Lets suppose we can turn a runway at LAX into a giant tredmill that has no top speed. If we place a 747 at the end, fire up the engines and throttle up, will the plane take off?
I’m pretty sure that a) this is not a ‘riddle’ and b) it was already pondered here in great depth…
No.
a) Sorry, “moderately interesting question for those of you who have Monday off like me and are looking for ways to entertain yourselff while you clean the house” was a tad long so I went with “riddle”.
b) Show me the money. Searched, got nothing.
Conceptual physics maybe?
I’m not sure, and I’m too lazy to search. I remember that it went onto multiple pages…
Treadmill-runway is a non-factor. The jet engines propel the plane forward, not the wheels. The plane takes off, the only difference being that the RPM of the wheels are considerably higher at lift-off.
The only case where it doesn’t take off is iff the engines are making the wheels turn, in which case, you don’t really have a plane, just a car with wings.
it’s air moving over and under the wings that creates the lift necessary to get the plane off the ground - ‘tredmills’ by definition don’t create forward movement . . .
now if you had asked about a huge mega-fan blowing at the plane, the plane would lift
Yikes…Everything you said is true, but your (implied) final conclusion is not.
These threads are fun to watch, and also a little scary.
How much force is applied to the wheels according to you?
Tisk, Tisk spot,
After your participation in this classic thread, I would have thought you’d change your tune…
Hahaha! You are right. I just reread that thread, and I stand corrected (again)!
Spot
“The only case where it doesn’t take off is iff the engines are making the wheels turn, in which case, you don’t really have a plane, just a car with wings.”
“Treadmill-runway is a non-factor. The jet engines propel the plane forward, not the wheels. The plane takes off, the only difference being that the RPM of the wheels are considerably higher at lift-off.”
Francois and Sphere are correct. Imagine you are running on the treadmill at any fixed speed. The treadmill is moving one direction, your legs are moving you the other direction at the same speed, therefore, you are stationary in space. Now someone comes up behind you and gives you a sharp push (like a jet engine). You move forward - faster than the treadmill is moving you backward - ie, you have forward motion generating lift ( if you’re an airplane).
on this LAX treadmill, if you get a push (as from jet engine, or if you are running like a push from a hand), the treadmill responds by going faster, and you don’t get lift.
if the treadmill does not increase with the planes speed, then you can just take the treadmill out of the equation and treat it like solid ground, in which case the riddle is no longer a riddle.
Francois and sphere are wrong. or the riddle is uninteresting.
Pure gold…
Riddle me this:
a) If a car is on a frictionless surface and spins its wheels, will it move forward?
b) If a plane is on a frictionless surface and fires up the engines, will it move forward?
What is the difference between the two forms of creating forward movement between a car and a plane?
P.S. The original riddle is interesting for precisely the reason you are stating the wrong answer.
on this LAX treadmill, if you get a push (as from jet engine, or if you are running like a push from a hand), the treadmill responds by going faster, and you don’t get lift.
But the treadmill has nothing to do with the lift, which comes from airspeed, not groundspeed. The airspeed comes from applying force to the air, not the movement of the ground relative to the wheels.
Maybe this will help…sigh…maybe not…
Think of standing on a real treadmill, wearing roller skates, and holding a fishing pole with your hands. Attach the fishing line to the wall. If you turn the crank on the reel, what happens to your body relative to the wall?
What difference does it make if the treadmill is running versus not running?
if the treadmill does not increase with the planes speed, then you can just take the treadmill out of the equation and treat it like solid ground, in which case the riddle is no longer a riddle.
See above. it really doesn’t matter what speed the treadmill is running, as the plane doesn’t apply any propulsive force via the wheels.
Francois and sphere are wrong. or the riddle is uninteresting.
Hmm.
Think about why aircraft carriers launch (and arrest, for that matter) into the wind. It has to do with maximizing the airspeed relative to the groundspeed.
The example that did it for me in the previous thread on this was if you had a buddy wearing roller blades on one of those airport moving walkways, and two folks on either side of him pushing him along…that made total sense to me.
Spot
a) If a car is on a frictionless surface and spins its wheels, will it move forward?
No - - ,unless it happens to be pointing downhill - - have you ever driven on ice?
b) If a plane is on a frictionless surface and fires up the engines, will it move forward?
