canard flying boat

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

On Wed, 20 Jul 2005 20:05:23 +1000, "......... :-\)\)"
wrote:

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.

picture an aircraft flying along which has a sudden brutal increase in
the resistance of the wheels (note that is hypothetical, thankfully
the snake oil plane wasnt built)
say for instance your canard clips the main wheels on the top wire of
a fence.
in that scenario, which is similar to what I predicted with the
hovercraft skirt, the sudden rotation forward and down would be
catastrophic in that the sudden downward movement of the canard would
probably increase its angle of attack beyond the stall (due to a
change in realtive wind direction).
that is what I was referring to.
other than that I have agreed with all that has been posted.
Stealth Pilot

Stealth Pilot wrote:
...
fredfighter wrote:

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

Correct.

picture an aircraft flying along which has a sudden brutal increase in
the resistance of the wheels (note that is hypothetical, thankfully
the snake oil plane wasnt built)
say for instance your canard clips the main wheels on the top wire of
a fence.

Or a less drastic scenario, the nosewhell plows thorugh a clump
of grass...

in that scenario, which is similar to what I predicted with the
hovercraft skirt, the sudden rotation forward and down would be
catastrophic in that the sudden downward movement of the canard would
probably increase its angle of attack beyond the stall (due to a
change in realtive wind direction).

Well, I thought that increasing the pitch (nose up) increased
the angle of attack and decreasing the pitch decreased the angle
of attack. Aren't both angles conventionally measured so that
an upward rotation of the nose of the plane is a positive change
in the respective angle?

E.g. lift increases with increasing angles until teh wing stalls.
If the canard is not stalled and then the nose of the aircraft
pitches downward the lift of the canard decreases pitching the
nose even more downward. In the air the aircraft would accelerate,
trading altitude for speed, thus increasing lift to compensate
for the reduced angle of attack. However on the ground there
is no altitude to trade for speed so the nose bounces.

Have I got it right yet?

--

FF

On 20 Jul 2005 10:27:47 -0700, wrote:



Stealth Pilot wrote:
...
fredfighter wrote:

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

Correct.

picture an aircraft flying along which has a sudden brutal increase in
the resistance of the wheels (note that is hypothetical, thankfully
the snake oil plane wasnt built)
say for instance your canard clips the main wheels on the top wire of
a fence.

Or a less drastic scenario, the nosewhell plows thorugh a clump
of grass...

in that scenario, which is similar to what I predicted with the
hovercraft skirt, the sudden rotation forward and down would be
catastrophic in that the sudden downward movement of the canard would
probably increase its angle of attack beyond the stall (due to a
change in realtive wind direction).

Well, I thought that increasing the pitch (nose up) increased
the angle of attack and decreasing the pitch decreased the angle
of attack. Aren't both angles conventionally measured so that
an upward rotation of the nose of the plane is a positive change
in the respective angle?

E.g. lift increases with increasing angles until teh wing stalls.
If the canard is not stalled and then the nose of the aircraft
pitches downward the lift of the canard decreases pitching the
nose even more downward. In the air the aircraft would accelerate,
trading altitude for speed, thus increasing lift to compensate
for the reduced angle of attack. However on the ground there
is no altitude to trade for speed so the nose bounces.

Have I got it right yet?

your mental picture is a practical one but you are looking at an
aircraft with a gently changing angle of attack and your assumptions
are correct I think.

but picture if you will a *very sudden* pitchdown. the canard would
see the direction of the incident airflow change from directly ahead
to somewhere down in front of it. ...like a gust.

dont lose too much sleep over this, the scenario was hypothetical.
besides I have some more of my Turbulent's fuselage beckoning to me
for attention :-) ...and that vw engine I picked up yesterday for
converting.
Stealth Pilot

Stealth Pilot wrote:
On 20 Jul 2005 10:27:47 -0700, wrote:

...

Have I got it right yet?

your mental picture is a practical one but you are looking at an
aircraft with a gently changing angle of attack and your assumptions
are correct I think.

The only part I questioned was the algebraic sense of the direction
or rotation. E.g. which direction was an increase and which a
decrease.


but picture if you will a *very sudden* pitchdown. the canard would
see the direction of the incident airflow change from directly ahead
to somewhere down in front of it. ...like a gust.

Thinking about it, a *very sudden* pitchdown _during takeoff_ will
also re-orient the thrust line downward. Given the proximinty of
the ground during takeoff that may prove to be the dominant
consideration.

--

FF