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December 15, 2006
Light Friday: 'Snow'-tipped Space Mountains, Santa Science, Can the Plane Take Off?
We have a brainteaser for engineers and physics-minded readers. Also, some of the principles involved in Santa's epic annual journey: advanced knowledge of electromagnetic waves, the space/time continuum, nanotechnology, genetic engineering, and computer science.
Can the Plane Take Off?
OK, this “thought experiment” has been zipping about the Internet. Here's the brainteaser for our engineers and physics-minded readers, as presented at Straight Dope and Pogue’s Posts (NYT):
“Imagine a plane is sitting on a massive conveyor belt, as wide and as long as a runway. The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Can the plane take off?
“I say no, because the plane will not move relative the ground and air, and thus, very little air will flow over the wings. However, other people are convinced that since the wheels of a plane are free spinning, and not powered by the engines, and the engines provide thrust against the air, that somehow that makes a difference and air will flow over the wing.”
Please leave your answers or ideas in our Comments section below.
Snowy Space Mountains
The tallest known mountains on Saturn's moon Titan have been revealed in the latest images from NASA's Cassini spacecraft.
During a flyby to obtain the highest resolution infrared views of Titan yet, Cassini resolved surface features as small as 400 meters (1,300 feet). According to Science Blog, the images reveal a large mountain range, dunes and a deposit of material that resembles a volcanic flow. These data, together with radar data from previous flybys, provide new information on the height and composition of geologic features on Titan.
The mountains appear in images from the Visual Infrared Mapping Spectrometer (VIMS) instrument. Though the features have been seen before, this time the instrument also spotted a hazy shadow stretching away from the range, which itself is about 150 kilometers (93 miles) long. The length of the shadow reveals that the mountains are about 1500 meters high (nearly a mile).
Deposits of bright, white material, which may be methane "snow" or exposures of some other organic material, lie at the top of the mountain ridges.
Santa Science
Dr. Larry Silverberg, professor of mechanical and aerospace engineering at North Carolina State University, tells Machine Design he can explain the science and engineering principles that could allow Santa Claus to pull off his epic journey to so many children in one night. Some of the principles involved: advanced knowledge of electromagnetic waves, the space/time continuum, nanotechnology, genetic engineering, and computer science.
Santa knowing what all those children want for Christmas
Silverberg says that the fat character has a listening device that combines technologies currently used in cell phones and EKGs. A sophisticated signal processing system filters the data, giving Santa clues on who wants what, where the children live and, of course, who’s been naughty or nice. An onboard sleigh guidance system provides Santa with the most efficient delivery route.
Crisscrossing 200 million square miles in a single night
Silverberg says Santa uses his knowledge of the space/time continuum to form “relativity clouds.” The engineering professor tells Machine Design, "Santa recognizes that time can be stretched like a rubber band, that space can be squeezed like an orange and that light can be bent. Relativity clouds are controllable domains — rips in time — that allow him months to deliver presents while only a few minutes pass on Earth.”
Getting inside 80 million homes
The same aforementioned “relativity cloud” technology also allows Santa to “morph” into people’s homes, where he sets up his workshops. Using nano-toy-making technology, he creates presents atom by atom out of snow and soot, much like DNA can command the growth of organic material like tissues and body parts.
"This is our vision of Santa's delivery method, given the human, physical, and engineering constraints we face today," Silverberg says.
Remainders
1) Apparently, Ford Motor Co. is so desperate to destroy one of the few cars the automaker has done right over the last few decades that it is considering a “mustang” wagon. According to a report in the magazine AutoWeek, the next generation of the Ford Mustang could include some previously unthinkable variants, including a four-door sedan and a station wagon. “To a Mustang purist, this is blasphemy,” said Bob Gritzinger, AutoWeek's senior editor for news. What do you think? (via CNNMoney)
2) India, considered one of the world's top polluters, said yesterday that it was not doing any harm to the world's atmosphere despite increasing emissions of greenhouse gases, reports Reuters. 
