Wing tips. The bits on the end of the wing. The tips that arguably are hotly
contested
at all points of pilots meeting and talking about aeroplanes. The bits
that cannot be improved upon. The bits that cause drag. The bits that attempt
to stop the air under the wing meeting the air at the top of the wing. The bits
that look awesomely cool and go faster. The bits that without the aircraft
would still fly fairly well. The bits that get researched by every aerospace
grad
student in an effort to discover something that has never been discovered
before. The bits that get smashed off as the aircraft gets rolled back into the
hanger. The bits that are swappable depending on prevailing conditions. Some
are painted. Some are plastic. Some are metal. Some are left off all together.
I am left convinced without a doubt that humans that fly aeroplanes are fixated
upon wingtips just as a dog is fixated upon the hubcaps of a car that drives
past. That would make me just as insane as well.
So now that the pre-amble is dealt with sufficiently we can focus upon what was
achieved and how it was achieved.
33 different variations of wing tips were modelled up and each attached onto our
trusted test pig of a clean unadorned Europa Classic plug, one at a time, and
fed into the digital wind tunnel to compare and contrast the results. Some
discoveries
were made and some expectations were surprisingly successful and others
unexpected. It was a fairly involved process that took a fair amount of time
to work through.
At this point its important to remember that these tests are not finite or
matched
exactly to the real world. They are comparative and trend specific only, and
can only be used as indicative due to each aircraft being individually built
by hand and therefore the performance of each finished aircraft can differ
immensely.
Each model received exposure to exactly the same environment variables. So the
only alternation across the entire study has been the wing tip; there has been
no other variable applied.
Our fluid Air environment was set to standard baseline Zero AMSL values. The AoA
of the aircraft was set to Zero degrees; so the Alpha (AoA of the wing to the
Free Stream Air) will equal the Europas Wing Incidence. Our Velocity was set
to 51ms (100kt).
On each Solution Run of 100 iterations only two basic criteria were recorded
Force
X (Drag), and Force Z (Lift). Because the study was comparative all that was
required was to know how x wing tip affected the numbers taken from our baseline
model developed upon Europa 181s airframe. Results are presented as x,z
format and are in Nm.
The Mesh resolution of each Solution was set to high.
Additional criteria reviewed upon completion of each solution were focused upon
wing tip vortex diameter; were they significantly larger or smaller than
provided
by the baseline model.
This is the part in the youtube video that I say So lets get into it in a really
super excited fake way and point my finger at the camera lens.
First off the block is our trusty Pig, sorry Pug, zero alterations, and not
surprisingly
it spat out:
606 / 3569
At this point I am going to supplement the results just recently received on the
wing tips modelled off VH-CUA, as they are of a more rounded form than those
on 181 and were:
CUA - 563 / 3747
Back to solution runs, a bit unconventional and worth a review all the same:
A - 627 / 4218
Could not work out why the drag results were so high; please be suggesting
something:
B - 2322 / 3250
Everybody wants to look cool, Winglets:
C - 568 / 3911
Still looking cool, looking cool, very cool etc, tweaking things and seeing the
results
D 593 / 3941
E 572 / 3995
F 582 / 4109
G 583 / 4153
H 583 / 4152
I 603 / 3952
J 577 / 4130
K 592 / 4345
L 585 / 4078
M 594 / 4153
Things started to get boring, so just for the sake of it elliptical tips were
modelled
up. The version numbers of the different elliptical tips relate to the
distance in (mm) from the outer extremity of the wing span toward the wing root
where the ellipse begins. So the lower the number the closer to the tip the
ellipse begins. The formatting of the ellipse is fairly rudimentary and is based
around the Supermarine Spitfire ratio. Versions designated T relate to the
distance in (mm) added to the standard Europa wingspan of the 2950 Ellipse.
EL50 559 / 3865
EL100 547 / 3833
EL150 542 / 3864
EL250 531 / 3839
EL350 527 / 3793
EL500 523 / 3716
EL750 529 / 3690
EL1000 515 / 3646
EL1250 530 / 3533
EL1500 528 / 3425
EL2000 505 / 3394
EL2500 495 / 3359
EL2950 500 / 3249
T250 517 / 3410
T350 515 / 3669
T550 532 / 3927
T660 529 / 4170
What about chamfered wing tips, like those on a Sonex?
