h a l f b a k e r yContrary to popular belief
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The idea behind jet engine nozzles is very simple: let the high-pressure air coming out of the engine depressurize, and it moves faster; the plane gets better fuel-efficency.
The problem is, a nozzle sufficiently long to allow this would be far too long and too heavy to be practical. (Imagine nozzles
as long as from the wing to the tail!)
Enter this:
The engines are moved from the wing to the belly of the craft, and forward, like a fighter jet.(Several configurations are possible.)
Nozzles could then easily extend from the engine toward the rear of the plane, integrated into the shape of the undercraft.
Consequently, the engines would effectively move more air, making all sorts of things possible: smaller engines, greater range, fewer engines, etc.
About Nozzles on Jet Engines
http://wings.avkids.../components-01.html
Scroll down to the part "Turbine", where pressure and velocity are mentioned, and "Nozzle". [galukalock, Oct 04 2004, last modified Oct 05 2004]
Bell Nozzles
http://www.imprimer...s/Files/LUIZARD.pdf
Found something about bell-nozzles, and their weaknesses. Page 7 of this PDF. [galukalock, Oct 04 2004, last modified Oct 05 2004]
I've seen better looking airplanes . . .
http://bz.pair.com/fun/noz.html
28 Jan 03 | (illustration updated 30 Jan) [25Kb image] [bristolz, Oct 04 2004, last modified Oct 05 2004]
An alternate approach . . .
http://bz.pair.com/fun/noz2.html
Stardate 2003.0130 | "Look, Ma! No bells!" [27Kb image] [bristolz, Oct 04 2004, last modified Oct 05 2004]
[link]
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So you're saying, put a long straw on the back of the engine? I'm not sure this well help enough to justify the added weight and bulk and complexity. For one thing it will create a lot of backpressure on the engine... |
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//So you're saying, put a long straw on the back of the engine?// |
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Exactly. What I failed to mention is that jets already have them, but because of what you mentioned, they are VERY short, almost unnoticeable.
The key is the first sentence of the second paragraph: INTEGRATING the engines partly into the belly, thus adding much less weight than would normally be necessary, and less complexity. Also, there is already backpressure, but the incoming air overcomes it. The nozzle allows it to DEpressurize. |
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//How about, instead...attach a large axial fan blade that both feeds the compressor section and pushes accelerated intake air directly out through the duct, back into the airstream, like a very fast propellor?// |
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They already have these as well.
They're called turbofans, and they are used mostly on passenger planes because they're quieter. |
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Problem: efficiency is a function of many more factors besides the thermodynamics, and there are umpteen thousand reasons why they place passenger jet engines away from the plane body. |
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A few for starters: rumble / NVH. safety. maintenance access. Heat isolation from the passengers. interchangeability. Commercial implications. Cost. Engine size, since the turbine shell would effectively run the length of the body. |
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I don't think that integrating the engine on a large plane into the body would save weight at all. You're only really saving the weight of the faring, which isn't much to be concerned with; and now you have to worry much more about heat shielding. |
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Never mind the commercial impracticalities of having nearly non-swappable engines for different aircraft. |
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//there are umpteen thousand reasons why they place passenger jet engines away from the plane body.// |
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Actually, if you really think about it, there aren't so many reasons as to make it impossible, or even impractical.
Why does the government have planes (fighters) with integrated engines if there's such a heat problem? Insulation such as aerogel solves that problem easily. They use it on the Shuttle. As you say, however, there are adjustments to be made if it's to work properly. Good engine walls to keep shrapnel out of other stuff, like passengers, in case of engine failure, changing the overall shape of the belly to accomodate the engines, etc.
I take issue, however, with the cost part of your statement.
