Wingtip Vortex: Heavy, Clean, Slow - Why?

[Eric Nospam]

You get the strongest wingtip vortices when an aircraft is flying heavy,
clean and slow. But why?

It has to do with the amount of lift being generated by the wing. HEAVY I
can understand. Heavier aircraft - you need more lift to keep it aloft.

But CLEAN? SLOW? Why do you get less vortex with the flaps down? Why does
a slow-moving aircraft generate a stronger vortex than a fast-moving
aircraft generating the same amount of lift? Is it because of the larger
angle of attack necessary to generate the same lift at a lower airspeed? If
so, why?

Eric

[AJW]

>
>You get the strongest wingtip vortices when an aircraft is flying heavy,
>clean and slow. But why?

Re slow: the airplane stays aloft in effect because its wings are pushing air
down. If it's moving fast, the momentum of the displaced air is spread over a
larger distance, so it's more diffuse. Slower means there's much more energy in
the air.

Not so sure about dirty v clean, but at low speeds clean isn't very effecient
at generating lift. That means larger angles of attack, more air displaced,
maybe a bigger vortex.

[Wdtabor]

>
>Not so sure about dirty v clean, but at low speeds clean isn't very effecient
>at generating lift. That means larger angles of attack, more air displaced,
>maybe a bigger vortex.
>

Clean, the angle of attack of the WINGTIPS is greater for the same lift at the
same speed. The vortex happens at the wingtips, not the wing as a whole.


--
Wm. Donald (Don) Tabor Jr., DDS
PP-ASEL
Chesapeake, VA - CPK, PVG

[Dan Girellini]

(AJW) writes:

>>
>>You get the strongest wingtip vortices when an aircraft is flying heavy,
>>clean and slow. But why?
>
> Not so sure about dirty v clean, but at low speeds clean isn't very effecient
> at generating lift. That means larger angles of attack, more air displaced,
> maybe a bigger vortex.

From <http://av8n.com/how/htm/airfoils.html#sec-wake-vortices>:

You would think that...flaps extended would be the absolute worst, but that
is not quite true. The flaps do increase the circulation-producing
capability of the wing, but they do not extend over the full
span. Therefore a part of the circulation is shed where the flaps end, and
another part is shed at the wingtips. If you fly into the wake of another
plane, two medium-strength vortices will cause you less grief than a single
full-strength vortex. Therefore, you should expect that the threat from
wake vortices is greatest behind an airplane that is heavy, slow, and
unflapped.

d.

[Orval Fairbairn]

In article >,
"Eric Nospam" > wrote:

> You get the strongest wingtip vortices when an aircraft is flying heavy,
> clean and slow. But why?
>
> It has to do with the amount of lift being generated by the wing. HEAVY I
> can understand. Heavier aircraft - you need more lift to keep it aloft.
>
> But CLEAN? SLOW? Why do you get less vortex with the flaps down? Why does
> a slow-moving aircraft generate a stronger vortex than a fast-moving
> aircraft generating the same amount of lift? Is it because of the larger
> angle of attack necessary to generate the same lift at a lower airspeed? If
> so, why?
>
> Eric


A basic aerodynamic equation is the Prandtl-Meyer equation:

L = Rho*V*Gamma, where
L is the generated lift;
Rho is the air density;
V is the velocity of the airstream;
Gamma is the vortex strength.

Assuming L = constant, Gamma has to increase as V decreases to maintain
equilibrium.

Simple, eh?

[ShawnD2112]

Think of it in terms of angle of attack and you'll get a better picture of
it. With flaps down, you need a lower angle of attack to maintain the same
lift, so you'll have less vortex. Slow, you need a higher AOA to maintain
your lift or decent rate.

Watch a fighter pulling hard Gs at and airshow and you'll see the wingtip
smoke curl inward with the vortex and, in the case of an F-16, the two
wingtip streams often meet in the middle of the wake from the strength of
the vortex.

