Is a flying wing generaly more drag efficeint?

[Mat]

Is a swept wing more drag effecient?

[steve wenban]

sweep on a flying wing is = to dihedral not to drag

[raptor22]

The sweep DOES produce more drag. That's why there are planks.

However, the sweep also allows (with the combination of washout) the use of airfoils with a negative lift coeffecient (which are generally more effecient). This advantage is generally outweighed by the drag rise from the sweep in most planes (aka planks are more effecient).

--Alex

[raptor22]

Flying wings are usually more effecient on a long stretch (like drag racing or the B2 on intercontinental missions) due to their lack of stab and fuse, but are less effecient when alot of maneuvering is involved due to their lower stability and the lesser CLmax given to reflexed airfoils.

[vintage1]

..yeah, thats why teh U2 was a flying wing right?

Flying wings are always less efficient drag wise than long thin wings. Below mach 1 anyway. Above I am not so sure.

They have other advantges though. Small radar profile when seen head on. Useful charcteristics at supersonic speed.

If you look at full size aircraft, not one subsonic aircraft that I can think of is a flying wing, and you have to go back to the Avro Vulcan to actually remember one that was.

The stealth stuff is, but I suspect a lot of things are compromised to achieve a low radar profile.

[Petri Nygren]

It cannot be generalized like that. What can be generalized though is that the efficiency potential over traditional layout is there, but it is many times harder to extract.

Just a few examples:

- All birds are of flying wing configuration. No rudders there except for tight manuevering, in cruise the birds tail is often closed up completely or very close to that. Nature evolves towards optimum so flying wing IS the optimum dragwise. We humans cannot come up with such extremely complicated flight control system as there is in bird´s head, so we usually need to build tails to our flying machines or suffer lesser performance without. However there are good contrary examples of the last statement.

- There is more to span loading than the elliptical only. If you download the tips as in bell curve, you reduce the loads in wingroot and can thus reduce the structure and weight. Less weight = less drag. Horten idea, probably bird idea too.

- There are several projects aiming to flying wing airliners for very good reasons.

[banktoturn]

Is a swept wing more drag effecient?

These are two separate questions. A swept wing is generally less efficient than an unswept wing, for subsonic flight. Whether a flying wing is more efficient depends on the details of implementation, but I'm going to say that it has inherent advantages which should allow it to be more efficient. One of them is that a flying wing is, or should be, 'span loaded'. By this I mean that the weight carried in the airframe is distributed along the span, rather than being concentrated at the center. This allows greater efficiency by allowing the wing to be build lighter. Obviously, a flying wing is not necessarily span loaded, but since there is no fuselage at the center, it is natural to take most of the stuff that would have been in the fuselage and mount it along the span of the wing. It's probably best to leave the fuel tank (and engine) in the center, but everything else can be moved outboard, as long as the plane stays balanced from left to right. Another fundamental advantage is that all the lift is being generated by the wing. When a traditional plane is at an angle of attack, all the surfaces, including the fusalage, will generate some lift. A wing with a nice long span generates less lift-induced drag than any other part of the plane, so it is best to eliminate all the other surfaces which could generate inefficient lift.

There are trade-offs, of course. It is trickier to build a stable flying wing, and it will probably require sweep and a reflexed airfoil. Neither of these has to be a show stopper, but one could consider getting most of the benefit of a flying wing by having a bare minimum fuselage and tail, containing no more weight than absolutely necessary. A small tube with only lightweight control rods inside, leading back to a small tail. The horizontal stabilizer can be kept small by placing the CG so that the plane is nearly neutral in stability, so that the stab doesn't need to generate large loads. The vertical stabilizer might be a little trickier, as big, long wings do tend to need a fairly big fin. Overall, I don't know how the skinny-boom plane would compare to a swept flying wing for drag.

banktoturn

[macr0t0r]

As a person who has spent way too much time studying flying wing dynamics at Cal Poly:

First off, flying wings ARE more efficient under specific conditions. There is no free lunch, though. Basically, at sub-sonic cruise, the lack of a fuselage and stabilizer greatly reduce drag. Especially the stabilizer, as having another leading-edge to the wind adds quite a bit of drag. There is also good weight savings, which translates to increased efficiency (less induced drag).

