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The "modest" construction quality of modern, mid-line DSLR's
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The "modest" construction quality of modern, mid-line DSLR's
PeterN wrote:
On 5/7/2013 11:07 PM, J. Clarke wrote: In article , Likewise the Lockheed Electra. So much vibration was transmitted into the body of the plane that at least one airline provided soft cushions for the passengers' feet. IIRC, they still used them, even after they were required to reduce the plane's maximum speed by about 100 knots. But after the speed reduction the planes at least stopped falling apart in the air. The speed reduction was temporary until the cause of the problem could be determined. It was determined, it was fixed, and the Electra airframe continued in production for another 50 years. The vibration problem was also fixed--the two were not related. I believe that Lockheed was well aware of the risk, some safety engineers had warned management, but the warnings were ignored in the interest of timely delivery. Sounds like Challenger ... .... history repeats, it seems, for those who won't learn from it. -Wolfgang |
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The "modest" construction quality of modern, mid-line DSLR's
In article ,
says... On 5/7/2013 11:07 PM, J. Clarke wrote: In article , says... On Sat, 16 Mar 2013 02:27:15 +0100, Wolfgang Weisselberg wrote: : RichA wrote: : : Weakness, focus problems (very likely due to plastic's inherent : incapability to be machined as accurately as metal, : : Please provide proof : : it's tendency to : change shape/size radically with temperature changes. : : Quite unlike metal, which *never* expands or contracts. : : Please look up the de Havilland DH 106 Comet 1. : Fun when the planes disintegrate in the air after 20k hours : due to *metal* fatigue. Likewise the Lockheed Electra. So much vibration was transmitted into the body of the plane that at least one airline provided soft cushions for the passengers' feet. IIRC, they still used them, even after they were required to reduce the plane's maximum speed by about 100 knots. But after the speed reduction the planes at least stopped falling apart in the air. The speed reduction was temporary until the cause of the problem could be determined. It was determined, it was fixed, and the Electra airframe continued in production for another 50 years. The vibration problem was also fixed--the two were not related. I believe that Lockheed was well aware of the risk, some safety engineers had warned management, but the warnings were ignored in the interest of timely delivery. You can believe anything you want to. I was an engineer at Hamilton Standard and I worked on that propeller. These events happened before my time but I had access to everything that was known about that propeller and there was a lot in the files about figuring out the vibration modes in support of Lockheed's effort to resolve the problem. The people who worked the problem were still there and this is one of the projects that they liked to talk about--from an engineering viewpoint it was "interesting". You know what a resonance is? Where you keep feeding energy into something at the same frequency at which it vibrates when struck? Well this was the problem. It wasn't that Lockheed was careless--they had designed to cope with every vibration frequency that the engine and propeller were known to produce--this wasn't horribly difficult as the propeller and engine turned at a constant speed. The trouble is that the propeller in this case started vibrating in a way that had not been anticipated (by anybody, not just Lockheed) that was hitting a resonant frequency of the engine mount. This is not something that a "safety engineer" could predict. This is not something that the propeller manufacturer could predict. This is not something that, in 1959, the entire scientific community working together could have predicted because the computing power to predict it simply did not exist in the world. It had to be determined by extensive testing, which was a significant engineering project in itself, and by analysis of the wreckage, which showed unexpected damage to the bearings on the blade shanks. Hamilton ultimately did figure it out and at that point, knowing the nature of the problem, Lockheed was able to work up a reliable fix. Note that the whirl mode vibration that was breaking the airplane was different from the vibration that causing the comfort problems and the two fixes were unrelated--the comfort problem was fixed by changing the angle of incidence of the nacelles, the structural problem required very extensive modifications to the engine mount and the outer wing structure. |
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The "modest" construction quality of modern, mid-line DSLR's
On 5/8/2013 9:12 PM, J. Clarke wrote:
In article , says... On 5/7/2013 11:07 PM, J. Clarke wrote: In article , says... On Sat, 16 Mar 2013 02:27:15 +0100, Wolfgang Weisselberg wrote: : RichA wrote: : : Weakness, focus problems (very likely due to plastic's inherent : incapability to be machined as accurately as metal, : : Please provide proof : : it's tendency to : change shape/size radically with temperature changes. : : Quite unlike metal, which *never* expands or contracts. : : Please look up the de Havilland DH 106 Comet 1. : Fun when the planes disintegrate in the air after 20k hours : due to *metal* fatigue. Likewise the Lockheed Electra. So much vibration was transmitted into the body of the plane that at least one airline provided soft cushions for the passengers' feet. IIRC, they still used them, even after they were required to reduce the plane's maximum speed by about 100 knots. But after the speed reduction the planes at least stopped falling apart in the air. The speed reduction was temporary until the cause of the problem could be determined. It was determined, it was fixed, and the Electra airframe continued in production for another 50 years. The vibration problem was also fixed--the two were not related. I believe that Lockheed was well aware of the risk, some safety engineers had warned management, but the warnings were ignored in the interest of timely delivery. You can believe anything you want to. I was an engineer at Hamilton Standard and I worked on that propeller. These events happened before my time but I had access to everything that was known about that propeller and there was a lot in the files about figuring out the vibration modes in support of Lockheed's effort to resolve the problem. The people who worked the problem were still there and this is one of the projects that they liked to talk about--from an engineering viewpoint it was "interesting". You know what a resonance is? Where you keep feeding energy into something at the same frequency at which it vibrates