The moving runway thought puzzle

[J D 8]

Plane on a conveyour belt  by Mythbusters avaiable on Youtube 

 Your first assumption is wrong LHF. Plane does not care if on static or moving surface, it moves forward and lifts off in the normal way.

[PatMc]

7 hours ago, Robert Cracknell said:

No, because the aircraft will rise due to the airflow over the wing not the rotation of the wheels, Unless the conveyor is fitted with some sort of blade to move the air the wing is effectively in still air. If this worked aircraft carriers would not need catapults.

 

 

 

 

Aircraft carriers don't necessarily need catapults, in fact the very first flight from a ship didn't have a catapult assisted launch. 

[Philip Lewis 3]

"Your first assumption is wrong LHF. Plane does not care if on static or moving surface, it moves forward and lifts off in the normal way."

 

Correct because the force from the airflow is against the static air behind it, the difference is airspeed V groundspeed, the wheels, assuming no friction at all, (unrealistic but not a lot anyway) would simply turn and the plane would not move, bit like if you are standing on a piece of floating wood and try to jump forward, the wood will simply move in the opposite direction and the lighter and less resistance it has will mean you will move forward less and less.

 

When the belt starts up if the wheeels offered no friction at all the plane would not move as there would be no force acting against the mass of the plane, (think of it as like the trick of pulling a table cloth away from underneath a load of crockery on top of it), if then, when it throttles up the force of the AIRFLOW against the static air behind it would then move it forward RELATIVE to the static air, the speed the wheels are turning is irrelevant.

Edited March 30, 2023 by Philip Lewis 3

[Martin Harris - Moderator]

I suspect that this question will always divide opinion because people aren’t reading it analytically. 
 

The question states that the machine is designed to match the wheel rotation. If energy is being used for thrust on the airframe and the airframe isn’t moving forward then it must be being diverted to powering the machine - energy cannot simply disappear. 
 

OK - but how is this achieved?  Well, I don’t think there is any method of doing this so the question is invalid and therefore there is no correct answer. 

[PeterF]

The plane takes off because the thrust against the static air is much greater than the rotational friction of the wheels. If wheel friction was significant then planes would not get airborne. There is no direct connection between the thrust generator and the wheels. The wheels only rotate when the plane is moving forwards. The belt matches that speed. Therefore if the plane is moving forward at 10mph the belt matches that backwards. The wheels are effectively turning as if the plane was travelling at 20mph. The only time the plane may take off is if the wheel friction was so large compared to the engine thrust that without the belt running the plane only just reached rog velocity.

 

If you like these sorts of questions, you are going up in a frictionless lift with a lit candle on the floor when the wire raising the lift breaks and the lift falls down the shaft, what happens to the candle and why.

[Philip Lewis 3]

The answer is nothing, because in the absence of any ventilation there is no airflow over the candle flame.

[Wookman]

8 hours ago, PeterF said:

The wheels only rotate when the plane is moving forwards. The belt matches that speed. Therefore if the plane is moving forward at 10mph the belt matches that backwards. The wheels are effectively turning as if the plane was travelling at 20mph.

That is what I said. 

[Nigel R]

the model is actually also being suspended by a thin piece of string 

as this ridiculous scenario starts, the model is raised up

the conveyor belt is instantly rendered irrelevant

conveniently, it is also a massively overpowered 1/3 scale 3d extra

model flies off

everyone is happy

except the 40% of the forum users who think 3d models are heretical

 

[Engine Doctor]

 

Silly discussion really as flight for fixed wing relys on air flow / speed over the flying surfaces.

Sitill a good discusion with a few pints thrown in at the pub 🥰.= plenty of pints = anything possible. 

Almost like asking if a man weighs 12 stone and he can lift 6 stone in each hand , if he then stands in two buckets can he lift himself up ?🙃💪

[Dickw]

11 hours ago, PeterF said:

...................

If you like these sorts of questions, you are going up in a frictionless lift with a lit candle on the floor when the wire raising the lift breaks and the lift falls down the shaft, what happens to the candle and why.

The shape of the flame as we normally see it is created by convection air currents, These convection currents are caused by the candle heating up the local air relative to the rest of the air and gravity moves the air because of the now different air densities.

In a falling sealed box like lift there is effectively no gravity, so no convection currents, so nothing to shape the flame and it will adopt a random shape – probably tending towards spherical.

 

Interestingly there is now nothing to hold the candle on the floor either, so that might also start to move around propelled by the flame 😀 (think of it as a small very inefficient rocket motor)

 

Dick

[Nigel R]

Quote

flight for fixed wing relys on air flow / speed over the flying surfaces.

 

Says who!

 

Wheels? check.

Wings? check.

VTOL? check.

 

 

 

 

[Alan Hilton]

14 hours ago, J D 8 said:

Plane on a conveyour belt  by Mythbusters avaiable on Youtube 

 Your first assumption is wrong LHF. Plane does not care if on static or moving surface, it moves forward and lifts off in the normal way.

Quite right the wheels just rotate at double speed .See mithbusters on youtube

[Christopher Wolfe]

To reverse the original question.

