Canards PAin In The Ass To Land? (was: Mechanics of Elevator Trim. In Detail.)

On Tue, 10 Jun 2008 18:14:41 GMT, Tauno Voipio wrote:

Tina wrote:
On Jun 10, 1:09 pm, Tauno Voipio wrote:

Tina wrote:

One point about the lift fairy sitting on the tail I'd like to
understand is this -- actually a serious question. As I understand
it, nearly aways the tail is exerting a downward force, since the
center of lift is aft of the center of gravity on general aviation
airplanes (that is true, isn't it -- that the cg is forward of the
center of lift?). If so the tail really is imposing an increased load
on the airplane, adding to its effective weight. The question I have
is, how many pounds of weight is imposed aerodynamically for an
airplane that might be loaded with its CG at the forward limit? I
don't know where the center of lift is on ga airplanes -- a third of
the way aft of the leading edge of the wing is an ok approximation,
but a few inches error on an airplane weighing what ours does at max
could make a huge change in the required force to overcome the nose
heavy moment.

A rule of thumb is that the force on the horizontal tail
is 5 to 10 per cent of the wing lift. This translates
to a loss of 10 to 20 per cent of the raw gross lift
availbale from the horizontal airfoils.


I'm obviously thinking about increased efficiency -- extra weight
added because of either fat people, full fuel, or aerodynamically
imposed, all cost horsepower (OK, watts for you purists) to move
around.

This is the reason why modern military aircraft are designed
aerodynamically unstable, and the electronic gnomes of the
flight control system have to work all they can do.

The loss of gross lift is the proce to pay for simple and
safe longitudinal stability.

--

Tauno Voipio
tauno voipio (at) iki fi

Thanks for the rule of thumb, Tauno. I have watched how busy the
flippers are on fighters when they are in the flare -- no human pilot
is working that hard for control. I knew the fighters are designed to
be aerodynamically unstable.

So the aerodynamic longitudinal stability the tail provides might
cost us 5 to 10%, The obvious question is, do canards buy back that
fraction? They would be offering positive lift, and if they stall
first would provide the same sort of longitudinal stability, wouldn't
they?

Yes - they do bring back some, and this is the reasoning behind
e.g. Rutan's Voyager,

The price is that the canard (front wing) has to stall first
unless you want to fall to ground in reverse when the thing
stalls. The rumours are that the canards are a PITA to land
nicely.

Apparently only to those who don't know how to fly one.