Airplane physics question

Garretto that is true (and why my example states drive away quickly) except the car would fall off earlier than 200mph unless it's isolated from the atmosphere.

Werewolf, I'm not sure if I'm visualizing your example the same way as you are trying to describe it.

 

Not without a static port and a calculation.

 

I'll get back later ... promise! Because you have indulged me, i hope to return the favor (rationally). But i'll be tied up for a bit ..

 

Of course, car will fall off if there's frontal air dragging, but was thinking more of a carrier truck like those in which the cabin would parapet cars behind.

 

^ I see. Here a flatbed is common phrase for an open tow truck. (with a flat bed of course)

Yes I mentioned as far back as years ago now that there would be a minimal force transmitted due to tires/bearings. Not trying to say there's zero friction, just minimal.

A car moving forward because the engine is attached to the chassis and routes power to the tires isn't an equal comparison to a conveyor moving an object on free spinning wheels, which is why I made that example.

 

OK ... we've agreed that an accelerating conveyor can apply a thousand pounds of force or more, through friction, to the tires of an object on the conveyor. Of course, we also agree that a constant-speed conveyor applies zero pounds of force to an object on the conveyor (a constant-speed conveyor has no hope at all, of keeping the plane stationary).

So the last remaining question is: In addition to the obvious rotational effect on the wheels, does this thousand-pound force result in a translational force on the plane itself? A more simple form of the same question:
Does the force applied to the edge of a freely-rotating wheel result in a translational force on the axle?

Please consider 2 simple experiments:

First, imagine a freely-rotating wheel on a fixed axle, to which we will apply 1000 pounds of force to the edge of that wheel. But in this experiment, we will apply 500 pounds on one edge, and 500 pounds on the opposite edge of the wheel. The total "rotational" force is, indeed, 1000 pounds, but the 1000 pounds is "split" ... 500 pounds on one edge, and 500 pounds on the opposite edge. Both forces tend to rotate the wheel in the same direction, but from the axle's perspective the forces are "opposite" ... or "balanced". I think we would agree ... based on symmetry if nothing else ... that there is no net translational force on the axle in this case.

Second, imagine the same freely-rotating wheel, to which we will apply 1000 pounds of force to the edge. But no "split" or "balance" this time ... we just apply 1000 pounds of force to ONE edge of he wheel. How does this situation compare to the first? Reason alone dictates that the 2 scenarios are quite different ... in the first example, a 500 pound force was applied in one direction to a body (the wheel), while a second 500 pound force was applied in the opposite direction to that same body (both forces conspiring to rotate the body in the same direction). But in this second case, the force applied is "unbalanced" ...

What happens in this second example? The answer is: the 1000 pounds of force applied to rotate the wheel, also applies as a net translational force on the axle.

Why? It's a principle in the field of mechanics (dynamics, specifically) that a force applied off-center on a body can be decomposed into two components: a "rotational torque", and an equivalent "translational force" applied to the "center of mass" of the body. You simply can't apply a single force anywhere on an extended body, without that force causing: BOTH a "rotational torque" (if that force is applied off-center), AND a "translational force" on the center of mass of the body.

How? In our case, the force applied to the tire's edge is not translated to the axle through friction in the wheel bearings (we assume there is none) ... but rather, the force exerted on the tire's edge is applied to the axle through a lateral pressure in the wheel bearings. Yep ... it's lateral pressure, not rotating friction, that applies the force at the tire's edge to the tire's axle.


bottom line: that 1000 pounds (or more) available as friction at the tire/conveyor interface, generated by an accelerating conveyor, is directly applied as a translational force on the tire's axle ... and, then, also applied as a translational force on the center of mass of the plane itself.

 

The situation is fundamentally no different than an accelerating car. Imagine the front wheels of a car, absolutely freely rotating with wonderful, friction-less bearings. BUT, these wheels are "driven" by the magnetic field of an electric motor. The electric motor applies only a ROTATIONAL TORQUE on the front axle ... a front axle with tires connected, a front axle that's absolutely free to rotate.

We would all agree, that once the motor is turned-on, the car accelerates ... being pushed forward by the friction at the tire/road interface

But ... how? How does that frictional force at the tire/road interface "push" the electric car forward? Can't be through any rotational friction in the wheel bearings ... there is none. Can't be some force counter-acting the magnetic field ... the magnetic field is a rotational torque only. BUT ... NOTHING ELSE is EVEN TOUCHING THE DRIVEN AXLE or WHEELS!!

How is THIS WHOLE CAR car pushed forward, by the friction force at the tire/road interface?

The answer is the same: the friction at the tire/road interface pushes the electric car forward through lateral pressure, not rotational friction, at the wheel bearings.