The cost would of course be large for R&D initially, but as planes were built and 'got mileage', fuel savings would be enormous, depending, of course, on the quality of the design. My submission on the site is intended to 'get the idea out there' so that someone with brains (and guts) will try it out and see how well it works. If every commercial airliner had this engine design (assuming they make it safe, etc.), airlines would save gigabucks on fuel, get out of bankruptcy, lower ticket prices, etc. I don't expect that to happen anytime soon, but I have a reasonable knowledge of physics and economics, and I can tell you that it can work. It just depends on whether some airline (or manufacturer) wants to implement it. |
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F-14 and F-15 are *very* similar. |
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[UB] - you are correct. The pictured bird is an F-14, NASA tail number 991, Navy serial 157991, photographed over Dryden in 1979 during spin-control studies. |
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Somedays, Google loves me. |
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I could never tell them apart :O) |
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In this idea, the engines wouldn't have to be 'buried' in the fuselage, just-- 'melded' to it, so that the nozzles wouldn't be so heavy, bulky, and breakable by leverage. |
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Check out on Google what the function of a nozzle is. You won't have to go through to many pages before you encounter a comment about how much more efficient jets would be if they had longer nozzles.
As regards bypass ratio, you've seen 777s, right? How much bigger can they make the engines? |
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One more thing: the manufacturing of an airplane is already complex. Truth be told, this idea more or less requires its own plane(s). It's not meant for retrofits. Engineering from the ground up is a lot easier than RE-engineering. You can figure out where to put stuff from the computer instead of ripping it out of the 747 or whatever. |
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Ok, I'll bite. I have googled a lot of pages, and have not found the quotes you mention. Would you mind linking one? |
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// Why does the government have planes (fighters) with integrated engines if there's such a heat problem? // |
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Because military jets are built like tanks compared to jetliners, using hordes of spare-no-expense materials, and price into the millions of dollars per fighter. Add some of that money into what a jetliner already costs, and you've lost your market to the lower bidder. |
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The development costs of manufacturing a plane would be astronomical compared to the miniscule fuel savings. The pressurized hull problem is just one example of what would be at issue. |
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What people usually miss on engineering issues like this is that they have to survive the justifiable business case in the business world. In order to meet the development costs, engines have to be made as common as possible. |
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Planes are terribly complex, yes. But they still ascribe to the same physics that we all live by. |
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There was a proposal awhile back by Boeing for a huge flying wing-type passenger liner that never got off the ground. Too costly to develop, not enough market. |
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[galukalock] - you've got the expansion thing correct. You've just missed a few important points. |
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As the very hot gasses leave the engine, they tend to expand and become cooler gasses under lower pressure. Your point is that while it's still high pressure, you want it to keep pushing on the airplane. Until it gets all the way down to atmospheric pressure, you should still be able to get work out of it. |
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Imagine the shape of a rocket nozzle. The ogive tapered form allows the expanding gas to push the nozzle forward. Look at the nozzle from the back. All of that area that you see is where the gas is able to apply a pressure vector (which goes normal to the surface, mind) that adds force in the direction you would like it to go. |
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Now imagine looking into the back of a pipe. There's nothing to push on in the direction you want it pushed. The pushing still happens - it's just balanced equally in all directions around the perimeter of the pipe, with zero net result. In actuality, not only do you have zero gain, but heat lost through the sides of the pipe reduces the expansion of the outflowing gasses so a longer pipe will be less effective than a shorter one. The backpressure increases so the longer pipe will be less effective than a shorter one. |
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There are many fighter aircraft which have an engine airway entirely embedded inside the aircraft. With few exceptions, these craft have the engine mounted all the way out in the tail. The exceptions are where performance is being sacrificed for the sake of stealth. It's not because the tail is the easiest place for the design - it's actually pretty crappy in terms of center of gravity. To improve the situation with the COG, you can pull the engines forward - check out the F-4 Phantom to see what that looks like. |
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Embedding the engines in a passenger jet fuselage isn't exactly a new idea. Take a look at the Boeing 727 and the Lockheed L-1011. |
