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ineon

Everybody uses canards

Question

Ok, so that was probably a bit of an overstatement to say that absolutely everybody uses canards all the time, but it still got me wondering...

I had never heard of canards before playing KSP. I might have seen a plane with canards IRL, but I don't recall it if I have. Most planes that I see do not have have them (although this might be a biased sample as the majority of planes are passenger airlines, and have a very specific purpose and therefore a very specific design). If I were to draw a 'typical' plane then it would have wings and a tail with elevators, and I might even be able to stretch to a delta-wing with elevons. I'm not saying that they don't exist IRL, but they are just a lot less common (to me)

So why is it this way? Probably about 30% of atmoplanes and spaceplances I see on this forum have canards. (Why) are canards better for KSP than in real life? Or to turn the question around, why don't we see as many planes IRL with canards?

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So, first of all, the planes you've seen aren't spaceplanes.  Because there's no such thing as a spaceplane IRL, yet.  :)

If you're just building a low-speed atmospheric plane in KSP, you can get by just fine without canards; a typical wings-and-tail arrangement works just fine.

The reason you tend to see a lot of canards on KSP spaceplanes is simple:

  • The mass tends to get concentrated at the back of the plane (because that's where the engines are, and orbit-capable spaceplanes need a lot of engine power, and engines are heavy)
  • This means that the wing area needs to be concentrated back there, too (to keep the CoL behind the CoM)
  • This means that control surfaces (ailerons, elevators) on the wings (and tail, if there is one) have very little lever arm to work with (they're close to the CoM)
  • Which in turn means that they have very little pitch authority, so control becomes a problem

Putting canards way up at the front of the plane, they have a big lever arm (they're way out in front of the CoM), so they have a control authority all out of proportion to their size.

Edited by Snark

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IRL canard designs are a lot less stable than a rear tailplane, hence why the majority of aircraft you see with them are fighters (Typhoon, Grippen, etc) which are designed to be inherently unstable.

With a conventional tailplane you have the CoM in front of the CoL of the wing, and the tail provides downforce.  If a gust cause you increase pitch the wing generates more lift (higher angle of attack), but the tail generates less downforce (reduced angle of attack because it's at negative pitch to start with), and the aircraft pitches back down.  The opposite happens with a gust that pitches you down, so you end up with a very stable aircraft.  Great for airliners where you don't want the self loading cargo spilling their gin and tonic.

With a canard design both the wing and canard provide lift, so in the above pitch increase/decrease example you have a problem.  For stable aircraft you design the canard to have a very high wing loading so it will stall way before the main wing, and pitch the aircraft back down again.  For military aircraft you have computers constantly adjusting the canard position and they're unflyable if you lose the computer.

In KSP I suspect the aero model isn't detailed enough to show such differences, and canards look cool

ETA:  There's also some issues about aerodynamic efficiency.  In straight and level flight a canard is more efficient because it's generating lift, rather than a tail generating downforce that ahs to be offset by extra wing lift, however in other flight conditions it can be less efficient, eg use of flaps on the wing would increase the loading on the canard, and because the canard has to stall before the wing you never load the wing as much as you could on a conventional aircraft, so again for a passenger aircraft (nice low landing speed being useful) they're not a great idea, but for a space plane with a long runway it might make more sense.

Edited by RizzoTheRat

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There is nothing inherently less stable about a canard design.   It's all about centre of lift vs centre of mass, and choosing an airfoil section for the canard that looses lift at a lower angle of attack than the main wing, so that the nose pitches down to avoid a stall.

Given that both canards and tailplaned aircraft can be designed to be stable,  the one that will be chosen is the most efficient design.   For most transport/utility aircraft, that is the tailplane.

The main reason is that the wing is operating in clean air, that has not been disturbed by passing over another lift surface.  Same reason that monoplanes won out over biplanes and tandem wings.

Also, in level flight, at relatively high speed, making only enough lift to support the plane's weight, the aircraft flies with a comparatively small nose-up angle. There is little nose up force required so the canard won't be providing much assistance to the main wing.

Supersonic fighters, might benefit from a canard design.  

The first is because of the bow shock generated at supersonic speeds.  Keeping the wing tips inside the bow shock means mounting them as far back as possible, with heavy components to the rear and a very short tail movement arm.

