Mastropiero
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Do Airplanes Handle Differently? (same Type)

Mon Apr 04, 2011 8:24 pm

I was wondering if, as pilots, you´ve come across aircrafts within your company´s fleet that handle differently than others (I am talking, obviously, about same type), perhaps due to the engines/airframe having more cycles or any other time/wear related issues?

I would assume aircrafts of the same type will handle very similarly, if not identically. I just wonder wether time and usage have the same effect they have on other machines and, if so, if you guys can "feel" this differences.

Thank you very much!

[Edited 2011-04-04 13:27:30]
 
DashTrash
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RE: Do Airplanes Handle Differently? (same Type)

Mon Apr 04, 2011 10:21 pm

Short answer, Absolutely!
 
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nitepilot79
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RE: Do Airplanes Handle Differently? (same Type)

Mon Apr 04, 2011 10:57 pm

I love airplanes, and I also love guitars. I think IMHO that maybe airplanes, as well as instruments, have individuality no matter how uniform their production might seem.
 
Fly2HMO
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RE: Do Airplanes Handle Differently? (same Type)

Mon Apr 04, 2011 11:55 pm

Very much so. No two planes are alike.

If you ask almost any major airline pilot, and more so MX personnel, I bet they'll have a couple of planes they hate to fly/service, or some planes that have some weird quirks. There were several like those at my former airline. There was always a few that were always the hangar queens, or the ones that never flew straight no matter how many times they got re-rigged. Like with some cars, some planes really do have a temperamental personality so to speak.

[Edited 2011-04-04 17:02:40]
 
dw747400
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RE: Do Airplanes Handle Differently? (same Type)

Tue Apr 05, 2011 12:47 am

Ask any dispatcher and mechanic, and they can tell you what aircraft at their company causes the most trouble. One specific aircraft (a 767) earned a horrible reputation with a major, but was offered for sale at the right price and management bought it over the objections of staff that had worked with it in the past... needless to say, it was an expensive mistake!

Not transport category aircraft, but the best example I can think of is a pair of 172's I instruct in.

Same year, or a few frames apart on the line. Both have the exact same STC providing them with a gross weight of 2,550 pounds and 180 horsepower. They even have the same paint. Both have been maintained and rigged by the same mechanic for several years. The only difference is that one has a mid-range S-Tec autopilot, the other does not.

One of the aircraft cruises at 110 to 112 knots at typical power settings, the other 120 to 125 knots. The faster plane also climbs 20% faster and has so much more glide performance than any other 172 I've flown--to the point that (for low time and student pilots) it requires as much difference training on engine outs as it would to transition to another airframe like a 152 or 182.

Personally, though, my favorite 172 is an old 1967 model that trues 137 knots at 70% power. Fastest one I know of!
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ThirtyEcho
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RE: Do Airplanes Handle Differently? (same Type)

Tue Apr 05, 2011 1:40 am

Quoting dw747400 (Reply 4):
Personally, though, my favorite 172 is an old 1967 model that trues 137 knots at 70% power. Fastest one I know of!

Sounds like you found the single C172 from that era that isn't out of rig. Seriously, are you sure that isn't a C175? The 175 flew way above book if you flew it by the POH numbers. Most pilots from that time flew the 175 like it was a 172 and got lousy airspeeds and short TBOs as a result. I have 600 hours in one from that time and it was a heck of a good machine.

I don't remember when the flap limitation to 30 degrees happened but that 1967 bird may have the full 40 degree flap travel. Properly trained, you will learn to love that but you need to have an experienced instructor show you how to use them and the attendant warnings about them.
 
Fly2HMO
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RE: Do Airplanes Handle Differently? (same Type)

Tue Apr 05, 2011 3:08 am

Quoting dw747400 (Reply 4):

Personally, though, my favorite 172 is an old 1967 model that trues 137 knots at 70% power.

Hmmm, you sure its last pitot/static inspection wasn't when it rolled out of the factory?  
 
