Canards vs CG question

[U2Steve]

Help! I know the routines for determining CG on a "conventional" model, but how does having canards affect things? I'm building one of these (it's from a Pat Mattes column here) and I'm wondering if I need to do something special to determine the balance point.

TIA,
Steve

[Ollie]

The same principle applies to all configurations: flying wing, tandem wing, canard or, conventional. Just put the CG at or, up to about 10% ahead of the neutral point depending on the purpose of the plane and the skill of the pilot.

Build a 1/4 scale balsa profile glider of the model in question. Adjust the CG and decalage until the glider goes straight whether you throw it soft or hard. With that set up the CG will be at or very, very near the neutral point.

[U2Steve]

So how does the neutral point relate to the mean aerodynamic chord? Or does it relate at all? All the methods for determining CG I've seen have to find the MAC, then take a fraction of it as the point to set CG.
Is this an aerodynamic neutral point, where air pressure forces balance? How do you determine where it is?
You said the CG is around 10% ahead of NP. 10% of what value?

Sorry for the ignorance. I'm trying to fix that, as we speak! :)

Steve

Edit- when you say adjust decalage, does it matter that the canard is all flying, and is actually my pitch control?

Geez- how far am I in over my head, anyway?

[Ollie]

Steve,
You have been using a rule of thumb that works well only with a conventional configuration of typical proportions. It works because you have been using a combination of tail area and tail moment arm that place the neutral point behind your predetermined CG location.

When you depart from the typical configuration you have to use the universal and more fundamental relationship between the neutral point and the CG.

Consider what happens when you morph a conventional configuration into a canard. The aerodynamic center of the wing is near 25 %of its mean aerodynamic chord. The aerodynamic center of the tail is near 25% of its mean aerodynamic chord. The aerodynamic center of the combination is between the two and is spaced inversely proportional to their respective areas. The wing is the big fat kid on the teeter-totter and is closest to the neutral point, while the tail is the little skinny kid that needs a longer moment arm to balance the wing. So, the neutral point is the aerodynamic center of the wing-tail combination. As we morph by making the wing smaller and the tail larger, the neutral point moves aft so that the product of the area times its moment arm to the neutral point is the same for both the fore wing and the tail (aft wing). For stability, the CG must be ahead of the neutral point, no matter what the configuration is. The distance that the CG is ahead of the neutral point is call the static margin and is a measure of stability. The static margin is commonly expressed as a percent of the mean aerodynamic chord of the larger wing. Static margins in the range of 5 or 10% are usually safe starting points for flight testing

The above description is the main idea but there are refinements that need to be applied for extreme accuracy. For example, the wake of the fore wing and the turbulence of the fuselage reduce the speed of air flow over the aft wing and render its area somewhat less effective. A few airfoils have their aerodynamic centers a percent or two aft of the assumed 25%. At very low reynolds numbers the aerodynamic center can shift some with changes in angle of attack because laminar seperation bubbles in the flow over the airfoil shift with angle of attack, causing a small shift in the airfoil's aerodynamic center. The data to calculate all these variations is often not availabe or hard to calculate.

Start with a generous static margin for safety and fine tune the CG between flight tests. That is a very practical approach that works well for prototype models of unfamiliar configurations.

[U2Steve]

So I think I get it- the neutral point is between the MACs of the canard and wing, but closer to the wing, because it has more area. And if (for instance) the wing was 5 times the area of the canard, the neutral point would be 5 times closer to the wing? That is to say, the moment arm of the canard would be 5 times that of the wing? Izzat right?

Thanks, Ollie. My brain hurts, but thank you.

Steve

[Ollie]

Yes, you have it right.