Surface area and lift questions

[zultone]

:D I've been thinking of ways to increase lift on a wing without adding length or width.

And it's my understanding that the more surface area you have, the more lift can be generated.

What got me thinking of this was how a lot of our organs are almost corrugated, like our lungs, lots of tiny folds to get as much surface area as possible in the smallest area.

So, would this work -

Corrugated wings, lots of medium sized folds on the outside of the wing running paralell to the body so that the air can travel over them smoothly.

And on the inside of the wing, something like a hollow honeycomb scructure, airfoil shaped, so the air can travel through the wing, and create lift.

But at the same time, this seems like it would create extreme amounts of drag also.

I made a diagram.

I wouldn't mind making a wing similar size to my Slo V wing and trying it out.

[Montag DP]

Unfortunately, your understanding is wrong so your solution is wrong as well...

Having an airfoil section with lots of surface area means that you have a thick and/or highly cambered airfoil, which generally produces greater lift than a thin and slightly cambered one, but that's not because of the greater surface area. Adding surface area in the way you're thinking will only increase drag and decrease lift.

The organs in our body (especially the lungs) have high surface area for the purpose of increasing the rate of diffusion of things to and from the blood, and they do a good job of that. Lungs wouldn't work too well to produce lift though, unless maybe you filled them with helium. :D

[wtl0715]

The reason that extra surface area generally creates more lift is because air is forced to move faster across the top of the wing than the bottom. Fast-moving air creates low pressure - the relatively high pressure under the wing pushes the wing upward in an effort to even the pressure above and below the wing. If you have folds, bumps, or other imprefections in the wing, it will likely slow the flow of air over the top of the wing. Ideally, a wing will be perfectly smooth so the air can move very fast over it while creating minimal drag (I imagine there may be a case in which this is not true, but as a general rule, smooth is good).

-TL

P.S. If I explained something you already knew or didn't answer your question, I apologize. I don't mean to preach to the choir.

[zultone]

Unfortunately, your understanding is wrong so your solution is wrong as well...

Having an airfoil section with lots of surface area means that you have a thick and/or highly cambered airfoil, which generally produces greater lift than a thin and slightly cambered one, but that's not because of the greater surface area. Adding surface area in the way you're thinking will only increase drag and decrease lift.

The organs in our body (especially the lungs) have high surface area for the purpose of increasing the rate of diffusion of things to and from the blood, and they do a good job of that. Lungs wouldn't work too well to produce lift though, unless maybe you filled them with helium. :D
I tried the helium thing!

I didnt float, but sure did talk funny.

So it'd basically be a super draggy wing?

[Texas Buzzard]

:cool: :cool: :) If you built a wing with the same planform area BUT made the surfaces convoluted like a wash board you'd be disappointed.

What would happen is First the Drag would go way up, 2ed the lift cooeficcient would GO DOWN.
In short it is a bad idea. :)

[nmasters]

And it's my understanding that the more surface area you have, the more lift can be generated.


Corrugated wings, lots of medium sized folds on the outside of the wing running paralell to the body so that the air can travel over them smoothly.



And on the inside of the wing, something like a hollow honeycomb scructure, airfoil shaped, so the air can travel through the wing, and create lift.


Apparently that's exactly what Alexander Graham Bell (http://www.cit.gu.edu.au/~anthony/kites/tetra/bell/) thought. Note that he's generally not recognized as one of the great pioneers of aviation :( As it turns out this is not the thing to do because, as has been pointed out, the increased surface area increases drag but also the lift force is a result of pressure and pressure always acts perpendicular to the surface. Since much of a corrugated surface is not horizontal much of the lift that that area could be producing is not going to support the weight of the airplane. This increases the induced drag which is in addition to the friction drag from the excessive surface area. The result was that Bell's tetrahedral airplane took way too much power to be practical but the concept works well for kit[e]s where the power is free.

--Norm

[zultone]

That's really interesting, thanks for the link!

[Derfy]

Corrugated wings, lots of medium sized folds on the outside of the wing running paralell to the body so that the air can travel over them smoothly.


