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#21
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On Wed, 11 Aug 2004 02:31:22 GMT, Roger Halstead
wrote: Now that number concerns me. A piece of steel tube the length of a normal muffler weighs almost that much. IF they claim no performance degradation the muffler must be straight through and would hardly be very effective, or at least hardly more effective than the stock units which weight a *lot* more than 2#. Are you sure that wasn't Kg? Straight through mufflers can be deceptively effective. They aren't literally just a straight through pipe, the pipe is actually drilled with many holes which allows the exhaust pulse to bleed off into an outer chamber. On street cars, this outer chamber is often packed with fiberglass, producing "Glass Packs", famed for their suppressed rumble. The "Swiss Muffler" is a variation on that theme. It's a long drilled tube, or a tube rolled from stainless steel mesh, surrounded by a solid outer tube. The space between the two is packed with stainless steel wool. So the exhaust pulse can go straight through, but it's energy is bled off through the holes in the inner tube. This is very effective, but the ones I've seen are routed outside the body. Since they parallel the wind stream, they don't greatly effect overall drag, but they definately are there to see. http://www.piteraq.dk/flight/muffler.html My opinion is that anti noise regulations for GA aircraft are likely in the future here in the USA. We know it isn't impossible to do because the entire European continent flies with such laws. Corky Scott |
#22
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On Wed, 11 Aug 2004 04:15:37 GMT, "G.R. Patterson III"
wrote: It's going to depend entirely on whether or not the aircraft in question is "overpowered". If the engine and prop combination is such that there's power to spare, adding a blade can effectively use that power. You'd actually do better with a larger 2-blade prop, but there will be excellent reasons why this can't be done with that aircraft (if it could, the manufacturer would have installed one). If the aircraft *doesn't* have any power to spare (mine doesn't, for example), adding another blade (or a larger prop, for that matter) will only decrease performance. Maybe, maybe not. I'm no prop design expert but I was intrigued by a prop planform design written up in the latest Contact! magazine. It's for experimental airplanes only, but it offered very good performance and low noise at maximum rpm. It had a sort of triangular planform, with the outer portion of the blade narrowing way down outside the cowl. This seemed counter intuitive to me because I'd always heard that the fuselage cross section basically blanked out much of the thrust produced by the prop close to the hub, and because the prop didn't turn very fast at that point anyway, there just wasn't much thrust to be had. But this designer found that wasn't true. He narrowed the tips and cut them off abruptly because they operate at Mach .85 or so and wide tips at those speeds create lots of drag and noise. So the prop looks really berzerk. It has an extreme pitch angle at the hub, then the planform widens dramatically to the outer edge of the cowling where it abruptly and also dramatically begins narrowing down. It sweeps inward then begins straightening out to the point where by the time the tip is reached, the planform is essentially straight. The pitch angle flattens out as it gets further away from the hub. The airfoil is does not have a flat back either, it's airfoil shaped on both sides, and I gather the airfoil is different at different locations from the hub. This is a relatively high speed prop for a high speed airplane, a Lancair 360. The designer was listening to a prop "expert" lecture (I think this was at Oshkosh but don't recall when). The guy repeated the old wives tale about multi bladed props being less efficient than two bladed props, and that a single bladed prop was the most efficient propeller of all. The designer pointed out that at cruise, the rate of advance for a standard rpm prop was in the neighborhood of nearly 15 inches PER BLADE, on a three bladed prop. That meant that each blade bit into air unsullied by the previous blade. So when exactly did a three bladed prop become inefficient? The expert paused for a moment, then said that the point of greatest inefficiency occured while the airplane was completely stopped. That's a "so what". Know one cares how efficient a prop is while standing still, we only care how efficient it is while pulling the airplane through the air. And while at cruise, the rate of advance guarranteed that the prop blades, even three or four bladed props, always saw clean, undisturbed air. Corky Scott |
#23
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On Wed, 11 Aug 2004 at 11:26:14 in message
, Todd Pattist wrote: So we have a blade hitting air that is 15" away from air that was hit by another blade. The pressure disturbance from the first blade propagated at the speed of sound over that 15" and has arrived at the "new air" 15" ahead long before the airplane and the second blade arrive , so there's some effect there. I'm also not a prop designer, but you need more to convince me than just saying they are 15" apart. Let us assume a cruise rpm of 120knots == 202 ft sec Then assume cruise rpm is 2000rpm @ 2000 rpm is 33 revs per second advance per revolution = 202/33 or 6 feet. So three blades would arrive at the same spot every 2 feet. I am not sure that means much because we know the flow is rotated by the prop as well. -- David CL Francis |
#24
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Roger Halstead wrote:
