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"Its a common procedure at Cessna. I am not aware of any accidents
attributed to it so if it is dangerous as you say don't you think it would show
up in the accident statistics?
Jeff Edwards
LIVP"
Jeff and others....
Haven't there been enough Lancair fatalities already without adding to the
list to try to prove the point?
Have you read the compilation of Lancair accidents done by Lee Metcalfe?
Noticed the number of stall/spin accidents? The number of fatalities (dreadful
word fatalities, each fatality was a warm living breathing person just like you
and me and everyone else) from these stall/spin accidents? Perhaps wing
incidence/washout differences are very much a factor in the all too common
propensity of Lancairs to depart controlled flight and crash except it's
rather difficult to reassemble the broken bits of aeroplane to check what
was the original incidence washout/incidence and if it was a factor.
Cessna wings are to Lancair wings as chalk is to cheese. Cessna wings with
big fat leading edges, thick sections, slow speeds, big tolerances and
docile behaviour in that after max Cl lift falls off gradually. Compared to
Lancair wings with laminar flow, high speeds, small leading edge
radii, low tolerance for building error and in the case of IVs and ESs,
NACA 64212 tip profiles, quoting Martin Hollman "a good profile with a high
coefficient of lift but a sharp stall"
Just how sharp is the stall? 3.5 degrees after max Cl (15 degrees
AoA) 20% of lift is lost. 7.5 degrees after max Cl more than 35% of lift is
lost. Little differences in incidence and especially tip incidence start to
become significant, especially at higher speeds, double the speed and the
lift quadruples. The LIV and ES wings have 2 degrees of washout, does
anyone seriously contend that a wing built with 1.7 or 2.0 degrees more
washout (an enormous 85% to 100% error in construction) than it's partner
wing can have it's incidence fudged by a degree or so with eccentric
bushes to average the lift in cruise flight and that that aeroplane has
safe stalling characteristics through ALL of the flight envelope? That the stall
will iniatiate at the wing root symmetrically and progress outboard
symmetrically as is the design intent?
Gust loads and the structural limits are interesting.
Consider an ES cruising along at 8,000 feet at a weight of 2,800 lbs,
maintaining 220 mph. The MAC AoA is 2.32 degrees, the wing is working at a
coefficient of lift of .205. The aeroplane encounters a vertical gust,
lets say 50 feet / sec. The wing at MAC now sees an AoA of 11.12, the Cl is
.98968 and the wing lift is 13,515 lbs. Divide this by the gross
weight and the g is 4.83, more than the normal load limit of the aeroplane.
Lesson learnt...stay in the green arc of the ASI.
While the MAC is working at a Cl of .98968 the wing tip with the correct
washout is working at 1 degree less AoA therefore the Cl = .90068. Plug the
numbers into the formula for lift and calculate the lift for one sq ft at the
tip equals 87.85 lbs lift. If the opposite wing tip has 1 degree more AoA
it's working at the same AoA and CL of the MAC and merrily producing 96.5
pounds of lift. That's 8.7 lbs difference each square foot at the tip, the
difference progresively becoming less towards the root, averaging about 300 lbs
difference acting at MAC.
Now our ES, after surviving that encounter slows down to 170 mph. The AoA
is 3.88 degrees, the Cl is .3439. Again it encounters that 50 fps vertical gust,
the AoA peaks at 15.28 degrees at MAC, the Cl is 1.36, total lift is
11,071 lbs and the g is 3.95. Hooray! the airframe survives.
But the wing root has stalled (at 15.5 degrees) as it works at 1
degree more than the MAC, however this is not a great problem as the falloff of
Cl past 15.5 degrees for the root profile is not so great (about 1/3 of the
falloff of the tip profile), and even if the incidence is a little different
side to side the rolling moment produced is not so much.
With a wing built with 1.7or 2.0 degrees more twist at the
tip than the other installed on our hypothetical ES, in this
situation you've gone into test pilot land. It's feasible one complete wing has
stalled and part of the other wing, even the tip is still merrily working
away at max Cl. The downgoing wing sees an upwards vertical component in
it's airflow which effectively increases the AoA, which increase the drag, the
aeroplane starts to yaw...you get the picture..??
Purists will know that my figures are for illustrations purposes only. In
calculating gust loads there are finite delays allowed in the gust
onset and aeroplane response that skews the figures but not by such an
amount that these figures are meaningless. Nature doesn't always oblige
with nice convenient 50fps gusts or downdraughts etc and any aeroplane can be
involved in such a situation.
Give yourself the best chance of survival and build 'em light
and symmetrical.....and keep 'yer tips flying!
Ciao,
Roberto d'Italia.
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