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notaxonrotax
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RE: Do Airplanes Handle Differently? (same Type)

Tue Apr 05, 2011 4:19 am

Quoting dw747400 (Reply 4):

Same year, or a few frames apart on the line. Both have the exact same STC providing them with a gross weight of 2,550 pounds and 180 horsepower. They even have the same paint. Both have been maintained and rigged by the same mechanic for several years. The only difference is that one has a mid-range S-Tec autopilot, the other does not.

One of the aircraft cruises at 110 to 112 knots at typical power settings, the other 120 to 125 knots.

That is amazing, I can´t grasp that!!
What would cause that?
Would a change of engine turn this tide around……..or perhaps swapping the wings between the 2 would do the trick?
I´d be curious what causes such a large difference between identical aircraft!!!??

At our club we have a few "sisters"; planes that were close enough on the production line, but they have been kitted out different by the several owners over the years……….and still perform very similar--> speed wise I mean. (they´re all faster than me!!)
This is confirmed by the club mechanic that flies them all, at times.

1 is a little faster because of only 1 wing support each side instead of 2 both sides--> a slightly newer model!

But no mysterious differences like dw747400 reported!

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ThirtyEcho
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RE: Do Airplanes Handle Differently? (same Type)

Tue Apr 05, 2011 6:33 am

I'm somewhat amazed at the pitot-static problems being reported here in Cessna 172s. Does it really cruise at 145KIAS? No, you didn't get to FL210 or land at 15KIAS. Try getting the dead wasp out of the tube and the itsy-bitsy spider out of the static port..
 
wilco737
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RE: Do Airplanes Handle Differently? (same Type)

Tue Apr 05, 2011 7:52 am

Well, on the 744s I flew so far they were very similar. Only minor differences in handling. I would never be able to tell just by the handling of the airplane which registration it is.

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Goldenshield
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RE: Do Airplanes Handle Differently? (same Type)

Tue Apr 05, 2011 10:06 am

Quoting dw747400 (Reply 4):
One of the aircraft cruises at 110 to 112 knots at typical power settings, the other 120 to 125 knots. The faster plane also climbs 20% faster and has so much more glide performance than any other 172 I've flown--to the point that (for low time and student pilots) it requires as much difference training on engine outs as it would to transition to another airframe like a 152 or 182.

So, instead of 9:1, it's 10:1?   

Still, I can understand what you mean, having flown 152s an 172s in the past; although, suffice to say, even with a low performance training glider, that 172 is still a rock.   
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mrskyguy
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RE: Do Airplanes Handle Differently? (same Type)

Wed Apr 06, 2011 4:05 am

John Nance spoke of a 727 in the fleet that simply "few funny" and never trimmed out quite right. Many pilots reported that while she flew, she just didn't feel comfortable to anyone. I believe it was Ship 408, which through some mistake in communication between management and the line, was painted by Calder and therefore got more public attention than any other bright Braniff jet. Lipstick on a pig, if you will.

I'm pretty sure that's the plane, because Nance spoke of considerable fuselage wrinkling just aft and below of the wing box from one (or many) very hard landings (something not exclusive to this aircraft, but apparently more noticeable on ship 408 than other BNF 727s).. take a look for yourself:


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dkswim
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RE: Do Airplanes Handle Differently? (same Type)

Wed Apr 06, 2011 10:11 am

In my reading about U-2s pilots would report slightly different handling from plane to plane. Slightly different stall speeds and other handling differences. Some were notorious hanger queens. One story was of on landing one wing would stall out 5-10th knots faster then other side, after repented attempts to rig the plane it was decided to pull wings off and measure them found a little difference on wing to the other of no more then thickness of sheet metal. Chalked difference up to that and subtle differences of one off airplanes.
 
Mastropiero
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RE: Do Airplanes Handle Differently? (same Type)

Wed Apr 06, 2011 12:17 pm

Thank you all for your replies. I did assume that, as every mass-produced machine, airplanes would be slightly different from eachother, much like cars are. However, considering the much higher standards of quality applied to aircrafts production, I was reluctant to use that comparison.
 
