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CeddP
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777 & A350 ailerons behavior vs fuselage length

Thu Mar 07, 2019 11:19 am

Hi all,

On the 777-200, outboard aileron droop with flaps down then back to neutral at flaps 25 and 30. We can observe this sequence in reverse during flaps retraction here : https://youtu.be/PxBEHTAqtcM?t=740
On the 777-300, outboard aileron droop with flaps down then deflect upward at flaps 25 and 30. See : https://youtu.be/6CH5h3UKwzg?t=711

Now we can observe the exact same behavior on the A350 with the -900 vs -1000. See for the -900 : https://youtu.be/OtBIkqPPaGY?t=497
And for the -1000 : https://youtu.be/J8dGnemm5So?t=540

So it seems fuselage length is a factor here. Is anybody able to explain why such an aileron behavior and what exactly B&A engineers managed to address doing this?
 
mpdpilot
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Re: 777 & A350 ailerons behavior vs fuselage length

Thu Mar 07, 2019 5:19 pm

I am sure someone with more knowledge than me will have more detailed explanation, but a few thoughts.

First, the videos are all just of one side of the aircraft. It is possible that aileron deflection during that stage of flight is due to control inputs from the cockpit. After landing, I can think of two reasons the control surfaces are used: 1. to counter a cross wind while taxing, 2. simply moving the controls out of the way (less of an issue on an airbus), while not standard procedure, not uncommon to take your hands off the controls after landing with the controls not in their zero position.

Second thing to think about is the purposes of flaps at 5 or 15 degrees verse 25 or 30 degrees. At flaps 5, the flaps are used to increase the wing area to improve lift, using the ailerons to help this effect allows the wings/flaps to be slightly smaller than needed otherwise (saves weight). At flaps 30, the flaps are mostly used to increase drag, to allow a steeper approach, (a flaps 0 approach/landing would look/feel quite odd, if you have never seen/done one, and it is dangerous in an airliner) so now the ailerons aren't needed to increase lift, they are needed for control, slower speeds mean more deflection for the same force, so they are returned to their zero position.

A step further, the 787 and I think A350 use the ailerons full up on landing for increase ground spoiler affect, this puts more weight on the landing gear and increases braking action. This full up position and the down position for takeoff is secondary though to the control inputs from the pilot. Say for example, the full up position of the aileron was 30 degrees up, if during rollout, the pilot needed to use some left aileron, the left aileron would move down from 30 degrees to increase lift on that wing, it wouldn't necessarily go below 0 though, because it is also providing that spoiler effect, just less of it. Same thing on takeoff, the ailerons can still move to provide the pilot with control, they just move relative to their new position, not their zero position.

Remember how the aileron works, down increases lift on that wing slightly causing a roll, up decreases lift on that wing causing a roll. Using them together to increase lift for take off, or decreasing lift on landing can be helpful, but can't take away from their primary purpose which is control. It is this last part that limits how far they go down/up for takeoff and landing, this limit is to give the pilots adequate control.

A further piece of information, Boeing uses an inboard and outboard aileron. This allows Boeing to build a slightly more flexible wing, than Airbus, you can see this in the wing flex of an A330 verse a 787. The outboard aileron when used causes a twisting force on the wing that has to be accounted for. Airbus makes their wings a little stronger, Boeing uses an aileron closer to the wing root to dampen the twisting action. The outboard aileron on Boeing aircraft are locked at high speed and high altitudes so as not to inadvertently cause an excessive amount of that twisting action. One isn't better than the other, just different, Boeing saves some weight in the structure of the wing, but adds weight in the form of a second set of control surfaces. Boeing also only seems to do this on wide-body aircraft, as the 737 only has outboard ailerons, but that is a totally different system and limits that twisting force in a different way.
One mile of highway gets you one mile, one mile of runway gets you anywhere.
 
GalaxyFlyer
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Re: 777 & A350 ailerons behavior vs fuselage length

Thu Mar 07, 2019 5:33 pm

B707 and B727 had inboard and outboard ailerons. On the 727, the outboards unlocked at Flaps 2.

GF
 
mpdpilot
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Re: 777 & A350 ailerons behavior vs fuselage length

Thu Mar 07, 2019 8:04 pm

GalaxyFlyer wrote:
B707 and B727 had inboard and outboard ailerons. On the 727, the outboards unlocked at Flaps 2.

GF


My bad, I forgot about those.

I wonder why Boeing opted for the single aileron on the 757 when the 767 has both? The 737 makes sense as it is iterative and the -100 had a small enough wing that it probably wasn't that big of a deal, but the 757 has a larger wing than the 727, and was designed along side the 767, interesting.

Perhaps Boeing realized after building the 757 that the inboard aileron was helpful, and that is why every new design since, has come with them.
One mile of highway gets you one mile, one mile of runway gets you anywhere.
 
CeddP
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Re: 777 & A350 ailerons behavior vs fuselage length

Thu Mar 07, 2019 8:23 pm

Thanks mpdpilot for your reply.

First, the videos are all just of one side of the aircraft. It is possible that aileron deflection during that stage of flight is due to control inputs from the cockpit.

