High thrust line on canard design?

I read a while ago in a thread that someone said canard design doesn't
like high thrust line but he didn't elaborate. I like canard design but
I have little aerodynamic konwledge so I am begging an explanation for
this statement.

The only reason I can think of from my layman's view is high thrust
line (say, an engine mounted on a pylon) on a canard airplane will
cause a large nose-down force and this force is specially bad for
canard airplanes which have elevator in the front. Many conventional
airplanes have high thrust line engine installation (like Lake
amphibian) and handle fine. So how more severely and to what extend
does this nose-down force affect canard design than a tail airplane,
not even allow a canard take off at all?

Thank you in advance.

Shin

On 3 Mar 2005, Shin Gou wrote:

I read a while ago in a thread that someone said canard design doesn't
like high thrust line

Having flown a canard for the last six years, I say that you are correct.
They usually need over 1000 ft for takeoff, and a high thrust line would
lengthen that somewhat.

But, I just opened an ASF safety mailing, about accidents while
manuvering,
the graph showed that about one half of those accidents were stall/spins,
the other large percentage was buzzing accidents. Very comforting to
a canard pilot.


George Graham
RX-7 Powered Graham-EZ, N4449E
Homepage http://bfn.org/~ca266

Shin Gou wrote:
I read a while ago in a thread that someone said canard design doesn't
like high thrust line but he didn't elaborate. I like canard design but
I have little aerodynamic konwledge so I am begging an explanation for
this statement.

The only reason I can think of from my layman's view is high thrust
line (say, an engine mounted on a pylon) on a canard airplane will
cause a large nose-down force and this force is specially bad for
canard airplanes which have elevator in the front. Many conventional
airplanes have high thrust line engine installation (like Lake
amphibian) and handle fine. So how more severely and to what extend
does this nose-down force affect canard design than a tail airplane,
not even allow a canard take off at all?

Think of it this way-

A significant advantage to a canard is you can design it so the
foreplanes to stall before the wing. This improves the handling, in
that when the airplane "stalls," the main wing is still "flying" and
providing lift. The canard airplane and conventional airplane will
tend to drop their noses when stalled, but the canard loses much less
altitude recovering than a conventional airplane. (When a
conventional airplane "stalls," the main wing stalls and provides
much less lift, so you lose more altitude.)

Now, if you have a large nose-down moment from a high thrust line,
the foreplanes will have to make more lift to overcome that. There
are two ways to make the foreplanes produce more lift.

One way is leave the foreplanes physically unchanged but make the
airplane go faster- as you say, this requires a higher takeoff speed.

The other way is to make the foreplanes produce more lift at any
given speed by physically changing them (larger, different airfoil,
whatever). During flight if you take away the nose-down moment, for
example testing the airplane's handling in a power off stall, and
suddenly the foreplanes are able to produce much more lift than
necessary. If they produce so much lift that the main wing stalls
first, the airplane will suddenly pitch up and who knows what next.

I hope that makes sense the way I explain it.

Thank you, Jim. It makes sense. Now what I need to do is some serious
caculation to figure out how much distance between thrust line and
verticial position of CG is tolerable for a canard design. Any idea?

Shin

Or vary the sweep of the canard as Beech did with the
StarShip.

Jim Carriere wrote:
Now, if you have a large nose-down moment from a high thrust line, the
foreplanes will have to make more lift to overcome that. There are two
ways to make the foreplanes produce more lift.
One way is leave the foreplanes physically unchanged but make the
airplane go faster- as you say, this requires a higher takeoff speed.
The other way is to make the foreplanes produce more lift at any given
speed by physically changing them (larger, different airfoil,
whatever). During flight if you take away the nose-down moment, for
example testing the airplane's handling in a power off stall, and
suddenly the foreplanes are able to produce much more lift than
necessary. If they produce so much lift that the main wing stalls
first, the airplane will suddenly pitch up and who knows what next.