Does the helicopter take off?

[ryan256]

Very vague OP!

However.... assuming that:
1- The tail rotor is unable to compensate for the spinning of the turntable.
2- Its an unmanned helicopter so no pilot blacks out.
3- There is no slippage between the helicopter struts and the turntable.
4- The turntable spins opposite the direction of the blades.

No.. the helicopter would not take off. Why?? No air flow over the blades. No air flow = no lift

/thread

 

[Specop 007]

Originally posted by: ryan256
Very vague OP!

However.... assuming that:
1- The tail rotor is unable to compensate for the spinning of the turntable.
2- Its an unmanned helicopter so no pilot blacks out.
3- There is no slippage between the helicopter struts and the turntable.
4- The turntable spins opposite the direction of the blades.

No.. the helicopter would not take off. Why?? No air flow over the blades. No air flow = no lift

/thread

Which is the same reason the lift characteristics of the blades change the faster you travel in a forward direction. Airflow over the blades.

 

[sswingle]

Originally posted by: brikis98

Originally posted by: ScottSwingleComputers

Originally posted by: brikis98
i am not an expert on helicopters by any means, so i'm humbly guessing. but, wouldn't the main rotor of a helicopter be "free spinning" wrt (with respect to) the helicopter's body? if so, let's imagine looking at the helicopter from ABOVE:

1. the main rotors is spinning clockwise and the turntable spins the body of the helicopter counterclockwise. wrt the body of the helicopter, the main rotor is now spinning twice as fast. however, wrt the ground, the rotor is still spinning at the normal rate and therefore generating lift. helicopter takes off.
2. the main rotor is spinning clockwise and the turntable spins the body of the helicopter clockwise (the OP's intent, i believe). wrt the body of the helicopter, the main rotor is not spinning at all. wrt to the ground... it's still spinning at it's normal rate, generating lift and the helicopter takes off.

for the above analysis, i'm ignoring the tail rotor (although at most, i believe it would only affect the body of the helicopter) and the effects of torque of a entire helicopter body spinning around (which would probably throw it off the platform).

now, if the main rotor is not free spinning wrt the helicopter body... then the only way the OP makes sense "no net motion" is if both the turntable and main rotor are at a standstill, which seems like a trivial case.

please comment/correct me, esp. on the mechanics of a chopper

Both dont make sense. Neither situation would change the blade speed in relation to the body. Think about it a bit more.

I wish I knew how to make a simple animation.

not sure what you're trying to say... it's the blade speed wrt the ground that matters, right? so if the rotor is free spinning, spinning the body of the helicopter has no impact on this... right?

The helicoptor engine is going to spin the blade the same speed no matter what anything else is doing. The blade to body speed remains constant. Lets say the blades are doing 10 RPM clockwise.
Now. lets say the turntable is going 10 RPM counterclockwise. Each motion cancels the other out. The only thing now spinning is the turntable and the helicoptor.

Raise the speeds, and the result is the same.

 

[Leros]

Originally posted by: brikis98

Originally posted by: Leros

Originally posted by: brikis98

Originally posted by: Leros
We dont care about the relation of the blades with resepct to the ground. We care about the relation of the blades with respect to the air around the blades. Since the blades are not spinning relative to the air around them, there is no lift generated.

fair enough. except i believe that if the rotors are spinning wrt the ground, they are likewise spinning wrt the air.

The problem that some people are having is that they are seeing the rotating platter as the ground.

And if we call the the "ground" the stationary ground around the rotating platter, then the blades are still not moving wrt to the ground.

i agree, the ground != the rotating platter. but i still argue that if the helicopter blades are free spinning, then they spin at their normal speed wrt the ground.

The OP said that the blades are spinning at the opposite velocity of the rotating platter (I'm assuming we are talking rotational velocity). The OP even said there is no net motion. Therefore the blades are stationary to the ground.

I don't what else there is to argue.

 

[uberman]

Is this a belt driven turntable or is it direct drive? If it is belt driven, how old are the belts?

 

[Specop 007]

Originally posted by: Leros

I don't what else there is to argue.

French fries or baked potato with your steak?

