Flight and the Conveyor Belt

Cecil's Columns/Staff Reports
1

Cecil, always enjoy your column, however you’ve got this absolutely wrong

So, if I tie a seaplane to a mooring in a rapidly running river I had better be careful because the aircraft might fly away? How far would it go? Oh, this example doesn’t apply because the engine isn’t running. I thought all we needed the engine for was to keep the aircraft in place as the conveyor moved beneath it. Replace the conveyor belt with the rapidly flowing river and the conditions are met for the aircraft to take flight. Who needs an engine? Just as long as the ground moves beneath the aircraft it will fly, right? Are you sure?

Cecil, the answer is in your column, but you fail to identify it. Regardless of means of propulsion, unless the wing moves through the air no lift is generated. Just because the ground moves beneath the aircraft does not matter and only distracts those who ponder this question.

Think of Airspeed as meaning the speed of the wing through the air.

When an aircraft takes off “two” things occur: movement over the ground and movement through the air. Airspeed. Without airspeed the result of which moves the wing through the air no lift will occur. The only thing you really is airspeed. Don’t confuse movement over the ground with movement through the air. The aircraft on the conveyor though providing the necessary propulsion to remain stationary as the conveyor moves beneath it does not provide movement of the wing through the air or airspeed. No forward motion. No motion through the air. No airspeed. This is the heart of the question plane and simple.

Take this into account. Let’s suppose an unlikely, but poignant example. Let’s say an aircraft attempting to take off has a tailwind that continually equals the aircrafts forward speed over the ground as it accelerates. No movement of aircraft would be met with no tailwind. However a speed of sixty mph would be met with a tail wind of sixty mph and so on. This would make the airspeed relative to the wing constantly remain at zero. No lift will be generated because there is no airspeed. Hence, the aircraft will not take off no matter how fast the ground moves beneath it. This is in essence the argument and also goes on to explain why an aircraft takes off into the wind: to shorten the take off distance by taking advantage of the airspeed of the air over the wing even before movement over the ground commences.

Conversely, if a headwind of sufficient velocity occurred as an airplane attempted to take off that matched the airspeed the aircraft needed to leave the ground and remain in the air; the aircraft could fly through the air and remain stationary over its initial position on the runway. No movement relative to the ground is necessary. If after lift off the wind suddenly died and the aircraft was of sufficient height to recover from the sudden decrease in wind speed, the aircraft would continue to fly by moving its wing forward through the air and subsequently over the ground. It would continue to generate airspeed though its speed relative to the ground would change.

An aircrafts movement relative to the ground is not enough to make it fly. Period.

Think windshear.

In this case an aircraft is moving rather swiftly relative to the ground at the time a severe wind shift occurs, this being most dangerous when the aircraft is taking off or landing. Though movement over the ground never ceases the movement of the air relative to the wing - in this case a headwind becoming a tail wind - causes the aircraft to crash because the air over the wing is suddenly not of sufficient velocity or airspeed to generate the lift needed to keep it airborne. This also explains why aircraft have airspeed indicators used to accurately measure “airspeed”. Without it an aircraft cannot fly. The aircraft on the conveyor generates no airspeed though its propulsion system manages to keep it stationary on the conveyor and therefore cannot fly.

I’ve been a pilot for over thirty years and am currently employed by a nationally recognized air carrier.

2

Link to the column?

3

I read this in a City Weekly and just about choked on my lunch! hehe I’m glad Strafe responded. As he stated you correctly mentioned you have to be careful how you frame the scenario. In the column you changed it once, made a correct observation regarding the wheel speed, then did not recognize its all about your frame of reference.

As Strafe stated, if the belt speed matched the airplane speed there would be no movement relative to the air, hence no airflow over the wing, no lift, no flight. The airplane is only moving relative to the belt and nothing else.

In the original scenario, I could sit on a ladder OFF the conveyor belt and hold the end of the wing of the airplane. The pilot could fire up the engines and go 600 knots relative to the belt but I could still sit there and hold the wing. No airflow due to no forward movement relative to anything OFF the belt. This is the same as running on a treadmill. Do you feel a wind blowing against your face running on a treadmill? No. You are only moving relative to the belt and nothing else. As Einstein said…its all relative to your reference system. The airplane would NOT fly.

I expect a correction in the next City Weekly and acknowledgment of the brilliance of Strafe and myself. BHAAAHAHAHAHA.

4

Oops, having thought about this a little more I see that I am wrong.

However, you have to make a couple of assumptions. If you do the aircraft could certainly fly.

I’ll say no more having embarrassed myself enough.

5

Okay, let me clarify things here.

First, forget about the damn conveyor belt and wheel speed. What a red herring, sheesh. Just tether the damn plane to a tree. All you’re trying to do is keep the plane from moving forward with respect to the ground, presumeably (that’s what treadmills/conveyors do). The conveyor is misleading: a plane will move forward as it pulls itself through the air (of the runway): the wheels on takeoff, after all, are not powered, they roll only because the plane is pulling itself forward. Running the conveyor will not slow the plane down, or keep it from rolling off the end of the conveyor. C’mon, think about it.

