Can we increase the wing area vertically?

[russ_watters]

If you mean by "efficeincy" the content that this word nomally wil have the realtionship between the lift and the drag, it is true that a thick wing allways will produce a lot of drag, so it not be "effecient" in that way.

Yes, in general, the higher the aspect ratio, the higher the l/d ratio due to reducing the effect of wingtip vortices.

If the word "efficiency" should mean something else, something to "how to produce as much lift as possible with a wing that has a small area", then the situation is changed.

True, but that isn't a typical situation for airplanes. In any case, there is another word for that: wing loading. It is the weight per unit area that a wing supports (ie, lb/sq ft). Delta wings typically have lower wing loadings than higher aspect ratio wings for the same reason described above: it takes more area to produce the same amount of lift on a lower efficiency wing.

Wing for bigger aircrafts is allways built with a leading edge slat and a trailing edge flap, to increas "the aerodynamic thicness" of the wing while flying at low speeds.

No, that's not the purpose of either flaps or slats. The purpose of flaps (leading or trailing edge) is to increase camber and the purpose of slats is to provide boundary layer control to enable flight at higher angles of attack (which is what your wik link says in it's first sentence). Neither significantly affect the chord length of the wing. Ie, with them deployed, you may double (or more) the lift while only increasing the chord by 10%. Your last link with the flow visualization also shows that very little lift is generated by the slat itself.

It is true that if you should make as much as possible lift on a wing with as small as possible area, it should make be made aerodynamically "thick".

No, that is not true and it contradicts the sentence that follows. Obviously, a wing with that consumes more fuel to keep flying has a lower l/d ratio.

You've got an awful lot of misconceptions going on here. You're doing good research, but you aren't reading what you find! Also, I get the impression you have a specific application in mind - if there is something unusual about it that changes the pros/cons, we may be able to help with it if you explain your application to us.

 

[caslav.ilic]

Some bits regarding the geometry influence on efficiency. Things sometimes take strange turns when one moves from analysis to design.

First things first, usually we know how big we want the airplane to be. This practically means its total weight, which will determine the price, all other things equal. So, the weight is somehow a fixed, external element of the design.

Let's think of cruise flight (forget flaps, slats, etc.)

The first "deviation" from the analysis is that the induced drag no longer depends on wing aspect ratio, but solely on span. That is to say, given a span, the induced drag will be the same no matter what aspect ratio the wing has.

Having further fixed the span, the aspect ratio actually influences the viscous, or skin drag. The higher the aspect ratio, the lower the skin drag. Unfortunately, also the higher the wing structural weight.

Aerodynamically speaking, the wing thickness now has only detrimental effect to drag -- the thicker the wing, the higher the skin drag. However, also the lower the wing structural weight.

How can these facts be used for design?

Suppose that you have a working airplane, and you want to tweak its wing shape to increase the efficiency (reduce drag) without touching either the total weight or the span (the span limit say due to ground support facilities, as largest box-size dimension for low-speed airplane). Furthermore, suppose that the airplane is structurally optimal, that all elements weigh only as much as needed to support the expected loads plus safety limits.

Well, the first piece of bad news is: you're screwed, can't improve efficiency at all :) So let's relax a bit: miraculously, some elements of the existing configuration can become lighter (e.g. due to use of shiny new materials). Now you have some weight reserve, which you can use to improve the efficiency of the wing.

So, span is fixed, means induced drag is fixed. You therefore start increasing aspect ratio in order to decrease skin drag, which increases the wing weight too, until you either again hit the total weight limit, or the limit of airfoil stall.

The airfoil stall limit may happen for the following reason. Having fixed span, increasing aspect ratio means reducing wing area. Hence, to carry the same total weight, the wing has to be under higher angle of attack. Eventually, the airfoils will stall. So if the stall limit happens before the weight limit, then airfoils have to be exchanged for some that will admit higher section lift, if possible.

If the weight limit is hit before the stall limit, than it is a bit more interesting. Remember the wing thickness? Providing that you keep the airfoil stall characteristics, you can increase the wing thickness so to gain more structural weight, which you can then keep throwing at increasing the aspect-ratio. (Usually the effect of skin drag increase for thicker airfoils is more than offset by skin drag decrease due to higher aspect-ratio.)

Basically, for cruise efficiency, this all boils down to increasing the wing loading within given weight limits, as Russ mentioned. Unfortunately, high wing loading will have detrimental effect to some other possibly important characteristics, like climb performance, takeoff/landing distance, and maneuverability. That's why commercial transports and airliners have very complicated slat/flap systems -- their basic wings are efficient cruise wings, and just by themselves would need much longer runway lengths and too high takeoff and landing speeds.