I just provided one exemple via YouTube for each case, but you'll find this aileron behavior on every single video, filmed left or right side, calm wind or close to thunderstorm conditions etc... What you see on videos I linked is the actual neutral position of the aileron as decided by the FBW computer with the current flap setting. I choose the just before retraction flaps stage because at this moment PF is busy taxiing with the tiller while PM is busy with ATC and after landing actions, nobody will play with flight controls at this stage ; then during actual flaps retraction, we can clearly see the different "neutrals" : 0° -> drooped -> 0° on the -200 and -900, few °s up -> drooped -> 0° on the -300 and -1000. You can check on all videos you find, left or right side, it will always be the same, so this discards the flight control inputs…

Second thing to think about is the purposes of flaps at 5 or 15 degrees verse 25 or 30 degrees. At flaps 5, the flaps are used to increase the wing area to improve lift, using the ailerons to help this effect allows the wings/flaps to be slightly smaller than needed otherwise (saves weight). At flaps 30, the flaps are mostly used to increase drag, to allow a steeper approach, (a flaps 0 approach/landing would look/feel quite odd, if you have never seen/done one, and it is dangerous in an airliner) so now the ailerons aren't needed to increase lift, they are needed for control, slower speeds mean more deflection for the same force, so they are returned to their zero position.

Completely agree with you on that, hence the question, why for the -300 and -1000 is the neutral position equivalent to a slightly upward deflection with landing flaps setting?

A step further, the 787 and I think A350 use the ailerons full up on landing for increase ground spoiler affect, this puts more weight on the landing gear and increases braking action. This full up position and the down position for takeoff is secondary though to the control inputs from the pilot. Say for example, the full up position of the aileron was 30 degrees up, if during rollout, the pilot needed to use some left aileron, the left aileron would move down from 30 degrees to increase lift on that wing, it wouldn't necessarily go below 0 though, because it is also providing that spoiler effect, just less of it. Same thing on takeoff, the ailerons can still move to provide the pilot with control, they just move relative to their new position, not their zero position.

Agree again. That being said, what you describe for landing (upward deflect a la spoiler) is valid for the 787, A340-500/600, A380, recent A320 family, but not always on the A350, and that's a secondary question of mine. Exemple on the link of my first post (A350-900 case), you see it deflecting fully upward just for a second or two when the aircraft is already starting to vacate the rwy... I really can't understand the logic there…
 
GalaxyFlyer
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Re: 777 & A350 ailerons behavior vs fuselage length

Thu Mar 07, 2019 8:30 pm

A little background on Boeing. The B-47 and B-52 were the original swept-wing, pod engine designs. The B-47 had lots of flex and aileron reversal problems. Crews would dive to around 380 knots, apply aileron until they found the speed where control reversal was found and used that as limiting speed on low levels to the target. So, using inboard and outboard ailerons was a logical solution. Obviously, all much advanced later.
 
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trpmb6
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Re: 777 & A350 ailerons behavior vs fuselage length

Thu Mar 07, 2019 8:43 pm

The second post above is a great summary.

I'm not sure if Boeing and/or Airbus utilize their ailerons as "flaperons" at certain flap positions (essentially changing the neutral position) but that is basically what you are describing.
 
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Starlionblue
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Re: 777 & A350 ailerons behavior vs fuselage length

Thu Mar 07, 2019 10:57 pm

On the A350, the ailerons deflect fully up at full ground spoiler extension. You can see this in the -1000 landing at TPE around 8:00. Unlike the spoilers, the ailerons don't start deflecting up until both mains are on the ground. Can't tell you why the -900 at SIN isn't doing it. Perhaps there are still lateral inputs on the stick?

- On the A350-900, the ailerons droop with flaps 1+F, 2 and 3 (in the latter case only on take-off, not approach). They are neutral with Flaps 1 and Full.
- On the A350-1000 they also deflect slightly up with Flaps Full.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
CeddP
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Re: 777 & A350 ailerons behavior vs fuselage length

Thu Mar 07, 2019 11:33 pm

Thanks a lot Starlionblue.

- On the A350-1000 they also deflect slightly up with Flaps Full.

Do you have any idea why that is?
 
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Starlionblue
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Re: 777 & A350 ailerons behavior vs fuselage length

Thu Mar 07, 2019 11:47 pm

CeddP wrote:
Thanks a lot Starlionblue.

- On the A350-1000 they also deflect slightly up with Flaps Full.

Do you have any idea why that is?


It would obviously alter the spanwise lift distribution, but I don't know the purpose.


BTW if you look closely you can see the A350s spoiler gap control function in action. When not extended, the spoilers deflect downwards with flap extension to control the gap between spoilers and flaps. At flaps 3, they are already deflected noticeably downwards. Going to Full, the outermost spoiler goes back to neutral as the rest deflect further downwards. See 5:30 in the -1000 video here: https://youtu.be/J8dGnemm5So?t=330
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
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Francoflier
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Re: 777 & A350 ailerons behavior vs fuselage length

Fri Mar 08, 2019 3:50 am

It has more to do with weight than length (though the 2 are somewhat related).

