Good analysis Frans,
If the tail plane is perfectly balanced and friction in the system nil,
the trim tab is centered.
A too heavy counter weight is offset by the trailing edge of the tab up.
A difference in flettner strips can cause the same.
Flutter is dependent on the center of mass and the aerodynamic center.
A horizontal control surface, mechanically balanced, with a symmetrical
airfoil surface with the aero center co-located with the center of mass
tends to stay neutrally stable. Camber angle and or trim tabs change
that of course.
When the center of mass is behind the the center of pressure, the
trailing edge goes down due to gravity, aerodynamic forces then try to
lift the trailing edge and in some cases overshoot the center line, so
gravity and the aero forces push the TE down, then it overshoots again
and flutter occurs. This is especially a concern of a cambered aileron
not mass balanced.
I don't have time to go into the drawings, but a symmetrical airfoil
contributes no moment to its normal force at low angles. That is, if it
is perfectly balanced at its aerodynamic center, it will stay at its
angle of attack unless acted on mechanically and requires no force
(beyond mechanical friction in the mechanism) to move it. It will, by
previous statement, also apply no counter force when moved (so no pilot
feel). This is why we have the anti servo tab, to create feedback to
the pilot. It also cleverly applies trim.
If the flettner strips are not perfect, and or the mass balance is
slightly off, your trim tab will show the effect of that. The
asymmetric flettner strips of course act as a flap and become not an
anti-servo tab, but a servo tab which drives the trim tab, which drives
the tail.
As for the unbalanced anti-servo tab. The tab itself does not
contribute to flutter if it is fixed. If the tab is loose, the tail
plane bushings or trim bushings are missing, loose, damaged or not
installed, flutter will occur. This is why, I am such a zealot about
stabilators, loose pins, and bearings. I would even prefer to try to
mechanically balance the trim tab for the guy who mistakenly fails to
hook up the trim tab pins to the trim bar.
Hope that doesn't add to the confusion Pete.
Bud Yerly
Tech Support
----- Original Message -----
From: Frans Veldman<mailto:frans@privatepilots.nl>
To: europa-list@matronics.com<mailto:europa-list@matronics.com>
Sent: Wednesday, May 12, 2010 9:52 AM
Subject: Re: Europa-List: Re: Effects of Flaps and Ailerons on Pitch
Trim
<frans@privatepilots.nl<mailto:frans@privatepilots.nl>>
On 05/12/2010 03:22 PM, Peter Zutrauen wrote:
> Your thesis assumes that the tailplane is aerodynamically balanced
wrt
> to the torque-tube position, and the only aerodynamic component
which is
> stabilizing it and keeping it from flutter is the anti-servo tab.
Stabilizing, but not necessarily keeping it from flutter. I think that
the mass balance weight is to keep it from flutter, just as with the
ailerons (that don't need to have a trim tab for that either).
> I would expect/hope that the tailplane would remain aerodynamically
> stable without the anti-servo tab, and that the tab is there only to
> provide for progressive stick forces, and trim.
This is how it works I believe. At least is seems so on the PH-DIY.
;-)
Frans
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