Hello Everyone/Happy New Year
I have noticed that sometimes after they land aircraft can bounce about for a little bit. The past weeks i have flown 4 flights on A321s and i noticed that all of them would bounce on landing. My question is do certain main landing gear structures allow this bouncing to happen more than others? because i have noticed these bounces on some aircraft significantly more than other, or is that just a coincidence.
Thank You
dimik747
RE: Bouncing On Landing
Airbus and Boeing both use landing gear build by Goodrich. I would venture to say that all of them are pretty similar and that the bouncing you witnessed was purely coincidental.
I support the right to arm bears
-
- kimberlyRJ
- Posts: 249
- Joined:
RE: Bouncing On Landing
I have not really noticed that much of a bounce on landing on any of the Airbus A318/A319/A320A321 family...
Oh the days of landing on the Boeing 727 used to be really bouncy and fun!!!
Kimberly.
Oh the days of landing on the Boeing 727 used to be really bouncy and fun!!!
Kimberly.
RE: Bouncing On Landing
| Quoting dimik747 (Thread starter): My question is do certain main landing gear structures allow this bouncing to happen more than others? |
No!
The "bounce" is totally pilot induced.
RE: Bouncing On Landing
| Quoting dimik747 (Thread starter): do certain main landing gear structures allow this bouncing to happen more than others? |
I would argue yes...albeit in reference to small general aviation types. After flying an airplane with damped landing gear...the Oleo struts on a Piper Cherokee, for example...and then flying an airplane with undamped, spring-steel gear, like a Cessna 170, you'll find that the undamped gear is more prone to wallowing and bouncing around.
Intentionally Left Blank
RE: Bouncing On Landing
Read "Fate is the Hunter" by Ernest Gann. It is true that he wrote about the 30's and 40's, but I think the principle still holds. The long and short of it is that different aircraft definitely have different landing characteristics; some are easy to land without bouncing and some are not. From articles I have read this definitely applies to modern airliners; some are really easy to grease on the runway and others very difficult. Just because they all have similar landing gear is not the issue; the aerodynamics affect it much more. If you think about it, what makes for a hard or soft landing is really the vertical speed at touchdown, and what causes that to slow down dramatically just before touchdown is ground effect. This will be different for different designs; how strong it is and how fast it builds up as the plane gets closer to ground. Flap design is going to have a big effect on it.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
-
- etherealsky
- Posts: 211
- Joined:
RE: Bouncing On Landing
| Quoting 2H4 (Reply 4): |
That's what I was thinking too - I'm used to bouncing all over the place in Cessnas even just during taxi, and the first time I sat in a Piper and started up the engine the difference felt excellent.
Of course, some people are still able to make oleo struts bounce pretty well.
http://www.youtube.com/watch?v=NMmHYWjEmkY
"And that's why you always leave a note..."
RE: Bouncing On Landing
And of course this one is always fun to watch too:
http://www.youtube.com/watch?v=OoK_dbLDfA0
http://www.youtube.com/watch?v=OoK_dbLDfA0
RE: Bouncing On Landing
| Quoting dimik747 (Thread starter): My question is do certain main landing gear structures allow this bouncing to happen more than others? because i have noticed these bounces on some aircraft significantly more than other, or is that just a coincidence. |
Bouncing isn't really related to gear design for large jets...they all use essentially the same oleo system and it's not nearly as much of a spring as it is a shock absorber.
Two big factors are geometry (how the angle of attack changes with touchdown and derotation) and the pitch dynamics. On a FBW aircraft, the transition from air to ground control laws can be tricky.
Tom.
RE: Bouncing On Landing
| Quoting tdscanuck (Reply 8): Bouncing isn't really related to gear design for large jets...they all use essentially the same oleo system and it's not nearly as much of a spring as it is a shock absorber. |
I opine that trailing-link versus non-trailing-link designs have a greater impact (no pun intended) on what the pax feel.
Intentionally Left Blank
-
- Starlionblue
- Posts: 19485
- Joined:
RE: Bouncing On Landing
| Quoting Phen (Reply 7): And of course this one is always fun to watch too: http://www.youtube.com/watch?v=OoK_d...LDfA0 |
Is it just me or was there a sidewind gust right before touchdown?
