The airfoil of the wing of any aircraft with a non symmetrical section will
produce
a negative pitching moment (-cm) around the aerodynamic center of the wing.
This means that without the tailplane producing an equal but opposite moment
the acft will pitch over into a vertical dive. The all flying tailplane on
the Europa has no fixed angle of incidence so to produce an opposing positive
torque moment to the wings negative one the trim tab must be held out of plane.
At higher speeds then the tailplane must provide greater downforce then
requiring
even more trim tab offset. We have to pay a small drag penalty to achieve
a level trim condition in cruise flight. This penalty can be reduced somewhat
by loading the acft towards the aft cg limits requiring less trim tab
offset reducing the downforce the tailplane must produce to maintain level
flight.
The tailless flying wing derives it's pitch stability due to the outer
portions
of the wing being reflexed up to produce that necessary positive pitching moment
but also introduces that small drag penalty again. There is no free lunch
in aerodynamics! Every desireable aerodynamic quality (like pitch stability)
has a price that must be paid.
Glenn
Fred Klein <fklein@orcasonline.com> wrote:
>
>On Apr 12, 2010, at 6:24 AM, david miller wrote:
>
>> I think that for most efficient flight the incidence of the wing and
>> tail should be such that at cruise there is no up or down trim on
>> the elevator or all flying tail.
>
>Fascinating discussion...as a non-aerodynamicist, I can't resist the
>temptation to add a comment...
>
>I've always understood that the elevator or an all-flying tail
>(horizontal tail plane) has a "job"...namely, to exert a force, either
>up or down, in order to balance the lift of the wing and stabilize the
>flight path of a normally configured aircraft (i.e., not a canard
>type). To do this "job" requires "work"; the tail plane is never
>neutral. And w/ our Europae...with it's trim tabs acting in
>opposition to the all-flying tail plane thru a direct mechanical
>connection...whenever the tail plane is doing its job (at whatever
>angle of incidence), the trim tabs will be doing their job as well
>(that of counterbalancing forces on the tailplane), and will
>necessarily be raised or lowered w/ reference to the tailplane.
>
>Of course if this explanation is correct, one might ponder the
>aerodynamics of my tail-less biplane...pix below.
>
>Fred
>
>
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