Canard vs Tail

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dwightlooi

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Unread post23 Jan 2008, 07:23

Let's not clutter other threads with this polarizing topic. There are advantages and disadvantages to both configurations. I am of the opinion that unstable wing-tails offer better performance overall, but feel free to present your views and the reasonings behind it.

I'll fire off the initial volley.

One of the traditional advantages to a canard layout is that in the canard-wing layout both the canard and wing are lift producing entities and this promotes better lift-drag performance. However, this advantage is reversed with unstable designs. With unstable wing-tails being lift-lift arrangements, whereas unstable canards are downforce-lift arrangements. The following picture illustrates this...

Image
Last edited by dwightlooi on 23 Jan 2008, 15:27, edited 1 time in total.
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sprstdlyscottsmn

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Unread post23 Jan 2008, 08:08

I think a lot of people are also confusing canard aircraft with canard deltas. A stable canard is better than a stable wing tail, and a stable canard delta is better than a delta, but delta wings have there own issues, more internal volume and less weight/unit area, but that is only due to the low aspect ratio. The high sweep angle loweres maximum lift and steepens the Cl Alpha curves. This means a lot of drag in a turn. Dwight, WONDERFUL pictures. They do a great job of illustrating the difference between stability and instability in bolt configurations.

As to your topic, I think for low speed designs in general aviation that a canard has a lot of advantages. One being that when you stall such a plane the canard stalls first so you get the desired nose down pitching moment but the wing is still producing most of its lift so the sink rate is not as bad. Plus such designs are stable so there is the drag benefit.

For a combat aircraft there are advantages to all configurations. Canards can be used as brakes better than a tail. Weather you pitch a tailplane up or down it will try to lift one set of wheels off the ground while a down-pitched canard would just push down into the ground. to make an unstable canard to me seems to be foolish, really. It takes away all of the benefit of canards!! Are the figures for the F-35 based on planform shape and main wheels? if so... I had no idea its pitching moment arm was going to be so large, and it has large surfaces. I notice that there are no canarded "supermaneuvering" aircraft. Yes the Su-35/37 HAS canards, but it already had sufficiant supermaneuver pitch with its large tails and they added TVC as well. I guess then Tails allow for greater non aerodynamic maneuvering (pitch moment based, or post stall). Now I know its all still aerodynamics but I think of aerodynamic turning as a "sustained" Cl pulling the whole aircraft around, not a pitch moment exceeding the AOA limits.
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AeroG33k

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Unread post23 Jan 2008, 08:28

Excellent summary, but (I may be wrong) this seems to apply mainly to level flight, whereas in a hard turn, a canard will still produce additional 'up' force whereas a tail will still produce down force, thus making the aircraft "more unstable" in a turn. This may account for the reported high instantaneous turn rates of the Eurofighter, but possible worse sustained turns due to the delta wing? (that's how I remember it anyway, thought I don't have a source right now). Also, canards can be more effective at high-AOA as they operate in 'clean' air and produce vortexes over the wing root (notice the 'notch' vortex generator on the typhoon which seem to be designed to re-energize the canard vortex). This can also improve STOL performance as seen on the Gripen. So, as was said before, I'd agree that canards would be a good complement when a delta-wing is desirable for an aircraft (desired for speed, low wing loading, high internal volume, stealth, enhancing vortex lift and making up for shortcomings in maneuverability as it's often difficult to apply a tail to delta wings within the Mach cone). There's a lot of advantages to canards in CONTEXT of a configuration, but not necessarily by itself. Of course, in different context and requirements, a conventional tail may perform much "better" (meaning more suited for requirements).
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johnwill

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Unread post23 Jan 2008, 08:43

Nice work, Dwight. Let's hope your drawings will clarify the explanations you've been trying to make. One comment, with fly-by-wire it is easy to schedule the wing trailing edge control surfaces. That makes it possible to get just about any load distribution you want on canards and tails. The F-16 and F-18 both do that under some conditions. On the F-16, the flaperons are raised 2 degrees when supersonic to reduce trim drag by unloading the horizontal tail. The F-18 schedules the flaps with increased AoA.
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johnwill

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Unread post23 Jan 2008, 09:02

AeroG33k wrote
Excellent summary, but (I may be wrong) this seems to apply mainly to level flight, whereas in a hard turn, a canard will still produce additional 'up' force whereas a tail will still produce down force, thus making the aircraft "more unstable" in a turn.


