I was wondering what the pros and cons about various comparatively rare aircraft design approaches are.

For example the V-tail as seen i.e. on the Beech 35 Bonanza or the Eclipse Concept Jet:

View Large View Medium Photo © Andrei Bezmylov

View Large View Medium Photo © Bruce Leibowitz

It seems that omitting an entire control surface would decrease weight and thus be more efficient, but what are the trade-offs?

Another kind of "V-Tail" (I'm sure that's not the right name here...) is the extra vertical stabilizer as seen in military aircraft such as the F-18 or F-22:

View Large View Medium Photo © George Canciani

View Large View Medium Photo © Alastair McBean

These are also slanted outward like the V-Tail but additionally there is the horizontal stabilizer. What are the reasons for a design like this and could there be feasible (reasonable) implementations in civil aviation?

And thirdly, what about canards as seen in i.e. the Piaggio Avanti?

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Photo © Mark Carlisle

Piaggio claims the canards would allow for much smaller overall lifting surfaces and thus less drag and thus less fuel consumption. It seems if there are no disadvantages about this design more aircraft manufacturers would apply it so what is the reason it is rarely seen?

Thanks in advance for all your input!

Quoting Flexo (Thread starter): what about canards as seen in i.e. the Piaggio Avanti?

I know that the canard wing will stall before the main wing, so the nose will drop, increasing airspeed before the main wing stalls, making it hard to stall the whole airplane.

As why its not used more? harder to design and build maybe?

but then again what do I know.


Dan in Jupiter

Quoting Flexo (Thread starter): These are also slanted outward like the V-Tail but additionally there is the horizontal stabilizer. What are the reasons for a design like this and could there be feasible (reasonable) implementations in civil aviation?

The two vertical stabilizers provide a way to get more tail volume while not adding to the height or length of the aircraft.

As far as why they are canted... right angles are unfavorable for maintaining small radar cross-section and should be avoided.

Quoting Flexo (Thread starter): For example the V-tail as seen i.e. on the Beech 35 Bonanza or the Eclipse Concept Jet:

Pros: -less drag than the three separate aerodynamic surfaces that you would otherwise have.

Cons: -I hear the V-tailed doctor killer (a.k.a. "Bonanza"   ) is known to have problems with dutch roll. Also, there are reasons the plane has the nickname it does...the V-tailed Bonanza is known to be one slick S.O.B., and if you point the nose below the horizon, she picks up speed rather quickly (in other words, too little drag...).

I know that the V-series Bonanzas have been the victim of several expensive AD's concerning the structure in the tail...not sure if that's specifically Beech's design that's at fault or a consequence of being V-tailed or even a consequence of keeping a 50-plus year old hot rod of a metal plane in the sky  

The V tail on the Bonanza produces a twisting moment on the fuselage which has caused some crashes. The fuselage of those planes should be inspected more than a conventional tail aircraft.

Quoting KELPkid (Reply 3): I hear the V-tailed doctor killer (a.k.a. "Bonanza"

Quoting Futurecaptain (Reply 4): V tail on the Bonanza

That's what I always thought, but Wikipedia (did a search to verify), seems to say that the V-tailed design having a liking for killing doctors is just folklore   

Quoting Starlionblue (Reply 5): If there was more willingness to look at options, half the aircraft would look like Rutan designs.

I know you were joking, but you did provide a real life example: the BE2000 "Starship." It was in fact, designed by Rutan, and was also revolutionary. However, due to failed promises and it's "out of this world" design, it was a failure.

Quoting Flexo (Thread starter): I was wondering what the pros and cons about various comparatively rare aircraft design approaches are. For example the V-tail as seen i.e. on the Beech 35 Bonanza or the Eclipse Concept Jet:

It seems that omitting an entire control surface would decrease weight and thus be more efficient, but what are the trade-offs?[/quote]

The main trade is more complex stability and control. Normal cruciform tails have less pitch/roll and yaw/roll coupling than a V-tail. You also loose some redundancy, I suspect, because loss of a normal elevator doesn't affect your yaw capability but loss of one V-tail surface would give you strong yaw/roll coupling.

Quoting Flexo (Thread starter): Another kind of "V-Tail" (I'm sure that's not the right name here...) is the extra vertical stabilizer as seen in military aircraft such as the F-18 or F-22:

Shorter, more damage resistant, allows lower radar cross section with equal tail volume (you can't pratically cant a single fin).

Quoting Flexo (Thread starter): And thirdly, what about canards as seen in i.e. the Piaggio Avanti?

Piaggio claims the canards would allow for much smaller overall lifting surfaces and thus less drag and thus less fuel consumption. It seems if there are no disadvantages about this design more aircraft manufacturers would apply it so what is the reason it is rarely seen?[/quote]

They're right, in general, about the advantages but you pay for it with a much more restricted allowable CG range and having a great whopping structural member running right through the fuselage in an inconvenient spot.

Tom.

