The biggest issue with a canard is that the CLmax of the configuration is
low compared to a conventional configuration. This is for several reasons:

1. The smaller canard must stall first and that means that the wing will
never stall and hence never develop it maximum lift. Of course the opposite
is true for a conventional airplane. The larger wing stalls and developes
its maximum lift whilst the smaller tailplane remains unstalled.

2. Unless you do some tricky stuff you cannot really put a flap on a canard
because it is difficult to trim out the nose down pitching moments.

Low CLmax means that the configuration will not develop as much lift at a
given speed and hence the airplane will not be as suitable for short fields
as will a conventiona configuration.

For those without an engineering background, CLmax is simply a measure of
how much lift a given wing will produce per unit area at a given speed.
CLmax is the maximum lift coefficient.



"Marc J. Zeitlin" wrote in message
...
fredfighter wrote:

Hmm. ISTM that a canard does not stall when the aircraft pitches
down, it stalls when the aircraft pitches up.

Correct.

.... Thus each time the
aircraft hits a bump or wave the nose pitches up stalling the canard
so that the nose of the aircraft comes down hard into the next
wave or onto the next bump and then nosedives under the wave or
bounces higher and stalls again.

Possible.

Thanks, now I have a much better understanding of the rough field
take-off problem with a canard.

Well, you would if that was the reason for the rough field issues, but
it isn't.

I fly a COZY MKIV, and what happens on a rough field, due to the
geometry of the nosegear (and NOT dependent solely on the fact that it's
a canard aircraft) is that as high grass or bumps cause the nose gear to
flex somewhat, the nose of the plane drops a couple of inches, causing
the AOA of the canard to decrease, and decreasing lift. If the drag
from the grass/dirt, etc. is high enough, the canard cannot reach a
speed or AOA where it can rotate the aircraft.

So the problem is one of inability to rotate due to drag on the nosegear
and resulting geometry changes that lower the AOA, NOT on canard
stalling.

I have taken off from a few paved runways that are very bumpy (AFN in NH
comes to mind), and if anything, the bumps can help to get the nose of
the plane in the air at speed, and never come close to raising the nose
far enough to stall the canard.

--
Marc J. Zeitlin
http://marc.zeitlin.home.comcast.net/
http://www.cozybuilders.org/
Copyright (c) 2005