Pyrex
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Strange Control Surface On The Wing Of The 767

Sun Mar 30, 2008 6:49 pm

Hi guys,

I just did a flight for the first time in a 767, a Delta -300(ER?) from ATL to GIG and, while landing, noticed on the wing what seemed to me a strange control surface. It was large, between the inboard and outboard flap on the trailing edge of the wing, took about 25% of the chord and had an awkward shape. At first I thought it was another spoiler but it didn't move in tandem with the spoilers. It did, however, move when the aircraft was turning.
I think it might be some kind of roll damper but I had never seen anything quite like that. If it is, in fact, a roll damper, it mustn't be very efficient to have such a large moving surface just for that. Can anybody help me identify this control surface?
Thanks,

Miguel
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futureualpilot
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RE: Strange Control Surface On The Wing Of The 767

Sun Mar 30, 2008 6:58 pm

High speed aileron IIRC.
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lowrider
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RE: Strange Control Surface On The Wing Of The 767

Sun Mar 30, 2008 7:00 pm

Off the cuff, it sounds like an inboard aileron. Can you find a picture? That would settle it.
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2H4
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RE: Strange Control Surface On The Wing Of The 767

Sun Mar 30, 2008 7:01 pm



Quoting Pyrex (Thread starter):
It was large, between the inboard and outboard flap on the trailing edge of the wing, took about 25% of the chord and had an awkward shape.

Sounds like the inboard aileron:


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Pyrex
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RE: Strange Control Surface On The Wing Of The 767

Sun Mar 30, 2008 7:17 pm



Quoting 2H4 (Reply 3):
Sounds like the inboard aileron:

That is exactly it. Is it normal for widebodies to have these or is it just the 767 aileron's that have little roll authority?
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lowrider
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RE: Strange Control Surface On The Wing Of The 767

Sun Mar 30, 2008 7:20 pm

It is a fairly normal design feature. On may types of aircraft, the outboard aileron will lock out to limit the twisting stress on the wing at high speeds.
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UAL747
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RE: Strange Control Surface On The Wing Of The 767

Sun Mar 30, 2008 8:26 pm



Quoting Pyrex (Reply 4):
That is exactly it. Is it normal for widebodies to have these or is it just the 767 aileron's that have little roll authority?

This is the high speed aileron, or inboard aileron. It's common on Boeing widebody aircraft, although the 787 will feature only outboard ailerons. Modern Airbus widebodies do NOT have inboard ailerons.

The outboard ailerons lock after the flaps are fully retracted or at a certain speed and only the inboard ailerons are used in cruise. The theory is that at slower speeds, the outboard ailerons kick in because they will have the most effectiveness because they are farthest away from the center of gravity. They lock out at high speeds because the inboard ailerons are sufficient and give less loading on the wing when in action.

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cptspeaking
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RE: Strange Control Surface On The Wing Of The 767

Sun Mar 30, 2008 9:20 pm

Ok, I understand the theory behind limiting the twist of the wing at high speeds, but the effectiveness theory doesn't sound right to me...

Airplanes that have the most "effectiveness" rolling would have to be the T-38, the Pitts S-2, etc...but the common denominator here is SHORT wings and less distance from the flight controls to the CG. Seems to me that a control deflection 20 feet out on the wing gives you twice the roll rate of the same deflection at 40 feet out on the wing. If you watch videos of these high speed ailerons, you'll see that they're really moving around and working a lot at LOW speeds. Am I just way off on this?
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lowrider
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RE: Strange Control Surface On The Wing Of The 767

Sun Mar 30, 2008 10:40 pm

Quoting CptSpeaking (Reply 7):

The aircraft you mentioned also have ailerons that are relatively large compared to the wing they are on. The short span combined with the large ailerons help generate the dramatic roll rate.

As to your question on the deflection of the ailerons, all other things (air pressure, altitude, etc) being equal, a control surface will have to deflect further at a lower airspeed to generate the same result as a smaller deflection at a higher airspeed. This is why the outboard ailerons can be locked out at high speed with no penalty. If they fail to unlock during approach, then you do suffer penalties. The simplest way to visualize it is to imagine a boat and note the effectiveness of the rudder at 5,10, and 20 knots.

Hope this thumbnail sketch helps.

[Edited 2008-03-30 15:40:46]
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Jetlagged
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RE: Strange Control Surface On The Wing Of The 767

Sun Mar 30, 2008 11:48 pm



Quoting CptSpeaking (Reply 7):
Airplanes that have the most "effectiveness" rolling would have to be the T-38, the Pitts S-2, etc...but the common denominator here is SHORT wings and less distance from the flight controls to the CG. Seems to me that a control deflection 20 feet out on the wing gives you twice the roll rate of the same deflection at 40 feet out on the wing. If you watch videos of these high speed ailerons, you'll see that they're really moving around and working a lot at LOW speeds. Am I just way off on this?

