747 Conveyor Belt (again)

[pasir]

I feel quite stupid asking this which I assume has already been dealt with however I am prepared for plenty of flak. The question being - If a 747 is placed on or over a very wide conveyor belt that is travelling in the opposite direction to a/c wheels . Will it take off?

[TurningFinalRWY36]

no it will not, no forward motion = no airflow over the wings

[EESDL]

......you should receive a lot of Flak
You know when they say that ‘there is never a stupid question’?
They were wrong ;-)

[neilki]

It's a great question and needs to be better explained.
In theory (and it's just that because no one has a conveyer belt wide enough)
Yes.
The problem is with Accelerate/Stop distances. Should an engine fall below V1 the length of conveyor belt required to bring the aircraft to a halt will be greater than the length of the belt itself, and will result in an overrun.

As an aside, an Airbus won't takeoff, due to Control Law protections. They've tried to make them idiot proof...

[PDR1]

Oh gawd - not again!

Quick version - the question has no answer because it contains fallacies, and any attempted answer must make one of two assumptions to remove the fallacy (thereby making it an answer to a different question). The two optional assumptions will produce two different answers ("yes" and "no" respectively).

Summary:
The aeroplane will only take off if it moves with respect to the AIR. The aeroplane really doesn't care much about how fast the ground is rolling past beneath its wheels.

The principle fallacy is the idea that if an aeroplane is propelling itself forwards at 20mph on a runway that is moving backwards at 20mph that will stop the aeroplane. It won't. There might be a miniscule reduction in speed due to increased dynamic friction in the wheels, but it wouldn't really be measureable.

So that's the core fallacy in the question as it is usually phrased* - the rolling runway would not have the effect that the question setter claims, so the whole questi0on is just bunkum. To answer the question you need to assume either:

a. The rolling runway would stop the aeroplane; or
b. it wouldn't

In case (a) the aeroplane wouldn't move forwards, so it wouldn't take off.

In case (b) it WOULD move forwards, so it WOULD take off.

But either assumption makes it the answer to a different question.

Fiunal thought - I understand there is an bill going through parliament which will legalise hunting people who post this question on forums with dogs. Or at least if there isn't there should be...

:E

* "....on a conveyor belt runway configured to move backward as fast as the aeroplane is moving forwards" or "....on a conveyor belt runway configured to move backwards fast enough to stop the aeroplane moving forwards"

[FCeng84]

No airspeed, no lift, no takeoff. Wing does not care about wheel or ground speed, just airspeed and angle of attack!

[pattern_is_full]

On and over are different things - which do you mean? One means "wheels touching the belt" - the other means "wheels not in contact with the belt/ground" - a flyby.

How fast is the conveyor belt moving? 10 kts? 150 kts?

Aircraft fly (or not) based primarily on airspeed (excepting VTOLs/helos). Ground speed (speed relative to the ground - or in this case, the belt) is mostly irrelevant.

Scenario 1: 747 needs (let us say) 150 knots airspeed to take off. Assume conveyor belt is moving tailwards at 150 knots (effectively, a 150-knot tailwind). The 747 will still need to reach a total net airspeed of 150 knots, or 300 knots relative to the conveyor belt. And then it will fly (assuming the tires don't fail travelling at 300 knots along the conveyor belt).

Scenario 2: Assume the conveyor belt is moving 20 knots tailwards. Then the 747 will need to accelerate to 170 knots relative to the belt, for an airspeed of 150 knots (170 knots minus "tailwind" of 20 knots).

Scenario 3: reverse the direction of th conveyor belt so that it carries the 747 forward. Leave the 747 throttles at idle and the brakes set, and increase the belt speed to 150 knots. The 747 now has an airspeed of 150 knots, and can lift off (BUT will not retain its forward speed of 150 knots once it loses contact with the belt - the source of its speed - better throttle up quickly!)

[vapilot2004]

Answers to this question often seem to assume motive power comes from the wheel/belt interface, which as any pilot knows, does not. Cars and tractors, yes, aircraft, no.

The principal limiting factor in preventing forward movement is the friction of the rolling tires and wheels against the surface of the belt. Although quantifiable*, that friction is minuscule compared to available thrust.

*Rolling friction and brake drag are why a transport category aircraft will not begin rolling forward after brake release without applying thrust above idle.

It is helpful to picture the scenario from a point off the side of the aircraft. In order to keep the aircraft from moving forward with thrust applied, the belt would need to run at such a great speed that the friction of the freewheeling tires prevents the aircraft from moving forward. If this belt could be ran at such a speed, tire failure would occur prior to reaching the limiting forward friction requirement.

Also, it is helpful to recall thrust provided by the fan air and (lesser) turbine output works primarily in relation to the air mass surrounding the aircraft, not the ground upon which it sets.

[RandomPerson8008]

235 mph tire speed limit. The tires would all blow out, destroying the engines, wings and killing all occupants. Were it not for that, then yes, it would take off.

