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View Full Version : If a tiny helicopter was hovering inside a moving car, would it move with the car?


AnabolicDoberman
04-28-2006, 09:33 PM
Or would the back glass slam into it when the car moved forward?

HMS Irruncible
04-28-2006, 09:47 PM
What? Like on a treadmill?

jake1964
04-28-2006, 09:54 PM
When a fly is trapped in your car, it doesn't slam into your back window. God knows I wish it would. Seems to me that it's the same as your hypothetical helicopter.

saoirse
04-28-2006, 09:58 PM
I'm just glad that I'm not the only one concerned with the ramifications of miniaturazation on helicopters.

asterion
04-28-2006, 09:59 PM
What frame of reference? Relative to a stationary car, relative to a moving car, or relative to the ground? There has to be a frame of reference for there to be an answer. But if it's hovering in a stationary car so that's it is also stationary with respect to the ground, yes, the car then accelerating will have the back window hit the helicopter.

tomndebb
04-28-2006, 10:01 PM
Any device (or creature) that is supported on air moves in relation to the air. Do you feel the air move past your face as you drive with the windows up? The helicopter would hover stationary to the relative speed of the air (which, inside the car has a relative speed of zero).

Shagnasty
04-28-2006, 10:07 PM
Before this thing goes all to hell, we need a definition of "tiny" and the materials that it is made of.

I was told that a helium balloon will actually move forward in a car that accelerates suddenly because the air gets compressed to the rear and it "rises" to the front.

Conversely, a 5 pound helicopter with an incredibly powerful motor will tend to have much more inertia that makes it stay in place and hit the rear.

It is an interesting question. The helicopter is flying in its medium (air) and that medium is moving so, under most circumstances, the helicopter will move with it. We just don't know the rate of change given the variables.

I am a perpetual flight student and I always read about it in the academic sense. One thing that most people have trouble imaging is that sustained wind does not blow against the plane. The plane is in the air and will move with it just as a boat in a current does. However, we have several factors to work with here and the change is sudden.

HMS Irruncible
04-28-2006, 10:07 PM
Any device (or creature) that is supported on air moves in relation to the air. Do you feel the air move past your face as you drive with the windows up? The helicopter would hover stationary to the relative speed of the air (which, inside the car has a relative speed of zero).
No way, dude. A hovering helicopter exerts force only relative to the vertical axis. If the car and the air in it moved up or down, then yes, the helicopter would stay stationary. But if the car moves in any lateral motion, the helicopter moves relative to the car. Perhaps the motion is attenuated by the "wind" produced, but it will move.

HMS Irruncible
04-28-2006, 10:10 PM
If the car and the air in it moved up or down, then yes, the helicopter would stay stationary.
Clarification... would oscillate around, and eventually return to, its starting point. It wouldn't be exactly stationary.

Squink
04-28-2006, 10:13 PM
But if the car moves in any lateral motion, the helicopter moves relative to the car. Perhaps the motion is attenuated by the "wind" produced, but it will move. By analogy to lighter than air balloon which moves forward when car accelerates (http://paer.rutgers.edu/PT3/experiment.php?topicid=8&exptid=79), heavier than air helicopter should move back.

AnabolicDoberman
04-28-2006, 10:24 PM
No way, dude. A hovering helicopter exerts force only relative to the vertical axis. If the car and the air in it moved up or down, then yes, the helicopter would stay stationary. But if the car moves in any lateral motion, the helicopter moves relative to the car. Perhaps the motion is attenuated by the "wind" produced, but it will move.

What if the windows were down? Wouldn't this decompress the air inside the car and prevent the helicopter from staying in place?

Sam Stone
04-28-2006, 11:12 PM
You need to draw a distinction between a car that is accelerating, and one that is moving at a constant speed. In the constant speed case, the helicopter will hover just fine. If you are accelerating, all kinds of wacky things happen. First, feel yourself being pressed back into your seat when you accelerate? The helicopter will feel the same force. So yes, it will be slammed into the back of the car when you accelerate, unless you compenate with the helicopter.

