Wingtip Vortex: Heavy, Clean, Slow - Why?

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

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.

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

emove (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.

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

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?

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

"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.

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

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