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#31
05-07-2006, 01:01 PM
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Re: Ask d10 about flying helicopters and win $$$
[ QUOTE ]
I just watched a Giro de Italia stage. When they showed the footage from the helicopter camera there was a very distinct 'whap-whap-whap' sound from the rotors. Nothing unusual. Just your normal chopper sound. But I wondered why it goes 'whap-whap-whap'. I assume that the rotors go around at a uniform speed. And since the cabin is located at the middle of the rotors there shouldn't be any doppler effect. So I thought it was weird. [/ QUOTE ] I can answer that. It has a uniform angular velocity, but not a uniform linear velocity. Imagine I am swinging a ball on a rope around. and it makes a rotation every 1 second. if It is 1 meter away, It goes around a circle with radius 1m every one second. so it travels 2*pi*1 m/s. Ok, now lets add an additional meter to the string. It is still traveling 1 rotation every 1 second. however the circumference of the circle it is now making has increased. It now travels 4*pi m/s I get this because it is traveling 1 circumference of the circle every second, the circumference is 2*pi*r. Melch 
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#32
05-07-2006, 02:20 PM
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Re: Ask d10 about flying helicopters and win $$$
[ QUOTE ]
[ QUOTE ] I just watched a Giro de Italia stage. When they showed the footage from the helicopter camera there was a very distinct 'whap-whap-whap' sound from the rotors. Nothing unusual. Just your normal chopper sound. But I wondered why it goes 'whap-whap-whap'. I assume that the rotors go around at a uniform speed. And since the cabin is located at the middle of the rotors there shouldn't be any doppler effect. So I thought it was weird. [/ QUOTE ] I can answer that. It has a uniform angular velocity, but not a uniform linear velocity. Imagine I am swinging a ball on a rope around. and it makes a rotation every 1 second. if It is 1 meter away, It goes around a circle with radius 1m every one second. so it travels 2*pi*1 m/s. Ok, now lets add an additional meter to the string. It is still traveling 1 rotation every 1 second. however the circumference of the circle it is now making has increased. It now travels 4*pi m/s I get this because it is traveling 1 circumference of the circle every second, the circumference is 2*pi*r. Melch [/ QUOTE ] That sounds about right, I've never put much thought into the acoustics. Other factors that probably influence the sound are your flight path in relation to the blades. A hover sounds different from moving forward and a climb sounds different from a descent. During a descent if certain conditions are met the "whap whap whap" sound becomes significantly louder and more pronounced. I don't know exactly what those conditions are, I think a certain rate of deceleration is part of it, possibly wind direction/velocity as well, but sometimes you'll hear it coming in on an approach and sometimes you won't. It sounds like the entire rotor system is about to rip itself apart but it's actually just the acoustics acting just right. Rotor blades twist and bend and move all around in the course of a single rotation, so there's definitely some doppler effect potential.
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#33
05-07-2006, 02:26 PM
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Re: Ask d10 about flying helicopters and win $$$
[ QUOTE ]
What unit are you in now? What units have you been in previously? [/ QUOTE ] Currently in B Co, 1-145 (the Army's flight school), previously I was in the 542nd Medical Co (medevac), which at the time was stationed at Camp Page, Korea, and C Co, 5-101st (air assault) stationed at Ft Campbell, KY.
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#34
05-07-2006, 02:43 PM
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Re: Ask d10 about flying helicopters and win $$$
[ QUOTE ]
That sounds about right, I've never put much thought into the acoustics. Other factors that probably influence the sound are your flight path in relation to the blades. A hover sounds different from moving forward and a climb sounds different from a descent. During a descent if certain conditions are met the "whap whap whap" sound becomes significantly louder and more pronounced. I don't know exactly what those conditions are, I think a certain rate of deceleration is part of it, possibly wind direction/velocity as well, but sometimes you'll hear it coming in on an approach and sometimes you won't. It sounds like the entire rotor system is about to rip itself apart but it's actually just the acoustics acting just right. Rotor blades twist and bend and move all around in the course of a single rotation, so there's definitely some doppler effect potential. [/ QUOTE ] Part of it is due to the blade's shockwave. At a certain speed, the next blade will hit the shockwave and make it even more loud, and etc. Sound is just vibrations in the air essentially, and thats what the blades are doing. Melch
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#35
05-07-2006, 02:45 PM
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Re: Ask d10 about flying helicopters and win $$$
[ QUOTE ]
What kind of helicopters were we seeing in that movie with the doors song playing about that [censored] war? Also, if I put the blades and the rudder-thingy of the helicopter on upside down, could I fly the helicopter upside down? [/ QUOTE ] I don't know the movie you're talking about. If by putting the blades upside down, you mean leaving the rotor system at the top of the helicopter but inverting the way the blades work, so that by flying upside down you mean you have the rotor system low and you're trying to balance the helicopter fuselage on top of it, not really. With significant design changes beyond inverting the rotor system it's theoretically possible, there are some RC helicopters that are designed to be able to fly upside down. You can fly briefly upside down in a standard helicopter if you maintain G forces towards the floor of the helicopter, such as in a barrel roll. Apache pilots do this occasionally, even though they aren't supposed to and they could lose their jobs if the wrong person found out. Black Hawks are capable of it, I've seen it done in a demonstration video. I've never heard of a regular pilot attempting it, but I'm sure someone somewhere has.
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#37
05-07-2006, 03:14 PM
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Re: Ask d10 about flying helicopters and win $$$
Do you think the Commanche project should have been canceled?
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#38
05-07-2006, 03:43 PM
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Re: Ask d10 about flying helicopters and win $$$
Why are helicopter pilots, and even more specifically Army helo bubbas, TERRIBLE on the radios? Do they not teach you guys comm brevity? [img]/images/graemlins/grin.gif[/img]
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