 

[brikis98]

Originally posted by: ScottSwingleComputers

Originally posted by: brikis98

Originally posted by: ScottSwingleComputers

Originally posted by: brikis98
i am not an expert on helicopters by any means, so i'm humbly guessing. but, wouldn't the main rotor of a helicopter be "free spinning" wrt (with respect to) the helicopter's body? if so, let's imagine looking at the helicopter from ABOVE:

1. the main rotors is spinning clockwise and the turntable spins the body of the helicopter counterclockwise. wrt the body of the helicopter, the main rotor is now spinning twice as fast. however, wrt the ground, the rotor is still spinning at the normal rate and therefore generating lift. helicopter takes off.
2. the main rotor is spinning clockwise and the turntable spins the body of the helicopter clockwise (the OP's intent, i believe). wrt the body of the helicopter, the main rotor is not spinning at all. wrt to the ground... it's still spinning at it's normal rate, generating lift and the helicopter takes off.

for the above analysis, i'm ignoring the tail rotor (although at most, i believe it would only affect the body of the helicopter) and the effects of torque of a entire helicopter body spinning around (which would probably throw it off the platform).

now, if the main rotor is not free spinning wrt the helicopter body... then the only way the OP makes sense "no net motion" is if both the turntable and main rotor are at a standstill, which seems like a trivial case.

please comment/correct me, esp. on the mechanics of a chopper

Both dont make sense. Neither situation would change the blade speed in relation to the body. Think about it a bit more.

I wish I knew how to make a simple animation.

not sure what you're trying to say... it's the blade speed wrt the ground that matters, right? so if the rotor is free spinning, spinning the body of the helicopter has no impact on this... right?

The helicoptor engine is going to spin the blade the same speed no matter what anything else is doing. The blade to body speed remains constant. Lets say the blades are doing 10 RPM clockwise.
Now. lets say the turntable is going 10 RPM counterclockwise. Each motion cancels the other out. The only thing now spinning is the turntable and the helicoptor.

Raise the speeds, and the result is the same.

again, you're assuming that the rotor of a helicopter is NOT free spinning wrt to it's body. since i'm not an expert on helicopters, i don't know if that's the case. if the blades spin freely wrt the body, then the turntable spinning the BODY of the helicopter has NO effect on the blades.

and if they are not free spinning... wouldn't both the turntable and the blades be at a standstill?

 

[brikis98]

Originally posted by: Leros

Originally posted by: brikis98

Originally posted by: Leros

Originally posted by: brikis98

Originally posted by: Leros
We dont care about the relation of the blades with resepct to the ground. We care about the relation of the blades with respect to the air around the blades. Since the blades are not spinning relative to the air around them, there is no lift generated.

fair enough. except i believe that if the rotors are spinning wrt the ground, they are likewise spinning wrt the air.

The problem that some people are having is that they are seeing the rotating platter as the ground.

And if we call the the "ground" the stationary ground around the rotating platter, then the blades are still not moving wrt to the ground.

i agree, the ground != the rotating platter. but i still argue that if the helicopter blades are free spinning, then they spin at their normal speed wrt the ground.

The OP said that the blades are spinning at the opposite velocity of the rotating platter (I'm assuming we are talking rotational velocity). The OP even said there is no net motion. Therefore the blades are stationary to the ground.

I don't what else there is to argue.

"no net motion" with respect to WHAT? for example, the blades could have no net motion with respect to the helicopter's body (such as when they are off), but if the whole system is spinning, they still have net motion with respect to the air/ground. in that case, ignoring the odd effects an entire helicopter must have when spinning, the blades would still generate lift.

if there's "no net motion" with respect to the ground, then the whole system is at a standstill and this is a stupid/trivial problem.

 

[ryan256]

Originally posted by: Specop 007

Originally posted by: Leros

I don't what else there is to argue.

French fires or baked potato with your steak?

Tater please! Hold the sour cream.

 

[Leros]

Originally posted by: Specop 007

Originally posted by: Leros

I don't what else there is to argue.

French fires or baked potato with your steak?

fires are never good, I'll have the potato

 

[swtethan]

Originally posted by: ryan256
Very vague OP!

However.... assuming that:
1- The tail rotor is unable to compensate for the spinning of the turntable.
2- Its an unmanned helicopter so no pilot blacks out.
3- There is no slippage between the helicopter struts and the turntable.
4- The turntable spins opposite the direction of the blades.

No.. the helicopter would not take off. Why?? No air flow over the blades. No air flow = no lift

/thread

#4 is in the OP

take out #1 and #3 and the helicopter will take off

none of this could be possible with a pilot anyway

 

[mordantmonkey]

how are the blades free spinning? aren't they connected to a gear box and engine, just like a cars wheels are connected to the drivetrain?
this is different han the airplane question because the airplane wheels are free spinning whereas the helicopter rotor is not.

 

[SoulAssassin]

Originally posted by: mugs
Is this helicopter African or European?

nice one

 

[brikis98]

Originally posted by: mordantmonkey
how are the blades free spinning? aren't they connected to a gear box and engine, just like a cars wheels are connected to the drivetrain?
this is different han the airplane question because the airplane wheels are free spinning whereas the helicopter rotor is not.

i've been trying to ask that the whole time. i honestly don't know. if they aren't free spinning, what happens when, during normal flight, the helicopter starts rotating in place along a vertical axis... does it lose lift suddenly?

 

[sao123]

then only way you could guarantee no airflow would be if the center point of the spinning ground matches the center point of the rotating blades.