Some scientists argue that unchecked greenhouse gas emissions could see global temperatures rise by 2-3 degrees Celsius in the next 50 years and could result in devastating climate change. Although India is not required under the Kyoto Protocol to cut emission levels at this stage, experts say its emissions are rising due to its rapid economic development and could become a significant contributor to global warming. The country’s environment minister told parliament India's emissions were insignificant compared to those of richer nations, which should take the lead in curbing greenhouse gases.
3) South Korea, on the other hand, opened the world's largest garbage-fuelled power plant on Tuesday. The 50-megawatt plant, 
which began operating this week, is designed to provide power to more than 180,000 households. Sitting on a mammoth garbage dump in the city of Incheon west of Seoul, it uses only the methane gas naturally generated from the decomposing garbage on the site for fuel. The plant is expected to save the country the import of 500,000 barrels of heavy oil and to reduce greenhouse gas emissions by 1.37 million tons per year, according to AFP.
4) A superior court judge in Reading, Pa., overruled a county court judge in August and declared that Miller Genuine Draft is, in fact, an actual beer. According to GovPro, the county judge had said that the prosecutor had failed to show that MGD was on the state beer list, but the superior court judge said there was other evidence that MGD is beer. Tax dollars at work, folks.
Cheers.
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47 CommentsThe plane is stationary, therefore no air is flowing over the wing. BTW, the conveyor belt only needs to be as wide and as long as the distance between the outside of the wheels and the length between the nose wheel and the wheels under the wings, as the plane is not going to move.
December 15, 2006 1:14 PMThe plane would take off. Imagine a rear-wheel-drive car. The front wheels are free wheeling against the pavement; as the pavement moves under the wheels at the same speed the wheels turn, you still have forward motion brought on by the thrust of the rear wheels. This principle would apply to the plane with the thrust of the engines moving the plane, not the wheels.
December 15, 2006 1:27 PMUnlike a helicopter (or the AV8B Harrier), I believe that forward motion is a prerequisite for flight in a typically configured airplane. If the conveyor is successfully canceling out any attempt at forward motion, I believe that what you would be left with is an extremely loud and frantic 'parking lot'!
December 15, 2006 2:27 PMThe movement of the conveyor belt would cause the air over it to start flowing. Eventually it would give the airplane just enough lift to get the wheels off of the coneyor belt, breaking the resistance, allowing the plane to move forward and take off.
December 15, 2006 3:43 PMThe plane cannot move relative to the air, so it does not take off. Wheel speed has nothing to do with the problem or planes could take off in tailwinds.
December 15, 2006 6:59 PMThe speed of the tires is irrelevant. The aircraft only cares about the air that is flowing over its wings.
December 17, 2006 8:47 PMThis is a stupid question. The plane would obviously NOT take off because there would be no air flow under/over the wings. Movement through the air is what makes a plane leave the ground, and the plane in this example is not MOVING. DUH!
December 18, 2006 9:53 AMDoesn't matter if the plane's moving because of jets, props, or hundreds of frantic midgets with tow ropes. If there's air moving around the wings, lift will be created (eventually) and it'll fly. Remember that old high-pressure/low-pressure thing?
December 18, 2006 10:24 AMAbout the Mustang... Yes, it's blasphemous in an American sort of way. Is it a bad idea? No. Relatively high-performance, car-based pickups and wagons are and have been commonplace in, for example, Europe and Australia for years. Here in the U.S., the Dodge Magnum wagon is a good example that it can be done -- and the public will embrace it. Using the Mustang platform (a very good one) for such a performance wagon or sedan would be a very smart move. Making it look like and calling it a Mustang, however, would be one of the Greatest Auto Industry Blunders Ever.
December 18, 2006 12:12 PMTo clarify, the plane would 'fly' as in lift off of the conveyor belt for a moment. Since the plane has no means of self-propulsion, however, it will almost instantaneously lose speed (as contact with the conveyor is lost) in the air, and come to rest back on the belt. Lather, rinse, repeat. It will lift or "fly", but it will not take its passengers (real or imagined) to any destination other than the conveyor belt itself. Now, a catapult system...
