Comment by AWildC182 | Hacker News Reader

Comment by AWildC182 | Hacker News Reader

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Also interesting is that they settled on what would now be called a canard design with the pitch control surface on the front rather than the tail. This is actually a superior arrangement in most cases as both the wing and the stabliator are both generating positive lift. Most conventional aircraft have the center of gravity in front of the main wing, such that the system is dynamically stable (try throwing a dart backward and you'll see what I mean). To control pitch, they then have control surface in the rear pushing down to keep the nose up. In a canard, the CG is still forward of the wing (and the center of lift) but the control surface helps lift the nose up instead of pushing the tail down improving efficiency. This also has an important safety feature in that you can add a couple degrees to the canard's angle such that it will stall before the main wing. In effect, this means the pilot cannot fully stall the aircraft and put it into a spin, it'll simply mush about, descending in and out of canard stall until the control input is relaxed.

Edit: an excellent example of a modern canard would be the Rutan LongEZ and it's accompanying, rather ridiculous, performance numbers: https://en.wikipedia.org/wiki/Rutan_Long-EZ

The canard design is unstable, which is why the first flight was of such short duration.

Curtiss put canards on his racing airplanes, because he copied the Wrights. On one race, the pilot hit a fence and damaged the canard. He cut off the wreckage and rejoined the race. He then told Curtiss that the airplane flew better and faster without the canard. Curtiss immediately pulled them off of his other airplanes.

It's only unstable if the aircraft is designed incorrectly. You can achieve the same situation with a far-aft CG in a traditional aircraft. It doesn't matter what the setup is, the aircraft will be unstable if the center of lift is near or in front of the center of gravity.

Modern canard designs also use active controls to compensate for the instability. You're right about the CG location relative to the center of pressure, but to make the canard effective it has to be big enough to also make it unstable.

Can you provide any sources on that? The only one I can think of would be military aircraft (ie. the Eurofighter) and they likely have relaxed stability (CoL ~= CG) as a design goal. All the GA examples (Long/VariEZ, the Velocity line, Cozy Mk*, etc.) have straight mechanical linkages and don't have any stability issues when operated correctly.

Here's a paper that mentions the instability of the Wright Flyer: https://www.researchgate.net/publication/315549654_A_STUDY_A...

A bit about active controls for canards: https://en.wikipedia.org/wiki/Canard_(aeronautics)#Computer_...

I hate to do this but the relevant part of the linked section itself has [citation needed] :)

I don't doubt that the Wright Flyer was unstable though. Canards are kinda picky about angle, airfoil, and aspect ratio so if you don't fully understand the system as was the case at the time, it can lead to some problems.

https://authors.library.caltech.edu/21217/

"The paper shows that the 1903 Flyer was readily controllable about all axes but was intrinsically unstable in pitch and roll, and it could barely be stabilized by a skilled pilot."

From the researchgate paper:

> Thus, it did not exceed the stick free neutral point and it is static stable for the entire operating envelope of the aircraft.

It looks like the canard design in the paper is stable, and the blanket statement that canards are unstable is, as yet, unsupported.

Try shooting an arrow with the feathers in the front :-)

Or make a paper airplane and toss it backwards.

Or buy a toy glider and launch it backwards.

They are unstable.

I built a paper and drinking straw canard glider in the 7th grade for a class project. I took Flight Dynamics and Control in college, while obtaining my degree in aerospace engineering. Your misapplied intuition about arrows is not at all convincing. Simply, you do not know anything about the stability of aircraft.

EDIT: WalterBright edited his comment to add examples and commentary after the arrow example. IOW, he added:

> Or make a paper airplane and toss it backwards.

> Or buy a toy glider and launch it backwards.

> They are unstable.

after I replied. His original comment was just:

> Try shooting an arrow with the feathers in the front :-)

A shitty move. I'll only address the "launch it backwards" comment, as the remaining bits are even more idiotic.

A canard airplane is not simply a backwards airplane. The criterion for stable forward motion, in pitch, is that dCm/d_alpha is negative, where Cm is the moment coefficient, and alpha is the angle of attack, and that the center of gravity be ahead of the center of lift. A "reversed" airplane, in either conventional or canard configuration, would have the center of gravity behind (in the new direction of travel) the center of lift, and would therefore be unstable.

Yaw stability is obtained by the vertical stabilizer exerting a moment opposite to the current yaw, by virtue of being behind the center of gravity and center of lift and, when yawed, being angled toward the midline of the aircraft. This is why, on a canard craft, the vertical stabilizer(s) are at the wing tips, or behind the center of lift and center of mass. A reversed aircraft, in WalterBright's imagination, or at his desk--It is difficult to trust anything he says, at this point.-- would have its vertical stabilizer ahead of the centers of lift and mass, and therefore destabilize the aircraft.

These two, that the center of mass is behind of the center of lift in the reversed configuration, and that the vertical stabilizer are ahead of it, are the actual reasons that WalterBright's toy glider, surprise, prefers to fly in the direction for which it was designed.

(He mentions the glider at his desk in his reply to this comment, which I will reproduce here, in case he deletes it: "I have a glider in my office. I launched it backwards. It promptly flipped over and proceeded in the normal direction.")

It would be interesting if, for example, WalterBright could produce a discussion from, say, Aerodynamics, Aeronautics, and Flight Mechanics, by Barnes W. McCormick, for example, warning the unawares aeronautical engineer in training of the dangers of the supposedly unstable canard aircraft.

The discussion above is all elementary flight mechanics, of which WalterBright would be aware if he'd had any training, or any expertise. Contrary to his repetition, his intuition does not suffice, and will not overturn these facts.

I have a glider in my office. I launched it backwards. It promptly flipped over and proceeded in the normal direction.

It would seem that Walter is not so Bright.

Several times now, people have pointed out to you canard general-aviation aircraft with no computer control. How do you reconcile this with your theory?

In the paper I linked to, the wing is moved back, and seems to function as a stabilizer does.

A reasonable explanation:

https://www.boldmethod.com/learn-to-fly/aircraft-systems/can...

Take a look at the planform of the airplane in the paper (figure 2). The wing is moved so far back, it is in the position the horizontal stabilizer is normally found. There it provides enough stability to overwhelm the little surface at the front.

The canard design is most certainly not unstable. I have built model canards almost by accident, from scratch, from first principles. Nor can you simply remove the canard from a properly designed aircraft, any more than you can delete the tail from a Cessna.

Take a wing shaped piece of card or balsa and attempt to fly it by itself. You will find that it rapidly pitches up and stalls, flips over on its back, and repeats the process as it flutters it the ground. You can prevent it from doing this by taping a pole sticking to the front - a pencil, perhaps - as a counterweight to the pitch-up moment. This arrangement flies, but is very difficult to trim and has a tendency to suddenly invert as soon as the angle of attack goes negative. To stabilise it, all you have to do is put a small horizontal surface on the end of the pole. You have made a canard glider.

(If you actually try this, you will also need some vertical stabilisation.)

Taping a pole to a wing makes it an arrow. The bulk of the air surface is behind the CG. The wing becomes the "tail section".

Your argument is that anything with a center of lift behind the center of gravity is an arrow? In that case all aircraft are arrows.

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