Physics Question for you.....

[Russianwolf]

I have a question for Russian Wolf. Did you have an answer for this when you popped it to us?

My answer is on page 5.

She will fly.

It's a "trick" question in that the "trick" is the junction between the conveyor and the plane.

The wheels.

If the plane were dependent on the wheels for propulsion, it would not be able to fly as it would remain in one place.

But as the wheel are free spinning and not needed for propulsion, the plane can take off almost as normal (wheels will spin at twice rpm as normal but not impede the takeoff).

[scorrpio]

In my original statement I had same error as many others had - I was forgetting that plane rolls not due to torque applied to the wheels - but due to engine thrust. I about changed my mind, but started thinking more about the problem, and now the answer will be: depends on how much plane's landing gear can take.

Now, in order for the plane to keep moving forward, it has to overcome two forces: air resistance and friction in its wheel bearings. Gravity is not in the equation yet. Once plane gains some speed and oncoming air pressurises under the wings, main purpose of thrust becomes 'cramming' enough air under the wings to keep the plane aloft, but in the beginning, rolling friction is the main enemy.

Now, the only thing we know about the conveyor, is that 'it is tracking the plane speed trying to match it backwards'. It seems that system is essentially trying to keep the object motionless. If this system detects that object is moving forward, it accelerates the belt further. If the propulsion principle of the object is based on pushing off the surface it travels on (i.e. torque to the wheels, rowing a boat etc), then yes, the object will stay motionless.

However, the propulsion is independent from the belt. What happens? The object(plane) starts moving forward. The conveyor, detecting this, will increase its speed. Conveyor goes to 100mph, causing wheels to spin madly, yet plane keeps moving forward. Conveyor goes to 200, 300, 1000, 5000mph, trying to keep the plane motionless. How quickly does the conveyor accelerate? Does it have a limit?

Potential scenarios:
1. As RPMs increase, friction forces go beyond bearing ability to handle. Either plane thrust is unable to fight friction or ensuing heat melts the bearings.
2. Wheels, subjected to ultra high RPMs, burst.

However, this can go in a different direction. Consider phenomenon known as hydroplaning. In heavy rain, a film of water forms on the road. Tires of cars traveling over the road, have treads to channel that water away from between the tire and the pavement in order to keep traction. When treads cannot channel the water away due to either high speed or insufficient tread capacity, tire floats on a cushion of water, losing traction with road (bad). But in this case, a fast moving conveyor will be dragging along a film of air, nad if speed is high enough, it might cause 'aero-planing', so plane essentially rolls over a cushion of air, and takes off.

Question is, which happens first.

[Stytooner]

Okay, the conveyor is on a mountain for a new test.
Start the coveyor running toward the bottom of the mountain. Now set the plane on the conveyor with the nose pointing toward the top of the mountain. The plane will be moving down hill with the conveyor and may actually start to outpace the conveyor by rolling down some depending on it's weight and the conveyor speed.
Now fire up the engines and get to full throttle. At some point, the plane will stop moving downward even though both the conveyor and gravity are working on it. Thrust will eventually hold it momentarily still and then the plane will move forward toward the peak of the mountain against both gravity and any speed on the conveyor. The plane's tires are still rolling with the conveyor, but the plane moves separately from the roll of its wheels.

It's quite simple really once you overcome the plane to wheel relationship. The tires only bear the weight of the plane and have no relation to any of the planes movements. The plane moves forward because of thrust and not because of anything the wheels do.

[LarryG]

Because large objects like conveyor belts big enough for a plane) tend to have large masses, they tend to start slowly.

That's exactly my point, Loring -- also Lee's inclined conveyor, which illustrates how gravity continues to act on the plane regardless of what the conveyor belt is doing. The conveyor will start slowly. The plane will start rolling slowly, even if the throttles are firewalled abruptly. Assuming the conveyor's speed tracking mechanism is able to exactly and precisely match the plane's movement in the opposite direction, as the OP stated it can, the plane won't move relative to the fixed ground.

