Centrifugal Impellor in place of Axial Ducted Fan

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Culleningus, Feb 4, 2010.

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  1. Feb 4, 2012 #41

    Aircar

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    In regard to centrifugal fans for creating thrust - www.entecho.com.au --turn this 90 degrees and Voila! ( the designer was principal engineer with the Orbital engine company which was a 'non revolutionary' engine from the early 70s (it didn't revolve but 'orbitted' -irrotational circular motion of the rotor ) that didn't pan out but led to superior fuel injection technology for two stroke and other engines . I proposed and did a preliminary design report for the Orbital firm in 1981 for an "orbital" aircraft (project Boomerang, --send it off to the East and it returns from the West.... --a flight right around Earth in one go ergo orbit, and submitted the design and drawings via the bloke who had involved the Broken Hill corporation (BHP-Australia's biggest company) with the engine inventor and whom I knew through gliding.
    I met Kim Schlunke at the 2009 Avalon airshow where their VTOL using the centrifugal fan was doing demo flights IN the booth (and looking just like a jellyfish or one of those 'flying jellyfish' from Avatar --bumping into people and the walls harmlessly --almost see through semi transparent and quiet.)

    My orbital proposal was considered either 'impossible' or something but of course the Voyager went on to show that it was doable (with almost identical parameters --my design three view was published in a model aircraft magazine and at least referred to in print in 1982 so can be verified pre Voyager ) SCALED built the GM Ultralight concept car to showcase the Orbital engine --it occasionally turns up in futuristic SCI FI films as a prop .
     
  2. Feb 4, 2012 #42

    gordonaut

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  3. Feb 5, 2012 #43

    Aircar

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    The really neat thing about their electric model was that it was steered by means of a ring that deflected a series of 'petals' (hinged plates) so as to offset the exit circle to give side forces --for vertical trim the ring was raised or lowered along a tapered conical shape formed by the overlapping plates around the exit -- just like a variable nozzle afterburner -- the exit size could also easily be varied by manipulating the plates independent of the speed of the efflux and power (it would make a great toy for flying inside and be just like the Avatar flying 'things' ) anti torque is provided by a stator on the big ones but could be done by pleating the exit cone for a very small one or vanes on the 'petals' .
     
  4. Feb 5, 2012 #44

    bob989

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    Hi Gordon,

    You raise some interesting points. I will respond first by reiterating and amplifying the arguments in favour of small engines, and then I’ll try to see if there is some common ground in the way you and I approach the surrounding issues.

    What I am contemplating is the "commoditization" of airplane engines. Engines would be manufactured in clean rooms by robots. This kind of manufacturing paradigm leads to very low cost, as evidenced by the plethora of modern technological gadgets that are all around us. Arguably, a hard disk drive or a digital camera is not enormously less complicated than an aircraft engine, but these gadgets sell for tens or hundreds of dollars, not tens of thousands or hundreds of thousands. So I believe that the cost of manufacturing aircraft engines is currently too high, perhaps much higher than it should be. This high cost seems to result from low production volume and large size. An aircraft engine is a specialty product that necessitates large facilities.

    There is an essential difference here between airplanes and engines. Airplanes are necessarily large, specialty products. But engines could be small, commodity products. This thinking goes against the recent trend in the aviation industry for fewer, larger engines. We now cross oceans on airplanes that have only two engines, something that for safety reasons was prohibited not too long ago. When you hit a flock of birds, you then have the tricky problem of trying to land safely in the Hudson River.

    If the commoditization of aircraft engines lead to a steep drop in prices, that would justify economically the re-engineering of airplane wings. The question then becomes, where is the gotcha? Is there some basic aerodynamic, structural, aeroelastic or other engineering issue that cannot be overcome?

    The engineering justification for the current practice of using a small number of large engines seems to be mainly about the efficiency of the propellers or fans. A large number of small propellers is not an efficient propulsion system. A large number of small ducted fans might be. At least, that is my thinking. If a large number of small fans is reasonably efficient, then the other economic arguments in favour of engine commoditization would carry the day. For example, I might be willing to sacrifice a factor of maybe 1.5 or 2 in fuel consumption, if I could save 90% of my total engine costs. But I would not be willing to sacrifice a factor of 20.