The connection to the plane/conveyor is this:

In our electric car example, the car is "pushed" by the friction at the road/tire interface ... a friction created by spinning (accelerating) the wheels. In the plane/conveyor example, the plane is "pushed" by the friction at the conveyor/tire interface ... a friction created by spinning (accelerating) the conveyor.

 

I disagree, only part of your 1000 pound example would be applied as a translational force.

If the wheels were mechanically locked, all of it would be applied, tire wouldn't spin, vehicle moves with conveyor.

Wheels free spinning, a very small amount would end up a translational force as most of the energy would end up accelerating the tire.

Your examples of cars are not even comparable. Yes they have wheel bearings and you still need to end up with a mechanical connection to transfer the torque, be it an internal combustion engine and it's driveline or an electric motor.

For a car to apply it's drivelines torque to the tires it has to be connected

So, a torque path from motor/engine to the ground in order for that to end up as a lateral force against the wheel bearings. Remove a link in that path (or, have an aircraft) and that torque transfer does not exist.

Plane vs car comparison is simply not valid.

Plane wheels are free to spin, car wheels are mechanically tied to the drivetrain and chassis -when- said drivetrain is capable of moving the vehicle and disconnected when not.


Or your electric example. When the motor is transferring torque to the tires, -it- is the torque path from the tires to chassis.

Unpower the motor and the armature is free to spin. Torque applied to the tires via conveyor would result in simply........ free spinning the wheels/tires/cv shafts/motor armature.

You lost the torque path.

 

Physics and the question say that the acceleration of the conveyor matches that of the plane, these forces cancel each other out, means there that the plane doesn’t accelerate in the air and develop lift to fly. A treadmill is a good example. The air flow generated by the conveyor is a boundary layer along the surface of the conveyor which likely would not be deep enough to significantly effect the laminar flow of air over the wings, ie it is likely to be turbulent around the wings as the wind speed generated by the conveyor shears against the static surrounding air.

The key to flight is the air flow over the wings creating lift, ground speed is not material to the argument, it is typically what is used to generate enough airspeed to creat the lift.

 

You are both sort of right, but your arguments are ignoring all of the other forces that need to be overcome by the plane to fly. The two major forces are the friction forces due the air resistance which push against the plane and gravity which attracts the plane to the centre of the earth. Don’t forget force is directional. There are a lot of parasitic forces that your arguments are ignoring like friction at the boundary of every moving surface, with the net result that the sum of all forces needs to be in a forward and upward direction for flight to occur.

Anyway sorry to interrupt your discussion, please carry on.

 

How is it the conveyor acting upon the tires cancels thrust?

 

Of course certain requirements have to be met in order for a plane to fly. I wouldn't say the lack of stating the obvious is ignoring anything.

The only difference between a normal flight and this example is a conveyor beneath the plane which can do nothing but move free spinning wheels. (and trick people into thinking planes work like cars)

 

It does not, the plane’s engines thrust is not enough to overcome gravity until the plane has enough air speed for the wings to generate lift. The wings generate lift upwards, gravity is a downwards force, thrust is a forward force which the conveyor reacts to by accelerating in the opposite direction. The tires are not frictionless devices and take reasonable amounts of forward thrust to overcome the rolling resistance.

 

Tires are not frictionless but it is miniscule in comparison to thrust.

A conveyor acting upon the tires could never hold it stationary.

And yes I understand how an aircraft works, and that gravity exists.

 

For those interested, the same problem has been on mathworld for about 20 years where scholars from all across the globe opine on the subject. It’s a cleaner read than what we have here.

 

This has been debated on FChat for a whopping 17 years. Wow.

 

If the conveyer belt matches the speed of the tires, then the speed of the tires will remain zero. The plane will move forward in relation to the ground (not the conveyer belt) because the prop pulls it forward, not the tires. The conveyer belt to tire would stay identical until the plane takes off. Tires would not spin because if belt matches speed of tires, speed of tires remains zero, despite the plane gaining airspeed.

Spray paint an “x” on the conveyer belt In front of the nose of the plane. The “x” would remain in that spot until the plane takes off.

It’s simple, the conveyer belt would be moving WITH the plane and the tires wouldn’t be spinning.

This is because he said the treadmill matches the speed of the tires. The speed of the tires won’t ever change because no power goes to them. Even if the treadmill went 1mph backwards for every 1mph forward the plane goes, it doesn’t matter… the wheels would just be spinning faster than the ground speed of the plane. The treadmill could be going 200mph underneath the plane and the plane would quickly go forwards atop the treadmill, gaining forward airspeed, even if the planes top speed is only 100mph. Sure, fractionally added friction maybe but incalculable because the wheels are irrelevant to the planes forward movement. Hell, tires are spinning during flight on a fixed gear plane. The plane is still flying! Lol. My statement is for a low frictional drag relationship between the tire and treadmill of course.