I had always heard the stories of the "dutch roll" in the V-tailed Bos, but had also been told by Bonanza experts that when flying one, if you cannot see the tail, you cannot tell whether you're in a V-tail or a straight-tailed (33) Bonanza.

Having flown both straight-tail Bonanzas (a lot), and V-tailed Bonanzas (enough to judge the difference), I can tell you that the V-tail flies, for all intents and purposes, just the same. The "tail wag" for which they have a reputation is here- in the 35s and the 33s. If you are in bumpy air, the simple trick of "guarding" the rudder pedals (essentially, resting your feet on them to stop them from freely waggling) damps the wag to nearly nothing.

And no, there is nothing inherently unsafe about the V-tailed Bo- if the pilot is adequately trained and competent to fly it. It is among the best-flying planes out there, light of touch, stable and yes, fast. They do, indeed, pick up speed in a hurry if you point 'er down... but so do the stright-tailed Bonanzas.

I love the planes... enough that I intend to own one... very soon!

Quoting Sprout5199 (Reply 1): I know that the canard wing will stall before the main wing, so the nose will drop, increasing airspeed before the main wing stalls, making it hard to stall the whole airplane.

On the other hand, this means that the wing cannot get to its maximum lift AoA (since the nose would drop first), leading to a wing that can never be as efficient as it could be when paired with a normal horizontal stabilizer - you'll always have some wasted lift capability.

-Mir

Quoting Starlionblue (Reply 5): Aircraft manufacturers and operators are by and large a conservative lot, which is why most civilian designs are tube with wings with a rear empennage

I kind of thought that conservativeness would play a big role...

Quoting Tdscanuck (Reply 7): Piaggio claims the canards would allow for much smaller overall lifting surfaces and thus less drag and thus less fuel consumption... They're right, in general, about the advantages but you pay for it with a much more restricted allowable CG range...

I assume with CG you mean center of gravity? Why do you have a more restricted CG range with a canard?

Quoting SCCutler (Reply 8): Having flown both straight-tail Bonanzas (a lot), and V-tailed Bonanzas (enough to judge the difference), I can tell you that the V-tail flies, for all intents and purposes, just the same

That's really interesting, I always thought flying a V-tail would require extra training to learn to compensate yaw/roll momentum from the V-tail with the ailerons. So I guess even the old Bonanza compensates yaw/roll couplings without extra input by the pilot?

Quoting Tdscanuck (Reply 7): The main trade is more complex stability and control

This might be a problem in small airplanes, but I guess the complexity of stability is easily solved in fly-by-wire aircraft. I always wondered why V-tails are not used in commercial planes. If a plane such as the F-117 has a V-tail, an airliner or any other GA would be controllable too. So I guess there are more problems associated with this, which I'm not aware of. Or is it just conservativeness?

Quoting Flexo (Reply 10): Why do you have a more restricted CG range with a canard?

Wikipedia cites as an advantage of the canard design the "more useful range of center of gravity". But I wouldn't trust the article, it also shows a Mirage III as an IAI Kfir  

The Fouga Magister was quite a success and it was a v-tail. This was a design from the '50s

BRGDS
SB03

Quoting N231YE (Reply 6): I know you were joking, but you did provide a real life example: the BE2000 "Starship." It was in fact, designed by Rutan, and was also revolutionary. However, due to failed promises and it's "out of this world" design, it was a failure.

Well, from what I've heard, the FAA certification folks didn't like that one characteristic of canards...no well-defined, clean stall. They insisted that the Starship actually stall with a break, so Beech had to modify the plane accordingly to get certification. The resulting aerodynamic mods knocked about 35 knots off of the top cruise...   Thus the failure to meet promises.

However, the Achille's heel of the Starship was the noise level inside...the promise was that it was supposed to be far quieter than any turboprop, however once the plane was a reality, it was realized that that wasn't the case, and was in fact louder in many regimes of flight.

So executives and others who made such decisions saw a plane with the King Air's engines and roughly the same cruise speed, but was louder inside than a King Air and cost a few million more than the King Air...guess which plane they chose.

Quoting Futurecaptain (Reply 4): The V tail on the Bonanza produces a twisting moment on the fuselage

It certainly does and this is a common explanation, but doesn't the classic rudder?

Quoting Mir (Reply 9): On the other hand, this means that the wing cannot get to its maximum lift AoA (since the nose would drop first), leading to a wing that can never be as efficient as it could be when paired with a normal horizontal stabilizer - you'll always have some wasted lift capability.

I'm afraid this is absolutely not true. First - you don't need to have the same airfoil on both surfaces and you probably will not; so - different range of useful angles. Second - even with the same airfoil the size of wing is a factor for angle of attack (although this I'm not sure).

BTW, for chauvinism sake   V-tail is an invention of Polish engeineer Rudlicki and AFAIK was first used on some Hanriot plane in 1920s. We call it also butterfly empennege.

Quoting Flexo (Reply 10): They're right, in general, about the advantages but you pay for it with a much more restricted allowable CG range... I assume with CG you mean center of gravity? Why do you have a more restricted CG range with a canard?

