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Old 07-10-2013, 05:11 AM   #1
mclarkson
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Default Canard CG - can it be out from under the wing?

I'm playing around with a little scratch built canard-equipped pusher.

When I use the Canard CG Calculator, it tells me the CG should be way in front of the main wing's LE.

Is that ... right? Will it be unflyable? Have I just punched in the wrong numbers?


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Old 07-10-2013, 09:14 AM   #2
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It's perfectly normal. I have a couple of simple canards and both balance between the wing and foreplane. If you're worried about it just make a "scale" chuck glider out of foam/wood and try it .

Steve
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Old 07-10-2013, 09:21 AM   #3
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Yep, it depends on the size of the canard but typically the CG will be some way ahead of the main wing LE. The larger the canard the further forward the CG goes.

Canards are one good reason why the old 'rules of thumb' for setting CG are dangerous.
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Old 07-10-2013, 10:37 AM   #4
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Thanks, guys. I appreciate the help. I ran it through two different canard CG calculators and they both gave me the same result but ... it seemed weird not to have the CG sitting under the wing.

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Old 07-10-2013, 10:49 AM   #5
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Originally Posted by mclarkson View Post
Thanks, guys. I appreciate the help. I ran it through two different canard CG calculators and they both gave me the same result but ... it seemed weird not to have the CG sitting under the wing.
It Seems more Weird to me Flying a Air plane backwards

I may be getting Older, But I Refuse to grow Up I am Having to much Fun to Grow Up LOL
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Old 07-10-2013, 02:09 PM   #6
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To be technically correct, if the CG isn't under the wing it's a tandem-wing arrangement, not a wing-canard arrangement.

And of course, technically correct is the best kind of correct.

That probably isn't useful info, but recalling that all rotations (pitch, roll, yaw) occur about the CG might explain some behaviors you might notice.

Ask me why your DX5e is doomed... and how to fix it.
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Old 07-10-2013, 04:07 PM   #7
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Originally Posted by CNY_Dave View Post
To be technically correct, if the CG isn't under the wing it's a tandem-wing arrangement, not a wing-canard arrangement.
I don't think that's necessarily true, most if not all canards that have straight wings will have the CG ahead of the main wing. It would be very hard to design a straight wing canard plane that had it's CG on the main wing, the canard would have to be tiny.

'Tandem wing' is when both wings are similar in size, but I don't think there is a precise definition of exactly 'how similar' they have to be.

Some make the mistake of calling the canard foreplane a 'stabiliser', it's not. The canard foreplane itself is highly unstable, it's the main wing on a canard that acts as a stabiliser.
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Old 07-10-2013, 05:33 PM   #8
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Many Canards will have the CG forward of the main wing leading edge.

The method for finding proper CG of a canard is the same as used for tandem wing and in a way a canard is a tandem because the small forward surface is a lifting surface.

CG is determined by a "balance beam" calculation to put the same wing loading on the main wing and the forward surface.
Use the areas as if they were weight and the 25% MAC of each as the locations of the weights... where in the distance between will they balance?
Fore-plane * A = Main area * B
A + B = (distance between 25% MAC points)
Find A and B
CG is A behind 25% MAC of fore-plane = B forward of 25% MAC of main wing.

The fore-plane and main wing stabilize each other. (same as a tandem wing)

You can also use a lifting tail on a "conventional" (big main wing, small horizontal stab at the rear) configuration aircraft. This used to be very common in Free Flight. (Might still be common)
Any time the CG is behind center of lift for the main wing on a conventional aircraft your tail is lifting. If the tail drops as airspeed goes down then you are using a lifting tail. (common with 3D aerobatic models to push the CG back this far)
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Old 07-10-2013, 07:11 PM   #9
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Originally Posted by fhhuber View Post
The fore-plane and main wing stabilize each other. (same as a tandem wing)

I know I'm nit picking so apologies in advance, but this isn't technically correct.

It's the rear surface that makes the plane 'stable' the front surface gives it 'trim'. Stability and trim are separate things.

If you take the canard off the plane will still be very 'stable'. It wont fly but it will be stable in the same way as a dart or bomb is 'stable'. i.e. it will return to a state of equilibrium pointing in it's direction of motion (going straight down!)

The canard foreplane adds 'trim' so that the plane will actually fly, but it reduces stability not increase it. As proof all you have to do is increase the size of the canard sufficiently and the plane will become wildly unstable.

Sorry again for the blatant pedantry
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Old 07-10-2013, 09:11 PM   #10
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To add to the mix, the canard also does not have to be lifting.

Ask me why your DX5e is doomed... and how to fix it.
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Old 07-10-2013, 10:56 PM   #11
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Originally Posted by CNY_Dave View Post
To add to the mix, the canard also does not have to be lifting.
I seem to be disagreeing with everything in this thread

The canard must produce positive (upward) lift. If fact the canard must in normal flight operate at a higher coefficient of lift compared to the main wing.

From the book 'mechanics of flight':


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Old 07-10-2013, 11:05 PM   #12
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That would be accurate if it said "If the aircraft is designed such that the canard carries some weight", that is usually the case but there is no physical reason it must be so.

It might be getting into the area of fly-by-wire only regarding stability, but I don't think there's an aerodynamic 'must'.

I'll concede for non-fly-by-wire my statement should probably read 'the canard does not have to carry a significant percentage of the weight'.

Ask me why your DX5e is doomed... and how to fix it.
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Old 07-10-2013, 11:22 PM   #13
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The first line states
'the canard must always support a proportion of the aircraft weight'
this that is perfectly correct, and there is a very good reason why it must be so.. that reason being that the plane would not fly if it were otherwise..

The only way a canard forplane could be trimmed to not carry any weight would be if the CG was way back behind the 1/4 chord point of the wing. Set like that the plane would be massively unstable. It would be so unstable as to be totally un-flyable, probably un-flyable even with the most advanced artificial stability systems because even the most advanced of systems can only react fast enough to cope with quite small degrees of instability.

On any stable canard (i.e. flyable by a human without computers) the foreplane MUST actually carry more than it's 'fair share' of weight based on it's area compared to wing area.
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Old 07-10-2013, 11:33 PM   #14
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Now y'all got me nervous about my canard aspect ratios and such!

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Old 07-10-2013, 11:42 PM   #15
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I think the Eurofighter Typhoon has the center-of-lift ahead of the CG when subsonic (oh those delta wings...), meaning subsonic the canards must produce negative lift.

The Control-loop lag may be why the canards on that plane are further forwards than 'normal'. Even the modern thousands-of-adjustments-per-second might not have been fast enough.

Ask me why your DX5e is doomed... and how to fix it.
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Old 07-11-2013, 08:34 AM   #16
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Originally Posted by mclarkson View Post
Now y'all got me nervous about my canard aspect ratios and such!
Don't worry about all the people getting picky about obscure definitions and full size fast jets with multiple computers controlling them. You aren't building one of them .

As I said earlier just make a small-scale glider with wing and foreplane shapes you want and give it a chuck. It WILL fly once you get the CG roughly right. Oh and you'll probably want a bit of positive incidence on the foreplane to make sure it lifts .

Steve
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