Winglets

[Dennis Ashworth]

I understand winglets improve fuel economy on certain aircraft.
Aerodynamically, what occurs that result in this improved fuel economy?

Dennis
Battle Ground, WA

[Parvaz]

As a wing moves through the atmosphere it produces at least two
distinct types of aerodynamic drag (the more technical the analysis
the more types of drag will be introduced). Parasite drag is the
resistance that is generated by the leading edge of the wing impinging
on the molecules present in the static atmosphere through which it is
passing. Parasite drag is what you feel when you stick your hand out
the window of a moving car. If the car is going fast enough it can
feel as though the force is powerful enough to rip your arm out of
your shoulder socket. Engineers try to design airplanes to be as
aerodynamically efficient as possible to help overcome the effects of
parasite drag.
Wings produce lift by separating the flow of air over their specially
engineered surfaces thereby creating differential pressure between the
upper and lower surfaces of the wing. The air has to travel "farther"
over the top surface of the wing than the bottom surface of the wing -
creating lower pressure "above" the wing, providing "lift" to the
airplane. By design, one side of the wing causes higher pressure to
exist than is present on the opposite side of the wing.
There is tremendous drag generated when the two different pressures
rejoin past the trailing edge of the wing. Much of this drag is
directed outward and focused at the wing tip. Induced drag is the
name given to this second type of aerodynamic drag that is created by
the violent remixing of the differential pressure above and below the
wing in flight. Not only does the high and low pressure air regions
bled behind the wing, they also blend at the wing tips. Induced drag
is an unavoidable byproduct of producing lift.
There are immense forces in play at the wing tips where the two
different pressures come together. A phenomenon called wake
turbulence and wing tip vortices takes place where virtual tornadoes
are created at the wing tips of a heavy jet in flight. These rotating
furies beginning at the wing tips in flight and trailing behind and
slightly below the wing are exceedingly powerful.
Properly designed winglets installed at the wing tips reduce, but do
not eliminate, the turbulence and friction (induced drag) that the two
differing air pressures create as they blend back together. Winglets
cause a reduction in induced drag resulting in greater aerodynamic
efficiency which yields significant fuel savings. At our airline we
are experiencing somewhere between 3% to 5% greater fuel efficiency on
airplanes with the winglets over those that don't have them. The fuel
savings is significant enough to justify the $50K+ cost of installing
winglets on the jets that don't have them. The airline for which I
fly is in the process of retrofitting all (those that will
structurally accept the modification) of its next generation Boeing
737 with winglets (they originally bought some -800s & -900s without
winglets).

Special thanks to R.P.S., master electrical engineer & father-in-law
extraordinaire for his help with this response.

[Dennis Ashworth]

Thanks for the very informative response!

A follow up question:

I know departing in a small aircraft (like a Cessna 172) several
minutes following the departure of a large jet may result in an
exciting ride from (what I assume are) wake turbulence and wing tip
vortices. Would the addition of winglets reduce a pilot's concerns
for these turbulences in a following aircraft, or are any reductions
largely insignificant?

Dennis
Battle Ground, WA

==

>--
>No virus found in this incoming message.
>Checked by AVG.
>Version: 7.5.519 / Virus Database: 269.21.7/1329 - Release Date:
>3/14/2008 12:33 PM

[Parvaz]

No, winglets will not reduce a pilot's concern regarding wake
turbulence. Winglets are engineering improvements that yield
aerodynamic efficiencies, thereby delivering better fuel economy, but
they do not mitigate the dangers of wake turbulence. Having winglets
installed probably reduces (speculation on my part) wingtip vortices
and wake turbulence, but that reduction isn't going to be enough to
diminish the danger in any significant amount.

Dennis below is a link to a FAA document called an Advisory Circular
on wake turbulence and wingtip vortices. An Advisory Circular is a
publication put out to educate the pilot population on some particular
point that the FAA determines needs to be better understood. I think
you will find it interesting. It is 17 pages long but easy reading
with many illustrations and diagrams.

http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/1472662a19f6603b86256c1600733da7/$FILE/AC90-23f.pdf

Since you live close to PDX, and if you have some time on your hands
you might consider driving out to the airport. When PDX is landing to
the west you can stand right under the approach path of airplanes
landing on 28L (the south runway). I forget the street number 86th,
84th, or 186th or something like that it runs north/south and
intersects the main road to the airport. If you pick a day where
there isn't much wind, and you stand right under the landing path of
the incoming jets, you should be able to hear the wake turbulence/
wingtip vortices crackle and snap overhead shortly after the jet has
landed. It should sound something like a cracking whip. Don't
confuse the sounds generated by the engines or the sound generated by
the airplane itself as it passes overhead, this will be a different
sound that will follow shortly after the jet passes overhead.
Something big and heavy like a FedEx or UPS heavy jet will yield the
most dramatic example. Boeing 757's are knows to generate strong wake
turbulence as well. Any large jet will do.