I don't believe a helicopter of any mass could match the balloon trick. The balloon does what it does because it's floating in air - it's bouyant. A helicopter isn't. A balloon will respond to differential changes in air pressure, and a helicopter won't (at least, only a tiny fraction as much).

At a constant velocity with the wind kept out, a helicopter hovering in the car has absolutely no idea how fast it's going. In fact, neither does the car, since it's also rotating with the earth, orbiting with the earth, orbiting the galaxy with the solar system, and moving through the universe with the galaxy.

garygnu
04-28-2006, 11:41 PM
This sounds like a job for the Mythbusters.
What about a tiny helicopter inside a plane on a giant treadmill.

AnabolicDoberman
04-29-2006, 12:00 AM
This sounds like a job for the Mythbusters.
What about a tiny helicopter inside a plane on a giant treadmill.

I was thinking about a plane...if you were on a plane and standing in the aisle and jump up, the back of the plane won't slam into you. But what if the plane was not pressurized?

Critical1
04-29-2006, 12:18 AM
I was thinking about a plane...if you were on a plane and standing in the aisle and jump up, the back of the plane won't slam into you. But what if the plane was not pressurized?


if you jumped up and the plane took off while you were airborn you would deffinitly hit the back wall, or at least fall on your ass.

Buck Rogers
04-29-2006, 12:38 AM
f=ma.

What is the force?
What is either the mass or acceleration?

Different windows open or closed are going to create different pressure drops. If the helo was tilted away from a pressure drop and it was utilizing similar force it would not move.

More data is needed to answer this correctly.

GusNSpot
04-29-2006, 12:40 AM
If the little helicopter pilot wants to live, he will work hard to keep the back window from hitting him or the front window, depending on if the car is accelerating or decelerating. When it's motion is constant, the helio will be in a hover unless it is really itty bitty and can fly around like a fly. But if it is the size of a fly, what about it's mass and ability to overcome the 3 G's of acceleration produced by the 454 ground pounder? Maybe it could handle all a 350 could deliver? Is it a Bell, Hiller, Sikorsky, Hughes, or some other design? Is it NOTAR? turbine or piston? How big is the car?

We need more info ..........

RaftPeople
04-29-2006, 02:38 AM
Just in case anyone doubts the answer that upon acceleration there would be movement relative to the car, consider whether you would leave a hot cup of coffee without a lid on the dashboard as you either braked hard or accelerated quickly. It's an even more stable situation than the helicopter (friction and air molecules trying to keep the cup in place instead of air molecules only for the helicopter), and the cup would certainly move/tip.

GuanoLad
04-29-2006, 02:56 AM
I always thought an insect's flight as it flies into a moving car through the window showed amazing agility.

Tomcat
04-29-2006, 05:56 AM
Seems like an interesting question that one can test easily with a RC helicopter and a semi trailer.

I think we can all visualize a helicopter hovering inside a semi-trailer, and if the trailer had the back doors opens while it moved forward, the helicopter would most certainly remain stationary as the truck drove off.

-Tcat

CC
04-29-2006, 08:30 AM
I think we have a fairly good analogy with helium balloons in cars, which most of us have experienced. When we maintain an equilibrium such as when we are driving at a constant velocity down a highway, the balloons just stay in one spot, hovering like a tiny helicopter. But when we stop, or turn, they move one way or the other. And I think that a tiny helicopter would do much the same, because the air in our car does, in fact, move somewhat independently of the car. It has a mass and has its own momentum. I've often noticed the inertia of the heated air in my car when I turn in the winter. Air that is being ducted to some part of the car suddenly is blown toward me from some place else. It's clear that some mass of air that had been moving forward wants to continue to move forward while the car is being made to turn. Thus, a tiny helicopter would have shifted positions inside the car while I turned, as well. I think this is sort of what the OP was talking about. If not, it's certainly what I'm talking about. It's my story and I'm sticking with it. Even if I'm not articulating it very well.