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Now imagine a passenger jet with the engines amidships embedded in the body. It's going to have to be on the centerline, as your expansion bell is going to need to be concentric to avoid sideloaded thrust. A nice parabolic curve is going to run out to maybe thirty feet in diameter by the time you get to the exhaust end. It's going to take a mighty big engine to provide enough pressure to make that big a nozzle worthwhile. |
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Well, actually, the way depressurization works in a nozzle is, the 'backpressure' is pushing the plane forward. Also, as the air tries to push against the pipe, it can't; it can ONLY expand forward (propelling the plane), and backward (gaining velocity, acting as a reaction motor, or rocket). Thus, it does lose temperature, but it does so by gaining velocity. |
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Heat would not be lost in any great quantity, because of above-mentioned aerogel insulation. Besides, once the nozzle is heated up, what more heat is to be lost into it? |
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//millions of dollars per fighter// |
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Passenger planes cost millions, too... |
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//parabolic curve is going to run out to maybe thirty feet// |
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Or, to keep the air confined to do the maximum work it can do, keep it the same diameter throughout so that the air's expansion occurs ONLY forward and backward (the directions that move the plane). |
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No. You're not getting it. Go find out why rocket nozzles are bell shaped. |
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And that's not the link you wanted - there's nothing in it that states or implies a longer nozzle would have better efficiency. |
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// Passenger planes cost millions, too. // |
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Well yeah. But you're not trading dollar for dollar, you're doubling the cost in this case. |
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Easiest way to tell a 14 from a 15: swing-wings. |
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To galuk's credit, there is an optimal length of a jet nozzle because you don't want to lose pressure out the sides of an exhaust too abruptly, and I would imagine that it gets traded off for other design factors in many cases. But I doubt that it would be nearly the length as to warrant a gain from integration into the hull. |
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[drew] was due to come back after christmas. I will give him a wake-up call ! |
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Well, what we need is for people to experiment and find out how well it works. THEN, and only then, will we have some idea whether it's practical. |
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//you don't want to lose pressure out the sides of an exhaust too abruptly// |
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That is the whole point of a nozzle's existence: to make the air expand ONLY in the directions that move the plane. |
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And yes, I would like some input from someone who really knows aircraft. |
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<quote from my link>As the pressure drops, the velocity of the flow, of exhaust gases, increases. As these gases leave the engine (turbine) they generate thrust.</quote from my link> |
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Therefore, the longer the nozzle, the more the pressure drops, the faster the air goes, the faster the plane goes, the smaller the engines can be to achieve the same effect. |
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<quote from www.gbnet.net/orgs/skylon/pap_pcsl.htm>conventional bell nozzles have high divergence and overexpansion losses</quote> |
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Wow, [bristolz]. Amazing illustration. Not sure that galukalock deserves it after having dismissed your earlier commentary so rashly. Perhaps he'll think a bit more before doing so again. |
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Well [galukalock] I have to admire your persistence and
gusto. |
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[bristolz] that's really great! |
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Knowing [bristolz] the damned thing has VTOL capability...... |
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Heh. The modifications to this morning's illustration are even funnier than what I saw last night. I like the fact that the plane now appears to "smile" as you've extended the nozzle/ tampon applicator thingy into the tail pipe. Reminds me a bit of the advertising slogan that Continental Airlines used in the late 1960s: "We really move our Tails for You!" The three fish carcasses painted up by the co-pilot's window are priceless. I'm adding my paltry plus to the idea just for the merit of the illustration. |
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Great illo, [bris], the smiling mouth reminds me of those anthropomorphic vehicles in the Thomas the Tank Engine shows. |
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Maybe as soneone who works alongside and with the people who design said aircraft and engines, I can provide some insight. When creating a new aircraft design, there are a huge number of parameters to be adjusted. Everything is a tradeoff. I'll keep my explanation to the engines only for now.
