madog
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Boeing 777 Outer Ailerons Behaviour On Finals

Thu Feb 16, 2012 12:31 pm

During final approach, I've noticed that in all of the 777s that I've flown in the past several years that the outer ailerons deflect upwards. It may be a stupid question, but why is that? I though the outer ailerons were supposed to give better handling when the aircraft is at low speeds i.e take off and approach. when it is deflected upwards does it suppose to give some kind of special handling quality????
 
chimborazo
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RE: Boeing 777 Outer Ailerons Behaviour On Finals

Thu Feb 16, 2012 6:15 pm

I'm by no means an expert on the 777 flight controls but I should imagine each time you were landing in a cross-wind so a little aileron was being used. I presume you couldn't see out the other side and that the other aileron was down a little?
 
roseflyer
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RE: Boeing 777 Outer Ailerons Behaviour On Finals

Thu Feb 16, 2012 9:54 pm

I am not sure what you are seeing. What you are saying that you see is actually opposite of what the airplane does. When the flaps are deployed, the outboard aileron droops 8 degrees trailing edge down. The droop is automatic. The only way that they would not droop is if the airplane is approaching above recommended speed which results in the droop being removed so that the airplane will pitch up and slow down.

You could be seeing the autopilot reacting to a cross wind, but that wouldn't result in a continuous upward position of the aileron.
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CXB77L
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RE: Boeing 777 Outer Ailerons Behaviour On Finals

Fri Feb 17, 2012 1:11 pm

I think this might be what the OP is referring to:


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Photo © Tony Printezis



The outboard aileron appears to have a slight upward deflection compared with one in a flush position:


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Photo © Andrew Hunt - AirTeamImages

Boeing 777 fanboy
 
CM
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RE: Boeing 777 Outer Ailerons Behaviour On Finals

Fri Feb 17, 2012 4:36 pm

Quoting CXB77L (Reply 3):
I think this might be what the OP is referring to:

I believe what you are seeing in these photos are evidence of roll command, and the opposite wing would not look the same. This is particularly true for the top photo, as the flaps are already at a high setting, which causes the ailerons on both sides to droop - this is true for all 777 models.
 
roseflyer
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RE: Boeing 777 Outer Ailerons Behaviour On Finals

Fri Feb 17, 2012 4:44 pm

Quoting CXB77L (Reply 3):

The outboard aileron appears to have a slight upward deflection compared with one in a flush position:

It appears the first picture that the aileron is reacting to a bank.
If you have never designed an airplane part before, let the real designers do the work!
 
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DocLightning
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RE: Boeing 777 Outer Ailerons Behaviour On Finals

Sat Feb 18, 2012 2:10 am

Quoting Roseflyer (Reply 2):
You could be seeing the autopilot reacting to a cross wind, but that wouldn't result in a continuous upward position of the aileron.

Why not, if it's a steady crosswind?
-Doc Lightning-

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goboeing
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RE: Boeing 777 Outer Ailerons Behaviour On Finals

Sat Feb 18, 2012 3:48 am

Quoting Roseflyer (Reply 2):
You could be seeing the autopilot reacting to a cross wind

Or the pilot making the same control input, since it's obviously the same thing out on the wing.

Quoting DocLightning (Reply 6):
Why not, if it's a steady crosswind?

Because the flight path is still coordinated (not in a slip or skid condition) all the way down final. The only time it gets uncoordinated in a crosswind is when the crab is taken out with the rudder, and this part requires opposite aileron.
 
madog
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RE: Boeing 777 Outer Ailerons Behaviour On Finals

Sat Feb 18, 2012 3:45 pm

I know that the outer ailerons are designed to droop down as a function of flap setting. during my last three landings sitting on either side if the 777-300ER in the last month I have seen the same thing happening. at full flaps the outer alerion deflect upwards, although it does move a bit what I saw was that it never went into downwards position during the last two flap settings. I Will take a video the next time I fly in a few eeks time to show what I exactly mean.
 
OldAeroGuy
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RE: Boeing 777 Outer Ailerons Behaviour On Finals

Sun Feb 19, 2012 3:27 pm

Quoting CM (Reply 4):
This is particularly true for the top photo, as the flaps are already at a high setting, which causes the ailerons on both sides to droop - this is true for all 777 models.

The outboard ailerons droop at takeoff flap settings for the 777. At landing flap settings, they are not drooped.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
mandala499
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RE: Boeing 777 Outer Ailerons Behaviour On Finals

Sun Feb 19, 2012 4:53 pm

Quoting CM (Reply 4):
I believe what you are seeing in these photos are evidence of roll command

By that deflection, the roll spoilers should be playing a bit. If there is a little play in the roll spoilers, then it should be a roll command.
I've seen this on the 320, on final approach, both ailerons deflect upwards slightly. I can't remember seeing that on the 777 but then, I could be wrong!   
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Mir
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RE: Boeing 777 Outer Ailerons Behaviour On Finals

Sun Feb 19, 2012 8:51 pm

Quoting DocLightning (Reply 6):
Why not, if it's a steady crosswind?

Because an upward aileron (assuming the other side isn't up as well) imparts a roll moment to the airplane. To make a turn in an airplane, you turn the controls in the desired direction until you get the angle of bank you want, then return them to center (in reality, you have to fiddle a little bit due to various aerodynamic factors, but you're not holding nearly as much control pressure once you've established a bank as you do in order to establish that bank).