--
Chusslove Illich (Часлав Илић)

 

[Langbein]

You've got an awful lot of misconceptions going on here. You're doing good research, but you aren't reading what you find! Also, I get the impression you have a specific application in mind - if there is something unusual about it that changes the pros/cons, we may be able to help with it if you explain your application to us.

Well, I thaught I did not do any resaerch at all.

I don't know which of my "applications" you are thinking about.

Well normally they have not used to fall down to often even though I thought that small little thing on the tip of the wings that goes down (the slat) and that cute little thing at the rear, the flap were to increase the lift.

As I believed that possibly not all of us have a professional background from the airline industry, so I thought that the person behind the question might mean something else with the expression "thick wing" that I first thaught he ment.

He might be right about one basic consideration:

If you make a wing with the tip tuned down (like a slat) and the rear of the wing tuned down (like a flap) so that the wing will be formed in a courved way, like a bird wing than this wing will produce a bigger lift on a lower speed.

This little "bird wing principle" is used on allmost all jet airplanes when they are landing.

If you make a aircraft wing with the same basic geometry as a landing jetliner you will get a wing that produces a lot of lift and drag.

When slats and slats are used they will be used togeteher with the proper flight procedures for that certain aircraft.

On most aircraft models the correct use of slats and flaps together the proper flight procedures (nose up) will produse more lift, as the verticall area of the airstream the the the airfoil will work on will increase. (If you like to call that a "aerodynamicly thick wing" you can do that. If you don't like to you can call it something else, but it will be working on a thicker part of the airstream.)

Which aircraft model does not work this way ? (I was doing military fighther planes for some years ago, and I have some doubts about the F-16, did it have a leading edge slat (with no boundary control) or did it not. I believe it had both slat and flap (flapron) without any boundary control. I will try to check on that.)

By the way, I have still not made any research, yet, but I might look for some material :-)

 

[Langbein]

hmmm... i was just wondering, if we increased the area of a wing and sacrifice drag, we should get more lift right? and if we got more lift, should we not be fly with a low velocity?

I think it looks like I do remember correctely about the F-16. It has electro hydraulic actuators to turn the leading edge down and the trailing edge also down, to form it as a bird wing, for increased lift at low speed (typical at landing.) There is, as far as I remember no airleakage trought the wing and no boundary control that way. If one like to call that a aerodynamically thicker wing as it works on a thicker airstream. (When the leading edge slat goes down the aircraft nose will go up, to increase angle of attack.)

You can actually see the slat and the flap is actuated on this picture. (I hope that the link work.)

(See the wing left for the pilot.)

http://rides.webshots.com/photo/2367729320011758446TfOPer

 

[Langbein]

One more picture of a fighter aircraft the shows the "bird wing shape" due to a leading edge slat and trailing edge flap. This makes the wing work on a thicker layer of the airstream and produces more lift.

http://www.sirviper.com/index.php?page=fighters/su-27/index

(See the last picture, typically leading edge down, traling edge down, and node up so that the airstream will see a "thinker wing". If you made a simular picture of an F-16 or a Boing 747 at low speed, you will se that they do the "bird wing trix" as well.)

The_Thinker was actually right in his initial thaughts that you can increase the lift by increasing the area of the wing or the horizontal area of the airstream that the wing will meet. (If I understood him right.)

I do not believe that he was right if he think that a aircraft with such a wing design will fly for very long time using a battery and an electric motor as the power source.

I still have not made any research but I need to find some pictures to explain some stuff from "the good old days".

 

[Langbein]

The bird wing principle once more:

http://www.flightlevel350.com/aviation_video.php?id=8467

At this one one can se the front slats is actuated at the moment the weight off wheel switches is operated both at take off and landing.

http://www.flightlevel350.com/aviation_video.php?id=6349&vcm

And in the end possible the best exaple of a heavy lift low speed design using the slat and the flap (pluss nose up) to give increased lift at low speed.

http://www.flightlevel350.com/aviation_video.php?id=4419


In the end .. some low speed propeller aircrafts is made with "rubber boots" for deicing on the wing front. Those does not use leading edge slats, but the prinsipple of increasing the "thickness" of the airstream are still the same.

Which aircraft model is designed and flown from prinsiples unlike those described above ?

 

[russ_watters]

I don't know which of my "applications" you are thinking about.

Sorry - lost track of the thread and thought you were the OP... and I think we had somewhat of a miscommunication there because of it. The OP is misuing the word "thick" to indicate an increased chord length. It appears you were using the word "thick" in the more conventional sense, which then makes most of what you said correct.

