Is vortex lift a thing?

[UndeadFirefly]

Is vortex lift a thing? I was trying to recreate the F-18/F-16 in KSP and was wondering if the highly swept wing root leading edge extensions would do anything in FAR

[18Watt]

4 minutes ago, UndeadFirefly said:

Is vortex lift a thing?

Not sure how FAR works, but no, vortex lift is not a thing.  That would be action at a distance, and is not supported by actual physical principles.

Yes, wingtip vortices are real.  Once generated they have exactly zero effect on an aircraft.  They are in the past as far as the wing or aircraft is concerned.

The exception would be if you could take a different aircraft and fly it through the wingtip vortices of the first aircraft.  In that case, yes the vortices are very real, and can cause control issues.  But for the aircraft that generated the vortices, those vortices are in it's past, and will not affect the flight of the aircraft that generated them.

 

[UndeadFirefly]

10 minutes ago, 18Watt said:

Not sure how FAR works, but no, vortex lift is not a thing.  That would be action at a distance, and is not supported by actual physical principles.

Yes, wingtip vortices are real.  Once generated they have exactly zero effect on an aircraft.  They are in the past as far as the wing or aircraft is concerned.

The exception would be if you could take a different aircraft and fly it through the wingtip vortices of the first aircraft.  In that case, yes the vortices are very real, and can cause control issues.  But for the aircraft that generated the vortices, those vortices are in it's past, and will not affect the flight of the aircraft that generated them.

 

Sorry I'm a bit confused. I went down the wikipedia rabbit hole reading about leading edge extensions and I'm pretty sure there's a wikipedia page on vortex lift. As I understand it, highly swept wing root extensions help delay stall on the main wing at high angles of attack. If FAR doesn't simulate it, I completely understand.
https://en.wikipedia.org/wiki/Vortex_lift

Also why do wingtip vortices not have any effect on an aircraft? Don't they create more drag? Unless you're talking about FAR, then I guess that makes more sense. 

[18Watt]

7 minutes ago, UndeadFirefly said:

Also why do wingtip vortices not have any effect on an aircraft?

Because the vortices are happening away from the aircraft.  For them to have any effect at all on the aircraft would require action at a distance.  Which does not happen in real life.

Watch this:  

You are talking about several unrelated concepts.  Yes, wing design has an effect on drag and lift performance.  But wingtip vortices are something which has already happened, and can have no effect on the wing or aircraft that generated it.

As a suggestion, I would advise not taking everything you see on wikipedia to be accurate.  Even worse is assuming that your visualization of how lift works is accurate, it's a concept which is very hard to describe.

[softweir]

45 minutes ago, UndeadFirefly said:

Is vortex lift a thing?

Yes, but FAR doesn't emulate it. The computations needed to simulate it is beyond PC computing power in a real-time application like FAR running under KSP.

36 minutes ago, 18Watt said:

vortex lift is not a thing. 

I think you are confusing wing-tip vortices and leading-edge vortices. As you say, wing-tip vortices have no lift effect, only drag! However, leading-edge vortices do generate lift -@UndeadFireflyhas linked to the appropriate Wikipedia article. Leading-edge vortices are constrained to the space immediately above the wing, so no "action at a distance" is needed. There are many other articles to be found which describe leading-edge-vortex, you don't need to depend on Wikipedia!

Edited April 17, 2023 by softweir

[18Watt]

12 minutes ago, UndeadFirefly said:

I went down the wikipedia rabbit hole reading about leading edge extensions and I'm pretty sure there's a wikipedia page on vortex lift.

Just because there is a wikipedia page on something does not make it a real thing.

[UndeadFirefly]

2 minutes ago, softweir said:

Yes, but FAR doesn't emulate it. The computations needed to simulate it is beyond PC computing power in a real-time application like FAR running under KSP.

Ah that makes sense. Thanks for the answer. I might still keep the extensions for the aesthetics 

[Vanamonde]

Discussion split from an unrelated thread. 

[18Watt]

25 minutes ago, softweir said:

I think you are confusing wing-tip vortices and leading-edge vortices.

You are, of course, welcome to believe whatever you want.  My experience with leading edge vortices relates to devices on the leading edge of wings, which are intended to produce vortices.  We also call them BLEs, or Boundary Layer Energizers.  They are simply bits of metal projected into the airstream along the leading edge of a wing, intended to add energy (induce some turbulence) to the airflow over the wing.  They do not produce lift in any way, the benefit is that they keep the airflow over the top of the wing at high angles of attack more energetic, and thus delay flow separation.  The vortices do not produce any lift.

Howver, another way to look at those devices is that they delay the loss of lift at low airspeeds and high angles of attack.  So they allow the wing to continue to produce adequate lift in more adverse conditions than would occur without them.  So in a way, I suppose you could say they benefit lift.  I would not say they generate lift, but rather delay the loss of lift, or perhaps expand the envelope in which the wing can generate adequate lift.

I am not familiar with FAR at all, other than having been impressed with the attention to detail the mod has striven for- for over 10 years now, I think.  I suspect (but do not know for certain) that FAR does not model or take into account Boundary Layer Energizers, which I could also call leading-edge vortex generators.