Yes, I meant center of gravity. The aerodynamic gain from a canard comes about by reducing the total lift (and hence induced drag) of the main wing. However, having a lifting surface in front of the wing is a destabilizing influence. If you make the canard too big you become unstable. If you make it small enough to stay stable, you have to restrict the allowable CG range to make sure you have enough canard authority to maintain pitch control.

Quoting Keta (Reply 12): This might be a problem in small airplanes, but I guess the complexity of stability is easily solved in fly-by-wire aircraft. I always wondered why V-tails are not used in commercial planes. If a plane such as the F-117 has a V-tail, an airliner or any other GA would be controllable too. So I guess there are more problems associated with this, which I'm not aware of. Or is it just conservativeness?

Just because stability can be controlled by fly-by-wire doesn't mean you actually want to do that in commercial aircraft. Military fighters, F117 included, are often neutral or unstable and are only flyable because of the FBW system. It is exceedingly unlikely that any regulatory agency would certify a neutral or unstable commercial aircraft because it's an additional risk for no gain.

That's a bit of a tangent, since you can have a stable V-tail.

Quoting 3MilesToWRO (Reply 15): Quoting Mir (Reply 9): On the other hand, this means that the wing cannot get to its maximum lift AoA (since the nose would drop first), leading to a wing that can never be as efficient as it could be when paired with a normal horizontal stabilizer - you'll always have some wasted lift capability. I'm afraid this is absolutely not true. First - you don't need to have the same airfoil on both surfaces and you probably will not; so - different range of useful angles. Second - even with the same airfoil the size of wing is a factor for angle of attack (although this I'm not sure).

Mir is right...for a canard design to have a controllable stall, the canard must stall first. Regardless of which airfoil and which incidence angle you use for each surface, the canard must stall before the main wing does. That means the main wing can never reach it's maximum AoA (the canard will stall first and decrease AoA).

Tom.

Quoting Tdscanuck (Reply 16): Mir is right...for a canard design to have a controllable stall, the canard must stall first. Regardless of which airfoil and which incidence angle you use for each surface, the canard must stall before the main wing does. That means the main wing can never reach it's maximum AoA (the canard will stall first and decrease AoA).

You never operate near max AOA, so not quite reaching it is hardly an efficiency penalty. Wing efficiency is primarily measured by L/D ratio not CLmax, as Mir said. The canard wing is just as efficient at normal AOA.

Quoting Flexo (Thread starter): Piaggio claims the canards would allow for much smaller overall lifting surfaces and thus less drag and thus less fuel consumption.

The Piaggio actually doesn't have a canard, because the Piaggio's front wing doesn't control pitch. AFAIK it's the only production aircraft in the world that has a foreplane, main wing, and horizontal tail that all provide lift. That's probably why the cg is restricted. But you can fly it was just two pilots up front. I remember reading an early flight report by Richard Collins who said that you really noticed the lack of weight on the ground.

Quoting Jetlagged (Reply 18): You never operate near max AOA, so not quite reaching it is hardly an efficiency penalty. Wing efficiency is primarily measured by L/D ratio not CLmax, as Mir said. The canard wing is just as efficient at normal AOA.

Yes and No.

The issue with canards is not max efficiency, and in fact, canards climb just as well as conventional aircraft, (which you are correct, is determined by L/D ratio).

However, saying that aircraft do not operate near max AOA is patently false, that is the fundamental definition of a conventional aircraft's stall speed, and herein lies the rub with canards.

As a canard's CG moves aft, more lift is carried by the main wing and less by the canard (by definition), therefore in trim, the canard flies at a lower AOA. Conversely, move the CG forward, and the canard flies at higher relative AOA to the main Wing for any given airspeed to maintain trim.

Now, at the aft most CG limit in a canard, with the canard flying 'flatter' and hense at the same AOA as the wing, then yes, conventional aircraft and canards are roughly equivolent (only roughly, because the canard still must stall at a lower AOA than the wing in all cases, including control margins). Start moving the CG forward, however, and the canard must fly at a higher AOA simply to compensate for the forward weight. More AOA to carry this load, means less AOA available for lift at the same low airspeed, means a higher airspeed necessary to keep the canard flying.

Thus your limiting factor for minumum stall speed is no longer the critAOA of the wing, but instead the critAOA of the canard, which must by definition be lower than that of the main wing, meaning your not getting as much lift from the main wing. This means you either need a bigger main wing, or a main wing with a higher Lift coefficient to carry the same weight at the same speed. Either case means more drag.

Thus, while a canard will climb just as effeciently as a conventional aircraft, which as you correctly state has nothing to do with critAOA, it will NOT be able to utilize its entire wing planform for higher AOA operations as a conventional aircraft can, so while Vy will be the same (and in fact, the Canard will probably climb better due to more lift going into climb rather than fighting tail downforce), Vs will always be higher for the Canard. Thus if you want the same Vs for a canard and a conventional aircraft, you will need a larger/higher lift (either way, more draggy) wing for the canard, regardless of Vy or L/D max.