Mangetout
04-29-2006, 09:23 AM
I was told that a helium balloon will actually move forward in a car that accelerates suddenly because the air gets compressed to the rear and it "rises" to the front.I don't think it's so much pressure difference that moves the ballon, but acceleration, which is the same as gravity, causes the air to 'sink' toward the back window and the ballon to 'float' toward the front.

Thrasymachus
04-29-2006, 12:20 PM
Well, it would hit the back glass of course. Doesn't matter if windows are open or shut.

Here's one my dad told me a while back that I like. A cargo plane takes off with a helicopter in the hold.

Sometime during level flight, the helicopter revs up and and starts hovering inside the plane. Is the plane now heavier, lighter, or the same as before?

And you'd be surprised how many Aero E's fresh out of college think that a rocket won't launch if the ground isn't there under the launch pad for it to "push against". :rolleyes:

David Simmons
04-29-2006, 12:47 PM
I don't think the question can be answered precisely with the information given. If the auto is stationary and starts moving, at first the air around the helicopter will be stationary. However there will be a pressure wave starting at the back window when the auto starts to move. This pressure gets the air inside into motion that ultimately matches the motion of the auto. The pressure wave will tend to move the helicopter forward with an acceleration that depends upon the aerodynamic cross section of the copter, coeffients of drag, lift, etc. I suspect this force would be small, but then we don't know much about the caracteristics of the helicopter.

After the air is in motion with the car and before the helicopter pilot has time to react there will be a relative wind accelerating the helicopter forward. Here again, how much acceleration results depends upon the aerodynamics of the helicopter as in the case above.

If the pilot reacts in time to tip the vertical thrust vector forward to as to give the helicopter forward motion to match that of the car the relative wind will fall to zero and the copter will hover normally as if the auto were stationary.

I think the pilot's reactions will be adequate to the job and he or she will be able to assume a stable hover without getting banged by the read window.

Eonwe
04-29-2006, 12:53 PM
This sounds like a job for the Mythbusters.
What about a tiny helicopter inside a plane on a giant treadmill.

And what if there are snakes in the helicopter?

Sam Stone
04-29-2006, 01:36 PM
Forget about a 'pressure wave' of air in this situation. The effect of the pressure differential of the air will be completely swamped by the inertia of the helicopter.

Is this really that hard a question? This is basic grade 10 physics. Try this - get a skateboard, and put a hot wheels car on it. If you pull the skateboard around at a constant speed, the Hot Wheels car will sit on top of the skateboard fine. However, if the skateboard is accelerated, the Hot Wheels car will want to roll off the back.

Or, if you want to try the situation in your own car, grab a marble before you go out today, and put it on the passenger floor. Observe its behaviour. When you first move forward, the marble will roll towards the back of the car. When you brake, it will move forward. If you are driving at a costant speed, it will just sit there.

At a constant speed (assuming no wind rush), all forces are in equilibrium. There is nothing acting on the helicopter inside the car to make it want to move in any particular direction.

Squink
04-29-2006, 01:42 PM
Or, if you want to try the situation in your own car, grab a marble before you go out today, and put it on the passenger floor. Observe its behaviour. When you first move forward, the marble will roll towards the back of the car. When you brake, it will move forward. If you are driving at a costant speed, it will just sit there.I did an interesting variation on this just after graduating drivers Ed. I tried simultaneously adjusting the seat position and stepping on the brakes; got a nice steering wheel shaped bruise on the forehead for my troubles.

David Simmons
04-29-2006, 02:08 PM
Is this really that hard a question? This is basic grade 10 physics. Try this - get a skateboard, and put a hot wheels car on it. If you pull the skateboard around at a constant speed, the Hot Wheels car will sit on top of the skateboard fine. However, if the skateboard is accelerated, the Hot Wheels car will want to roll off the back.