When a nacelle is designed, there are several things you want:
Smooth inlet flow
Minimum nozzle loss
Minimum weight
Minimum impingement (don't blow hot exhaust on aluminum)
Low risk in the event of catastrophic failure |
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While you (galukalock) are correct in stating that a longer nozzle can produce higher speeds, this is only true in a vacuum. What you want is to expand the exhaust plume until it is at atmospheric pressure. Any more expansion, and you end up with reduced thrust. In addition, longer nozzles start to run into problems of frictional losses due to boundary layer turbulence, making a longer nozzle less efficient. The nozzles on large jets are designed to operate most efficiently at cruise altitude of approximately 35,000 feet.
To address the smooth inlet flow, the easiest way to do this is to use a short inlet, with the engine out away from the fuselage, where the air is already smooth.
Putting the engines out on the wings also reduces impingement and reduces weight, because now you don't have to make the fuselage out of heat-resistant steel. (notice the blue color of the exhaust cone on the 777? That's heat-resistant steel there...turned blue by extreme heat)
In terms of catastrophic failure, the farther away from the fuselage you put the engine, the less damage a piece of broken engine can do to the plane. (I could show you pictures of actual damage, but I'd probably get fired). |
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You also mentioned stronger attachments to the fuselage: The engine mounts are actually designed, so that if the engine fails and starts to vibrate too much, the mounts will fail and the engine will leave the plane before the wing structure fails.
You wondered why fighter engines are buried in the fuselage. The main reason for this is simply that the engine is not built inside the fuselage, but rather the rest of the plane is built around the engine. A fighter plane is pretty much just a big engine with wings and a cockpit.
OK, I have to get back to work now... |
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Sorry I took so long getting back to this. |
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Great job, [bz]. Wish I could do that kind of stuff. However, I was thinking more rounded, like the turbines, and a bit smaller, reflecting the smaller engine(s). The steps don't seem right. I hate to pile criticism on such hard work, but I had no bell nozzle in my concept, either. |
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I read that paper from imprimerie, and encountered a description of exactly what you were talking about, [FF]. However, keep in mind that the engine compresses the air to around 4000 PSI, so I'm sure you can get plenty of gain from a longer nozzle than current designs; however, let me emphasize the need for TESTING by someone with the resources (supercomputer?) to determine *how much* there is to be gained. It's entirely possible that the gain is not sufficient to warrant a major change in plane design. But until someone tests it, we'll never know. |
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While I'm down here, I'll bring up altitude. Who's to say that 35,000 ft. is the best altitude for any given plane? Could be that this design would work better at 50,000 or 20,000. Once more: it needs to be tested. |
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Thank you all for your input, by the way. |
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I wasn't trying to 'be rash' with anybody. I apologize if anyone took it personally. |
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Um, my illustration is supposed to be ridiculous. Clearly, I didn't make it ridiculous enough. |
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Put an *engineer's pump* on there - it'll increase airflow *and* look ridiculous. |
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Mission accomplished with the latest illustration, Bris. It's entirely laughable. I like the fact that I can see the tail lettering a bit more clearly on this version, you removed the co-pilot's private accessway, and fixed the potential problem about the landing gear location. I'm still not sure how you resisted the urge to add a few more fishbones to the paint job ... You could have made the flier of this plane a real "Ace". |
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The second version has a certain Star Trek appeal to it as well. |
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As I said, I work for the company that designs these things. In a business environment where 1 or 2 percent difference in performance may be the difference between winning and losing a multi-billion dollar sale. I get to see first hand the results that come out of the supercomputer (a very large, very powerful CRAY machine) and please believe me when I say the analysis is thorough. The designs are very well tested. When the airplane is designed, the engineers begin with a target altitude and speed, and design specifically for those conditions. The nozzles are shaped very carefully to provide the optimum pressure at the end of the nozzle. So, yes, the current designs are very close to optimum, and they have been tested, both in computers and in real life. A recent development meant to make the 747 quieter by using a LONGER nozzle (more room to stabilize and quiet the airflow) actually produced a lower thrust. One of the only reason that the nozzles aren't shorter is that there needs to be room for the thrust reverser mechanisms. The combustion chamber may produce pressures around 4000 psi, but much of this is converted to work in the turbine. Bypass air from the fan is actually much lower pressure. The very careful design of the nozzles does producethe proper pressures when the plane is cruising along at design altitude.