when struck? Well this was the problem. It wasn't that Lockheed was careless--they had designed to cope with every vibration frequency that the engine and propeller were known to produce--this wasn't horribly difficult as the propeller and engine turned at a constant speed. The trouble is that the propeller in this case started vibrating in a way that had not been anticipated (by anybody, not just Lockheed) that was hitting a resonant frequency of the engine mount. This is not something that a "safety engineer" could predict. This is not something that the propeller manufacturer could predict. This is not something that, in 1959, the entire scientific community working together could have predicted because the computing power to predict it simply did not exist in the world. It had to be determined by extensive testing, which was a significant engineering project in itself, and by analysis of the wreckage, which showed unexpected damage to the bearings on the blade shanks. Hamilton ultimately did figure it out and at that point, knowing the nature of the problem, Lockheed was able to work up a reliable fix. All I can and will say, is that my belief is not based upon mere speculation. Andthere are times when memos disappear from files. Note that the whirl mode vibration that was breaking the airplane was different from the vibration that causing the comfort problems and the two fixes were unrelated--the comfort problem was fixed by changing the angle of incidence of the nacelles, the structural problem required very extensive modifications to the engine mount and the outer wing structure. True. -- PeterN |
#6
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The "modest" construction quality of modern, mid-line DSLR's
In article ,
says... On 5/8/2013 9:12 PM, J. Clarke wrote: In article , says... On 5/7/2013 11:07 PM, J. Clarke wrote: In article , says... On Sat, 16 Mar 2013 02:27:15 +0100, Wolfgang Weisselberg wrote: : RichA wrote: : : Weakness, focus problems (very likely due to plastic's inherent : incapability to be machined as accurately as metal, : : Please provide proof : : it's tendency to : change shape/size radically with temperature changes. : : Quite unlike metal, which *never* expands or contracts. : : Please look up the de Havilland DH 106 Comet 1. : Fun when the planes disintegrate in the air after 20k hours : due to *metal* fatigue. Likewise the Lockheed Electra. So much vibration was transmitted into the body of the plane that at least one airline provided soft cushions for the passengers' feet. IIRC, they still used them, even after they were required to reduce the plane's maximum speed by about 100 knots. But after the speed reduction the planes at least stopped falling apart in the air. The speed reduction was temporary until the cause of the problem could be determined. It was determined, it was fixed, and the Electra airframe continued in production for another 50 years. The vibration problem was also fixed--the two were not related. I believe that Lockheed was well aware of the risk, some safety engineers had warned management, but the warnings were ignored in the interest of timely delivery. You can believe anything you want to. I was an engineer at Hamilton Standard and I worked on that propeller. These events happened before my time but I had access to everything that was known about that propeller and there was a lot in the files about figuring out the vibration modes in support of Lockheed's effort to resolve the problem. The people who worked the problem were still there and this is one of the projects that they liked to talk about--from an engineering viewpoint it was "interesting". You know what a resonance is? Where you keep feeding energy into something at the same frequency at which it vibrates when struck? Well this was the problem. It wasn't that Lockheed was careless--they had designed to cope with every vibration frequency that the engine and propeller were known to produce--this wasn't horribly difficult as the propeller and engine turned at a constant speed. The trouble is that the propeller in this case started vibrating in a way that had not been anticipated (by anybody, not just Lockheed) that was hitting a resonant frequency of the engine mount. This is not something that a "safety engineer" could predict. This is not something that the propeller manufacturer could predict. This is not something that, in 1959, the entire scientific community working together could have predicted because the computing power to predict it simply did not exist in the world. It had to be determined by extensive testing, which was a significant engineering project in itself, and by analysis of the wreckage, which showed unexpected damage to the bearings on the blade shanks. Hamilton ultimately did figure it out and at that point, knowing the nature of the problem, Lockheed was able to work up a reliable fix. All I can and will say, is that my belief is not based upon mere speculation. Andthere are times when memos disappear from files. OK, now you're going off into tinfoil hat territory. Note that the whirl mode vibration that was breaking the airplane was different from the vibration that causing the comfort problems and the two fixes were unrelated--the comfort problem was fixed by changing the angle of incidence of the nacelles, the structural problem required very extensive modifications to the engine mount and the outer wing structure. True. |
#7
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The "modest" construction quality of modern, mid-line DSLR's
On 2013.05.08 21:12 , J. Clarke wrote:
Note that the whirl mode vibration that was breaking the airplane was different from the vibration that causing the comfort problems and the two fixes were unrelated--the comfort problem was fixed by changing the angle of incidence of the nacelles, the structural problem required very extensive modifications to the engine mount and the outer wing structure. I recall flying the Grumman Tiger. The 180 hp Lycoming version of the aircraft (AA-5B) tach had a yellow-band of about 100 RPM in the normal operating (green band) range - you could not operate the airplane in that RPM band. (Fixed pitch prop). During the check ride you operated it there for about 10 or 20 seconds in order to recognize the vibration. Nice little airplane. Fast - IIRC we could get a nice round 130 kts at 65% / 8000 ASL. Do not drink coffee before flight. (5 hour legs were not unusual - flying eastbound, climb to just below oxygen required and catch a good 25 - 30 kt. tailwind at 11,500 - lower throttle setting for about 120 kts TAS. - that's economy cruise! Westbound, cruise at 70% for 135 TAS - keep it lower and out of the higher alt. headwinds - 4 hour legs). -- "A Canadian is someone who knows how to have sex in a canoe." -Pierre Berton |
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