This is elementary. Not sure why people think it wouldn’t fly.


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This is kinda like a plane being atop of a semi truck trailer. The semi truck is going 90mph down the road. The planes tires are not spinning at all, but the plane can still takeoff.

In the conveyer belt scenario, the prop or turbine or whatever pulls the plane forward through the air creating a forward speed over the ground. It doesn’t matter if the conveyer belt is going 100mph faster in the same direction of the plane, 100mph in the opposite direction, or is identically the same speed of the plane.

Assuming it’s a tread mill material and normal airplane tires, there would be minor friction but near irrelevant. The planes wheels have wheel bearings on the struts and spins completely irrelevantly for what the plane needs to fly. If the treadmill was tall grass and the plane has tundra tires and the grass/treadmill was going the opposite direction the plane was going then that would create frictional drag and would be relevant.

If the plane was stationary on the treadmill, then the treadmill launched 200mph backwards, the plane would take awhile to catch-up to that speed. Just like ripping a table cloth off and the dishes stay still. Now if the engine is on, the plane would so quickly overcome the tiny amount of friction between the tires and treadmill and quickly gain forward airspeed regardless of what the treadmill is doing.


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I figured out why tanks aren't as effective as they could be. They are all two wheel drive! ALL of them only have one set of drive wheels. M1 has them in the back, some have them in the front. If they put them in the back AND front, tanks would be unstoppable.

I can't believe no one has seen this very obvious flaw.

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Another way to think of the physics:

A plane needs airspeed. The tires don’t have any power going to them. If a plane need 60 kts airspeed to fly, and it’s pointed into the wind blowing 60 kts (pilot holding brakes to not roll backwards), then pilot adds adequate power to go 60kts airspeed, the plane takes off like a helicopter and the tires never rotated.

This conveyer belt scenario said the conveyer belt matches the speed of the tires. The speed of the tires would never change because no power goes to them. However, the planes prop/turbines would pull it forward through the air and the conveyer belt would stay in a constant position to the plane because the tires never rotated, but the plane would be gaining forward airspeed.

However, the conveyer belt is a distraction. It’s irrelevant (assuming there isn’t an insane amount of surface friction from it and the tire, but there isn’t in a normal circumstance).


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DEBATE OVER (not that it was ever a “debate” it’s like saying “the war is over” if the USA declared war on a single caterpillar)


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How is there 107 pages on this...lol

An airfoil requires airflow to create lift, longer surface on top causes the air transiting to go faster and thus creating lower pressure. No flow across the wing, no lift. More flow across the wing, more lift.

I guess in all 107 pages, someone else probably said this but I'm not reading it all. Just bored and felt like adding my $.02.

Good Morning.

 

More ways to explain this:

If the plane had roller ball wheels on it, the conveyer belt could be going forwards, backwards, left, or right and once the plane’s engines are on, it would pull forward, gain airspeed, and takeoff regardless of what the conveyer belt is doing. Sure, minuscule drag occurring with the tires/roller ball wheel but the planes power quickly overcomes this.

Planes takeoff on skis. Planes takeoff with floats. Both floats (especially) and ski’s would be giving far more drag than normal tires on a conveyer belt but they’d still overcome and takeoff and they don’t even have wheels.

It really doesn’t matter what the conveyer belt is doing. The plane will still propel forward from the prop/turbines and takeoff.

If the plane moves 1mph, and the conveyer belt moves opposite direction 1mph, sounds like the wheel wouldn’t be spinning. So it’s like a plane sitting a-top of a semi truck and as the plane accelerates, so does the truck. The planes tires never moves and it takes off. BUT, the whole conveyer belt and plane can go whatever speed over the ground, which means airspeed for the plane to fly. The conveyer belt is separate of the ground. In this scenario, we are assuming no winds so ground speed = airspeed.

Minimal drag is being transferred through the tires, wheel bearings, to struts/airframe.

Any debate is more related to the semantics and perceptions of the question. But, once the engine is on, it is going to pull forward through the air regardless of what the belt is doing underneath (assuming minimal drag between tires and belt) . Anyone who thinks it wouldn’t fly is wrong. Whether the conveyer belt is moving forwards or backwards or left or right to the plane, the planes prop or turbines pulls it forward through the air.

I understand the question is poorly written and can be perceived multiple ways. One way, implies the wheels and conveyer belt will reach speed of infinity, but the plane would simply be pulled through the air regardless of the conveyer belt and tire speed.



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^ Agreed, and it's all been said in one way or another.

Still, give it a little time and someone else will come along, read nothing and say they don't go anywhere when running on their treadmill.

 

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