h a l f b a k e r yExpensive, difficult, slightly dangerous, not particularly effective... I'm on a roll.
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Aircraft already have basically cylindrical fuselages, I know. But they (especially supersonic aircraft) also have pointy ends attached to those cylinders, which I think can be associated with a problem. In this Idea I hope to describe the problem and the solution.
As an aircraft moves through the
air, pointy end first, the pointy end shoves air to all sides of the cylindrical fuselage. The body of the aircraft MIGHT (rather imperfectly) be described as being surrounded by a layer of compressed air. At the tail of the aircraft a second pointy end allows the compressed air to expand, reoccupying the space it was located, before the aircraft passed through it.
More accurately, of course, we know that the pointy front end of the aircraft imparts considerable Momentum to the air it encounters, and as a result the pressurized air keeps moving away from the aircraft; there is no "layer" of compressed air surrounding the fuselage. We MIGHT be able to associate "drag" (air resistance) with SOME of the differences between this description and the description in the previous paragraph. After all, if the aircraft shoves air to all sides, and that air keeps on moving away from the aircraft, then when the aircraft passes on, a region of diminished air pressure is behind it. Progress through the air requires continuously overcoming the tendency for air in front of the aircraft to push it back into that region of diminished air pressure, behind the aircraft.
A more ordinary definition involving "drag" includes the extra words "coefficient of", and there is a precise comparison between Object A, which is aerodynamically shaped to minimize drag, and Object B, which has the same total "frontal surface area" as Object A, but the frontal area of Object B is flat; that is, it encounters the air flat-face first. (B is basically a cross-section of the front view of A.) By definition, the coefficient of drag of Object B is 100% (practice and theory don't always mesh; see link). It is possible for a well-designed vehicle to have a coefficient of drag of about 15%. It would be nice to go rather lower than 15%, if we could, of course. Well, that is what this Idea is about....
So, suppose we remove the pointy ends of the aircraft. For the moment, let's imagine a very thin cylindrical shell, moving through the air. The surface area that interacts with the air, used to compute the coefficient of drag, is the area of the EDGE of the cylindrical shell. The thinner the edge, the less air is pushed away from that edge; obviously the drag will be less, too.
Of course such a cylinder is not so good for carrying passengers, we typically want to fill the cylinder with seats and luggage-storage and so on. We can still do that here, provided the cylinder is larger than normal....
At the front of this aircraft design, we leave the cylinder "open". Even if the cylindrical fuselage is 20 meters in diameter, the front of this vehicle is still an open cylinder, a ductway leading TOWARD the rest of the interior.
However, after say a meter of duct (probably less), the interior of the cylinder begins to take on a funnel-shape. It narrows down to a tube maybe 5 meters in diameter (assuming an initial 20). This tube extends the length of the fuselage, and expands at the tail of the aircraft (another funnel shape).
There is more to it than just that, but first let me say that the "working volume" of this aircraft fuselage is the space between the 20-meter-diameter outer cylinder and the 5 meter-diameter inner cylinder.
Mathematically, the since the inner cylinder has 1/4 the diameter of the outer cylinder, it also has 1/16 the area of the outer cylinder. This means that air entering the front duct needs to compress by 16 times, for it all to flow down the inner cylinder, to the tail of the vehicle. IF IT DID, then that air would be exactly the amount needed to perfectly re-fill the space behind the aircraft, as it moves through the air. OUTSIDE the aircraft's fuselage (not counting the effect of wings and other structures), the effect of that cylinder passing through the air might be rather minimal (meaning: A Nice Low Coefficient of Drag).
Very likely we need to assist the process of compressing that air, to ensure it all actually goes down the central tube, instead of forming the equivalent of a "bow shock" wave inside the mouth of the 20-meter cylinder, spilling outside the cylinder and messing up the effectiveness of this design.
Well, it happens that air compression is a well-proven technology used in all jet engines. We could even consider the whole fuselage of this aircraft as a potential Giant Jet Engine. I won't especially recommend that, for safety reasons (the passengers would be pretty close to the hot throat of the thing, after all), but some front-end compressors should certainly be do-able. Some expansion turbines in the tail-funnel can recover much of the energy used to compress the incoming air. (Maybe a LITTLE fuel can be burned in that area, to provide just enough power for this purpose....)
A final note regarding the pilots. They won't be able to see straight ahead through cockpit windows, like most ordinary aircraft, of course. But exterior cameras can be embedded in enough places (remember the plane still has wings) for a full view to be constructed and displayed on monitors. Think of it as just another variety of "flying by instruments".
