Happy-flier
Topic Author
Posts: 287
Joined: Sat Dec 25, 1999 4:41 pm

When Outboard Ailerons Act As Flaps...

Sun Nov 15, 2009 4:39 am

What brought about the distinct change in design philosophy that has seen airliners that routinely use the outboard ailerons to augment flap function? Examples: 777, A330/340, MD11. The use of inboard ailerons only in the same manner - as on the 767, for example - was seen a bit earlier. And, going still farther back in time, no other major jets ever used any of the ailerons (inboard or outboard) in the same way (e.g. 707, 727, DC-10, L1011, 747).

Does having the entire wing trailing edge as "one flap", even if in sections, allow you to use fewer degrees of deflection overall, thus increasing lift while decreasing drag? Is that the whole point?

As a counterexample, consider the KC-135 wing. The flaps have a large deflection, while the ailerons do not perform any flap augmentation: this causes there to be simultaneous sections of high-lift, and other sections of perfectly clean wing.

Would love to read some thoughts on this whole thing.
May the wind be always at your back . . . except during takeoff & landing.
 
rwessel
Posts: 2448
Joined: Tue Jan 16, 2007 3:47 pm

RE: When Outboard Ailerons Act As Flaps...

Sun Nov 15, 2009 6:04 am

Flaperons are fairly common on sailplanes. They provide better roll performance, since the effective ailerons are now much bigger (remember that most sailplanes are pretty ponderous in roll - especially at lower speeds) - on some sailplanes essentially *entire* span* is covered by the flaperons - on others it's only the outboard half-or-so, and more effective flaps, since they're bigger too (no cutouts for the ailerons).

You do need a mixer to combine the two inputs. Presumably this is pretty straight-forward to implement on a FBW aircraft.

OTOH, you don’t see the complex flap types on gliders that you do on powered aircraft, so I’d assume it’s more treating the aileron as an additional section of simple flap to eek a few extra percent performance out of the wing at low speed.


*Although it’s not uncommon for there to be more than one segment.
 
swiftski
Posts: 1837
Joined: Tue Dec 19, 2006 6:19 am

RE: When Outboard Ailerons Act As Flaps...

Sun Nov 15, 2009 6:11 am



Quoting Happy-flier (Thread starter):
Does having the entire wing trailing edge as "one flap", even if in sections, allow you to use fewer degrees of deflection overall, thus increasing lift while decreasing drag? Is that the whole point?

Whenever you use flap, you increase lift (refer to lift formula if needed).

Whenever you show more of an aerofoil to the relative airflow, you increase drag.

Flaperons are designed to deploy less than flaps, not to act as a large combined flap all the way across the aerofoil.

Does that make sense? I'm not sure if I'm being clear.
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: When Outboard Ailerons Act As Flaps...

Sun Nov 15, 2009 6:38 am



Quoting Happy-flier (Thread starter):
What brought about the distinct change in design philosophy that has seen airliners that routinely use the outboard ailerons to augment flap function? Examples: 777, A330/340, MD11.

Primarily FBW. It's not required, obviously, but it's a lot easier. Implementing aileron function, flap function, and high-speed lockout all at once in a manual or manual/hydraulic system is not trivial (and heavier). In FBW, it's just more lines of software (and no weight penalty).

Quoting Happy-flier (Thread starter):
Does having the entire wing trailing edge as "one flap", even if in sections, allow you to use fewer degrees of deflection overall, thus increasing lift while decreasing drag?

Yes. Full-span flaps have lower induced drag than the same amount of lift coming from part-span flaps.

However, I suspect the weight advantage is much larger than the drag/deflection advantage.

Tom.
 
CosmicCruiser
Posts: 2049
Joined: Tue Feb 22, 2005 3:01 am

RE: When Outboard Ailerons Act As Flaps...

Sun Nov 15, 2009 2:25 pm

We used the drooped ailerons on our MD-11 for years but they were finally deactivated due to the increased maint.
 
B777LRF
Posts: 1471
Joined: Sun Nov 02, 2008 4:23 am

RE: When Outboard Ailerons Act As Flaps...

Mon Nov 16, 2009 12:23 am

The main benefit of drooping ailerons is that it'll allow for slight lower landing speeds, thus reducing landing distance required. It may also help to lift a bit more weight off a restricted runway or, conversely, reduce the Take-off distance required with similar weights.

Though it wasn't an option for early generation FBW Airbii, it became retrofitable through nothing more than a software update, demonstrating just how clever FBW can be.
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