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<< Lancair Builders' Mail List >>
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This question for the aerodynamicists out there (both real and amateur):
How come trim tabs don't have to be counter balanced? Seems that they
are subject to flutter too as are all foil surfaces. I suspect that trim
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One perspective from an amateur:
Fixed aerodynamic surfaces depend on shape and structural stiffness to
prevent flutter. Movable surfaces cannot be rigidly attached by definition,
so they depend on balance. A trim tab is considered fixed since it is held
in place, but there should be no slop in the linkage. Examples follow if
you're interested.
Fixed:
Consider the forward swept wing of an x29. A gust deflects the wingtip
upwards. If the wing is not stiff enough, the upward deflection of the
wingtip twists the wing and increases the angle of attack at the tip. This
generates more lift at the tip, further deflecting the tip upwards and
twisting the wing.
Wing strength will overcome a small deflection, and the tip snaps downward
to a negative angle of attack. The downforce thus generated further deflects
the tip down. If the wing is strong enough the wing snaps up and the cycle
repeats until the wing is either stabilized in some way, or the increasing
deflections rip it off. This is flutter.
Airliner wings are not stiff relative to their aerodynamic size, since
stiffness means weight. However they do use aft sweep, since this damps out
wingtip deflection. (the wing twists in the reverse of the direction
described above) The X29 has very stiff wings, and computer controlled
ailerons to damp out any oscillation.
A trim tab is aerodynamically very small relative to the stiffness with
which it is attached to a control surface, so it does not flutter.
Movable:
Consider an underbalanced (center of mass behind the hinge line) elevator. A
gust deflects the tail upwards. Since center of mass is behind the hinge
line, the elevator deflects downward. This increases lift (or reduces
downforce) of the tail, causing the upward deflection to be exaggerated.
At some point the tail stops rising and begins to drop. The elevator mass
continues upward, and deflects the elevator up, reducing lift. (increasing
downforce) This exaggerates the downward motion of the tail.
This exaggeration of deflection is flutter, and if it is strong enough or
continues long enough, the part that is fluttering comes off the aircraft.
By moving the center of mass to the hinge line, no exaggeration takes place,
so no flutter. If you overbalance the part, moving the center of mass ahead
of the hinge line, gust deflections (and control inputs) are countered
rather than exaggerated. You don't really want overbalance either, for other
reasons.
Peter Frost.
LIV: wings, tail, fuselage, no funds.
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