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Propellers In a Tunnel

  1. Sep 10, 2014 #1
    First off, please allow me to preface this by stating I am a HS graduate with a miniscule physics background, so if this is a stupid question, I humbly apologize. That being said...

    I have 2 questions, both of which are probably inane.

    #1. What would be the beneficial effect, if any, of linking several propellers in tandem, all mounted on a single shaft running the length of the tube? Would it provide more thrust? Would it even provide any thrust?

    #2. Would this arrangement provide any help with suppressing the prop noise? Is there such thing as a silent propeller?

    I'm working on a short story/novella, and I would like to be as accurate as possible. Thank you for humoring me.
  2. jcsd
  3. Sep 10, 2014 #2
    Astrophysics is the application of physics to astronomy (which is the measurement of the heavens). Your question is about fluid dynamics or mechanical/aeronautical engineering.

    If you're talking about aerospace engineering, what you describe seems somewhat similar to a jet engine, and having been around them, I am very skeptical that they can be anything but extremely loud.

    If you are talking about moving through some other fluid, such as water, the Navy has some pretty silent propulsion techniques and I do not know enough about it to comment.

    Again, you should ask in a different subheading. This has nothing to do with astrophysics and I know very little of aerospace engineering.
  4. Sep 11, 2014 #3


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    If the tunnel and propellers had diameters that decreased from front to back so that mass flow was constant as velocity increased, then each propeller could have more pitch than the propeller in front of it to futher accelerate the air.

    As an alternative, turbines have a "fat" axis of varying width in addition to the outer ducting diameter profile so that the cross sectional area for air and fuel decreases as flow speed increases since mass flow is constant. With a fixed pitch system, it's only optimal for a range of speeds.
    Last edited: Sep 11, 2014
  5. Sep 11, 2014 #4


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    Perhaps but why do you want "more thrust"? In general what you need to do is achieve the required thrust at the best efficiency. If you just want "more thrust" there might be better (eg more efficient) ways to do it such as using a larger diameter fan/duct. In short the overall solution is what usually matters not just one parameter.

    I doubt it. However in the case of single engine prop aircraft multi blade props are generally considered quieter than twin bladed props. I believe this is because they present a greater load on the engine so are run at a lower rpm via a gearbox and that reduces tip speeds and noise (but I'm not an expert).

    Perhaps look at the way modern high bypass jet engines work. They consist of two tubes one inside the other. Most of the air doesn't go through the jet engine in the middle tube, it goes around the outside through the bypass. Modern high bypass ratio engines are a lot quieter than older low bypass engines. Think of the outer tube and it's lower speed air as a sort of noise blanket around the noisy high speed jet in the middle.
  6. Sep 11, 2014 #5


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    #1 Two propellers rotating in opposite directions would help, because the air would tend to exit from the tube flowing "straight" instead of having a lot of circular motion, which consumes energy to create it but doesn't generate any thrust.

    To use more propellers you would need some static vanes in between them to straighten out the air flow, but more than two are probably not going to give much benefit.

    Post #3 is a good way to design an air compressor, but it's not an efficient way to generate thrust. From Newton's laws of motion, thrust = rate of change of momentum = (mass flow per second) x velocity. But to create the velocity you have to supply power from an engine, and the power = (1/2)(mass flow per second) x velocity2. So to make an efficient propeller, you want to keep the velocity low, and move a lot of air slowly, not a small amount of air very fast. Compressing the air a lot will produce a high velocity when it expands again.

    The high pressure itself doesn't create any extra thrust. If you think about it, any extra force from the "releasing the pressure at the back of the tube" is cancelled out by the force you had to supply at the front of the tube to create the pressure.

    #2 A Propeller in a tube will make less noise than one in free air. Much of the noise comes from the flow around the tips of the blades, and that flow pattern is completely different if the blades fit closely in a tube. Also, you can line the tube with something to absorb the sound.
    Last edited: Sep 11, 2014
  7. Sep 11, 2014 #6
    Thank you for all the great responses. I apologize for listing this under the wrong heading, but I didn't initially see the Aeronautical Engineering section. I will also repost this under that heading. If this helps with the answers, I also forgot to post (actually was just too plain reticent to post) that these questions sprang from a hypothetical I posed to myself: How will planes fly without fossil fuels? I came up with an idea for an electrical plane, but I wanted it to be as quiet as possible, while still able to cruise around .7 Mach. Is this possible? I don't need to know if we are able to do it presently, only if it may be possible if all the conditions were met & the science was right. Thoughts?
  8. Sep 11, 2014 #7


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    You can do anything if all the conditions were met & the science was right! Even teleportation would be possible and that would make aircraft obsolete.

    The problem today is that fossil fuels have a high energy density (so the fuel doesn't weigh much). By comparison todays batteries are very heavy.

    Some solar powered aircraft have already flown..
  9. Sep 13, 2014 #8
    I think you face 2 major problems with any sort of propeller driven aircraft, ducted or otherwise.

    Propellers like all mechanical devices have physical limitations which create upper and lower limits of efficiency, if not operation. This is why 0.7 Mach is the high upper limit of speeds achieved by propeller driven aircraft under ideal conditions (including tailwinds). Such ideal conditions also include doing no useful work eg: no payload other than the craft and generally, only one operator. For practical use 0.5 - 0.6 Mach is a given for propeller driven devices.

    A common multi-engine aircraft can be viewed as distributed parallel power, a distinct advantage over one huge engine and prop system. A ducted fan version must be viewed as "in series" since the system is closed within itself. Any volume or velocity of air output can only be equal to the input. Any advantage to multiple props and/or engines is outweighed by system complexity.

    The last, and possibly largest problem is power supply. If your novel assumes no fossil fuels, this effectively rules out any known chemical power, presently leaving only nuclear as a source. Here the problem is what has plagued would-be designers of cars, planes, anything Earthbound, since the idea was first formulated, namely radiation shielding. There apparently have been some successful nuclear aircraft engines such as Project Pluto for use within our atmosphere and engineers still love to argue about Bussard RamJets for use in Space, but to date nothing truly practical has surfaced.
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