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beauseizure on May 17th, 2007 11:54 am (UTC)
Regarding the helicopter:

http://en.wikipedia.org/wiki/Aliasing

See the section "Sampling Sinusoidal Functions," and note that the rotation of the helicopter blades is basically a sinusoidal function. The blades are spinning at a rate very near the sampling frequency of the camera.
Jeffrey Carl Fadenjeffreyatw on May 17th, 2007 05:06 pm (UTC)
That's what I originally thought, and what the majority of people who commented on the story thought. But people were arguing that it couldn't be the case because the helicopter seemed to be going upward and forward at points, meaning it would have to be changing the rotation speed of its blades.

Still think that's the case, though?
beauseizure on May 17th, 2007 07:48 pm (UTC)
Let me amend my original post to say that the blades are spinning very near an integer multiple of the sampling frequency. As the helicopter ascends and descends, the positions of the blades relative to the body do change, but at a very slow rate. If the camera is sampling at 25 Hz, and the helicopter blades are rotating at 50 Hz, they would appear stationary. If the blades rotated at 50.02 Hz, the apparent rate would be 0.02 Hz. Similarly, if the sampling rate of the camera changed or wavered, we'd see a corresponding change in the appearance of the blades.

Regarding helicopters changing blade rotation speed to move around, I don't think that's true. I have very limited knowledge of helicopters, but if I recall correctly blade angle is operative variable.

Also, this is cool:

http://www.michaelbach.de/ot/mot_strob/index.html
Elliott Kelleyhastis_epigeios on May 18th, 2007 06:34 am (UTC)
to move laterally, the blade angle is important. to rotate, the rear blade frequency is important. to move vertically, the main blade frequency is important.

if it rotates, and stays at the same height, the blades do not follow the body
when it's ascending in the video, the blade frequency slightly increases, but they increase the rear blade's frequency to compensate, to make it look like the blades follow the body.
in the beginning, when it's flying forward and turning around, you'll notice that the blades are always facing the same direction, and not rotating with the body. that's because the helicopter is staying at the same height.

so yeah, it's definitely the sampling frequency thing (or at least appears to be).
Jeffrey Carl Fadenjeffreyatw on May 18th, 2007 06:36 am (UTC)
What about here?

http://www.youtube.com/watch?v=HZJJK-XAnso

Looks like there is ascent and descent.
Elliott Kelleyhastis_epigeios on May 18th, 2007 03:52 pm (UTC)
in that one it's moving forward. the ascent and descent is the same as it is when gliding.
Elliott Kelleyhastis_epigeios on May 18th, 2007 04:07 pm (UTC)
oops, i was wrong. looking at it again, it's a combination of all 3, gliding physics, collective pitch adjustment, and throttle (rpm/frequency) control.

in the beginning when it's tilted forward and descending, the blades face the same direction. on the first turn, the blaces are still facing the same direction. on the second turn, when it begins to ascend, the blades begin to follow the body of the helicopter.

it's the same as the first video, only a little bit more advanced.
beauseizure on May 18th, 2007 12:23 pm (UTC)
Again, I've never flown a helicopter, but the internet tells me it's blade angle, rear blades included, which makes sense to me; that's way less work than modulating the frequency.

http://www.rc-airplane-world.com/how-helicopters-fly.html
http://www.bbc.co.uk/dna/h2g2/A245611
http://en.wikipedia.org/wiki/Helicopter
Elliott Kelleyhastis_epigeios on May 18th, 2007 04:03 pm (UTC)
according to wikipedia it's both.
i never heard of increasing the collective pitch before, but that makes sense.
but they use the throttle too. "Simultaneously increasing the collective and adding power with the throttle causes the helicopter to rise."
look a little bit lower and it mentions the throttle in more detail.

that's good, i was wondering why the frequency wasn't increasing as much as i thought it should.

and modulating the collective pitch isn't much different from modulating the frequency, in terms of difficulty. modulating the frequency requires modulating the fuels dispension to the engine. modulating the collective pitch requires modulating a seperate motor that modulates a platform (the swashplate) up and down.
beauseizure on May 18th, 2007 04:35 pm (UTC)
again, i don't drive a helicopter to school, but i think throttle != rpm. it seems to me from the articles i've read that the throttle is modulated specifically so the rpm can be held at a consistent frequency.

the wiki article goes on to say:

"Helicopter rotors are designed to operate at a specific RPM. [...] A single (higher) RPM is used in order to minimize resonance design requirements and add a safety margin to rotor stall RPM. Usually only in autorotation are different RPMs used to increase rotor efficiency [...] In general, RPM must be maintained within a tight tolerance, usually a few percent, and the RPM indicator dial is marked accordingly."