December 18, 2006 1:06 PMNo. Scratch my previous comments. Misread the question. If the plane were being moved on the conveyor belt, it would lift as described. With the plane stationary and conveyor moving opposite the wheels, however, it's not going anywhere, in any direction.
No airflow=no lift.
My bad. I need to RTFM next time.
December 18, 2006 2:30 PMThe plane would easily take off. The thrust of an airplane is produced on the air around it, not on the ground it is sitting on. How would a ski plane or float plane take off if thrust was produced on the ground beneath it? The conveyor would have no effect on it!
December 19, 2006 1:48 PMThere are several factors not specifically stated in the set-up. No mention of engine power, applying the brakes and the conveyor direction is somewhat uncertain, i.e., "opposite direction", and the ambient air conditions, velocity and direction. So the possibilities follow.
The wheels of the airplane will rotate relative to the conveyor belt movement in addition to any relative aircraft movement. Since there is no mention of power applied to the aircraft engine, it must be considered that there is none. As the conveyor belt speeds up, the aircraft wheels will rotate with little effect on the aircraft itself, so the airplane will remain in place and no lift will be produced. Induced air movement from the moving conveyor belt will be insignificant velocity and mass.
If the pilot were to apply the brakes and the conveyor was moving in a direction that would take the aircraft forward, then at such time as the speed exceeded minimum operational velocity through the ambient air, the aircraft would indeed lift off. But it would shortly return to the ground without another thrust source, i.e., it's own power.
Aircraft fly when the air over the wings is of adequate velocity to produce the needed lift. So if there is thrust for the machine, this thrust -- jet or propeller -- will propel the aircraft forward relative to the air, totally irrespective of the conveyor movement; and when sufficient airspeed is achieved, the aircraft will take off.
December 19, 2006 5:08 PMThe plane's wheel can rotate maybe, but as there is no air blown by the convere belt, there would be no upthrust generated. Plane doesn't takes off unless its nose lifts, and how is it possible? The experiment should be changed: a good blower of air should be used, not the conveyer belt.
December 20, 2006 2:41 AMIn the statement, "The plane is sitting on a conveyor belt":
If the belt is not moving, the plane can take off as normal using the conveyor belt as a runway.
If the belt is moving at speed as engineered and the plane sitting on the belt moving in the opposite direction, the wheels' speed will be twice that of normal, but forward velocity will not be affected and such the plane lifts off.
Hope this helps!
December 20, 2006 5:02 AMAssuming the engines are in fact turned on and applying thrust sufficient to cause the plane to lift off from a standard runway, then the plane would take off from the conveyor belt. The engines are not thrusting against the ground, but the air.
As the aircraft (and its wheels) moved forward against the air, the conveyor belt would move forward with it at the exact rate of speed required to keep the wheels stationary relative to the conveyor. Once the aircraft had achieved sufficient airspeed, it would take off, without the wheels ever spinning. The conveyor at that point would be moving forward at a speed equal to the airspeed of the aircraft.
Is there a cash prize for this?
December 20, 2006 9:04 AMFirst, if the plane gets all of its movement from pushing air with its engines and moves with respect to the air about it, to make its wheels turn. That is, you cannot have the wheels turning and the plane be standing still no matter what the conveyor does.
Second, if the plane moves, it will take off when it gets the proper take of velocity.
Finally, regardless of the conveyor speed, the plane has to move forward with respect to it if its propulsion sytem is working properly.
December 20, 2006 10:13 AMThe key is, people are missing the fact that the function of the conveyor isn't to stop the aircraft from moving, but to *stop the wheels from spinning.* Since it can't provide any opposing force to the thrust provided by the engines, the only way the conveyor can do that is to move with the plane.
December 20, 2006 11:46 AMWhen the airplane engines are ground tested, without the airplane moving forward, the pilots move all the surfaces and the airplane goes nowhere. Airplane only takes off if speed (i.e. forward momentum) is applied. That creates the necessary pressure differential on the wing and at a certain speed, with the nose up, the airplane is airborne. What really happened was that the force due to pressure differential was enough to counter the inertia of the airplane. A stationary (with respect to ground), airplane will not have the required pressure differential on the wing to lift it. What is going to create the differential? Only helicopters can create upward thrust in a stationary condition on ground, but that is another story.Tire movement has nothing to do with it.
