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Old Nov 08, 2008, 09:39 PM
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Springfield, Missouri, United States
Joined Jul 2002
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Quote:
Originally Posted by Don Stackhouse
I presume you're referring to turbulation to prevent boundary layer separation. That really only applies to lower Reynolds numbers, such as models or on relatively slow full-scale aircraft. At higher Re's, the efficiency-improvement problem is usually more about trying to keep the flow from becoming turbulent.

There are cases of turbulation (such as vortex generators) used on higher Reynolds number applications, but there it is generally to prevent or delay stall of that portion of the flying surface. It does not generally improve efficiency (in fact it normally reduces it), but it can increase the max lift coefficient possible from the surface. It's often used as an add-on to fix handling problems, such as loss of aileron effectiveness near stall, or to increase control authority.

For example, used on the fin of a twin-engined aircraft they can allow more lift to be squeezed out of a given size fin+rudder, allowing the tail to handle more asymmetric thrust in an engine-out situation. This can fix an existing problem, or it can allow a new design to get away with a smaller vertical tail than it would need without vortex generators. However, the benefit here is not from a direct increase in fin efficiency in cruise (as before, it reduces it), but rather by allowing the fin (which in a twin is sized by an emergency situation) to be smaller. Note, vertical tails on twins are normally several times larger than what yaw stability alone would call for, just to get enough authority for the engine-out situation.



A canard with a control surface on it IS a variable geometry canard. The majority of canards already out there fall into this category. A canard with variable sweep (such as on the Beech Starship) is a more radical example of variable geometry. In that case they have a wing with flaps, and need a multi-mode canard to keep the canard's lifting ability matched to that of the wing in its different modes. Beech paid a pretty high price in the form of a fairly complex and extremely flight-critical jackscrew linkage coupling the canard sweep to the wing flaps in order to pull that off. A failure of that mechanical system could immediately make the airplane incontrollable. Failure modes end up making a lot of the decisions in aircraft designs.

In all cases, for decent stall characteristics you need to make sure the canard stalls before the wing, unless you can limit the canard's capability by some other means, such as through a very "smart" fly-by-wire system. If you don't, you run the risk of a violent pitch excursion at stall. As Burt Rutan pointed out to me once, you almost never want to add more lifting ability to a canard (the typical reason for more elaborate variable geometry techniques) unless you first increase the lifting ability of the wing.

In any case, it's important to remember that it's the wing's job to support the plane, and the tail's (or canard's, in this case) job to take care of stabilizing and controlling the wing. When you start to blur those two tasks, overall aircraft efficiency generally goes down.
Well dang Don.
You took the word right out of my mouth once again.
Thanks for such great points to ponder

Larry
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