Hi Jos,
I am very sorry but I have to disagree with nearly everything in your recent
posting!
Your points are relevant to a "conventional" fixed tailplane with separate
elevators and trim tab, but not to the Europa tail which is a masterpiece
of design!
The thing is - your comments about "needing trim tab deflection" apply to a
conventional
elevator - which is in effect a trailing control surface or tab itself.
This conventional elevator will always need to be "held" against a restoring
force from the airflow and the trim tab deflection provides this "holding
force", or trim as we call it.
The Europa tail principle is different however. The job that the horizontal
tail
surface does is the same on ALL conventional fixed wing aircraft (Europa, C-150,
Piper Cub, whatever) - it provides a small downforce to balance the nose
down pitch effect generated by the wing. It will always be a downforce needed
---From the tail because the C of G is always forward of the Point or Centre of
the Lifting force generated by the wing. It is only a small downforce needed
---From the tail because of the distance the tailplane is from the wing - you
have
a long spanner!! As the Europa has a symmetrical section tailplane then this
downforce has to be generated by a slightly negative angle of attack of the
Relative Airflow to the tailplane. (Look at the line drawings of the aircraft
- the tailplane's incidence in the neutral position is slightly negative to
the fuselage waterline/ reference line - the wing's incidence of course is 2.5
deg positive). The downforce can be varied by the pilot adjusting the angle
of the tailplane with the stick (control) or by changes to the Relative Airflow
by say a gust (the starting point of the aircraft's natural longitudinal
stability).
Now the clever bit regarding the Europa! The tailplane is "AERODYNAMICALLY
BALANCED"
which means that the tailplane can provide this "lift" force (downforce)
BUT REGARDLESS OF THE PRODUCTION OF THIS DOWNFORCE AND THE AMOUNT OF IT THE
TAILPLANE(OR ELEVATOR CONTROL) WILL NOT BE DEFLECTED AWAY FROM IT'S CURRENT
POSITION
EVEN WHEN THE GENERATED DOWNFORCE CHANGES BY CHANGING RELATIVE AIRFLOW.
THE TAILPLANE WILL NOT BE DEFLECTED BY THE RELATIVE AIRFLOW, EVEN IF THIS
AIRFLOW
CHANGES IN DIRECTION OR SPEED RELATIVE TO THE TAILPLANE. This is due to this
very clever aerodynamic balance. Basically any "rotating" force on the
tailplane
created by the airflows's angle of attack will be the same behind the pivot
(torque tube) as that created in front of it. In other words the slight
negative angle of attack causes the airflow to "push down" on the tailplane from
above with an equal average force behind the torque tube as in front of it.
This is true regardless of speed and the angle of the airflow and so the
tailplane
will never be deflected by just the airflow even if the airflow changes.
This is very hard to visualise because of course the surface area of tailplane
behind the torque tube is very much greater than in front of it - but THE
SHAPE or cross section of the tailplane is also very different fore and aft of
the torque tube and due to the beautiful design this AERODYNAMIC BALANCE IS
THERE!!
Now then - TRIM TABS!! The job of the tabs on the Europa is to hold the
tailplane
in the position that the pilot wants it to stay in! THE TAILPLANE WILL STAY
IN IT'S CURRENT POSITION ONLY WHEN THE TRIM TABS ARE FLUSH WITH THE TAILPLANE
SURFACE. IF THE TABS ARE NOT FLUSH THEN THEY WILL BE CREATING A ROTATING FORCE
ON THE TAILPLANE THAT WILL WANT TO MOVE IT FROM IT'S CURRENT POSITION TO
ACHIEVE THE TABS-FLUSH STATE AGAIN. If the tailplane moves out of the chosen
trimmed position - maybe the pilot inadvertantly moves the stick - then because
of the clever linkage design, the tabs move from their flush position in the
same direction. (Tailplane trailing edge moves up - tabs move up). The tabs
thus provide an aerodynamic "restoring force" to put the tailplane back where
it was - in other words it's original trimmed position with the tabs flush
again!
The trim switch/ control on the Europa merely resets or adjusts the tailplane
position where the tabs are flush with the surface. The required tailplane
position simply depends on how much downforce you want it to make which depends
entirely on what you are doing with the aircraft!
So, the functions of the tabs are:
1/ To hold the tailplane in the chosen position (tabs flush when achieved) and
provide a restoring force to the tailplane if it is moved away from it's chosen
trimmed position - this is known as "feel" that is provided to the pilot through
the stick - the further you move the stick away from the chosen trimmed
position (without re-trimming) the greater the tab deflection and thus the
greater
this restoring force will "feel". If there were no tabs then the tailplane
would just stay wherever the pilot put it, regardless of speed/ airflow etc
- but there would be no "feel" to the elevator at all (zero feel).
2/ To be pilot-adjustable so that you can reset the trimmed (tabs flush)
tailplane
position as required during flight.
Note that none of this theory specific to the Europa tailplane control is
connected
to the aircraft's longitudinal stability theory - that is a different subject
and has no relevance specifically to balanced tailplanes or otherwise.
Longitudinal
stability theory is the same on all conventional fixed wing aircraft
regardless of tailplane design.
Variances: A perfectly constructed and balanced tailplane will always have it's
tabs flush with the tailplane in steady correctly trimmed flight, regardless
of speed and attitude. Variation in build MAY affect this perfect state. If
the aerodynamic balance of the tailplane is not quite perfect (not quite the
perfect tailplane profile section) then the balance of forces (created fore and
aft of the torque tube) might not be quite perfectly equal with the trim tabs
flush and so a slight tab deflection in the appropriate direction will be
permanently
needed in steady trimmed flight to balance the forces up again. Not
a problem, just very slightly less efficient due to the slight increase in drag
caused by the permanently deflected tabs. Also if the counterweight (mass
balance weight) in the fuselage is not perfectly correct then this too will
create
an unwanted rotating force on the tailplane that would require an (unwanted
and continuous) tab deflection to counter/ balance out this unwanted force.
Also, if the pilot flies "out of trim" for some reason then a tab deflection
will be required in the appropriate direction to oppose the pilot's push or pull
force on the tailplane. This one can easily be corrected though!!!
The only real way to tell if your tailplane system is absolutely perfect in
balance
(aerodynamically as well as in weight) is to trim the aircraft perfectly
in steady flight (any steady speed) and get a mate to come up behind you and
take
a close up photo of your tabs. In a perfect set up they will be flush with
the tailplane!!!!
Cheers, Jon
--------
G-TERN
Classic Mono
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