 

[Specop 007]

Originally posted by: Leros

Originally posted by: Specop 007

Originally posted by: Leros

I don't what else there is to argue.

French fires or baked potato with your steak?

fires are never good, I'll have the potato

 

[mordantmonkey]

Originally posted by: brikis98

Originally posted by: mordantmonkey
how are the blades free spinning? aren't they connected to a gear box and engine, just like a cars wheels are connected to the drivetrain?
this is different han the airplane question because the airplane wheels are free spinning whereas the helicopter rotor is not.

i've been trying to ask that the whole time. i honestly don't know. if they aren't free spinning, what happens when, during normal flight, the helicopter starts rotating in place along a vertical axis... does it lose lift suddenly?

uh dude they aren't free spinning. If the helicopter where able to rotate that fast (fast enough to counteract the rotor) the passengers would be in serious trouble (g force). the tail rotor counteracts the gyroscopic motion of the main rotor. and it's the tail rotor that adjusts speed or angle in order to yeild or fight that gyro motion in order to turn the body of the craft.

have you ever seen a heli lose it's tail rotor function? yeah the body spins wildly out of control and the blades do lose some of their lift.

 

[IEC]

Ignoring the fact that the original question is a theoretical construct that could not happen in "reality", theoretically the heli's rotors and turntable should exactly cancel, thus making the rotors appear to be at a standstill in relation to the turntable.

In reality the passengers would pass out/die and/or the helicopter would slide off the turntable, allowing it to fly due to the lift generated by the rotors.

I've never taken a physics class. Ever. But my intuition tells me this is true.

 

[brikis98]

Originally posted by: mordantmonkey

Originally posted by: brikis98

Originally posted by: mordantmonkey
how are the blades free spinning? aren't they connected to a gear box and engine, just like a cars wheels are connected to the drivetrain?
this is different han the airplane question because the airplane wheels are free spinning whereas the helicopter rotor is not.

i've been trying to ask that the whole time. i honestly don't know. if they aren't free spinning, what happens when, during normal flight, the helicopter starts rotating in place along a vertical axis... does it lose lift suddenly?

uh dude they aren't free spinning. If the helicopter where able to rotate that fast (fast enough to counteract the rotor) the passengers would be in serious trouble. the tail rotor counteracts the gyroscopic motion of the main rotor. and it's the tail rotor that adjusts speed or angle in order to yeild or fight that gyro motion in order to turn the body of the craft.

so then explain something i'm just curious about. say that in normal flight, the main rotor is spinning clockwise. say the helicopter (from an above view) wants to rotate in place counterclockwise, which i believe it should be able to do. if the main rotor isn't free spinning from the body of the helicopter, how would the body be able to turn rotate in the opposite direction of the rotor? i'm having trouble picturing it...

 

[Kelemvor]

I have the image in my head of the guys in Spies Like Us going around on that centripetal force machine. heh.

But no it wodln't take off since if the rotors don't move relative to the air, then there's no lift.

 

[jagec]

Originally posted by: swtethan
You cannot fly a helicopter or will it liftoff without a tail rotor, or a gyro for the tail rotor. the gyro will always automatically compensate for any kind of spin unless you forced it to spin using the controls, anything outside of the controls of the helicopter, it will compensate for.

The helicopter is ON THE TURNTABLE. Unless your tail rotor is sufficiently powerful to cause the helicopter to "break traction" on the turntable and cause the body of the helicopter to come to a halt with regards to ground (with the turntable doing its best rotary sander impression on the skids), it ain't gonna happen.

Originally posted by: brikis98
car tires aren't free spinning... afaik, a helicopter's rotors are... or else, how would it spin (around it's vertical axis) in place during normal flight?

Helicopter rotors are NOT free spinning, they are powered. Much like a car's tires. The speed of the rotors is directly controlled by the engine. You can't suddenly increase or decrease that speed without an external force.

You're getting confused because of the airplane problem.

 

[mordantmonkey]

Originally posted by: brikis98

Originally posted by: mordantmonkey

Originally posted by: brikis98

Originally posted by: mordantmonkey
how are the blades free spinning? aren't they connected to a gear box and engine, just like a cars wheels are connected to the drivetrain?
this is different han the airplane question because the airplane wheels are free spinning whereas the helicopter rotor is not.

i've been trying to ask that the whole time. i honestly don't know. if they aren't free spinning, what happens when, during normal flight, the helicopter starts rotating in place along a vertical axis... does it lose lift suddenly?

uh dude they aren't free spinning. If the helicopter where able to rotate that fast (fast enough to counteract the rotor) the passengers would be in serious trouble. the tail rotor counteracts the gyroscopic motion of the main rotor. and it's the tail rotor that adjusts speed or angle in order to yeild or fight that gyro motion in order to turn the body of the craft.

