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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