Coefficient of Drag
http://en.wikipedia...ki/Drag_coefficient
As mentioned in the main text --not to be confused with the percentage of time a man wears women's clothing. [Vernon, May 18 2009]
Flying by instruments
http://en.wikipedia...rument_flight_rules
As mentioned in the main text [Vernon, May 18 2009]
On jet engines
http://en.wikipedia.org/wiki/Jet_engine
You probably know some of this already. So? One thing I need to mention is that it probably will not be possible to build a 20-meter-diameter compressor (the centrifugal effect would destroy it). But several smaller ducted compressor fans ought to work. And, of course, we could always go for a larger-diameter inner cylinder. For example, if it was 1/3 the diameter of the outer cylinder, only a 9:1 compression ratio is needed, not 16:1 (easier to do). [Vernon, May 18 2009]
English Electric Lightning
http://images.googl...e=UTF-8&sa=N&tab=wi
The aeroplane designed specifically to rapidly scramble, climb and shoot down an incoming ICBM. It can climb at 50,000 ft/minute. [hippo, May 19 2009]
X-zyLo
http://explore4fun.com/xzylflyincyl.html
would it look anything like this? [jaksplat, May 19 2009]
Coefficient of drag queens
http://www.thecriti...unway_terri_506.jpg
Somebody had to do it... [normzone, May 19 2009]
SNECMA Coleoptere
http://www.aviastar...ecma_coleoptere.php
VTOL Research craft - annular wing [FlyingToaster, May 19 2009]
Caproni Flying Barrel
http://www.youtube....watch?v=xYqr2h_xQRk
ducted-fan [FlyingToaster, May 19 2009]
Patented
http://www.freepate...ne.com/6607162.html
Seems that 'ring wings' or 'annular wings' is baked. [pocmloc, Sep 30 2009]
What I thought was meant from the title.
http://aero.stanfor...rwings/RingWing.jpg
[FlyingToaster] that link is great. [2 fries shy of a happy meal, Jan 08 2013]
[link]
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This is a halfbaked scheme in the best sense. I very much look forward to discussion of it. This would not be a hard thing to model, either. |
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There are several java programs to show aerodynamics, but the set of shapes is limited: airfoils and such. |
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[21 Quest], yes, but that's why I specified a larger-than-normal fuselage diameter. I gave some example dimensions of 20 meters outer diameter and 5 meters inner diameter. That means 7 and 1/2 meters (about 25 feet) in-between the two cylinders (surrounding the inner cylinder), all down its length. An alternate description:
( 7 & 1/2m + (5m inner) + 7 & 1/2 m) = 20m outer dia. |
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[bigsleep], your pods would increase the coefficient of drag, defeating the purpose of this Idea. |
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[bigsleep], you can't put as many passengers in your pods as could fit in the fuselage. And you also will have problems with emergency exits, people needing to use the toilets, and supplying them with food, drinks, etc. |
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Have you considered building a prototype? |
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For the same frontal area as a 'normal' fuselage, you would have less useable space.
I also think (I could think wrong...) that because the air is forced to compress into the central tube, as opposed to the gradual pressure decrease around a 'normal' fuselage, it would be less efficient.