The next is that fighters and space planes aren't optimised solely for cruise economy.  They may be pulling hard maneuvers which means generating many times the aircraft's weight in lift , space planes may only be shooting for 1g but are trying to operate in absurdly thin air at ridiculously high altitude.  Both cases mean pulling the nose up hard and having the canard contributing to total lift as opposed to a tailplane having to generate a very high downforce to compensate for the short movement arm can be an advantage.

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My theory is that it is because of differences in mass distribution of the various parts of an aircraft between RL and KSP.

KSP planes, especially craft made to look like they go fast, tend to be slightly more nose heavy than their RL counterparts. So an easy fix is to put canards on it, to move center of lift forward, too.

But even so, CoM still tend to be closer to the tail than to the nose, and you simply get more control authority by having the control surfaces further from CoM.

Another reason could be the tendency of placing the main landing gear too far behind CoM, making it very hard to lift the nose off the runway. Canards will help a lot more in that situation, again because they are further from the pivot point (landing gear in this case).

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Personally I use canards as a cheat. In kerbal, the CoM is generally much further back then what it would be IRL because of the way the engines are made. (in real life engines on most small jet fighters are half the length of the plane rather then just a short module at the rear)

And with the mass so far back, it is hard to get any leverage from rear mounted tailplanes. So canards are the next best option, and they do work pretty good.

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The (hypothetical/future) Skylon includes them, which may influence some player designs. Also, I'm not sure how valid this is, but I've seen an argument that if most of your pitching is upwards (say, because you're trying to get to orbit), then it would make sense to use an upward force (thus at the front) rather than a downward one (at the back).

Skylon_diagram.jpg

 

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Canards are used a lot in ksp for

  • looks
  • balancing of lift
  • manuvering

Because canards are in a spot to add agility to a craft, IRL planes that have them tend to have fly by wire etc.

For instance the Dassault Rafale

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22 hours ago, ineon said:

Ok, so that was probably a bit of an overstatement to say that absolutely everybody uses canards all the time, but it still got me wondering...

I had never heard of canards before playing KSP. I might have seen a plane with canards IRL, but I don't recall it if I have.

There are a few modern fighter aircraft that use them - SAAB Viggen, some Russian SU-30/35s and the Eurofighter are probably the most widely known - but there's also a few versions of the Mirage and the Chinese J10. .  The XB70 Valkyrie bomber also had them, but that wasn't a production aircraft. 

According to Wikipedia, the Wright brothers Flyer also used them - news to me - so they've been out there a while.

Wemb

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37 minutes ago, Wemb said:

According to Wikipedia, the Wright brothers Flyer also used them - news to me - so they've been out there a while.

If you define a canard as a pitch control surface in front of the lifting wing, then yes, the Wrights did use canards. But I don't think they called them that. As far as I know, they called them elevators. The name "canard" appears to have come from a French airplane (as one would expect, it being a French word) in 1906.

Edited by mikegarrison

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Outside military aircraft (ie designed for agility), the only relatively modern designs  I can think of with Canards are the Long-EZ and Vari-EZ, Beech Starship, Piaggia Avanti and the TU-144.  And the first 3 were all designed by the same bloke, Burt Rutan who was also behind the design of Star Ship 1, White Knight and Global Flyer, so known for slightly non traditional designs.

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For commercial aircraft. There was the ill fated  Tupolev Tu-144 . Where they help balance out the heavy nose if landing speed was too low. Although the was not a commercial success next to Concord. Ironically it would later be used by NASA to do basic testing of a space shuttle concept.  

NASA did extensive testing of further applications. There is an advantage in certain situations. For those that would like to study the matter further I can provide the link to some of the results. http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19870013196.pdf  

 

Edited by nobodyhasthis2

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18 minutes ago, AeroGav said:

There is nothing inherently less stable about a canard design.

Really?  I would have said that any time you have an airfoil surface in front of the center of mass, then that's a recipe for instability.  Any time you have any AoA other than zero, it's generating a torque away from prograde, and that force gets bigger as the AoA increases.  That's pretty much the definition of instability.

This doesn't necessarily mean that "if you have a canard then your ship is unstable", because you could have other surfaces elsewhere that generate enough stability to offset the canard's instability.  But the fact remains, the canard is a source of instability.  It's just a question of whether it's a big enough source to cause a problem for the overall stability of the craft.