Ideally a muffler could combine pulses into a steady string and average them out. The higher the RPM and the more cylinders the higher the frequency. The larger the pulse volume the more difficult to smooth, The higher the frequency the easier to smooth. Agreed. Playing with a bit of math and an over simplification where I always make at least one mistake: I'm not particularly "challenged" in any way, but frankly, I don't understand the point of your calculations. But the principle is simple and I agree: The bigger the volume of one (!) pulse, the bigger the volume of the muffler. Everything beyond this pronciple is beyond me, too, and left to the specialized engineers. All I know is that I can hear the difference between a good and a bad muffler. Stefan |
#25
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Newps wrote:
Any muffler, no matter how well engineered, puts back pressure on the system. No. As the exhaust system is an oscillating system, a well tuned one even *reduces* back pressure to the engine! If it didn't it wouldn't muffle anything. Back pressure to the engine relates to one side of the system, noise is created on the other side. Rip off the exhaust system from a modern car. You'll be surprised how much power you loose! (Don't ask me why I know.) That's because the computer is now befuddled In my particular case there wasn't any computer in the car. Ok, I should have avoided the "modern" part. Stefan |
#26
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Roger Halstead wrote:
Why less RPM? Speed maybe, but why more fuel consumtion? To get less noise with a particular prop you have to reduce the RPM. Yes, but we talked about changing the prop. To go with more blades to reduce the noise is reduced efficiency, This needn't to be so. The noise from the 2 blade prop is on take off. The noise which causes most problems is the take off noise. En route noise can be reduced by flying high and avoiding populated areas. Of course a quiet plane is a nice thing en route, too. To me, working on the muffler(s) is like trying to keep the mouse from squeaking while the lion is roaring. Nobody prevents one to work on both. Stefan |
#27
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Roger Halstead wrote:
One thing between Europe and the US, in general they are not flying the same kind of planes, or the ones they do are in the lower performance area with only a couple exceptions. Flying over there is unbelievably expensive compared to the US. That's certainly so. There are several reasons for this: Generally more dense population (I'm not talking Manhattan, of course), hence more real noise problems. Smaller distances, hence private planes are considered luxury. And generally more environmentally minded, hence the high taxes on fuel and more strict noise regulations. Oh, and no colonies for an ensured oil supply! (duck and run) Stefan |
#28
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On 11 Aug 2004 11:26:14 -0500, Todd Pattist
wrote: So we have a blade hitting air that is 15" away from air that was hit by another blade. The pressure disturbance from the first blade propagated at the speed of sound over that 15" and has arrived at the "new air" 15" ahead long before the airplane and the second blade arrive , so there's some effect there. I'm also not a prop designer, but you need more to convince me than just saying they are 15" apart. Not sure I need to convince you Todd. I'm just stating what the guy who IS a prop designer is saying. If you want to debate, I'll look the guy's name up and you can contact him. But from what I can discern, the rate of advance is such that each prop blade is biting into clean, undisturbed air. You should see this prop, it's really unusual looking. Corky Scott |
#29
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On 12 Aug 2004 09:21:10 -0500, Todd Pattist
wrote: Corky Scott wrote: Not sure I need to convince you Todd. I'm just stating what the guy who IS a prop designer is saying. If you want to debate, I'll look the guy's name up and you can contact him. You don't have to convince me, I'd just like to know the answer. Monoplanes are more efficient than biplanes, so it makes sense to me that single blades are better than multiblades. How much "better" is the question. I would expect anything within about a prop "span" to have some effect. The designer of the prop I mentioned wrote in the article about the fallacy of single bladed prop. Regardless the dubius advantage of biting into clean air, the problems associated with the unbalanced thrust produced by the single blade spinning around, despite it being counter balanced weightwise, are for all practical purposes insurmountable. The prop tries to rip the engine out of it's mounts all the way around. The imbalance is pretty much impossible to dampen out and results in monumental vibration. Corky Scott |
#30
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On 13 Aug 2004 08:20:13 -0500, Todd Pattist
wrote: I was looking at the issue from the theoreticalaerodynamic sense and whether 15" of separation is enough. I agree there are lots of practical difficulties with single blade thrust, but that does not mean they are insurmountable in all cases. It's not unusual to see them on indoor model aircraft where efficiency is particularly critical and the "unbalanced thrust" is low and less of a problem. Not absolutely positive, but I think that the difference in Reynolds numbers between one of those 2 oz indoor mylar covered rubber band powered airplanes and a Lancair 360 would skew the information so much as to be non comparible. Practically speaking, there appears to be a point where a two bladed propeller simply cannot deal with the available horsepower and another blade is necessary. That is why during WWII you can see the progression from two bladed props, to three bladed, four bladed and eventually, five blades. Corky Scott |
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