I guess this brings us back to design altitude. The altitude is another compromise, very carefully chosen. If you fly lower, the air is denser, and hence takes more thrust to reach a given speed. If you fly higher, the air is thinner, needing bigger wings (again, more drag) to hold up the plane at a given speed. Going higher means it takes longer to get down, should the cabin depressurize. I almost hate to tell you this, but the oxygen system isn't supposed to keep you awake, just alive long enough to descend to 18,000 feet (the altitude where you can survive on just air). If you increase the time to descend to a safe altitude, you put lives at risk. Going higher also means you have to build the fuselage stronger to withstand the pressurization, which means more weight, which means more drag, which means more thrust needed, which means more fuel burned.
I could go on and on for hours about the merits of the current design, but I don't want to bore the rest of the half-bakers. To sum it up, the designs have been exhaustively tested, and they work just fine how they are. |
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. . . . and one should have confidence in an aeronautical authority whose name is "Freefall." Akin to the secure feeling you get when you notice the taxi driver's nickname is "Crash." |
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Actually, I offer a sincere thanks to you, [Freefall], for taking the time to explain this. I have gained knowledge by your comments and that is a thing of value. |
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Nice job, [bz]. Definitely laughable. Man, I wish I could do that stuff. |
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Thanks [thcg] for that link. Having already read the paper, I had no idea the link wouldn't work. |
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Interesting point, [Freefall] about the altitude bit. I had thought about that, and you're at least partly right. Also, I had wondered how well oxygen masks could be expected to work under low pressure conditions. |
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As regards putting more strength into the fuselage, I agree. However, I pointed out earlier that this design concept isn't meant to be built around existing planes, and some of the consequences described reflect how well it DOESN'T work with 7x7's, etc. This means that we shouldn't worry about adding more weight, etc. We can simply make this plane out of composites or some other such material (simply isn't as simple as it sounds; I'm just brainstorming here). |
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I guess what I'm getting at is that although you're quite correct on this, it's sort of beside the point because this engine doesn't apply to existing aircraft. It's up to you to discuss this with your fellow engineers and see how feasible it is to use this idea in *future* airliners. |
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Again, thanks for your contributions to this discussion. |
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Wait a minute. Isn't the max altitude you can survive on air 11,000 ft.? |
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Of course, I'm not talking about Himalayan natives. |
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The FAA requires oxygen for pilots in unpressurized aircraft flying at or above 12,500 if they are at that altitude for over 30 minutes and continuously if above 14,000 feet. You can bet they have a fairly good margin of error built in. |
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I have suffered the effects of hypoxia but only subtly. It was on a night flight and we had to go over unexpected weather and didn't have oxygen on board (very, very stupid). I think we were at about 14,500 or 15,000 for about an hour and things seemed fine. Upon descent, though, the colored lighting of the instruments became quite vivid and it was then that I realized my vision had become nearly monochromatic at altitude. It was a little spooky because it was such a gradual loss of color perception and only realized when the acuity was restored. Lesson learned. |
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I guess that, sometimes, you don't know what you're missing until you get it back. |
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I think, if I remember right, that the survivability at 30,000 feet is only a couple of minutes and at 40,000 a few seconds. |
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Perhaps this is one reason that they don't pressurize the cabin to *full* atmospheric pressure? |
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Off the subject, just how many half-bakers are from UK, Canada and Oz? Watashi wa Amerika-jin desu--I mean I'm American. |
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