And as others have noted, the Avanti (which I have a bit of a fetish for) is _NOT_ a canard aircraft, and in fact, 3LS is a very elegant way of getting (almost) the best of both worlds... but that is a rant for another time.

Quoting KELPkid (Reply 14): Well, from what I've heard, the FAA certification folks didn't like that one characteristic of canards...no well-defined, clean stall. They insisted that the Starship actually stall with a break,

Ehm, what do they need this stall for, after all? I see it as a manouver like many others, so it looks a little bit for me as if they denied certification because, say, the plane can't do a loop.

Quoting EridanMan (Reply 19): The issue with canards is not max efficiency, and in fact, canards climb just as well as conventional aircraft, (which you are correct, is determined by L/D ratio).

My point about efficiency was only about this.

Quoting EridanMan (Reply 19): Start moving the CG forward, however, and the canard must fly at a higher AOA simply to compensate for the forward weight. More AOA to carry this load, means less AOA available for lift at the same low airspeed, means a higher airspeed necessary to keep the canard flying.

In this case the canard is providing lift, which reduces the amount the main wing must produce. In a conventional aircraft the tailplane provides a downward force to maintain trim, meaning the main wing must create more lift for a given weight.

The point about the effect on stall speed is valid, but rather than increase wing size or modify the section, the canard designer can always either use better high lift devices (heavier of course), or simply accept a slightly higher stall speed. More drag in high lift configuration is not a problem, it may even be desirable.

Quoting Jetlagged (Reply 21): More drag in high lift configuration is not a problem, it may even be desirable.

Drag is a problem for a takeoff high lift configuration as it may limit airplane takeoff weight to meet second segment climb gradient requirements.

On the canard question, consider that the first and second airplane designs to achieve powered flight were canards. Therefore the conventional wing - tail designs were a departure from these initial designs. For almost 100 years, canard configurations have been the exception rather than the rule. If a canard configuration was inherently more efficient than a wing - tail configuration, doesn't this seem a little odd?

Quoting SCCutler (Reply 8): They do, indeed, pick up speed in a hurry if you point 'er down... but so do the stright-tailed Bonanzas.

...And so do Mooneys. More so, in my experience. It's simply a characteristic, though, and as SCCutler points out, there is nothing inherently unsafe about it.

Quoting Keta (Reply 12): But I wouldn't trust the article, it also shows a Mirage III as an IAI Kfir

The author of that article wouldn't last a day in Identify This...  


2H4

Quoting Tdscanuck (Reply 16): Just because stability can be controlled by fly-by-wire doesn't mean you actually want to do that in commercial aircraft. Military fighters, F117 included, are often neutral or unstable and are only flyable because of the FBW system. It is exceedingly unlikely that any regulatory agency would certify a neutral or unstable commercial aircraft because it's an additional risk for no gain.

The thing is, you can have either a stable or unstable aircraft with a V-tail (e.g. Bonanza and F-117). So, you could perfectly use a V-tail in a commercial jet. What is the reason they don't use it? The original poster's question remains unanswered:

Quoting Flexo (Thread starter): It seems that omitting an entire control surface would decrease weight and thus be more efficient, but what are the trade-offs?

Quoting 2H4 (Reply 23): The author of that article wouldn't last a day in Identify This...

Not even Jane's guides' writers would last a day...  

Quoting Keta (Reply 24): So, you could perfectly use a V-tail in a commercial jet. What is the reason they don't use it? The original poster's question remains unanswered:

I don't know the reason for certain, but I have to assume it's a controllability issue. The weight savings is too obvious so there has to be a compelling reason for not having done it.

Trim might be the killer issue...with a V-tail, to provide horizontal trim you'd have to generate more lift force on the tails than a horizontal tail (because the tail surface lift vector isn't orthogonal to the pitch axis). That means more induced drag on the tails than the equivalent horizontal design. You also would need physically stronger (heavier) tails, even though there would be one less surface. For small aircraft, the weight savings probably justifies it because the pitch trim forces are small but on a commercial airliner they can be huge.

Tom.

Quoting OldAeroGuy (Reply 22): Quoting Jetlagged (Reply 21): More drag in high lift configuration is not a problem, it may even be desirable. Drag is a problem for a takeoff high lift configuration as it may limit airplane takeoff weight to meet second segment climb gradient requirements.

The debate had become centred on not being able to achieve CLmax. For takeoff or climb, there's no reason why a canard should have more drag, because it is nowhere near the stall AOA in these configurations. I was talking specifically about the landing case, where a little more drag might even be a benefit if the overall shape was very slippery. That is, after all, why extreme landing flap angles are used: extra drag.

Quoting OldAeroGuy (Reply 22): If a canard configuration was inherently more efficient than a wing - tail configuration, doesn't this seem a little odd?

I haven't said the canard is inherently more efficient, have I? I only countered some of the claims of inefficiency, which I didn't think were justified, or were being exagerated. It's not inherently less efficient either, apart from the stall AOA limitations.