 

[Langbein]

OK. No problem. The idea just struck me that the OP might have ment something that I did not understand at first reading :-)

By the way, I'm not sure that my english and my teminology is 100 % as I haven't looked into these textbooks for a while. (But used to work with that stuff for a couple of years.)

 

[Langbein]

Normally the leading edge slat is used to increase lift at low speed takeoff and landing only, but it looks like this Sukhoi Su-27 is flying with the slat extended all the time (while doing the airshow) to increase lift and manouverability at low speed. I think I have not seen that befor (??!)

http://www.flightlevel350.com/aviation_video.php?id=4663&vcm

 

[The_Thinker]

Wow! Guys thanks for all the info... I've really learned a lot from this... A lot of stuff that I had no idea about... Now I have a better understanding... Although a some of it is over the head...

Anyway...
Yeah, I am familiar with the F-16, and the hydraulic design on that, thanks for the all links Langbein...

But... yeah I get the general Idea... by increasing the chord length one can get more lift at low velocities, which is why planes land nose up, showing more of their wing to the wind, that is as you put it a thicker airstream (I hope I got this right...) but, the system would be very inefficient... Hope I got the point right...

 

[Langbein]

The increase of the cord length while the slat and the flap is extended is a part of the effect. The main effect will be as the slat in front goes down which will permit the the aircraft nose to go up so wing to meet more air in the vertical direction. The extended plaps will increase this effect, and the extended flaps will also make a drop in the pressure at the upper side of the wing, and a pressure buildup under the wing, and then all in all, a lor more lift and a even more increase in drag.

For setting the aircraft down while landing the increased drag is a good thing to reduce the forward speed. As much as possible lift at as small speed as possible while doing aerodynamically breaking, makes a smooth and nice landing. For landings you typically use a lot of slat and flaps, espesially at the last final. Landing without flaps will be an emegency sitation for most airplanes, as this situation will reqire a rather high landing speed, to keep the aircraft flying until landing.

For takeoff you use not that amount of slat/flap because the drag will be to great.

For the Sukhoi Su-27 the slats is something that you use also for doing airshows .. Didn't know that before I saw the film - quite interesting :-)

In the end I found two links that explain the slat/flap thing quite easy:

http://travel.howstuffworks.com/airplane13.htm

http://travel.howstuffworks.com/airplane14.htm

 

[FredGarvin]

Just be careful with "How Stuff Works." I treat them like Wkipedia in that they are a good first source, but try not to take them as gospel without corroborating sources. I once read an article they did on counter-rotating helicopters that was so badly wrong it was apparent that they never even looked at one let alone understood what was going on.

You guys are pretty close to what is going on, but I have to add one little thing...The effect of slats and flaps is to increase the AoA of the wing without altering the orientation of the fuselage. For example, in the slides that show the flaps in Langbein's first link, the flap in the down position took a wing chord with essentially no AoA and took it to approximately 20°, without changing the orientation of the major section of the airfoil. To really see the effect, draw a line from the center of the leading edge to the tip of the flap, that is your AoA.

 

[Langbein]

"The effect of slats and flaps is to increase the AoA of the wing without altering the orientation of the fuselage."

That's true. It can be used that way. Sorry for the inaccuracy.

 

[Langbein]

"I once read an article they did on counter-rotating helicopters that was so badly wrong .."

How does the Cariolis effect work on counter-rotating helicopters. Will the "90 degres delyed forces" be canceled out or what ? Will you have to have two set of swash plates where one is tilted to the left and one to the right when you want to get forward ? (It should work this way, shouldn't it ??)

 

[FredGarvin]

The Coriolis acceleration is not at issue. You are referring to precession and no, the counter rotation has no effects on the gyroscopic precession. It still is there. However, they do cancel out torque created by the other head, hence no need for a tail rotor.

If I understand your point about the swashplates correctly, yes, each rotor head has it's own swasshplate. However, due to mixing and flight control issues, it's not quite as cut and dry as you describe. There is some tilting of the rotor disk, but the swashplate effects the pitch of each blade in an articulating rotor head. Each blade pitch changes at a specific point in the rotation, depending on the cyclic input. If the collective is changed, the entire swashplate moves at the same inclination.

 

[russ_watters]

You guys are pretty close to what is going on, but I have to add one little thing...The effect of slats and flaps is to increase the AoA of the wing without altering the orientation of the fuselage.

Hmm, didn't think about that - how big of an issue is that? Older planes used to have leading-edge flaps instead of slats, so their geometric angle of attack didn't change much, if at all when they were deployed. Is the major benefit just that you don't need tall landing gear and a droopy nose like on the Concorde?