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
-
- etherealsky
- Posts: 211
- Joined:
RE: Bouncing On Landing
| Quoting tdscanuck (Reply 10): |
The first thing that comes to mind is the IL-76 (but only the nose, I think?)
Photo © Dashu
And how can we forget these guys?
(An-124 & An-225)
 Photo © Shary - Iranian Spotters |  Photo © Radek Oneksiak |
Photo © Michael Fritz
The Tu-334 is also guilty of trailing-linkery:
Photo © Max Bryansky - Russian AviaPhoto Team
However, the largest western airliners I can think of with trailing link gear are the ERJ & CRJ with their mains.
 Photo © Bjorn Van Brussel |  Photo © Bruce Leibowitz |
Oh! And the BAe146 and 328JET.
 Photo © Maciej Nowak (medMatthew) |  Photo © Leor |
Of course, those depend on your definition of 'large'.
"And that's why you always leave a note..."
-
- Viscount724
- Posts: 19316
- Joined:
RE: Bouncing On Landing
| Quoting tdscanuck (Reply 8): Quoting dimik747 (Thread starter): My question is do certain main landing gear structures allow this bouncing to happen more than others? because i have noticed these bounces on some aircraft significantly more than other, or is that just a coincidence. Bouncing isn't really related to gear design for large jets...they all use essentially the same oleo system and it's not nearly as much of a spring as it is a shock absorber. Two big factors are geometry (how the angle of attack changes with touchdown and derotation) and the pitch dynamics. On a FBW aircraft, the transition from air to ground control laws can be tricky. |
After hundreds of flights on 727s and 737s of almost all models, I always found 727s much more prone to bouncing than 737s which seemed to stay on the ground even after quite firm landings. I've also read in many sources that 727s are notoriously difficult to land smoothly with any consistency.
RE: Bouncing On Landing
| Quoting kimberlyRJ (Reply 2): I have not really noticed that much of a bounce on landing on any of the Airbus A318/A319/A320A321 family... |
Neither have I, even after quite a bit of time flying it. I find the A-320 series the easiest airliner to consistently land other than the L-1011, which is just a delight to roll on. The 737 and 744 are quite easy, too. The DC-9 and MD-80 are somewhere in the middle of the pack; the 727 is definitely the hardest aircraft to land consistently that I have ever flown. Of course any aircraft can be made to bounce with enough vertical speed at touchdown, but I have never found it be troublesome to overcome after being properly trained on any type (except perhaps the 727.)
| Quoting tdscanuck (Reply 8): Two big factors are geometry (how the angle of attack changes with touchdown and derotation) and the pitch dynamics. On a FBW aircraft, the transition from air to ground control laws can be tricky. |
That's interesting, and I don't think I had ever thought about that. I have not flown the 777, so my FBW experience is limited to Airbus models, but I have found them very nicely behaved in the flare and at touchdown.
| Quoting 2H4 (Reply 9): I opine that trailing-link versus non-trailing-link designs have a greater impact (no pun intended) on what the pax feel. |
I do agree with that, though as was pointed out trailing-link gear are none too common on airliners. I have flown them on GA aircraft and they really do flatter pilot technique!
| Quoting Viscount724 (Reply 13): After hundreds of flights on 727s and 737s of almost all models, I always found 727s much more prone to bouncing than 737s which seemed to stay on the ground even after quite firm landings. I've also read in many sources that 727s are notoriously difficult to land smoothly with any consistency. |
Your recollections and readings are absolutely correct in every way!
RE: Bouncing On Landing
| Quoting PGNCS (Reply 14): my FBW experience is limited to Airbus models, but I have found them very nicely behaved in the flare and at touchdown. |
I do regularly notice one thing I notice as a pax in the A319 and A320...often, immediately after the tires touch down very smoothly, the aircraft then, for lack of a better term,"bounces" up and down a couple of times without the tires leaving the ground. Almost as if the spoilers dump the lift so instantly, the struts go through a couple complete cycles of travel before the "drop" is damped out.
