| 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
|