Timeline for Can wing-tip vortices be reduced/eliminated with a rear-facing propeller near the wing-tip?
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| Aug 13, 2019 at 1:10 | comment | added | Zak | This thread is becoming ricidulously long. I suggest you look at @AEhere's answer and the linked paper there: Fig 24 nicely shows how lift is almost unaffacted by moving the prop to the wingtip, but drag is nicely reduced. There's more figures showing how the vortex is weaker if the prop is turned on. I'm tapping out now. | |
| Aug 12, 2019 at 18:13 | comment | added | Jan Hudec | @Zak, what you do to the vortex later will not affect the lift applied by the bound vortex to the wing. However, if you want to actually cancel it and leave no vortex behind—as opposed to pushing it further apart, which does of course improve efficiency—you have to take back the momentum of the downwash. So it will get the opposite force to the lift applied to it, and if it is part of the same aircraft, it will undo the lift (convert it to tension and torque, not nothing at all). | |
| Aug 12, 2019 at 17:58 | comment | added | Zak | @JanHudec Yes, you need the bound vortex -- which is why it's helpful to make more use of it by having e.g. longer wings, before it goes away (more lift, same wingtip vortex!). With infinite wings (or a profile between parallel walls) there isn't even wingtip vortex. And that's the key: You don't need the wingtip vortex. It's unavoidable on finite wings, not necessary. So anything you do to that vortex later, e.g. putting a propeller in it, will not reduce lift. (Unless of course, if you point the propeller downwards or something) | |
| Aug 12, 2019 at 16:32 | comment | added | Jan Hudec | @Jimmy, and where did I say that you can't reduce the induced drag? I only said you can't remove it completely. | |
| Aug 12, 2019 at 14:36 | comment | added | JZYL |
@JanHudec You also can't generate lift without generating induced drag for reasons of plain old conservation of momentum and energy. I don't know where you got that from. The circulation theory of lift, including lifting line (from which the term induced drag was first derived), is based on conservation of momentum, mass and energy. It clearly predicts that the longer the span, the less the induced drag per unit lift. The theory is so successful that it's used in modern aircraft design.
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| Aug 12, 2019 at 13:24 | comment | added | Jan Hudec | @Jimmy, the naming of by-product and cause and effect are a bit arbitrary in this situation actually. The effects are all tied together, but the way the laws of physics are interconnected, you can explain why they are tied together starting from different points. | |
| Aug 12, 2019 at 13:16 | comment | added | Jan Hudec | @Jimmy, yes, I am not disputing that. But they are a necessary by-product. You can't generate lift without generating wingtip vortices. You also can't generate lift without generating induced drag for reasons of plain old conservation of momentum and energy, with vortices just happening to be how these are satisfied. And, most importantly, you can't undo that induced drag without also undoing the lift. | |
| Aug 12, 2019 at 13:02 | comment | added | JZYL | @JanHudec I invite you to take a look at my answer to this question. Wingtip vortices are a by-product of lift, not the other way around. | |
| Aug 11, 2019 at 16:32 | comment | added | Jan Hudec | @Zak, I also think the situation is symmetrical for lift and induced drag—the bound vortex is what creates lift, but you already need the energy, i.e. the induced drag, to create it, and the wake is carrying away both the momentum and the energy. That might not be according to the lifting line theory though; I am relying more on basic laws of motion in this regard. | |
| Aug 11, 2019 at 16:30 | comment | added | Jan Hudec | @Zak, that's correct. But my point is that whichever way you look at it, you can't cancel the wake vortex without cancelling the lift as well. | |
| Aug 10, 2019 at 22:51 | comment | added | Zak | @JanHudec I don't think the comment section is a good place to discuss this. I'm very certain of what I'm saying here. I took my aerodynamics exam twice because I got some bits quite wrong the first time round. The whole vortex thing is only accurate in potential flow, which is a pretty easily misunderstood abstraction in the first place. I have no time to write a book here but I suggest you have a look at the Wikipedia article on lifting line theory, and the "horseshoe vortex" one -- the circulation around the profile ("lifting vortex) creates lift; the trailing vortices reduce lift. | |