Indeed you are wrong. The load distribution that Dwight shows applies to level flight and turns. To initiate a turn there will be a brief time when the canard load is up, but then very quickly the load must again be down to prevent exceeding g and AoA limits. Whether at 1g or 9g, the same pitch balance is required, so the canard load must be down. The same thing applies to tail load, to start the turn the tail load will briefly be down, the rapidly change to up, again to balance the airplane it pitch.
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AeroG33k

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Unread post23 Jan 2008, 09:35

johnwill wrote:AeroG33k wrote
Excellent summary, but (I may be wrong) this seems to apply mainly to level flight, whereas in a hard turn, a canard will still produce additional 'up' force whereas a tail will still produce down force, thus making the aircraft "more unstable" in a turn.


Indeed you are wrong. The load distribution that Dwight shows applies to level flight and turns. To initiate a turn there will be a brief time when the canard load is up, but then very quickly the load must again be down to prevent exceeding g and AoA limits. Whether at 1g or 9g, the same pitch balance is required, so the canard load must be down. The same thing applies to tail load, to start the turn the tail load will briefly be down, the rapidly change to up, again to balance the airplane it pitch.


I think this would be the case when initiating a climb or a small course change when all you need is pitching the lift vector, but not in a hard sustained turn or loop where you'd keep pulling the stick back.The whole point is to generate a moment to turn throughout the entire turn. Without a moment, there is no turn and in dwight's illustration, it seems to be that the moments cancel each other around the Cg for level flight, whereas in a turn a deflection is required to create a moment. :? Graphically, the control surface vectors on dwight's illustration would be 'longer' in the direction of the turn.
Even so, it seems to me that it would be easier to create a larger moment arm with a canard, while the wings control surfaces and the canard are further separated and can work together, while a tail is rather closely coupled to the wing. And so my reasoning went that instant turn performance IS improved (which you seem to admit) and the rest of my argument for canards is still valid. Afterall, the Eurofighter IS reported to have excellent agility in this respect (rather than sustained turns) and it would just make sense to me that the canards have something to do with this, especially since this isn't a delta-wing's forte...

You seem to be very knowledgeable as far as aircraft controls go, so I don't mean to argue or undermine your knowledge in that respect, I'm just trying to understand the issue better myself :whistle:
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dwightlooi

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Unread post23 Jan 2008, 16:10

sprstdlyscottsmn wrote:For a combat aircraft there are advantages to all configurations. Canards can be used as brakes better than a tail. Weather you pitch a tailplane up or down it will try to lift one set of wheels off the ground while a down-pitched canard would just push down into the ground. to make an unstable canard to me seems to be foolish, really. It takes away all of the benefit of canards!! Are the figures for the F-35 based on planform shape and main wheels? if so... I had no idea its pitching moment arm was going to be so large, and it has large surfaces. I notice that there are no canarded "supermaneuvering" aircraft. Yes the Su-35/37 HAS canards, but it already had sufficiant supermaneuver pitch with its large tails and they added TVC as well. I guess then Tails allow for greater non aerodynamic maneuvering (pitch moment based, or post stall). Now I know its all still aerodynamics but I think of aerodynamic turning as a "sustained" Cl pulling the whole aircraft around, not a pitch moment exceeding the AOA limits.


This video shows the F-35 speed brake deployment.

http://www.jsf.mil/video/f35test/AA-1%2 ... e_high.wmv
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dwightlooi

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Unread post23 Jan 2008, 16:21

AeroG33k wrote:I think this would be the case when initiating a climb or a small course change when all you need is pitching the lift vector, but not in a hard sustained turn or loop where you'd keep pulling the stick back.The whole point is to generate a moment to turn throughout the entire turn. Without a moment, there is no turn and in dwight's illustration, it seems to be that the moments cancel each other around the Cg for level flight, whereas in a turn a deflection is required to create a moment. :? Graphically, the control surface vectors on dwight's illustration would be 'longer' in the direction of the turn.