Running the corrugations spanwise seems to work for the dragonfly. :D

http://www.public.iastate.edu/~huhui/paper/2007/AIAA-2007-0483.pdf

:cool:

[JetPlaneFlyer]

Zultone,
There is a modern day application of something similar to what I think you are taking about.. That’s 'grid' fins on rockets: http://en.wikipedia.org/wiki/Grid_fin
For rockets they have specific benefits such at packaging maximum area into a small volume, and making actuation and folding flat of the fin easier.

For wings on a conventional aircraft they could be made to work but in this application the drag would be much greater than a normal wing of similar 'lifting capability'. They would be very inefficient but could make an interesting head turning model if efficiency was not your main aim..

Steve

[eflightray]

Zultone

While your in Wikipedia, (above post), have a look at the Explanation of Airfoils (http://en.wikipedia.org/wiki/Airfoils)

For the same wing span and chord, the wing section, airfoil, makes abig difference to the lift and drag of a wing. Choosing the right, (most efficient), airfoil for the flight characteristic required is quite important.

[HerkS]

Lift is proportional to speed squared. A little more speed - lots more lift. (all else being equal - which they are not of course :-)

But then - that's not what you asked. :rolleyes:

[Ralph Walton]

[QUOTE=zultone]:D I've been thinking of ways to increase lift on a wing without adding length or width.

QUOTE]

Give the wing more camber by lowering the trailing edge a couple of mm. This is regularly done in high end rc gliders, but can work on lower spec planes too. It works better on some foils than others, It depends on the original design.

Don't forget that more lift equals more drag. There's no free lunch

A higher camber airfoil could be used as well.

It depends what you're trying to achieve as an end result. More info would help us give you a better answer.

[VeeOneRotate]

This is an interesting question. Firstly, forget the honeycomb, if the air is passing through the wing the airfoil won't work.
Secondly, these corrugated wings would not produce more lift because the shape of the airfoil has not really changed in the important aspects. BUT, these corrugations may help reduce the spanwise flow of the air caused by the generation of lift, reducing wingtip vortices and thus reducing drag. Much like wing fences and winglets do. Less drag means more excess thrust, which means greater speeds and better climb performance.

But then thats all just conjecture.

I say give it a try and see what happens!

[Brandano]

or see if the Ford Trimotor or the Junkers JU 52 had better performance than smooth skinned planes with the same powerplants

[eflightray]

Lift is proportional to speed squared. A little more speed - lots more lift. (all else being equal - which they are not of course :-)

But then - that's not what you asked. :rolleyes:

Does that still hold true for a fully symmetrical section ?

[VeeOneRotate]

Does that still hold true for a fully symmetrical section ?

I would guess yes. The lift formula goes:

Lift=1/2 air density X speed squared X coefficient of lift X lifting surface area

A symetrical airfoil produces lift only at positive angles of attack. so if you kept the coefficient of lift constant (ie a constant angle of attack) and increased the speed, then the lift will increase proportionally with the square of the speed.

I've been thinking about idea of corrugated upper wing surfaces again, yeh, I see where you're coming from Zultone. The increase of surface area might just cause an lower air pressure and increase lift, but the advantage would be negligible, and it would be easier to just build a deeper section airfoil. So the point is moot. :p

[BMatthews]

The honeycomb style wing doesn't work for the same reason that biplanes and other multi wing planes need to maintain a decent gap between the wings. When the gap gets down to only one chord in size the drag due to interference becomes fairly high and if you try to reduce it more the drag rises faster than the gap closes. This is why biplanes have an interwing gap that is typically 1 to 1.5 times the chord of the biggest wing.

Eflightray, yes that rule applies to symetrical wings as well. A symetrical wing supplies lift just as well as any other shape. It's only downside is that, all else being equal, it will tend to stall earlier than an airfoil with camber.

[JetPlaneFlyer]

I've been thinking about idea of corrugated upper wing surfaces again, yeh, I see where you're coming from Zultone. The increase of surface area might just cause an lower air pressure and increase lift, but the advantage would be negligible, and it would be easier to just build a deeper section airfoil. So the point is moot. :p

VeeOne,
Corrugated surfaces would not increase lift at all. The only wing 'area' that produces useful lift is that which is apparent when you view the wing from the top i.e. the projected wing span x average chord. Any lumps, bumps or curves or whatever in the wing surface does not increase the wing lifting area. Even a 'deeper section airfoil' does not increase lifting area.