Intentionally Left Blank
RE: Bouncing On Landing
In any aircraft you can experience a "greaser" or conversely,...in any aircraft, you can dribble your way down the runway like a basket ball...or...like my last landing last Saturday in a 172R..man it sucked!...It comes down to pilot feel and control...some days you got it...some days you don't!
RE: Bouncing On Landing
| Quoting PGNCS (Reply 14): Quoting tdscanuck (Reply 8): Two big factors are geometry (how the angle of attack changes with touchdown and derotation) and the pitch dynamics. On a FBW aircraft, the transition from air to ground control laws can be tricky. That's interesting, and I don't think I had ever thought about that. I have not flown the 777, so my FBW experience is limited to Airbus models, but I have found them very nicely behaved in the flare and at touchdown. |
I should have been clearer...if the OEM has done their job right, it shouldn't be tricky at all for the pilot. What's tricky is how you design the control law transition so that it looks smooth to the pilot (and the passengers). Once it's done right by the flight control engineers, it should be very predictable and stable to the pilot.
Tom.
RE: Bouncing On Landing
| Quoting tdscanuck (Reply 18): I should have been clearer...if the OEM has done their job right, it shouldn't be tricky at all for the pilot. What's tricky is how you design the control law transition so that it looks smooth to the pilot (and the passengers). |
Does the 777 even have much of a control law transition there?
RE: Bouncing On Landing
| Quoting 2H4 (Reply 15): Quoting PGNCS (Reply 14): my FBW experience is limited to Airbus models, but I have found them very nicely behaved in the flare and at touchdown. I do regularly notice one thing I notice as a pax in the A319 and A320...often, immediately after the tires touch down very smoothly, the aircraft then, for lack of a better term,"bounces" up and down a couple of times without the tires leaving the ground. Almost as if the spoilers dump the lift so instantly, the struts go through a couple complete cycles of travel before the "drop" is damped out. |
Interesting. You can watch the spoilers come out on the lower ECAM page in the cockpit, but I have never noticed the exact issue you are describing, likely because I am too busy at the time!
Depending on the wind, control inputs, and the touchdown the "bottoming out" sensation does differ a bit from landing to landing. Sorry I don't have a better explanation.
| Quoting 474218 (Reply 16): Quoting PGNCS (Reply 14): I find the A-320 series the easiest airliner to consistently land other than the L-1011, which is just a delight to roll on. Hard to beat that DLC and flying stabilizer! |
Let me go one step further if you'll permit me: IMPOSSIBLE to beat the DLC and flying stab!
| Quoting tdscanuck (Reply 18): I should have been clearer...if the OEM has done their job right, it shouldn't be tricky at all for the pilot. What's tricky is how you design the control law transition so that it looks smooth to the pilot (and the passengers). Once it's done right by the flight control engineers, it should be very predictable and stable to the pilot. |
I actually thought that's what you were getting at. I am sure that the 777 is quite predictable and docile from talking to friends that fly it. My point was that the Airbus FBW guys did a great job of the transition of the control laws during the final approach and touchdown. You engineers sure are a wily bunch!
RE: Bouncing On Landing
| Quoting PGNCS (Reply 14): the 727 is definitely the hardest aircraft to land consistently that I have ever flown. |
I've got a good streak going right now, like 8 of the last 10 have been pretty decent. That being said, it's over the next flight, here comes a bouncer lol.
I've seen a few similar to this, man I hate that crash sound! http://www.youtube.com/watch?v=XxRAnDQ_iM8
[Edited 2011-01-06 18:18:17]
Too lazy to work, too scared to steal!
-
- Starlionblue
- Posts: 19485
- Joined:
RE: Bouncing On Landing
Here are some nice bouncers. Wind, wind and more wind. Love the QF 767 pitching down sharply at 0:35+.
http://www.youtube.com/watch?v=P_LaAkAyoz0
http://www.youtube.com/watch?v=P_LaAkAyoz0
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
RE: Bouncing On Landing
| Quoting Klaus (Reply 19): Does the 777 even have much of a control law transition there? |
There must be at least two...all FBW airliners that I'm aware of do some kind of pitch law modification to provide a realistic flare (C*-like control laws will render ground effect mostly invisible to the pilot without modification), and any yaw control law that looks at sideslip goes screwy on the ground (since you can't sideslip on the ground without skidding the wheels).