| Aug 10, 2019 at 21:53 | comment | added | Jan Hudec | @Zak, the wake vortices move down as a whole, which corresponds to the downward momentum of the wake from lift. That is because the vortices form due to the wake behind the wing moving down and the air outside the span not moving. Momentum being vector quantity, the momentum of the circular motion sums to zero and is not relevant for this analysis. | |
| Aug 10, 2019 at 20:49 | comment | added | Zak | @JanHudec "momentum came from lift". Lift is an upwards force -- it should cause downward momentum change in the flow. However, the momentum in the vortex is mostly around the center, in circles. What the vortex does is turn the flow seen by the wing close to the tip downwards, thus reducing lift because the wing can't turn it as much anymore.You may be confusing the lifting vortex ("in the wing") with the wingtip vortex ("lifting vortex where wing ends"). This is why long slender wings are so efficient: Use a longer lifting vortex, and a smaller part of the wing affected by the vortex. | |
| Aug 8, 2019 at 17:55 | comment | added | JZYL | @JanHudec If you look at the derivation of the lifting line theory, for elliptical distribution, induced downwash is inversely proportional to span. In the limit where the span becomes infinite, induced downwash vanishes, unit lift remains finite (of course, total lift goes to infinity) and is a function of the 2D airfoil lift slope and free-stream incidence only. I can't process the derivation in a comment. But if you form a question, I'd be more than happy to show my reasoning. Of course, this is just academic; for all intents and purposes, we live in a finite span world. | |
| Aug 8, 2019 at 15:24 | comment | added | Jan Hudec | @Zak, the vortices contain energy, which came from drag, and momentum, which came from lift. Therefore they “cause” drag and lift in exactly the same way. You should either consider them causing both or neither, but not one only…. | |
| Aug 8, 2019 at 14:20 | comment | added | Zak | @ Jan Hudec: Vortices and drag: The vortices contain moving air, right? That means they contain kinetic energy, right? So, where did that come from, and who keeps supplying it? -- you're not completely wrong, though: The propeller could only counter the vortex (which is mathematically infinite, decaying with inverse distance) by applying swirl (not the same as vorticity) to the central part of the vortex, making it dissipate quicker and re-using some of its energy. So no, you won't eliminate the vortex. However, the vortices are unavoidable for finite wings, not necessary. | |
| Aug 8, 2019 at 10:08 | comment | added | Jan Hudec | @Jimmy, actually, the vortices don't contribute any additional drag either, because they are already past the wing. The lift and induced drag are both applied to the same point. Also with infinite span, you only get zero induced drag if you generate finite lift over it, which means you generate zero lift per unit of span. If you generate non-zero lift per unit of span, you get non-zero induced drag no matter whether the span is finite or infinite. | |
| Aug 7, 2019 at 14:33 | comment | added | Michael Hall | @JanHudec - I see you edited your answer from what I quoted in my first comment. I have canceled my downvote. | |
| Aug 7, 2019 at 12:47 | comment | added | JZYL | @JanHudec You are correct in saying lift cannot exist without vortices. Michael and Zak are also correct in saying wingtip vortices do not contribute to additional lift. Wingtip vortices are what contribute to the induced drag, a finite-span 3D phenomenon. In infinite span, induced drag would disappear and you would get the 2D result, and you will still have bound vortices. | |
| Aug 7, 2019 at 10:53 | history | edited | Jan Hudec | CC BY-SA 4.0 |
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| Aug 7, 2019 at 10:51 | comment | added | Jan Hudec | @MichaelHall, spilling the vortex at the wingtip doesn't produce any additional lift, indeed. It does not produce any additional drag either. Both were already produced by the bound vortex. However, the spilling vortex is carrying away the momentum needed to produce the lift and if cancel it, you have to apply the opposite momentum, therefore cancelling the lift as well… | |