although it would be fun to have a fleet of black helicopters play für elise, they do, in fact, sound like giant flying blenders.

also, i'm not talking about work in the "difficulty" sense, but in the mechanical work sense.
Elliott Kelleyhastis_epigeios on May 18th, 2007 06:18 pm (UTC)
Wikipedia:
"In many piston-powered helicopters, the pilot must manage the engine and rotor RPM. The pilot manipulates the throttle to maintain rotor RPM and therefore regulates the effect of drag on the rotor system. Turbine engined helicopters, and some piston helicopters, use servo-feedback loops in their engine controls to maintain rotor RPM and relieve the pilot of routine responsibility for that task."
"Helicopters maneuver with three flight controls besides the pedals. The collective pitch control lever controls the collective pitch, or angle of attack, of the helicopter blades all together, i.e. equally throughout the 360 degree rotation of the rotor. When the angle of attack is increased, the blade produces more lift. The collective control is usually a lever at the pilot's left side. Simultaneously increasing the collective and adding power with the throttle causes the helicopter to rise."

BBC:
"The balance of the three controls is difficult because each interacts with the others in a complicated manner. For example - if the pilot increases the collective pitch then he must balance the machine by using the rudder pedals; the use of the rudder pedals must then be balanced by the cyclic control and the collective then adjusted to suit the new position."

RC-Airplane-World:
"The throttle control is a 'twist-grip' on the end of the collective lever and is linked directly to the movement of the lever so that engine RPM is always correct at any given collective setting.
During normal flying, constant engine speed (RPM) is maintained and the pilot only needs to 'fine tune' the throttle settings when necessary.
The engine RPM, and hence main rotor blade speed, can be kept at a constant rate because of the cyclic and collective pitch control of the main rotor blades."


so it looks as though you're right. the throttle (which controls power, and thus rpm and torque, but != rpm [or you could say that the throttle = acceleration of rpm]) is used to increase torque to compensate for the increased amount of work required to lift from the increased collective pitch.
the throttle can either be manual or automatic. in both cases, there is no such thing as perfect.



however, while i was wrong about how helicopters work, i was still right about the videos. the rpm does increase a small amount when the helicopter ascends in both videos. and the helicopter turning does not affect the rotor rpm.
the difference between what i first thought and the truth is that the increased rpm could be actually due to a few things, and the throttle doesn't have to be one of them. but i'm going to guess that it's designed to do that, for one reason or another.



and relating tho mechanical work. initially i thought that increasing the rpm (by increasing power) and increasing the collective pitch (and thus requiring an increased torque/power) individually to increase altitude would equate to about the same amount of work. but after thinking about it a bit more i realized that with a lower collective pitch the thrust is spread out, instead of directed, resulting in power loss, so increased power (turning to rpm) has less of an effect. so my guess is that there is a maximum efficiency rpm that varies depending on the collective pitch (or visa-versa, or co-dependant), but the limiting factors are resonance frequency, the speed of sound, and a few other things, all requiring that the rpm remain relatively constant.
Elliott Kelleyhastis_epigeios on May 18th, 2007 04:32 pm (UTC)
oh, and you're right that i was wrong about the rear blade.
Nikolausoneirist on May 17th, 2007 03:12 pm (UTC)
One of the lawyers' names is Cheryl Justice. She should totally become a supreme court justice and be called Justice Justice.
Ranagan_Labardinelabardine on May 17th, 2007 06:09 pm (UTC)
God, Wolf Blitzer is a douchebag.
שירן shiranne シラーン 冉施安gogalucky on May 17th, 2007 08:23 pm (UTC)
Elliott Kelleyhastis_epigeios on May 18th, 2007 04:20 pm (UTC)
that's awesome.
Elliott Kelleyhastis_epigeios on May 18th, 2007 06:44 pm (UTC)
wait a minute. i don't remember if i was responding to that video or this one
http://www.youtube.com/watch?v=Q5VeaUW12pY&mode=related&search=


(also, i'm sorry for taking up about half of the comments on this lj entry)
Jeffrey Carl Fadenjeffreyatw on May 18th, 2007 05:08 pm (UTC)
Yyyyeah, I can imagine that MySpace would censor giant bulletin spam about anything. As people point out in the comments, they can write other things about Ron Paul that aren't giant blocks of HTML code just fine.

This is reactionary tin-foil hat conspiracy theory bullshit.
Elliott Kelleyhastis_epigeios on May 18th, 2007 06:35 pm (UTC)
really? lol.
aww, i was hoping it was real.
Elliott Kelleyhastis_epigeios on May 18th, 2007 06:38 pm (UTC)
wow, i guess i should read those comments more often.