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#61
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Spinner strobing as a "Bird Strike Countermeasure"
If any of you have tried to use a strobe light to determine rotational
speed, you'll remember harmonics are a serious problem. If there's a single mark on a shaft, it will appear stationary if the flash rate is equal to the time it takes the shaft to turn once. It will also appear stationary if its rate is half of that time, (it will blink on the spot every other time) a quarter of that time, and so on. It gets worse. If the blink rate is twice that of the shaft speed, the spot will appear stationary, but only half as intense, since one flash will 'lock' the spot, and the next will illuminate the opposite side of the shaft at an unmarked place. To make this relevant to aviation, replace spot with prop blade. On Dec 4, 11:17 am, Harry K wrote: On Dec 3, 7:21 pm, Harry K wrote: On Dec 3, 2:45 pm, Just go look it up! wrote: On Mon, 03 Dec 2007 21:52:04 -0800, Airbus wrote: In article , says... When observed directly under artifical light that "flickers", the most obvious being a strobe light, but there are other types of artificial lights that have flicker. -- Fine - but which ones cause you to see the propellers turning in apparent reverse? Do you frequently operate your airplane indoors? Propellers are usually observed in natural light, which does not flicker. At night, on the rare occasions where you actually see the props clearly, it is from the aircraft's own lighting, which is DC. I have nbever seen the props turning backwards on a real plane - see it frequently in movies though. . . Night, near one of those big off-amber ramp lights, run the RPM up and down, there's a range where it will look like it's going backwards. I thought it was kind of interesting. It's something similar to the poor-man's "is my RPM somewhat right" test, it'll appear stopped at (I forget what RPM now) RPM and if your tach is somewhat near, viola.- Hide quoted text - - Show quoted text - Been a long, long time but my rusty math skills says it would be about 3600 unless I am wrong (per wife that is my normal state). That is the 1/2 harmonic of the rpm/flicker rate. 60 X 120 = 7200. The phenomenon should appear at 1/2, 1/4, double rate etc intervals. Harry K- Hide quoted text - - Show quoted text - After thinking that over... It gets worse. There will be multiple rpm that will show the effect under strobe conditions. With a 2 blade prop it can be sychronizing every 1/2 rev. 3-blade prop every 1/3 or 2/3 rev, etc. in addition to synching on the harmonics. Harry K- Hide quoted text - - Show quoted text - |
#62
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Spinner strobing as a "Bird Strike Countermeasure"
In rec.aviation.piloting Harry K wrote:
On Dec 3, 7:21 pm, Harry K wrote: On Dec 3, 2:45 pm, Just go look it up! wrote: On Mon, 03 Dec 2007 21:52:04 -0800, Airbus wrote: In article , says... When observed directly under artifical light that "flickers", the most obvious being a strobe light, but there are other types of artificial lights that have flicker. -- Fine - but which ones cause you to see the propellers turning in apparent reverse? Do you frequently operate your airplane indoors? Propellers are usually observed in natural light, which does not flicker. At night, on the rare occasions where you actually see the props clearly, it is from the aircraft's own lighting, which is DC. I have nbever seen the props turning backwards on a real plane - see it frequently in movies though. . . Night, near one of those big off-amber ramp lights, run the RPM up and down, there's a range where it will look like it's going backwards. I thought it was kind of interesting. It's something similar to the poor-man's "is my RPM somewhat right" test, it'll appear stopped at (I forget what RPM now) RPM and if your tach is somewhat near, viola.- Hide quoted text - - Show quoted text - Been a long, long time but my rusty math skills says it would be about 3600 unless I am wrong (per wife that is my normal state). That is the 1/2 harmonic of the rpm/flicker rate. 60 X 120 = 7200. The phenomenon should appear at 1/2, 1/4, double rate etc intervals. Harry K- Hide quoted text - - Show quoted text - After thinking that over... It gets worse. There will be multiple rpm that will show the effect under strobe conditions. With a 2 blade prop it can be sychronizing every 1/2 rev. 3-blade prop every 1/3 or 2/3 rev, etc. in addition to synching on the harmonics. It isn't that bad. Synchronization only occurs on integral fractions and engine RPM is usually 1000 and about 2700 RPM so the possibilities are limited. For a 2 bladed prop: 7200/2= 3600 - 1800 RPM 7200/3= 2400 - 2400 RPM and 1200 RPM 7200/4= 1800 - 1800 RPM and 900 RPM etc. I leave it to someone else to show how many blades you see at each RPM. -- Jim Pennino Remove .spam.sux to reply. |
