plane question

[Geoff St. Germaine]

For anyone who isn't aware, a google search shows that this is a trick question that a lot of people have posted on various forums in the past few months. Most people think that the plane won't take off and the trick is that they're thinking about it more like a car than a plane. A car accelerates itself by pushing on the road, and thus accelarates relative to the road. Thus, if the road is a conveyor, then the car is moving relative to the conveyor. If the conveyor is moving at the same speed as the car's tires are moving relative to the conveyor, but in the opposite direction, then the car will be held in place relative to the ground. The difference is that the plane doesn't rely on the ground to accelerate, it accelerates relative to the air, so it doesn't really matter what the ground is doing.

Think of it this way: if a planes take off speed is 100 km/h and it is sitting on a conveyor that is moving in the direction opposite to the way the plane is facing, can it take off? The answer is yes, the plane will be moving at 200 km/h relative to the conveyor and 100 km/h relative to the ground/air assuming there is no wind.

 

[bmc]

Assuming no wind it will be moving relative to both. Twice as fast relative to the conveyor belt than relative to the ground. With no wind the groundspeed will also be the airspeed.

Maybe I'm reading the original post wrong. I interpretted it to say that the conveyor belt will always move at the same speed as the airplane but in the opposite direction as the plane is going. Based on that, the airplane is motionless with wheels spinning. If, on the other hand, the plane is moving faster than the conveyor, at somepoint it hopefully will reach its take off speed and fly.

 

[bmc]

For anyone who isn't aware, a google search shows that this is a trick question that a lot of people have posted on various forums in the past few months. Most people think that the plane won't take off and the trick is that they're thinking about it more like a car than a plane. A car accelerates itself by pushing on the road, and thus accelarates relative to the road. Thus, if the road is a conveyor, then the car is moving relative to the conveyor. If the conveyor is moving at the same speed as the car's tires are moving relative to the conveyor, but in the opposite direction, then the car will be held in place relative to the ground. The difference is that the plane doesn't rely on the ground to accelerate, it accelerates relative to the air, so it doesn't really matter what the ground is doing.

Think of it this way: if a planes take off speed is 100 km/h and it is sitting on a conveyor that is moving in the direction opposite to the way the plane is facing, can it take off? The answer is yes, the plane will be moving at 200 km/h relative to the conveyor and 100 km/h relative to the ground/air assuming there is no wind.

So you are saying the airspeed indicator in the cockpit is registering a 200km wind, correct?

As a math equation:

100mph (airplane speed) + (-100 conveyor speed) = ?

The only way that airplane attains a takeoff speed of 100mph is when it has 100 mph of wind going over the wings and indicated on the airspeed indicator as 100mph. If the conveyor moves backwords at 100mph, the plane needs to move forward at 200 miles an hour relative to the conveyor and 100 mph relative to the ground.

 

[Dan1099]

Maybe I'm reading the original post wrong. I interpretted it to say that the conveyor belt will always move at the same speed as the airplane but in the opposite direction as the plane is going. Based on that, the airplane is motionless with wheels spinning. If, on the other hand, the plane is moving faster than the conveyor, at somepoint it hopefully will reach its take off speed and fly.

That's the problem. The plane WON'T remain motionless. The conveyer will just spin the wheels faster.

 

[Davidoc]

For anyone who isn't aware, a google search shows that this is a trick question that a lot of people have posted on various forums in the past few months. Most people think that the plane won't take off and the trick is that they're thinking about it more like a car than a plane. A car accelerates itself by pushing on the road, and thus accelarates relative to the road. Thus, if the road is a conveyor, then the car is moving relative to the conveyor. If the conveyor is moving at the same speed as the car's tires are moving relative to the conveyor, but in the opposite direction, then the car will be held in place relative to the ground. The difference is that the plane doesn't rely on the ground to accelerate, it accelerates relative to the air, so it doesn't really matter what the ground is doing.

Think of it this way: if a planes take off speed is 100 km/h and it is sitting on a conveyor that is moving in the direction opposite to the way the plane is facing, can it take off? The answer is yes, the plane will be moving at 200 km/h relative to the conveyor and 100 km/h relative to the ground/air assuming there is no wind.