I recommend you read up on how lift really works. This is a good page: http://regenpress.com/
This page also de-bunks the false 'hump theory' which may be leading to your confusion.

Steve

[VeeOneRotate]

VeeOne,
Corrugated surfaces would not increase lift at all. The only wing 'area' that produces useful lift is that which is apparent when you view the wing from the top i.e. the projected wing span x average chord. Any lumps, bumps or curves or whatever in the wing surface does not increase the wing lifting area. Even a 'deeper section airfoil' does not increase lifting area.

Ok, I have my doubts about this idea as well, but this is my line of thought: Granted the lifting surface of the wing has not changed, but the overall surface will have increased. The air has to cover a greater distance, and so it must move faster, thus reducing the pressure and creating more lift. Though as i said before, the increase will most likely be so infinitesimally small that it's not worth the effort.

A deeper section airfoil DOES increase lift. A high-lift slow-speed wing will have a much deeper airfoil than a high-speed wing (lets assume sub sonic to keep it simple). Again, the air moves further, creating a lower pressure, creating more lift.

[JaRaMW]

The air has to cover a greater distance, and so it must move faster, thus reducing the pressure and creating more lift.

There is no physical reason for any air particle to move faster only because it travels along a longer path than its formerly adjacent particle. The air travelling along the top of a lifting airfoil even arrives at the trailing edge before the air below.
Furthermore, not a mysterious variation of an air parcel's speed causes a pressure drop. It is the pressure drop that causes the increase in speed. It is just convenient to back-calculate a pressure distribution from a known velocity field, but this is not the physical cause.
Air molecules follow Newton's laws just like any other mass: a force causes acceleration (and not vice versa). Only that we are obviously not willing to calculate the forces on every single molecule here but rather concentrate them in sufficiently small air parcels and calculate the pressures on their surfaces (pressure * surface = force).
Saying the pressure difference results from a change in speed is like saying a falling object creates the gravitational force by altering its speed.

[JetPlaneFlyer]

Ok, I have my doubts about this idea as well, but this is my line of thought: Granted the lifting surface of the wing has not changed, but the overall surface will have increased. The air has to cover a greater distance, and so it must move faster, thus reducing the pressure and creating more lift. Though as i said before, the increase will most likely be so infinitesimally small that it's not worth the effort.

A deeper section airfoil DOES increase lift. A high-lift slow-speed wing will have a much deeper airfoil than a high-speed wing (lets assume sub sonic to keep it simple). Again, the air moves further, creating a lower pressure, creating more lift.

Study the page I linked to in my last post and JaRaMW's reply.. Lift has nothing to do with any greater distance air has to travel over the wing compared to under the wing. Also increasing distance DOES NOT make the air travel faster. You have been missguided by the incorrect 'Hump' (sometimes known as 'equal time of transit') lift explanation.

If you dont believe me then maybe you will believe NASA: http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html Or ask yourself how on earth can a plane fly upside down!

Regarding the 'deeper' airfoil.. it all depends what you mean by 'deeper'. Conventional terminology for airfoils uses the terms Thickness and Camber.. 'Deepness' is not conventional terminology. I'm assuming by 'deeper' you really mean 'thicker'?
A thicker airfoil DOES NOT by itself usually result in more lift. In fact for slow flying models a thick airfoil will usually produce less lift than a thin one, everything else being equal. The biggest variable in deciding how much lift an airfoil will produce (other than angle of attack and airspeed) is camber.. The mean camber line is a line drawn midway between the upper and lower surfaces of the airfoil and the more camber it has the higher the maximum lift potential of the airfoil, at least up to a point. Camber is unrelated to thickness. it's perfectly possible and very common to have a very thin yet highly cambered airfoil.

Look at wings on slow flying WWI or pre WWI aircraft and you will see that they have very thin but very cambared airfoils. Also look at indoor freeflight duration models or 'Slow Stick' type park flyer RC models.. These have extremely thin but highly cambered wings. On these wings the top and bottom surfaces are the same length yet they produce more lift than just about any other airfoil and they fly very slowly indeed.