Or, if you want to try the situation in your own car, grab a marble before you go out today, and put it on the passenger floor. Observe its behaviour. When you first move forward, the marble will roll towards the back of the car. When you brake, it will move forward. If you are driving at a costant speed, it will just sit there.The helicopter wouldn't be hovering in the first place without a pilot, small though he or she might be. So will the pilot just sit there like a marble? Now it it's an autopilot, what you say might be true since the autopilot doesn't sense the automobile motion.

magellan01
04-29-2006, 02:11 PM
Theoretically, the helicopter wold hit the back glass. I think of it this way:

I'm in a very long, very, very tall airplane—a mile tall—and I'm a very, very high jumper. If I jumped 5,270 feet, I would be in the air for X amount of time. During that time, the plane would continue its forward travel and my landing spot would travel with it, so I would land further back in the plane. There is some initial forward momentum as I jump, but the force of travel in the forward direction is limited and will eventually putter out. But the forward travel of the plane continues at a constant. If acceleration is involved, the difference in forward travel is increased and I will hit the back of the plane even quicker.

Sam Stone
04-29-2006, 02:28 PM
Theoretically, the helicopter wold hit the back glass. I think of it this way:

I'm in a very long, very, very tall airplane—a mile tall—and I'm a very, very high jumper. If I jumped 5,270 feet, I would be in the air for X amount of time. During that time, the plane would continue its forward travel and my landing spot would travel with it, so I would land further back in the plane. There is some initial forward momentum as I jump, but the force of travel in the forward direction is limited and will eventually putter out. But the forward travel of the plane continues at a constant. If acceleration is involved, the difference in forward travel is increased and I will hit the back of the plane even quicker.

I'm not understanding you. Are you saying that if a plane a mile high were flying along at a constant speed, and you dove off a mile-high platform inside the plane, that you'd move towards the back of the plane? If so, you're wrong.

If you're saying that if you dive as the plane accelerates and increases speed then you'd hit the back wall, then yes.

Again, this is very simple physics. It's an application of Newton's first law. An object in motion tends to stay in motion, unless some external force acts upon it. In the case of a helicopter hovering inside a car driving a constant speed, or a diver diving inside a mile-high airplane traveling at a constant speed, there is no force being applied, and therefore they maintain their original velocity. The skydiver will accelerate straight down, because the force of gravity is acting on him, but he won't move towards the front or back of the airplane, because no force is causing him to do so.

If the airplane or car are accelerating, however, then because the helicopter or skydiver wish to remain in place, the back of the vhicle will start to move towards him. It not that the skydiver or helicopter are 'thrown back', but that they stay in place while the back wall starts to move towards them. So now the skydiver will hit the back wall, or the helicopter will have to start applying a force of its own to maintain its relative position inside the vehicle.

David Simmons
04-29-2006, 02:45 PM
I'm not understanding you. Are you saying that if a plane a mile high were flying along at a constant speed, and you dove off a mile-high platform inside the plane, that you'd move towards the back of the plane? If so, you're wrong.
...

The skydiver will accelerate straight down, because the force of gravity is acting on him, but he won't move towards the front or back of the airplane, because no force is causing him to do so.
Actually in the absence of air resistance you would stay exactly under you starting point relative to the plane and you would move forward relative to the spot on the ground.

How come?

Well, the spot directly under the plane is moving at a velocity in miles/hour of w*r*cos(lattitude) where w is is the radial velocity of the earth in radians/hour at the spot in question and r is the radius of the earth in miles. The plane's velocity is w*(r+1)*cosine(lattitude). Obviously the plane's velocity is higher and without air resistance the diver will retain that velocity and strike the ground ahead of the spot in the direction of the plane's motion.

spingears
04-29-2006, 02:47 PM
What frame of reference? Relative to a stationary car, relative to a moving car, or relative to the ground? There has to be a frame of reference for there to be an answer. But if it's hovering in a stationary car so that's it is also stationary with respect to the ground, yes, the car then accelerating will have the back window hit the helicopter.The inertia on the mini-copter will follow the laws of physics. Accelerate the car faster or slower at a rapid rate and a crash is likely to ensue.
Any departure from an initial steady state results in displacement.

David Simmons
04-29-2006, 02:49 PM
Actually in the absence of air resistance you would stay exactly under you starting point relative to the plane and you would move forward relative to the spot on the ground.

How come?