That would make sense only if the plane was stationary on the conveyor belt, and the belt was moving in the direction the plane was facing. Then the pieces would fit with that explanation; the plane would have no net acceleration with the belt, but would be accelerating relative to the air, causing it to take off.

If the belt is moving in the opposite direction as the plane at the same velocity, then that explanation does not fit. The plane would have double acceleration relative to the belt. (not none at all), and would have no acceleration relative to the air.

 

[Davidoc]

That's the problem. The plane WON'T remain motionless. The conveyer will just spin the wheels faster.

The conveyor will spin the wheels faster, but the plane will remain motionless.

 

[Dan1099]

That would make sense only if the plane was stationary on the conveyor belt, and the belt was moving in the direction the plane was facing. Then the pieces would fit with that explanation; the plane would have no net acceleration with the belt, but would be accelerating relative to the air, causing it to take off.

If the belt is moving in the opposite direction as the plane at the same velocity, then that explanation does not fit. The plane would have double acceleration relative to the belt. (not none at all), and would have no acceleration relative to the air.

He's not saying that it doesn't accelerate relative to the conveyer belt. He's saying the conveyer belt does not effect the acceleration of the plane. The plane does not use the ground to accelerate, like a car does. The plane pulls on the air to accelerate. The wheels in the plane will just spin faster.

 

[Davidoc]

He's not saying that it doesn't accelerate relative to the conveyer belt. He's saying the conveyer belt does not effect the acceleration of the plane. The plane does not use the ground to accelerate, like a car does. The plane pulls on the air to accelerate. The wheels in the plane will just spin faster.

But the plane will not be able to pull the air if it's not moving through it.

 

[bmc]

That would make sense only if the plane was stationary on the conveyor belt, and the belt was moving in the direction the plane was facing. Then the pieces would fit with that explanation; the plane would have no net acceleration with the belt, but would be accelerating relative to the air, causing it to take off.

If the belt is moving in the opposite direction as the plane at the same velocity, then that explanation does not fit. The plane would have double acceleration relative to the belt. (not none at all), and would have no acceleration relative to the air.

Exactly.

 

[bmc]

He's not saying that it doesn't accelerate relative to the conveyer belt. He's saying the conveyer belt does not effect the acceleration of the plane. The plane does not use the ground to accelerate, like a car does. The plane pulls on the air to accelerate. The wheels in the plane will just spin faster.

The conveyor belt does not affect the acceleration of the plane? If it's trying to pull it backwards at the same speed as the plane is trying to go forward, I would argue that it absolutely affects the acceleration of the plane. The plane isn't going anywhere.

If you're on a jogging machine and it's set to go at the speed you run at, how much distance will you cover in an hour, relative to the stationary floor around you? Not much. You will not have left the building while standing on the jogging machine, correct?

Look at another way. You're having a race between two traffic lights. You're standing on the jogging machine and have programmed in a speed of 30mph, because you're one fast lad. Your jogging machine is set up on a road next to Ferrari. He sets his speed to 20 mph and never exceeds it. You're both 1 mile from the lights. Who gets there first? You on the jogging machine or the Ferrari?

 

[AxtoOx]

Exactly.

It's a trick question, I was taken in by the same thing thinking air speed is 0. It's not. I'm pretty embarassed because I fell for it, and I used to work the flight line. Like I said my Dad has seen this before, he's ex-Air Force and an engineer. The plane will take off. The question is decieving. I think he's still laughing at me.

 

[SnoMan]

If you're on a jogging machine and it's set to go at the speed you run at, how much distance will you cover in an hour, relative to the stationary floor around you? Not much. You will not have left the building while standing on the jogging machine, correct?

Your legs push off the tredmill for propultion....like a car on the ground.

The plane pushes the air to gain movement.


You can't compare apples to airplanes.