Steve

[VeeOneRotate]

I humbly apologise. It appears I was taught the incorrect theory. Thanks for the link, I've spent the last hour going over it, I understand what you mean now.

It's genuinley scary that after investing 3 years and £100K on flight training, I didn't know how an airfoil really worked. You would have thought SOMEBODY in a professional training school would have explained the correct theory!

[Accu157]

The reason that extra surface area generally creates more lift is because air is forced to move faster across the top of the wing than the bottom. Fast-moving air creates low pressure - the relatively high pressure under the wing pushes the wing upward in an effort to even the pressure above and below the wing. If you have folds, bumps, or other imprefections in the wing, it will likely slow the flow of air over the top of the wing. Ideally, a wing will be perfectly smooth so the air can move very fast over it while creating minimal drag (I imagine there may be a case in which this is not true, but as a general rule, smooth is good).

-TL

P.S. If I explained something you already knew or didn't answer your question, I apologize. I don't mean to preach to the choir.

No no no. :) That's not true either. Glad to see others beat me to it. FWIW, there is hardly any pressure change going on in our models. However there is redirection of air. The reduced pressure airfoil thing is a common myth, it will take a while to wither away. :D

[Montag DP]

No no no. :) That's not true either. Glad to see others beat me to it. FWIW, there is hardly any pressure change going on in our models. However there is redirection of air. The reduced pressure airfoil thing is a common myth, it will take a while to wither away. :DWhat do you mean by that? The pressure force on the wing from the air flowing around it is the same as the force caused by the downward flux of air being redirected - you can't have one without the other. It's conservation of momentum.

[JetPlaneFlyer]

I humbly apologise. ..

Not necessary at all. The 'hump' theory is a very persistent and popular myth that i think is accepted because it is so simple to understand plus it mixes in a bit of genuine science in the shape of Bernoulli.. You can still find it in many popular publications and I'd not be surprised if your flying instructors believe in it too.
There are actually people out there whose belief in 'the hump' is so strong that they deny that planes can actually fly upside down, even though they have seen it with their own eyes... Never ceases to amaze me how the human mind can delude itself.

I take it you learned to fly as Inverness airport?

Steve

PS.. Accu157; Lift is directly a result of a difference in (static) pressure between the top and bottom surfaces of the wing. Other than drag then pressure is the only force that air can actually apply to the wing surface, and lift has to be the result of some force applied somewhere... It's the cause of the difference in pressure that is the point of debate.....

[BMatthews]

No no no. :) That's not true either. Glad to see others beat me to it. FWIW, there is hardly any pressure change going on in our models. However there is redirection of air. The reduced pressure airfoil thing is a common myth, it will take a while to wither away. :D


Actually the pressure and redirection factors exist at the same time because they are different aspects of the same process.

Old time testing by NACA using multiple manometers to measure the pressure differential between the top and bottom of the wing can show that the differences over the whole wing add up to the force needed to support the plane.

However if you look at the vertical velocity component of the air before and after the wing passes the mass of air and the downward velocity imparted ALSO adds up to the weight of the plane.

You can't have the one without the other. It comes down to the old chicken or egg thing. Does the downward accelleration of the air result in creating the pressure difference or does the pressure difference result in the air being accelerated downwards? Does it really matter? No, since they are both aspects of the same action and you cannot have one without the other.


One old aerodynamicist's tale that IS wrong is the one about equal time flow over the top and bottom. A few months back there was a video, YouTube perhaps?, linked from one of the forums that showed interrupted smoke streams passing over an airfoil. It was VERY clear that the equal time transit claim is a myth with absolutely no basis in reality. The lower surface smoke was reaching and shedding off the trailing edge while the upper surface streamline "dashes" were only about 2/3 to 3/4 along their own journey.

[VeeOneRotate]

JetPlaneFlyer - thank you for pointing me in the right direction, it's opened up a whole other world of understanding to me!
Yep, I learnt to fly at Inverness, then did my CPL IR with Oxford Aviation Training in Oxford and Phoenix AZ. Now I'm just completing my instructor rating in Warsaw before returning home to bonnie Scotland to teach out of Inverness. Mon the teuchters!
Jamie

[JaRaMW]

I already wasted a lot of my time reading tons of crap about how wings supposedly create lift - and I believed a lot of it. Needless to say it confused me a lot. Repeating the same old fairy tale over and over again doesn't make it true. But unfortunately, this happens a lot in textbooks and especially in teh internets and sometimes it is sold in a way that sounds so seductively good...