Whether the plane has any lift, and actually gets airborne, depends on whether there is a sufficiently smooth flow of air at sufficient speed over a sufficiently large amount of the wing surface. In most planes, the engines are arranged to mostly pull/push lots of air through, providing thrust, which then (upon attainment of sufficient air speed, beyond stall speed) begins to lift the plane. This air speed is attained by the plane zipping down the runway. The thrust of the engines does not translate directly into lift, only into forward air velocity, which then (playing upon the entire wing surface), translates into lift.

However, I imagine it could. An old multi-engine prop plane, with 4 (8, 16) engines producing a reasonably laminar flow of air across the wing might be able to produce sufficient lift to raise the plane. To say it another way, a plane will fly at a certain air speed, in smooth enough air. It doesn’t matter if that’s produced by a headwind on a front-tethered glider, a fan in a wind tunnel, or the engines of the plane itself. BUT the configuration of most modern planes WON’T fly in the (rear) tether mode because the engines are arranged to produce thrust (forward) and not a laminar flow on the wings which would result in lift.

Well, at least I’ve convinced myself.

Keith

6

First of all, the columAn airplane taxies in one direction on a moving conveyor belt going the opposite direction. Can the plane take off?, dated today, but not indexed on the homepage yet.

Second, Cecil is right, or at least as “right” as you can be when discussing a nonsense question (which he correctly identifies as such).

You guys are wrong who are saying, “Let’s just replace the conveyor belt with a tether.” The question isn’t, “An airplane is tethered in place, while its engines run. Can the plane take off?” and Cecil clearly demonstrates that the two questions are not equivalent.

7

An important part of Cecil’s treadmill analogy is that the wall that the rope is attached to provides a force in reaction to one’s pulling on the rope.

Let’s isolate the forces acting on the plane. (No wind; the air is stationary relative to the ground.)

Gravity pulls the plane down. The wheels and struts in contact with the ground provide a normal force upward. At first, there’s no lift, so the plane doesn’t move vertically.

The plane’s engines throw air out the back. The air reacts (Newton’s third law) with a force pushing the plane forward.

The conveyeor pushes the plane backwards at its wheels by rolling friction. The wheels react by spinning faster; there’s ideally no friction in the axle. However fast the conveyor moves, the wheels react to cancel out the force imparted on them. The axle is fixed to the plane, so the wheels will feel the same force forward as the rest of the plane. The net force on the wheels is equal to the rest of the plane’s body.

So there’s a net horizontal force forward. F=ma. The accelerating plane moves air over the wing surfaces. The wings generate lift. Takeoff.

There’s an added angular momentum in the wheels, but that’s probably not significant given how much more massive the plane’s fuselage and wings are compared to its wheels.

8

Sorry, Cecil, the name is Albert Einstein.

The salient issue on this conveyer-belt problem is a question of lift which is derived by airspeed – the wind beneath our wings.

The first problem is there seems to be the assumption that airspeed=groundspeed which is absolutely incorrect. The difference of a stationary aircraft on a runway poised for takeoff with 15 knots of wind streaming under the wings even though the plane has a groundspeed of zero and one facing the opposite direction with a -15 knots of airspeed and a groundspeed of zero could well be the difference between flying and crashing at the end of the runway. This difference is also why airplanes fall out of the air whilst flying through windshear and explains why, when flying in the winter in the US, it takes significantly longer to fly upwind to LAX than downwind to LGA.

Whatever. The root problem here is an unstated assumption is being made: That the air within which the aircraft’s wings reside is moving like the conveyer, not like the (further assumed) stationary air around the conveyor belt.

So it all devolves to a question of Relativity as per Uncle Al:

If the air surrounding the wing is moving relative to the conveyor at, say, 135 knots, the aircraft will rotate and fly. If the wind moves relative to the observer at 0 knots, the wings will droop normally and wonder what all the hub-bub is about.

The rest is all herring red.

9

Einstein didn’t invent relativity. He merely reconciled it with the negative results of the Michaelson-Morley experiment (and with certain theoretical implications of Maxwell’s wave equations).

For the rest, since it is not at all peculiar on this planet to have weather conditions in which the wind speed is negligible compared to airplane rotation speeds, Cecil is quite right not to have introduced this quibble.

10

The question is only a question because it was framed so badly. The real question is: does an airplane fly because of its engines or because of its wheels? Stated like that, the answer is obvious.

The secondary question, which is still interesting, is: does a plane (specifically, a passenger jet, not a Harrier jumpjet) need to move forward to fly, or can it rely on its engines to go up?

Keithwins has answered to his satisfaction that a plane which pushes air across the wing (e.g., old front-mounted props) might lift even if not allowed to move forward, but a modern jet would not, because it relies more on that pushing forward. I’m not convinced. I think even a prop plane might get a little light on its wheels, so to speak, but I doubt it would go up. It is a plane, not a rocket.

