who have already solved the plane on a treadmill lets move onto the helicopter on the turntable. Anyone got any thoughts on this. Lets assume the turntable moves cw in one situation and ccw in another.

1/28/2008 8:59:46 PM

already a thread on this

1/28/2008 9:00:15 PM

10/10

1/28/2008 9:00:28 PM

that would be funky

1/28/2008 9:01:26 PM

the shear stresses on the body of the helicopter tear it apart

it doesn't take off

1/28/2008 9:03:12 PM

Lets assume an ideal situation where the helicopter wouldn't rip apart

1/28/2008 9:03:54 PM

the pilot gets sick because of all the spinning and cancels the experiment

it doesn't take off

1/28/2008 9:04:55 PM

autopilot take off. Don't try to ruin this shit you will fail.

1/28/2008 9:05:25 PM

http://www.brentroad.com/message_topic.aspx?topic=506930

1/28/2008 9:05:35 PM

the helicopter rips apart or falls over and then rips apart

1/28/2008 9:05:44 PM

has this myth been submitted to mythbusters?

1/28/2008 9:06:26 PM

Assuming that the helicopter isn't going to break...

If the turntable is moving in the same direction as the helicopter blades, I don't see any reason why it wouldn't take off.

If the turntable is moving opposite the blades at the exact same speed, it wouldn't take off... if this scenario is even physically possible (seems like it would be). The blades would not be moving relative to the air at all, they would be stationary and the helicopter's body would be spinning beneath them. Again, this is assuming the scenario can physically be done, and that the helicopter is not going to break.

1/28/2008 9:07:02 PM

B.A. says "i ain't gettin in no chopter," and face and murdoch can't leave without him

it doesn't take off

1/28/2008 9:08:41 PM

a helicopter made of a sturdy enough material to withstand the rotational shear forces and various torsions and filled the the necessary electronics to autopilot a helicopter would be too heavy

it doesn't take off

1/28/2008 9:08:50 PM

Alternatively, the turntable and helicopter act as a massive drill, and the helicopter would anti-take-off, drilling a hole to the center of the earth.

1/28/2008 9:09:35 PM

Quote :

"If the turntable is moving opposite the blades at the exact same speed, it wouldn't take off... if this scenario is even physically possible (seems like it would be). The blades would not be moving relative to the air at all, they would be stationary and the helicopter's body would be spinning beneath them. Again, this is assuming the scenario can physically be done, and that the helicopter is not going to break."

You are neglecting the function of the tail rotor.

Imagine if it were a dual rotor helo, assuming the turntable was reasonably slippery, it'd take off. Same with a regular helo, except the tail rotor is what cancels out the angular momentum of the turntable.

1/28/2008 9:10:50 PM

Obviously the tail rotor is neglected in my explanation.

...Or it could be assumed that the turntable is spinning with an additional amount of force such that it compensates for the tail rotor... if this wouldn't invalidate the whole damn myth anyway.

1/28/2008 9:14:45 PM

Quote :

"...Or it could be assumed that the turntable is spinning with an additional amount of force such that it compensates for the tail rotor... if this wouldn't invalidate the whole damn myth anyway."

The turntable is irrelevant. Just like the treadmill for the plane.

The only difference is how much force the tail rotor has output to overcome the force of friction (just like in the treadmill scenario with the friction in the wheel bearings). Once this force is overcome, the speed of the turntable becomes completely irrelevant to the motion of the helicopter.

Obviously, unlike the plane/treadmill, you couldn't reconstruct this scenario without destroying the helicopter (other things factor in where the helo will just lose control). Probably what would happen is that while the helicopter is taking off, the turntable would tip it over. If the turntable was VERY slick, almost frictionless, then maybe you could take off without issue.

1/28/2008 9:21:04 PM

If the turntable was frictionless/near-frictionless, it would take off easily.

I think the scenario assumes that there is actually an outstandingly large amount of friction, such that the helicopter is basically horizontally fixed to the turntable, and can only move by taking off in the vertical axis. That's the way I pictured/interpreted it, anyway.

1/28/2008 9:26:28 PM

Well, if you imagine near infinite friction, then why can't the helicopter have near-infinite engine power?

Can God microwave a burrito so hot even he/she can't eat it?

1/28/2008 9:29:47 PM

Quote :

" Can God microwave a burrito so hot even he/she can't eat it?"