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Zkpilot
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RE: Do Airplanes Handle Differently? (same Type)

Wed Apr 06, 2011 8:54 pm

Quoting notaxonrotax (Reply 7):
Quoting dw747400 (Reply 4):

Same year, or a few frames apart on the line. Both have the exact same STC providing them with a gross weight of 2,550 pounds and 180 horsepower. They even have the same paint. Both have been maintained and rigged by the same mechanic for several years. The only difference is that one has a mid-range S-Tec autopilot, the other does not.

One of the aircraft cruises at 110 to 112 knots at typical power settings, the other 120 to 125 knots.

That is amazing, I can´t grasp that!!
What would cause that?
Would a change of engine turn this tide around……..or perhaps swapping the wings between the 2 would do the trick?
I´d be curious what causes such a large difference between identical aircraft!!!??

Main thing that would cause that would be the prop... one probably has a coursepitch and the other fine.
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dw747400
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RE: Do Airplanes Handle Differently? (same Type)

Wed Apr 06, 2011 10:32 pm

Quoting Zkpilot (Reply 14):
Main thing that would cause that would be the prop... one probably has a coursepitch and the other fine.

You would think so, but in general he course should be better in cruise and the fine better in climb... in this case, the same airplane wins in both.

Quoting ThirtyEcho (Reply 5):
Sounds like you found the single C172 from that era that isn't out of rig. Seriously, are you sure that isn't a C175?

Well, I did leave out the fact it's a USAF T-41C, so it does have 210hp up front  ... still, I've flown several of them and they rarely make it above 115 KTAS (most have climb props though, this one has a cruise pitched prop--still, it is a heck of a difference).

Quoting ThirtyEcho (Reply 8):
I'm somewhat amazed at the pitot-static problems being reported here in Cessna 172s. Does it really cruise at 145KIAS? No, you didn't get to FL210 or land at 15KIAS. Try getting the dead wasp out of the tube and the itsy-bitsy spider out of the static port..
Quoting Fly2HMO (Reply 6):

Hmmm, you sure its last pitot/static inspection wasn't when it rolled out of the factory?

Hah! Quite sure it is current. In addition to the logs, we've verified the the speed with GPS in multiple directions to try and account for wind. Not perfect, of course, but I'm quite confident that number is within 5 knots or so of what she is actually doing. And she can do it heavy too--we've hit 135KTAS at a few pounds under gross (2500). Commuting from Baltimore to NYC she knocks almost 20 minutes off the round trip travel time compared to her sisters.

To make things more fun, IAS is in MPH, so passengers enjoy seeing that dial just a hair under 150!
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ShyFlyer
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RE: Do Airplanes Handle Differently? (same Type)

Thu Apr 07, 2011 2:09 am

Quoting Mastropiero (Reply 13):
However, considering the much higher standards of quality applied to aircrafts production, I was reluctant to use that comparison.

Right off the factory assembly line, there will likely be very little, if any, detectible difference between two aircraft. One they are out "in the field," each of those aircraft will be exposed to different flying styles, usage rates, maintenance encounters, etc even if they are in the same fleet. That is, barring any sort of "incident" occurring at the factory that would give a specific aircraft a "personality trait."

In my case, I flew 4 different Cessna 172s belonging to a local FBO. Two of them R models, the others SPs. With the R models, "16U" was my favorite. Despite a pilot side door that was difficult to latch at times, it always flew like a dream. "062," on the other hand always required more nose down trim. Always.

