winglets

OK, I can get the theory (OR can I?) with winglets to reduce wingtip vortices.

Surely the best direction for the tip is DOWN!, I understand this may be difficult for actually making, especially with gliders and groundloops, aesthetics, etc, but can someone explain why they are pointing up?

Consider this..
The HIGH pressure area is UNDER the wing, we want to keep this from migrating to the LOW pressure area ABOVE the wing. Our current wingtip style will restrict the LOW pressure from bleeding sideways (and why would it anyway) yet the majority (?) of our lift is generated from the HIGH pressure BELOW the wing.

This high pressure is what we want to retain and utilise, yet it is still allowed to bleed out sideways. Sure the current shape of wingtip will reduce the vortices caused by the two pressures meeting, and thus the drag, but it wont increase the lift available by retaining the HIGH pressure UNDER the wing, where it is most useful.

I also note the return of the "plate" style winglet on latest jetliners, half above, half below the wingtip, instead of a large winglet.

Enlighten me, O knowledgable ones!

Wayne

Originally Posted by bagmaker

OK, I can get the theory (OR can I?) with winglets to reduce wingtip vortices.

Surely the best direction for the tip is DOWN!, [...] Enlighten me, O knowledgable ones!

Wayne

i'm certainly not that knowledgeable person, but i have heard people whom i would consider quite knowledgeable say that the orientation of the winglet really doesn't matter - it could be flat, or pointed down. either way it reduces induced drag. however, costly hangar space is costlier if the wingspan is wider, and i imagine crosswind landings could become more interesting with winglets pointed down.

:-)

frank

It does matter. The pressure on the top side of the wing I a lot lower
that the surrounding air, the overpressure on the downside only a bit
higher. That way it makes sense.
For avery angle of attack (Cl) there is an optimum configuration,
usually a double eliptical shape. (Bend up like a Discus-2, a bit the
shape of an ls-6/8)
Alhoewel real life deals with Reynolds and sideslip whick makes is a
lot more complicated.

For the jetliners, a very small "extra" inverted winglet clearly lowers
the vortices. Also dependant of cruise Cl, Reynolds, mach number.

Jarno Nieuwenhuize
The Netherlands.

At 04:12 07 January 2006, Bagmaker wrote:

OK, I can get the theory (OR can I?) with winglets
to reduce wingti
vortices.

Surely the best direction for the tip is DOWN!, I understand
this ma
be difficult for actually making, especially with gliders
an
groundloops, aesthetics, etc, but can someone explain
why they ar
pointing up?

Consider this..
The HIGH pressure area is UNDER the wing, we want to
keep this fro
migrating to the LOW pressure area ABOVE the wing.
Our current wingti
style will restrict the LOW pressure from bleeding
sideways (and wh
would it anyway) yet the majority (?) of our lift is
generated from th
HIGH pressure BELOW the wing.

This high pressure is what we want to retain and utilise,
yet it i
still allowed to bleed out sideways. Sure the current
shape of wingti
will reduce the vortices caused by the two pressures
meeting, and thu
the drag, but it wont increase the lift available by
retaining the HIG
pressure UNDER the wing, where it is most useful.

I also note the return of the 'plate' style winglet
on lates
jetliners, half above, half below the wingtip, instead
of a larg
winglet.

Enlighten me, O knowledgable ones!



It's not so much about blocking the flow since it's
all going to mix once it gets aft of the wing anyway.
The winglet actually reduces the vortex by creating
circulation off the vortex itself - this is why a winglet
is an airfoil and not just a plate. It flies in the
local flow field at the tip and creates a lift vector
that is inward and slightly forward (that's right forward
lift - thrust!).

I honestly don't know if pointing up versus down is
for practical or aerodynamic reasons - I can't think
of a reason why it would matter, you'd have to reverse
the airfoil and reoptimize for the underside flow field.

9B

Hi Wayne,

yet the majority (?) of our lift is generated from the
HIGH pressure BELOW the wing.

No, it only generates 1/3 of the lift. The rest is generated by lower
pressure on the upper side.

I recommend you read
http://www.soaridaho.com/Schreder/Te...lets/Masak.htm
which explains the design and function of winglets.

Greetings, Erik.

