http://www.straightdope.com/columns/060303.html
Why on earth hasn’t Cecil forced Little Ed or some other lackey into roller skates and onto a moving treadmill with a fire extinguisher?
Or go to O’hare and get on a moving walkway, same equipment.
What exactly is the thrust of a fire extinguisher?
http://videos.streetfire.net/player.aspx?fileid=35E964D9-38DB-4EFD-BE8D-D6BA1A43A06B
You would think a video like that would help ‘prove’ that Cecil isn’ t in the wrong here
Was it just me, or did Unca Cec’ sound a little extra snarky today? I dare say this airplane on a conveyor belt seems to have touched a nerve. 
I would also say that, taken in conjunction with the first article, this question is answered. Mods, can we now have anyone who starts debating this topic summarily rounded up and [del]shot[/del] re-educated?
Who cares what it would prove? It should be done.
The proof is in Cecil’s original column - a treadmill that automatically adjusts to the plane’s speed violates the rules of physics.
There is a ~very~ simple explanation as to why there is such debate over the plane on a treadmill.
Everything stems from a semantic problem with the word 'speed'. I think the 'everyday, intutive' definition of the speed of an object includes an implicit reference to the 'zero speed' reference point. For almost everyone (not discussing astrophysics), the 'zero point' is the ground. Therefore, if the treadmill 'continually adjusts it's speed' while the engines keep pushing harder, the net result is that the plane stays in the same spot - relative to the ground. The seemingly massive quantity of air that is forced 'past' the wings by the engines is actually 'diverted' - passed through the engine rather than over the lifting surface. No lift.
This is ~not~ a restatement of Basic Realization #2. In fact, both BR #1 and BR #2 are only useful for people that do ~not~ consider the ground to be a zero reference point - and need a clever way to get to back to the original problem - the definition of the word 'speed'.
Best regards,
Greg
OK, now lets suppose there is a headwind strong enough that the plane can hover off the runway and take off? Huh? How 'bout that?
It seems to me that the distinction everyone might be missing is best exemplified by a simple thought experiment - imagine the difference between a winged car and a winged airplane, both on the treadmill. The distinction is that the propulsive force of the airplane is decoupled from the treadmill, but the propulsive force of the car is not (yes, I know I’m ignoring the friction/acceleration effects). No one would have trouble understanding that the car is not going to “fly”, but it might be easier to understand that the airplane would. Just a thought.
Sorry, but no. There are two basic points of disagreement between the two camps:
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Whether or not the treadmill has the ability to prevent forward motion of the jet. This is covered in Cecil’s article (answer: we don’t know; the problem is very poorly stated). Assuming a real-world treadmill and a real-world jet and a real-world mechanism for adjusting how fast the treadmill runs, no, it doesn’t prevent the jet from moving, but that assumption is not covered by the problem as stated.
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Whether or not the jets blow air over the wings, thus causing lift. No, they don’t blow air over the wings, but that’s not what causes lift.
My problem is not with the word “speed,” but with the word “move.” According to Cecil’s Basic Realization #1, “The plane moves forward at roughly its usual speed relative to the ground and air, generates lift, and takes off.” I’m no physicist, but from where I’m standing the plane does not appear to “move.” The only part of the plane that is in motion are the wheels, and they are merely rotating. My instinct tells me that no matter how fast those wheels rotate, if the plane does not “move” horizontally then it will not generate lift.
Somebody set me straight!
You haven’t hit BF#1 yet…
The tires have nothing to do with the over all motion of the plane…
The tires do not DRIVE the plane… (if this were a car, and the method of locomotion were via a car your mental picture would work)…
Cecil,
Normally you are right on track, but I think maybe your aeronautics expertise might have been exceded with this one.
Admitting the unreality of the scenario, there is a key difference between taking off and flying through the air. In terms of the latter, your answer (i.e., force/thrust being the key factor) but taking off is a bit different. To get the lift, the wings have to be moving through physical space fast enough to create the pressure differences that yield lift. The airstrip (in the case of Navy aircraft carriers can be turned into a headwind, to help add to that movement of air, thuis helping the take-off process, but otherwise the aircraft has to move forward quickly (i.e., roll down the runway), through the airspace, to develop the lift necessary to get up off the surface.
If, somehow, the treadmill could move opposite to the take-off direction, at the “forward” speed of the aircraft, then I can see where there would never be enough relative motion of air across-and-under the wings to allow the plane to take off.
Remember, a rocket works on the basis of thrust-to-weight ratio, but planes (even an F-15, with TTW ratio geater than one) really need runways down which they taxi until reaching take off speed.
So…why do they even bother with long decks on Aircraft Carriers?? Why not just
put a BigAssTreadmill on them…and…let the planes take off from there???
I am not sure about the landing part…but…one thing at a time…!!!
Kyle :dubious:
Can God create treadmill that won’t allow his personal airplane to take off?
They do, it’s called a catapult. But they don’t make it strong enough to launch the plane by itself, they just make it strong enough to work in conjunction with the engines so that the plane will take off in a reasonably short distance.
You seem to be leaving out two key points: 1) wheels roll and have relatively little friction; 2) the wheels do not “push” the airplane, the engines do.
That’s a bit of a strong statement. If you’re talking about Cecil’s statement that
This language leads to a paradox: If the plane moves forward at 5 MPH, then its wheels will do likewise, and the treadmill will go 5 MPH backward. But if the treadmill is going 5 MPH backward, then the wheels are really turning 10 MPH forward. But if the wheels are going 10 MPH forward . . .
then he’s simply wrong (or, rather, he’s not accounting for the forces he discusses in the current column, and he’s not explicitly ignoring them).
