Lifting Canards...

[LearningTOfly 0]

For an aircraft utilizing a lifting canard instead of a conventional horizontal stabilizor, is it true that the aerodynamic centre of pressure/ lift must act always behind the centre of gravity in order to maintain stability about the normal axis in all(most) usual attitude manuevers? ...otherwise a gust would trigger a 'goundloop' type maneuver, but in the air.(?)

I'm assuming that that is the reasoning in designs like Rutan's e-z's or the starship to sweep the 'main' wing, but want to confirm with anyone who has a bit more knowledge of the stuff...

[BGill 0]

asphinctersayswhat?

[DrunkMonkey 0]

I like pie. Mmmmm, pie.

[Frenchy68 0]

"Canard" in French means "Duck".
Are you using a lifting duck instead of a conventional horizontal stabilizor?
If so, I think you're really cool, but will never board your freakin' aircraft. Good luck...

"For once you have tasted Absinthe you will walk the earth with your eyes turned towards the gutter, for there you have been and there you will long to return."

[AggieDave 6]

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Are you using a lifting duck instead of a conventional horizontal stabilizor?

Aflak!

Speaking of ducks...its been too long since I've enjoyed a nice freshly killed duck roasted to a nice juicy finish.

--"When I die, may I be surrounded by scattered chrome and burning gasoline."

[Frenchy68 0]

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Speaking of ducks...its been too long since I've enjoyed a nice freshly killed duck roasted to a nice juicy finish.

Still into road kills, uh?

"For once you have tasted Absinthe you will walk the earth with your eyes turned towards the gutter, for there you have been and there you will long to return."

[AggieDave 6]

Road kill duck? Hmmmm...never hit a duck before. I was thinking more of a friend that is really into hunting ducks and seems to always bring back the limit.

--"When I die, may I be surrounded by scattered chrome and burning gasoline."

[Frenchy68 0]

I meant roadkill Finns!

"For once you have tasted Absinthe you will walk the earth with your eyes turned towards the gutter, for there you have been and there you will long to return."

[TomAiello 26]

I'm pretty sure it depends on the specific wing shape used.

Canard (tail in front) ought to generate more total lift, because the "tail" pushes up to balance the inherent rotation of the main wing, rather than down (which it would if placed in front). But as far as CG relative to wing? I dunno, but I think it's going to depend on a lot more complex things than I can think of right now.

-- Tom Aiello

Tom@SnakeRiverBASE.com
SnakeRiverBASE.com

[ryoder 1,590]

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For an aircraft utilizing a lifting canard instead of a conventional horizontal stabilizor, is it true that the aerodynamic centre of pressure/ lift must act always behind the centre of gravity in order to maintain stability about the normal axis in all(most) usual attitude manuevers? ...otherwise a gust would trigger a 'goundloop' type maneuver, but in the air.(?)

I'm assuming that that is the reasoning in designs like Rutan's e-z's or the starship to sweep the 'main' wing, but want to confirm with anyone who has a bit more knowledge of the stuff...

For the a/c as a WHOLE, the CG and the CoL must be colocated during level flight. If the CoL moves aft of the CG, the plane will be pitching nose down. If the CoL moves forward of the CG, the plane will be pitching nose up.

Now since conventional a/c have negative lift generated by the tail, the CG must be forward of the WINGS CoL, to balance against the tails downward push.

In a canard design, the CG is in front of the WINGS CoL, since the canard is lifting. The canard operates at a higher AoA, so it will stall before the wing, forcing the nose to drop in a near stall.

Also note that some conventional LOOKING a/c have a LIFTING tail, (eg F-16). These will by neccessity, be fly-by-wire systems.

"There are only three things of value: younger women, faster airplanes, and bigger crocodiles" - Arthur Jones.

[Salsa_John 0]

in plain english..

Canards can decrease stall speed and variable pitch canards can increase lift.

Generally good

wing tip winglets decrease wingtip vortices

"You did what?!?!"

MUFF #3722, TDSM #72, Orfun #26, Nachos Rodriguez

[pilotdave 0]

I think you're on the right track. I believe it's not the center of pressure/lift, but rather the aerodynamic center of the wing that you're talking about.

