Wake Turbulence & Wingtip Vortex Facts

[kojdog]

EDIT 2/10/2019

I've become aware of something called vortex lift that has drastically different characteristics than what's discussed below. It's generally present in sharp delta wings but can also be generated by other means (strakes, etc). Curiously, the most composed answer I can find about this phenomenon is Stackexchange answer - The FAA and NASA have failed to give me a good article to link for you:

https://aviation.stackexchange.com/questions/21069/what-is-vortex-lift

It's possible this is what is being shown by ED's Hornet image with the Madonna cones.

 

 

REASON FOR POSTING

We'll soon be getting Wake Turbulence/Wingtip Vortex effects in DCS!

I'm thrilled, but had a small concern about the perfect cone shaped structures illustrated in the news letter. I understand those are likely approximations of the funnel but the way they extended off the aircraft is what got me thinking about this. So, this post is to try and get some information in front of people so we're all working from correct perceptions of what this phenomenon is. Also, I'm over-caffeinated and have just covered this topic in Flight School, so I'm probably a little overzealous.

I also understand that this was one tiny screenshot and may not be showing even a portion of what they're calculating to create the effect. I also understand that the cones are an approximation and I do not expect to see swirling tornadoes being calculated in some complex Navier-Stokes w/ Viscosity method because this isn't Digital Fluids Simulator. I'm sure they'll get a satisfactory product once they call it 'done'.

That said, the picture in question:

 

EVIDENCE PRESENTED

There's an FAA circulated document on the topic that wonderfully illustrates the nature and 'shape' of these vortices and also shows that a cone coming directly off the rear of the plane, in the same flight path, may not be desirable. Also, the positioning of the vortices is an important aspect to capturing their effect. You can find that publication here (also attached):

https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_90-23G.pdf

 

BRIEF SUMMARY FOR REFRESHING KNOWLEDGE

"VORTEX GENERATION. Lift is generated by the creation of a pressure differential over

the wing surfaces. The lowest pressure occurs over the upper wing surface and the highest

pressure under the wing. This pressure differential triggers the rollup of the airflow aft of the

wing resulting in swirling air masses trailing downstream of the wing. After the rollup is

complete, the wake consists of two counter-rotating cylindrical vortices"

 

PRIMARY CONCERNS ILLUSTRATED

The point of all this is illustrated in the following images from that document -

 

VORTEX DESCENT:

As seen in the image, the vortices from an aircraft's wingtip descend from the airplane and do not extend linearly in the path of the airplane's travel. To quote the document -

"at higher altitude the vortices from large aircraft sink at a rate of several hundred feet per minute (fpm), slowing their descent and diminishing in strength with time and distance behind the wake-generating aircraft".

Unfortunately they didn't offer any handle rule-of-thumb descent rate for these vortices but the description would lead me to expect a natural logarithmic decay relative to the environment (and I have no idea what I'm talking about :huh:).

 

VORTEX POSITION:

The issue in this case is with the positioning of the Vortex funnels inboard of the wingtips in DCS' image. The below quote from the document (emphasis mine) is the reason I've included this section -

"VORTEX CIRCULATION. The vortex circulation is outward, upward, and around the wing tips when viewed from either ahead or behind the aircraft. Tests with large aircraft have shown that the vortices remain spaced slightly less than a wingspan apart, drifting with the wind at altitudes greater than a wingspan from the ground"

 

I would think the funnels should be positioned more outboard from the fuselage and perhaps narrower. In large aircraft, according to the document, the vortices are primarily within the wingspan of the generating aircraft.

 

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This is really what I wanted to put out for discussion/thought. The below part is just candy.

 

Thanks for your time if anyone decided to stick around and read (and especially if thanks if you read the document; we all learn something and are a better people for it! :smartass:)

 

I would also love to be corrected/educated by anyone who has the knowledge... this post is almost a cry-for-help for knowledge from knowledgeable people. If you do choose to be generous with your time, then thank you in advance!

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VORTEX AND WIND/GROUND:

The preceding three images all illustrate what happens to the vortices near the ground but also show the effects of crosswinds. This is probably beyond what DCS should try to model but it's food for thought. It's probably better if I let the FAA speak for their own images:

 

"WIND. A crosswind will decrease the lateral movement of the upwind vortex and increase the movement of the downwind vortex. Thus, a light wind with a cross-runway component of 1 to 5 knots (depending on conditions) could result in the upwind vortex remaining in the touchdown zone (TDZ) for a period of time and hasten the drift of the downwind vortex away from the runway (possibly toward an adjacent runway if one exists). Similarly, a tailwind condition can move the vortices of the preceding aircraft forward into the final approach and TDZ. A light quartering tailwind requires maximum caution, as it presents a worst case scenario where a wake vortex could more likely be present along the final approach and TDZ."

 

"LATERAL MOVEMENT. When the vortices of large aircraft sink close to the ground (within 100 to 200 feet), they tend to move laterally over the ground at a speed of 2 or 3 knots at an altitude of slightly less than one-half the wingspan"

AC_90-23G.pdf

Edited February 11, 2019 by kojdog
Vortex Lift acknowledgement; Addition of Vortex positioning issue

[mytai01]

I concur! This is the same information that Air Traffic Controllers are taught about the subject...I just hope the effect is modeled right off the runway and can impact small aircraft performance (potential flip over) behind heavy jets just as in real life...

[litzj]

Nice attempt !!

[kojdog]

Probably weird to reply to your own thread, unprompted, but I came across something called Vortex Lift. I've added something to the top of the original post along with a link that has some information in plain English. It's worth mentioning because I wonder if that's what the bizarre cones in the ED Hornet image are.

[GumidekCZ]

What is the angle of the vortex cone behind end of each wingtip. I personaly think, that the angle is to large and you have to fight this if in wingmen position, even if you have pretty large lateral spacing off the leads wingtip. Because the actual cone angle is more than 40°.

Can any real pilot witness here, if in parade formation F-16/F-18 they need to compensate the leads vortex?

Edited November 27, 2021 by GumidekCZ