In other words, the airplane will not take off.
December 20, 2006 12:35 PMThe question is "CAN" the plane take off. Yes, it can take off if the engines are lit to provide thrust. It will move and it will fly. Conveyer will do nothing to prevent this. Since planes does not get speed from torqe applied to wheels.
December 20, 2006 1:44 PMIf the direction of travel of the conveyor is opposite the rotational direction of travel of the wheels and IN MOTION under the conditions described, there is no way for the plane to produce lift if it is not moving forward. Lift is created by the movement of air around the wings as the plane is thrust forward by the engine(s). With no forward movement, the plane will literally just sit there and spin its wheels on the moving conveyor.
As to whether the plane "can" take off, it can if it's flightworthy and not spinning its wheels on such a moving conveyor....or, if the pilot applies the brakes once flight velocity is achieved on the conveyor (assuming the conveyor doesn't then stop as well) and releases them before the end of the runway. Or, of course if the conveyor belt isn't in motion!
December 20, 2006 3:12 PMHere's the key. I think:
"The conveyor belt is designed to exactly match the speed of the wheels, moving in the opposite direction."
Clearly, the teaser wasn't designed by an engineer. (We don't know, for example, why the wheels are moving and what put them in motion. The conveyor itself?) What does 'exactly match the speed of the wheels' mean? Does that mean that the plane's wheels are already moving forward/counter-clock? If so, the conveyor's headed in the opposite direction. Thus, the plane will be motionless. (Kinda like walking Up a Down escalator, speed-matched.)
As Mr. Nighman says, it'll just sit there and spin its wheels. No airflow around wings=no lift.
December 21, 2006 9:29 AM“Imagine a plane is sitting on a massive conveyor belt, as wide and as long as a runway. The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Can the plane take off?"
We were discussing this here at Boeing, and it even puzzled a few of the engineers here. The final answer however, is that airplanes take off by pushing on AIR, not on the tarmac. That is why a Cessna outfitted with skis can take off from solid ice in Alaska.
In the case of the Cessna, the plane can takeoff because the friction force between the airplane and the ice is a small fraction of the force of thrust from the engine.
Thought experiment:
(1) Let's imagine for a moment that the frictional force is actually zero. It should be even EASIER for the airplane to take off than from ice, right?
(2) If the frictional force between the airplane and the ground is truly zero, this would be much like hanging the airplane from a track or from an infinitely long sting such that wheels don't actually touch the tarmac.
Again, the airplane pushes against the AIR, and the plane moves forward until it generates enough lift from the wings to support itself.
(3) When the wheels aren’t touching the ground, as in example (2) above, it doesn’t matter if the tarmac is stationary or moving forwards or backwards. There simply isn’t any interaction between the tarmac and the airplane.
This is also true for a truly frictionless surface. The surface can move in any direction and the airplane will still be able to take off.
(4) In the real world, we make sure the wheel bearings in airplanes are close to frictionless as possible. Only a small fraction of the approx 170,000 pounds of thrust a 777 can generate on takeoff goes to wheel bearing friction.
I don’t know that actual number, but let’s pretend that 10% of the thrust is used to overcome friction. That means that at takeoff speed (approx 180 mph) the friction force would be 17,000 pounds in the direction opposing thrust.
To keep a 777 standing still at takeoff thrust, the moving tarmac would have to going fast enough to generate 170,000 pounds of force in the reverse direction! If the friction forces are linear, the tarmac would have to move backwards at 1800 mph to keep the airplane standing still!
Thus, the airplane takes off.
-Farcast
December 21, 2006 9:33 PMYou need only know if you are traveling a like speed against the air, wheel speed has nothing to do with lift, lift created by wing curvature @ speed, you need lift to take off not ground speed but air speed. The rst of the conversation is moot.
December 4, 2007 6:51 PM