[vapilot2004]

See my post above, RP. We generally agree. :ok:

[tdracer]

Mythbusters did an episode on this a few years back - I'm sure it's on YouTube somewhere. They do a good job of explaining things in layman's terms.
But the short answer, as others have noted, is the conveyor belt doesn't do squat - airplane performance is governed by airspeed, not ground speed.

[Helix Von Smelix]

is this in still air?

[PDR1]

Assume conveyor belt is moving tailwards at 150 knots (effectively, a 150-knot tailwind)

No, it isn't. That's the whole point. It is effectively a zero tailwind but with some extra rolling friction in the wheels. Just that - nothing more.

Sheesh - how many times do we have to explain this!!!

[wiedehopf]

well it all depends on the frame of reference doesn't it.

if the runway start and end points are drawn on the conveyor belt and the plane starts stationary on the belt it is indeed a tailwind. but that's even less sane than that question.

also maximum wheel speeds can get out of the specs i'd say.

so might bust some wheels before taking off therefore not take off :)

now i feel silly having replied in this thread, oh well!

[RudderTrimZero]

There is one condition under which the 747 might be able to takeoff....

If you put the conveyor belt brakes on so hard that it launches the thing into the air.

[Mechta]

We have wind gradient due to friction with the surface in the real world, and wall effect in wind tunnels which is much the same. It is therefore reasonable to assume that a moving conveyor belt would induce some movement in the air above it. Clearly the belt would need to be going a lot faster than normal take off speed to induce an airflow at wing level that is at take off speed, and the comments about tyre speed would apply. The aircraft would need to be restrained at some fixed point ahead of the conveyor belt to prevent it being shot backwards until it is in the air or engine thrust carefully balanced to match the drag from the induced airflow acting on the airframe.

As all the others have said, it is airflow over the wing that generates the lift that allows a conventional aircraft to fly, not speed on the runway. Here is a hang glider proving the point:

https://www.youtube.com/watch?v=vas0nVTLyCM

[mustangsally]

You have to be kidding..... Someone has had a couple to many pints!

[NSEU]

Or has forgotten his flying basics...

Quoting a previous post by Pasir...

I was a Junior with KLM at Croydon Airport in '46 and would wander around what I feel certain were JU52s in various...

[megan]

Now, if the 747 was a cargo ship with a full load of canaries on board, and they all took wing at the same time, that there 747 would levitate off that conveyor belt in no time at all. :eek:

[PDR1]

We have wind gradient due to friction with the surface in the real world, and wall effect in wind tunnels which is much the same. It is therefore reasonable to assume that a moving conveyor belt would induce some movement in the air above it. Clearly the belt would need to be going a lot faster than normal take off speed to induce an airflow at wing level that is at take off speed, and the comments about tyre speed would apply.


Right, of course. A 40-foot-thick boundary layer... :rolleyes:


Here is a hang glider proving the point:


It's proving *a* point, certainly. But sadly I fear not one which is relevant to this question.

[Dont Hang Up]

Imagine a wine bottle lying sideways on a checkout conveyor.
When the belt starts to move the bottle shows a very strong tendency to remain exactly where it is by starting to roll. Indeed if we could ignore the pesky complications of rolling friction and rotational inertia it will never reach the cashier. The faster the belt goes the faster the bottle will roll.

A 747 with its engines not running would show a very similar characteristic. Nomatter how fast the belt goes it will just sit there - the wheels rolling faster and faster to match the belt speed (don't forget we are ignoring rolling resistance of the tyres or the rolling inertia of the wheels).

So now we start the engines. The engines generate reactive thrust by accelerating air through themselves. That thrust is applied directly to the airframe - nothing happening with the ground, the wheels, or the belt have any bearing on that force. So the aircraft begins to accelerate forwards. When it has accelerated to take of airspeed we can rotate and away we go.

[pattern_is_full]

(don't forget we are ignoring rolling resistance of the tyres or the rolling inertia of the wheels).

If you ignore physics you can fly by flapping your arms - but so what? ;)

Fly a lot of wine bottles, do we?

[PDR1]

These can be ignored because their magnitude is tiny compared to engine thrust. For some strange reason aircraft are designed to have low rolling resistance - something to do with the twin desires to (a) take off, and (b) not have the wheels melt in the process.

Of course it can make it more difficult to stop when landing, but the designers mitigate this problem by adding features like brakes and thrust-reversers.

Heck, most 4-engined jets won't even stand still in the asphalt with even two engines running at ground idle unless the brakes are applied - that gives an indication just how small the rolling resistance really is.

[Dont Hang Up]

You ignore them because it is a thought experiment (in fine Einstein tradition). It has no practical application but is intended to demonstrate the distinction between the ground and air frames of reference when considering aircraft dynamics. So you only consider the primary factors and ignore the practical limitations.

Once you start to include all of the practical limitations then you end up going nowhere other than arguing about what all those practical complications are.