Jason’s Case of the Plane and Conveyor Belt riddle is confusing very smart people, so I thought I might explain it. First, go read Jason’s post, then come back here.
So, a lot of people assume that the thrust of the airplane is somehow effected by the speed of the magical conveyor belt it rides on. Let me first say this: it’s not, and rather than talk about how the thrust only acts on the air, and not the conveyor belt (the two are totally unrelated) I’ll talk about something I know well: skateboards.
Picture this - you’re standing on a skateboard that is riding on a treadmill. One person is standing in front of the skateboard on firm ground, and the two of you are holding a rope. This person pulls on the rope that you’re holding so that the rope moves exactly an inch per second, advancing you forward. No matter what speed the treadmill is going, as long as that person maintains the same rate of pull, you’ll advance forward an inch per second. Your skateboard wheels might go faster or slower in relation to the speed of the belt, but you’ll pretty easily advance forward. Change the rope to a stick, and the conveyor belt can travel in either direction at either speed and be just as irrelevant.
The airplane’s engines provide the forward force, pushing against the air behind their outputs. The air is like the person standing firm (as firm as air can be) and the engines pushing against that air provide the same kind of force that someone pulling on the rope provides. In both cases, the speed of the conveyor belt has no correlation with the force that the rope or engines produce against the air or the person standing firm.
Granted, in both cases, wheel friction will come into play. With a skateboard and treadmill, friction might be noticeable. With the kind of forces a jet turbine can produce the wheels would probably melt off before the engines noticed anything.
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I recall a time when we were in a Price.com office together and I confessed that even though I had flown several times on airplane, how they actually managed to fly was a big mystery to me.
You drew some stuff on the white board explaining how the shape of the wings cause the airflow to make what I now can only recall as being a suction action that makes “an airplane flying on air about the same as a boat floating on water” (if I can recall things you said some ~6 years ago).
Now… that being said (that the airflow over the wings is what is vital to the airplan being able to fly or not), aren’t the wheels and the ground irrelevant? I would also venture to say that to some degree, the airflowing through the engines don’t matter either. As I understand… the engines pushing air is only half the equation, the effect of the engine *needs* to be that air is passing around the wings, else there would be zero _lift_.
I think if there was an airplane on a conveyor belt, the conveyor belt would have to move at an extremely high speed to counteract the engines pushing off the air propelling the plane forward. And if the conveyer belt did succeed in keeping the plain stationary, that the plane would then not have enough airflow over the wings and thus would not have lift off.
So my thinking is that the plane actually moving forward is vital. Unless the engines are aligned in some manner to pull air directly around the wings (above and below) to simulate the wing cutting through the air.
But that’s the thing - the conveyor belt can go a thousand miles per hour or two miles per hour - the thrust the engines produce will still push the airplane forward because they’re acting on the air, not the belt.
If you took a car, slapped some wings on it, and drove fast enough, you might lift off the ground, but that trip into the air would be short lived because the wheels were producing thrust by interacting with the Earth’s tendency to not want to move when wheels are rotated against it. That action has to have an equal and opposite reaction, and since the Earth ain’t gonna budge the wheels roll forward pushing the car forward. The very second the wheels leave the ground, the forward force is gone (because wheels spinning in the air tend to do just about nothing), and the car would quickly drop back to the ground. Put a conveyor belt under that car, counteract it’s speed with a belt moving in the opposite direction, and it would never produce airflow over the wings to produce lift, and therefore, it’d sit there.
And yes, all that junk I said about how airplanes fly is true, and of course still applies here. The wings move forward through the air because the engines push them, oblivious (totally and completely) of what the ground is doing beneath them.
Another way of looking at it - if you take off in a tail wind, it’ll take you a lot longer to actually take flight. That’s because the air, relative to forward motion produced by the force of the engines, is less than if the air were simply stagnant. Same principle works in reverse if taking off into a head wind (which most planes try to do if the wind is strong enough to have it become a factor). It’ll take less space to take off.
You want to think the plane won’t take off because the author of the riddle makes the ground such a big deal, when really, it’s about as important as to whether it’s night or day.
Mike, a plane taking off has everything to do with lift and little to do with forward motion. Sitting the conveyor belt, the plane would generate no lift since there is no increase in the air flowing over the wings. It cannot fly without lift.
I could not wrap my mind around this problem until I realized that the plane’s wheels can spin as fast as they want (and, as you said, in either direction) and it will have no effect on the plane’s direction or speed (friction aside). Thanks!
Even though I eventually got a B+ in college physics (and so I do understand this) I would have probably understood a lot sooner if you had been my physics teacher!
In the fantasy land of hypothetical situations, many things are possible. In reality, you will need to deal with: conveyor belt material (airplane tires are designed for concrete), weight of the airplane on the belt (planes are heavy, belts break), friction and counter-torque due to tire deformation (planes are heavy, tires deform), and wind resistance (there is wind in the real world). My bet is the plane nosedives or veers off the (concrete?) belt before ever reaching a lift speed.
Okay guys, I’ve only been a pilot for about 5 years. The plane will not lift off in this given scenario, unless the pilot mashes the throttles forward and runs down the conveyor belt to achive lift off speed. Granted, if he did that while the belt was full speed in the opposite direction his air speed indicator in the cock pit may show let’s say 100 knots his wheel speed might read 2 or 3 times that but the plane would not lift off until his liftoff speed was reached, regardless of wheel speed.
Here’s one to ponder>……………………….
How about the same airplane, with a pilot in it, No conveyor belt and a VERY large fan in front of the airplane capable of 600 mph winds
and let’s say you run that fan upto 200mph while at the same time the pilot matches your windspeed with thrust. Could the pilot in fact keep the plane stationary above a certain point on the ground while lifting off the ground?
Riddles, such as this one, serve at least one somewhat obscure purpose. I serve as a flight instructor for a model-airplane club and I use this aerodynamic riddle, and others, to sharpen my teenage-students’ interest in aerodynamic principles in general. Sneaky of me, perhaps, but it does work.