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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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17 minutes ago, wumpus said:

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

And that's what I mean by "spaceplane."  If you can't actually get to orbit, it's not a spaceplane, it's a suborbital plane.  :P

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In real aircraft with the usual wings-and-tailplane layout it's not uncommon for the whole tailplane to be adjustable to trim the aircraft in pitch, especially in tailed supersonic aircraft but also in some large airliners. That's problematic to do in KSP which probably hurts the conventional design. There are a few small all-moving fins but then you can get twitchy controls, and there's no good option for larger planes. Then again stock KSP doesn't feature the "Mach tuck" effect that motivates that powerful pitch trim adjustment. (FAR does, and I often have trouble with planes that can't hold the nose up through transonic speeds).

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Canards on mil fighters are generally close coupled - at least the big ones rather than any damper devices - and there mostly to interact with vortex generation over the main wing. The Tu-144's canards were there because they messed up the aerodynamics and it didn't work as well as Concorde :P but perhaps also because Concorde has ( analog! ) fly-by-wire it didn't need anything else ( if you read a bit about Concorde it apparently got *very* fun if you cut the fbw out in flight ).

Ignoring close-coupled canards, there's lift-generating ones which usually have a fixed area, and pure control ones. Lift generating ones are stable, especially if you make them stall before the main wing can. The advantage of both is all your surfaces are generating positive lift when you're pitching the nose up, which is obviously a good thing for a heavy orbit-bound spaceplane. Pure control surface canards are not so useful because it's very easy to stall them - if you've finely balanced your craft enough that your pitch controls don't have to generate lift in level flight, it won't really make any difference where you put them.

Edited by Van Disaster

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On 1/26/2016 at 7:19 AM, 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. 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?

Canards also make a very handy stepping platform for the pilot when they exit the cockpit.  Without it, they might just accidentally fall off the side or have trouble re-entering the cabin.

 

It's also useful in this instance in the video where you need the extra pitch authority to keep the nose up and the extra lift to make sure you land at a slower speed.

 

Edited by Edax

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33 minutes ago, Ketatrypt said:

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.

Which takes us back to the OP's questions, "what's with everybody using canards all the time", and this is the answer right here.  :) 

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To determine the stability of your plane requires not looking merely at one CoL/CoM matchup, but also to look at how it varies as you pitch up and down (and left and right, but that's rarely a problem). I do that by rotating the root node of my plane. Often I notice that as the angle of attack increases, my center of lift goes forward -- that's a recipe for a dead Kerbal, and requires fixing.

As mentioned up-thread, the key to stability is that you want the canard to gain lift slower than the main wing. So just rotate it up a notch.

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On 1/27/2016 at 4:46 PM, AeroGav said:

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.

Quoted for truth. I flew a RC airplane glider(it could be built to be powered or unpowered, it was built as an unpowered glider for ridge soaring) with a canard when I was little. Back when the controls were analog, and before all these computer stabilized multirotor drones.

Found a youtube video of one:

No computer stabilization

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On Tuesday, January 26, 2016 at 11:05 AM, RizzoTheRat said:

 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.

You realize you inverted that, right?  Lower wing-loading leads to a *lower* minimum landing and takeoff speed, not a higher one.  And canards that are designed to stall mean you have to load the wings more, not less (so the result of canards that are designed to stall leading to higher landing speeds was correct, but the reasoning given was inverted.)

Regards,

Northstar

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On 1/27/2016 at 10:10 AM, 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.

The key in real life is engineers can design the airfoil of a canard such that it will stall before the main wing does, with high confidence. At the angles real wings stall at, the moment from stall drag is minimal compared to that from main wing lift.

We don't have that ability in KSP. Airfoils largely have the same generic properties, and stall behavior is generally nothing to be particularly impressed with.

The main reasons we don't see them in more use in real life are probably because a) the chief advantage - that all wing surfaces provide lift rather than downforce - isn't that advantageous with all the other design options we have at our disposal.

Also, b) we understand very well and are used to wing-tailplane designs, which are practically demanded by single-engine tractor-propeller designs, which were easier to deal with in the early days of the industry. "Pusher" designs, which call for canards more often, demand multiple engines or (for single engines) more structurally complex or multiple tails.

And "design inertia" is a very difficult phenomenon to overcome. Fewer major design changes (like the lift paradigm) result in cheaper design and engineering costs. Which is probably why rear-engine jetliners use T-tails, even though canard configurations are possible with them.

In KSP, we use canards often because we don't have the same kind of fine control over fuel location (CoM) or wing design (lift/stall) that real engineers do.

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On 27/01/2016 at 5:21 PM, 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.


Sorry to drag this back up, but I can't agree with that.

Ordinary, small, non-computer-controlled aircraft do not have their CG behind the main wing. In normal use it is located right on or just in front of the wing's a.c. There is virtually no loading of the elevators at all. And that makes a tailplane design stable, where a canard - and especially a control canard - is not.

At the limits of its loading, or on larger aircraft, CG may well be just behind the aerodynamic centre of the main wing when cruising, but not when flaps are down. At the most critical times of its operation, therefore, stability is guaranteed.

This is also linked with this:

9 hours ago, pincushionman said:

The key in real life is engineers can design the airfoil of a canard such that it will stall before the main wing does, with high confidence.

I can't agree with that reasoning either.
That is the reason why it's possible to make canard designs relatively safe. However, if you are designing a stable plane, the one thing you do NOT want to do is design bits of it to stall in normal flight, ever. Real planes fly in cross-winds and turbulence all the time. If you have any surface that is going to stall, it'll stall unequally as soon as you are not heading straight into undisturbed air. Stalling is therefore bad, particularly at the crucial and dangerous low-speed phases of takeoff and landing. Stalling in flight is also extremely uncomfortable (sudden change in apparent gravity), so while it may be a good solution to dampen controls for RC planes, it isn't if you have anyone on board.

Stalling is only relevant - and a design necessity - to avoid catastrophic loss of control. Yes, you have to design it so that if it does stall, it does so predictably and recoverably (and this is where canard design requires greater care since, as you said, the canard has to stall before the main wing to allow recovery), but in normal (and again, non-computer-controlled) flight it should either provide lift dependably, or not, and not something in-between.

 

So yes, it is certainly possible to make canard designs relatively safe. That doesn't mean that they are anywhere near as naturally stable as a tailplane design.

I don't think design inertia has much to do with the reason why we don't see many canard designs IRL. I think it's mostly because the benefits of the design (less drag, possibly, and certainly greater maneuverability for unstable craft) are vastly outweighed by the design difficulties and other disadvantages (stability, ease of piloting, comfort) for anything other than fighters.

I suspect this will only change with improvements in electrically-powered flight. A small, rear-engined canard design can, it seems, gain slightly higher cruise speeds for the same engine compared to a classic design, but fuel location becomes more of an issue the smaller the aircraft. Therefore, if you get rid of the fuel and are free to locate batteries wherever you like instead (and, perhaps, benefit from a larger wing area for solar power), the canard design starts to look a bit more useful.

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