What I learned is this: never believe anything that you don't understand at a first glance, no matter how many scientifc titles or flight hours the author has. We are talking physics, not religion. Always ask why and never accept anything but a 100% clear and verifiable explanation. The whole thing is a bit more complex than many people want it to be, thus all the fallacies. Take your time and make every single step clear. The basic physics behind it are very simple and intuitive. But it takes a few bricks to build a house :D

[fnev]

The honeycomb style wing doesn't work for the same reason that biplanes and other multi wing planes need to maintain a decent gap between the wings. When the gap gets down to only one chord in size the drag due to interference becomes fairly high and if you try to reduce it more the drag rises faster than the gap closes. This is why biplanes have an interwing gap that is typically 1 to 1.5 times the chord of the biggest wing.

Eflightray, yes that rule applies to symetrical wings as well. A symetrical wing supplies lift just as well as any other shape. It's only downside is that, all else being equal, it will tend to stall earlier than an airfoil with camber.


Your statement regarding biplanes is not entirely true… What about the theory behind rhomboidal (diamond, closed, boxed, etc) wings? Do you know that ALL the major big names in the aerospace industry work on design around this concept?

Unfortunately it is still very confidential (as the potential benefits are huge) and even a heavy Web search will give you minimal information…

[JaRaMW]

Your statement regarding biplanes is not entirely true… What about the theory behind rhomboidal (diamond, closed, boxed, etc) wings? Do you know that ALL the major big names in the aerospace industry work on design around this concept?

Unfortunately it is still very confidential (as the potential benefits are huge) and even a heavy Web search will give you minimal information…

The statement is true, he is talking about the gap depth which would be unfavorably small in a honeycomb style wing, creating a lot of drag.
Box wing concepts offer several structural and stability advantages, depending on the details of the concept. They can reduce induced drag and exist without elevator, which again is an advantage since an elevator behind the wing has several aerodynamic disadvantages.

[vintage1]

Your statement regarding biplanes is not entirely true… What about the theory behind rhomboidal (diamond, closed, boxed, etc) wings? Do you know that ALL the major big names in the aerospace industry work on design around this concept?

Unfortunately it is still very confidential (as the potential benefits are huge) and even a heavy Web search will give you minimal information…

So let me get this straight. All aerospace companies are looking at something that you know about, but its nowhere published because its so secret.

Hmm.

And no one has stumbled on it years ago, when nature was making wings for birds and the bees, and designers tried just about every wing planform there is, before deciding the seagulls knew best when it came to aerodynamic efficiency?

The reality is there IS a secret program, and its beaming this stuff straight into your brain from zorg central. You need a tinfoil hat methinks..
:eek:

[fnev]

Well, in a rhomboid wing the 1 to 1.5 chord distance between the two wings doesn’t really exist…

I will agree that in the case of a biplane with “open” ends you need to keep a minimum space between the two wings for the reasons you mentioned. What I wanted to say was the fact that by changing the geometry between the wings this “rule” doesn’t apply necessarily anymore.

Here is an interesting link with other links embedded:

http://www.secretprojects.co.uk/forum/index.php?topic=4963.msg59894

Enjoy… but, as usual, take some of the statements with a pinch of salt… Nevertheless some very serious work by the top names in the aerospace industry.

[fnev]

So let me get this straight. All aerospace companies are looking at something that you know about, but its nowhere published because its so secret.

Hmm.

And no one has stumbled on it years ago, when nature was making wings for birds and the bees, and designers tried just about every wing planform there is, before deciding the seagulls knew best when it came to aerodynamic efficiency?

The reality is there IS a secret program, and its beaming this stuff straight into your brain from zorg central. You need a tinfoil hat methinks..
:eek:


I don’t think that your comments are very appropriate… It is border line to be not only rude but could be taken as an insult.

[JaRaMW]

I will agree that in the case of a biplane with “open” ends you need to keep a minimum space between the two wings for the reasons you mentioned. What I wanted to say was the fact that by changing the geometry between the wings this “rule” doesn’t apply necessarily anymore.