| Aug 7, 2019 at 10:49 | comment | added | Jan Hudec | @MichaelHall, if the wing spans the entire width of the wind tunnel, then the wind tunnel indeed immediately stops the vortices. The bound vortex must (and does), however, still exist, it just hits the wall instead of spilling off as the horseshoe vortex. | |
| Aug 6, 2019 at 23:29 | comment | added | Michael Hall | @YAHsaves - Yes, it would have. That is where and how the lift is produced. Spilling over into a vortice at the wingtip doesn't produce any additional lift though. | |
| Aug 6, 2019 at 20:54 | comment | added | YAHsaves | @MichaelHall Wouldn't the high pressure air already have delivered an "opposite reaction" to the wings downward push by the time it goes back on top of the wing to meet with the low pressure air? This answer shows some images of vortex wake that are causing effects long after the air craft has passed: aviation.stackexchange.com/a/49357/31421 | |
| Aug 6, 2019 at 16:44 | comment | added | Michael Hall | @Jan Hudec - If you had a wing that spanned the entire width of a wind tunnel you could have a positive AoA and produce lift without vortices. Vortices exist because in real life because the wing has a free end, and high pressure air beneath spills up and over the top. You are the first person I have ever heard staking a claim that this was a beneficial effect that actually produces lift vs drag. | |
| Aug 6, 2019 at 11:02 | comment | added | Jan Hudec | @MichaelHall, by the way, if you don't accept the vortices generate lift, they don't generate drag either—the aft component of the aerodynamic force on the wing does. After all, the vortices are behind the wing and no longer interact with it in any way. Of course when using vorticity to describe the situation (see lifting line theory) the bound vortices around the wing are the key part, and they generate both lift and drag. | |
| Aug 6, 2019 at 10:56 | comment | added | Jan Hudec | @Zak, if you cancelled the vortices, you would indeed cancel the lift, but fortunately the propellers won't do that. | |
| Aug 6, 2019 at 10:16 | comment | added | Jan Hudec | @MichaelHall, wingtip vortices are necessary part of the phenomenon of lift generation. There cannot be lift without wingtip vortices (more properly, without wake vortex sheet, which then composes to wingtip vortices), and at least one way of analysing the flow uses the vortices as explanation. Winglets are installed to move the wingtip vortices a bit further apart, which makes the overall process a bit more efficient, when it is not practical to do so by making the wing longer for other reasons, because aerodynamically that is always better. | |
| Aug 6, 2019 at 8:33 | comment | added | Bram | In case the wing-tip engine is providing a substantial amount of thrust, the problems indicated by Jan Hudec and Zak hold. For a distributed propulsion system, such as the configuration used by the NASA X-57, many of these drawbacks are prevented. | |
| Aug 5, 2019 at 23:27 | comment | added | Zak | Correct: single-engine handling would be horrible (and also putting such masses at the wingtips is a recipe for aerolastic catastrophes), and ailerons interacting with propellers must be a nightmare in terms of stability and control (and the propeller, too) -- but the propellers wouldn't generate very much lift. Also: making the wings longer isn't always an option. | |
| Aug 5, 2019 at 23:22 | comment | added | Zak | I agree with Micheal Hall. Otherwise, why not put some vortex generators on the wingtip to increase vorticity? Would putting propellers behind the wingtips to cancel the vortex out destroy the lift? The wingtip vortices are a side effect of having a lifting vortex in your wing (precisly: spanwise change of strength of the lifting vortex) -- which itself is as much a mathematical construct than an actual thing. | |
| Aug 5, 2019 at 21:33 | comment | added | Michael Hall | "The wingtip vortices is what generates the lift." Wrong, wingtip vortices are a by-product of lift generated by the wings, but they generate drag by themselves. Otherwise why add winglets? | |
| Apr 13, 2017 at 12:59 | history | edited | CommunityBot |
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| Jan 22, 2016 at 10:21 | history | answered | Jan Hudec | CC BY-SA 3.0 |