The start (beginning) of wingtip vortices
 

I have been looking around google and PPRuNe search, not yet found a reasonable answer.

Why do wingtip vortices start at lift-off point, and not before during the t/o roll? Actually is that statement true?

The same question for when they stop upon touch-down...

The reason is that you start producing lift at rotation and you stop producing lift at touchdown. I mean that the wingtip vortices are a direct consequence of the pressure difference between the upper side and the lower side of the wing, as you know, and the airflow passing over the tips of the wings trying to equalize those pressures.
You start producing significant lift - wingtip vortices- at the rotation point. And you stop producing significant lift - wingtip vortices - when you touchdown.
And the greatest vortices (wake turbulence) is when the a/c is :
slow
clean
heavy

i would have thought that as soon as there is spanwise flow, wingtip vorticies start. at low speed, they are just not very large. the wing will create lift at very low speed - just not enough to fly

As sir.pratt has said they start with the spanwise flow. I believe they only get to be significant (on take off) once the wing lifts the aircraft off the ground (i.e. after rotation).

Hope this helps.

Theoretically, wingtip vortices will start as soon as there in any pressure differential between the upper and lower surface if the wing, so even parked, facing into wind there will be small very weak vortices being formed.

When an aeroplane with a nosewheel is on the ground, the wing will have a very small angle of attack and therefore there won't be much of a pressure differential between the upper and lower surface of the wing. As the aircraft begins to move down the runway, the pressure differential will increase a small amount (ever notice how on large aircraft the wings begin to flex even before rotation?). At rotation the angle of attack is increased hugely, the pressure differential increases hugely and air below the wing attempts to move around the wingtip to the area of comparatively low pressure above the wing. As the aeroplane is moving at a relatively slow speed (compared to in the cruise) the vortices form over a short space and thus are more intense.

In the cruise there is the aircraft travels at a higher speed but lower angle of attack. There is the same pressure differential between the upper and lower wing surfaces as at rotation but the vortices are "stretched" over a longer horizontal distance and are therefore less intense. Landing has the opposite effect of takeoff; as the nosewheel is lowered to the runway surface, the angle of attack decreases and the pressure differential between the upper and lower surface of the wing decreases.

The use of flaps will decrease the intensity of wingtip vortices as vortices shed from the flaps will tend to mix with and disrupt the wingtip vortices.

sr

Aah Sir Pratt, you beat me to it.....

Thank you. It is how I analyzed it, I needed confirmation from more experienced people.

Practically speaking -

Wingtip vortices are a function of C sub l (lift coefficient) which is in turn a function of Angle of Attack. Thus, parked on the ramp in a headwind, AOA is (nearly) zero, and so are vortices.

And while they start at rotation, they don't stop at MLG touchdown - they still exist till the nosewheel is down. This can be seen in film shot in dust/sandy conditions, or in high humidity (condensation trails).

In addition to what speedrestriction and barit says there is one
maybe more important thing.

Ground effect - or to be more specific - Span Dominated Groundeffect.

When the aircraft is below about one span altitude, there is less
space for the vortex to build up below the wing. The leak of pressure
from the lower side of the wing gets less the closer to the ground you are.
This results in;
1. Less vortex buildup
2. Vortex being forced to the sides.
3. Less induced drag

http://www.se-technology.com/wig/images/ge_span.png

X-axis is altitude in % of span, Y is Induced drag

http://www.se-technology.com/wig/ima...selsberger.png

Cheers,
M

Don't understand this wingspan thing. Glider with huge wingspan gives small induced drag and little vortices. These little vortices start to diminish at 50 plus feet above ground?
An F104 with short wings and large induced drag and large vortices do not have ground interaction until a much lower altitude AGL than a glider?
Mr. XPMORTEN do you have some ideas?
Stan

Could it be that a glider with huge wingspan gives vortices that are weak (small peak airspeed) but nevertheless huge? Whereas a F104 or Concorde creates powerful vortices that are small?

Does the diameter of the wingtip vortices scale with the span of the wings, or with the chord of the wings? That is, in case of a glider, are the vortices concentrated in small volume around the tracks of the wingtips while the long wing in between leaves the air undisturbed? Or does the circulation of the vortices continue without much weakening compared to peak speeds to the middle of the airplane?

Stan, you are correct.

A glider has a high Aspect Ratio wing (long span, small area)
A F104 har a low aspect ratio wing.

The formula for aspect ratio is ;
AR= (Span)^2 / (wing area)

So a long thin wing has a high AR, a short fat one has a low AR
A high AR wing has less vortex and thereby induced drag than a low AR
wing because the "rest" of the wing is further away from the tip so
that the high and low pressure areas at the tip are smaller.
So, the spanwise lift distribution is a significant factor.

Since winglets increase the AR (span), thats one of the reasons they
reduce vortex.

Cheers,

M

As long as there is airflow, there is lift. As long as there is lift, there
is induced drag and vortex.

An airliner produces lift (and vortex) when rolling with all wheels on the
ground - even if the AoA is negative.

Below is a Boeing airfoil Cl diagram. You can see the wing produces lift
even at about -3 deg AoA. An airliner on all wheels will usually
have a slight negative deck angle, but the wing will also
have a positive incidence of maybe 2-3 deg. So, all in all
a normal loaded acf will produce alot of lift when rolling.
With flaps even more offcourse.

If the wing wasn't producing lift when rolling on
all wheels, you wouldn't need ground spoilers.... :}

http://www.xplanefreeware.net/morten/DOCS/757Cl.gif

Cheers,

M

Mr XPMorten. It is still the wingspan thing I don't understand. Small vortices start to diminish at 50 plus feet AGL for a glider, while stronger vortices don't start to diminish to 20 feet or below for a F104?

Right, as you can see on the first illustration,
and on the Wieselberger diagram, the effect starts at about
one span altitude. A high AR wing vortex will probably have
a bigger radius than a low AR wing since the lateral airflow
has a bigger distance to travel from root to tip. So it will get
in conflict with the ground sooner.

The percentage of drag reduction in the two cases will offcourse be the same,
but the F104 will loose drag faster and more than the glider.

M

I believe you're diagram, I just don't understand why.
A high AR wing has smaller induced drag generally, so how does this equate to bigger radius? With a glider with extremely long wings, for example, the pressure differential between top and bottom wings will be smaller than one with less wingspan. And this higher AR wing creates a bigger radius vortex??
And lateral airflow travels from root to tip??
The percent of drag losses from tips is the same?? Can't see why that would be the case either.
Anyone?

This might clear up a few things;

Tip Vortices and Ground Effect yy Dave Pickenpaugh
http://www.torks.com/Guides/THE%20PH...pter%20III.PDF

Note, he is discussing the two types of ground effect as "one".

About lateral airflow, here is an example;
http://www.aerodyn.org/Wings/larw-comp.html
The middle image (wing bottom) shows lateral flow from root to tip.

M