Well, the spot directly under the plane is moving at a velocity in miles/hour of w*r*cos(lattitude) where w is is the radial velocity of the earth in radians/hour at the spot in question and r is the radius of the earth in miles. The plane's velocity is w*(r+1)*cosine(lattitude). Obviously the plane's velocity is higher and without air resistance the diver will retain that velocity and strike the ground ahead of the spot in the direction of the plane's motion.Actually this doesn't really made sense, does it? Let's say instead of a plane, a helicopter is hovering directly over a spot on the ground and stays there. Then the diver will move in the direction of the earth's rotation relative to the spot on the ground.

magellan01
04-29-2006, 04:05 PM
I'm not understanding you. Are you saying that if a plane a mile high were flying along at a constant speed, and you dove off a mile-high platform inside the plane, that you'd move towards the back of the plane? If so, you're wrong.

If you're saying that if you dive as the plane accelerates and increases speed then you'd hit the back wall, then yes.

Again, this is very simple physics. It's an application of Newton's first law. An object in motion tends to stay in motion, unless some external force acts upon it. In the case of a helicopter hovering inside a car driving a constant speed, or a diver diving inside a mile-high airplane traveling at a constant speed, there is no force being applied, and therefore they maintain their original velocity. The skydiver will accelerate straight down, because the force of gravity is acting on him, but he won't move towards the front or back of the airplane, because no force is causing him to do so.

But the jumper/diver has two forces acting upon him. One is gravity which pulls him straight down. The other is momentum in the direction of travel of the plane. Let's say a person jumps out of a plane. The moment he leaves the plane the force is dissapating, which means that a plane travelling 1,000 miles an hour will "throw" him further than a plane travelling 50 miles an hour. So, regardless of the speed of the plane, at some point the horizontal force generated by it will dissapate. If you looked at the jumper from a horizon vantage point his travel would look like half a parabola, not a straight diagonal line.

Maybe a simpler way to look at it is if I jump from a spot on the equator and it takes me one hour to reach the height of my jump and fall back to earth, the point that I launched from is now 1,000 to the West (assuming a 24,000-mile circumference). So, if I jump up in Colorado I'll come down in California. Of course, this assume no atmosphere, which is also moving at 1,000 miles an hour.

Then again, maybe you're right. At least in a vacuum. I'm too confused now.

Johnny L.A.
04-29-2006, 04:14 PM
Maybe a simpler way to look at it is if I jump from a spot on the equator and it takes me one hour to reach the height of my jump and fall back to earth, the point that I launched from is now 1,000 to the West (assuming a 24,000-mile circumference). So, if I jump up in Colorado I'll come down in California. Of course, this assume no atmosphere, which is also moving at 1,000 miles an hour.

Then again, maybe you're right. At least in a vacuum. I'm too confused now.
Remember that when you jump, you are also moving 1000 mph along with the planet. So is the air. Perfectly calm air is stationary from the viewpoint of an observer on the surface. From space, the air is moving 1000 mph. We know that an object in motion will remain in motion unless acted upon by an outside force. So you jump up. You're moving in laterally in space 1000 mph, but not moving wrt the Earth. Where's the outside force? It's not the wind, since there isn't any.

HMS Irruncible
04-29-2006, 09:01 PM
Maybe a simpler way to look at it is if I jump from a spot on the equator and it takes me one hour to reach the height of my jump and fall back to earth, the point that I launched from is now 1,000 to the West (assuming a 24,000-mile circumference). So, if I jump up in Colorado I'll come down in California. Of course, this assume no atmosphere, which is also moving at 1,000 miles an hour.
No, in this case, you are already in the earth's rotating frame of reference at the beginning of the change. In other words you've already inherited energy from the earth's rotation, so in the earth's frame of reference, you jump up and land in the same spot. It's relativity at its simplest.

In our helicopter experiment... if the helicopter started the experiment sitting on the seat of the car, then the car started moving, then the helicopter lifted into a direct hover, of course it would show no lateral motion relative to the car. Any maneuvering by the helicopter could be done in the moving car without taking into account the motion of the car, just as if the car were parked. This is how bugs in a moving car fly as they choose without smacking onto the rear window.

