And what you guys are not seeing is that the above statement isn't true. The speed of the wheels is a direct result of the thrust of the engines. If the engines stop, the wheels stop. If the wheels stop, the engines have stopped. You cannot separate the two. Wheel speed is a result of engine thrust.

 

do the front tires not turn just as fast as the drive tires on your truck (2wd only!)???? case closed!!!!

 

In a situation where there is no wind, air speed will ALWAYS equal ground speed. But, you said "on a windy day" you could be sitting on the ground and still have air speed. In this scenario we have no wind, so air speed equals ground speed. That bird ain't gonna get off the ground.

 

Or...since we're talking planes and some of these guys seem to know planes...let's look at a plane flying into a headwind.

Let's say you have an air speed of 150 mph and you are flying into a 150 mph head wind. What is your speed in relation to the ground?? 0 mph. Say you speed up so you have an air speed of 200 mph, but the headwind also speeds up to 200 mph. What is your speed in relation to the ground?? 0 mph.

The conveyor belt is exactly the same as the headwind...and since the plane is sitting on the ground and there is no wind then air speed and ground speed are equal. If the wheels are going 150 mph and the conveyor is going 150 mph, then how fast is the plane moving in relation to the ground?? 0 mph. Say the plane speeds up to 200 mph, but so does the conveyor. How fast is the plane moving in relation to the ground?? 0 mph.

With no ground speed you get no lift under the wings. With no lift, the plane doesn't leave the ground.

Does THAT make sense??

 

i'm back because i am frustrated

this statement is INCORRECT!!!

when the plane lands and applies reverse thrusters, the wheels do not spin backwards.

imagine riding a bike with training wheels. you provide acceleration due to the chain being connected to the rear TIRE which is pushing off the ground. the training WHEELS provide stability, as we all knew. if perhaps we were riding our bike with training wheels and came across an area where the two bike TIRES were still on the road but the training WHEELS came upon their own conveyor belt (and these two conveyor belts applied the same rules as this dreaded airplane), the bike TIRES would still provide forward acceleration but the training WHEELS would be going "nowhere".

at times the training wheels are in direct correlation to bike tires, but not always. you can lock the brakes on the tires but the wheels will continue to spin until the bike comes to a complete stop.

the analogy?
the bike tires are your turbines/jets/props to make it go. the training wheels are the wheels of the plane to provide stability and safety. granted in a plane the brakes are on the wheels and they have reverse thrusters, two forms of negative acceleration (NOT deceleration).

if you want to get into friction than apply it to the conveyor as well. it could never act instananeously!

no one will answer how a float plane will fly? it doesnt have a wheel to accelerate it, so how can it? how does a plane continue to fly in the air when its apparent reason for acceleration is lost? where is the driveshaft or gearset to the wheels to accelerate it?

the real reason the plane moves is the ENGINE/JETS/TURBINES that pull on the air to create thrust. why would they measure "thrust" of these if all of its power was transferred to the wheels? why not HP or TQ? because they dont transfer it to the wheels, its to the air. the same reason you feel a breeze from a fan on a hot day is how they impellers/propellers work!

the plane flies and i continue with my job building more airplanes...

 

OK my last post on this.
The myth is rather vague, so I will break it up into possibilities.

First off, does the conveyor belt go backwards at 'exactly' the same speed that the plane moves forward relative to the ground?

If you assume yes, and that seems to be the version we are assuming, then when the plane's engines generate enough thrust to accelerate it to 140MPH, then it will have reached minimum airspeed, and be able to take off. The conveyor belt will be moving backward at 140MPH, so the wheels will be spinning at 280MPH effective. The extra drag this will create will slow down the plane's acceleration, but not stop it, so eventually it will reach airspeed and take off, assuming that this magic conveyor belt is long enough. No mystery here, so I doubt that the myth ever intended to be like this.

The other possibility is that the conveyor belt is assumed to go backwards fast enough to completely negate the forward thrust of the airplane's engines with the drag induced by the rapidly moving belt. Thus the plane stays at the same geographical location (flag pole next to wing tip the whole time). Given that the plane is not moving compared to the earth, this makes for a more interesting question of can it take off. But, before we get to that, we have to decide if it is even possible to create this condition.