They've been called high speed ailerons here. All speed aileron is more accurate. The inboard/all speed ailerons work at low speeds and high speeds. Outboard/low speed ailerons work at low speeds, not at high speeds. These are generalisations, there are a lot of detail differences between aircraft types.

As for roll rates, an aircraft with a short stubby wing can generate a high roll rate because the roll damping from the wing is small (centre of pressure of the wing close to the CG) compared to one with high aspect ratio wings. They also have lower inertia about the roll axis, too. Ailerons are less effective closer to the CG (their leverage is less in layman's terms). Larger aileron area can make up for less leverage. Hence high rate of roll in the T-38, Pitts S-2, etc.

Quoting UAL747 (Reply 6):
Modern Airbus widebodies do NOT have inboard ailerons.

The A330 and A340 do have inboard and outboard ailerons but they are side by side, outboard of the flaps. In manual flight outboard ailerons only deflect at low speeds (< 190 kts) with flaps extended. In automatic flight and with certain failures they operate with the inboards up to 300 knots.
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RE: Strange Control Surface On The Wing Of The 767

Mon Mar 31, 2008 12:15 am



Quoting Jetlagged (Reply 9):
The A330 and A340 do have inboard and outboard ailerons but they are side by side, outboard of the flaps. In manual flight outboard ailerons only deflect at low speeds (< 190 kts) with flaps extended. In automatic flight and with certain failures they operate with the inboards up to 300 knots.

Ah, well, I learn something new everyday. However, I still consider those to be a "set" of outboard ailerons when compared to the distance of the inboard ailerons from the Boeing aircraft, but to each his own. I wonder then what they call all 3 of them on the A380? Outboard, Middle, Inboard Ailerons? BTW, speaking of the A380's ailerons, does anyone else think that their movements are really strange? It seems there is no method to the madness as they are ALL always moving, some down while others up.....strange...though I know that there is a reason for it, just can't explain it.

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RE: Strange Control Surface On The Wing Of The 767

Mon Mar 31, 2008 12:52 am



Quoting UAL747 (Reply 10):
...though I know that there is a reason for it, just can't explain it.

I seem to remember that the A380 was fitted with an active gust load alleviation system for the wing. Perhaps the aileron movements are due to this function  Confused .

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RE: Strange Control Surface On The Wing Of The 767

Mon Mar 31, 2008 12:54 am

Ahhhh. I always wondered what those things exactly were also.
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RE: Strange Control Surface On The Wing Of The 767

Mon Mar 31, 2008 1:55 am

The 707 and 727 also have outboard and inboard ailerons.


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cptspeaking
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RE: Strange Control Surface On The Wing Of The 767

Mon Mar 31, 2008 3:02 am



Quoting Jetlagged (Reply 9):

I'm pickin up what you're puttin down about the center of pressure, dampening, etc., but I'm still struggling with how ailerons closer to the CG are less effective...

I'll go read up a bit in my Aerodynamics books and see if I can find an answer I can visualize...
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RE: Strange Control Surface On The Wing Of The 767

Mon Mar 31, 2008 3:27 am

I've heard stories about 747 test flights with the outboard aileron lockout deactivated. At high speeds, the pilot was able to quickly turn the wheel to full deflection and the plane stayed put. This is because the wing twisted in the opposite direction enough to counteract the aileron.

Quoting CptSpeaking (Reply 14):
I'm pickin up what you're puttin down about the center of pressure, dampening, etc., but I'm still struggling with how ailerons closer to the CG are less effective...

I think they key has less to do with roll rate than with roll authority. Since they outboard ailerons are further from the CG, they've got far more leverage. This means they'll be more responsive at low speeds. Think of it this way: if you had to roll an airplane by grabbing the wing and pulling down, you'd have to apply much more force to get it started by holding the wing root. For the same amount of force, you'd get it started easier at the wing tip.
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RE: Strange Control Surface On The Wing Of The 767

Mon Mar 31, 2008 11:31 am



Quoting CptSpeaking (Reply 14):
I'm pickin up what you're puttin down about the center of pressure, dampening, etc., but I'm still struggling with how ailerons closer to the CG are less effective...

I'll go read up a bit in my Aerodynamics books and see if I can find an answer I can visualize...

It's not so much to do with aerodynamics, its forces and moments. The same size aileron twice as far away from the CG will produce twice as much rolling moment.
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RE: Strange Control Surface On The Wing Of The 767

Mon Mar 31, 2008 2:46 pm

Also on the 777 the inboard moves down when the flaps extend.
 