Now with the SPs, "2SP" was my choice. It was the only one of the four that I could fly with my fingertips. I rarely had to touch the trim and when I did, I only needed to make tiny adjustments. Never had a bounced landing in that one. "3SP," while it had a nicer (leather) interior and an autopilot, wasn't one I looked forward to flying. I was convinced it was the only aircraft on the planet that didn't want to fly. It required more back-pressure on the yoke to get it airborne. Handled like a truck in the air too.
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RE: Do Airplanes Handle Differently? (same Type)

Thu Apr 07, 2011 8:35 pm

What can even be more interesting is when a plane goes into maintenance (heavy check) and comes out handling completely different then before it went in. I have flown a 1900 that flew great and was a real joy to fly, until one month it went in for a heavy check. That aircraft never quite came out the same and handled completely different then from before. So in short, absolutely, and their are many different variables that can contribute to the way an aircraft can handle and fly.
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fsnuffer
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RE: Do Airplanes Handle Differently? (same Type)

Fri Apr 08, 2011 11:04 am

I see a lot of posts about Boeing aircraft, how about Airbus FBW aircraft? I would assume the software would "null" out any differences and make them all seem the same to fly.
 
Magcheck
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RE: Do Airplanes Handle Differently? (same Type)

Fri Apr 08, 2011 7:31 pm

Hey dw747400, I realize from your posts that I know you from a certain *distinguished* flying club in Maryland, and you're right about our T-41's being different! My personal fav is -93, so stay the heck out of it! hahaa

Back on subject, I would suspect that differences in overall rigging and feel would be much more detectable on smaller aircraft, especially because hydraulics/FBW systems would dampen a lot of the "feel" you get when you're dealing with bellcranks and pulleys and cables. I've selected certain light aircraft to fly based solely on my perception (real or imagined) that they're "nicer" or "feel better" than others, even if they're essentially the same aircraft.

I notice that our 172's and even 152's seem to fly a lot smoother than the T-41's-- in my early days I was chasing the T-41's all over the sky during steep turns and other maneuvers, while the other aircraft would "lock in" and stabilize very nicely. I still swear I can fly them better than I ever could the T-41's.
 
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RE: Do Airplanes Handle Differently? (same Type)

Fri Apr 08, 2011 11:33 pm

Quoting fsnuffer (Reply 18):
I would assume the software would "null" out any differences and make them all seem the same to fly.

Oh, the magical wonders of Fly By Wire.

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KELPkid
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RE: Do Airplanes Handle Differently? (same Type)

Sat Apr 09, 2011 4:31 am

I swear, one of the flight school's 172s had it out for me, someone must have tweaked the rudder trim tab, as it would wander off course in a heartbeat if you weren't watching the turn and bank like a hawk, especially when tracking the localizer down at decision height...   P.S. Squawking the bird for its squirrely directional control tendencies did no good, and resulted in snarky comments from maintenance  
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Aircellist
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RE: Do Airplanes Handle Differently? (same Type)

Sat Apr 09, 2011 1:41 pm

...That planes comparison reminds me of that day when I tried 22 cello bows for a student... So much difference! Yet, those were all supposedly "the same": same price, same makers, same specs, etc...
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KELPkid
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RE: Do Airplanes Handle Differently? (same Type)

Sat Apr 09, 2011 6:32 pm

Quoting Goldenshield (Reply 10):
Still, I can understand what you mean, having flown 152s an 172s in the past; although, suffice to say, even with a low performance training glider, that 172 is still a rock.

Not as much as a PA28  The 172 is a sailplane compared to the Cherokee...  
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Mastropiero
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RE: Do Airplanes Handle Differently? (same Type)

Sat Apr 09, 2011 7:55 pm

Quoting Aircellist (Reply 22):
...That planes comparison reminds me of that day when I tried 22 cello bows for a student... So much difference!

I thought about bringing up this comparison too, being a professional violinist myself. 22 bows? My hat is off to you, that takes some patience!!! I remember once I spent a whole morning trying to decide between only two of them.....

Sorry for going off-topic!  
 
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Zkpilot
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RE: Do Airplanes Handle Differently? (same Type)

Sun Apr 10, 2011 7:25 pm

Quoting dw747400 (Reply 15):
Quoting Zkpilot (Reply 14):
Main thing that would cause that would be the prop... one probably has a coursepitch and the other fine.