A wing creates lift by moving the air down (don't give me that Bernoulli crud, find yourself a modern book). Since most of the "down-push" happens behind the CG, this causes negative pitch. The stabilizer counter-acts this with negative lift. The longer the tail moment, the less negative lift required, therefore the less overall lift lost.

However, the lack of a good tail moment on a flying wing means that the pitching must be done by pushing down on the trailing edge. Basically, your overall lift gets trashed when you need to pull-up. Flying wings actually lose quite a bit of speed in high-alpha maneuvers. The Zagi gets away with this by being so incredibly light-weight. You never need much of an angle of attack to get it to come around.

Conclusion: More efficient at cruise (where there is a very low angle of attack). Less efficient in every other situation. Models of today have become so lightweight, that the weight-savings of tail-less designs is minimal. Notice that thermalizers are not flying wings. All efficiency goes to pot as soon as they have to rely heavily on wing-lift during sparse-thermal conditions.


As for sweep: sweep in conventional planes enable a subsonic plane get closer to mach speed before the wing's airflow becomes transonic. Since lift is a matter of perpendicular wingspan, increasing sweep while maintaining wingspan will provide higher subsonic speed at a cost of increased wing structure and almost no additional lift.

Sweep on a flying wing has a bonus advantage of providing stability. It increases the moment-arm for pitching. So, a little sweep is good for flight characteristics. With enough sweep, you can add your vertical fins to the wingtips instead of building a small fuselage to hold a tail behind the trailing edge. Once again, I think the Zagi has a pretty good "sweet-spot" for sweep.

As for real-world applications of the flying wing, full-size aircraft don't have the luxury of a ridiculously light wing-loading like us modelers do. The lack of a good tail-moment means you can't really "flare" for a landing. The B2 requires a ridiculous amount of runway to land. This makes a flying wing useless as a transport (where they often must use small runways...look at how the C-17 is built).

Also, there is a very low-frequency altitude oscillation in flying wings. It can't be easily taken out using control feedback. The B2's targeting computer actually compensates for this oscillation. Supposedly, some pilots get pretty sick from the motion. I don't think it would make an enjoyable airliner for this reason.

Last but not least, flying wings do not handle a change in CG very well. The B2 is designed to spread its load around the CG. We electric flyers don't have to worry either. However, most aircraft need to handle a CG differential based on number of passengers, loading of cargo, and depletion of fuel. It would make a precarious personal aircraft.

Conclusion: VERY few applications for flying wings in the real world. I like flying wings for their simplicity as an RC model, but I often prefer the better maneuverability of a conventional airplane when I want to really tear up the skies.

There ya go!

[raptor22]

..yeah, thats why teh U2 was a flying wing right?
.

Northrop built many flying wings full scale, and almost put a bomber wing into production as well as a wierd anti-bomber fighter that was intended to hit them to cut them in half while still surviving the impact.

They had great long range performance, but were not used because of the stability issues I mentioned.

The stability is the reason flying wings are not too popular.

Plus, the SR-71 was tailless.

--Alex

[raptor22]

Conclusion: VERY few applications for flying wings in the real world. I like flying wings for their simplicity as an RC model, but I often prefer the better maneuverability of a conventional airplane when I want to really tear up the skies.

There ya go!

have you ever flown a plank? Many have ALOT of maneuverability.

--Alex

[macr0t0r]

@raptor: True, the barn-door has enough moment for pretty good maneuvering due to the massive chord-width and light wing-loading. I still like something with a stall-able tail for proper spins.

[kallend]

A wing creates lift by moving the air down (don't give me that Bernoulli crud, find yourself a modern book).

Funny you should write that. I spent this very afternoon watching a group of undergraduates doing wind tunnel tests on an instrumented airfoil, to measure the pressure distribution over top and bottom surfaces.

The pressure distributions measured are exactly as Bernoulli's equation predicts. The pressure distributions exactly match the lift force. The lift force is predominantly generated by the top surface of the airfoil.