Since an airplane of any type gets its forward momentum from propelling air backwards, not torquing its wheels against the ground like a car, it will still try to accelerate. To prevent the plane form moving forward simply by moving the conveyor belt backwards, you must generate enough drag forces to completely counteract the thrust of the airplane (propeller or jet, doesn't really matter). The only major drag forces that the moving conveyor belt cause against the airplane are rolling friction in the wheels, and air resistance from the entrained air in the boundary layer next to the moving conveyor belt. Since the rolling friction of the wheels DOES increase with rotation speed, it is theoretically possible to move the conveyor belt fast enough to create enough drag to completely counter the engine thrust. But in real life, the belt might have to move 500MPH, or even thousands of miles per hour! Not very practical for a belt that must support the weight of th airplane! The second component is the entrained air. Every time something moves through the air, a layer of air very close tot he surface moves along with it. The larger the surface, and the more smooth flowing the air, the thicker this boundary layer becomes. To make the boundary layer of the conveyor belt think enough to create enough air resistance on the plane, the belt would likely need to be several times the width of the wings wide, and ten to twenty times longer than it is wide. Having a conveyor belt 400 feet wide and 2 miles long, moving at hundreds of miles per hour, and able to support the weight of a jet plane sounds very unlikely. So No, it is not possible to create the myth conditions in a full scale test. It _MIGHT_ be possible in a scale model test.

If you were actually able to create the test conditions, would the plane eventually fly away? No. Since it needs air flowing over and under the wings to fly, in the first case, with small conveyor belt and wheels only drag, there would little to no wind flowing over the wings, so no lift. In the second, giant conveyor belt case, it is possible to generate enough wind around the wings to create enough lift to lift off the conveyor belt, but since the plane is still stationary relative tot he ground, it will not be able to fly away, only hover in ground effect.

 

We're not talking about a plane that is landing. We're talking about a plane that is trying to take off ona moving conveyor belt.

 

continue reading

***edit: the point of that is there is NO direct correlation between wheel speed and thrust. yes, at that moment there is, but not in a generality.

i think we need to build a giant conveyor...

 

u need to read your disclaimer. it applies here

 

You're forgetting one thing. In this scenario we said that the conveyor was designed to ALWAYS match the speed of the wheels. In theory, that is impossible, but that was a condition put on this scenario. With your scenario above it would be even more impossible because the drive wheel of the bike is pushing against the ground...not the air.

Again, we're talking about a plane trying to take off. Not a bicycle locking it's tires up. Apples and oranges. If you don't lock the tires up on the bike, the training wheels will ALWAYS go the same speed as the drive tire (assuming no slip, remember??).

Again, we're talking about taking off. Not landing. Apples and bananas.

Agreed, but a condition put on this scenario was that it COULD act instantaneously.

That has been addressed. It doesn't need a wheel to accelerate it. The wheels do not provide the thrust. The wheels DO provide resistance when the plane is sitting on the ground though.

Nobody is saying that the wheels drive the plane.

The plane never moves and I continue to think that someone who builds airplanes should know better.

 

Your first assumption is wrong. The conveyor moves at the same speed as the wheels. As such, the wheels can NEVER have a speed greater than that of the conveyor. The faster the plane TRIES to to go the faster the conveyor will go. Equal and opposite forces mean the plane doesn't move. You have effectively tied the plane to the ground.

THAT is the way the scenario was set up. Whether it's possible or not doesn't matter. As you said, given the scenario we have...and that there is no wind....the plane stays at the same geographical location....so it doesn't move. If it doesn't move it doesn't get lift. Without lift it stays on the ground.

Exactly!! But, we never said it was possible. It is just the scenario that Geico created for us. In the real world this could never happen. In theory though, the plane would never move.

Bingo!!!! FINALLY somebody gets it!!!!

 

every one chant!!! HOSS! HOSS! HOSS! HOSS!

 

Whew!!! I'm tired. I need to go take a nap now.

 

1. you answered it correctly in the first quote: "THE WHEELS DO NOT PROVIDE THRUST". the thrust is from the engines. F=ma. you have a Force which is thrust, you have a mass of the airplane, and you now have an acceleration.

2. all the non-believers that the plane wont fly are essentially saying the wheels drive the plane. they are free wheeling like the wheels on the front of a 2wd rear wheel drive pickup. they can brake but they cant provide forward/reverse movement. ***edit: im assuming the resistance of these wheels doesnt come into play, just like the resistance of the conveyor belt spinning at an awesome rate doesnt come into play.***

3. i will continue to build them but this thread keeps getting in the way...


agree to disagree

 

1. Correct. They don't provide thrust. They DO provide RESISTANCE...and THAT is what is keeping this plane from moving. Force = Mass x Acceleration, but you have no acceleration because the force of the engine is negated by the force of the conveyor belt pushing BACKWARDS on the plane.

2. NOBODY has said that the wheels drive the plane. They are definitely free wheeling as you say. However, the rolling resistance provided by the wheels because of the conveyor will prevent the plane from moving.

3. Maybe you should go back to building them and leave the debate to those of us that have nothing better to do than sit around and ponder on these sorts of things.

Hmmmm....who can we get settle this once and for all??