To which one could add: with *enough * thrust, the plane would go up regardless. Hence the jumpjet.

11

For me the issue is this…

…and this…

…that confuses me :confused:

I understand that what makes a plane lift off is the movement of the wings through a flow of air from which an upward thrust is created. As a plane moves across a runway, the wings move through an increased flow of air and lift is generated.

If the wheels were fixed then the conveyor belt could drag the plane back with the same force the engines move it forward and it would be stationary. If there was no wind, then no lift.

Now, if the wheels were rotating, it gets awkward for me to visualise. What if the plane is moving across the ground at 20mph and suddenly the ground underneath moves at 20mph in the opposite direction? It seems to me it would be the same as a sailing boat moving up a lake and suddenly moving onto a river flowing quickly backwards. There would be a point at which the wind would be insufficient to make the boat move against the flow of the river. How would that compare to the plane on the conveyor?

12

This can only end in tears.

13

Damn you, Cecil. Now the baby’s crying again.

14

Engines do not make airplanes fly, whether they are propellers, or jets.

It’s the wings. It’s ALWAYS the wings. The engines are there to push the airplane fast enough to push sufficient air over the wings to generate lift.

Airplanes without wings are called missiles – they fly, but not for very long (propulsion is a lot less efficient way to fly than aerodynamic lift). Airplanes without engines are called gliders (sailplanes).

Gliders fly because somebody tows them up to flight speed, then, depending on the skill of the pilot, the glider trades off airspeed for altitude, takes advantages of winds and thermals, and flies. All without engines.

So let’s restate the problem:

A glider is being towed behind a car, which is on a big conveyor belt. The conveyor belt turns to counteract the forward speed of the car, such the car never moves forward or backwards along the runway. Does the glider take off?

NO! Because there is never any net airflow over the wings. This is exactly the situation for the airplane.

Okay, so I’m exaggerating when I say it’s ALWAYS the wings. There are always exceptions: the Harrier, et al, redirects the engine exhaust to push itself directly off the ground. So in some extreme cases, there will be some component of the engine exhaust velocity normal to the conveyor belt, which might generate a little bit of lift. But that’s splitting hairs.

15

Nobody out there at all with a treadmill and a radio control model plane? Seems there must be a way to settle this empirically…

FTR, my $0.02 is with the no-airspeed, no-lift camp. Although if this goes on long enough, the stakes could rise well beyond a coupla lincolns.

16

That’s what I had thought and it seemed settled that way in GQ until this thread started again :smack:

17

Here is a previous thread on the same subject which has generated over 400 replies.

I don’t think we’re going to come to a concensus here.

18

Pre-coffee post here, so please bear with me.

First: A propeller does move air over the airframe. Only (let’s assume a single-engine aircraft) it does not push air over the entire span of the wings. So let’s assume a single-engine jet. A-4 Skyhawk, BD-5J, whatever. Air goes into the intake and comes out the back as thrust, with no flow over the flying surfaces. Let’s further assume that the experiment is taking place outside on an absolutely calm day.

So you fire up the turbine. The aircraft tries to move forward through the airmass. But the conveyor belt moves beneath it at the same speed, so the aircraft remains stationary in space.

If the aircraft remains stationary in space, and thus the flying surfaces are not moving through the airmass, where does the lift come from?

19

“A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction). Can the plane take off?”

OK, OK, I think the explanation might be a little unclear…

If you RUN on a treadmill, YOU are stationary. But how can you be stationary if you are running? Because your GROUNDSPEED is 0 MPH. Because you are propelling yourself forward at the same speed the treadmill is going backwards. Now suppose you start running twice as fast as the treadmill, you are going to move forward into the wall or whatever. Because your GROUNDSPEED is now 10 MPH.

Now, read the sentence carefully, it’s all in the semantics, the plane MOVES in one direction, ie, it’s GROUNDSPEED is 160 MPH forward, while the treadmill moves 160 MPH in reverse. The wheels are spinning at 320 MPH, reference Cecil’s comment about good wheels and bearings. So the plane is MOVING foward, GROUNDSPEED 160 MPH and can take off. It doesn’t matter what the treadmill does, quite simply because the wheels aren’t powered. The plane is powered by thrust from the engines which will move it forward to takeoff velocity regardless of what the runway is doing underneath the plane.

Further, if a seaplane takes off against a 60 MPH current, would it simply drift backwards 60 MPH? Of course not, it would move forward, GROUNDSPEED being 220 MPH at takeoff speed of 160 MPH AIRSPEED.

See? The treadmill goes one way, the plane goes the opposite, no where in the question does it say the airplane is standing still. As a matter of fact, if the engines weren’t on and the wheels were free-rolling, the plane could sit in one spot without moving. Unlike when you stop running on a treadmill and it launches you backwards into the nearest wall. Or Aerobics Instructor.

Frank

20

The whole point is that lift is aquired because the plane DOES MOVE FORWARD ON THE TREADMILL. Like he said, the wheels just move twice as fast, so the plane still moves forward. Read the damn column again, it’s very obvious.