I don't recall any of Bernoulli's writings in which he stated that momentum was not conserved, or anything suggesting that air was not deflected downwards, so what exactly is the "crud" to which you refer?

[Sparky Paul]

@raptor: True, the barn-door has enough moment for pretty good maneuvering due to the massive chord-width and light wing-loading. I still like something with a stall-able tail for proper spins.
.
My Evans Simitar spins quite well, after I changed to elevators and ailerons from elevons.

[macr0t0r]

@kallend: I've done similar tests at the Cal Poly WInd Tunnel. I should clarify that Bernoulli's Venturi analysis is not exactly wrong, since the numbers do add up. However, it is misleading where the lift comes from. If his theory was 100% correct, planks would not fly. The problem is that pressure differential is an effect, not the source of lift. Pushing air down against itself does raise pressure, but the bottom line is strictly a matter of equal and opposite forces: air goes down, plane goes up. Here's a very good presentation from NASA about it:
http://www.grc.nasa.gov/WWW/K-12/airplane/wrong3.html

And for those willing to brave the equations supporting this (the conclusion is well written if you want to skip to the end for curiosity):
http://www.uni-frankfurt.de/~weltner/Mis6/mis6.html

[vintage1]

It cannot be generalized like that. What can be generalized though is that the efficiency potential over traditional layout is there, but it is many times harder to extract.

Just a few examples:

- All birds are of flying wing configuration.



(i) They are not. The vast majority have and use tails.

(ii) they fly by flapping. not wth engines.

(iii) If the soaring types, most analagous to aircraft, they all have and use tails.

(iv) birds are not necessarily optimised for efficient flight. Cf Chickens and Ostriches :D


No rudders there except for tight manuevering, in cruise the birds tail is often closed up completely or very close to that. Nature evolves towards optimum so flying wing IS the optimum dragwise.


Bad data, bad assumptions.

Birds do have at least a horizontal tail, which they twist for lateral manuvers.

Nature evolves towards survival. Flying is not the only thing birds have to worry about. Woodland birds for eaxmple, merely use flying to escape predators and generally only fly short distances. They don't feed in the air.

Raptors generally soar as do gulls - and they are perjaps teh most effcient flyers. But even they also need to be optimised for diving speed as well.

Ducks geese and other migratory birds, are efficient flappers, and don't have much tail I agree. But flappinmg flight is not teh same as gliding or pwered glding.

[/quote]

We humans cannot come up with such extremely complicated flight control system as there is in bird´s head, so we usually need to build tails to our flying machines or suffer lesser performance without. However there are good contrary examples of the last statement.

- There is more to span loading than the elliptical only. If you download the tips as in bell curve, you reduce the loads in wingroot and can thus reduce the structure and weight. Less weight = less drag. Horten idea, probably bird idea too.

- There are several projects aiming to flying wing airliners for very good reasons.[/QUOTE]

However, like birds, flying is not teh only thing airliners do.

The cost of stripping out fully embedderd engines is huge compared with hanging them on pods. Which is why they get hung on pods.

Agrred that although flymng wins are not the most efficient aerodynamically, iif it makes for a lighter cheaper airplane, its worth looking at...

[Petri Nygren]

(i) They are not. The vast majority have and use tails.

When maneuvering, yes. In cruise, as I wrote, not even nearly always. Check any gull or albatross in glide. Tail moment is very short, and tail is closed up. Most, if not all, stability and control of the efficiently flying birds in cruise comes from wing only. There are exceptions as always, but denying these facts probably just means better binoculars are needed! :rolleyes:

Bad data, bad assumptions.
Well, I strongly feel the same of yours Sir! :D


However, like birds, flying is not teh only thing airliners do.

