Wake Turbulence

[Luca Santoro]

Hello all!

Currently doing my ATSIMTEST C1.

I have a strange question here. Does anyone know if and how wake turbulence is influenced by speed?
I would personally answer proposition 2 and 3.

Thanks
Luca

wake_turb_c1.jpg

[Chiara Paganini]

Simplified answer that is good enough for ATPL level aerodynamics but nowhere near good enough for aero(-space) engineering.

Wake turbulence is the result of air moving from the underside of the wing to the top around the wing tip creating a vortex.

For factor weight the correlation is obvious, more weight needs more lift for the same load factor, and more lift needs a larger pressure differential leading to a stronger vortex (This btw also works by adjusting the load factor, the 150t zero-G A300 creates less of a vortex in its parabola than a Da-40 in level flight)

The correlation with speed is a little less obvious but here's my attempt at explaining it: If you have a 3D aerofoil you don't just have the streamlines moving over the wing front to back, you also get spanwise flow, especially on swept wings as a result of the general deflection of the air downwards which creates the positive pressure under the wing needed for lift. If you go faster you get more air mass flow over the wing so you need to deflect each unit of air less,, creating less spanwise flow and thus a smaller wake vortex.

In summary vortex strength is:

• roughly proportional to weight
• roughly inversely proportional to the square of the air speed
• roughly inversely proportional to the aspect ratio of the wing

If there's anyone with more of an insight into how it actually works in detail I'd be very interested too though!

[Luca Santoro]

Thanks for your answer

If you go faster you get more air mass flow over the wing so you need to deflect each unit of air less,, creating less spanwise flow and thus a smaller wake vortex.

You lost me there

The faster you fly, the smaller the wake turbulence will be? I find this oddly confusing ...

My reasoning would that the strengh of the wing tip vortex will increase when lift increases, no?
That being sad, flying faster does not mean the lift necessarly increases. In the end, we have to compensate for the same weight...

[Michael Kühne]

Slow and heavy -> lots of wake.
In addition these aircraft have their flaps deployed, increasing the turbulence even more.

[Thomas Wand]

The faster you fly, the smaller the wake turbulence will be? I find this oddly confusing ...

Think of it in terms of angle of attack. If you're slower, you need a higher angle of attack in order to maintain altitude. A higher angle of attack means that you mess the air up a lot more.

[Nicolas Ammann]

Wake turbulence consists of various components but the most important component is wingtip vortices, as other components are less stable and disappear within seconds (while wingtip vortices are stable for minutes and strictly speaking are not turbulence, as they are not chaotic).
Wingtip vortices occur due to the pressure difference between the upper side of the wing and its lower side. The pressure difference increases with higher angle of attack and flaps, which will increase the strength of wingtip vortices and hence the "wake turbulence".