Or perhaps I'm still misunderstanding the exact device or principle you are referring to.  Misunderstandings are common these days in forums, and a frequent cause of flame wars.  I hope I'm not starting one now..

Edit to add:  I'm dubious of the overall effects of BLEs, or leading-edge vortex generators.  This is from my personal experience with them.  In theory, the shape of them is critical, and they should not be sanded or polished or smoothed out.  In reality, the crews that come to clean the plane and polish the shiny bits don't really care about the BLEs, and run the polishers right over the BLEs.  So we end up with smooth BLEs, which probably don't do nearly as much as the engineers intended.  The result?  The plane still flies just fine, I can detect no difference at all.

[Meecrob]

Root extensions are there to lower the stall speed/increase the operational range of degrees of angle of attack...kinda the same thing but from two different directions.

Vortex generators are there to basically un-turbulate airflow in regular flight (low AoA). They are not there to help you if you decide to do aerobatics.

Root extensions are specifically for high angle of attack flight. Obviously they are like one big vortex generator, but the design regime is totally different, hence the different aerodynamic shapes.

 

Edit: These are vortex generators:

 

This is a root extension:

 

 

They are two totally different things...

Edited April 17, 2023 by Meecrob

[Nuke]

according to bees, yes.

[RCgothic]

You can think of lift and drag in two functionally equivalent ways.

1) As a pressure difference over the wing area. The force vector produced holds the wing up, but also pulls it back as drag. If air spills around the wing tips, the pressure difference gets reduced so a greater pressure difference is required. At constant airspeed this is acheived by a higher angle of attack, so the ratio of lift to drag changes unfavourably as the lift vector points more aft.

2) As conservation of momentum. If a wing accelerates downward an aircraft's mass of air each second, by conservation of momentum the aircraft is held up against 1g. The wing also drags the air forward, and by conservation of momentum the wing is dragged back, with the ratio depending on angle of attack. Vortices spilling around the wingtips don't contribute to  linear momentum change, as circular motion has no overall direction. This is in effect wasted effort as the wing converts its linear momentum into useless angular momentum.

Edited April 17, 2023 by RCgothic

[mikegarrison]

Wow this whole discussion is confusing, apparently because people are talking about a whole bunch of different things without really defining what they are talking about.

The Kutta–Joukowski theorem says that lift is equal to airspeed * air density * circulation, which is the net rotational flow around the wing. Thus, the "wingtip vortex"(which is something of a misnomer, because the vorticity is actually shed all along the wing, not just at the wingtip) ends up being directly proportional to the lift. So you don't have lifting wings without vortices.  Which I suppose might be taken to mean "vortex lift is a thing", but you really need to define what you mean by "vortex lift" before such questions can actually be answered.

Those wing strakes on fighters do provide some lift, but their main purpose is more to energize the airflow over the wing when at high angle of attack, to try to keep the wing working in a regime where it otherwise would completely stall out. I don't know for sure whether FAR is able to simulate this sort of aerodynamics, but I doubt it.

10 hours ago, Meecrob said:

Vortex generators are there to basically un-turbulate airflow in regular flight (low AoA). They are not there to help you if you decide to do aerobatics.

So close, but not.

They do not "unturbulate" the air over the wing. First off, because that is impossible. (Once air is turbulent you can't make it laminar again. The best you can do is scrape off the boundary layer and try to re-establish a new laminar flow.) But even more importantly, they are actually trying to make the flow turbulent, not the reverse.

The issue is that laminar flow is lower drag, but for the same reason it has lower boundary layer energy. In an adverse pressure gradient, the boundary layer loses energy until it separates from the wing. Since laminar flow has less energy to start with, it separates easier. Turbulent flow brings energy from outside the boundary layer into the boundary layer, thus working against the energy loss from the adverse pressure gradient, thus resisting separation of the boundary layer from the wing.

So those vortex generators are meant to energize the flow and increase its turbulence, not decrease it. But that helps the flow stay attached to the wing.

[UndeadFirefly]

For what it's worth what I meant with my original question was whether vortex lift, as I understand it, leading edge vortices that span the entirety area of a highly swept wing root extension delaying flow separation and stall on the main wing, was a thing in FAR specifically. Did not expect it to generate such lively discussion . Sorry if I caused any confusion. 

But on that topic I'm wondering how wing root extensions interact with leading edge slats? AFAIK they're more or less trying to accomplish the same thing. Do they have different operating regimes? Would there be a situation where one would hurt the other?

I have very little aerodynamics knowledge and all of this is very new and exciting to me 

[mikegarrison]

1 hour ago, UndeadFirefly said:

But on that topic I'm wondering how wing root extensions interact with leading edge slats? AFAIK they're more or less trying to accomplish the same thing. Do they have different operating regimes? Would there be a situation where one would hurt the other?

This is correct AFAIK. The idea with gapped slats is to create a small wing, so to speak, with its own circulation, and that acts in much the same way as these leading edge vortices are supposed to do. Although, multi-element wings are quite complicated, and slats may not be gapped, in which case they are then acting more to change the shape and area of the wing.

I think delta wing and other highly-swept wing airplanes don't often have slats, but I'm not an expert in high-lift devices or delta-wing airplanes, so I may be wrong there.