Yes, and thats one of the reasons why there are number of flying wing airliner projects in all major aerospace countries. Good for us, not everybody limits their thinking:

http://www.ave.kth.se/staff/flight/martinc/ELSA/index.html
http://aero.stanford.edu/BWBProject.html
http://oea.larc.nasa.gov/PAIS/BWB.html
http://www.aerosite.net/bwb.htm
http://www.wingco.com/bwb_jeb_profiles.htm
http://aero.stanford.edu/bwbfiles/AerospatialeBWB.html
http://www.twitt.org/bldwing.htm

[banktoturn]

@kallend: I've done similar tests at the Cal Poly WInd Tunnel. I should clarify that Bernoulli's Venturi analysis is not exactly wrong, since the numbers do add up. However, it is misleading where the lift comes from. If his theory was 100% correct, planks would not fly. The problem is that pressure differential is an effect, not the source of lift. Pushing air down against itself does raise pressure, but the bottom line is strictly a matter of equal and opposite forces: air goes down, plane goes up. Here's a very good presentation from NASA about it:
http://www.grc.nasa.gov/WWW/K-12/airplane/wrong3.html

And for those willing to brave the equations supporting this (the conclusion is well written if you want to skip to the end for curiosity):
http://www.uni-frankfurt.de/~weltner/Mis6/mis6.html

macr0t0r,

The fact that a lifting wing will necessarily deflect air downward does not make it valid to declare that this is the 'real' cause of lift. It is simply not valid to declare cause and effect in this situation. I can just as legitimately point out that the specific shape of an airfoil 'causes' a particular velocity field around it, which 'causes' a particular pressure distribution, which thereby 'causes' lift. The pressure distribution observed on the wing is well predicted by the Bernoulli equation, so referring to it as 'Bernoulli crud' is ridiculous. Viewing the deflection of air as an alternative, rather than an accompanying phenomenon, is not valid. The Bernoulli equation is perfectly valid, when applied correctly. The fact that the erroneous 'venturi' model is frequently used to explain lift does not change that. Nor do any of the other incorrect interpretations and explanations of lift. If you lean against a wall, what is 'causing' the force, you or the wall?

banktoturn

[kallend]

Well, now you've changed the parameters. Neither Bernoulli nor I said anything about venturis now, did we?

You cannot deflect air without a pressure differential. Where does that come from?

A fluid can only exert a normal force on a surface by virtue of pressure. Integrate the pressure over the surface of a wing and you get the lift.

And it works just fine for a plank if you do it correctly.

[macr0t0r]

@kallend: Tell you what, I think my problem here is that my original statement SHOULD have said, "don't give me that Venturi crud, find yourself a modern book." I have somewhat erroneously tied Bernoulli to the Venturi concept, when the Venturi concept is a misapplication of Bernoulli's principals.

HOWEVER: I still say wings fly because the push air down. ;) Air is a fluid. Pushing on air increases pressure. This pressure gets measured. Relating this to me leaning on a wall is kind of apples/oranges, since the wall is not a fluid.

A better question is how a heavy object can swim? Water is mostly incompressible, so how do the hydrafoils work?

[banktoturn]

@kallend: Tell you what, I think my problem here is that my original statement SHOULD have said, "don't give me that Venturi crud, find yourself a modern book." I have somewhat erroneously tied Bernoulli to the Venturi concept, when the Venturi concept is a misapplication of Bernoulli's principals.

HOWEVER: I still say wings fly because the push air down. ;) Air is a fluid. Pushing on air increases pressure. This pressure gets measured. Relating this to me leaning on a wall is kind of apples/oranges, since the wall is not a fluid.

A better question is how a heavy object can swim? Water is mostly incompressible, so how do the hydrafoils work?

macr0t0r,

I don't think the analogy is inappropriate, though I may not have made the connection very clearly. My point is that force (or pressure) is an interaction between two things. It could be your hand and a wall, or a wing and some air. It is meaningless to declare which of the two is pushing on the other, or more specifically, which one is causing the pressure. When a wing is lifting, the necessary pressure distribution exists, and the air deflection occurs. You can't escape either one of them, and neither one can be singled out as causing the other. It continues to mystify me that people feel the need to line up behind one of these 'theories'.

banktoturn

[chlee]

macr0t0r wrote:
I should clarify that Bernoulli's Venturi analysis is not exactly wrong.