In the OP's question, though, although the car contains the helicopter, the forces on them put them effectively in 2 different frams of reference. The car is not exerting any force on the helicopter whatsoever. Thus, when the car accelerates, the helicopter does not, and it will quickly meet the surface opposite the direction of motion. At that point the motion has been transferred to the helicopter, and assuming it isn't damaged, it can now maneuver inside the car as if it the car were stationary (until the car turns).

Sam Stone
04-29-2006, 09:34 PM
But the jumper/diver has two forces acting upon him. One is gravity which pulls him straight down. The other is momentum in the direction of travel of the plane. Let's say a person jumps out of a plane. The moment he leaves the plane the force is dissapating, which means that a plane travelling 1,000 miles an hour will "throw" him further than a plane travelling 50 miles an hour. So, regardless of the speed of the plane, at some point the horizontal force generated by it will dissapate. If you looked at the jumper from a horizon vantage point his travel would look like half a parabola, not a straight diagonal line.

Maybe a simpler way to look at it is if I jump from a spot on the equator and it takes me one hour to reach the height of my jump and fall back to earth, the point that I launched from is now 1,000 to the West (assuming a 24,000-mile circumference). So, if I jump up in Colorado I'll come down in California. Of course, this assume no atmosphere, which is also moving at 1,000 miles an hour.

Then again, maybe you're right. At least in a vacuum. I'm too confused now.

No, your momentum doesn't 'dissipate'. It takes some kind of force to change it. If the air inside the airplane is stationary with respect to the jumper, there will be no force to move him. In fact, if there are no windows in this airplane, there is no experiment you can devise within its walls that can tell you where the airplane is standing still or going 1 milllion miles per hour. There's no such thing as 'absolute' speed. It's all about the frame of reference you are in.

So why does a jumper's forward velocity dissipate? Simple. Because of wind resistance. If there were no wind resistance, he would continue along merrily outside the airplane. Think of spacewalkers leaving the shuttle. They're going close to 20,000 mph around the earth. But they can push themselves away from the shuttle and float alongside it just fine.

David Simmons did throw a bit of a wrinkle into this - when discussing movement on the earth, we have to remember that we're in a rotating reference frame, which you can detect. If you throw a ball up in the air while spinning around on a merry go round, it will appear to curve away from you because of this. We call this the Coriolis force. But for the sake of this argument, assume we are flying along a flat plane at a constant speed. Under those conditions, there is absolutely no way to tell from inside the airplane how fast you may be going.

GusNSpot
04-29-2006, 10:57 PM
::: Searching through the wreckage looking for survivors.... :::

magellan01
04-30-2006, 12:14 PM
Sam Stone, Brain Wreck, Johnny L.A., and others,

Yes, you are, of course, correct. I kept thinking of some type of resistance that wold impede the forward travel, which would cause it to dissaipate. :smack:

Johnny L.A.
04-30-2006, 02:25 PM
David Simmons did throw a bit of a wrinkle into this - when discussing movement on the earth, we have to remember that we're in a rotating reference frame, which you can detect. If you throw a ball up in the air while spinning around on a merry go round, it will appear to curve away from you because of this. We call this the Coriolis force. But for the sake of this argument, assume we are flying along a flat plane at a constant speed. Under those conditions, there is absolutely no way to tell from inside the airplane how fast you may be going.
I'd forgotten about Coriolis. Or rather, I ignored it because I was thinking on the small scale of jumping in a moving jet. In that case, it does make sense to think of moving laterally in a flat plane. But indeed when an object with lateral velocity moves away from the center of rotation, it must travel faster to remain over the same spot.

But with respect to the OP, Coriolis is so small as to make no odds.

Chronos
04-30-2006, 02:44 PM
I don't think it's so much pressure difference that moves the ballon, but acceleration, which is the same as gravity, causes the air to 'sink' toward the back window and the ballon to 'float' toward the front.Six of one, half-dozen of the other. The air "sinking" forward causes the pressure difference, and the effect of the pressure difference is what you called "floating".

And if I had a dime for every time a physics student thought of momentum as a force, it'd probably double my stipend.