With zero trim on the tail an unstable wing-tail will continue to turn but probably not at the maximum rate.

In anycase, the argument about sustained turn performance shouldn't be between whether the lift (or the lack thereof) from a tail during a turn compared to a canard. It really should be between canard based high AoA lift enhancement, and that from the LEXes and body shaping of non-canard designs.
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Unread post23 Jan 2008, 16:29

dwightlooi
Where did you get the illustrations with the CG?
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dwightlooi

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Unread post23 Jan 2008, 16:32

ACMIguy wrote:dwightlooi
Where did you get the illustrations with the CG?


I took a bunch of side view illustrations of appropriate aircrafts, put them on the same picture and drew the arrows. The moments are NOT supposed to be accurate, they are only there to show what's ahead of what and whats going up or down.
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Pilotasso

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Unread post23 Jan 2008, 16:49

dwightlooi wrote:
One of the traditional advantages to a canard layout is that in the canard-wing layout both the canard and wing are lift producing entities and this promotes better lift-drag performance. However, this advantage is reversed with unstable designs. With unstable wing-tails being lift-lift arrangements, whereas unstable canards are downforce-lift arrangements. The following picture illustrates this...

Image


Theres a subtile snag that makes you mistaken about this.
The part in bold is only true in stable canard designs, i.e. Manualy actuated aircraft.
However in the typhoon since design is unstable in the sense that the aircraft wants to pitch all the time means in a turn the canards direction and intensity of force varies with time accordingly to the pilots demands and the dynamic situation of the aircraft. In adition to that the downforce necessay to keep the aircraft leveled is relatively reduced. The arm moment is large thus the force necessary is smaller, similarly like a lever.

A typhoon making a hard turn will have its cannards ease off the downforce, or even use lift.

Most tail elevators are located nearer to the centroid of lift wich in turn is behind the CG. That necessitates a larger downforce that subtarts that of the wings.


Comparing both concepts, the tail configuration aircraft will aquire higher angles of attack than a given aircraft with a similar wing loading.


Similarly to the typhoon, the F-16 albeit with tail elevators doesnt mean they are producing downforce all the time. During pitch for example in a take off, the elevators direction of force varies from down to up lift several times per second. That happens because the planes got the CG behind the lift centroid making it unstable but in a different way the typhoon is.

An f-16 or F-35 during a hard turn will have both downforce at the moment of initial pitch and up force once the aircraft reaches the desired AOA for the main wings to produce the necessary lift. when that hapens the elevators literaly hold the aircraft heavy a$$. A very different efect than that of the typhoon.

did this made sense to you?

You can conpensate the disavantages of tail planes with a lower wing loading but that also requires more thrust due to extra drag. During the turn that extra drag is ofset by smaller AOA induced drag. But for the lobgest part of the mission the plane will spend more fuel.

Usualy canard unstable deltas have more vicious spin charestics that are harder to defeat by design. That is in my opinion why US has stayed away from it since they can conpensate that with a large wing and the most powerfull engines in the world.
Last edited by Pilotasso on 23 Jan 2008, 17:27, edited 1 time in total.
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Unread post23 Jan 2008, 17:15

It's simplistic to consider canard-versus-tail-aft separately from the design of the aircraft. It's actually secondary to the overall goals of the aircraft design.
For instance, on the Euro-Canards, a major starting point was a delta wing. Light, low supersonic drag, lots of space for weapons (being stiffer than a trapezoid). Bad news (classic Mirage, F-106) is that it;s not very agile if naturally stable, partly because of high sweep and partly because there's only so much room for control surfaces for pitch and roll.
Now look at the airplane. Where can I add a pitch control surface? At the back? Short coupled, small moment, big surfaces even if I stick them on booms (F-22). Ah, nice long nose. Put them there. Robust FBW to avoid divergence.
Most of the time, though, I'm not carrying a lot of load on the canard, up or down, nor do I want to. The control surfaces are shorter than the wings and don't produce lift as efficiently. The only time that really applies is on take-off and landing, where I have all the power I need and more, I don't care about L/D so much as maximum lift coefficient, so I crank that canard to rotate my wing to the high alpha that a delta likes. That's why Gripen and 'phoon are respectable STOL performers and Rafale can land on a carrier.
So the "efficiency" argument of carrying an upload on the canard, versus a download on the tail, is a ...errrm.... canard.
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Unread post23 Jan 2008, 17:35

Actualy no, you cannot reduce the advantages and disavantages of both designs like that...