 

If one was to land on a conveyor belt that was travelling in the in the opposite direction to the aircraft's heading, would the landing run be shortened?

 

I suspect not by much, it would depend upon the friction in the wheel bearings and not matter very much until the wheel brakes were applied.

 

This is sounding a bit like the dreaded 'Downwind Turns' debate that raises it's head on a regular basis 🙃

 

Chris

[Brian Cooper]

People seem to assume that the aeroplane is solidly attached to the runway. 

The runway and the aeroplane are two completely different entities. 

 

The aeroplane couldn't care less if the runway happens to be a conveyor belt.... the aeroplane will independently move forward under its own power, it will gain flying speed and it will takeoff regardless of the speed of the conveyor belt. 

 

The worst that can happen is the wheels might be turning a bit faster. 

[Cuban8]

QED......I think...

https://www.youtube.com/watch?v=YORCk1BN7QY

[Robin Colbourne]

Here's a light aircraft in a strong wind that takes off by itself.  No forward movement, no propeller thrust, just the airflow over the wings.   Light aircraft gets airborne in a strong wind.

 

and here's a hang glider doing something similar:  Hang Glider vertical take off in a strong wind

Edited March 31, 2023 by Robin Colbourne

[Martin Harris - Moderator]

But as I said before, the question states that the machine is designed to match the wheel rotation.  If this condition is satisfied, whatever the real world physics of an aircraft on a moving runway, there can be no forward motion relative to air that the moving runway is located in.  The aircraft and runway start in a static state.  Thrust is applied but unless the whole runway is moved, nothing can happen other than lots of hot wind being produced behind the engines or propellers.  Say the aircraft moves an infinitely small amount forward...the runway has to move the same infinitely small amount backwards but this breaks the conditions of the question as there must be an element of mismatch to start any movement.

 

None of the demonstrations completely match the conditions laid down.

[John Muir]

If I remember correctly the the problem itself is invalid because it contains a physical impossibility. It states that the conveyor belt matches the rotational speed of the wheels, but this simply is not possible. The rotational speed of the wheels will be defined by the speed of conveyor belt plus the takeoff speed of the aircraft. It can't be anything else because you cannae change the laws of physics (captain). It is therefore impossible for the conveyor belt to match the speed of the wheels. No matter how fast you make the belt go the wheels will always go faster.

 

 

[Alan Hilton]

The crux is that the wheels don’t drive the plane it’s the prop/ turbine .If the plane was replaced by a car the the car wouldn’t move because it’s driven via the wheels .The plane  will move the wheels just rotate more quickly

[J D 8]

             Motor cycle jumper needs to hit the ramp at 40mph to clear the three busses.

             If he were to accelerate along a a conveyor that was going 20mph in the other direction he would have to hit 60 to clear. Were the conveyor going 20 toward the busses he would only need to reach 20 indicated or over shoot.

  I was at a show when a motor cycle jumper went way to far over 10 busses missed his landing ramp and came down very heavy, some how he stayed on until the bike slowed down and fell over with him holding his nether parts.

  Just so happened my nephew who had knocked himself silly crashing his Gocart earlier that day and ended up in the hospital bed next to the jumper. Flattest, blackest set of man bits you ever saw ha said.

[Alan Hilton]

3 minutes ago, J D 8 said:

             Motor cycle jumper needs to hit the ramp at 40mph to clear the three busses.

             If he were to accelerate along a a conveyor that was going 20mph in the other direction he would have to hit 60 to clear. Were the conveyor going 20 toward the busses he would only need to reach 20 indicated or over shoot.

  I was at a show when a motor cycle jumper went way to far over 10 busses missed his landing ramp and came down very heavy, some how he stayed on until the bike slowed down and fell over with him holding his nether parts.

  Just so happened my nephew who had knocked himself silly crashing his Gocart earlier that day and ended up in the hospital bed next to the jumper. Flattest, blackest set of man bits you ever saw ha said.

Yes because the motorbike is driven by its wheels

[paul devereux]

1 hour ago, Alan Hilton said:

The crux is that the wheels don’t drive the plane it’s the prop/ turbine .If the plane was replaced by a car the the car wouldn’t move because it’s driven via the wheels .The plane  will move the wheels just rotate more quickly

I think this may be the solution. 

The wheels are immaterial to the problem. If they weren't turning, the conveyor belt would be stationary. If they were turning at such a speed as to make the conveyor move 100mls per hour (backwards), the plane would still be stationary (although they would only turn if the plane was moving). So it is only the force of the prop that moves the plane. So if the plane reached a velocity of 100mph...nope, I'm stumped again.

[J D 8]

                  The thrust from prop has no connection to the conveyor. The plane wheels are not driven and are free to turn at whatever rate they need until plane reaches flying speed and lifts off.

[Alan Hilton]

1 hour ago, J D 8 said:

                  The thrust from prop has no connection to the conveyor. The plane wheels are not driven and are free to turn at whatever rate they need until plane reaches flying speed and lifts off.

Exactly

[Andy Stephenson]

There's a lot more common-sense talked about this ridiculous conundrum here than there was over on RCG. It even came to a point where the mods had to step in and suspend members for attacking each other.