If you’re talking about the inability to physically build a treadmill that will hold back an F16 (as Cecil appears to be doing in the current column) then that’s reasonable. However, nothing in the original question specifies the type of plane. I would think you could build a treadmill that is capable of holding back, say, an underpowered RC model for a short length of time.
Actually, I think he nailed it. Most people are coming to correct solution, they just interpret speed differently and diverge from there.
Group 1 (Cecil’s flying group) say that when you say speed you are talking relative to a nearby building or a section of ground that is not the treadmill.
Group 2 (the stuck to the groud group) define speed relative to the ground immediately beneath the plane, aka, the moving surface of the treadmill.
Thus, for group 1 there is an increasing backward force on the wheels of the plane as it moves faster relative to a stationary object, but nowhere near enough to stop it from taking off.
For group 2 however, the plane has a speed V relative to the surface of the treadmill. The surface of the treadmill has a speed -V relative to a stationary object. Thus (neglecting relativity) the plane has a speed 0 relative to a stationary object. An observer of the plane that has his view of the treadmill obstructed will not notice any motion in the plane. With no motion there is no lift and therefore the plane stays on the ground, whatever V you are talking about.
So, depending on what you define speed relative to, you have two very different problems.
And, by the way…
Although this week’s column is substantially better than the earlier one, Cecil still seems to fundamentally misunderstand the requirements of the problem.
If you interpret the original question as requiring the treadmill to match the tire rotational speed (not the only interpretation, for sure, but not uncommon), that requires the treadmill to hold the plane motionless relative to the ground. It could be held motionless as Cecil explains in the current column, but that requires the forces to balance.
Thus exactly counteracting the plane’s force is a necessary requirement of the original problem statement requiring matched velocities. Not at all “a departure from the original question,” as Cecil claims, but a necessary extension.
Regarding WildWriter quoted below: I have to agree with the conceptual exercise posed by HFTobeason@pandaemonium.com above (re: winged car on treadmill vs. winged plane on treadmill). The reason this exercise is useful is that it clearly distinguishes the difference between where forward momentum generated with the car and the plane. In a car, forward momentum - relative to the ground - is generated through the wheels via contact with the ground. On a treadmill, this forward momentum is perfectly counteracted by the backward movement of the treadmill belt. Net motion - relative to the ground - is zero.
However, with a plane or jet, forward momentum is generated by the propellers/jets PULLING the plane through the air. As the earlier writer astutely points out, this force is completely decoupled from the ground/treadmill. The treadmill could spin as fast as it wants and, assuming the wheels of the plane freewheel to negate the motion of the treadmill belt, it would not affect the plane’s position relative to the air (and that’s important). So you fire up the engines and suck the air through the turbines. In doing so, you PULL the plane through the still stationary air and the plane begins to move forward regardless of how fast the treadmill is moving. I think a concept lost in this argument is that people envision the treadmill as something just the length of the plane and can’t conceptualize a plane slowly “levitating” off a treadmill, as though the plane is tethered to the treadmill and needs to lift straight up. Don’t get me wrong, a plane would still need to move forward - probably the same distance as a traditional take off - so the hypothetical treadmill would need to be runway length. But the key element is that the treadmill alone does not prevent forward motion by the plane (relative to the air and ground around the treadmill) for the reasoning above. You could have a runway-length treadmill and the plane would take off just the same as on stationary ground.
I disagree with the physics involved with the Thrust To Weight (TTW) ratio as an explanation of why a plane needs stationary ground in order to take off - it only explains why a plane would need more or less runway space for a horizontal take off. The lower the TTW ratio, the longer the runway needed to generate enough speed to create lift. But consider a Harrier Jet - it’s TTW is significantly lower than a rocket, and I would presume less than the F-15 you mention - and yet it is still able to generate enough thrust for a perfectly vertical take off. One reason we have horizontal flight is that wings use air (via lift) to reduce the energy required to keep a plane suspended and moving forward. Rocket powered, wingless planes would require so much fuel and energy that it would not be practical for long, routine, sustained flights.
In fact, the example of a Harrier Jet is perfect proof of why the treadmill analogy would work. Why? Because the Harrier doesn’t even need to create lift over its wings to take off - the primary argument of why a plane couldn’t take off from a treadmill. The Harrier “pulls” itself upward through the air, using the stationary air alone (i.e. the ground, ground speed/motion, is completely irrelevant). So, if a Harrier can pull itself through the air without regard for the ground, the same principle applies to a forward moving plane on a treadmill. Once the plane begins to pull itself forward through the air with enough speed (relative to the air) it will create lift with the wings and take off.
Now, in the redux of this scenario, Cecil and readers begin to tinker with normally insignificant forces such as inertia and friction by increasing acceleration exponentially. In doing so, I can’t necessarily say the situation will still hold water because at some point, if you infinitely increase the force of friction or inertia, you will begin to overcome the thrust capability of the planes engine and thus prevent take off. But as the original scenario was posed, I see no problem for flight.
It sort of frightens me to even post on one of these threads, but I’m going to try one more way to get people to BR#1.
As I’m sure you’re aware, most planes retract their landing gear (the wheels) after takeoff. Thinking about that may help you understand that the engines make it go forward without the help of the wheels, whether it is flying or is still taking off, with its wheels touching the ground.
What, in your mind, is the difference in the physics of a plane flying over a treadmill with its landing gear stowed and a plane that still has its wheels in contact with the treadmill?
It is just the small frictional force transmitted through the wheel bearings, right? Is that small force enough to counteract the force of the engines, which is clearly enough to keep the plane flying when it isn’t touching the treadmill?
Do you imagine that if the wheels-up plane were to lower its gear while flying low over a treadmill that it would be stopped in mid-flight?