You got a point there, but the negative interference effects for a biplane (or multiplane) are not only due to induced vortices, but also due to the interference of the flow fields of both wings.
Box wing, or any other connected wing designs, profit from a reduction of induced drag due to the closed geometry at the wing tip. I don't see what advantages, in case of a honeycomb-wing, additional connections along the wingspan would bring. A reduction in spanwise flow maybe, but there are more efficient ways to do that. The additional parasite drag of all those additional connections would sure outweigh this advantage.

[mnowell129]

Regarding Biplanes and honeycomb wings, etc. A good mental experiment is to consider two thing wings separated by one air molecule. It should be fairly obvious that this setup will behave like a single wing. If you then vertically separate the two wings they will start to behave more and more like two wings and make twice the lift. At close distances they interfere with each other since the flow fields are overlapping. Certainly an infinite distance apart you get two wings worth of lift. There is some spacing of the wings where the additional drag is tolerable to gain the structural efficiency (World War 1 Airplanes) but with modern structural materials the inefficiency of the biplane is not a viable option (anyone seen a biplane jetliner proposal lately?) The honeycomb is many wings very close together with all the resultant inefficiencies of two closely spaced wings.
As to corrugations, even if you ignore the causes of lift and were to assume that for whatever reason you had an increased area over which to apply the low pressure, this wouldn't increase the lift. The pressure operates perpendicular to the surface and the low pressure on the sides of the corrugations opposes the low pressure on the other side of the corrugation, canceling out. Thus only the projected horizontal area "counts" in lift production on a corrugated wing.

[fnev]

Absolutely, the big killer in the biplane design is (are) all the wires and struts with their drag. It is interesting to note that the early airplane designs (biplanes, tri planes) were not “that stupid” the problem was the available technology and materials at the time.

Someone heavily involved with closed wing designs went as far as saying that this is because of these very limiting (necessary) devices (struts, wires) that the cantilever wing became so successful. But modern designers/engineers are not so sure anymore that this is the best choice for performances and strength. The future might well be with multi surfaces (wings) aircrafts…!!!

[biber]

Box wing, or any other connected wing designs, profit from a reduction of induced drag due to the closed geometry at the wing tip.Can't imagine where that would come from.
Closed geometry wings have often been proposed for such induced drag reasons, but I never have seen a valid example or explanation.

In terms of induced drag it's all about wingspan and a nice lift distribution.
A boxed wing would require one wing to downlift in order to minimise induced drag.
You could play that game down to zero induced drag at zero net lift.

biber

[vintage1]

Absolutely, the big killer in the biplane design is (are) all the wires and struts with their drag. It is interesting to note that the early airplane designs (biplanes, tri planes) were not “that stupid” the problem was the available technology and materials at the time.

Someone heavily involved with closed wing designs went as far as saying that this is because of these very limiting (necessary) devices (struts, wires) that the cantilever wing became so successful. But modern designers/engineers are not so sure anymore that this is the best choice for performances and strength. The future might well be with multi surfaces (wings) aircrafts…!!!

The struts and so on were there to brace what were originally 'curved plate' wings. Because power was so low that worked..the wings were permenent flaps down..and they could fly high too. Camel tops out at 17000 ft, and did.

Once power started to go up, the drag from the wings started to get dominant, so the monoplane was born: for ultra low drag a long thin wing is best, but it has structural issues, so its as thick as it has to be for strength.

But its all a compromise. Speed planes benefit more from small stubby wings, but then they can't turn corners..

In teh absence of any other constraints the best efficiency is always a long thin elliptical wing, possibly with end plates.

Two long thin wings connected by endplates may just have something to offer: But I remain to be convinced.

[fnev]

Well, I did give some Web links: it is up to you to spend the time and even try to do a better search. You might be very surprised if not convinced…

BTW the need for the cantilever wing was to fight drag and (at the time) for a perceived mass gain. Today things are changing. It won’t happen overnight and it is still a LOT of work to be done but it is very promising. So promising that, as I said already, it is very difficult to get access to any sensible (sensitive!) data.