You would think so, but in general he course should be better in cruise and the fine better in climb... in this case, the same airplane wins in both.

Yes that was the part that troubled me... I'm thinking that the course pitch aircraft perhaps has a slightly better performing engine and maybe slightly lighter with a CoG more ideally located. That would allow it to climb better but get the faster cruise speed as that is the harder task to accomplish in a prop.
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jetstar
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RE: Do Airplanes Handle Differently? (same Type)

Mon Apr 11, 2011 5:43 pm

One of many reasons that 2 identical airplanes can handle differently in flight is in the rigging of the flight controls.

You can take airplanes that are a few years old and check the rigging and will probably find the rigging out slightly. As an airplane ages, the flight control cables stretch slightly, and this can affect the tension on the cables.

In my years working on small airplanes at an FBO, I did my fair share of trouble shooting problems on why the airplane did not fly straight and level.

On small single engine Cessna’s, the 2 problem areas are the aileron and rudder cables. On the ailerons, they use a 3 cable system, one cable goes from the control wheel to the left aileron bell crank located in the wing by the aileron, the other goes from the right aileron bell crank and the third goes from either aileron bell crank though the cabin ceiling to the other aileron bell crank.

I have found excessively worn bearing ends in the rods from the bell crank to the aileron and pulleys where the bearings were also worn. In one case the bolt through the bell crank was also worn so there was excessive play in there.

On the rudder cables, there are 2 springs that keep the rudder centered, they are located by the rudder pedal bars at the base of the control column. The springs can loose tension over the years or even break and if so will affect the centering of the rudder in flight. In one case I found a larger spring installed on one side, it appeared that someone had replaced a spring with one bought from a hardware store. I always recommended that if one spring had to be replaced, replace both with Cessna parts so they are equal in tension.

One time on a complaint about the airplane not flying straight and level, we checked all the cables and everything checked out, what we found was the nose wheel strut did not fully extend with no weight on it. Cessna nose wheel struts have a self centering cam inside the strut so in the last bit of strut extension, the nose wheel centers to prevent the nose wheel from acting like a small rudder in flight, especially if equipped with the wheel fairing. We found the drag links had not been greased in a long time and bound up so it prevented the nose strut from fully extending and engaging the self centering cam.

Another time we found that one of the flaps was not fully retracting, it stuck out about one eighth of an inch and this acted like an inboard aileron, the problem there was a broken pulley.

I won’t go into detail on how to rig the flight control system, The airplanes maintenance manual has the specific instructions and should be done by a properly qualified mechanic, but if the airplane is out of rig, then there should be some performance improvement when then airplane is properly rigged. The Cessna Pilots Association occasionally runs courses on how to properly rig the flight control cables.

JetStar
 
daviation
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RE: Do Airplanes Handle Differently? (same Type)

Mon Apr 11, 2011 8:04 pm

A bit off-topic, but I own a Steinway 6-foot grand piano. Before my purchase, I tried several of these pianos in the studio, the exact same size and model, all manufactured around the same time. Steinway isn't some knock-off brand. They pride themselves on being one of the world's great piano manufacturers. And yet each piano was so completely different from its sister, it was as if they were distant cousins. Of course you cannot compare a Steinway with a Boeing or Airbus, but there is hand-manufacturing in all of them, and these have to account for some differences.
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Aircellist
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RE: Do Airplanes Handle Differently? (same Type)

Tue Apr 12, 2011 2:44 am

Hey, Mastropiero, Daviation, this must be a first! An A.net thread hijacked by musicians! Yay!

Maybe we should create the A.net's Musician's Guild, open to amateurs and professionals?

Back to the topic (to prevent deletion...), nothing has been said about Airbus planes yet, although the question has been asked: are they really identical in handling?

If so, that will indeed be quite remarkable.
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tdscanuck
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RE: Do Airplanes Handle Differently? (same Type)

Tue Apr 12, 2011 2:57 am

Quoting Aircellist (Reply 28):
Back to the topic (to prevent deletion...), nothing has been said about Airbus planes yet, although the question has been asked: are they really identical in handling?