You just want to reduce the wing bending moment in high lift configuration and also probably leave more roll authority at landing configurations as the spoilers are less effective at higher AoA.
...just my guess.
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WIederling
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Re: 777 & A350 ailerons behavior vs fuselage length

Fri Mar 08, 2019 2:04 pm

Starlionblue wrote:
CeddP wrote:
Thanks a lot Starlionblue.

- On the A350-1000 they also deflect slightly up with Flaps Full.

Do you have any idea why that is?


It would obviously alter the spanwise lift distribution, but I don't know the purpose.


less lift from the outer wings reduces loads on the center wing box. ( the -1000 is quite a bit heavier.)
in a regime where you already have high loads there due to low fuel.
Murphy is an optimist
 
mpdpilot
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Re: 777 & A350 ailerons behavior vs fuselage length

Mon Mar 11, 2019 11:58 pm

Starlionblue wrote:
CeddP wrote:
Thanks a lot Starlionblue.

- On the A350-1000 they also deflect slightly up with Flaps Full.

Do you have any idea why that is?


It would obviously alter the spanwise lift distribution, but I don't know the purpose.


BTW if you look closely you can see the A350s spoiler gap control function in action. When not extended, the spoilers deflect downwards with flap extension to control the gap between spoilers and flaps. At flaps 3, they are already deflected noticeably downwards. Going to Full, the outermost spoiler goes back to neutral as the rest deflect further downwards. See 5:30 in the -1000 video here: https://youtu.be/J8dGnemm5So?t=330


I think this sums it up well, I will just add my final thoughts, sorry for the delay, I was traveling (on a 787 interestingly enough).

The only thing I would add, is that they FBW controls do not just move the ailerons up when the controls are moved, they move them however needed to accomplish the inputs from the pilots, so it is possible, that a slight gust of wind required an adjustment, it is hard to tell in that the FBW system is taking in many different parameters and putting in various movements to accomplish the inputs from the cockpit. It is a little mind boggling the calculations done to achieve this.

As for the controls while taxing, the 777, and A350 are still airplanes and they are still affected by wind on the ground, and as such while taxing, pilots use the ailerons to prevent the plane from flipping. If I had to guess in this day and age, the older pilots still do this due to habit, and the newer pilots do not. http://flighttrainingcenters.com/traini ... ring-taxi/

While on the 787, during the takeoff role, the inboard aileron did a full cycle of its movement and returned to matching flaps 5. I don't have an explanation for that except that their could have been a cross wind that the aircraft/pilot was correcting for until they reached a higher speed.

And I will leave you with one last thought, some FBW systems are so advanced, that to adjust for turbulence, they actually move back in forth effectively countering the turbulence, or in the case of the F-22, increasing instability, making the aircraft more maneuverable, or so I am told...
One mile of highway gets you one mile, one mile of runway gets you anywhere.
 
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Starlionblue
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Re: 777 & A350 ailerons behavior vs fuselage length

Tue Mar 12, 2019 1:45 am

In airliners, there is no need to hold control surfaces during the taxi to compensate for wind. The aircraft is too heavy to be materially affected. If the wind is strong enough to move the aircraft on the ground, you shouldn't be going flying. :)

During the take-off roll on the A330/A350, forward stick is held until around 100 knots to compensate for the pitch up moment of engine thrust. Aileron into the wind can be held to compensate for crosswind, but input should not be so great as to deflect the roll control function of the spoilers.

Yes, advanced FBW does counter for turbulence, with features such as load alleviation.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
CeddP
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Re: 777 & A350 ailerons behavior vs fuselage length

Tue Mar 12, 2019 2:28 pm

While on the 787, during the takeoff role, the inboard aileron did a full cycle of its movement and returned to matching flaps 5. I don't have an explanation for that except that their could have been a cross wind that the aircraft/pilot was correcting for until they reached a higher speed.

Once again no pilot input involved there but a bypass mode activated during first part of TO roll to prevent wear on the flaperon jack (same on 777). Starlionblue created a thread about it few years ago : viewtopic.php?t=743941
 
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Starlionblue
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Re: 777 & A350 ailerons behavior vs fuselage length

Tue Mar 12, 2019 2:31 pm

CeddP wrote:
While on the 787, during the takeoff role, the inboard aileron did a full cycle of its movement and returned to matching flaps 5. I don't have an explanation for that except that their could have been a cross wind that the aircraft/pilot was correcting for until they reached a higher speed.

Once again no pilot input involved there but a bypass mode activated during first part of TO roll to prevent wear on the flaperon jack (same on 777). Starlionblue created a thread about it few years ago : viewtopic.php?t=743941


Thanks for the trip down memory lane. 14 years ago... OMG.... I wasn't even contemplating a PPL back then....
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
WIederling
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Re: 777 & A350 ailerons behavior vs fuselage length

Tue Mar 12, 2019 5:18 pm

mpdpilot wrote:
............ it is hard to tell in that the FBW system is taking in many different parameters and putting in various movements to accomplish the inputs from the cockpit. It is a little mind boggling the calculations done to achieve this. .


not 777 or a350 but A380:
https://www.google.com/search?q=Valse+Des+Ailerons

quite intriguing to look at.
Murphy is an optimist

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