This is the root of the debate. It seems mainly to be a matter of biography and names, rather than physics. In your previous posts, if you were to replace some of the "Bernoulli" references with "Venturi model," then I think that we would largely be in agreement.

Chung

[macr0t0r]

Okay, I apologize for really de-railing the topic here. Usually, I'm admonishing others for going going way off topic. My fault, this time! :p

Let me close this up: true, with any fluid, force causes pressure and pressure causes force, because they really are just different ways of describing the same effect (F=P*A). That being said, I concede that its pointless to argue wether the wing pushing air down causes pressure as a side effect, or if the wing causes pressure differentials which cause a force.
"The carton has 6 eggs."
"No, it has half a dozen, you doof!"

So, I'll respectfully retract my statements on Bernoulli by stating that banktoturn and kallend are also correct, and that it is mostly a case of personal perception. Let's get this thread back on track with the efficiency of Flying Wings!

Birds and tails: In cruise, the bird's entire back end is the stabilizer. Just because the tail isn't spread out, doesn't mean it's still not being used. The tails does spread when maneuverability is needed.

Flying Wing Airliners: Boeing is working on one (Blended Body Transport). They pump fuel to adjust the CG. Apparently, there will be some form of vectored thrust at the nose and tail to provide additional pitch-control on takeoff and landing. Not a bad idea. The trick will be if the weight/drag savings of removing conventional surfaces will be enough to balance the extra weight of thrust-control.

[banktoturn]

Okay, I apologize for really de-railing the topic here. Usually, I'm admonishing others for going going way off topic. My fault, this time! :p

Let me close this up: true, with any fluid, force causes pressure and pressure causes force, because they really are just different ways of describing the same effect (F=P*A). That being said, I concede that its pointless to argue wether the wing pushing air down causes pressure as a side effect, or if the wing causes pressure differentials which cause a force.
"The carton has 6 eggs."
"No, it has half a dozen, you doof!"

So, I'll respectfully retract my statements on Bernoulli by stating that banktoturn and kallend are also correct, and that it is mostly a case of personal perception. Let's get this thread back on track with the efficiency of Flying Wings!

Birds and tails: In cruise, the bird's entire back end is the stabilizer. Just because the tail isn't spread out, doesn't mean it's still not being used. The tails does spread when maneuverability is needed.

Flying Wing Airliners: Boeing is working on one (Blended Body Transport). They pump fuel to adjust the CG. Apparently, there will be some form of vectored thrust at the nose and tail to provide additional pitch-control on takeoff and landing. Not a bad idea. The trick will be if the weight/drag savings of removing conventional surfaces will be enough to balance the extra weight of thrust-control.

macr0t0r,

Don't apologize for going off-topic; that's where a lot of the good stuff is to be found.

banktoturn

[vintage1]

So, I'll respectfully retract my statements on Bernoulli by stating that banktoturn and kallend are also correct, and that it is mostly a case of personal perception. Let's get this thread back on track with the efficiency of Flying Wings!


Indeed. When is a square wave an infinite series of superimposed sine waves all harmonically realted and phase locked..:D


Birds and tails: In cruise, the bird's entire back end is the stabilizer. Just because the tail isn't spread out, doesn't mean it's still not being used. The tails does spread when maneuverability is needed.


Yes. Variable gemotery to account for when they change CG after splattering your windscreen with you-know-what. Amongst other things. Very much optimised for 3D flying rather than efficient cruise, most birds :D




Flying Wing Airliners: Boeing is working on one (Blended Body Transport). They pump fuel to adjust the CG. Apparently, there will be some form of vectored thrust at the nose and tail to provide additional pitch-control on takeoff and landing. Not a bad idea. The trick will be if the weight/drag savings of removing conventional surfaces will be enough to balance the extra weight of thrust-control.

Agreed. Its the drag associated with pitch stability surfaces that makes a tailless wing inefficient.

However, I dread t think what such a machine would hadle like if it lost thrust.

Elevators have the virtue of smplicity at least...

[kallend]

There is nothing in the least inconsistent between Newton's 3rd Law (or the 2nd, for that matter) and conservation of energy (which is what Bernoulli's equation describes).