 

[bmc]

It's a trick question, I was taken in by the same thing thinking air speed is 0. It's not. I'm pretty embarassed because I fell for it, and I used to work the flight line. Like I said my Dad has seen this before, he's ex-Air Force and an engineer. The plane will take off. The question is decieving. I think he's still laughing at me.

I've been in the aviation industry for 25 years. I have been a licensed pilot for 10 years. I have flight experience in Cessnas, Cherokees, Katana's, Citabria's, Twin otter's, B737 and some highly illegal time in an A340 over Europe. My flight experience has told me that I could actually rotate an aircraft sooner that recommended and just keep it ground effect until I have built up sufficient airspeed to climb away. But unless there is sufficient airflow over my wing that generates lift, we ain't goin anywhere. So...using Geoff's example, I need 100mph over them wings to give me my get up an go. I know there will be 100mph there for me to pull back on the stick because a) my airspeed indicator says so or b) I pull back on the stick and plane responds. In both a) and b) there is airflow.

The first post said that the conveyor is pulling backwards at the same speed the plane is trying to go forward. Where's the trick?

I'm sitting her laughing, along with your father.

 

[Bassic83]

Oh, man, this whole thread is a waste of gas!

If you are in a plane, you absolutely need airspeed in order to fly. The way that question is worded leads me to believe that the "moveable runway" will be matching speed with the forward motion of the plane. As it accelerates through the air, lift is generated. However, in this scenario, it is prevented from accelerating because it's forward motion is being canceled out by the backwards motion of the "treadmill" that couples the wheels to the "pavement". It cannot go forward, all advances in speed are offset by the treadmill matching speed in the opposite direction. Sure, the engine will be getting hot, and you'll be spending a lot of money on fuel, and the prop will be madly clawing at the air, but unless the aircraft is a jumpjet or a helicopter, you're not going to achieve the amount of airspeed you need to achieve flying speed.

I've experienced the effects of "negative windspeed" on takeoff during my training. One day, my instructor took me to the end of the runway, and after checklists were completed, and equations run, and double-checked, had me take off with a tailwind. We took off, all right, but at the assigned "rotate speed" of 65, the airplane's elevator lacked the authority to persuade the tail to go down, and about 12 mph later, finally bit. Our airspeed was showing 70, but GPS revealed our groundspeed to be closer to 82. Also, the wings of an aircraft require a certain amount of air to be flowing over the wings in order to generate lift, which is why airplanes can't hover in still air.

I've also flown backwards. In a 45-knot headwind, in a C-150. I had full flaps deployed, and about 20% power, in slow-flight configuration, stall horn wailing, nose high... I watched as an intersection slid slowly ahead of my wing strut, 5000 feet below me. GPS confirmed a 3 mph net loss of speed, yet we were still flying, beacuse of the headwind providing enough relative airpseed over the wing to keep the plane from seeking the earth.

 

[bmc]

Your legs push off the tredmill for propultion....like a car on the ground.

The plane pushes the air to gain movement.


You can't compare apples to airplanes.

Nope. It's wind over the wings that generates lift. Not the big fan at the front end.

 

[bmc]

Oh, man, this whole thread is a waste of gas!

Thank you brother.

Good example of tailwind and airspeed indicator/GPS reading, btw.

 

[bmc]

Your legs push off the tredmill for propultion....like a car on the ground.

The plane pushes the air to gain movement.


You can't compare apples to airplanes.

SnoBassMan you are partially right in that the plane pushes air to gain movement, but in this example, the conveyor is matching the speed but pulling it in the opposite direction. If a speedometer was attached to the wheels, it would show 100mph. But the airspeed indicator would show 0 mph.

 

[Bassic83]

Go post the same question over at studentpilot.com, and see how quick you get shot down!

So, bmc, are you a pilot too?

 

[Davidoc]

I don't even get how this is a trick question. No acceleration through the air, no lift. BMC's treadmill comparison is a good one.

 

[Bassic83]

I think it is trying to be a trick question due to the coupling of the wheels to the treadmill. People think the big fan up front is to pull the airplane through the air. It's not. It's a fan to keep the pilot cool. Want to prove it? Stop it in flight once, and watch the pilot start sweating!