Wayne,

I have created a page containing links to various winglet articles. Most
are based on Mark Maughmer's research at Pennsylvania State University.
http://www.soaridaho.com/Schreder/Te...ts/PSU_Ref.htm

Wayne
HP-14 N990 "6F"
http://www.soaridaho.com/



"Erik Braun" wrote in message
...
Hi Wayne,

yet the majority (?) of our lift is generated from the
HIGH pressure BELOW the wing.

No, it only generates 1/3 of the lift. The rest is generated by lower
pressure on the upper side.

I recommend you read
http://www.soaridaho.com/Schreder/Te...lets/Masak.htm
which explains the design and function of winglets.

Greetings, Erik.

"The rest is generated by lower pressure on the upper side."

hmmmm ...

http://www.eskimo.com/%7Ebillb/wing/airfoil.html

http://www.aa.washington.edu/faculty/eberhardt/lift.htm


Erik Braun wrote:
Hi Wayne,

yet the majority (?) of our lift is generated from the
HIGH pressure BELOW the wing.

No, it only generates 1/3 of the lift. The rest is generated by lower
pressure on the upper side.

I recommend you read
http://www.soaridaho.com/Schreder/Te...lets/Masak.htm
which explains the design and function of winglets.

Greetings, Erik.

Ken Kochanski (KK) wrote:
"The rest is generated by lower pressure on the upper side."

hmmmm ...

http://www.eskimo.com/%7Ebillb/wing/airfoil.html

http://www.aa.washington.edu/faculty/eberhardt/lift.htm

Can you point out where these articles compare the pressures on the top
and bottom surface, or some reference that gives the relative
magnitudes? I was unable to find it.

--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

Eric Greenwell wrote:
Can you point out where these articles compare the pressures on the top
and bottom surface, or some reference that gives the relative
magnitudes? I was unable to find it.

For "classic" airfoils, try the book Theory of Wing Sections, by Abbott
& VonDoenhoff, THE reference for us old aero types.

You'll see section pressure distributions that clearly show far more
suction on the top (area under the curve of pressure coefficient vs.
chord), than high pressure on the bottom.

People are forgetting that there are other factors affecting the impact
of winglets, including the height of the winglet and more importantly,
the toe in -- which some have claimed generates "thrust."

I won't pretend to be a winglet expert (flying a non-wingletted 1-26),
but it has been pointed out to me that winglets tend to be point design
items (e.g., Global Flyer, Voyager), or compromises that provide
different benefit at different angle of attack (or C-L).

In some bathroom stall (at Boeing, Douglas or Northrop -- I've worked
for all 3), there was posted a truism:
"There is no substitute for span."

However, more span means more wing root bending (trying to pull the
wing tips up until they touch...), and some other problems (e.g., stall
characteristics, tail power required...). The complaint about hangar
space is in there, too, but look at the span of the 777 and the A380...
If you build it, they'll make room. Or maybe the Eta?

Winglets have competition from their more contemporary cousin, the
raked tip (fashionable on aircraft such as the 767-400). The raked
tips begin to lose thier effectiveness at higher angles of attack
(e.g., near stall), and thus mitigate some of the downsides of winglets
or added span.

You'll still have vortices and wake. One day I flew through the wake
of the late Mark Navarre (OD), and told him he "thumped" me. He
complained that his glider (ASW-20?) wasn't supposed to have a wake.
If you're heavier than air and you're flying, you ARE going to generate
a wake.

You'll still have vortices and wake. One day I flew through the wake
of the late Mark Navarre (OD), and told him he "thumped" me. He
complained that his glider (ASW-20?) wasn't supposed to have a wake.
If you're heavier than air and you're flying, you ARE going to generate
a wake.

Back in the sixties, during a contest at Grand Prairie, TX, I was in the
reurn part of an O&R task -- approaching Fort Worth. Ahead, crossing my
path and well below was a B-36 taking off with all ten engines working
mightely. About two minutes later, wake turbulence threw my Schweizer
1-23 almost inverted. Luckily, after recovery enough height remained to
enable a "good finish".
--
Charles Yeates

ZS Jezow PW-6U & PW-5
http://www3.ns.sympatico.ca/yeatesc/world.html