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Please disregard my winglet post from 4/26. That post was a draft copy and
should be replaced with the following:
My winglet design methods incorporate a more highly loaded and effective
blended winglet technology. Currently Lancair uses a Whitcomb (circa 1972)
winglet with a minor wing/winglet blend radius modification. One of the
aspects of the Whitcomb design is high leading and trailing edge sweep
angles which tend to decrease the effectiveness of the winglet. Sweep will
reduces effectiveness on any surface. For example, the old 172 straight
tail, once Cessna swept the tail they had to increase its size to make up
for the loss in effectiveness. The Mooney vertical tail has zero leading
edge sweep, is relatively small, and is very effective.
The 747 cruises at Mach .8 to .85 with a lift coefficient of .5 to .7
(depending on MTOGW). The Lancair IV cruises at Mach .45 to .5 with a
cruise lift coefficient of .18 to .20. Speeds above Mach .7 (transonic
region is between Mach .7 to 1.2) require swept surfaces to control Mach
effects on the wing, tail, and control surface aerodynamics. Also winglets
(particularly ones that are not aerodynamically loaded) do not work well
below lift coefficients of approximately .5 That means that on the Lancair
IV there is very little chance that a winglet will reduce drag at cruise
lift coefficients, particularly one that uses Whitcomb design methods. The
purpose of the winglets on the IV were to improve high altitude stability
issues, in particular dutch roll mode. Several test pilots have verified
this mode as objectionable on the Lancair IV and have stated that it becomes
worse with altitude and aft CG. This is not uncommon to high speed high
altitude aircraft. Many high performance aircraft are required to have yaw
dampers engaged above a specified altitude. The winglets were also added on
the Lancair IV to decrease stall speed and improve roll stability or
dihedral effect. Currently roll stability is neutral without them, not
necessarily a bad thing but not a good thing either. Several sources have
reported that after several design iterations Lancair was able to reduce the
cruise speed impact of their winglets from approximately 20 kts to 5 to 10
kts. This was primarily accomplished by reducing the size of the winglet
which also reduced their effectiveness. The result was a winglet that does
increase stability and reduces stall speed over the Lancair IV without
winglets. It is hoped that my winglet design will be an improvement over
the Lancair design and further decrease the cruise speed impact to zero,
further reduce the stall speed, and produce even more climb rate.
The Whitcomb winglet made some assumptions of winglet geometry that were
considerably different from the main wing. What we later discovered through
computational fluid dynamics (CFD), wind tunnel testing, and flight test was
that the geometry of the winglet (except for the selection of airfoils and
airfoil thickness) should mirror the wing very closely. The first person to
explore this in detail was ex-Chief of Boeing Aerodynamics Bernie Gratzer
through his company (Aviation Partners). The blended winglet is a departure
in that it has a very large blend radius, an unloaded intersection and
reloaded and twisted winglet section, and larger aspect ratio. The winglet
section has similar geometry, though smaller scale, as the wing. It is
almost like tipping a smaller version of the main wing up on edge and
hooking it together with a very large blend radius. Just before I left
Boeing Aerodynamics to work on the American Engine project we did a detailed
study on every wing tip device you can imagine on each of the Boeing models
from 737 to 777. Bernie had left the company many years before and was
actually having a hard time getting Boeing to buy into the blended winglet
concept though we were studying something similar. Bernie finally got
Boeing to fully buy into the program after he did a set of Winglets for the
Gulfstream II and showed over 7% fuel burn improvement at cruise compared to
the 1% we showed on the Whitcomb winglet on the 747. His winglets are now
flying on the Boeing Business Jet, take a look at Bernie's web site
www.aviationpartners.com and the Boeing web site at www.boeing.com. What
you will notice about blended winglet technology is less sweep, more aspect
ratio, longer root chord, and more twist distribution. One draw back to a
highly loaded and effective winglet is the impact it has on wing structure.
Many structural changes were required on the 747 and the Boeing Business
Jet, beyond the attachment structure, to accommodate the winglet
installation. Some of these changes included, stringer, spar, and skin
gauge increases. However, with sufficient structural margin in the wing,
which the Lancair IV does appear to have, a less radical winglet can be
designed for retrofit which does not require extensive modifications to the
wing structure. This is the approach taken by many companies that have
retrofitted winglets to existing airframes, including the Lancair IV.
The reason for using a large blend radius is to smooth the transition from
wing to winglet where most of the interference drag and lift losses occur.
The result is a much more effective winglet giving you better lift
coefficients and better drag reduction at high lift coefficients (high
angles of attack). As you may know the way a winglet reduces drag is by
reducing wing tip vortices at the wing tip which are produced by the higher
pressure lower surface air rolling up toward the lower pressure upper wing
surface air. The 3d effect at the wing tip produces a rotating vortex sheet
that is shed from the wingtip. This vortex sheet produces induced drag
(drag due to lift) through energy losses from the wing. The blended winglet
does a better job of controlling and recapturing this vortex than the
Whitcomb winglet.
What this meant for me on the Lancair IV was optimizing a winglet for our
wing geometry, lift coefficient, and cruise speed. Going in I felt that if
I could get all the benefits of the bigger wing area and the resulting
stability, climb, and stall speed improvements without it costing me
anything in cruise speed I would be successful. According to my engineering
analysis, which must still be verified through flight test on Derek Hine's
Lancair IV, is that at cruise speed they will cost us from 0 to 2 kts at max
and mid weights respectively. They should also have a pronounced
improvement in increased climb rate and reduced stall speed. Remember that
the higher the lift coefficient the better a winglet works which means the
slower and heavier we go and the higher the angle of attack the higher the
lift coefficient. This means that both Lancair's and my winglet will work
at the low end, but hopefully mine will work better in the low end and cost
us nothing at the big end.
Curtis M. Longo, President
Aviation Ventures Inc.
Email: avi@uia.net
http://www.americanengine.com/
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LML website: http://www.olsusa.com/Users/Mkaye/maillist.html
LML Builders' Bookstore: http://www.buildersbooks.com/lancair
Please send your photos and drawings to marvkaye@olsusa.com.
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