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At 06:48 PM 3/8/2007, Bob Mackey wrote:
Marv:
I think Mark hits this nail on the head. At first glance, it
might be nice to have the greater lifting area and coefficient
of lift (Cl) associated with Fowler flaps. It might seem that
simply lowering the hinge point would do the trick. In going
from the 235 to the 320/360, the hinge point is lowered from
the top skin to the lower skin. In the Legacy the hinge point
drops another 4-5 inches.
When you look into the Legacy wing you'll also see some other
changes: larger bond areas, stiffer skins, heavier ribs.
Notice the Legacy's drag spar -- much stiffer than the LNC2's.
Even heavier areas around the flap hinge mounts. Also notice
that the airfoil is different, and the tail is bigger and on
a longer boom.
All excellent points. This is why I thought I should ask the questions and start down the path to reasonable answers. I wonder if there is a scaled 3-view that overlays the 235/320/Legacy airframes? It would be an excellent tool to help put some of these things in perspective. Also, I thought that the LNC2 MKII tail was carried over into the Legacy.
All those changes are there in part to accommodate the greater
torsional loads from the Fowler flap design. The deeper camber
with the Fowler flaps requires a different pressure distribution
on the top of the wing to be effective. The deeper downwash
behind the wing changes the airflow at the tail. The more effective
flaps move the center of lift aft, further increasing the
tail loading. Perhaps those effects contributed to
Greg Cole's decision to put the tail where it is on the Legacy.
You wouldn't want to learn on your test flight that lowering
the flaps causes the tail to stall as you approach full up elevator.
Is it not true that the structures in each of these aircraft are designed for a specific weight range? That being the case, it explains why there are so many stiffer, heavier, stronger structures in the Legacy as compared to the LNC2. While it is pure speculation on my part, having done zero structural analysis, it seems that the incorporation of logically designed hard points and additional BIDs to help transfer the loads over a greater area within the LNC2 aft wing structure could be undertaken without too much concern. The original design is pretty well overbuilt as it is, and would the loads from Fowler flaps actually be that much greater than those imposed by the existing hinged ones? Logic would suggest that all of the current flap loads are taken up primarily by the lower wing skin and the aft spar shear web. If the hinge arms for the Fowler flaps were attached to hard points in the ribs that incorporated load-sharing BIDs that ran from the rib faces to the inner wing skins and the aft spar, then the ultimate load carrying ability would be increased dramatically. Far more, it would seem, than the modest increase in loading that I suspect results from going to the Fowler flap from the hinged style. This, after all, is pretty much how the Fowlers are installed in the Legacy wing. I repeat, this is speculation on my part, but it seems to be reasonable.
While getting the structural part of it right doesn't appear to be that great an obstacle, I agree with your assessment wholeheartedly regarding the aerodynamic impact of the modification. That part of it is way outside my purview, and I humbly defer to greater minds when it comes to those issues. Scott Krueger mentioned the importance of flap reflex in the cruise regime, and I'm sure that could be incorporated into the Fowler flap design. But again, an aerodynamic analysis would most certainly be in order and I'd love to learn more from you other aero types out there what other issues come into play.
I'm not saying that Fowler flaps for a 320/360 is a bad idea,
but there are quite a few ramifications to consider. At the very
least I would want to build and test a model (real or computed)
to determine how the new airfoil would work, and what loads it
would impose on the airframe. A structural engineering model
(real or computed) should be tested to determine what changes
would be needed to carry those loads. The new load and angle of
attack requirements for the horizontal tail would be estimated
from a tufted model or 3D-CFD calculation and re-engineered.
Then the new design should be re-analyzed for aeroelastic (flutter)
effects. Finally, the real airframe should be load tested on the
ground to see if the new design achieved it's modeled stiffness
and strength.
Again, I'm not saying it is a bad idea, but it would
definitely be earning the "experimental" badge.
Well, that's why they call them experimental airplanes. I'm with you, though, and would truly like to have that warm and fuzzy feeling that comes from a thorough analysis. Maybe when I'm rich and famous I'll get all that computer modelling done. I guess in the meantime I'll just continue to think about it as an interesting mental exercise. It's not like there isn't enough to do in simply building the airplane according to plan <g>.
<marv>
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