If the canard is steerable, then that can offset the instability effect-- it can use steering to compensate.  But there are two problems there.  First, it can only limit the instability effect within the deflection range of the canard; as soon as AoA gets bigger than the max deflection range, you get the instability.  Second, to be able to steer that way generally requires active stability management at a rate far higher than human reflexes can keep up with, so you need a computer controlling them.  (Which they do, on the modern fighter designs that have canards, and which KSP does when you have SAS turned on.)

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2 minutes ago, Snark said:

Really?  I would have said that any time you have an airfoil surface in front of the center of mass, then that's a recipe for instability.  Any time you have any AoA other than zero, it's generating a torque away from prograde, and that force gets bigger as the AoA increases.  That's pretty much the definition of instability.

 

I think you need to compare like with like.   Two aircraft with the centre of lift the same distance behind the centre of gravity, one as a tailplane, one as a canard.

You are right, the canard is ahead of the CG and is a source of instability.  But the wing in this design will be behind the CG, so the main wing is contributing to stability.

In the conventional layout, yes the tailplane will be behind CG and contributing to stability.   But in order to have the CoL in the same place, the wing will have been moved slightly ahead of CG, so the WING will be creating the instability.

The only thing that matters is CoL relative to CG.

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I know there were claims (from the Smithsonian exhibit where I saw it) that the X-31 was unstable due to putting the control surfaces in front of the wings (canards), and it was only kept stable by 3 computers on board.  Presumably the Rutan Varieze  (that now is exhibited in the Uvar-Hazy center) was buried too deep and nobody noticed that it used its canards as controls (without computers).

My guess is that the aero model still isn't "good enough" and that we typically design unstable aircraft just to get sufficient control.  No idea how different "sufficient control" is in real life vs. KSP (third person makes flying harder).

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12 hours ago, AeroGav said:

I think you need to compare like with like.   Two aircraft with the centre of lift the same distance behind the centre of gravity, one as a tailplane, one as a canard.

You are right, the canard is ahead of the CG and is a source of instability.  But the wing in this design will be behind the CG, so the main wing is contributing to stability.

In the conventional layout, yes the tailplane will be behind CG and contributing to stability.   But in order to have the CoL in the same place, the wing will have been moved slightly ahead of CG, so the WING will be creating the instability.

The only thing that matters is CoL relative to CG.

Absolutely.  That's exactly what I said in my post,

12 hours ago, Snark said:

This doesn't necessarily mean that "if you have a canard then your ship is unstable", because you could have other surfaces elsewhere that generate enough stability to offset the canard's instability.  But the fact remains, the canard is a source of instability.  It's just a question of whether it's a big enough source to cause a problem for the overall stability of the craft.

Burt Rutan has produced plenty of designs with canards and they fly just fine.

But the fact remains, a canard is a source of instability.  If you add them, it means you have to expend more design effort elsewhere in order to keep the overall craft stable, which means more engineering challenges.

Lots of KSP players have trouble getting their planes to fly in a stable fashion.  Old hands have no problems with this-- I'm sure you could design a stable spaceplane in your sleep.  :)  But the fact remains that in KSP, stability can be a tricky thing, and for players whose stability-design skills are marginal, canards can be tricky-- they raise the bar for the rest of the craft.

And canards are especially tricky if you don't really grok how they work and what the potential issues are.  They can be your best friend or your worst enemy.  There are some designs that cry out for "put a canard on the front" (this is my answer for lots of people trying to build shuttles that look exactly like NASA's, who don't understand why it's horribly unstable on reentry, because they've got the CoM way in the back right next to all their control surfaces).  There are plenty of other designs where "gosh, this is having problems, maybe if I stick a couple of canards on there" is the worst possible thing to do.  They are a source of instability, which is why you have to know what you're doing when you're using them.

As compared to control surfaces on the tail, which are pretty darn close to idiot-proof.  "Stick fins on the back" is pretty safe advice; it's hard to go wrong with that.  :)

Edited by Snark

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55 minutes ago, Snark said:

 

  "Stick fins on the back" is pretty safe advice; it's hard to go wrong with that.  :)

Of course,  if you can find any room left at the back of the plane by all means add a lifting surface.  I still prefer my controls at the front, but passive fins on the back are fine , if there's room (in my designs, there never is).