    Since low-speed aircraft have already successfully demonstrated the use of centrifugal fans, the focus of my investigation is toward somewhat higher airspeeds. You have raised several complicating factors for the supersonic and high subsonic range, and I don't know how to deflect those criticisms. I found in my own calculations that in terms of area ratio, the centrifugal fan would have no advantage above about Mach 0.68, and there would be no advantage in creating a vacuum in front of the intake. But this is only part of the problem. It seems to me that an axial fan approaching Mach 1 necessarily generates shock-wave drag on the blade tips, and perhaps elsewhere, but I don't see why a centrifugal fan would necessarily generate shock waves. I can imagine long, slender impeller designs where the flow would never hit the solid surface at more than Mach 1. Too close to Mach 1, as for the wing itself, there would be shock waves in lower-pressure parts of the channel, analogous to what happens above critical Mach number on the upper surface of the wing. Perhaps some sort of a Whitcomb design might allow the impeller to operate efficiently closer to Mach 1. I would be very happy if reasonable efficiency could be achieved at Mach 0.8. And a large number of small turbofan engines with axial fans would also be acceptable, so long as the efficiency is not severely degraded.

    But I'm more interested in what happens at lower Mach numbers, where propeller airplanes now reign supreme. I really want to know the answer to this, so I will re-frame the question in another way, to try to stimulate your thinking. Start with a low-speed aircraft that uses a centrifugal fan, like a hovercraft or a flying saucer. Now begin changing the airspeed continuously upward. At each airspeed, redesign the entire aircraft to have optimum efficiency at that airspeed. That process should result in a continuous family of designs that vary smoothly from a flying saucer to an airplane. They will smoothly start to grow little winglets that become wings. The angle of the propulsion system will smoothly vary from near vertical (propulsion system generating lift) to more and more horizontal (wings generating lift).

    In this context, the question becomes, is there some airspeed at which the design process does not produce an efficient airplane with centrifugal fans? That is, is there some point at which you would abandon centrifugal fans in a duct in favour of one or two large propellers out in the freestream? Now ask the question again, in the context of commodity engines. If you want a small number of large propellers, are you willing to pay for large, expensive engines? And are you cleared for landing on the river?

    Your remarks about the Coanda effect and the Dyson fan are intriguing. I think that boils down to choosing the optimum place for the duct exit slot. It's possible that some amount of thrust augmentation would be desirable, and that my initial instinct for placing the exit slot near the trailing edge might have been wrong.

    Your other comments seem to be warnings about potential problems, which of course would have to be considered in any detailed design. You did not mention fire suppression, which of course would also be an issue for engines placed inside the wing. But I don't see a gotcha. At least, not yet.

    My preference for placing the intake on the leading edge is due to the use of a high vacuum to sweep air from both above and below. If I place the intake below the wing, the air flow from above would be blocked. That's not a disaster, and as you mention there might be problems with assuring adequate flow to both the propulsion system and the wing surfaces, which is just another way of saying that the overall system would have to be aerodynamically integrated and would have to be designed and tested as such.

    I see a possible way to experimentally prove (or disprove) my concept. One might start with the small turbine engines that are available off-the-shelf for R/C models. These have low thermodynamic cycle efficiency, but for a proof-of-concept demonstrator aircraft, that would be a side issue. Once the propulsive efficiency were known, the effort to re-engineer small turbine engines to have good cycle efficiency would be easier to justify.

    I don’t think the big engine manufacturers would ever attempt to develop commodity engines at low prices. They have too much of a vested interest in the current state of affairs.

