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I am wondering if enough lift can be created for a VTOL aircraft using a propeller that is extracting air from a cylinder which is closed at one end and has holes in the top half of the cylinder.

Please reference the conceptual drawing below.

I believe that when the propeller is turned on and air is flowing in through the holes and out of the cylinder, there will be low static pressure inside the cylinder and the cylinder will be lifted up by the ambient air pressure outside the cylinder pushing against the bottom half of the cylinder. Lift should occur because the bottom half of the cylinder has a larger surface area compared to surface area of the top half of the cylinder which has holes in it.

So, would a propeller extracting air from a cylinder with holes in it be an ideal lift apparatus for a VTOL aircraft?

enter image description here

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4 Answers 4

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It seems very inefficient. There's a couple of ways to see this.

From a momentum perspective, you're doing a a lot of work to accelerate the air downward. But once it gets into the tube, it has to be accelerated upward again because it strikes the bottom. You lose a lot of benefit here.

The other perspective is to look at where pressure at the boundary differs from atmospheric.

  • High pressure at the rear. Only contributes to thrust, not lift.
  • Low pressure only above the holes. Very small total area for the pressure to act on, creates a small amount of lift. If you try to increase the area of the holes, the fan can no longer maintain the same low pressure inside.

It looks to me like just turning the fan to blow downward and eliminate the tube would be much better.

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Already invented by an Italian priest in the 17th century. His version was simpler, having no engines or propellers...

https://en.wikipedia.org/wiki/Francesco_Lana_de_Terzi

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  • $\begingroup$ @ xxavier, thanks for posting that link. I did not know about that inventor nor about his ship design. I believe my design should create enough lift to lift small VTOL aircrafts like drones due to today's light weight materials and powerful motors. $\endgroup$
    – user36220
    Feb 8, 2019 at 23:37
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Your design will not work.

If you want to reproduce the concept of the Italian priest, then you want no holes in the top surface, because his concept was based on having a vacuum in the spheres (balloons) attached to his ship. Since you have holes, you will not get much reduction in static pressure, because air will flow in trying to refill the tube. And, as pointed out from a momentum perspective, it creates sideways thrust, not downwards thrust.

So, no, you will only get a very tiny bit of lift from a very small reduction in static pressure, but even with modern materials, you weight will be orders of magnitude higher.

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  • $\begingroup$ @ Penguin, actually I am not trying to create a total vacuum inside the cylinder, just a partial one. I am thinking that if each hole was say 6 inches in diameter and there are 3 holes, the total surface area of the three holes would be 84.9 sq in. So, if the static pressure inside the cylinder could be reduced by 5 psi that would produce 424.5 lbs of lifting force. This cylinder is only intended to create vertical lift for the VTOL aircraft. The aircraft would need another propeller to propel it forward through the air. $\endgroup$
    – user36220
    Feb 9, 2019 at 15:13
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    $\begingroup$ @HRIATEXP. I understand that, but it still doesn’t work. The lower static pressure does not only act on the area of the holes. It acts upon every surface it is in contact with. That’s your error. In your reasoning, the bigger the holes, the larger the lift force. But, the bigger the holes, the less drop you get in static pressure, because more air flows in due to the static pressure drop, so you cannot maintain a low static pressure. If what you want is lift, you would get more with no holes. You need to understand how buoyancy occurs. See Hobbes answer, point 2. We are saying the same. $\endgroup$
    – Penguin
    Feb 11, 2019 at 8:24
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No.

  1. A propeller can only reduce the air pressure in the cylinder by a small amount. When that value is reached, air will start flowing around the propeller blades back into the cylinder.

  2. The holes around the circumference will make that "small amount" even smaller: they leak more air into the cylinder.

  3. For argument's sake, let's say your idea works. You've closed off the holes in the top and developed a one-way propeller that does not allow air back into the cylinder. You turn on the engine, and the propeller pulls the air pressure in the cylinder all the way to a vacuum. Your cylinder now has to withstand 1 kg/cm2 of force, i.e. it needs thick walls.

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  • $\begingroup$ @ Hobbes, those are good points you've made. I wonder now if the propeller was replaced with a centrifugal impeller such as one used in a vacuum cleaner, if that would produce enough vacuum inside the cylinder to lift it. Yet as you pointed out, the cylinder wall would have to be thicken and that extra weight would probably cancel out the lift being created. $\endgroup$
    – user36220
    Feb 9, 2019 at 14:57

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