Fluids can only exert normal forces on surfaces by way of pressure, which Bernoulli's equation describes very nicely.

You cannot deflect air without a pressure difference any more than you can have a pressure difference without moving air.



What is wrong is the the incorrect use of Bernoulli's ideas by the people who drew the idiot pictures in some kids books and FAA publications.

[biber]

To come back to the threads topic: For some reason there are only few fullscale gliders built tailless. The most known are the horten types, the fauvel types and the SB 13. The SB 13 has quite good performance compared to uptodate gliders in FAI standart class, but its flight characteristics are a different deal. It tends to oscillate noseup nosedown allthetime when the air is not absolutely smooth. That causes the pilot to end up getting sick after about one or two hours of wobbling around. The only way found to reduce this is to move the CG backward and decrease stability margin ,wich causes other uncomfortable flight characteristics (e.g. spins). Some hortens performed rather well for their time but had a terrible adverse yaw, some where nice to fly, idiot proof, but did not perform. The fauvels are quite nice to fly eccept for the holding of during the landing. If pulled through to quick, an instant lose of lift follows and it smacks down. Or you are lucky to have had still enough altitude not to smack it down with this action, then you find yourself to high and way to slow after a second when the wing starts to respond with more lift. In addition to that the performance of these fauvel planks is poor. But in some very special cases tailless proved to best choice like zagi for combat :) .

[biber]

Forgot to mention the marske designs, wich i nevertheless don't know much about, but they can't be that much better than the old fashioned fauvels i think.

[grindel]

Forgot to mention the marske designs, wich i nevertheless don't know much about, but they can't be that much better than the old fashioned fauvels i think.

sorry for the flame, but doesn't really inspire confidense in what you say.

"I forgot to mention chocolate bars, but they're brown too so I'll call them cr@p"

[biber]

I assume this because i have never ever seen a plank so far that showed really good performance in a broad range of speed. The very most of them are specialists wether for thermaling or highspeed. The planks advantage is that it is easy to build and if you need good performance at only one CL (e.g. DS!) a plank is a considerable choice. But for fullscale gliders thats just not the case if you want to fly crosscountry. So i don't say planks are not good at all, but in most cases they are disadvantaged against other designs in terms of fullscale gliding requirements. BTW i wouldn't hesitate one single monent if i had the opportunity to get my ass in one of those marske wings, would shurely be fun to fly, although not fun because of pure performance.

[Mat]

Most flying wings from my current point of view have spiral instability, is this due to the lack of vertical area on the wing?

[macr0t0r]

Lack of tail-moment, mostly.

[Tim Green]

A simple home experiment can help a lot - create two airfoils from solid balsa. Both will be typical flat bottom airfoils - but one will have some slight up reflex added at the trailing edge - otherwise they'll be the same. So now you have two wing sections, each about 3 cm wide, and say 12 cm long. Thickness I leave to you. Put a pin through the fronts so that these little wings hinge on their leading edges more or less.

Now blow over their tops, using a straw to direct the airflow.

Only the one without the reflex will rise.

But I'll bet they both exhibit Bernoulli pressure differentials.

It's the Coanda effect throwing air down which causes the wing without the reflex to rise.

Try it.

[Antonsoarer]

A simple home experiment can help a lot - create two airfoils from solid balsa. Both will be typical flat bottom airfoils - but one will have some slight up reflex added at the trailing edge - otherwise they'll be the same. So now you have two wing sections, each about 3 cm wide, and say 12 cm long. Thickness I leave to you. Put a pin through the fronts so that these little wings hinge on their leading edges more or less.

Now blow over their tops, using a straw to direct the airflow.

Only the one without the reflex will rise.

But I'll bet they both exhibit Bernoulli pressure differentials.

It's the Coanda effect throwing air down which causes the wing without the reflex to rise.

Try it.

I think it's pitching not rising, there is a difference. Hold a spoon dangling loosely under a running tap, does it pitch or rise in the flow, it does indeed move into the flow? If you place wing on a flat surface and blow water or air only over the top of it will it rise?