[LCHIEN]

The scan Niki posted suggests that for the plane to move forward, it only has to overcome the frictional resistance of its wheels. But this assumes it is sitting on a non-moving piece of ground. When it's sitting on a conveyor belt that is moving backwards, it must counteract not only the frictional resistance but also the distance it is moved backwards by the conveyor belt.

Surely we can all agree that if the plane was sitting on the conveyor belt with its engines shut off, it would move backwards with the conveyor belt, at the same speed as the conveyor belt. What would make the component of gravity magically disappear just because the engines start and begin producing thrust?

Sigh, Larry,
if there were no initial friction of the wheels, moving the conveyor belt would actually leave the plane behind because of inertia, one of newtons laws, things in motion tend to stay in motion (and things that are still tend to stay still). Because large objects like conveyor belts big enough for a plane) tend to have large masses, they tend to start slowly. Friction has a component called stiction that is large when the object is at rest, that must be overcome to start it moving.
Try pulling a tablecloth quickly from underneath a table setting.

[Stytooner]

Okay, lets let gravity do the work then.
Say the conveyor is on the side of a mountain like a ski lift. Turn it on going 100 MPH going uphill. Sit a plane on this conveyor pointing downhill. Let it go.
It's going to only go uphill a little until gravity takes over. The wheels will be spinning, but the plane will be moving down even though the belt is turning trying to get the plane to the top. Gravity will pull it downhill. It will be moving pretty fast when it gets to the bottom of the mountain all the while the belt is moving rapidly uphill.

[LarryG]

The scan Niki posted suggests that for the plane to move forward, it only has to overcome the frictional resistance of its wheels. But this assumes it is sitting on a non-moving piece of ground. When it's sitting on a conveyor belt that is moving backwards, it must counteract not only the frictional resistance but also the distance it is moved backwards by the conveyor belt.

Surely we can all agree that if the plane was sitting on the conveyor belt with its engines shut off, it would move backwards with the conveyor belt, at the same speed as the conveyor belt. What would make the component of gravity magically disappear just because the engines start and begin producing thrust?

[Stytooner]

Until I separated the action of the tires on the plane, I thought the conveyor was canceling out forward motion of the plane. The only thing the conveyor acts on is the tires. The plane still moves forward regardless of the direction or speed of it's tires.


Think about this. Change the direction of the conveyor and now match the speed of the plane. The tires aren't even turning now, but the plane is moving down the conveyor and will take off.

[leehljp]

TK421,
Your premis about it being a glider wasn't the original statement.

A plane is standing on 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 opposite direction).

The question is:

Will the plane take off or not? Will it be able to run up and take off?

"Will it be able" implies directly to its self. Gliders are referred to as gliders. Powered flying wing vehicles are referred to a planes.

[TK421]

Yes it will fly.

I hope this hope this clears it up.

Lets say that the plane in question is actually a glider plane modified with a typical tricycle landing gear setup. The plane is on the conveyor belt runway attached via cable to a truck on a road running parallel to the runway. As the truck starts moving the down the road the plane starts moving as well. It has to! They are attached! It doesn't matter how fast the conveyor belt going or how fast the wheels are spining. Thats all the wheels are doing - spining.
The truck is providing the "thrust" that a real plane would get from its engine.

or

Lets say you had a real jet on the conveyor belt runway and it was tied down via tie down chains. With the engines at idle you bring the the conveyor belt up to speed (lets say 200 mph ~ 300 ft/sec). The plane is tied down its not moving, the wheels are just spining. But if one were careful you could attach the chains to a tow truck and pull it down the runway. It wouldn't matter how fast the conveyor was moving because once again, the wheels are free spining.

Thats the key. The wheels provide a 'frictionless' surface on which the plane rests while on the conveyor belt.

[niki]

Thank you Mpc
Everything that you said is totally correct except that the last time (that I know about) that a B-747 lost two engines during takeoff, it could not hold itself in the air and they crashed (EL-AL, cargo at Amsterdam) but maybe there were also other factors that we don't know.

The problem to understand the question and the answer is because most of the people thinks about airplanes in comparison to cars.

They are two totally different animals.