It's not really Airbus v. Boeing, it's FBW vs. non...the glory of closed-loop control laws is that they reject disturbances. There are other (good) reasons to do this but, as a side effect, they'll cover up all but the most egregious build differences. I do know of at least one FBW airliner that, although it handles perfectly normally, does perpetually fly just slightly one wing down.

Tom.
 
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RE: Do Airplanes Handle Differently? (same Type)

Tue Apr 12, 2011 3:21 am

Quoting tdscanuck (Reply 29):
although it handles perfectly normally, does perpetually fly just slightly one wing down.

IIRC the 737 classic that went down as Adam Air Flight 574, had a slow right roll that was contributing to the crash.
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Aircellist
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RE: Do Airplanes Handle Differently? (same Type)

Tue Apr 12, 2011 4:22 am

Thanks, Tom, again.

I'm somehow glad that there may still be a difference between those machines.

And I did not at all mean an A vs B question. You are right, it was FBW vs non... I could (should?) have included the 777 in my question, or any other FBW...

By the way, what is the one you're talking about, if you can say... ?
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tdscanuck
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RE: Do Airplanes Handle Differently? (same Type)

Tue Apr 12, 2011 5:27 am

Quoting PolymerPlane (Reply 30):
IIRC the 737 classic that went down as Adam Air Flight 574, had a slow right roll that was contributing to the crash.

Yep. If the aircraft had a smaller roll, it wouldn't have gone divergent as quickly and they might have caught it before they lost control. No excuse though for 1) disconnecting the autopilot, 2) not noticing they disconnected the autopilot, 3) not noticing the increasing bank angle, and 4) botching the recover.

Quoting Aircellist (Reply 31):
And I did not at all mean an A vs B question. You are right, it was FBW vs non... I could (should?) have included the 777 in my question, or any other FBW...

It depends somewhat on the control law. You can implement FBW as an open-loop control system, in which case you'll get the same variance you do with cable-rigged aircraft. But most FBW now is closed-loop, usually around some kind of C* law in pitch and some kind of roll-rate control in roll, both of which will make everything fly basically the same regardless of rigging (within reason).

Quoting Aircellist (Reply 31):
By the way, what is the one you're talking about, if you can say... ?

Can't say...it doesn't roll or anything, it does fly straight, it's just that the particular combination of surface position and incidence angles on that aircraft means that "straight and level" is really a tiny bank with a tiny rudder input.

Tom.
 
Aircellist
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RE: Do Airplanes Handle Differently? (same Type)

Wed Apr 13, 2011 3:27 am

Instructive again, Tom.

I get the difference between open-loop and closed-loop, for control response.

As fot the plane you can't tell much more about... What would be the difference in fuel consumption, between that one and a straight-flying one? If such a statistic exists...
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tdscanuck
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RE: Do Airplanes Handle Differently? (same Type)

Wed Apr 13, 2011 4:11 am

Quoting Aircellist (Reply 33):
As fot the plane you can't tell much more about... What would be the difference in fuel
consumption, between that one and a straight-flying one? If such a statistic exists...

I don't really know, but it's probably such a small variance that it's in the noise of the data (i.e. the delta fuel flow is smaller than the variance you get from different fuel loads, fuel meter inaccuracy, etc.).

Tom.
 
Mastropiero
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RE: Do Airplanes Handle Differently? (same Type)

Wed Apr 13, 2011 12:59 pm

Tom, Jetstar (and everyone else), thank you so much for your very instructive replies!   

Quoting Aircellist (Reply 33):
I get the difference between open-loop and closed-loop, for control response.

Unfortunately, I don´t....   

Would you mind explaining it (a little, at least) in layman´s terms, so I can get an idea?

Thanks!!!