Shawn
"Eric Nospam" > wrote in message
om...
> You get the strongest wingtip vortices when an aircraft is flying heavy,
> clean and slow. But why?
>
> It has to do with the amount of lift being generated by the wing. HEAVY I
> can understand. Heavier aircraft - you need more lift to keep it aloft.
>
> But CLEAN? SLOW? Why do you get less vortex with the flaps down? Why
> does a slow-moving aircraft generate a stronger vortex than a fast-moving
> aircraft generating the same amount of lift? Is it because of the larger
> angle of attack necessary to generate the same lift at a lower airspeed?
> If so, why?
>
> Eric
>

[Dan Girellini]

"ShawnD2112" > writes:

> Think of it in terms of angle of attack and you'll get a better picture of
> it. With flaps down, you need a lower angle of attack to maintain the same
> lift, so you'll have less vortex.

That doesn't seem to explain it to me. If the flaps are down you may well be
moving slower than you could with flaps up, requiring a larger aoa.

d.

[[email protected]]

For a given speed, flaps will lower the overall AOA. Even in a
172 we can see this. It's the increased camber of the wing, as well as
its increased area if they are Fowler flaps, that produce more lift and
allow the AOA to decrease at any given airspeed.
The vortices are mainly a product of the tips, but the whole wing
has input. Air on the bottom is being squeezed and tends to flow not
only back (chordwise) , but outward (spanwise), and the air on top,
being of lower pressure, is sucked inward and flows at an angle toward
the fuselage. The angles are more pronounced the farther out on the
wing we go, and at lower speeds, where AOA is higher, they get bigger
overall. The air leaving the trailing edge ends up with a twisting
motion, producing small vortices all along the TE and a really big one
at the tip, caused by air spilling over the tip. Winglets are supposed
to control this spill, thereby reducing the drag caused by vortices.
Dan

[[email protected]]

Actually, the reason that a dirty plane has vortices that are less
intense than a clean airplane is that the vortices coming off the flaps
themselves act destructively with the vortices coming off the wingtips.
Same principal is in play on a turbulent day when the vortices are
broken up more quickly than on a day with smooth air.

Dave
Eric Nospam wrote:
> You get the strongest wingtip vortices when an aircraft is flying
heavy,
> clean and slow. But why?
>
> It has to do with the amount of lift being generated by the wing.
HEAVY I
> can understand. Heavier aircraft - you need more lift to keep it
aloft.
>
> But CLEAN? SLOW? Why do you get less vortex with the flaps down?
Why does
> a slow-moving aircraft generate a stronger vortex than a fast-moving
> aircraft generating the same amount of lift? Is it because of the
larger
> angle of attack necessary to generate the same lift at a lower
airspeed? If
> so, why?
>
> Eric

[steve.t]

Is this why there are fences on wings? [I am assuming that the over
sized chord shaped flat metal blade on a wing is a fence]. That is to
keep the air flow straighter than it would otherwise flow?

And speaking of winglets, would a winglet provide sufficient efficiency
increase for piston singles to make it worth the cost of the
modification (field approval or STC)?

Later,
Steve.T
PP ASEL/Instrument

[Morgans]

"steve.t" > wrote in message
oups.com...
> Is this why there are fences on wings? [I am assuming that the over
> sized chord shaped flat metal blade on a wing is a fence]. That is to
> keep the air flow straighter than it would otherwise flow?
>
> And speaking of winglets, would a winglet provide sufficient efficiency
> increase for piston singles to make it worth the cost of the
> modification (field approval or STC)?
>
> Later,
> Steve.T
> PP ASEL/Instrument
>

Without getting into a bunch of searching for proof, what I recall reading
is that winglets can be thought of as more wing, to keep the vortices from
coming off the wing tip. Vortices still come off the winglet. Some of the
energy is reclaimed by making the vortices from the winglet flow over the
airfoil, adding lift. Angle and size are important, as being slightly wrong
can soon destroy any gains.

They are most efficient at speeds above what piston singles can attain.
Fences and other trick wing tips (drooped, angled) are better at low speeds.
You are better off adding more wingspan to increase the aspect ratio, at our
size.