The car is using its engine to power (rotate) the wheels and by that advancing the car forward (or back).

Airplanes are using the engine(s) just to push it forward (in some cases also back). It's not an "engine" in normal terms but it is a "Propulsion Unit" meaning, it produces work by "Action and Re-action".
The "Action" is accelerating mass of air. The "Reaction" is, push force created in the opposite direction to the "Action".

I gave an example in reply #57 (Page 6) and reply #63 (page 7), I will try different approach:

Put the airplane on the conveyer. Hang the airplane with hot air balloon so the wheels are lifted to 1" above the conveyer. To emphasize, the airplane tires does not touch the conveyer but are 1" above it.

Now, start the airplane "Propulsion unit" or "Power Plant" (if you want, call it an engine).
Push the "Thrust lever" (jet) or "Throttle" (piston) to maximum power and see what will happen.

The airplane will accelerate forward gaining airspeed, and at some point will liftoff. The tires did not rotate and the airplane liftoff.

Let's imagine, just for the experiment, that at some point during the forward acceleration, you are going under the airplane and start to rotate the tires. Will it stop or reduce the forward acceleration of the plane? of course not, because the wheels are separate unit from the plane and are connected to the plane through bearings and if you rotate them or not, it does not have any influence on the airplane or the Propulsion unit(s).

Now lets make the same experiment like above, but without the hot air balloon, the airplane will sit with the wheels on the conveyer.
What dragging or friction or stopping force is added now? only the tires friction on the conveyer and the tire bearing friction of the wheel. That's the ONLY parameter that changed.

And as Mpc stated, we shall need only small additional Propulsion (or thrust) to overcome this additional drag of the tires and bearing friction.

Just to visualize how much extra thrust we need to overcome the tires and bearings drag, go to your car, gear Neutral and brake Off and push it...
Call a few gays more and you can push also the 747...

niki

[mpc]

No air speed = no flight, everybody agrees on that. But I don't agree with the conclusion that if the belt is moving opposite what the airplane thinks it's doing that the actual air speed is zero.

Original statement: 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 opposite direction).

Okay, the belt is going gangbusters backwards. So why does that make the airplane NOT move when the engines push it forwards? Niki's little car tugged by a string while it rides on a belt sander is the same thing. The string wins. The car's wheels, like the airplane's wheels, are spinning way faster than the rest of the car is traveling but that does not mean car speed relative to the fixed pole is zero.

The belt can do anything it (or it's controller) darn well pleases; because there's no rigid/strong connection between the belt and aircraft body center of gravity it can't cancel the body speed. There will be ground speed relative to a fixed pole, and there will be air speed. If the engines are off, and the airplane's parking brake is set (yes, they really have such a thing) then whatever the belt does the plane will do - the wheels are now "locked" to the belt. If the engines are off, and the brakes are off, for the most part the airplane will ignore what the belt does... only that 15% rolling friction/drag term will affect it... so the airplane will move, slowly accelerating until it's moving at the same speed as the belt.. but it will not accelerate at the same pace as the belt. Just like yanking the tablecloth out from under the dishes... do it slowly, the dishes move with the tablecloth. Tug it a bit more quickly and they sort-of move, sort-of stay put - they do not move as fast as the tablecloth because you've broken the "static friction" and turned it into the much smaller "dynamic friction." The "rigid connection" of static friction is gone... only a smaller term remains. Yank the tablecloth really fast and there isn't enough time for the dinky dynamic friction to do anything to the dishes - they barely move. The rolling wheels of the airplane are dinky friction relative to the mass of the airplane. The drag force = 15% of the weight (approximately, and it's fairly independent of speed too) so the aircraft could accelerate backwards at 0.15g's maximum; if the belt accelerated at 0.3g's backwards the airplane would only do 0.15g's backwards still. That's all the force that can be transmitted from the belt, through the wheels, to the center of gravity of the airplane.