That said, the central tube could be used as a wind-tunnel, for model testing and such (easy to change speed, air pressure, etc. by flying somewhere else). |
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[bigsleep], regarding the weight of the inner cylinder, this depends on what it is made of, and just how much pressurization is needed. I described 16:1 in the main text and mentioned 9:1 in one of the link-texts. That means the inner cylinder needs to withstand 16 (or 9) atmospheres of pressure. Since I wasn't planning on it also withstanding stress of combustion heat (except maybe some at the rear of the aircraft), something like a relatively normal (these days) carbon-fiber composite material should work fine. |
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A little residual concern about the airworthyness of the plane in a stall type situation. |
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Had you considered turning the concept inside out? |
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Imagine, if you will, the incoming air is ducted to an annular space while the inner space is left for passengers etc. as in a conventional aircraft. |
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What we have, then, is a conventional aircraft with a thin wall tube around the fuselage containing the displaced air. In this embodiment it is much easier to see the comparison to conventional design. |
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It would be worth looking at the drag of a conventional fuselage and the extra weight it could carry if that drage were, say, halved. That would give a quick guide to the faesibility or otherwise of the idea. |
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If you go for a cylindrival fuselage with a big air intake at the front, you get the English Electric Lightning, one of the most exciting jet fighters ever (link). |
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i do not even see you path of reasoning. |
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For certain forms you have (in a certain range of sizes) a linear relationship of drag with frontal area. Now instead of using the form with lowest coefficient of drag known to man (a drop shape) you propose a new one - this is fine, this site is about new ideas. But from the truism, that a tube has no drag (in this equation, as long as the thickess of its walls is zero, and therefore the frontal area is zero... in reality it has drag, of course, otherwise the needed pressure for a fluid streaming through a tube would not scale with length) you for some reason jump to the conclusion that a thick-walled tube has negligible drag also. Why? |
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As you corecctly state, // we need to assist the process of compressing that air // - in other words, the air does not want to go there, we have to force it along. Now what difference does it make in you book whether we force a pointy-end tube through the air, or the air through an open tube (hint: its the efficiency of needed power versus fillable-with-cargo volume)? |
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Or, to break it down a little: imagine to see an axial cross-section of your plane. now imagine zoomin in on the lower edge, losing from sight the upper edge: How does this differ from a wedge-shaped aircraft (F117 or similar)? |
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by the way: where do you cite from saying // the coefficient of drag of Object B is 100% // - wikipedia gives 1.28 as Cd for a flat plate perpendicular to flow, and ~2.0 for the Eiffel Tower... |
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//Technicalities. //
I think that tradition requires sp. "<dismissive hand-wave> Technicalities </dhw>"
//shoot down an incoming ICBM// Oh come now, [hippo] - you made that last bit up. |
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[loonquawl], the description I gave regarding coefficient of friction came from something I read years ago, about how it was defined. Maybe something about that definition has changed in the interim? |
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I agree that the interior of a cylinder, when filled with stuff, represents a "face" that would be associated with increased drag over an empty cylinder. But I spelled out I thought quite clearly an advantage over the conventional design, since the air behind the body of this aircraft will not have reduced pressure, after the aircraft goes through it. That is, drag has two main components, the pressure built up in front of the object, and the reduced pressure behind the object. We are still using power to overcome the first, but we are simultaneously eliminating the second, in this Idea. |
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So you pose the aerodynamics of a powered shape against those of an unpowered shape? In fact, any object that picks up every molecule in front of it, and replaces it afterwards to the exact same location, will not disturb the fluid around it. The question is as to the power used in this endeavour. A normal tear-shape can be said to do just this, as can be any shape that lets airflow be laminar. |
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a rocket in this reasoning has fabulous aerodynamics, as it actually increases pressure behind the body... |
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Also, for a tube of same volume as a stick you double the 'wetted' surface area, while increasing either the velocity of the air streaming inside, or the density, both being detrimental for drag. |
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There are two different factors to drag. The first is the air
displaced at the face of the aircraft (and suction caused as
it returns at the back) (dynamic drag), the second is
"stickiness" of the air moving along the surface of the
aircraft (viscous drag). While an infinitely thin walled tube
has none of the former, it still has a noticeable amount of
the latter. In fact it has something like twice as much as a
closed cylinder, since it experiences this on the inside and
outside.
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This design would minimize the suction at the rear, and if
it was turbine assisted it would eliminate the
displacement at the front. However, the energy to
turbine assist would, I suspect, completely offset any
savings in fuel due to lower drag. Especially since your
inner tube does experience the surface drag over it's
whole length, considerably more than a normal (short) jet
engine.
