once in a very, very heavy airplane out of LRD right as we lifted off on a windy night, that will get your attention. Another time just randomly while cruising along in smooth air at FL370 out of OAK going to YIP, that will wake you up too. I'd rather have that one fail so put the junk one in the middle! Not that it was much of a big deal, about 2 spins of the rudder trim to get it close and she flew just fine with a pod out.
Too lazy to work, too scared to steal!
 
PGNCS
Posts: 2260
Joined: Sun Apr 15, 2007 5:07 am

RE: The Centre Engine On A 3 Holer

Sat Oct 24, 2015 11:11 pm

Quoting tb727 (Reply 23):
Quoting PGNCS (Reply 16):
Are you saying that airlines would cherry pick the engine with the best surge/stall margins and put it in the center consciously? I flew the 727 for a while and am aware of the issues involved, but I had never heard airlines would do something like this, but I'm a pilot nor a mechanic so that's entirely possible. Could you elaborate on this a bit? Thanks!

The only compressor stalls I've had on that airplane it was on #2. Once in a very, very heavy airplane out of LRD right as we lifted off on a windy night, that will get your attention. Another time just randomly while cruising along in smooth air at FL370 out of OAK going to YIP, that will wake you up too. I'd rather have that one fail so put the junk one in the middle! Not that it was much of a big deal, about 2 spins of the rudder trim to get it close and she flew just fine with a pod out.

I have also only had #2 compressor stall on the 727 as far as I can recall, though the only one I ever had outright fail on the 727 was #1. Same thing as you for the most part: windy with a gusty crosswind on the takeoff roll is when I remember it being most common. Also agree that directional control was not an issue engine out on the 727.
 
Max Q
Posts: 8452
Joined: Wed May 09, 2001 12:40 pm

RE: The Centre Engine On A 3 Holer

Sun Oct 25, 2015 2:49 am

Quoting tb727 (Reply 23):
Yep, on the type and PC's the Feds had been wanting the 2nd engine failure at 400' on the 2 engine go-around, it was a handful but I kinda knew it was coming so I maybe got her climbing a little quicker to get a couple hundred more feet for the pattern!

Good technique that, I saw a few people crash on this exercise !

Quoting tb727 (Reply 23):
When I went into training on the Airbus they kept waiting for me to declare an emergency when we lost an engine. I had to get out of the mindset that an engine out was an abnormal in the 727, not an emergency!

Thats funny, it was nice to have three engines.



During strong crosswind conditions we would set power normally on #1 and 3 then slowly bring the power up on #2, seemed to work pretty well in avoiding compressor stalls.
The best contribution to safety is a competent Pilot.


GGg
 
ThirtyEcho
Posts: 1411
Joined: Tue Jan 01, 2002 1:21 am

RE: The Centre Engine On A 3 Holer

Sun Oct 25, 2015 4:31 am

Then there is the dreaded B52 seven engine approach.
 
User avatar
TheRedBaron
Posts: 3276
Joined: Tue Mar 29, 2005 6:17 am

RE: The Centre Engine On A 3 Holer

Sun Nov 08, 2015 5:38 am

Quoting ThirtyEcho (Reply 26):
Then there is the dreaded B52 seven engine approach.

LOL....good one! But those are not engines they are hairdryers!

TRB
The best seat in a Plane is the Jumpseat.
 
113312
Posts: 671
Joined: Sat Apr 16, 2005 9:09 am

RE: The Centre Engine On A 3 Holer

Sun Nov 08, 2015 3:37 pm

As the years went by, it was not uncommon to have a mix of JT-8D versions installed on B727 aircraft. They could be operated with equal EPR to produce nearly equal thrust. Some installations, and operators, had approvals for maximum takeoff thrust to be determined by the individual version of the engine and thus slightly different maximum thrust.

A late and limited modification to the -200 model involved replacement of the pod mounted engines with JT-8D-217C engines, thrust reversers, and cowls similar to MD-83 aircraft. The Valsan conversion. Space and weight constraints did not allow the center, #2 engine, to be replaced in this manner and remained as a JT-8D-17 version. The result was that there was a large difference in maximum thrust available between the center and pod engines. All performance considerations for the loss of two engines were predicated on the worst case which would be having only the center engine operating.
 
User avatar
Classa64
Posts: 323
Joined: Fri Apr 03, 2015 5:40 pm

RE: The Centre Engine On A 3 Holer

Mon Nov 09, 2015 12:58 am

Quoting glen (Reply 2):
Quoting wilco737 (Reply 3):
Now I am fling 744/ 748 and we do 2 engine out approaches in the sim all the time. No big deal. Even once we tried a 3 engine approach at Max landing weight. It was still possible, but just barely

Am I guessing right in saying on a 3 engine with two out or a 4 engine with 3 out that the one engine is basically allowing you a longer glide slope? Basically keeping you up for a longer duration and distance but still not able to climb?