Now turn the engines on... that 15% drag is small compared to engine forces acting on the airplane - the engines will win, accelerating the aircraft forwards. A 130,000 pound MD90 aircraft has 2 engines with 28,000 pounds of thrust each. That's 130,000 pounds responding to 56,000 pounds of thrust... acceleration is 0.43 g's; well over the 0.15g's the belt can pull it backwards. (yes, I know I'm using "pounds weight" instead of "pounds mass" in the F=ma equation, but fortunately both F and m need the same correction factor so it cancels anyway). As the aircraft builds airspeed, rolling drag will decrease (as the wings lift some of the weight off the gear) and aeroydnamic drag will increase: the basic "profile drag" (drag of shoving something through air) and the "induced drag" (drag due to making lift). Engines will still win though. Why? Because another aerodynamic/performance equation basically boils down to Climb Rate = (Thrust - Drag)/Weight. We know the aircraft can climb with one engine conked out (cutting Thrust a lot)... so Thrust has to be greater than drag (with a konked out engine) to keep Climb Rate positive. With all engines operating, Thrust is waaayy bigger than drag. More than 0.15g's worth.

I doubt the original intent of the question included the 15% drag anyway; like most folks I'm sure it intended zero rolling drag. I'm putting too much "reality" into the answer. Without it though the aircraft has even more reason to overcome the belt and take off.

Niki's string on his toy car represents the airplane engine thrust. All it takes is enough thrust to overcome the rolling friction... which the airplane has in spades. So do you when you tug on the string. You can make the car move forwards regardless of the sanding belt.

Oh, one other clarification based on a prior post: for most commercial jetliners, the job of the engines is not to draw/blow air over the wings; in fact the designers try to lessen this effect. The engines just create forward thrust to overcome drag to make the aircraft mass accelerate to some velocity... enough velocity relative to the air to generate enough lift over the wings to fly. Unless it's done very carefully, engine airflow actually screws up wing airflow more than it helps.

Some airplanes do use "augmented lift" techniques - military "STOL" aircraft (Short Takeoff or Landing), "blown flaps" airplanes like the C-17 heavy lifter, etc. This type of technology though is risky for commercial airliners - if an engine dies, the augmented lift on that wing would drop to nothing... but the other engine+wing would still be making loads of augmented lift. The aircraft would roll over - rolling towards the dead engine - which is a challenge for pilots to control. The FAA and NTSB don't like the phrase "challenge to control" when passengers might be on the airplane. Prop airplanes do get some augmented lift from the propwash but it's not huge and there are other things added to help the pilot if the engine does fail. The FAA rules for performance, on commercial airplanes, do not allow engine thrust induced lift to be used in performance computations either - all aircraft capabilities are computed based on the "bare wing" lift only... so the prop airplane doesn't get credit for the propwash induced lift anyway. Many turboprop airplanes "cross shaft" the engines too - the engines are connected so if one dies, the other engine still turns both props to eliminate the asymmetric lift and roll problem. Jet engines induce so little flow over the wings (by design - it's a lot of effort to position jet engines actually, to design the pods and pylons, etc.) that the asymmetric roll is not a challenge.

mpc

man I wish I'd kept my fingers shut. I'm just going to annoy somebody and that's not my desire. I'm going to (try to) refrain from any more posts in this thread.

[leehljp]

If IF those who think that it will fly by virture of enough thrust to break it free, then it would have to be a thrust factor to the extent it would not need wings. At that point, all flight would be as a result of thurst force and thrust direction, not wings.
IN this case,
1. we are not talking about an airplane (except maybe a Harrier) and
2. if it had that kind of thrust it would not need to "roll" to begin with.

If rolling forward were needed for flight speed, then it would never happen if the conveyor matched the planes wheel speed.

[JTimmons]

The question is:

Will the plane take off or not? Will it be able to run up and take off?

No!

71 replies now counting mine, surely someone has gotten the right answer. Russianwolf, care to end the madness?

Edit: Missed reply #44 completely, just read it, thanks to Ray!

[Tom Slick]

assuming air speed is 0, the plane doesn't fly, period. relative ground speed can be mach 1 and it still won't fly. it may bounce around but with no airflow over the wings it's not going to be in controlled flight.

It would be the same as (previously mentioned) it sitting still with it's brakes on.