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An idealized fuselage shape, (I think a double pointed tear
drop, but I'm not certain) would minimize dynamic drag by
slowly pushing the air out in front, and allowing it to
return smoothly in back. Aircraft are not designed this
way for practical reasons, but I don't think this design
would carry any advantage over the current closest
equivalent. |
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the idealized fuselage is (for subsonic velocities) a tear-drop shape. Having a pointy end in front, too, is worse than the rounded solution every subsonic aircraft applies. |
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As I said, wasn't sure, fluid dynamics is not my specialty. |
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So you're compressing the air on the inside as opposed to "pushing it out of the way" on the outside (assuming Mach < 0.3)? Both require power. If indeed this scheme decreased drag, your engines would have to generate less thrust, but still would have to power the compressor(s). |
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The noble goal here is to eliminate the "base drag" at the fuselage rear. This is actually quite small compared to other types of drag, and in getting there you're likely to encounter twice the skin friction drag and as [lurch] said, supersonic flow through your nozzle, leading to some kind of internal wave drag. (Some might consider this cooling drag, since your nozzle is, more or less, an engine) |
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This could be interesting as a NASA project, but would most likely be wildly inefficient and horribly complex. Since there are no flight regimes I know of where base drag is a limiting factor, I could only see this as a curiosity. |
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I get the idea, but at a guess I'd agree with the consensus
that it wouldn't be more efficient. If I understand
correctly, the hoped-for improvement comes from the
fact that the air which is compressed by the passage of the
plane is trapped inside the inner tube, and is then
available for re-expansion behind the plane. |
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Well, yes, sort of, but you haven't quantified how much
drag on a conventional plane comes from the partial
vacuum left behind it. My guess is: not much. First, I
suspect that the rear taper of the plane does indeed allow
most of the displaced air to re-expand and reduce
"suction". Second, whatever "suction" there is acts only
on the cross-section of the fuselage, which is not that
huge. |
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In your system, you've increased the cross-sectional area
of the fuselage considerably. It is still "streamlined", but
in an inverted sense (ie, the streamlined taper now goes
inward from the nose of the plane instead of outward), but
a bigger cross section is still a bigger cross section. |
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Also, there's a difference between vacuum and pressure: a
vacuum can only be as great as atmospheric pressure (if
you see what I mean) whereas pressure can be much
higher. A conventional plane may have some "vacuum"
behind it sucking it back, but your design will have a much
greater positive pressure in front of it. In short, I think
this ain't gonna work. |
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However, as has been said, this would be very easy to
model in something like Comsol. |
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By the way, why ask the pilots to fly by TV? Just put the
cockpit on the bottom. Gives 'em an incentive not to
screw up the landing, anyway. |
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an entire of generation of fighter-jets had the engine intake in the nose so it's not that innovative except for the scale of course. |
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Another solution of a more conventional nature would be to have the jet engine intake as a ring around a normal fuselage... depending on the airspeed you might be able to capture all the air (okay probably not). |
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anyways <links> (Coleoptere and Caproni) |
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//engine intake in the nose so it's not that innovative//
Yes, but very few passengers reported a happy travel
experience whilst sitting inside the fuselage. |
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This idea is new in that the central "hole" is not primarily a
jet-engine throat, but is largely passive. However, I still
don't think it'll work. |
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//the central "hole" is not primarily a jet-engine throat//
without doing the calculations (without looking the calculations up and trying to figure out which ones are relevant), seems to me that tripling (at least) the exposed area is going to cause more drag than that of pushing aside x sq metres of air, so you may as well be using it for *something*. |
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Try this idea on a ground vehicle and you won't need to deal with supersonic flow. |
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Also (and this may be more half-baked still) can you decrease the drag of the air flowing through the central tube by compressing it to a much higher density, so the same mass flow can happen at a smaller velocity? |
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The posted idea appeals to me: rather than pushing air out of the way and then having its momentum (therefore kinetic energy) unrecoverable, handle it internally so it can be placed at rest behind you with no kinetic energy. Of course if you increase drag too much doing this it's inefficient, but there's still a kind of elegance to it. |
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It seems like there ought to be a way to get this to work, but I don't really know how to use any aerodynamic modelling programs. |
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From a more whimsical perspective, I like the idea of hanging the passenger pods from the wings, and suggest moving the pilots to either the tail or the landing gear wheels for increased visibility. |
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Bags not being in the passenger compartment when this thing throws a turbine blade. |
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A Mig-21 is essentially the above described idea, a
tube open on both ends with an engine in it and
little wings glued on. |
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But I think you're talking about cramming the
friction causing air hitting the front of the plane
into a little tube so you still have some usable
area in the plane. That takes work to accomplish.
Rather than pushing the air around the aircraft, shoving it aside into the relatively low
pressure ambient, you're taking that same air and
pushing it through a high pressure restricted tube
needing more energy to accomplish. Of course if
you have an engine in there anyway, might as well
take that friction causing air and burn it like in the
Mig-21, but putting a duct down the middle of a
passenger jet wouldn't be worth the extra engine
you'd need to compress the air into that tube to
blow it out the back. The additional weight of the tube would also cause more drag as the angle of attack or speed which keeps the plane up would need to be adjusted, not to mention the insulation you'd need around the tube if you didn't want to cook the passengers. If you went the passive route, any air that wouldn't fit into the flow through the high pressure tube would just pile up and re-direct around the outside of the plane anyway. |
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That being said, there was some testing done of laminar flow control wing
surfaces where slots in the wings sucked air in
using the engines somewhat along the lines of
your idea. I believe the gains weren't worth the
extra energy expended but you can feel good that a variation of your idea in theory was valid enough to warrant building an expensive x plane to test. |
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