Yes, but that’s a different question than what the riddle is asking . . . what the riddle is asking is: if a plane fires up its engines but does not move forward, will it rise (take off). The answer is No. The reason there is no forward motion of the plane is because the ‘ground’ moves with it - the ‘faster’ the plane ‘moves’, the ‘faster’ the treadmill moves. Until sufficient airflow exists, the plane behaves as any wheeled vehicle - it doesn’t matter if the vehicle is powered thru the wheels like a car or the wheels are a passive energy transfer like a 747 - - the wheels can not go faster than a treadmill with no top speed.
P.S. The original riddle is interesting for precisely the reason you are stating the wrong answer.
I would love to be wrong here, as there would then be a major loophole to solve all our energy problems.
if one assumes that the moving walkway has a top speed, or runs at a constant speed, it is easy to see how you could push your buddy faster than the walkway - -
but the OP stated that the LAX treadmill has no top speed -
Can you move your buddy forward on a treadmill with no top speed? The faster you push him, the faster the treadmill goes. If the treadmill has a top speed, then there is no riddle - the plane will take off - and your buddy will go flying into the concession stand.
If your buddy had aerodynamic wings, could you give him a lift if you can’t move him forward?
Remind me never to go soaring with any of you guys.
There are four forces acting on an aircraft in flight, be it a 747, or a beloved 304CZ glider:
- gravity pulling down on the aircraft,
2)lift from air flow acting on the wings lifting the aircraft, - forward thrust from either engines, or in the case of gliders converting potential energy to kinetic energy by decreasing the altitude (i.e. flying downhill), and lastly,
- drag due to air interacting with the wings and fuselage.
Scenario 1: Treadmill going in opposite direction of aircraft, full engine power. (Original post scenario)
Placing an aircraft on a giant treadmill, with the treadmill running opposite of the direction the aircraft is heading (as in the gym with a typical runner) will require thrust from the engines in order to maintain the position on the treadmill. If the treadmill goes fast enough, the aircraft could reach a speed relative to the treadmill that equals normal take off speed. However, at all times, there is no increase in the amount of lift the wings are generating, as the airspeed remains constant as the plane remains in the same position. Likewise, the amount of drag does not change (i.e. no drag), and the effect of gravity (which is always constant) remains unchanged. The sum of the forces is therefore just gravity pulling downward, zero lift, and zero net thrust, and zero drag.
Remember, it is speed with respect to surrounding air mass that is critical for flight.
747s probably top out around 500-550 knots, with take off speeds in excess of 140 knots. If the treadmill exceeds 400-450 knots, then there will never be enough air mass speed differential to generate sufficient lift from the wings.
Edit: This last statement is not completely accurate for a couple of reasons. The max velocity of a 747 is a combination of max engine thrust and air resistance/drag (just like us cyclists). However, on an infinitely fast treadmill, it would be the equivalent of accelerating in a vacuum, or outer space. There would be no increase in drag as drag remains zero. Thus, in this scenario, a 747 would probably continue to accelerate for as long as the engines were producing thrust.
Scenario 2: Treadmill runs in direction of runway, no engines
In the case in which the treadmill runs in the direction of the normal runway (e.g. like the people conveyor belts in airports), then if the treadmill exceeds minimum take off speed of the aircraft in question, it is possible for the aircraft to momentarily lift off the ground. In this case, the airmass is stationary, but the aircraft has been accelerated by other means. However, if no engine power is being used, once the aircraft has lifted off the ground, the source of acceleration is removed, and the drag on the aircraft will quickly reduce the airspeed of the plane. It will drop back down, in very short order, to the ground.
Thrust increases due to the conveyor belt, indirectly increasing airspeed which progressively increases both lift and drag until take off speed is reached (assumming the brake thingy business, see below), momentarily, until the loss of acceleration due to loss of contact with the treadmill decreases the required lift for flight.
This is not a likely scenario, either, however. As the treadmill is accelerating towards the aircrafts’ take off speed, the fourth force listed above, drag, is increasing. If no engine power is being drawn and the plane is just sitting on the treadmill, at a certain point, the relative airspeed created with respect to the treadmill will start pushing the aircraft backwards from its initial position on the treadmill. Then again, I suppose one could stand on the breaks until the plane lifted off.
Scenario 3: Forward running treadmill, full aircraft engine power = shorter, quicker take off.
In essence, the second scenario is precisely what aircraft carriers do when they launch aircraft. First, they turn into the wind, and then accelerate the ship, producing a wind over the deck of 30 knots or more. This reduces the velocity required by the catapult launch system. For example, if an F-18 Hornet take off speed is 170 knots (calm air), a headwind of 30 knots means that the F-18 only needs to produce the power required to reach 140 knots relative to ground speed.