 

[caslav.ilic]

The effect of slats and flaps is to increase the AoA of the wing without altering the orientation of the fuselage.

Hm, I wouldn't really put it that way. If airplane is in the air, it can change AoA by pitch control. If it's on the ground, it could do the same by telescoping nose wheel strut. In fact, the Vought Crusader even had variable incidence wing, which could pivot by seven degrees on takeoff/landing. Any of this would be simpler and structurally lighter than the slats/flaps mechanism, hence no need for them only to increase AoA.

More precise would be to say that slats/flaps increase the critical value of the effective AoA (the one measured from the zero-lift AoA), i.e. the value where the lift stops increasing with increase in effective AoA.

--
Chusslove Illich (Часлав Илић)

 

[caslav.ilic]

Older planes used to have leading-edge flaps instead of slats, so their geometric angle of attack didn't change much, if at all when they were deployed. Is the major benefit just that you don't need tall landing gear and a droopy nose like on the Concorde?

I don't exactly understand this "leading-edge flaps instead of slats" -- these are not synonymous in English? The way I had it, slat is just a short name for leading-edge flap, and as trailing-edge flaps they come in several designs. But all the slat designs are having qualitatively the same effect on aerodynamics, so either way I think there shouldn't be any differencing between them for purposes of this discussion.

Flaps increase effective AoA and decrease the geometric (fuselage) AoA at critical effective AoA, which is a benefit at landing. Slats too increase the critical effective AoA, but also the geometric AoA (i.e. they don't change the zero-lift AoA), and therefore would by themselves require an increased fuselage incidence. Therefore at landing slats are used as an addition to flaps, when even more lift is needed than by flaps alone. (Slats can be used by themselves in maneuvering flight, as they don't increase the drag as much as flaps, have nose-up pitching moment thus reducing trim drag, and simple slotted slats can even be deployed automatically when needed by a mechanical spring system.)

Regarding Concorde, it couldn't make use of flaps because the delta wing with flaps would have too high nose-down pitching moment with nothing to counter it. Delta wings also don't use slats, as they kind of get them "naturally": at high AoA starts the shedding of leading-edge vortices, which act in a manner similar to slats, increasing critical effective AoA and not changing the the zero-lift AoA (in fact, these LE vortices are even better, as the lift curve goes upwards "superlinear" from that point on). So Concorde had to land at very high AoA to exploit the LE vortices for enough lift at low speed, hence the droop nose.

--
Chusslove Illich (Часлав Илић)

 

[FredGarvin]

Hm, I wouldn't really put it that way. If airplane is in the air, it can change AoA by pitch control.

The whole point is to be able to change the AoA without using pitch control. That way the fuselage can remain relatively in the same position while the wing "thinks" it's at a higher AoA.

If it's on the ground, it could do the same by telescoping nose wheel strut.

If it's on the ground, then flaps and slats aren't really going to do much of anything, will they? What's your point?

In fact, the Vought Crusader even had variable incidence wing, which could pivot by seven degrees on takeoff/landing. Any of this would be simpler and structurally lighter than the slats/flaps mechanism, hence no need for them only to increase AoA.

A moveable wing like that is simpler than a flap system? I'd debate you on that point. It may or may not be. I would lean towards the latter.

More precise would be to say that slats/flaps increase the critical value of the effective AoA (the one measured from the zero-lift AoA), i.e. the value where the lift stops increasing with increase in effective AoA.

I would say that that is an aspect of it all, not the more precise statement.

 

[FredGarvin]

Hmm, didn't think about that - how big of an issue is that? Older planes used to have leading-edge flaps instead of slats, so their geometric angle of attack didn't change much, if at all when they were deployed. Is the major benefit just that you don't need tall landing gear and a droopy nose like on the Concorde?

I think those early systems were more of a way to help the flow remain attached on the wing at high AoA. I could be wrong there. There really isn't much of an "issue" per se as an added benefit of just getting that much more angle out of the same wing.

 

[caslav.ilic]

[In air] The whole point is to be able to change the AoA without using pitch control. That way the fuselage can remain relatively in the same position while the wing "thinks" it's at a higher AoA.

The critical angle of attack, after which the wing will stall, for a conventional clean wing is about 15 degrees. At landing approach this is not too much with regards to forward visibility for subsonic-nosed airplane. Also, in this case it is not directly the fuselage AoA which matters, but pitch angle to the runway, which is further 3-5 degrees less than AoA due to descent path angle. Given that stall angle shouldn't actually be approached too near, the total fuselage pitch angle would come at less than 10 degrees -- which is inside normal tail strike clearances, at least by the books :) (except for a stubby-legged, high-wing, tail-pipe aircraft like Crusader).