#63
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Spinner strobing as a "Bird Strike Countermeasure"
Gig 601XL Builder writes:
Wrong, exactly backwards wrong. Rutger was correct. The frame is a painted raster. The image is the complete set of visual data for simultaneous assimilation. |
#64
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Spinner strobing as a "Bird Strike Countermeasure"
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#65
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Spinner strobing as a "Bird Strike Countermeasure"
Yes - I have a name writes:
I'm sure I've seen wheels on cars that appear to be moving backwards. Are my eyes defective? No, but your memory is imperfect. You've never seen that in real life, only on TV and in films, or under stroboscopic lighting. |
#66
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Spinner strobing as a "Bird Strike Countermeasure"
Maxwell writes:
This phenomenon is apparent to the naked eye under continuous lighting conditions. No, it is not. You might need to wait for a three or four blade prop, but it DOES happen in real life. No, it does not. Continual assertions to the contrary will not change this. |
#67
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Spinner strobing as a "Bird Strike Countermeasure"
"Mxsmanic" wrote in message ... Yes - I have a name writes: I'm sure I've seen wheels on cars that appear to be moving backwards. Are my eyes defective? No, but your memory is imperfect. You've never seen that in real life, only on TV and in films, or under stroboscopic lighting. No it's because YOU have never seen anything in REAL LIFE retard. If it hasn't been on a tv or computer screen, you have so little experience YOU don't realize it exists. |
#68
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Spinner strobing as a "Bird Strike Countermeasure"
"Mxsmanic" wrote in message
... Yes - I have a name writes: I'm sure I've seen wheels on cars that appear to be moving backwards. Are my eyes defective? No, but your memory is imperfect. You've never seen that in real life, only on TV and in films, or under stroboscopic lighting. Translation: I don't understand it, so it can't happen. Looks like I've been MXed (The absolute denial of common knowledge). |
#69
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Spinner strobing as a "Bird Strike Countermeasure"
In a previous article, Mxsmanic said:
Yes - I have a name writes: I'm sure I've seen wheels on cars that appear to be moving backwards. Are my eyes defective? No, but your memory is imperfect. You've never seen that in real life, only on TV and in films, or under stroboscopic lighting. Wikipedia references some scientific papers that show three distinct cases of people seeing it non-strobing (ie natural sunlight). 1. People see the effect on car wheels, mostly because hubcaps have reflective surfaces that cause a strobing of the light in natural sunlight, but also because of other things changing the light, like shadows or the reflection off other objects. 2. People see the effect when their eyes are being vibrated. The article addresses people seeing it when humming, but I suspect the same effect could happen with the low frequency noise that permeates an airplane cockpit. 3. There is some controversy about an effect that *some* people see when they stare at a moving pattern of dots for periods of time greater than 30 seconds (some needing to stare as long as 10 minutes before they see it). Some scientists are saying that it shows that our brain processes vision in discrete frames. Others say it's a totally different effect. I've never seen it in the air, and I'd never heard of it until I read the wikipedia article, but it turns out that Maxwell and others are correct and you're wrong. I guess I should have known. -- Paul Tomblin http://blog.xcski.com/ "My band is called Imation CDR 74m 650 MB...have you seen our CDs?" |
#70
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Spinner strobing as a "Bird Strike Countermeasure"
"Gig 601XL Builder" wrDOTgiaconaATsuddenlink.net wrote in message
... Shut the **** up, MX. So Say We All! |
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