C01 572 / 3973
So what information can we garnish from the results of all the solution runs?
Like everything else that humans mostly do there needs to be an incentive. And
as long as there is the provision of quality and care there is no better
incentive
than a free lunch!
Firstly I need to confess that test B was not a serious concept, but I was
expecting
Darth Vader to turn up at any moment afterward to take his ship and fly
away with expedience.
Back to the free lunch; what do we want to see from these results? We want to
see
less Drag and more Lift; because we fly Europas not Drifters.
606 / 3569 is our baseline, so anything better than that is a winner; or is it?
Looking at the Elliptical numbers we see that as the ellipse is extended the
Lift
and Drag trend diminishes. However with the Winglet style tips we see a decrease
in Drag and increase in Lift trending. For example, Winglet K is signalling
68kgs of additional Lift and 1kg less Drag. Thats nothing to scoff at when
its placed on the table, and this appeared to be where the winglets were maxing
out. Fine tuning and tweaking could not really achieve any better results.
Winglet A2 was progressively trimmed back to a fairly small tip similar to a
Jabiru
and did not really alter the results greatly. Did the Winglets reduce the
size of the tip vortex, yes they did by about 30% on average; but not as much
as the Elliptical tips did.
The Elliptical tips are in short simply amazing at their ability to reduce tip
vortex size by some 90% to almost nothing. Regardless of the version tested they
all displayed the ability to dramatically reduce the diameter of the tip vortex;
even the 50mm Elliptical tip provided a dramatic advantage over the standard
style wing tip found on 181. If we look at the 150mm or 250mm Elliptical
tip we can see the similarity with those found on an old Biplane or Aviat Pitts;
handy to know, and lets come back to that.
Regarding the rounded tips on VH-CUA vs the flatter version found on 181. Well
CUA is delivering an easy to prepare canap that anybody can probably achieve
without
too much fuss.
The Chamfered wing tip results were surprising and not expected to be that
great.
But after looking at the numbers it was plain to see why this simple yet
affective
style of tip is so widely used; its simple, and its affective. The Chamfered
tip returned 40kg of free lunch and 3.4kg of less alcohol to slow things
down. But where it looses out is at higher angles of attack. Although the size
of tip vortex was close to the elliptical tip in the 0deg attitude, things
change as higher angles of attack are assumed with tip vortex sizes increasing.
However the Elliptical style of wingtip does not suffer this, as AoA is
increased
the tip vortex size remains quite small.
Another aspect observed of the Elliptical style tip was the extended working
area
of the wing span. Where as the standard 181 style tip sees maybe 85% of the
wingspan actively generating Lift, the Elliptical style tip sees almost 100%
of the wing span generating Lift. However this ability to generate more Lift
balances
out again as there is less comparative surface area for the same chord
and span sizing. So as we extend the ellipse inward toward the wing root the
amount of Lift generated diminishes due to a smaller overall surface area.
Additionally Elliptical style wing tips appear to maintain laminar airflow over
the aileron section of the wing at high angles of attack along with very small
tip vortex sizes, with the stall characteristics trending to an overall higher
AoA ability of the wing to remain flying which in itself assists STOL
capabilities.
Feeding all the results into a simple algorithm of (z/x) versions K, E250, E350,
T350, T550, T660 stand out as decent performers, all bouncing over the 7.15
mark with T660 on top at 7.88 by cutting Drag down some 7kg while increasing
Lift
by 61kg. However E250 and E350 at 7.22, 7.19 respectfully probably win due
to their more conservative gains of lets say 7kg reduction in Drag and 28kg
increase
in Lift while the wingspan remains unchanged.
So at the end of it all E250 and E350 appear to be an achievable style of tip
and
similar to those found on old biplanes and the Aviat Pitts and based upon
comparative
results a very efficient form of wing tip perhaps? Either way it would
appear that Elliptical style wing tips cannot be beaten when it comes to overall
aerodynamic performance and efficiency.
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=511240#511240
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