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I have been recently thinking about a conceptual VTOL aircraft thruster that should generate lift through the use of a drag force and I am very curious to know if such a thruster has ever been built and tested on a VTOL aircraft.

The basic principle of it would be a drag force acting upon the top surface area of an airfoil that would be partially embedded within a vertical pipe. The drag force (low air pressure area) would be created using pressurized air that would exit out of the vertical pipe at a high velocity and would flow around the top surface area of the embedded airfoil.

The outside ambient air pressure pushing against the exterior surface of the reducer section of the vertical pipe should result in lift/upward thrust. This would be due to there being an atmospheric air pressure imbalance between the outside surface area of pipe's reducer section and the top surface area of the airfoil.

To help illustrate and explain how this VTOL thruster would work, I have created a couple of CAD drawings and have posted them below.

(Note: I want to point out that four screws are being used to secure the airfoil to the vertical pipe, and that the airfoil is colored orange to help it stand out from the rest of the thruster.)

enter image description here

enter image description here

I have spent many hours searching VTOL/drone manufacturers' websites and aviation historical websites on the Internet for a VTOL thruster design similar to this one but have not found one yet.

Has there ever been a VTOL aircraft thruster that generated lift through the use of a drag force?

EDIT

Based on user FreeMan's comment below in which he suggested that a better design would be to have two very smooth curves directing a pre-divided air flow up and down, I redesigned this VTOL thruster to incorporate this design improvement.

In this new design, the airfoil in the top nozzle is more streamlined and the bottom nozzle is positioned within the bottom pipe so that air will have a harder time entering into the bottom nozzle due to turbulent airflow, causing a slight airflow restriction.

I am thinking that this slight airflow restriction will result in more of the inlet pipe's air mass to flow out of the top nozzle which hopefully will result in what I want to achieve, which is approximately 50% of the inlet pipe's air mass to exit out of the top nozzle and approximately 50% of it to exit out of the bottom nozzle.

enter image description here

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    $\begingroup$ I do not understand how your design is supposed to produce an upward force, you're directing air pressure up and down at the same time, which will cancel the forces out. $\endgroup$
    – GdD
    May 4 at 15:28
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    $\begingroup$ @Bianfable, a thruster that directs all air downwards is a better design. I am just curious if such a thruster like this one has been built and tested in the past. $\endgroup$
    – user57467
    May 4 at 15:37
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    $\begingroup$ I would think that the pressurized air exiting the horizontal pipe and entering the vertical pipe would create a huge amount of turbulence and back-pressure as it hits the vertical wall of the pipe. This would end up limiting the amount of air actually making it out of the vertical pipe. If anything, you'd want two very smooth curves directing a pre-divided air flow up and down. $\endgroup$
    – FreeMan
    May 4 at 16:53
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    $\begingroup$ @user57467 Do your calculations include the momentum of the air going up and the air going down? Ultimately, momentum has to balance. You either transfer momentum from the machine to the ambient air, or to the machine's exhaust air. If the machine is going up by transferring momentum to ambient air, that means some ambient air is going down. Where is the ambient air going down? It's not going down on the top part of the machine, I'm reasonably sure. $\endgroup$
    – user253751
    May 4 at 17:11
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    $\begingroup$ I can't give voice to the physics behind my hunch, but you get thrust and lift by accelerating a mass of air. Action/reaction. Pressure differential instinctively just feels like an extremely inefficient means of generating any significant amount of force. $\endgroup$ May 5 at 0:30

2 Answers 2

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No, there haven't been any VTOL aircraft that used a device like that. There's a very good reason for this: this device will produce very little, if any, lift.

If exactly 50% of the air exits out of each nozzle, and the two streams are at the same velocity, then there will not be any lift at all. But that's not what will happen. The upper path is more restricted than the lower, so the amount of air going up will be less than that going down*. This will generate a slight upward thrust.

You can increase the upward thrust by further restricting the top nozzle. This will force more air downward, thus creating more upward thrust. The most efficient form of this device is one in which the top nozzle is 100% restricted, at which point you just have a simple downward-pointing nozzle.

* Yes, the air exiting the upper nozzle will be at a higher pressure, which will increase the downward thrust somewhat, but not enough to compensate for the reduced mass flow rate.

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  • $\begingroup$ while designing this in my CAD program, I made the cross-sectional surface area of the upper and lower nozzles' exit area to be 25% of the size of the cross-sectional surface area of the main vertical pipe. I did this thinking that it would cause an equal amount of air to exit each nozzle. $\endgroup$
    – user57467
    May 4 at 15:57
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    $\begingroup$ @user57467 The geometry of the opening also matters. A thin but long opening is going to generate more drag than a round pipe of the same internal cross-sectional area. $\endgroup$ May 4 at 16:02
  • $\begingroup$ okay, I see what you are saying. $\endgroup$
    – user57467
    May 4 at 16:04
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Momentum is conserved, which means if something goes up, something else has to go down. That's just the way it is, unless you've invented new laws of physics, which I don't think you're claiming to have done.

If your machine goes up, then air is coming down somewhere. You say equal parts of the machine's exhaust go up and down, so that isn't what's doing it. In fact the air that goes up goes through a smaller restriction, so it should be moving faster and carrying more momentum than the air going down. You could fix that by making the ratio not 1:1.

So your exhaust air isn't intended to carry any net momentum. Then it must be coming from the ambient air. Does your machine make ambient air go downwards? I think it does not.

Another way to analyze this machine is: what happened to the downward pressure that the upper ambient air would have pushed on the top of the machine with, if the top of the machine was a closed cap? Well, now it is pressing on the top exhaust air, slowing it down. If you want the top and bottom air streams to have the same final speed, the top air stream has to come out faster from the machine (pushing the machine down). If they come out at the same speed from the machine, the top one is slowed down by the ambient air pressing on it, and you might generate lift, but it comes from the top air stream being slower than the bottom one, a well-established principle. If you accept that principle, you might as well send no air out the top, and send all of the air out the bottom, and then you have a plain old engine.

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  • $\begingroup$ @ user253751, I now realize that I did not take in account, when I was drawing this in my CAD program, that the four screws will reduce the amount of air that can exit out of the top nozzle. This would mean that the diameter of the bottom nozzle has to be reduced in size to compensate for the surface areas of the four screws so that an equal amount of air will exit the top nozzle and the bottom nozzle at the same time. $\endgroup$
    – user57467
    May 5 at 14:02
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    $\begingroup$ @user57467 I think the low-pressure area you indicated is going to suck the upper exhaust air back down (actually the ambient air will push it, which is what suction is), slowing it down - as I tried to write in the last paragraph. So the overall effect when you count the suction air as part of the machine, will be faster air coming out the bottom and slower air coming out the top, in equal amounts. $\endgroup$
    – user253751
    May 5 at 14:08
  • $\begingroup$ @ user253751, also, the overall idea of this thruster is that atmospheric air pressure will push the thruster up through the atmosphere, kinda like how an air bubble is pushed up to the surface of a body of water. I think the only way this thruster will work is if the low air pressure above the top surface of the airfoil is much lower than the ambient air pressure. For example, say that the ambient air pressure is 14.7 psia and the air pressure above the airfoil is at 8 psia or lower. $\endgroup$
    – user57467
    May 5 at 14:10
  • $\begingroup$ @ user253751, concerning your comment above, that's a good point you make. I think this is one of those things that looks good in theory, but physics proves that it will not work as intended. $\endgroup$
    – user57467
    May 5 at 14:18

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