I don't believe it would "ground loop" in the air... you'd have an unstable aircraft, but not ncecessarily uncontrollable. What you'd have is a pitching moment that would increase with an increase in angle of attack. A stable plane (in pitch) has a very constant negative pitching moment. At stall the pitching moment should increase like crazy in the negative direction. Otherwise you wouldn't be able to recover from a stall.

There are multiple reasons to sweep the wings on a canard pusher design. One is what you basically said... move the aerodynamic center of the wing farther back. It's gotta go way back because the rear engine gives a far rear CG. Another reason is to put the tails as far back as possible, for yaw stability and control. And it looks freaking cool.
Dave

[VectorBoy 0]

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...otherwise a gust would trigger a 'goundloop' type maneuver, but in the air.(?)

You do know a ground loop is more like pulling a donut or a spin in a race car than actually looping an aircraft right?

Canards fly at higher wing loadings and stall before the main wing. The crafts center of gravity is well aft of the canard and slightly forward on the main wing. Hope that helps.

[pilotdave 0]

A ground loop is caused by instability of a tail dragger on the ground because the CG is behind the main landing gear. He was comparing that to what might happen if the CG was out of whack in a canard aircraft. Just like a bump on the ground could suddenly cause the tail to swing around, a bump in the air could cause the same thing, though along the lateral axis. It was just an analogy.

In this case I don't agree... but picture a plane with the vertical tail mounted ahead of the CG.... what would happen when distubed in yaw?

[freakbrother 0]

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For an aircraft utilizing a lifting canard instead of a conventional horizontal stabilizor, is it true that the aerodynamic centre of pressure/ lift must act always behind the centre of gravity in order to maintain stability about the normal axis in all(most) usual attitude manuevers? ...otherwise a gust would trigger a 'goundloop' type maneuver, but in the air.(?)

I'm assuming that that is the reasoning in designs like Rutan's e-z's or the starship to sweep the 'main' wing, but want to confirm with anyone who has a bit more knowledge of the stuff...

The center of lift and aerodynamic center (AC) are not the same thing. The Cof L moves around depending on control inputs, and coincides with the CG if the aircraft has no angular acceleration in pitch. The AC is the point at which pitching moment coefficiants are invariant and (for the entire aircraft)needs to be behind the CG for stability. On a subsonic airfoils the AC is at 25% of the mean chord. The aircraft AC is found by weighted average of the contributions of the various components (wing, tail, canard, fuselage even).

.
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www.freak-brother.com

[flyingferret 0]

Canucks don't lift...they play hockey

--
All the flaming and trolls of wreck dot with a pretty GUI.

[LearningTOfly 0]

Thanks for the helpful input everyone... your info plus a bit more I've read has cleared things up for me... I was thinking that the center of lift for the wings was somehow related to the centre of pressure of the fuse... which it isn't, after all. Thanks again...

[VectorBoy 0]

but picture a plane with the vertical tail mounted ahead of the CG.... what would happen when distubed in yaw?

[pilotdave 0]

Absolutely not. Rutan's planes never count for stuff like this. They work only by the magic of voodoo curses.

Actually it makes perfect sense... compare the size of the winglets (which are really vertical tails on the wingtips, not the same as standard winglets) with the size of that nose rudder. The rudder disturbs the aircraft in yaw, but the aircraft still has yaw stability due to the vertical tails mounted well aft of the CG. (ie CG is ahead of the aerodynamic center).

A perfect example of that "ground loop" in the air a standard plane flying backwards. Not usually possible, unless the plane is in a tail slide. What happens to a plane in a tail slide that gets disturbed in yaw? CG is now behind the aerodynamic center. It'll flip right over unless the pilot makes constant fine adjustments to the rudder. Unstable, but controllable to a point. As the "angle of attack" on the tail increases (sideslip angle, beta) of a plane in a tail slide, the yaw moment will increase. This is opposite what will happen when flying forward. Increase beta, yaw moment decreases (increases in the opposite direction, really).

Dave