Tom.
RE: Bouncing On Landing
| Quoting Starlionblue (Reply 22): Here are some nice bouncers. Wind, wind and more wind. Love the QF 767 pitching down sharply at 0:35+. |
A classic. Every time I open that video, I have to watch it all the way through. I can only imagine the passengers' stomachs in their throats whenever I see that 767 drop.
What's interesting is none of those touchdowns appear to be particularly jarring, with the exception of the Dash 8 at the end. That'll wake up the sleepers in back
B-52s don't take off. They scare the ground away.
RE: Bouncing On Landing
| Quoting tdscanuck (Reply 23): There must be at least two...all FBW airliners that I'm aware of do some kind of pitch law modification to provide a realistic flare (C*-like control laws will render ground effect mostly invisible to the pilot without modification), and any yaw control law that looks at sideslip goes screwy on the ground (since you can't sideslip on the ground without skidding the wheels). |
Yeah, but the 777 is effectively in direct law all the time anyway, so the difference should be pretty minor by comparison.
On an Airbus the G-force feedback loop gets broken at low speeds when elevators and ailerons simply lose too much of their control authority for normal law to function, so it transitions to flare mode and then to direct law if I remember correctly, which is a rather fundamental change by comparison.
RE: Bouncing On Landing
| Quoting Klaus (Reply 25): On an Airbus the G-force feedback loop gets broken at low speeds when elevators and ailerons simply lose too much of their control authority for normal law to function |
That is not correct. The Airbus flight controls have sufficient authority for all flight regimes in normal law. Since in normal law unlike in Boeing products the Airbus has full time autotrim, the designers made for a more conventional feel in the flare by having the aircraft insert some nose down trim which has the result of making the pilot flare like in any other plane. At no time does the aircraft come out of normal law, although on the ground a direct relationship exists between the control surfaces and the sidestick being used. You are correct in saying there is a transition between flight and ground modes of normal law, but recognize that the aircraft is still in normal law at all times (even though the behavior of normal law changes as a function of phase of flight, which is almost completely transparent to the crew as the changes are nicely blended.) Perhaps I misunderstood your underlying point: I am certainly not trying to make this an argument over terminology.
| Quoting Klaus (Reply 25): Yeah, but the 777 is effectively in direct law all the time anyway, so the difference should be pretty minor by comparison. |
Tom, is that correct that in the 777 there is a direct correlation between the yoke deflection and control surfaces like in a conventional aircraft (i.e. direct law exists at all times)? I would have guessed not, but it is a Boeing.
| Quoting tb727 (Reply 21): Quoting PGNCS (Reply 14): the 727 is definitely the hardest aircraft to land consistently that I have ever flown. I've got a good streak going right now, like 8 of the last 10 have been pretty decent. That being said, it's over the next flight, here comes a bouncer lol. |
You know what they said: if you get one good landing in a 727 you're good; if you get two good landings in a row in the 727 you're lucky; if you get three good landings in a row in the 727 you're a liar!
Let me know how long your streak continues!
[Edited for clarity about normal law issue.]
[Edited 2011-01-08 11:49:36]
RE: Bouncing On Landing
| Quoting Klaus (Reply 25): Yeah, but the 777 is effectively in direct law all the time anyway |
No. The 777 normal pitch law is a maneuver command, not a surface command. Roll and yaw are effectively "direct" since surface deflection is proportional to control input in normal function, but the FBW system can override yaw and roll commands for certain protection functions so it's not pure direct.
Any flight control law is a lot more complicated than just the proportionality...from normal to direct, even in roll and yaw, you have changes in how the gains are scheduled, how the envelope protections are enforced, control forces, etc.
