From Dan:
The way i allus understood it was - the canard's forward wing, flying at a higher angle of attack in order to "lift the nose" generates drag equivalent to the conventional's tailplane in "pushing the tail down". In other words, there's 'no free lunch' in designing any airplane to be hands-off stable. Or mebbe i'm out to lunch here. :-) Bill(oc)
snipped-for-privacy@webtv.net (Bill Sheppard) wrote in news: snipped-for-privacy@storefull-3171.bay.webtv.net:
You're not out to lunch, but the canard story is actually worse than that - from a wing design standpoint, it's all bad.
First - big wings are more efficient at generating lift than small wings, so you'd like to have your big wing generating all the lift. With a canard, though, the forward wing has to be operating at a higher Cl (lift coefficient - think 'lift per square foot' in this context, because it's safe to assume that the front and back wings are moving at the same speed through air of the same density) than the aft wing, so you're already paying a penalty.
Second - since the front wing is operating at a higher Cl than the back wing, it follows that the back wing is operating at a lower Cl than the front wing. This means that the front wing will stall well before the back wing gets up to its maximum Cl, so to keep the airplane in the air, you have to fly faster. This fits with the other poster said about long ground roll for canards.
Third - in anything like a vaguely normal layout, the rear wing is flying in air that's been disturbed by the front wing, and that's never good. In a conventional layout, the rear wing is pretty small and we're not asking a lot out of it, so the penalty isn't too bad. With a canard, though, an awful lot of the big wing is flying in disturbed air, and we *are* asking a lot of the rear wing, so the performance penalty isn't trivial.
Of course, such is the nature of engineering that the more you look at a trade, the bigger and more complicated it gets.
First - and to my mind, most important - canards look cool.
Second - a properly-designed canard can be nigh-stallproof. It's a little eerie to watch. On aerobatic planes, of course, this would be terrible, but for more casual use, it's an advantage.
Third - if you have a heavy engine and want to use it as a pusher, it may be impossible to get a conventional layout to balance
Fourth - and related - there's something to be said, aerodynamically, for pusher engines. Tractors have all that prop blast hitting the fuselage, and that can't be good (the whole prop blast thing doesn't apply to jets, of course)
And, of course, of course, structure and visibility issues just complicate the story further.
Canards are more efficient in cruise, which is why Bert Rutan uses them for his spectacular achievements like nonstop round-the-world flights. The conventional layout does impose a drag penalty in creating stability, and Rutan couldn't accept this. Pusher props produce less drag on the fuselage, but pretty much any such advantage is eaten up in the disturbed air the prop has to fly in. The prop blade's AOAs are all messed up by the fuselage's and wings' wake. Canards look cool to some, but Beech couldn't convince the market with its Starship. People still bought King Airs and other similar turboprops, even though they were less efficient. Beech recently offered to buy back all of them, as it's been something of an embarrassment. Note that there are no birds with their tails in front. Canards such as the Long-EZ and Velocity have had a few very unusual accidents involving deep stall-type scenarios. Pilots experimenting with slow flight behavior have managed to get into a nose-high attitude where the front wing is still flying and won't drop the nose, but the main wing is partially stalled and the airplane descends very slowly and remains in this condition no matter what the pilot does with the throttle or flight controls. One of them pancaked into the ocean, power-off, descending at about 300 feet per minute (much slower descent than a normal glide of about 700 fpm) and the pilot was uninjured. The airplane floated (foam and glass) and was recovered and flew again. The pilot did it again, inverted this time, and was killed. The theory seems to be that a spanwise vortex forms along the top of the main wing, creating lots of lift at very low airspeed, and can't be broken away through any fancy maneuvering. Scientists are studying it as a possible new form of STOL flight. The bumblebee uses the same vortex arrangement to create lots of lift with thos tiny wings.
Center-of-pressure movement graph here:
More on balance here:
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