Years ago I saw a single engine Beechcraft airplane (like a Cessna
172) gets caught up in the wake turbulence of a DC-10 (3 engine, heavy
jet, about the size of a Boeing 747). What I witnessed was so violent
and it happened so quickly that at first my mind refused to believe
what I had just seen had actually happened to a "real" airplane. The
forces involved are tremendous!

Wake turbulence is dangerous! Having winglets installed on the
airplane in front of you will not reduce the danger of wake turbulence
in any substantial amount whatsoever- especially to a light aircraft
such as a Cessna 172. Rather than thinking of the danger in terms of
"an exciting ride" think of it in terms of "having the potential to
kill you" if you have the unfortunate experience of encountering
severe wake turbulence behind a heavy jet while you are close to the
ground on takeoff or landing. Not every encounter with wake
turbulence is fatal, but the phenomenon is exceedingly dangerous.

Dennis, how did you happen to come across this blog? Since I don't
advertise I am always interested in learning how people came across
it. I appreciate your interest and your questions!

Take care!

[pavas]

Another way of reducing the drag caused by wingtip vortices, though
impractical, is to fly in ground effect. Dennis, your 172 will
encounter significantly improved performance within 1 wingspan from
the ground. This is because the vortices are encountering friction
when impacting with the same ground. When I did my training in W.A., I
was taught to use this in case of coming short on a practise forced
landing. It is also what causes the float during an incorrect flare.

Since the introduction of RVSM, the definition on Wake turbulence
(Vortices) has changed slightly. Previously I was taught that these
extended 1000 feet below, however, nowadays they only seem to extend
800 feet below.

Although the 320 is not listed as heavy, it can leave quite a trail
especially in the flare, I had to GA in LHR once due to only 3 Nm
separation.

A colleage of mine was almost flipped over in a B737 over the gulf of
Liguria a few years ago after being vectored behind a B747.

Do not think that these only occur at TO and landing, avoid always and
respect separation minima.

P

On Mar 18, 7:10 am, Parvaz <tbdeut...@integrity.com> wrote:
>         No, winglets will not reduce a pilot's concern regarding wake
> turbulence.  Winglets are engineering improvements that yield
> aerodynamic efficiencies, thereby delivering better fuel economy, but
> they do not mitigate the dangers of wake turbulence.  Having winglets
> installed probably reduces (speculation on my part) wingtip vortices
> and wake turbulence, but that reduction isn't going to be enough to
> diminish the danger in any significant amount.
>
>         Dennis below is a link to a FAA document called an Advisory Circular
> on wake turbulence and wingtip vortices.  An Advisory Circular is a
> publication put out to educate the pilot population on some particular
> point that the FAA determines needs to be better understood.  I think
> you will find it interesting.  It is 17 pages long but easy reading
> with many illustrations and diagrams.
>

> http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular...

[Parvaz]

That is REALLY funny about the location of wake turbulence changing
since the introduction of RVSM to 800' from 1,000' --- that is
comical !!! I am looking forward to your article on flight deck CRM
Europian style!

-

[pavas]

This is a very in depth study of the phenomena of wingtip vortices,
probably further than the scope of this forum.
http://www.eurocontrol.int/eec/gallery/content/public/documents/PhD_theses/2006/Ph.D_Thesis_2006_Choroba_P.pdf

As my books are not with me, I cannot quote pages regarding the 800
feet, but I will. It is interesting and funny, I assume it is a result
of more in depth study more accurately defining the region of risk
rather than stretching a point to allow 1000 feet of vertical
separation. if you see the above link, you will note that 500 to 900
feet is specified.

At the same time, 1000 feet was and is standard below RVSM.

P

Slight correction to my previous, Dennis, your 172 being a high winger
will be less affected than would a low winger simply due to the
proximity of the wing to the ground.