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RE: Strange Control Surface On The Wing Of The 767

Mon Mar 31, 2008 4:00 pm

Nobody mentioned the A310 which has no outboard ailerons installed.

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RE: Strange Control Surface On The Wing Of The 767

Tue Apr 01, 2008 1:36 am

I know something too. Spoilers help too Big grin

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RE: Strange Control Surface On The Wing Of The 767

Tue Apr 01, 2008 2:09 am

Yup! A310 has no outboards.

Perhaps less known, but also true, is the fact that the Convair jets - the 880 and 990 - had no outboard ailerons, only inboards. Making it even more idiosyncratic, the CV990 had no inboard spoilers, just immediately inboard of the outer engines. This particularly good perspective of the NASA CV990 shows this peculiarity quite well.
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RE: Strange Control Surface On The Wing Of The 767

Tue Apr 01, 2008 3:19 am



Quoting CptSpeaking (Reply 7):
Seems to me that a control deflection 20 feet out on the wing gives you twice the roll rate of the same deflection at 40 feet out on the wing.

Actually, a control deflection 20 feet out on the wing would give you half the roll rate of the same deflection 40 feet out on the wing. This is just simple physics: force X arm = moment. If you're trying to turn a bolt with a wrench, you're going to have a much easier time if you hold the part of the wrench furthest from the bolt, because the additional arm will make your efforts much more effective.

Comparing the effectiveness of a 767's ailerons to those of a T-38 is comparing apples to oranges. Or, more accurately, apples to coffee tables - they're nothing alike.  Smile

Quoting CptSpeaking (Reply 7):
If you watch videos of these high speed ailerons, you'll see that they're really moving around and working a lot at LOW speeds. Am I just way off on this?

Since they are less effective than the outboard ailerons, they do have to move around more to achieve a similar result.

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RE: Strange Control Surface On The Wing Of The 767

Tue Apr 01, 2008 3:25 am



Quoting Troubleshooter (Reply 18):
Nobody mentioned the A310 which has no outboard ailerons installed.

Just like the early A300s. I can confirm that the TNT A300 which we have in the hangar at the moment only has an inboard aileron.

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RE: Strange Control Surface On The Wing Of The 767

Tue Apr 01, 2008 1:32 pm



Quoting Mir (Reply 21):

I think I'm starting to get it...my tiny little brain was thinking that if I push down on the wingtip 2 feet at a certain rate, the airplane will roll at x rate, and if I push down at half the distance for 2 feet at the same rate, I'll get twice the roll rate, and the wingtip will now be 4 feet down rather than 2 with the same force being applied, just in a different spot...what I wasn't considering was the difference in force required caused by the difference in arm/moment...got it now though, thanks  Smile
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RE: Strange Control Surface On The Wing Of The 767

Tue Apr 01, 2008 5:27 pm



Quoting Mir (Reply 21):
If you're trying to turn a bolt with a wrench, you're going to have a much easier time if you hold the part of the wrench furthest from the bolt, because the additional arm will make your efforts much more effective.

Lemmy's story suggests that you might also extend this analogy;

Imagine a seized bolt - one that resists turning. Gaining more leverage with an extension bar might get it undone, but it also might just shear the bolt - or bend the extension bar.
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RE: Strange Control Surface On The Wing Of The 767

Thu Apr 03, 2008 2:39 am

To Pyrex,

Regarding the wing of the Boeing 767, you probably saw the high-speed aileron.

Traditionally, the best place to put the ailerons on a wing are at the wingtips becuase it's position far away from the plane's center-of-gravity enables them to exert considerable leverage to roll the plane.

Certain airplanes though utilize inboard-ailerons (generally in conjunction with outboard ailerons) generally, if not always, because certain airfoils are rather flexible and bend rather easily particularly at the tips, particularly at high-airspeeds. When the ailerons are deflected at such speeds, the flexibility of the wing can cause the wing's leading-edge to twist in opposition to the ailerons which can nullify the ailerons movement and eventually reverse the roll.

