Why airliners don't have negative flaps?

[Turbavykas]

Hello

As you may know modern flapped gliders use negative flaps. It helps reduce drag at high speed. Well airlines always fly more or less near maximum speed while in cruise ... but at different weight. Maybe negative flaps could be used to always maintain one optimal angle of attack? I mean when the fuel is burned negative flaps are added.

[redsnail]

Let's just think this through.

You are suggesting that airliners shouldn't optimise their weight change as fuel is burnt off? If I am getting close to the MMo, then I simply reduce thrust. Voila.

The jet I fly uses spoilers to reduce speed (or increase drag). I think that's what you're talking about.

[Turbavykas]

Not. You are completely wrong. For example if you cruise fully loaded 747 say you need 5 degrees angle of attack. For fully empty maybe 1 degree is needed. With negative flaps you can design wing that is optimal for just one angle of attack.
It has nothing to do with brakes or engine trust. Glider I fly have flaps: Landing, +2, +1, 0, -1, -2. If I fly 180km/h I can use 0, -1 or -2. But -2 is the most optimal setting.

[KBPsen]

Airliners also have wings optimized for a specific angle of attack. If required, this AoA is maintained by adjusting speed in relation to weight. It is known as long range cruise or max range cruise.

[Piltdown Man]

Turbavykas has a point. An airliner's wing (and wing rigging angle) is optimised for one angle of attack. But having reflex flaps means that the wing can be rigged for minimum drag and the aircraft can be operated more efficiently over a wider speed range. After all, the required gubbins is already installed. Yes a mod. will have to be done for existing aircraft but that doesn't stop this feature being installed in new aircraft. Then, LRC might be slower but high speed cruise will certainly be more efficient - and let's face it, we often cruise in excess of LRC.

[KBPsen]

To be honest, I'm not sure Turbavykas understands the point he/she is trying to make.

Maybe it's a language issue.

[ShyTorque]

I think the OP is referring to "reflex" which is in use as he describes.

[Mad (Flt) Scientist]

Added complexity is likely one reason. Although the flaps are indeed "already installed" in most cases their actuation and load-bearing systems are designed for (relatively) low speeds - up to VFE and VF. Having to design a flap system for the whole flight envelope - likely up to VD, once you say they can move in cruise - might have significant weight impacts.

There would also be failure cases - once it's possible for the flaps to be legitimately in motion in a high speed condition, then detecting an erroneous motion becomes much harder.

And there would be certification issues - if I can be anywhere between 0 deg flap and -4 deg flap at MMO, I am going to have consider all of the possibilities for the upset conditions. The increased number of fairly high risk tests will be a burden.

Aerodynamically, it likely works, or could be made to work if designed-in from the start. But it's not going to be a free ride.

[Dan Winterland]

I understand it - but then I have also flown high performance gliders. And I have often thought the same - although I understood negative flaps to be more about dumping unwanted lift and also drag. But as some of the team who designed the modern Airbus aircraft had also designed gliders (the A320 has an aerofoil section based on a common glider section) I would be surprised if they hadn't considered it. The fact they don't have them probably means they weren't deemed to be workable.

[dixi188]

Don't some FBW ailerons already perform the function of reducing lift at high speed on the outboard part of the wing?

[Cough]

[guess mode]Don't we change level in order to maintain a more optimal AoA as weight changes? Or is this an over simplification?

[Linktrained]

A French National Gliding Centre used to be at Montaigne Noire, conveniently situated to the East of Toulouse, which may have helped to encourage thoughts about wings and air-mindedness generally, in the 1950s.

(I recall my wings sometimes creaking a little, on meeting up with a Thermal, or whatever, when gliding there, then !)

The Hermes aircraft always flew tail low. Our Chief Pilot tried to see if " inching down " a small amount of flap, whilst cruising could make a difference. He had a marked glass of water, acting as a spirit level. I don't think that it did, so far as I recall. But worth trying, anyway.


We had had the Air League of the British Empire, rather earlier.

( Other Gliding Centres were active at Troyes and at St. Auban sur Durance in Bas Alpes.)

[Intruder]

I think that the answer to the OP's question lies in the fact that airliners are optimized for a relatively narrow high-speed range already. In contrast, high-performance gliders need to be optimized for 2 distinct speed ranges -- min sink speed for thermaling, and max range speed for cruising between thermals. While the difference may be "only" 30-50 knots, it is a significant percentage increase. Therefore, the use of negative or reflex flaps makes sense when the basic wing has to be efficient at the lower thermaling speed (positive flaps would add drag; and leading edge devices would add drag, weight, complexity, and cost).

[John Farley]

Well said Intruder

[Tinwacker]

Don't some FBW ailerons already perform the function of reducing lift at high speed on the outboard part of the wing?

Yes, and most recently the B747-8 does that and before FBW the L1011-500 too..active ailerons.

[EXWOK]

The 787 does indeed reflex its flaps in the cruise.

As I remember it, there's a separate (electromechanical) actuation system which reflexes the flaps slightly according to the current position in the flight envelope.

Most glider aerodynamic innovations make it on to airliners after 20years or so, this one just took a little longer ;)

[Intruder]

I know of no such "reflex flap" implementation on the 748. Maybe you are confusing the Pitch Augmentation function of the spoilers, which functions when the flaps are extended...

The outboard ailerons on the 748 are also active, but AFAIK they are solely active to prevent flutter at high speeds. OTOH, that would not preclude them from being programmed to unload the wingtips in some conditions... They also droop when flaps are selected to 10 or 20 for takeoff.

EWOK: I initially read yours as 748 instead of 787. Sorry 'bout that!

[Piltdown Man]

Intruder - I don't think you quite have the handle on gliding. The flap system on a glider merely generates multiple polar curves; one for each flap setting. The speed for max. L/D increases as the flap angle reduces. For an airliner, this would mean that when cruising at high weights with high speeds, a certain amount if reflex flap could be used to obtain an optimum deck angle, thus reducing profile drag. At lower weights, lower speeds would be used and the flap setting would move towards zero from a negative setting, again optimising deck angle with respect to the relative airflow.

A glider's speed is also not binary as you appear to suggest. It's a little more subtle than that. It varies from minimum sink to best L/D to warp factor snot depending on the air it's travelling through and what the pilot expects next. If gliding solutions were applied to airliners we might actually save an appreciable amount of fuel, a number that might even impress the bean counters.

The reason we might not have reflex flaps were discussed by our mad scientist and he said it might result in the failure case being an expense we couldn't afford. But there again, we might reconsider this when we really have to start saving fuel

[Bpalmer]

Hello

As you may know modern flapped gliders use negative flaps. It helps reduce drag at high speed. Well airlines always fly more or less near maximum speed while in cruise ... but at different weight. Maybe negative flaps could be used to always maintain one optimal angle of attack? I mean when the fuel is burned negative flaps are added.

The 787 and A350 both have cruise flap functions that will -without pilot intervention or action - adjust the flaps in flight to optimize performance. Earlier models (A330) moved fuel to adjust the center of gravity to optimize performance, but that required a lot of plumbing. This generation adjust the center of lift, instead of the center of gravity for a similar result with less hardware.