so then explain something i'm just curious about. say that in normal flight, the main rotor is spinning clockwise. say the helicopter (from an above view) wants to rotate in place counterclockwise, which i believe it should be able to do. if the main rotor isn't free spinning from the body of the helicopter, how would the body be able to turn rotate in the opposite direction of the rotor? i'm having trouble picturing it...

for ever action there is an equal and opposite reaction. the engine creates force the rotates the blades, but this force also pushes the body in the opposite direction. fortunately the body has much more mass than the blades. (think if the blades were held fast and the body suspended in the air, the body would be spun by the engine.) the tail rotor counter acts this opposite spinning force, so in order for the body to turn in the opposite dir. of the blades, the tail rotor simply yeilds to that force.

 

[brikis98]

Originally posted by: mordantmonkey

for ever action there is an equal and opposite reaction. the engine creates force the rotates the blades, but this force also pushes the body in the opposite direction. fortunately the body has much more mass than the blades. (think if the blades were held fast and the body suspended in the air, the body would be spun by the engine.) the tail rotor counter acts this opposite spinning force, so in order for the body to turn in the opposite dir. of the blades, the tail rotor simply yeilds to that force.

i'm still missing something in the picture. here's a simple experiment i did:

i have a ruler with a hole drilled right in the middle of it and a pencil. while holding the pencil upright in my hand, i place the ruler on it so the hole fits snuggly around the pencil tip. i can now spin the pencil with my hands, which turns the ruler. this is like the engine of the helicopter turning the rotors.

now, i happen to be sitting in a swivel chair. as my hands spin the pencil/ruler (which is right above my head) i can use my feet to spin the chair. now, my feet are acting like the turntable on the "body" of the helicopter (the chair/me). as far as i can tell, the ruler is definitely still spinning with respect to the air around it, regardless of which direction i'm turning in the chair. if it was a helicopter rotor, it would be generating lift.

perhaps the right way to put what i'm thinking is that the rotor is free spinning with respect to the turntable, and therefore the turntable has no effect on it.

what am i doing wrong?

(edited for clarity)

 

[jagec]

Originally posted by: mordantmonkey
have you ever seen a heli lose it's tail rotor function? yeah the body spins wildly out of control and the blades do lose some of their lift.

Well, when I was shooting down Mi-24 Hinds in Afghanistan with the mujahadeen...

Originally posted by: brikis98
so then explain something i'm just curious about. say that in normal flight, the main rotor is spinning clockwise. say the helicopter (from an above view) wants to rotate in place counterclockwise, which i believe it should be able to do. if the main rotor isn't free spinning from the body of the helicopter, how would the body be able to turn rotate in the opposite direction of the rotor? i'm having trouble picturing it...

Easy, it decreases the force of the tail rotor, and the reaction force from the engine powering the rotor causes the helicopter to rotate.

Originally posted by: brikis98
now, i happen to be sitting in a swivel chair. as my hands spin the pencil/ruler (which is right above my head) i can use my feet to spin the chair. now, my feet are acting like the turntable on the "body" of the helicopter (the chair/me). as far as i can tell, the ruler is definitely still spinning with respect to the air around it, regardless of which direction i'm turning in the chair. if it was a helicopter rotor, it would be generating lift.

Sounds like you're just not spinning the chair fast enough, because you should be able to bring the "rotor" to a standstill, doing what you describe.

 

[brikis98]

Originally posted by: jagec

Originally posted by: mordantmonkey
have you ever seen a heli lose it's tail rotor function? yeah the body spins wildly out of control and the blades do lose some of their lift.

Well, when I was shooting down Mi-24 Hinds in Afghanistan with the mujahadeen...

Originally posted by: brikis98
so then explain something i'm just curious about. say that in normal flight, the main rotor is spinning clockwise. say the helicopter (from an above view) wants to rotate in place counterclockwise, which i believe it should be able to do. if the main rotor isn't free spinning from the body of the helicopter, how would the body be able to turn rotate in the opposite direction of the rotor? i'm having trouble picturing it...

Easy, it decreases the force of the tail rotor, and the reaction force from the engine powering the rotor causes the helicopter to rotate.

Originally posted by: brikis98
now, i happen to be sitting in a swivel chair. as my hands spin the pencil/ruler (which is right above my head) i can use my feet to spin the chair. now, my feet are acting like the turntable on the "body" of the helicopter (the chair/me). as far as i can tell, the ruler is definitely still spinning with respect to the air around it, regardless of which direction i'm turning in the chair. if it was a helicopter rotor, it would be generating lift.

Sounds like you're just not spinning the chair fast enough, because you should be able to bring the "rotor" to a standstill, doing what you describe.

your answer about the rotor makes sense, but i still think that the ruler is spinning wrt to the air... i'll try it again later when the boss isn't around