I read all of the responses, and saw nothing on the tail of the hornet. If you didn't know? The rudders actuate inward on takeoff to increase tail drag which forces the nose up. If you have ever watched a carrier launch, the pilot hands are on the hand holds on the canopy. The aircrafts for the U.S. Navy and Marine Corps are designed to to take the "Cat" shot hands off. It is completely at the option of the pilot, except the rudders. He can input and they will respond but they will go back to T.O, until a certain speed is reached. I spent a good bit of 10 yrs. on carriers. Have a nice Marine Cops day.

737tdi

Quoting Starlionblue (Reply 26): Or to quote one of the P&W engineers working on the original 747 engines: "You will sell your own mother for a 1% decrease in specific fuel consumption!"

I think it was Bill Lear who said he'd sell his grandmother to save a pound of weight.

2H4

Quoting Jetlagged (Reply 27): It's not inherently less efficient either, apart from the stall AOA limitations.

Canards are less efficient aerodynamically than conventional configurations in two primary areas:

1) Induced drag due to the high fraction of total lift that must be carried by a stable canard. Since the span of the canard is less than that of the wing, higher induced drag is the result.

2) Higher wetted area drag due to the inability of a stable canard to stall the wing. Since the wing cannot develop its full lift potential on a canard configuration, the total lifting surface area on a canard (canard + wing) is greater than that of a conventional configuration (wing + h. tail) for the same takeoff/landing operating speeds.

For these reasons, stable canards are less efficient than conventional configurations for all phases of flight. The number of canards airplanes built vs the number of conventional airplanes tell the tale.

Quoting Tdscanuck (Reply 25): Trim might be the killer issue...with a V-tail, to provide horizontal trim you'd have to generate more lift force on the tails than a horizontal tail (because the tail surface lift vector isn't orthogonal to the pitch axis). That means more induced drag on the tails than the equivalent horizontal design. You also would need physically stronger (heavier) tails, even though there would be one less surface. For small aircraft, the weight savings probably justifies it because the pitch trim forces are small but on a commercial airliner they can be huge.

That might be the issue. However, some aircraft are using fuel transfer, from the wings to the tail and other parts, as a method of controlling CG location. This reduces the need of trimming. I don't know if this is used in climbing and descending phases, though. This way, V-tails would be easier to implement?

Quoting OldAeroGuy (Reply 30): 1) Induced drag due to the high fraction of total lift that must be carried by a stable canard. Since the span of the canard is less than that of the wing, higher induced drag is the result. 2) Higher wetted area drag due to the inability of a stable canard to stall the wing. Since the wing cannot develop its full lift potential on a canard configuration, the total lifting surface area on a canard (canard + wing) is greater than that of a conventional configuration (wing + h. tail) for the same takeoff/landing operating speeds.

With great respect, we are back full circle.

1. The distribution of lift between canard and mainplane is up to the designer. Some canards have relatively small areas. Anyway, what about the induced drag of the tailplane on a conventional plane. It's still there and the tailplane is producing negative lift. The effect was known as trim drag when I was working at the RAE. Also, because the conventional wing has to produce more lift than a canard's wing it's induced drag is higher in all regimes.

2. What if you decide to accept the small penalty on landing speed for equal efficiency? It will only be a knot or two. Then your wetted area is the same.

Conventional wisdom is not always correct. If it were, no one would produce any canard designs at all.

Quoting Jetlagged (Reply 33): Conventional wisdom is not always correct. If it were, no one would produce any canard designs at all.

The rarity of canard designs is not indicative of "conventional wisdom" being incorrect. It's simply an indication that the manufacturers of the canard designs that have reached production placed more importance on outside factors, such as appearance, marketing, cabin space, etc. Those designers simply concluded that factors like these outweigh the inherently more efficient design of a conventional aircraft.


2H4

When I was running the local airport a homebuilt canard (I've forgotten which one) came in occasionally. The owner had built it, and we discussed the pros and cons of canards quite a bit. One thing that nobody has mentioned is the difficulty of putting flaps on a canard plane; the increased lift caused by flaps upsets the balance between the main wing and the canard, unless you put flaps on the canard as well. This is very complex, and would cause massive problems if they didn't deploy together. Because of that problem, his did not have flaps and consequently had very high landing speeds. The CG range was also much more restrictive than on a conventional design, and the consequences of exceeding it were also more severe, especially with aft CG. Consider a canard where the main wing stalls first (which is what would happen with CG too far aft). The plane would tend to fall with the nose pointed up instead of down, which would be extremely difficult to recover from.

Quoting SEPilot (Reply 35): The plane would tend to fall with the nose pointed up instead of down, which would be extremely difficult to recover from.

...Which is exactly what happened to this airplane, and what earned it the nickname of "Ass-Ender":





2H4

Quoting 2H4 (Reply 36): ...Which is exactly what happened to this airplane, and what earned it the nickname of "Ass-Ender":

Thanks for posting that; I had never heard of this plane before.

