|
|
Posted for "Robert Overmars" <robert.overmars@tiscali.it>:
Saluti tutti,
The LIV and ES wings are RXM 5-217 at the wing root and NACA 64-212 at the
tip with a linear interpolation between the root and tip. The RXM 5-217
airfoil, if I may quote from M.A.D. again "is a Jones-Jouskowski section which
features 50% favourable pressure gradient in cruise and a larger leading edge
radius than conventional laminar flow shapes. etc...This airfoil is designed
for high speed cruise... and is similar to NACA 63218" And later..."The NACA
64-212 has a very high maximum coeffieient of lift but a sharp stall.Since we
are not going to let the wing tips stall this should not be a problem" (The
'no-stallers' have won the day I'm afraid)
RXM 5-217 is a Rick McWilliams proprietory airfoil and I've yet to see a lift
polar for it, however as it's 17% thick and with a larger than normal leading
edge radius one would expect that it has quite gentle characteristics. Looking
at NACA 63218 to which it's stated to be similar this would appear to be the
case, certainly there's no evidence of hysteresis and the fall-off in lift
after max coefficient is quite gentle.
NACA 64-212 is the tip airfoil and is quite a different kettle of fish. When
moving inboard from the wing tip the wing section changes, becoming both
thicker and with a larger leading edge radius, obviously moving inboard the
hysteresis effect diminishes and eventually disappears however at what point
this occurs I don't know. If someone has the software to model the LIV & ES
wings it would be a very interesting exercise to model and see what sort of
differences in lift exist at the wingtip when one wingtip restablishes max
lift and the opposite stays at the lower coefficient of lift and to see how
far inboard this effect extends.
As far as Reynolds Numbers are concerned here's a simple formula; Reynolds
Number = 9360 x V (mph) x Length (chord in feet) For an LIV wingtip at 100
mph (86 knots) the RN is 9360 x 100 x 2.5 = 2,340,000 For an ES wingtip at
the same velocity the RN is 9360 x 100 x 3 = 2,808,000. Double the velocity
and the Reynolds Number doubles and is still less than 6 million and according
to the NACA 64-212 chart the hysteresis will still occur at this Reynolds
Number.
I'd like to quote this from John Dreese, producer of DesignFOIL: "there's no
such thing as a bad airfoil. Yes there are some unsafe ones, but the airfoils
that get a bad reputation are usually victims of bad construction...etc" I
don't truely believe that NACA 64-212 is a bad airfoil as such, the problem as
I see is that it's not tolerant of building error; as regards the leading edge
radius, differences in thicknesses between one wing and the opposite (the ATSB
investigator investigating the crash of VH-CIV told me he surveyed other LIVs
and found as much as 1/4" differences in thickness on wings), and also because
of the hysteresis effect the setting of angle of incidence is not just
important IT'S CRITICAL. In other words, if the wings are completely
symmetrical and the incidences are identical I don't think there's a
significant problem.
As regards the fitting of winglets, again, if the wing is symmetrical and
incidences identical and the winglets (about 3.75 sq feet each, I measured
today)are mirror images of each other there shouldn't be a problem...to my
mind anyway.
Following on from my last post looking at what could be happening when an LIV
or ES with a difference in wingtip incidence has stalled and the pilot pushes
forward stick to unstall....the first wingtip to come back to max coefficient
of lift has a much greater lift than the opposite wingtip and the aeroplane
rolls uncommanded, yaws and is entering spin. A smart spin trained pilot
stands on opposite rudder and stops the rotation...then pushes forward stick
to unstall...then the first wingtip to come back to max coefficient of lift
rolls the aeroplane again, which yaws, and the aeroplane is entering spin
again.... and again..
According to the accident investigator of VH-CIV "it's an interesting theory
which explains the facts but is hard to prove". Given that there are many LIVs
and ESs flying with less than symmetrical wings perhaps those most
knowledgeable about spin recovery might care to chip in with their thoughts on
how to possibly recover from such a scenario so hopefully the next pilot who
inadvertently stalls and enters spin might have some idea on how to recover.
I'd like to caution those who think about using aileron when stalled or close
to stall. Please back go and look at the lift curve polar for NACA 64-212. The
two curves on the left side with the peak coefficients of 1.9 and 2.4 are for
a simulated split flap deflected 60 degrees. It's interesting to note that an
aileron deflected down acts the same way as a flap in that the coefficient of
lift increases but the Angle of Attack at which the wing section reaches the
max coefficient of lift is less. Note that the peaks of these two curves are
displaced to the left of the other curves, by as much as 8 degrees to only 6
degrees AoA for the standard roughness simulated flap curve.
I'm curious to know if anyone has any information of the LIVs lost to
stall/spin/splat...were flaps extended or retracted?
ciao,
Roberto d'Italia.
|
|