In a steady crosswind, you'd see a deflection of the ailerons as the crab angle was established, then they would return to center until it was time to take out the crab again just prior to touchdown.

-Mir
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CeddP
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RE: Boeing 777 Outer Ailerons Behaviour On Finals

Thu Jun 20, 2013 8:37 pm

I dig up this post because the original question remains. I searched around the net looking for an answers and found nothing satisfying !

To clarify the question : on the 777-200, we can see an outboard aileron droop when flaps are selected down to 15 or 20. Then with flaps at 25 and 30, the aileron goes back to neutral.
We have a similar behavior on the 787 and 747-8.
Note a special feature of the 787 when intercepting the glide, where there is an "autodrag" function which consists in an upward deflection of the outboard aileron and the last two spoilers.
Description can be read there : http://www.aviationweek.com/awmobile...l/AW_12_10_2012_p46-522072.xml&p=3
Video (between 3:30 and 3:55) : http://www.youtube.com/watch?v=3PhMr57u-Pc
Some will point out that it's not the autodrag function because it uses a downward aileron deflection, as explained in the previous article and in the FCOM (see : 787 Spoilers And Ailerons (by n797mx Jan 14 2013 in Tech Ops)).
That's interesting indeed, but what do we see in this video then (and many more on utube) ? Spare me the roll command answer. You can't explain such an outboard aileron deflection with two spoilers out during 20 seconds without generating any bank at all with a roll command ! Please also spare me the "crosswind correction-uncoordinated flight during 20 seconds of the final approach" explanation.
Interestingly, you can hear a decrease of power from the engine juste before the aileron/spoilers deflect upward, and a spool up when they get back to "normal" position. All this really sounds like a kind of autodrag function...?

To go back to the 777, I've never heard of such an autodrag system. Yet we can observe an interesting behavior of the flight controls on this video : http://www.youtube.com/watch?v=tWKR4tEAeCA
Start watching at 8:10, pseudo "autodrag" mode using upward ailerons inboard spoilers from 8:15 to 10:20, then "normal" approach mode.

And here is the question : with the 777-300, the outboard aileron is systematically upward deflected when flaps 25 or 30 are selected.
777-300 video : http://www.youtube.com/watch?v=61RsJLZUM_Q
We can see during this approach that the outboard aileron is at least neutral, otherwise always up.
777-300 ER with few obvious roll command interferences : http://www.youtube.com/watch?v=3_HIcbpzpZw

Why this behavior on the -300 ? Is it a kind of continuous autodrag function to increase speed stability during final approach ? Or a way to change the lift distribution over the wing when flaps are in approach config ? And if so why such a difference in flight control logic between the -200 and 300 ?

Plenty of questions in this post, any insight on all this ?
 
BoeingGuy
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RE: Boeing 777 Outer Ailerons Behaviour On Finals

Fri Jun 21, 2013 2:47 pm

Quoting OldAeroGuy (Reply 9):
The outboard ailerons droop at takeoff flap settings for the 777. At landing flap settings, they are not drooped.

Wasn't this something that came out of test flying on WA001? I thought they originally drooped during Flaps 25 and 30, but John Cashman found some issues during stall testing, so the outboard ailerons come back up at 25 and 30 now.
 
roseflyer
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RE: Boeing 777 Outer Ailerons Behaviour On Finals

Fri Jun 21, 2013 3:57 pm

Quoting CeddP (Reply 12):

To go back to the 777, I've never heard of such an autodrag system. Yet we can observe an interesting behavior of the flight controls on this video : http://www.youtube.com/watch?v=tWKR4tEAeCA
Start watching at 8:10, pseudo "autodrag" mode using upward ailerons inboard spoilers from 8:15 to 10:20, then "normal" approach mode.

The 777 has a system called landing attitude modification. If the airplane detects an overspeed approach, it will automatically reduce the aileron and flaperon droop when flaps are at 25 or 30. Once the airplane is no longer in an overspeed condition, normal aileron and flaperon droop will resume.
If you have never designed an airplane part before, let the real designers do the work!
 
OldAeroGuy
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RE: Boeing 777 Outer Ailerons Behaviour On Finals

Fri Jun 21, 2013 10:32 pm

Quoting CeddP (Reply 12):
To clarify the question : on the 777-200, we can see an outboard aileron droop when flaps are selected down to 15 or 20. Then with flaps at 25 and 30, the aileron goes back to neutral.

Which is what I said in Reply 9

Quoting CeddP (Reply 12):
Note a special feature of the 787 when intercepting the glide, where there is an "autodrag" function which consists in an upward deflection of the outboard aileron and the last two spoilers.

This feature is not on 777 current models.

Quoting BoeingGuy (Reply 13):
Wasn't this something that came out of test flying on WA001? I thought they originally drooped during Flaps 25 and 30, but John Cashman found some issues during stall testing, so the outboard ailerons come back up at 25 and 30 now.

It was a controllability issue also identified by the 777 S&C Lead Engineer, Jim Kraft

Quoting Roseflyer (Reply 14):
The 777 has a system called landing attitude modification. If the airplane detects an overspeed approach, it will automatically reduce the aileron and flaperon droop when flaps are at 25
or 30

It looks like the LAM may functioning in the picture since flaperon is not in its full down position.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis

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