| Dear humble sevant... or is it 'savant'?
See, we often shackle our efforts to understand by having aforehand
made up conventions to better understand something else. I feel
motivared to offer some observations on understanding the L2s, if I
may.
Don't forget that you have a delete button if this gets too
boring.
This is how I see the Lancair's wing-flap-stability-stall, as
compared to recent LML discussions of same.
The CG range and the wing's characteristics is a good example of
thinking 'in the box'.. Guys working in the wind tunnels measuring
forces of invisible air found that at those small angles where wings
make a lot more z force than Y force -- those forces seemed to be
centered around 25% chord... which they first called a center of
pressure, and then 'aerodynamic center' or a.c., and so they made their
charts about this imaginary point and things work out great for
calculations.
The thing is, what's really going on is that as the moving wing
flows through the stationary air, a pressure bubble is generated, with
most of it being at the leading edge, and then tapering
'triangular-sish-ly' toward the trailing edge.... at least at higher
angles of attack. And the lower or cruise angle so attack the
bubble flattens out aft-ward... and is varied by moving a trailing edge
flap.
So this is getting at the importance of the bubble... that's what
we move around when we move the pitch control. With a flap, or an
aileron, or a tail flap thing.
And so, when we talk about flying with the CG range forward or aft,
we're really talking about the CG moving with respect to this
total-airplane-bubble... which we have to keep centered over our center
of mass. We do this by making little bubbles of pressure on the
tail, or in Lancair reflexing of the flap, by moving the bubble of the
wing. And that's what the CG or center of mass is hanging
from.
So when we drop a flap, it's obvious that we have moved that big
bubble, and have to balance it with changing the bubble size on the
horizontal tail.,, trim tabbing it.
The design characteristics of the L@'s reflexible airfoil are
referenced to the section with the flap not reflexed. Also,
therefore, it is most likely that Lance figured the CG that way, because
that's the way the aerodynamic data was available to him in the NASA
report, I think, on the NLF(1)-0215F... please hasten to correct me if
I'm wrong. : )
I just try to keep my (total airplane) bubble's center as close to
the CG, wherever that is, by making a little bubble over or under the
horizontal tail, with the pitch trim tab.
One problem is that we pilots don;t talk much abut the shape or
location of the pressure bubble at AOAs higher than the stall
angle... and that's a situation where designers then have to start
gluing yukky shapes of strakes and vanes on to correct this oversight.
What Lance should have done in the first place is wind-tunnel his
200 and -235 from zero AOA up to 90 degrees, and he would have seen a
big forward movement of the bubble's center resulting from the broad
cowl and skinny aft fuselage... imo.
Such testing was done by NASA for the Piper canard (after it
crashed) and on a configuration like the Dragonfly... which I discovered
when researching the Dragonfly we bought... and discussed in a Kitplanes
article many years ago. In the too frequent event that a new
design configuration is locked in by building before testing, the
economical remedy solution then is to fly with a forward CG, or to add
strakes aft, to keep that total-airplane high AOA bubble centered where
it belongs ... aft of the design CG,.. for a restoring pitching
moment
In the Dragonfly we flew at forward CG.
In L235/320 N211AL I have modified the horizontal tail to add slots
to prevent it from stalling at AOA higher than the wing's stall
AOA...( still testing that, but it worked on my Magnum.) Also I
added ballast to the engine mounts to be certain the CG stayed forward,
within Lance's original limits.
It's a beautiful little plane, and very efficient ... and this is
how it works -- I think.
Terrence
L235/320 N211AL
If you know or even care:
In general, LNC2's as originally designed seemed to better
tolerate a CG at the forward edge of the envelope rather than flight
at or towards the rear. This includes adequate elevator control
at flare during landing. Lancair tested the long engine mount on
an LNC2 that moved the forward CG edge +1.5" and there were no
flight problems. Hmmmmmm.........
Consider that the LNC2 wing has a dramatic change in pitch
forces when the flap is moved between its designed standard position
and into -7 degrees reflex. In my airplane at around 140-160
KIAS the difference in those two flap positions
is approximately a measured 6 degrees in attitude (couldn't
measure the AOA delta). It is clear that moving the flaps
sightly out of reflex (1 or 2 degrees) can help resolve uncomfortable
flight at rear CG conditions by pitching the nose down some and
altering the AOA. Perhaps the rear CG and small tail at cruise
leads to some flight instability that cannot be overcome by the size
of the tail?
So, here is the question: If the CG range was calculated
for the normal state of the wing (flaps not in reflex), is it possible
that the range is too far back for normal cruise flight with the flaps
in full reflex? If so, should the POH aircraft data include
two ranges based on these two flap positions? What does
such a change do to the forward CG limit?
Of course, this might raise the same question with the 200 series
aircraft. Why? Well, the faired in position of the flaps
for 200 series aircraft is the not-in-reflex position while the plane
cruises with the flaps reflexed and not faired in. The 300
series aircraft has the flaps in reflex when they are faired in to the
fuselage.
When considering an answer, remember that wings designed to
operate by changing shape (TE goes through some reflex angles) have
been primarily used in tailless airplanes and the TE
position controls the pitch balance of the airplane. I have
no idea how the CG range for such an aircraft is determined.
Your humble servant,
Grayhawk
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