If it's on the ground, then flaps and slats aren't really going to do much of anything, will they? What's your point?

I meant during takeoff.

A moveable wing like that is simpler than a flap system? I'd debate you on that point. It may or may not be. I would lean towards the latter.

I don't see why should it not be less complicated? All the load carrying elements are there anyway, it only needs pivot instead of rigid connection at rear spar and hydraulic instead of rigid connection at the front spar (or vice-versa for low-wing aircraft). That compared to flaps and slats which need a lot more hydraulic and telescoping elements and control links spread along the wing leading and trailing edges, all dead weight, and also complicating the wing structure and aeroelastic effects. Or, compared to simply increasing the landing gear height to provide more tail strike clearance. At least the Crusader designers considered pivoting the whole wing a better bet than increasing the landing gear height by 70-90 cm, and Crusader does have extremely stubby legs (they didn't nickname it "Hog" for no reason :)

But this is all moot, for the simple fact that a cruise-efficient airplane cannot afford enough wing area for clean wing near critical AoA to produce enough lift at landing. The slats and flaps both work to raise the effective critical AoA, but the flaps reduce the zero-lift AoA, whereas slats don't affect it; in fact, the slats are there precisely to allow higher effective AoA to be used, they do nothing at otherwise admittable AoAs. In sum, with flaps and slats deployed, the geometric AoA may be even greater, and so the fuselage may be at even greater pitch-to-runway attitude compared to that with clean wing. For example, see the http://adg.stanford.edu/aa241/highlift/highliftintro.html (the bottom-most diagram on the page), where it is shown that the geometric critical AoA is increased by 5 degrees with flaps and slats deployed, rather than decreased. This means that if a clean wing could provide enough lift for landing, flaps and slats would immediately be deleted altogether.

--
Chusslove Illich (Часлав Илић)

 

[russ_watters]

I don't exactly understand this "leading-edge flaps instead of slats" -- these are not synonymous in English?

Leading edge flaps are just hinged, well, flaps. There is no slot:

Leading edge flaps perform a similar function to slats (see above.) The difference is that leading edge flaps increase the camber of the wing, and do not create a slot.
As a result LE flaps increase lift at all angles of attack, and do not delay the stall as effectively as slats.

http://selair.selkirk.bc.ca/aerodynamics1/controls/Page5.html

I think the argument is getting buried in the minutae now, though, with what the bigger benefits are.

 

[FredGarvin]

I meant during takeoff.

Ahhhhh...Now I see.

I don't see why should it not be less complicated? All the load carrying elements are there anyway, it only needs pivot instead of rigid connection at rear spar and hydraulic instead of rigid connection at the front spar (or vice-versa for low-wing aircraft). That compared to flaps and slats which need a lot more hydraulic and telescoping elements and control links spread along the wing leading and trailing edges, all dead weight, and also complicating the wing structure and aeroelastic effects. Or, compared to simply increasing the landing gear height to provide more tail strike clearance. At least the Crusader designers considered pivoting the whole wing a better bet than increasing the landing gear height by 70-90 cm, and Crusader does have extremely stubby legs (they didn't nickname it "Hog" for no reason :)

I can see a lot of flight control complications by doing that. The Crusader had flaps anyways, because the wing incidence couldn't solve all of the problems with slow approach speeds but I think they also had other issues to deal with that are natural constraints due to carrier based aircraft. Also, the Crusader's wing was a 2 position setting, either up or down whereas flaps have multiple settings.

I think the argument is getting buried in the minutae now, though, with what the bigger benefits are.

Yeah. I think we've beaten this horse pretty well.

 

[gaming_addict]

Normally the leading edge slat is used to increase lift at low speed takeoff and landing only, but it looks like this Sukhoi Su-27 is flying with the slat extended all the time (while doing the airshow) to increase lift and manouverability at low speed. I think I have not seen that befor (??!)

http://www.flightlevel350.com/aviation_video.php?id=4663&vcm

Just to clear, Su-27 slat automatically deploy once a certain angle of attack(or stall angle) is reached.

When Su-27 which has high wingloading(relatively speaking) must have high angle of attack in low speed maneuvers in order to turn or stay aloft. The craft must be maneuvering well past the AoA needed to deploy the slats. Not only the su-27 is doing thing but also a host of modern jet fighters like F-16.

..Also to add, I've read this thread and some concepts are not absolute, some thick wings can be used for high speed travel, an example is the 'supercritical airfoils' which is in use with modern airliners. Will actually produce less drag at high subsonic that non critical airfoils that could be much thinner. It also produces superior lift at low speeds so most airliners use it, even the B-2 bomber