Fixed vs All Flying tail

[Kevin Neave]

Apart from the *better* handling of a fixed tailplane,
is there any aerodynamic advantage of a fixed tail/elevator
over an all flying tail?

I can't help feeling that when the stick's not in the
middle the fixed tail/elevator looks less efficient

(And I used to prefer the light stick forces and not
needing to constantly re-trim, when I flew a Cirrus,
rather than the Discus I have to put up with these
days - but NO I'm not offering to swap :-) )

Kevin

[Finbar]

Kevin Neave > wrote in message >...
> Apart from the *better* handling of a fixed tailplane,
> is there any aerodynamic advantage of a fixed tail/elevator
> over an all flying tail?
>

Summarizing what Fred Thomas has to say about it,

1. The elevator tail is a flapped airfoil where the flap goes the
"wrong" way: the elevator goes up for low speeds, but the tail needs
to generate positive lift at low speeds; while the elevator goes down
at high speeds, when the tail has a download on it. As a result, to
keep an elevator tail operating inside its laminar drag bucket, it has
to be bigger than an all-flying tail would have to be. Yes, the
elevator tail is less efficient, as it appears. Score one for the
all-flying tail.

2. Score a half for the elevator tail during a winch launch, because
then there is a download on the tail during the launch, while the
elevator is deflected upward (i.e., the airfoil flap is deflected the
right way for once).

3. It's harder to design an all-flying tail to have "good handling,"
which translates into appropriate stick forces. An elevator, hinged
at its leading edge, essentially trails in the breeze and contributes
nothing to pitch stability, but the fixed portion of the tail keeps
providing stability (of course trim can modify the "trailing" behavior
of the elevator a bit, but you get the idea). If you hinge an
all-flying tail at its leading edge, or anywhere ahead of its
aerodynamic center, the whole thing contributes essentially nothing to
pitch stability when stick-free: if you hinge it behind the a.c. then
the control surface naturally deflects in the right direction, but the
control deflection is unstable - if you're nose-high the tail will
automatically go to a "down" control input, but it will go to a FULL
down control input! There are lots of techniques available to improve
the handling of an all-flying tail, but that's the heart of the
problem.


So it comes down to handling and efficiency during winch launches, I
guess!