[Edited 2011-04-13 06:02:25]
 
tdscanuck
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RE: Do Airplanes Handle Differently? (same Type)

Wed Apr 13, 2011 1:42 pm

Quoting Mastropiero (Reply 35):
Quoting Aircellist (Reply 33):
I get the difference between open-loop and closed-loop, for control response.

Unfortunately, I don´t....   

Would you mind explaining it (a little, at least) in layman´s terms, so I can get an idea?

"Open-loop control" is where you don't have any feedback. The accuracy of the thing is implicit in the rigging. Suppose you pull the column back some amount (you're asking the airplane to pitch up). To get that result, the elevator should move some amount...say 10 degrees. In an open-loop system there is no feedback, so you pull the column some amount and the elevator ends up whereever it's going to end up based on the exact rigging, cable tension, state of the pulleys, slop in the linkages, etc. It might actually end up at 8 or 9 or 10 or 11 or 12 degrees and the only way to change that would be to re-rig the flight controls.

"Closed-loop control" has feedback...you have a sensor of some kind that measures where the elevator actually is and compares that to where you want it to be and modifies the commands appropriately. The most common implementation (although there are others) is to subtract the actual position from the commanded position and then try to drive that value to zero (i.e. the actual matches the command). Depending on the math you do on the difference to figure out the new command, you can reduce or eliminate error completely...the surface will always go exactly where it's commanded to go. This automatically covers up any slop/mis-rig/aging in the control system, within reason.

More complex control laws, like in most FBW systems, aren't controlling directly on surface position, they're controlling something more esoteric but more useful. For example, a very common roll control law is to control roll rate. You move the stick/yoke and the computer interprets that as a request for roll rate...the difference between that and your actual roll rate determines what happens to the control surfaces. The computer will do whatever it needs to do (within obvious limits) to give you that roll rate. This is, for example, why roll rate control laws have perfect spiral stability...if you don't touch the yoke, you're commanding zero roll rate, so if you just let go of the stick the aircraft will hold that bank angle "forever".

Tom.
 
Mastropiero
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RE: Do Airplanes Handle Differently? (same Type)

Wed Apr 13, 2011 2:02 pm

Tom, thank you for that, although I still have questions. Mainly one: what kind of feedback are we talking about? As in, the higher the pitch you want to command, the higher the pressure you have to apply on the yoke/stick?

Otherwise, in an open-loop system, if I got it right, only experience will help you know how far you have to pull on the yoke/stick in order to give you those 10 degrees, for the pressure you would have to apply would be just the same for 2, 5, 10 or 30 degrees, is that correct?
 
pilotpip
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RE: Do Airplanes Handle Differently? (same Type)

Wed Apr 13, 2011 7:08 pm

On my FBW aircraft (170/175) there are differences within our fleet. There are a couple aircraft I don't like flying as they require trim adjustments like a cessna (every power/speed/configuration change)
DMI
 
Got2fly
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RE: Do Airplanes Handle Differently? (same Type)

Wed Apr 13, 2011 9:08 pm

I find very few differences between our A319 fleet in terms of handling etc and even between the A319 and A320's...in fact it's quite easy to forget which one your actually flying. As others have mentioned though, there are always aircraft in the fleet which have a few little quirks. We have one which always seems to crash and reset the FMGS just after starting the engines. It's not a very popular aircraft surprisingly!
 
tdscanuck
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RE: Do Airplanes Handle Differently? (same Type)

Thu Apr 14, 2011 5:49 am

Quoting Mastropiero (Reply 37):
Mainly one: what kind of feedback are we talking about? As in, the higher the pitch you want to command, the higher the pressure you have to apply on the yoke/stick?

I'm talking about feedback within the control system itself (i.e. does the airplane know where the surface actually is and it is using that information to figure out what to tell the actuator to do). Feel feedback to the pilot (haptic feedback) is very important for pilot feel, avoiding PIO, etc. but has nothing to do with how a FBW control system decides where to position the flight control surfaces.