To avoid this, one solution is to place another aileron further inboard on a thicker area of the wing that doesn't tend to be as flexible. This tends to often be in between the flaps, however the A-330 and A-340 has them placed inboard of the low-speed ailerons. The outboard ailerons are locked-out either by flap position (Either all at once: Flaps go up, outboard aileron locks out, or incrementally -- like the KC-135, B-707, DC-10, early B-747's, as certain flap-settings are reached the outboard aileron is increasingly restricted or unrestricted as on the B-727 as a certain degree of deflection can occur without twisting the wing at least in some cases), or as a function of airspeed (As on the B-767, on the B747-400, and at least some refitted B747-200's, the B-777, and the A-330 and A-340 though I'm not sure if in correct order :-P ). The latter is more complicated apparently than the first (Although the DC-8 sort of used an inboard/outboard aileron, it was a split-aileron. A torque-tube is used to restrict the movement of the outboard section -- it first flew in 1958) Some planes have just inboard ailerons such as the F-100 Super-Sabre (Normally I wouldn't include a fighter, but it's an example), B-52, the Convair-880 and Convair-990, the A-310, and some A-300 models. Since inboard ailerons do not generally produce good roll-rates at low-speed (The F-100, CV-880 and CV-990 are exceptions although the first is a fighter, and the second two have rather large inboard ailerons. The A-330 and A-340 type inboard ailerons also wouldn't behave badly at low-speed because they were fairly far out down the span) usually asymetric-spoiler deflection (increases roll-rate by dumping lift on one side, asymmetrical drag also helps negate adverse yaw) are used to compensate. (As a note, many planes that have inboard and outboard ailerons, along with planes that have outboard ailerons too also often use roll-spoilers as well)

Another solution is to stiffen the wing, and/or reduce the wing-sweep in order to reduce the amount of flexing to avoid the whole aileron reversal problem in the first place. Stiffening can be easy in some cases, and harder in others. It depends on how big the wing is, and how big the span is. Weight can be a considerable addition. As for the wing-sweep, modern supercritical foils don't require as much of a sweep angle to produce the same cruise speeds as earlier jetliners.


To CptSpeaking and Jetlagged,

What's Roll-Damping?


To Mir,
That's how I always understood it. The further out the aileron is from the plane's center-of-gravity the higher the roll-rate except with very very long wings I think particularly at very low airspeeds like with glider-designs.
 
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RE: Strange Control Surface On The Wing Of The 767

Thu Apr 03, 2008 3:47 am



Quoting Blackbird (Reply 25):
What's Roll-Damping?

The amount of counter-torque caused by dragging the wings through the air as you roll. Thing of a rolling airplane as a paddle wheel with the wings as the paddles. The bigger the paddles, the more torque it takes to achieve a particular roll rate, and the faster that roll rate will drop off (damping) when you stop the control input.

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RE: Strange Control Surface On The Wing Of The 767

Thu Apr 03, 2008 5:27 pm



Quoting Blackbird (Reply 25):
To CptSpeaking and Jetlagged,

What's Roll-Damping?

Damping is a force opposing motion, due to velocity. So roll damping is a rolling moment opposing the rolling motion due to roll rate. If you initiate a high roll rate, then centre the controls, roll rate will reduce due to roll damping.

When the aircraft is rolling, the AOA on the down going wing is increased, the AOA on the upgoing wing is reduced. So the down-going wing gets more lift, opposing the rolling motion. The up-going wing produces less lift, which also opposes the rolling motion. The higher the roll rate, the greater the effective AOA change and so the greater the damping.
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Blackbird
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RE: Strange Control Surface On The Wing Of The 767

Thu Apr 03, 2008 10:55 pm

Tdscanuck and Jetlagged,

Thank you for the explanations


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RE: Strange Control Surface On The Wing Of The 767

Fri Apr 04, 2008 3:36 am



Quoting Blackbird (Reply 25):
The further out the aileron is from the plane's center-of-gravity the higher the roll-rate except with very very long wings I think particularly at very low airspeeds like with glider-designs.

No exceptions. The further out the aileron is, the higher the roll rate will be (all other things equal). FxA=M doesn't lie.  Smile

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Blackbird
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RE: Strange Control Surface On The Wing Of The 767

Fri Apr 04, 2008 4:39 am

Mir,

Quote:
No exceptions. The further out the aileron is, the higher the roll rate will be (all other things equal). FxA=M doesn't lie.  Smile

What about roll damping?
 
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RE: Strange Control Surface On The Wing Of The 767

Fri Apr 04, 2008 5:44 am



Quoting Mir (Reply 29):
FxA=M doesn't lie.

Neither does F=MA  Wink

Quoting Blackbird (Reply 30):
What about roll damping?

I assume he meant all other things being equal.
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Mir
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RE: Strange Control Surface On The Wing Of The 767

Fri Apr 04, 2008 2:05 pm



Quoting Blackbird (Reply 30):
What about roll damping?

Not part of the equation. I did say "all other things being equal". Granted, roll damping will make it harder for a long-winged aircraft to turn than it would for a short-winged aircraft. But that has to do with the length of the wings, not the position of the ailerons. If you take a glider that has X amount of roll damping and move the ailerons inward while leaving the wingspan the same, you'll still have about the same amount of roll damping, but the effectiveness of the ailerons will be reduced due to the shorter arm.

-Mir
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