Edit to add; I also realize if the plane is heavy or lite will depend on weather you could climb or not

[Edited 2015-11-08 17:06:18]
"Freedom is the miles i'm rolling on"
 
User avatar
Starlionblue
Posts: 20036
Joined: Fri Feb 27, 2004 9:54 pm

RE: The Centre Engine On A 3 Holer

Mon Nov 09, 2015 1:36 am

Quoting Classa64 (Reply 29):
Quoting glen (Reply 2):
Quoting wilco737 (Reply 3):
Now I am fling 744/ 748 and we do 2 engine out approaches in the sim all the time. No big deal. Even once we tried a 3 engine approach at Max landing weight. It was still possible, but just barely

Am I guessing right in saying on a 3 engine with two out or a 4 engine with 3 out that the one engine is basically allowing you a longer glide slope? Basically keeping you up for a longer duration and distance but still not able to climb?

Edit to add; I also realize if the plane is heavy or lite will depend on weather you could climb or not

In essence yes but it is very much dependent on weight. And it goes for any plane. A twin at FL390 that loses an engine will have a maximum climb gradient. The climb gradient will be negative but this does not change the physics. Once you get down to the drift-down altitude for that weight the climb gradient is zero.

The best climb gradient will be at L/Dmax, which is Vx speed, (designated "green dot" on Airbus). That's why in a drift-down situation you let the plane slow in level flight to Vx before descending, barring ATC or routing considerations which would make you descend before reaching that speed.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Viscount724
Posts: 19316
Joined: Thu Oct 12, 2006 7:32 pm

RE: The Centre Engine On A 3 Holer

Mon Nov 09, 2015 5:03 am

As a sidenote, the intake on the 727's #2 engine was changed from oval on the -100 to round on the -200. If not mistaken that was an attempt to reduce cases of #2 engine surges, which even happened on the 727's first flight.

727-100, oval intake:


View Large View Medium
Click here for bigger photo!

Photo © Jason McDowell
View Large View Medium
Click here for bigger photo!

Photo © Bill Armstrong


View Large View Medium
Click here for bigger photo!

Photo © Marlo Plate - Iberian Spotters




727-200, round intake:


View Large View Medium
Click here for bigger photo!

Photo © Gregg Stansbery
View Large View Medium
Click here for bigger photo!

Photo © Chris Coduto

 
User avatar
Classa64
Posts: 323
Joined: Fri Apr 03, 2015 5:40 pm

RE: The Centre Engine On A 3 Holer

Sun Nov 15, 2015 5:19 pm

Quoting Starlionblue (Reply 30):
A twin at FL390 that loses an engine will have a maximum climb gradient. The climb gradient will be negative but this does not change the physics.

How can a Climb gradient be neg? I have no flying experience other than flight Sim could you elaborate on this for me to understand.

Quoting Starlionblue (Reply 30):
The best climb gradient will be at L/Dmax, which is Vx speed, (designated "green dot" on Airbus). That's why in a drift-down situation you let the plane slow in level flight to Vx before descending,

Could you simplify this reply for me, lost now  

My thinking is; in a Twin at FL390 you loose an engine, your going down or loosing altitude weather your heavy or light correct? Because one engine at that altitude will not sustain enough forward speed to produce enough lift at that altitude. Once you get down to thicker air you gain more lift at lower speeds, but climbing is still out of the question I would think? Please correct me if I am wrong and maybe I could try it in my sim, just trying to wrap my head around it.
"Freedom is the miles i'm rolling on"
 
User avatar
Florianopolis
Posts: 333
Joined: Sun Mar 29, 2015 2:54 pm

RE: The Centre Engine On A 3 Holer

Sun Nov 15, 2015 6:17 pm

Quoting Classa64 (Reply 32):
How can a Climb gradient be neg?

It's just another way to think about it. In your typical cruise regime, you need the thrust from all your engines to keep you up there. Climb gradient is a function of available thrust, and when your available thrust decreases, the maximum rate you'll be able to climb may dip below zero.