Infinity/Infinity = Undefined.

Obviously we must take the limit and use L'Hospital's Rule to solve this... at least that's what I would have done in Calc 1,2, and 3.

1/28/2008 9:33:30 PM

the problem with this myth is that there is no defined set of parameters. Which way does the thing spin? are the rotor blades on?

1/28/2008 9:35:57 PM

the problem with this myth is that it was proposed as an obvious joke.

1/28/2008 9:36:45 PM

^^ http://www.brentroad.com/message_topic.aspx?topic=506930

^ yeah, it was

[Edited on January 28, 2008 at 9:39 PM. Reason : ]

1/28/2008 9:37:17 PM

^

Quote :

"yes, because helicopters are powered by noise and magic"

-fredbot3000


So far I think this is the most reliable answer.

[Edited on January 28, 2008 at 9:38 PM. Reason : user link]

1/28/2008 9:38:32 PM

Quote :

"If the turntable is moving in the same direction as the helicopter blades, I don't see any reason why it wouldn't take off."

still problem: The relative speed of the blades to the vehicle is lower than what's needed to take off actually, but the net rotation of the vehicle itself allows it to gain lift.

It gains lift, raises off the platform. How far though? Immediately resistance will be slowing the net rotational speed, and when starting from a just balanced system.

1/28/2008 9:57:45 PM

The movement of the blades relative to the air should be enough to create lift, and unless the blades stopped after takeoff, it should remain in the air. Bear in mind that I'm assuming the helicopter is running, not just turned off sitting on a spinning platform.

1/28/2008 10:00:58 PM

no, if the table is spinning in the same direction as the blades, then some fraction of the driving force for lifting it is coming from the table, not the craft. It leaves the table, and unless you instantly increase the drive force from the craft, lift is insufficient to keep it up.

1/28/2008 10:02:44 PM

Quote :

"It leaves the table, and unless you instantly increase the drive force from the craft, lift is insufficient to keep it up."

It woudln't have to be instant, because the helicopter would still have plenty of momentum, but it'd have to be quick.

Kind of like the plane on the treadmill, engines off, with the treadmill pushing it forward. If the treadmill were moving fast enough, the plane would gain lift then fall once it slowed, unless the engines could be spun up fast enough to maintain forward motion.

1/28/2008 10:05:56 PM

the idea whether or not the blades will be turning relative to the air at all

i'm trying to visualize this one, pretty rough...but the way i'm thinking to an observer it would look like the platform and chopper body are rotating furiously while the blades are stationary.

not taking the tail rotor into account...

1/28/2008 10:08:59 PM

^Assuming the table is rotating opposite to the spin of the blades, this is also how I visualized it at first.

^^I might be misunderstanding the parameters, but wouldn't the top blades relative to the air be moving at double the normal speed if the turntable is moving the same direction and the helicopter is on? Then be moving at normal speed once it lifts off (not taking into account the continued momentum of the helicopter spinning)? Again, I'm ignoring tail rotor (inaccurate, yes, I know).



[Edited on January 28, 2008 at 10:14 PM. Reason : .]

1/28/2008 10:11:33 PM

^^ yeah, but if you don't take the tail rotor, then you might as well not even say that it's a helicopter.

Helicopters can't fly without a tail rotor (or device/mechanism that serves the same purpose).

[Edited on January 28, 2008 at 10:13 PM. Reason : ]

1/28/2008 10:12:56 PM

then the velocity of the tail rotor would be insane, as it would have to be pushing the helicopter in the opposite direction to get the rotors to 'spin'

as in, having a stationary helicopter take off using only its tail rotor

1/28/2008 10:17:07 PM

Quote :

"^^I might be misunderstanding the parameters, but wouldn't the blades relative to the air be moving at double the normal speed if the turntable is moving the same direction and the helicopter is on? Then be moving at normal speed once it lifts off (not taking into account the continued momentum of the helicopter spinning)?"

The blades would be moving at double speed relative to the air, and then once it takes off, SLOWLY (as air friction slows the helicopter's spinning... because it's spinning at several hundred rpm) spin down to "normal" speed. Since the helo requires a minimum speed to maintain flight, once the spinning slows down enough, it'll start to fall, unless you could continue to throttle up before you dropped below the minimum threshold to stay aloft.