Firstly, because a mirage 5 is hugely different from a typhoon, gripen or Rafale.
The robust FBW system for the deltas divergence phenomena is aplied in the mirage 2000, it has no cannards and answers the problem in a different way.

Secondly, because a delta provides the best combo of high lift and low drag, for both high speeds and less fuel usage at a given speed (range for the same amount of fuel).

In a knife fight, the tail configs will have higher AOA, wich could lead to nose up snapshots, but the deltas will have less AOA and thus less induced drag.
That explains why the gripen can match an F-16 in a turn with a less trust to witgh ratio. And I bet it has more usefull lift too.
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johnwill

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Unread post23 Jan 2008, 18:28

AeroG33k wrote:
I think this would be the case when initiating a climb or a small course change when all you need is pitching the lift vector, but not in a hard sustained turn or loop where you'd keep pulling the stick back.The whole point is to generate a moment to turn throughout the entire turn. Without a moment, there is no turn and in dwight's illustration, it seems to be that the moments cancel each other around the Cg for level flight, whereas in a turn a deflection is required to create a moment. Confused Graphically, the control surface vectors on dwight's illustration would be 'longer' in the direction of the turn.
Even so, it seems to me that it would be easier to create a larger moment arm with a canard, while the wings control surfaces and the canard are further separated and can work together, while a tail is rather closely coupled to the wing. And so my reasoning went that instant turn performance IS improved (which you seem to admit) and the rest of my argument for canards is still valid. Afterall, the Eurofighter IS reported to have excellent agility in this respect (rather than sustained turns) and it would just make sense to me that the canards have something to do with this, especially since this isn't a delta-wing's forte...

You seem to be very knowledgeable as far as aircraft controls go, so I don't mean to argue or undermine your knowledge in that respect, I'm just trying to understand the issue better myself Whistle


First, I take no offense at any disagreement we may have. I have 40 years experience as an aero engineer, but I'm still learning things too.

Because the airplane is unstable, most of the pitch moment to turn the airplane comes from wing/fuselage lift acting forward of the cg, not the canard or tail. The canard and the tail both act to prevent too much pitch moment from the wing/fuselage lift. The canard must do it with down force and the tail must do it with up force. During abrupt pitch commands, where maximum pitch acceleration is needed, the canard will indeed be uploaded and the tail downloaded. But max pitch acceleration occurs at medium g (4-6), so at high g, the canard is again down loaded and the tail uploaded.

At a constant g, whether it is 1g or 9g, the total pitch moment is ZERO, so Dwight's diagrams apply to all constant g conditions.

I think if you look carefully you will see the F-35 tail moment arm is essentially the same as the Typhoon canard arm. The tails are MUCH larger than the canard, so have more pitch moment capability. However, that is not the whole story, as I suspect the Typhoon also uses wing trailing edge surfaces for pitch command. Working together, the canard and TE surfaces will have very powerful pitch command capability. With the TE loaded up (like a tail), the down load on the canard can be reduced or reversed.

You need to be careful about terminology (and it is confusing). When you say instant turn performance, I think you really mean g onset rate or pitch acceleration. Instant (or more correctly instantaneous) turn rate or g means the max available at a given mach/altitude/gross weight, allowing speed and altitude to decrease. Sustained turn rate or g means the same thing except speed and altitude must remain constant.

It should be obvious to everyone that a canard or tail can get the job done, and that one is not better than the other in all aspects.
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Unread post23 Jan 2008, 18:55

johnwill
I have a question, using the F-22 design as an example, would a canard and tail combo act the same as tail with vectored thrust?
The vectored thrust changes the dynamics of this discussion but is also very expensive.
The canard-tail combo may be a low cost way to achieve the same affect.
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