There are major handing changes going from normal to direct in a 777.
| Quoting PGNCS (Reply 26): Tom, is that correct that in the 777 there is a direct correlation between the yoke deflection and control surfaces like in a conventional aircraft (i.e. direct law exists at all times)? |
It's not a generally true statement. Within the normal envelope, roll and yaw have proportional surface movement to yoke/pedal movement (though the proportionality constant varies with airplane speed). Elevator/stab movement is not proportional to column movement and is, in no sense, a direct law.
At the envelope edges, the proportionality also falls out for roll/yaw where things like bank angle protection, thrust assymetry protection, etc. kick in.
My interpretation of the best description of a 777 in normal law would be:
Pitch: fully augmented control law
Roll/yaw: bounded speed-proportional direct law, with full augmentation outside the boundaries
Tom.
RE: Bouncing On Landing
| Quoting PGNCS (Reply 26): That is not correct. The Airbus flight controls have sufficient authority for all flight regimes in normal law. |
Well, when airspeed is decaying beyond a certain point, the FBW G-force request from the pilot just can't be satisfied any longer. It's been a while that I had learned about the transitions happening during takeoff and landing, but from a systems point of view the normal control loop would at low speed just lead to full control surface deflection on the slightest deflection of the stick which would not be useful.
And I was only referring to direct law in terms of behaviour on the ground; I've not looked up whether the terminology still covers this behaviour under normal law since it is not a pilot selection but an automatic operating mode.
I was aware of the trim transition during flare, I was just thinking only about ailerons and elevators in my certainly imprecise description above. I'm aware that precision is important here, so of course I defer to actual knowledge of these issues.
RE: Bouncing On Landing
| Quoting Klaus (Reply 28): It's been a while that I had learned about the transitions happening during takeoff and landing, but from a systems point of view the normal control loop would at low speed just lead to full control surface deflection on the slightest deflection of the stick which would not be useful. |
On the ground, pitch is "direct" (elevator deflection proportional to stick position). This stays in effect from liftoff until pitch attitude exceeds 8 degrees...over the next 5 seconds the control law ramps from ground mode ("direct") to flight mode (load-factor demand).
On landing, pitch control goes from flight mode to flare mode (flight mode with a ramped nose-down trim) over 1 second, starting when the aircraft passes through 50'. After being on the ground for 5 seconds and the pitch less than 2.5 degrees, flare mode starts ramping out and ground mode starts ramping in. The ramp takes 5 seconds. So, basically, on a normal landing, it takes 10 seconds to do the full transition from touchdown to ground mode.
Tom.
RE: Bouncing On Landing
| Quoting etherealsky (Reply 6): http://www.youtube.com/watch?v=NMmHYWjEmkY |
| Quoting Phen (Reply 7): And of course this one is always fun to watch too: http://www.youtube.com/watch?v=OoK_d...LDfA0 |
And heres my personal favorite:
http://www.youtube.com/watch?v=F2PwB5QCISQ
Had kinda similar landing in LCY with an LH E190, what a literary pain in the a..se! I praise the magic materials landing gears must be made out of....
-
- maxpower1954
- Posts: 1067
- Joined:
RE: Bouncing On Landing
Well, I guess i'm the liars club, because I never found the 727 (speaking only of the -200; never flew the -100) to be that hard to land consistently, but my technique was similar to making a wheel landing in a DC-3 - low flare, don't even think about pulling the power off until the descent is completely stopped, then relax the back pressure on the yoke, which had the effect of reducing the
sink rate of the main gear. Worked 90% of the time for me (the other 10% we don't have to talk about!) The guys I saw having the most trouble would pull the power off too soon and in the insuing dramatic flare rotate the main right into the ground. The DC-8, especially the 61/63 was a bigger challenge to me than the 727 ever was - still my favorite jet of all time. Russ Farris
sink rate of the main gear. Worked 90% of the time for me (the other 10% we don't have to talk about!) The guys I saw having the most trouble would pull the power off too soon and in the insuing dramatic flare rotate the main right into the ground. The DC-8, especially the 61/63 was a bigger challenge to me than the 727 ever was - still my favorite jet of all time. Russ Farris
Who is online
Users browsing this forum: No registered users and 12 guests