    -bob989
     
  5. Feb 5, 2012 #45

    gordonaut

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  6. Feb 7, 2012 #46

    bob989

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    Hi Gordon,

    There are a number of good ideas in your post. I'll focus on three of them: (a) the scaling argument that suggests an increase in drag, as a percentage of total power, in small engines, (b) the use of heat exchangers and other methods to enhance cycle efficiency, and (c) the optimum number and size of engines.

    (a) If it were true that smaller core engines would necessarily lead to lower thermal efficiency, that would be a potential gotcha, that is, an engineering issue that would ruin the rationale for the propulsion system. If the small engine were a scale model of a large engine, I agree that drag in the small channels would be proportionally greater. I see two ways to get around that problem. One way is to lower the velocity of the working fluid, by making the channels bigger. That is, use a different scale factor for the radial directions. The small engine would be fatter, that is, the ratio of diameter to length would increase. The second way is to reduce the axial length even further, by packing the compressor and turbines stages more closely together and using thinner blades, and by using a larger number of combustors so that flame length is reduced.

    (b) But your thinking is generally correct regarding other possibilities to further enhance thermal efficiency. The known techniques for improving thermodynamic cycle efficiency, including precooling, reheating, isothermal combustion, and the use of a secondary cycle driven by the waste heat from the primary cycle, are not implemented in aircraft engines because they make the engine more complicated. Complexity compromises safety, so people opt for a less efficient, but more reliable, engine design. I would characterize the reluctance to introduce these kinds of efficiency improvements as a kind of collective paranoia that results from the use of a small number of large engines. If we are going to put all of our eggs in one or two baskets, then we can justify all kinds of expensive ways to ensure that those baskets are safe, including the use of a crummy old Brayton cycle that is known to waste fuel. Fuel is not as expensive as accidents.

    But of course, if we don't begin by shooting ourselves in the foot, that is, if we deliberately choose not to put all of our eggs in one or two baskets, then we can open up the debate to include some really good ideas, including your own ideas, for increasing thermal efficiency. So long as the engines operate independently, the probability of an engine-related accident becomes the probability of a single failure raised to the number of failures needed to cause the accident. The larger the number of engines, that is, the more finely-grained the propulsion system is, the more exponentially difficult it becomes to cause an accident. So long as they operate independently.

    The increased engine complexity caused by including thermodynamic cycle improvements just means that the robots do a bit more work to build the engines. But that's peanuts.

    (c) Regarding the size of the engines, my instinct is that the lowest cost per unit thrust would be achieved for engines that you could hold in your hands. If you imagine a graph of cost/thrust plotted as a function of engine thrust, there are simple reasons to believe that the very high end and the very low end both go to positive infinity. A very, very, very large engine requires very, very, very large facilities. And a microscopic engine requires special technology. Somewhere in the middle, the curve has a minimum. Here, you're using ordinary robots built with off-the-shelf components by people working with hand tools. The assembly line is located on a few table tops in a clean room in an ordinary building. A few engineers in bunny suits keep it going. You ship the engines in UPS boxes.

    I think this is a really good discussion. We’re covering some interesting issues and I’m learning from this.

    Regards,

    -bob989
     
  7. Feb 8, 2012 #47

    Voyeurger

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    Gents,
    We lurkers are digging the discussion as well.
    Thanks.
    Gary
     
  8. Feb 8, 2012 #48

    delta

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    Without having first hand experience with them, I can maintain my partiality to four stroke weed whacker engines. A dozen or so aught to do it...

    Rick
     
  9. Feb 8, 2012 #49

    gordonaut

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    Last edited: Sep 25, 2012
  10. Feb 8, 2012 #50

    autoreply

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    I've always thought that the majority of the energy of an engine goes out of the exhaust in the form of heat. Any idea about the ratio's between that and pumping losses for a typical car/aircraft engine?
     
  11. Feb 8, 2012 #51

    gordonaut

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  12. Feb 8, 2012 #52

    autoreply

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    Thanks for the elaboration Gordon, that's clear :)
     
  13. Feb 8, 2012 #53

    bob989

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    Hi Gordon,

    I don't agree with your argument about engine scaling. You seem to hold this opinion quite strongly, so it may be an area where you and I would have to agree to disagree.