Airliners can't simply add more wing length, and still fit in the gates and
hangars, and keep the spars strong enough, so they put on the winglets.

I'm sure that some of this is not quite right, but I think the general
concepts are correct.
--
Jim in NC

[Hilton]

Dan Girellini wrote:
> From <http://av8n.com/how/htm/airfoils.html#sec-wake-vortices>:
>
> You would think that...flaps extended would be the absolute worst, but
that
> is not quite true. The flaps do increase the circulation-producing
> capability of the wing, but they do not extend over the full
> span. Therefore a part of the circulation is shed where the flaps end,
and
> another part is shed at the wingtips. If you fly into the wake of
another
> plane, two medium-strength vortices will cause you less grief than a
single
> full-strength vortex. Therefore, you should expect that the threat
from
> wake vortices is greatest behind an airplane that is heavy, slow, and
> unflapped.

Yeah. Go tell Denker to fly behind a flapped 757 on an approach.

http://www.aopa.org/asf/asfarticles/sp9810.html
http://www.aopa.org/asf/asfarticles/sp9403.html

Hilton

[[email protected]]

If I'm not mistaken fences prevent the spanwise flow across the wing,
delaying the onset of a stall. For the airplanes that we typically fly
the wing stalls from the root first then spanwise to the tips, the
fences help to reduce this. Vortex generators on the other hand do
exactly what their name imply, create vortices that re-energize the
boundary layer on the wing. Slots also serve a similar purpose, except
that higher energy flow from beneath the wing is allowed to travel to
the upper surface and re-energize the boundary layer.

Dave

[[email protected]]

But the reason that you can fly slower in the first place is because
the flaps are down. :-) If you look at a CL curve for a wing with and
without flaps you will notice that with flaps extended the curve moves
up and to the left. Up and to the left. (Sorry about the JFK movie
bit). So while the AOA with flaps is reduced for the overall wing, the
local lift coefficient in the section with flaps extended is higher.
The common belief is that downwash causes vortices which in turn
increase drag. Actually, it's the wing tip vortices and vortices bound
to the wing surface that deflects the airflow in the vicinity of the
wing downwards, contributing to the creation of downwash and induced
drag.

Dave

[steve.t]

Dave:

Actually, you want the root to stall first so that you can maintain
control. If the tips stall first, you will lose aileron effectiveness
immediately.

And I think this is why "stall strips" are put on the inboad sections
of wings on certain aircraft, to force the root to stall before the
wing tips.

In looking at airfoil designs and their stall characteristics, the
Piper "Hershey Bar" design gives the best stall handling as it
naturally stalls from the root moving outward to the tips.
Regards,
Steve.T
PP ASEL/Instrument

[steve.t]

Jim:

I've noticed and have flown C-15x with the exagerated wing tip (folded
down) and I've not really noticed any improvement at low speed (which
is what I think it is for, better handling at low speed for approach).

What I am interested in is increased lift, and better performance (in
terms of fuel usage) which is what the winglets do for the B-747
(something like 30% improvement in fuel economy! if memory serves me
correctly).

So if we could obtain the equivalent above 6000' MSL without wrecking
the approach speed performance, I was wondering if it would be worth
it. Particularly when looking at US$3/gal for 100LL.

In my case, with speed mods for wheel pants, wing tips, rudder and flap
gap seals, I get 1 Gal/hr better than the POH calls for with a fully
run out engine (I've just had it overhauled and don't have current
figures). What that means is, at 70% power, POH calls for 9.3 GPH at
6000' MSL (STD day). I get 8-8.3 GPH and 4-7 Knots better speed and a
lower actual stall speed. This was rather expensive to do (all the STCs
cost more than I would get back even flying 100+ hours/yr -- thankfully
the plane already had them when I bought it). And I do not do aggresive
leaning (I only lean until I start to get an engine drop of any RPM and
then enrich 1.5 turns).

But if a single mod could take advantage of the vortice energy to
recover 30% at cruise, that might make it worth doing. Particularly if
it added to climb rates (which I also see with all the STCs I currently
have).