1/28/2008 10:17:22 PM

huh? if the turntable and blades were spinning the same direction, the helicopter would just take off in half the time since the acceleration time to get to take off speed would be cut in half

1/28/2008 10:21:22 PM

Quote :

"then the velocity of the tail rotor would be insane, as it would have to be pushing the helicopter in the opposite direction to get the rotors to 'spin' as in, having a stationary helicopter take off using only its tail rotor"

How insane it would be would depend on friction between the skids/turntable. I'm pretty sure i've seen videos on those Crazy Home Videos shows of a helicopter on a landing pad spin out due to some type of failure. So i'd guess the tail rotor has enough power on its own to scrape the helicopter accross the ground.

Also, it'd likely be a gradual process of the tail canceling out the turntable, which is where problems arise, because if while the helo is slowing down relative to the turntable, teh turntable catches a skid and pushes the helo over, the craft would be destroyed, and the pilot would be dead.

1/28/2008 10:24:51 PM

^^ yeah, it would take off in half the time, but it's not clear if it would be able to keep the rotors going fast enough before ti started to fall again.

1/28/2008 10:26:08 PM

Why would it fall if the blades are spinning at the same rate as a non-turntable-aided helicopter?

1/28/2008 10:27:38 PM

Quote :

"It woudln't have to be instant, because the helicopter would still have plenty of momentum, but it'd have to be quick. Kind of like the plane on the treadmill, engines off, with the treadmill pushing it forward. If the treadmill were moving fast enough, the plane would gain lift then fall once it slowed, unless the engines could be spun up fast enough to maintain forward motion."

very true.

in real reality, however, the coefficient of static/dynamic friction comes into play. Instead of lifting off, the blades would just eliminate enough normal force for it to slide, it then slides until it slows down enough for friction to grab again, and this would repeat. Just a PY205 model.

what you would want to do is latch it onto the table until it is pulling up, then release it so the momentum can make it travel some distance sufficient to throttle up the driving force in the craft.

1/28/2008 10:29:08 PM

well, it would take off in half the time, but once off the ground the blades wouldn't be going fast enough and it would fall....

k, reread what you wrote and it sounds like you were talking about the blades themselves spinning 2x required speed, rather than the whole unit spinning 2x

1/28/2008 10:30:11 PM

Quote :

"Why would it fall if the blades are spinning at the same rate as a non-turntable-aided helicopter?"

Because since this helicopter took of in 2x the time (as you noted) then a non-turntabled-helicopter would have still been on the ground.

1/28/2008 10:31:14 PM

threads like these make me happy.

Just imagine if this was a message board for Chapel Hill.

1/28/2008 10:33:36 PM

^^Ah, thanks for that explanation there. Mentally kicking myself for missing that particular fact.

[Edited on January 28, 2008 at 10:35 PM. Reason : .]

1/28/2008 10:34:11 PM

^ Don't feel bad... on the original plane-on-a-treadmill thread in Lounge, I argued for a good 2 pages it wouldn't take off (with engines on)

Chew on this though...

if the treadmill pushing the plane forward doesn't help it, why to aircraft carriers use a catapult?

[Edited on January 28, 2008 at 10:38 PM. Reason : ]

1/28/2008 10:36:54 PM

wtf do you mean by friction.

if blades for a normal craft to stay suspended in normal flying conditions is x rad/s, and you're saying that table is spinning at x rad/s, and the blades are spinning 2x rad/s relative to the ground, then there would be a net force m upwards equal to the mass of the craft. You would be tethering it down for it to not fly away before this point.

^^ responded to that, and i think it got edited

[Edited on January 28, 2008 at 10:37 PM. Reason : ]

1/28/2008 10:36:59 PM

Chew on this though...

if the treadmill pushing the plane forward doesn't help it, why do aircraft carriers use a catapult?

[Edited on January 28, 2008 at 10:38 PM. Reason : reposted for great justice ]

1/28/2008 10:38:26 PM

it does help it get up.

doesn't help it stay there.

1/28/2008 10:39:53 PM

Quote :

"if the treadmill pushing the plane forward doesn't help it, why do aircraft carriers use a catapult?"

You can't be serious

1/28/2008 10:41:15 PM

it's not a trivial question.

it's a philosophic one.

1/28/2008 10:42:54 PM

What does the catapult mean?

1/28/2008 10:43:38 PM