Quoting SEPilot (Reply 35): the increased lift caused by flaps upsets the balance between the main wing and the canard, unless you put flaps on the canard as well

(Not to mention the change in pitching moment.) Just as it does with a conventional plane. You don't have to have flaps on the horizontal stab, but variable incidence stab trim helps. Same for the canard.

Quoting 2H4 (Reply 34): The rarity of canard designs is not indicative of "conventional wisdom" being incorrect. It's simply an indication that the manufacturers of the canard designs that have reached production placed more importance on outside factors, such as appearance, marketing, cabin space, etc. Those designers simply concluded that factors like these outweigh the inherently more efficient design of a conventional aircraft.

Some of what you say is true, but it is possible to produce an equally efficient canard with certain compromises. Aircraft design follows fashion as well as economics. However, a bad canard design is much more likely to be unsafe or less efficient than a conventional design. In that sense the canard layout is a more risky 'bet' for an aircraft manufacturer.

However even Boeing (conservative as they are) consider the canard layout on occasion (as in the Sonic Cruiser concept)

The early canard designs handled badly because they had no fixed fin and stabiliser surfaces. The most successful early aircraft had the conventional layout (with fixed surfaces) and others followed suit. In the early days many designers thought the monoplane was inherently dangerous, and less efficient biplanes ruled the skies for years because of this.

I don't want to be characterised as "Mr Canard", but I think conventional wisdom has a lot to do with peoples' attitude to the layout. Some notorious canard failures add to this view. I just think people should be more open minded on the issue, and less dismissive.

Quoting Jetlagged (Reply 38): (Not to mention the change in pitching moment.) Just as it does with a conventional plane. You don't have to have flaps on the horizontal stab, but variable incidence stab trim helps. Same for the canard.

The difference is that with the conventional layout the horizontal stabilizer is just providing balance, while on a canard it is sharing duties with the main wing in providing lift. In a conventional layout you do have a change in trim with flap deployment, but all the lift is still being provided by the wing. With a canard when you add flaps to the wing but not the canard, you are causing a pitch down moment that is much more severe than what you encounter in a conventional plane. Without flaps you may not have enough control authority left in the canard to flare sufficiently. It is not at all the same situation as in a conventional layout.

Quoting Jetlagged (Reply 38): I don't want to be characterised as "Mr Canard", but I think conventional wisdom has a lot to do with peoples' attitude to the layout. Some notorious canard failures add to this view. I just think people should be more open minded on the issue, and less dismissive.

I initially thought that the canard was a great idea, too. But the more I learned about it the more problems I discovered, and have come to the conclusion that it is very, very difficult to implement it in a way that equals the conventional layout in safety, flexibility, and pilot-friendliness. I have studied many canard designs (usually in homebuilt or kitbuilt aircraft) and have not seen one that lived up to expectations. Most of them have defects (like the landing speed of the one referred to in my earlier post-it would not have been certifiable) that would preclude general use and the others do not offer any improvement of performance over conventional layouts. It's not as if it's a new idea that just has not been explored; some very, very smart people have been trying for years to make them work, and so far have failed to offer any significant advantages. It may yet happen, but at this point I remain skeptical.

Quoting SEPilot (Reply 39): The difference is that with the conventional layout the horizontal stabilizer is just providing balance, while on a canard it is sharing duties with the main wing in providing lift. In a conventional layout you do have a change in trim with flap deployment, but all the lift is still being provided by the wing. With a canard when you add flaps to the wing but not the canard, you are causing a pitch down moment that is much more severe than what you encounter in a conventional plane. Without flaps you may not have enough control authority left in the canard to flare sufficiently. It is not at all the same situation as in a conventional layout.

You don't think that the tailplane's lift might not be adding to (or rather subtracting from) the overall lift of a conventional aircraft then? Some law of physics means it is nulled out if behind the wing, but not if in front?

The canard foreplane is only providing a balancing moment, just like a tailplane. If you put flaps down in a conventional aircraft you significantly increase nose down pitching moment. The tail has to provide a lot more nose up trim to balance this by increasing the download on the tail. This in turn offsets the lift increase from the flaps. In a canard, the increase in nose down pitching moment is countered by nose up trim provided by an increase in lift on the foreplane. There is no absolute need for a flap on the canard. A variable incidence foreplane with elevators could do the job.

As soon as you get away from the mindset that the canard is "sharing the lift" then it becomes clear that there is no big difference between the two apart from the stall case. The only other difference is that the canard is operating in freestream air, while the tailplane operates in the downwash of the wing. Obviously not everything in the canard universe is rosy, but it's not all negative either.

Quoting Jetlagged (Reply 33): Also, because the conventional wing has to produce more lift than a canard's wing it's induced drag is higher in all regimes.

In cruise, the CG on a conventional transport config. is more often than not near the wing center of lift. The h. tail has little load either up or down. Therefore total lift is essentially due to wing lift and induced drag is the result of wing lift only.

