This old chestnut

[RichardM]

I think the conveyor will mash the aircraft wheels and undercarriage prior to it reaching take off speed. There will be a big bang and the emergency crews will be called upon to put the fire out, unfortunately they too have conveyor belts fitted to the trucks wheels, as designed by a certain H. Robinson, but in this case there will be no forward motion! The fire trucks will not get to the plane which will continue burning.
Happily due to Covid and having conveyor belts strapped to the wheels there are no passengers inside, only the flight engineer, pilot and co-pilot. The emergency chutes activated and they all survived, but were later charged with gross misconduct, not for losing the aircraft but because they didn't take their shoes off before using the slide!

 

[mike os]

Does that mean it falls as soon as it retracts it’s undercarriage?

 

[Barking Mat]

Does that mean it falls as soon as it retracts it’s undercarriage?

Ooh interesting point.

It's the lack of momentum caused by the conveyor belt, which worries me. And are the engines powerful enough to cause that much lift on a stationary 747 ?

 

[AVI_8]

Ooh interesting point.

It's the lack of momentum caused by the conveyor belt, which worries me. And are the engines powerful enough to cause that much lift on a stationary 747 ?

Engines supply thrust
Wings supply lift
As long as an aircraft’s engines are producing more thrust than the total drag the aircraft is experiencing it will accelerate, (Airspeed) once the airspeed matches the calculated speed for take off (weight and altitude dependent) it can then take off, the wings will produce enough lift in order to overcome gravity.
It will then take off leaving the hypothetical conveyor belt behind irrespective of what speed the wheels are travelling at

 

[redlig]

They do, the anti-skid depends on it

I must look and see the speedometer the next time I pilot a Jumbo. Surely they use speed sensors not speedometers.

 

[AVI_8]

I must look and see the speedometer the next time I pilot a Jumbo. Surely they use speed sensors not speedometers.

They’re called air speed indicators

 

[redlig]

They’re called air speed indicators

i was talking about the wheel speed in the case of ALB NOT speed of air craft through the atmosphere.

 

[redlig]

I was just about to start discussing momentum.

Thank you.

Please Don't . It's difficult to stop once started.

 

[AVI_8]

i was talking about the wheel speed in the case of ALB NOT speed of air craft through the atmosphere.

Yeh each main wheel has a wheel speed sensor for the anti skid braking system if that’s what you’re asking

 

[Maker]

The only thing you know is that the conveyor is designed to match the wheel speed.

There lies the answer. If you assume it's an ideal conveyor which always perfectly matches the surface speed of the wheel, with no top speed, it's impossible for the centre of the wheel to move relative to the ground, the aircraft is stationary, no airspeed, no takeoff. Any thrust produced by the engines must be balanced by drag at the wheels' bearings, which means as soon as the engines start producing thrust, the conveyor starts moving very bloody quickly. No idea how quickly, definitely fast enough to burst the wheels...

If the conveyor is only designed to match the wheel speed, it won't perfectly match it, so the aircraft will move, and eventually take off.

 

[slim_boy_fat]

Imagine a 747 is sitting on a conveyor belt, as wide and long as a runway. The conveyor belt is designed to exactly match the speed of the wheels, moving in the opposite direction.

Can the aeroplane take off?

It's an old one but a goodie, once you know the correct answer it's a bit boring, but there are loads of variations on this question which have different answers...

I'll have some of what you've been having

 

[E T]

Imagine yourself running on a treadmill in the gym. That's a conveyor belt right?
When you put your hands on the handrails you can pull yourself forward with your arm muscles.
So now your arms are like the jet engines generating forward thrust irrespective to what your feet (wheels) are doing on the conveyor.

 

[Cobbler]

Surely the wheels speed must match the speed of the conveyor, they are sat on it, can’t do anything else!

 

[mike os]

Ooh interesting point.

It's the lack of momentum caused by the conveyor belt, which worries me. And are the engines powerful enough to cause that much lift on a stationary 747 ?

In terms of thrust, which is what moves it forwards, more than enough it doesn’t use the wheels to move, they just stop it rubbing on the ground. If an aerofoil moves through air lift is created…

 

[Munkul]

The problem is that the only thing you gain from the question is that the conveyor is moving and the plane is on it. Without further info, or extrapolation of likely events, you can't tell if the plane has airspeed or not.

Yes you can. The conveyor has speed relative to wheel speed. Not to the plane speed. The plane doesn't need wheel speed to take off, it just needs low rolling resistance.

 

[Munkul]

As I've stated the question, the scenario is not physically possible. Think about what the wheels and conveyor will start doing the INSTANT the plane starts to move forward.

 

[Pete.]

Yes you can. The conveyor has speed relative to wheel speed. Not to the plane speed. The plane doesn't need wheel speed to take off, it just needs low rolling resistance.

The original question only says that the conveyor is moving hence my original response that wheel speed is not important. At first reading I took it that they were asking if the plane would fly if the conveyor got the wheels up to speed. What it actually requires you to infer is that if the plane IS getting to take-off speed but the conveyor is holding the wheels stopped by it's motion will the plane take off? If you do take that as the question then yes it will of course, otherwise seaplanes would be boats.

 

[Maker]

As I've stated the question, the scenario is not physically possible.

It's entirely possible to design a conveyor to match the wheels speed, it just won't do it very well or for very long.

 

[sg66]

Mythbuster covered this years ago

Awful quality sadly.

Yes it will takeoff normally

Only scary bit is the fact pilot thought he wouldn't move let alone take off

 

[Munkul]

It's entirely possible to design a conveyor to match the wheels speed, it just won't do it very well or for very long.

Under the conditions of the question, as soon as the plane moves forward a fraction, the wheels and the conveyor belt start turning to infinite speed with infinite acceleration.
Think about it... the wheels and conveyor won't just be going "bloody fast", they will literally accelerate to infinity.

The plane moves forward and the wheels turn a little.
The conveyor moves backwards exactly the same amount.
The wheels have to turn a bit more.
The conveyor moves backwards exactly the same amount,.
All the time, the wheels and the conveyor are picking up speed. And in a true theoretical scenario, this isn't a nice steady slow increase with a delay in rate change - the MOMENT the plane moves from zero, the wheels and conveyor go to infinity.
The plane will still take off, if you ignore rolling resistance, but infinite wheel speed will also equal a infinite proportion of rolling resistance. That's also ignoring the inevitable mechanical failure.

Both infinite acceleration and infinite speed are impossible