Regards,
Steve.T
PP ASEL/Instrument

[[email protected]]

Steve,

I think my original post mentioned that the wing stalls first at the
root and then move spanwise toward the tips, and that stall fences
delayed the seperation from moving out towards the wingtips and hence
the ailerons. A more common way to ensure the root stalls first is
washing out the wing. Of course this doesn't apply to swept, tapered,
or elliptical wings which either stall uniformly or at the tips first.
Dave

[Blueskies]

"steve.t" > wrote in message oups.com...
> Jim:
>
> I've noticed and have flown C-15x with the exagerated wing tip (folded
> down) and I've not really noticed any improvement at low speed (which
> is what I think it is for, better handling at low speed for approach).
>
> What I am interested in is increased lift, and better performance (in
> terms of fuel usage) which is what the winglets do for the B-747
> (something like 30% improvement in fuel economy! if memory serves me
> correctly).
>

Oh, no, not 30%! If that were true then every plane would have them.

http://www.boeing.com/commercial/737family/winglets/wing2.html

[Morgans]

"steve.t" wrote

> Jim:
>
> I've noticed and have flown C-15x with the exagerated wing tip (folded
> down) and I've not really noticed any improvement at low speed (which
> is what I think it is for, better handling at low speed for approach).

The fact that all small planes do not have that type of wingtip, is a
testamony to what little benefit they must provide. If they were
noticeable, by simple observation, everyone would be using them. I have no
doubt that they have a measureable gain in slow speed performance, when
measured in a wind tunnel, but enough for you to notice on the
plane?....Gimmic?... They do look cool. <g>


> What I am interested in is increased lift, and better performance (in
> terms of fuel usage) which is what the winglets do for the B-747
> (something like 30% improvement in fuel economy! if memory serves me
> correctly).

30% seems a little, or a lot high, from my memory.

Longer wingspan, high aspect ratio wings are almost always more efficient.
Look at sailplanes. That is the primary way winglets improve efficiency.
Also, remember my comment about them being better at higher speeds than
where we operate piston singles, or even piston twins.

> So if we could obtain the equivalent above 6000' MSL without wrecking
> the approach speed performance, I was wondering if it would be worth
> it. Particularly when looking at US$3/gal for 100LL.

Unlikely. See above.

> But if a single mod could take advantage of the vortice energy to
> recover 30% at cruise, that might make it worth doing. Particularly if
> it added to climb rates (which I also see with all the STCs I currently
> have).

Climb rates are at slower speeds, so even less gain than cruise.

I am far from an expert on all things aerodynamic; I just read all I can.
My take on winglets for small planes is like the argument I made on the
drooped tips, earlier. If they were significant for use in small planes,
everyone would be using them. Are all of the major manufactures of light
planes using them? Nope. Wrong tree. (you're barking up) ;-)
--
Jim in NC

[[email protected]]

Jim and Steve,

Winglets work by 'flying' in the tip vortex; they are placed on the
wing with a slight 'toe out' such that their lift vector is inward and
slightly forward. They fly with a posivitve AOA at this toe out angle
because their local relative wind is the top (inward rotating) part of
the tip vortex. The forward component of their lift vector shows up as
a reduction of induced drag.

Winglets are effective only on aircraft that cruise at or near the
stall speed, where the vortex is large. This is true on most bizjets
and transport type jets; while their true airspeed is very high, their
indicated airspeed is nearly down to the stall. Most of these types of
aircraft cruise in this little 'corner' of the envelope, just above the
stall speed and just below the limiting Mach numer. A little slower
and you stall, a little faster and you get into bad Mach effects
(buffeting or Mach tuck.) It is commonly called the coffin corner.

Winglets are effective on these planes not because they are flying
fast, but rather because (in indicated airspeed) they are flying slow
(close to the stall). The low IAS means they fly at a high AOA (nearly
stalled) and create strong vortices for the winglets to fly in.

Winglets are typically of no or even negative value on light aircraft
because our cruise speed is typically about twice the stall speed.
This means we cruise with a much lower AOA, and therefore create much
weaker wingtip vorticies. Without the strong vortex, there is little
rotating flow for the winglet to fly in, and it just adds parasite
drag.