With a stable canard, the CG must always be ahead of the wing center of lift, some times by a considerable amount. This means the lower span canard is carrying a relatively large amount of lift, with a corresponding "lump" in the span load distribution. It's the poor span load distribution that causes a canard to have higher induced drag than a conventional design.

Quoting Jetlagged (Reply 33): What if you decide to accept the small penalty on landing speed for equal efficiency? It will only be a knot or two. Then your wetted area is the same.

It's not just a knot or two. Unless you are willing to accept a complex lift system were the wing and canard have the same trailing edge and/or leading edge high lift devices, a canard configuration will have about a 40% reduction in CLstall compared to a conventional configuration with high lift devices on only the wing.

Suppose you have a conventional config. with an MLW approach speed of 140 kts. A canard configuration with wing high lift devices only will have an approach speed of 168 kts or will need 40% more lifting surface area for a 140 kt approach speed.

Quoting Jetlagged (Reply 40): You don't think that the tailplane's lift might not be adding to (or rather subtracting from) the overall lift of a conventional aircraft then? Some law of physics means it is nulled out if behind the wing, but not if in front? The canard foreplane is only providing a balancing moment, just like a tailplane. If you put flaps down in a conventional aircraft you significantly increase nose down pitching moment. The tail has to provide a lot more nose up trim to balance this by increasing the download on the tail. This in turn offsets the lift increase from the flaps. In a canard, the increase in nose down pitching moment is countered by nose up trim provided by an increase in lift on the foreplane. There is no absolute need for a flap on the canard. A variable incidence foreplane with elevators could do the job.

On a conventional layout the center of lift is always behind the CG, and so the tail is providing down force. It is balanced so that even with flaps extended the amount of downforce required is within the capability of the elevators. If the center of lift is in front of the CG, then if it stalled it would fall tail first just like the "Assender." This is what limits rearward CG.
On a canard design the canard is not just a balancing surface. The key to canard designs is that the canard is providing lift also. The primary motivation for a canard is that it is wasteful to have the tail providing downforce, as that requires the wing to have more lift than would be otherwise necessary. But in order for a canard to be safe, the canard must stall before the wing, or you get the "Assender" situation. This is where the disadvantages come in, such as restricted CG range and problems with high lift devices and slow speed performance. It all comes down to the fact that there still ain't no free lunch.

The canard is primarily there to balance pitching moment, not provide lift. However it can do so with a positive upforce. The fact that this contributes to lift in a small way is not a disadvantage. A tailplane works in exactly the same way, with similar induced drag but a negative lifting component.

You simply cannot say that a tailplane is primarily for balance but canard foreplane is primarily for lift.

Positive lift on the foreplane is not the only reason to choose a canard config. Being able to place the engines aft, yet still providing bending relief on the wing is another. Internal layout may be improved too, depending on the requirement.

There have been canard designs which were more like tandem wings, which is a different proposition all together. The whole thing is a matter of degree, and the balance is in the designer's hands. To generalise about these things is not useful.

To use an abject failure like the Ascender as an example is not particularly fair either. Designers now know about the stalling issue. There have been equivalent conventionally tailed failures.

Quoting Jetlagged (Reply 33): Anyway, what about the induced drag of the tailplane on a conventional plane. It's still there and the tailplane is producing negative lift.

Yes, but the tailplane is flying in an opposite downwash from the wing, which actually lowers the tailplane's induced drag. A canard is in the opposite situation.

Quoting Jetlagged (Reply 38): However even Boeing (conservative as they are) consider the canard layout on occasion (as in the Sonic Cruiser concept)

Sonic Cruiser was, effectively, a delta wing. Whole different ball game.

Tom.

Quoting Starlionblue (Reply 32): The Mad Nitpicker strikes: It reduces the need for aerodynamic trimming. The need for trimming remains

You're right. I think it was me, that I was hoping that the reason why they were not employed was conservativeness, and tried to take away other options  

Quoting Jetlagged (Reply 44): The canard is primarily there to balance pitching moment, not provide lift.

If this was true, there would be a substantial amount of fuselage behind the main wing, which I have never seen on any canard design. The CG on a normal plane falls somewhere near the leading edge, as can be seen from the location of the landing gear. If a canard was to use the canard for balance only, the CG would have to be in the same place. If that were true, every one I have ever seen would need one heck of a heavy engine. The attraction of a canard design is that theoretically it can be stall-proof, as it is designed so that the canard stalls before the main wing, which will cause the nose to drop and reduce the angle of attack rather than going into a full stall. The catch is that it only works when the CG is within design limits, which turns out to be a very narrow range. I will not buy the idea that a canard can be just a balancing surface unless you can show me a design that has a chance of having the CG near the leading edge of the main wing. As I said in my earlier post, I believe that if you did that it would have very undesirable stall behavior.

Quoting SEPilot (Reply 47): If a canard was to use the canard for balance only, the CG would have to be in the same place. If that were true, every one I have ever seen would need one heck of a heavy engine.

The relative location of the wing, engine and CG is up to the aircraft designer. The canard layout does not impose any limits on this freedom. You seem to assume that a canard's main wing must be at the extreme aft end of the fuselage. I can't think why you believe this to be a requirement.