[JaRaMW]

Can't imagine where that would come from.
[...]
A boxed wing would require one wing to downlift in order to minimise induced drag.

Reduce induced drag, not cancel it. The box geometry would of course not prevent the pressure difference from forming vortices anywhere - they would still form along the trailing edge due to spanwise flow and behind the wing - but the box configuration would create a smoother vortex distribution resulting in less induced drag.

[Ralph Walton]

And now for something completely different..........

http://www.custerchannelwing.com/


Its related to this discussion, and definitely in the more lift same wingspan basket.

[fnev]

And now for something completely different..........

http://www.custerchannelwing.com/


Its related to this discussion, and definitely in the more lift same wingspan basket.


A very interesting subject but very complex and not easy to implement.

[graupman]

It seems to me that there are many factors not being considered. Multi-wing designs may be capable of producing more lift in a smaller package, and they certainly have structural advantages, but I am not sure there is any argument for any type of multi-wing being more efficient.

Even the rhombus shape planform (which may not suffer from the same interference between airfoils as a typical biplane) will still operate at a reduced efficiency due to one wing operating within the influence of the other. A subsonic lifting surface influences the air well in front and well behind of the leading and trailing edges. If you look at high performance gliders, you'll notice the fuselage droops slightly in front of the wing to follow the stream lines created by the wing acting on the air in front of it's leading edge.

The bottom line is no matter what you do, the lift to drag ratio or the amount of lift per unit of surface area will be reduced, resulting in a heavier structure. The only way to make this kind of thing work is to get the structural advantages or some other consideration to outweigh the reduction in efficiency. I am sure there are certain cases where that might be true, but for most applications the boring typical airplane shape really is the best.

[Martin C Wendel]

.................................
A few months back there was a video, YouTube perhaps?, linked from one of the forums that showed interrupted smoke streams passing over an airfoil. It was VERY clear that the equal time transit claim is a myth with absolutely no basis in reality. The lower surface smoke was reaching and shedding off the trailing edge while the upper surface streamline "dashes" were only about 2/3 to 3/4 along their own journey.

Sorry to go back a ways but is this the video clip?

http://www.youtube.com/watch?v=6UlsArvbTeo

Was there perhaps a typo in your post? Looking at the "stop action" of the pulsed smoke it looks like to me like the upper surface "dashes" have left the TE before the lower surface are even close to shedding. What I'm seeing surprised me and seems opposite to what you were saying. Maybe I'm misinterpreting what I'm seeing and reading.

Martin C Wendel

[Montag DP]

Sorry to go back a ways but is this the video clip?

http://www.youtube.com/watch?v=6UlsArvbTeo

Was there perhaps a typo in your post? Looking at the "stop action" of the pulsed smoke it looks like to me like the upper surface "dashes" have left the TE before the lower surface are even close to shedding. What I'm seeing surprised me and seems opposite to what you were saying. Maybe I'm misinterpreting what I'm seeing and reading.

Martin C WendelYes, the upper surface streamlines definitely reach the TE first. Mr. Matthews must have misspoken.

[JetPlaneFlyer]

Yes, you would expect the air to reach the TE first when the airfoil is producing lift. This is not directly to do with the wing shape (i.e. the longer path over the top rubbish).... Its simply a result of the fact that on on a wing that makes lift the air pressure on the top is less than the bottom and in a freestream the lower pressure air must have a higher velocity in order for overall energy to be conserved.

[BMatthews]

My bad, I remembered one side being faster than the other and got it wrong when I said the upper surface was slower. And yes that is the video I was reffering to.

The main point was that the layman's theory we've all read about the upper and lower surface air meeting back up at the same point is wrong.

[Martin C Wendel]

BMatthews thanks. Wasn't trying to be a smart :censored: but found the video clip and threw me into reverse. The "stop action" is really neat but not sure I've rationalized it in my own mind. You guys are so far ahead of me it's pitiful.

Martin

[BMatthews]

Don't sweat it Martin. Like I said I totally mis-remembered which side was faster.

And while I knew some things exchanging threads like this back and forth over the past few years really helped with the more detailed understanding a lot. There's a lot of folks on here that were in the industry for years as designers and wind tunnel specialists and know their stuff not to mention the likes of Drs Selig and Drela that drop in now and then.