CG range question

[Mike Switzer]

I managed to make some time to work on my homebuilt design today (for the first time in ages). One of the things I was struggling with was moving things around to narrow the CG range as much as possible.

Which brings up a question. Why is it the Canard type aircraft based on Rutan's designs have approx. a 5 inch allowable CG range, when your average Cessna has a 12" range of allowable CG?

[WLIU]

The answer has to do with the requirement that the canard stall first in every flight condition. And the allowable CG range is not directly related to how much you can load into the aircraft. Because the canard and the main wing are both contributing lift, the effect of loading to different CG's is different than with a more tradititional configuration where the main wing is supplying lift and the horizontal stab and elevator are actually lifting in the opposite direction.

Static and dynamic stability is one big factor that sets the aft CG limit for traditionally configured aircraft. In contrast, the aft CG of a canard configuration is set by the requirement to load the canard so that it stalls before the main wing under all flight conditions. This also limits the ability of the designer to add lift enhancing devices to the main wing like fowler flaps. You have to pair that with proportional lift enhancement with the canard, which adds complexity and weight. An example of this is the Beech Starship that had the canards move from a swept position to straight when flaps on the main wing are deployed. Complicated and additional weight over a "simple" fowler flap system.

But as noted above, traditional vs canard designs have different design factors that drive the determination of practical CG range limits.

Hope this discussion helps,

Wes
N78PS

[Mike Switzer]

Helps a little. Part of the reason I like the design is the ability to make the canard stall first.

The baggage load & fuel aren't that much of a problem as I can get them pretty near the CG, and trying to move the engine to the CG (like the original E-Racer) doesn't help that much (since it is a fixed weight it's position doesn't affect the CG range much) - the biggest problem I am having is allowing for the range of weight in the pilot & copilot seats - I am trying to allow for anything from one 150lb pilot to two 250 pounders & that creates a pretty wide range. If I use moveable ballast like the Cozy it "fixes" the problem (I can get the CG range down to about 3 inches) but I don't like the idea of carrying ballast around.

[martymayes]

Rutan said many times at forums that limiting c.g. range is the best way to control stall behavior and that a canard design is the easiest way to limit c.g. range.

If you want the best of all worlds, you need a third horizontal surface.

[Mike Switzer]

If you want the best of all worlds, you need a third horizontal surface.

I have actually been considering that, although it will add some drag. Also makes the calculations more complex.

[martymayes]

I believe Piaggio claimed their 3LSC design allowed for an overall 34% reduction in drag vs a conventional configured aircraft. Claimed less weight as well.

[WLIU]

Here is an interesting Piaggio fact. Are you aware of the 3rd control surface failures? At least one crew had the aft control surface depart the airplane. Never noticed it until multiple hops were completed. See http://www.flyingmag.com/news/piaggio-avanti-flies-missing-elevator

Very interesting food for thought.

Happy New Year,

Wes
N78PS