It's important to draw a distinction between the aircraft and the pilot when talking about flight controls because the pilot is, in themselves, a very sophisticated and versatile closed-loop controller. In all the stuff I was talking about above I'm just talking about the airplane side, not the pilot.

Quoting Mastropiero (Reply 37):
Otherwise, in an open-loop system, if I got it right, only experience will help you know how far you have to pull on the yoke/stick in order to give you those 10 degrees, for the pressure you would have to apply would be just the same for 2, 5, 10 or 30 degrees, is that correct?

Not exactly...even in an open-loop system, the pressure you apply changes with how far you pull the yoke/stick/column. This is called the force gradient (how the force changes with deflection) and is a very important certification issue for handling qualities. On direct cable-rigged aircraft, you get automatic force feedback because the aerodynamic force on the surface increases with deflection so you need to push/pull harder on the cable to more the surface more. This is called a reversible control system (force at the surface comes back to the pilot). Modern hydraulic powered control systems are usually irreversible (force on the surface is counteracted at the actuator and doesn't get back to the pilot). To cover this up, modern systems use springs/actuators/cams/etc. to provide artificial feel to the pilot.

To illustrate the difference between open-loop and closed-loop, think about cruise control in a car. A really simple way to implement cruise control would be some kind of lock that holds the pedal in one position. The throttle would stay at whatever position you put the cruise control on and, if you hit a hill the vehicle would slow down because you're not adding power. The car sees pedal position as a command for throttle position and puts the throttle where you said without taking any information about the actual state of the car into account...that's open-loop.

Closed-loop cruise control controls around speed (usually). When you hit cruise control, you say "I want this speed." What the controller actually does is subtract the real speed from the commanded speed and use that difference to drive the throttle. If you go up a hill, the car slows and the difference between actual and commanded speed goes up, causing more throttle to come in and the car speeds back up. This is closed-loop control because you're feeding the actual state (the actual speed) back into the calculation of what you want the throttle to do.

The phrase "closed-loop" comes from the drawings that control engineers do of how the various variables are flowing...in open-loop you input a command, that goes through some stuff (actuators, engines, cables, control surfaces, whatever) and an result comes out...a straight series of arrows from left to right with no loops (i.e. the loop is "open"). If you take the result and feed it back you literally "close a loop" in the diagram.

Feeding back the simple difference between the commanded and actual variable (usually times some constant number to make the units work out) is called proportional control...the input to your actuator is *proportional* to the difference between what you want and what you've got.

More exotic controllers can control on the rate of change (derivative) or cumulative sum (integral) of various differences and get much finer control. In a real-world control system like an aircraft, it's way more complex than this.

Tom.
 
Mastropiero
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RE: Do Airplanes Handle Differently? (same Type)

Thu Apr 14, 2011 7:34 am

Quoting tdscanuck (Reply 40):

Wow! That is a 10/10 reply if I´ve ever seen one.      

Thank you so much, Tom, now I understand.      
 
Aircellist
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RE: Do Airplanes Handle Differently? (same Type)

Thu Apr 14, 2011 1:02 pm

Thanks again, Tom. I had understood that the closed-loop meant that the FBW is programmed so that all the aircraft in the family do respond the same way to a given command indeed, but those last explanations go much beyond what I'd grasped.
"When I find out I was wrong, I change my mind. What do you do?" -attributed to John Maynard Keynes
 
tdscanuck
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RE: Do Airplanes Handle Differently? (same Type)

Fri Apr 15, 2011 4:41 am

Quoting Aircellist (Reply 42):
I had understood that the closed-loop meant that the FBW is programmed so that all the aircraft in the family do respond the same way to a given command indeed, but those last explanations go much beyond what I'd grasped.

To have the same response across different aircraft pretty much requires closed loop control, but similarity doesn't define closed-loop control. Done the other way, you can use closed-loop control to make one aircraft fly like pretty much any other aircraft that has a slower natural response...this is how NASA makes a biz jet fly like the space shuttle to train their pilots.

Tom.

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