Quoting Classa64 (Reply 32):
Could you simplify this reply for me, lost now

Think about it this way: An airplane can fly level at 100 knots, or 200 knots. The lift is the same at 100 or 200 (we're not climbing or descending, so lift=weight). But the drag at those two airspeeds is different. Hence, the L/D is different between those two airspeeds. For a given airplane weight (and hence the same lift), there is an airspeed that will give you the lowest drag to maintain level flight, and thus requires the least amount of thrust (let's not get into accelerated climbs or descents). Since thrust from your engines (or gravity in a descent) is what keeps your airspeed, flying at the best L/D keeps you as high as you possibly can be given your available thrust. In cruise you're probably flying faster than your best L/D speed, so when you lose an engine you will bleed off airspeed at level flight before descending at the drift-down speed.
 
User avatar
Starlionblue
Posts: 20036
Joined: Fri Feb 27, 2004 9:54 pm

RE: The Centre Engine On A 3 Holer

Mon Nov 16, 2015 3:57 am

Quoting Classa64 (Reply 32):

Quoting Starlionblue (Reply 30):
A twin at FL390 that loses an engine will have a maximum climb gradient. The climb gradient will be negative but this does not change the physics.

How can a Climb gradient be neg? I have no flying experience other than flight Sim could you elaborate on this for me to understand.

If a cimb gradient is negative, the airplane is in a negative climb, or in other words descending. Semantics, perhaps, but there is an important point about speed. The best lift to drag ratio speed is dependent on weight and altitude. It will give you the best angle of climb, which may be negative. The speed required is the same. And if you are actually descending, you will be descending at the lowest possible descent gradient, which is still the highest possible climb gradient, albeit a negative number.

Quoting Florianopolis (Reply 33):
Quoting Classa64 (Reply 32):
Could you simplify this reply for me, lost now

Think about it this way: An airplane can fly level at 100 knots, or 200 knots. The lift is the same at 100 or 200 (we're not climbing or descending, so lift=weight). But the drag at those two airspeeds is different. Hence, the L/D is different between those two airspeeds. For a given airplane weight (and hence the same lift), there is an airspeed that will give you the lowest drag to maintain level flight, and thus requires the least amount of thrust (let's not get into accelerated climbs or descents). Since thrust from your engines (or gravity in a descent) is what keeps your airspeed, flying at the best L/D keeps you as high as you possibly can be given your available thrust. In cruise you're probably flying faster than your best L/D speed, so when you lose an engine you will bleed off airspeed at level flight before descending at the drift-down speed.

Thanks for clarifying.

Quoting Classa64 (Reply 32):
My thinking is; in a Twin at FL390 you loose an engine, your going down or loosing altitude weather your heavy or light correct? Because one engine at that altitude will not sustain enough forward speed to produce enough lift at that altitude. Once you get down to thicker air you gain more lift at lower speeds, but climbing is still out of the question I would think? Please correct me if I am wrong and maybe I could try it in my sim, just trying to wrap my head around it.

Correct. However as Florianopolis explained it all depends on excess thrust. If you have excess thrust you are using some of the engine thrust to stay at FL390, and some to fly faster than L/Dmax. If you have a deficit of thrust (because you lost an engine), you can still fly at L/Dmax, but you would be drifting down. The higher you go, the less thrust you have and the less lift due to thinner air. So at a certain altitude (dependent on weight), you no longer have excess thrust and you're level at L/Dmax. The concepts don't change whether you have both engines or one (barring some drag from the asymmetric thrust).

The logic can be seen again in the performance figures once you've lost an engine. You can continue at L/Dmax at your drift-down altitude (the max altitude where you can maintain L/Dmax on one engine) or accelerate to Engine-Out Long Range Cruise while sacrificing some altitude.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
User avatar
Classa64
Posts: 323
Joined: Fri Apr 03, 2015 5:40 pm

RE: The Centre Engine On A 3 Holer

Tue Nov 17, 2015 1:56 am

Quoting Florianopolis (Reply 33):
Quoting Starlionblue (Reply 34):

Great answers thanks so much, I understand a lot better now.

Cheers
"Freedom is the miles i'm rolling on"

Who is online

Users browsing this forum: Crackshot, swisslue and 9 guests

Popular Searches On Airliners.net

Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

Military Aircraft Every type from fighters to helicopters from air forces around the globe

Classic Airliners Props and jets from the good old days

Flight Decks Views from inside the cockpit

Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

Cargo Aircraft Pictures of great freighter aircraft

Government Aircraft Aircraft flying government officials

Helicopters Our large helicopter section. Both military and civil versions

Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

Night Photos Beautiful shots taken while the sun is below the horizon

Accidents Accident, incident and crash related photos

Air to Air Photos taken by airborne photographers of airborne aircraft

Special Paint Schemes Aircraft painted in beautiful and original liveries

Airport Overviews Airport overviews from the air or ground

Tails and Winglets Tail and Winglet closeups with beautiful airline logos