The voyager had winglets (until Dick scraped them off) because that
aircraft was designed to fly at the airspeed of maximum range, which
was near enough to the stall speed to make the winglets effective.
That the aircraft made it around the world without them attests to the
fact that the induced drag reduction is incremental (a few percent.)
Regards,

Gene

[Morgans]

> wrote in message
ups.com...
> Jim and Steve,
>
> Winglets are effective only on aircraft that cruise at or near the
> stall speed, where the vortex is large. This is true on most bizjets
> and transport type jets; while their true airspeed is very high, their
> indicated airspeed is nearly down to the stall.

Thanks. Of course; one part I forgot.
--
Jim in NC

[[email protected]]

As I remember hearing Burt talk a long time ago, they needed a vent for the
tanks that wouldn't vent
fuel before/during launch. The wings sagged a lot. Running a vent inboard
has a different set of
problems when the wing is bending up a lot. The winglets were a way to get
the vent higher and
do it without losing much efficiency. The aircraft was not a span limited
design - ie it didn't have
to taxi up to a gate and it wasn't limited by rules on setting the record. I
don't think they were
influenced by any marketing/sales group. Ergo - the usual reasons for
winglets weren't really in play.
Grinding on them on launch was not part of the plan.

wrote in message
om>...
>Jim and Steve,
>
>Winglets work by 'flying' in the tip vortex; they are placed on the
>wing with a slight 'toe out' such that their lift vector is inward and
>slightly forward. They fly with a posivitve AOA at this toe out angle
>because their local relative wind is the top (inward rotating) part of
>the tip vortex. The forward component of their lift vector shows up as
>a reduction of induced drag.
>
>Winglets are effective only on aircraft that cruise at or near the
>stall speed, where the vortex is large. This is true on most bizjets
>and transport type jets; while their true airspeed is very high, their
>indicated airspeed is nearly down to the stall. Most of these types of
>aircraft cruise in this little 'corner' of the envelope, just above the
>stall speed and just below the limiting Mach numer. A little slower
>and you stall, a little faster and you get into bad Mach effects
>(buffeting or Mach tuck.) It is commonly called the coffin corner.
>
>Winglets are effective on these planes not because they are flying
>fast, but rather because (in indicated airspeed) they are flying slow
>(close to the stall). The low IAS means they fly at a high AOA (nearly
>stalled) and create strong vortices for the winglets to fly in.
>
>Winglets are typically of no or even negative value on light aircraft
>because our cruise speed is typically about twice the stall speed.
>This means we cruise with a much lower AOA, and therefore create much
>weaker wingtip vorticies. Without the strong vortex, there is little
>rotating flow for the winglet to fly in, and it just adds parasite
>drag.
>
>The voyager had winglets (until Dick scraped them off) because that
>aircraft was designed to fly at the airspeed of maximum range, which
>was near enough to the stall speed to make the winglets effective.
>That the aircraft made it around the world without them attests to the
>fact that the induced drag reduction is incremental (a few percent.)
>Regards,
>
>Gene
>

[[email protected]]

The tapered, swept or elliptical wings tend to start stalling at
about midspan at the TE, according to the texts I have here. Not ideal,
but the planforms have advantages that outweigh stall behavior, I
suppose. I wouldn't want ever to fly a wing that stalled tips first. I
wonder if it's even legal to build an airplane that does that.

Dan

[Orval Fairbairn]

In article . com>,
wrote:

> The tapered, swept or elliptical wings tend to start stalling at
> about midspan at the TE, according to the texts I have here. Not ideal,
> but the planforms have advantages that outweigh stall behavior, I
> suppose. I wouldn't want ever to fly a wing that stalled tips first. I
> wonder if it's even legal to build an airplane that does that.
>
> Dan


Of course, it *is* legal to build a plane that stalls tips first -- it
would always be licensed EXPERIMENTAL, and would never pass standard
certification tests.

OTOH, it would be stupid to build one that way, as the plane would be a
bear at low speeds.