Assuming the player knows how to bring up the yellow and blue balls though,  i think this is where he should concentrate.  Too many try to emulate Youtube tutorials where some expert player knocks up a design that without any adjusting has the blue marker one pixel behind the yellow one and says "ok we'd normally adjust the position of the wings at this point, but that's perfect so let's go".

In fact so long as you aren't running out of pitch authority or getting excessive trim drag I think you should be playing it much more safe in a space plane.  Sure , it won't win any agility awards but you're getting massive CG shifts as the fuel burns off and you offload the cargo.

Also look at TacFuel balancer and think long and hard about where you put the cargo bay.

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3 hours ago, RizzoTheRat said:

Outside military aircraft (ie designed for agility), the only relatively modern designs  I can think of ...  and the TU-144.  And the first 3 were all designed by the same bloke, Burt Rutan who was also behind the design of Star Ship 1, White Knight and Global Flyer, so known for slightly non traditional designs.

And the latter was designed by BAC and Sud Aviation   - apart from the canards - I'd forgotten that Concordski had those. I assume they were Tupolev's contribution to the design.  :-)

Wemb

Edited by Wemb

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Yeah, their wing wasn't anywhere near as good so needed extra lift at the front for landing, the folded away for normal flight.

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16 hours ago, Snark said:

If the canard is steerable, then that can offset the instability effect-- it can use steering to compensate.  But there are two problems there.  First, it can only limit the instability effect within the deflection range of the canard; as soon as AoA gets bigger than the max deflection range, you get the instability. 

Hence the maximum deflection on them tends to be huge

220px-Typhoon_f2_zj910_canard_arp.jpg

A Canard doesn't automatically mean it's unstable, but you need to be a lot more careful with the design to ensure it's not, AeroGav's spot on with his comment above about canard aerofoil design. If you look at non military designs the canards are all a very high aspect (long and narrow) design, the idea being that the canard gains lift more slowly than the wing with increasing angle of attack.  Plus as mentioned before they should stall before the wing so you'll never get a catastrophic pitch up.

The other big difference in KSP for real life is we tend to hang engines on the back.  In fighters the engines tend to be around the CoM, and the reheat system behind is relatively light in comparison.

Edited by RizzoTheRat

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7 minutes ago, RizzoTheRat said:

Hence the maximum deflection on them tends to be huge

220px-Typhoon_f2_zj910_canard_arp.jpg

 

That degree of deflection isn't used in flight, it's only there for braking and so that the pilot can get in and out without the canard surface being in the way.

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Depends on the aircraft, not sure what sort of angle of attack the Typhoon hits safely, but check out the canard angle on the X-31, I believe that managed stable flight at over 60 degrees AoA, and even barrel rolled it at over 45 degrees AoA which must really mess up the pilots sense of direction, although obviously he's cheating slightly as it's got vectored thrust too.

520px-X-31_at_High_Angle_of_Attack.jpg

Edited by RizzoTheRat

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First class liners may have Canard IN plane

I don't use canard on plane

canard-colvert.jpg

I prefer them on a plate !

recette-e22256-canard-gras-dans-tous-ses

 

This discussion about Canard made mu hungry :D

(off topic but irresistible !)

Edited by Warzouz

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On 2016. 01. 26. at 4:57 PM, Snark said:

If you're just building a low-speed atmospheric plane in KSP, you can get by just fine without canards; a typical wings-and-tail arrangement works just fine.

Silly me... That was an eye opener. As a spaceplane enthusiasts, most aircrafts I saw were well.. spaceplanes. Canard was always a natural choice.

Now it feels so OP delightful to fly tail controlled atmospheric crafts. ^_^

Edited by Evanitis

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On 1/26/2016 at 10:57 AM, Snark said:

So, first of all, the planes you've seen aren't spaceplanes.  Because there's no such thing as a spaceplane IRL, yet.  :)

If you go to the Smithsonian, when you walk in, SpaceShip 1 is to your right and the X-15 is out of sight*, but slightly to your left (and above) and near the escalators.

But IRL spaceplanes only just touch space (yay!), not go into orbit.

* or not (while the layout is pretty clear in my mind, I'm not sure what's blocked).  Usually the other stuff in the entryway commands your attention and you see the X-15 when walking by it on the second floor.

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