    Even so, I think we agree that if the smaller engines do indeed have reduced thermal efficiency, we could compensate for that by incorporating some known thermal-efficiency improvements. From a very narrow theoretical point of view, it is always possible to improve any engine's thermal efficiency by making it more complicated. You can get as close as you want to Carnot efficiency if you are willing to go to extreme lengths to do so. For example, you can have multiple cycles, each cycle being driven by the waste heat from the previous one in the chain. An infinite number of cycles would capture all of the wasted energy up to the physical Carnot limit.

    But there are always the questions of cost and (for airplanes) weight and volume. If we don't have reasonable thermal efficiency in the primary cycle, then the use of enhanced heat-recovery techniques might become too expensive, heavy and/or bulky. So it is important to know why a small engine cannot be thermally efficient.

    At the risk of beating a dead horse, I will ask you to reconsider your point of view without assuming equal scale factors for the three spatial directions. This is where you and I seem to differ. You are basing your argument on an assumption that the three directions scale with the same scale factor. I want you to abandon that idea. I want you to recognize that in a turbine engine, the axial direction is qualitatively different from the two radial directions, and it would be foolish to insist on using the same scale factor. For a piston engine, the displacement direction is qualitatively different from the other two directions, and it would be foolish to insist that the stroke length scales with the same scale factor as the cylinder diameter. No one would do that.

    A small engine is not a scale model, in 3 equivalent spatial dimensions, of a large engine. If you insist that it is, I would have to agree with your conclusion. But I think it is foolish to insist on using the same scale factor when that is a choice that no reasonable person would make. Faced with an unacceptable amount of drag, any designer would go back and review the assumptions that led to that situation. It would become glaringly obvious that treating the special direction as if it wasn't special is a design constraint that leads to a certain, very undesirable, conclusion. What happens if we relax that constraint?

    Regarding tip clearances, I think that the drag resulting from the interaction of the tips with the stationary walls could be reduced considerably by changing the geometry of the blades. I imagine rotors and stators as thin foils, like the foil on an electric shaver, with channels cut into them. The channels do not run the full radius, and the tips would be joined together in a smooth circle. The blades might also be joined together at other radii, and for structural stiffness, the whole thing might be cone-shaped instead of a flat disk. The thinness of the foil would mean that the air circulating in the gap between the "tips" and the stationary walls would be sheared over a small volume. Again, I am not suggesting that the blades would be scale models of the blades on large engines.

    Your comments about angle of attack are correct. The combined system would have an angle of attack intermediate between a pure wing (5 degrees or so) and a pure propulsion system (which is typically 2 or 3 degrees to help generate lift). Let's call it 3.5 degrees for the average angle of the air coming off the trailing edge of the combined system.

    So the future of aircraft propulsion is riding on the answer to one last, big question. Can small engines be thermally efficient?

    Regards,

    -bob989
     
  14. Feb 8, 2012 #54

    autoreply

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    Yes, they can. Some of the smaller diesels (VW I think) go over 40% efficiency with a turbodiesel and less than half a liter of displacement per cylinder, while the ship diesels (turbocharged 2-strokes) and the big turbofans go just over 50%. I can see the theoretical sizing argument, but I presume those are relatively minor contributions to engine efficiency and thus the reduced efficiency of going for a smaller displacement is very low.
     
  15. Feb 8, 2012 #55

    WonderousMountain

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    The surface area of a cylinder 1/2 the size is 27% more per volume. This is where most of the heat is lost in small cylinders. The ideal arrangement for reasonable balance and cooling efficiency is drumroll..... 2 horizontally opposed pistons. This is probably why we see so many on small models and in the 100cc range.
     
  16. Feb 8, 2012 #56

    gordonaut

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  17. Feb 9, 2012 #57

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  18. Feb 9, 2012 #58

    gordonaut

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  19. Feb 9, 2012 #59

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  20. Feb 9, 2012 #60

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