Quoting SEPilot (Reply 47): The CG on a normal plane falls somewhere near the leading edge, as can be seen from the location of the landing gear.

You're generalising again. Actually it can be anywhere between around 0% MAC and 40% MAC on an airliner, actual limits varying with the type involved.

Quoting SEPilot (Reply 47): The attraction of a canard design is that theoretically it can be stall-proof, as it is designed so that the canard stalls before the main wing, which will cause the nose to drop and reduce the angle of attack rather than going into a full stall.

What? This was claimed to be a disadvantage earlier in the thread: that it must stall before the main wing, thus reducing CLmax.  

Quoting SEPilot (Reply 47): I will not buy the idea that a canard can be just a balancing surface unless you can show me a design that has a chance of having the CG near the leading edge of the main wing.

I see, so you are right unless I can prove you wrong. I am not playing that troll's game. Your words show your mind is already closed.

To assess the lifting capability of the foreplane, look at the relative area of it to the wing. In some canards it is very small, so the lift produced as a percentage of the total is small too. In others it is big, more like a tandem wing in fact.

Both tailplane and foreplane designs balance pitching moment and CG with lift on a moment arm. The foreplanes lift is positive, the tailplanes lift is negative. Can't you see this? You conveniently ignore the negative lift produced by a tailplane, which requires more AOA of the main wing and so more induced drag.

I include this link merely to show the vectors involved because I can't be bothered to draw it myself.

http://www.grc.nasa.gov/WWW/K-12/airplane/trim.html

Quoting Tdscanuck (Reply 45): Sonic Cruiser was, effectively, a delta wing. Whole different ball game.

So some canards aren't canards? I suppose the Viggen, Gripen and Typhoon don't count either? The kind of canard SEPilot is going on about is more like a tandem wing, whole different ballgame.

Quoting Tdscanuck (Reply 45): Yes, but the tailplane is flying in an opposite downwash from the wing, which actually lowers the tailplane's induced drag. A canard is in the opposite situation.

Downwash has an effect, and if you read my posts you will see I have already pointed out this difference, but it is still wrong to say the the canard produces induced drag while the tailplane does not.

To summarise:
Tailplane: produces less induced drag, but negative lift
Foreplane: produces more induced drag, but positive lift

Sort of balances out really, don't you think?

I am not claiming the canard is superior. All I'm saying is that it is not inherently inferior either. This is a plea for open mindedness, nothing more.

Quoting Jetlagged (Reply 48): The relative location of the wing, engine and CG is up to the aircraft designer. The canard layout does not impose any limits on this freedom. You seem to assume that a canard's main wing must be at the extreme aft end of the fuselage. I can't think why you believe this to be a requirement.

But the airplane designer is still limited by the laws of physics. I have never seen a canard design that resembles a conventional aircraft flying backwards, which is what you are implying is possible. The point is, I have never seen a canard design that did NOT have the main wing at the extreme aft of the fuselage; have you?

Quoting Jetlagged (Reply 48): You're generalising again. Actually it can be anywhere between around 0% MAC and 40% MAC on an airliner, actual limits varying with the type involved.

We're arguing about semantics; what you say is consistent with what I meant (but perhaps did not say clearly.)

Quoting Jetlagged (Reply 48): What? This was claimed to be a disadvantage earlier in the thread: that it must stall before the main wing, thus reducing CLmax.

In fact it is a disadvantage exactly as the previous poster said; the attraction, as I said, is that theoretically the main wing cannot stall. But as I also said, this is true only if the CG is kept within design limits, which turn out to be quite narrow.
The real case against a canard comes when you consider what happens when you totally lose control, which can be likened to tossing the plane in the air at random. How will it fall? With a conventional design, the CG is somewhere on the wing, and the greatest leverage over it will be at the tail. Therefore the tail will tend to have more air resistance relative to the weight on it, so the plane will tend to fall nose first, which will give a chance to recover control. If you turn the plane around, and have the same situation but the control surfaces at the nose, the plane will ALWAYS fall tail first, guaranteeing that it cannot be brought under control. If the control surfaces are forward, the CG must also be far enough forward to guarantee that the nose falls first, requiring that the control surfaces are lifting surfaces. This brings up my point that the balance of lift between the canard and the main wing must be carefully chosen, and the resulting restrictions exist on allowable CG range.

Quoting Jetlagged (Reply 48): I am not claiming the canard is superior. All I'm saying is that it is not inherently inferior either. This is a plea for open mindedness, nothing more.

Open mindedness cannot be extended so far as to ignore physical laws. The Viggen, Gripen, and Typhoon are a) delta wings and b) military aircraft, both of which put them in a different category. Military aircraft all have ejection seats, and are considered expendable, so they almost never have the degree of stability and recoverability that is required by civil aircraft. In fact, most of the newest ones are deliberately unstable, and are only flyable by FBW because the computer keeps them from going out of control. We are not yet at the place where this would be acceptable on a civil transport.