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Salutti tutti,
Referring to Mike Easley's posting of a measured
difference of 2.7 degrees between the wingtip incidence of his Lancair
leaves me completely astounded! If Lancair has such poor workmanship and
quality control and not just on ESs but across the range of Lancairs as I
suspect, I strongly believe that Lancair Company is significantly
responsible for the extraordinary rate of fatalities in Lancairs. ANY aeroplane
that has such an enormous difference in wing(tip) incidence is nothing short of
a deathtrap in my opinion. Mount such a wing any way you like to
average the lift in cruise flight so the aeroplane flies more or less wings
level and at or close to max AoA the wing(tip) with the higher AoA
will stall first and drop....with yaw...into spin. In NO WAY is 2.7 degrees
difference in wintip incidence acceptable, ground the aeroplane, return the wing
to Lancair, DEMAND a new wing built properly, your life and the life of whoever
flies with you is at risk....
Over time I have come to believe that the quality
and accuracy of Lancair's workmanship is unacceptable. My own experience of
Lancair fast build is that the accuracy is poor and for instance on the
last unfortunately "fast build" Lancair IV I built it took longer to
do the remedial work than to do the work correctly in the first place. Just
a few days ago we all saw the pictures of the fast built Legacy that lost
it's stub wing upper surface panel, now Mike Easley writes of an
extraordinary 2.7 degrees difference in wingtip incidence...what will it be
next?
Suspecting that differences in wing(tip) incidence
may be a factor in the all too common Lancair stall/spin fatality rate I
have been calculating some numbers to give some perspective. I'm so astounded at
Mike Easley's measued difference of 2.7 degrees in wing tip incidence that even
though I don't have all the material ready which I want to include in my
post I will write this tonight from memory as my notes are back in my
workshop; for a Lancair IV wing flying at 200 knots, using sea level
density, a difference of 0.1 degrees between the port and stbd wing results
in a difference of 56.6 lbs of lift. As the lift curve slope of the wing is
linear it follows that for a wing with 0.5 degrees difference between port and
stbd wings the lift of the wing with the lessor A0A is 56.6lbs x
5 = 282.5 lbs. To get a perspective on this imagine a spring balance hanging
from a hypothetical sky hook and attached to your "heavy" Lancair
IV wing at about mid span, for the aeroplane to fly straight and
level the spring balance will have to hold 282.5 lbs of force!
And that's for a wing with "only" 0.5 degrees of difference in incidence. If you
use aileron to hold up your "heavy" wing--- beware! Getting down to stall speeds
is interesting....the outboard wing section with down aileron will stall at a
lessor angle of attack, probably less than the inboard. It doesn't take
great imagination to understand the consequences.
Looking at what happens at slower speeds is
interesting. A clean and straight LIV wing has a maximum coefficient of
lift of 1.4, the calculated stall speed is a just a little less than 80 knots in
this configuration. Calculating, a difference of 0.5 degrees of wing
incidence means the wing with the higher AoA will stall first by 1.25 knots,
but that's not taking into account on the other wing using the aileron
to hold up the "heavy" wing. It may well be that the "heavy" wing outboard
section will stall first with interesting consequences to say the
least....but my calculations haven't gotten that far yet so I have no solid
information.
And I haven't even started to look at differences
in washout (twist) but I have no doubt that the numbers will be very interesting
and revealing. And the numbers don't lie!! Then combine port and stbd
differences in incidence and washout, asymmetric flap rigging... and
it all gets very interesting...
It's EASY to set up a jig to build a straight
and clean and SYMMETRICAL wing. Measuring wing incidence to 0.1 degrees or
better is not difficult. Those persons, (or Lancair Company) who can't
measure to such accuracy shouldn't be doing such work, get help from persons who
can. Your life depends on it.
ciao,
Roberto d'Italia.
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