There was another item
observed during the trim tab examination that deserves mentioning. I
have seen this on other Lancairs as
well. The concept really applies to any hinged control surface.
Wherever flow
transitions from one flying surface to another, i.e. wing to aileron or
flap, horizontal stab to elevator, or elevator to tab, care must be taken to
ensure the trailing surface is in line with or slightly protrudes
into the slip stream. (see sketch) Flow readily separates if
the in-trail surface is not in proper alignment. Best case is one
merely suffers a drag penalty. Worst case is the effectiveness of the
surface is compromised. Many control surface designs will intentional
project the leading edge into the slipstream during control surface
deflection to assure flow remains attached. In the case of the
the trim tab, the separated flow served as an excitation on the loose
tab. This, in turn, got the elevator oscillating which was felt in the
control stick.
Chris
Zavatson
N91CZ
360
std