Airplane physics question

I think you might be conveniently ignoring forces like gravity, friction, drag, etc. We are not operating in a vacuum and in the absence of gravity.

 

I think you need to study some basic physics ... so you can understand the fundamental difference between objects moving at constant velocity, versus objects that are accelerating. They are not kinda pretty much the same thing In one case, a net force is involved ... but not in the other case.

Once again: A constant-speed conveyor imparts no net force on the object it's conveying, but an accelerating conveyor does impart a net force (in order to accelerate the object conveyed).

The presence, or absence, of a force from the conveyor belt is rather fundamental to the question at hand wouldn't you agree?

A constant-speed conveyor belt imparts no force on the plane, and therefore cannot possibly keep the plane stationary. The same cannot be said for an accelerating conveyor belt. You really need to understand this fundamental difference.

 

This thread is still funny. Now explain to me how you can pull the tablecloth from under the dishes….it does accelerate but yet….

 

No friction. Table cloth accelerates, but it can't impart a net force to the dishes because ... no friction. Would the tablecloth trick work with rubber dishes?

 

Let’s put those rubber dishes on roller skates.

 

I'm glad Vincent355 took up the battle. Common sense can't overcome misapplication of physical laws.

 

Roller skates have rubber wheels, which have friction.

Which means, there's still a net force applied from the accelerating table cloth to those wheels

Constant-speed table cloth: NO net force
Accelerating table cloth: YES net force

Force "present", versus Force "absent" ... the difference is not negligible

Thanks for playing!

 

I love gravity. I only drive south. Saves on gas.

 

Whatever. The real world beckons. Good luck.

 

It's where I live. Physics and the real world recognize when a net force is present, compared to when a net force is absent.

My world recognizes friction ... it's the principle required for an accelerating conveyor to impart a force on an object that's being conveyed (a force that's entirely absent with a constant-speed conveyor).
My world recognizes gravity ... without gravity, there's no "normal" force to activate friction (see above).

Not clear what world you live in (where there's no difference between accelerating bodies (or conveyors) and constant-velocity bodies, no difference between inertial and non-inertial frames of reference, etc etc) but i won't be needing much "luck" at all in the real world.

 

It doesn't matter what the conveyor is doing. Accelerating conveyor can impart a very small force to the airframe through tire rolling resistance and bearing drag.

Certainly not enough to overcome those turbofans.

Throw it on a conveyor, accelerate the conveyor and hold the plane stationary with a fish scale. You believe that would impart a higher force than engine thrust?

Do it backwards, park the plane on the conveyor (engines off) now accelerate as fast as you can forward. Will it reach take-off speed?

No, because the conveyor couldn't transmit enough force through free spinning wheels. Even if slowly accelerated the airframe would reach a point of drag where speed stalled and further acceleration of the conveyor would result in reverse wheel speed.

 

So this conveyor takes the atmosphere in it's entirety along with it?

Conveyor can impart minimal force through tire rolling resistance and wheel bearings, it cannot cancel thrust.

 

Afraid there won't be enough friction, at the tire/conveyor interface? There's enough friction between the tire/road interface, to accelerate the cars we love at a pretty rapid rate ... yes, that friction IS the force that's accelerating our cars (our cars move forward because the road pushes them, through friction). Do a quick F=ma calculation on a 4000 pound car being accelerated from zero to sixty in 3 seconds, and we know that at least that much force can be provided through tire friction. Or, see if any road car can approximately match the acceleration of a plane before take-off ...

Furthermore ...

The original question has no numerical, or time, bounds or limits. How heavy is the plane? How long must the conveyor keep the plane stationary? How much force is the plane prop providing? (The original question was written as "plane" not "jet"). How much electrical power is available, to supply the increasing current needs of an accelerating conveyor?

Several points:

- There's a real, fundamental difference between a constant-speed conveyor and one that is accelerating. One of these can NOT supply a force to counter the propeller, the other CAN. This difference is by no means trivial or negligible. The presence vs. absence of a force can't be "waived away" as non-material.
- The friction force supplied by an accelerating conveyor is, fundamentally, the same friction force that accelerates our cars on the road (i.e. friction at the tire/road interface)
- The plane will remain stationary ... until the accelerating conveyor bearings wear out, or until the accelerating conveyor circuit breakers flip, or until the plane tire explodes from excess heat, etc etc. If that time period is only 3 or 4 seconds ... is the original question satisfied?

 

It's not fundamentally the same force as accelerating our cars on the road.

How fast have you gone in neutral?

Of course that much force can be applied through tire to ground interface, by a driveline that's attached to said tires, they aren't just free to spin.

The original question doesn't state "how long must the conveyor keep the plane stationary" because it's incapable of doing so.

 

Interestingly enough, I once waxed the propeller on my 182. I couldn't take off! It would only fly if I put tire chains on the prop.

 

It absolutely IS the same force that's accelerating our cars on the road: friction at the road/tire interface. This friction accelerates the car. Do you think some "other" force ultimately accelerates our cars? Nothing else is "touching" our cars; there's no other force except friction at the road, pushing our cars forward ...

In the car's case, the friction at the road/tire interface is created by spinning the wheels. In the airplane's case, the friction at the road/tire interface is created by spinning the road (aka an accelerating conveyor).

Please remember: the conveyor must be accelerating; a constant-speed conveyor will impart no force, as no force is required to move an object at a constant speed.

Thousands of pounds (if not tens of thousands, depending on the normal force) of force are "available" at the road/tire interface, through friction. You can create/realize/experience that force through spinning the tires, or spinning the road ... it matters not.

(Your car-in-neutral analogy is meaningless, of course. In this case, neither the car's wheels nor the road are being forced to spin ... therefore no friction force is applied or created, to cause acceleration)

 

LMAO, the engine is putting power through the driveline to the tires. Again, how far does your car go in neutral? Are you even reading what you're writing?

The conveyor can put as much speed/force to the tires as it wants, and that gets transferred to the plane how? Through the free spinning wheel bearings?

 

Do you agree or disagree: the friction force at the road/tire interface is ultimately the force that accelerates our cars?

YES or NO. Simple question.

 

YES and also irrelevant as the car is DRIVEN by that interface and not by thrust.

The reason that is true is a mechanical connection between the tires, driveline, engine then chassis.


There is no such mechanical connection from tire to chassis on an aircraft. Simply a free spinning wheel bearing.


So, explain how any torque applied to an aircraft tire is transferred to the chassis/airframe through a free spinning bearing.

 

YES, is the correct answer our cars are accelerated by the friction force at the road/tire interface. Thank you, for indulging ...

Next (if you care to indulge me further): Assuming the normal forces are the same, can I "create" that same friction force by forcing the road to rotate beneath the tire, instead of forcing the tire to rotate on the road? Same pressure (normal force) involved ... does the friction itself "care" if i force the tire to rotate, or force the road to rotate? Can i "create" the same friction force, either way?

 

You can create the friction force either way. Yes, of course.

Now, in order to apply that force to the car it would need to still complete that mechanical link between the road surface and the chassis.

That could be accomplished by having the car in park, or having the brakes locked.

If the car was in neutral and the wheels able to freely spin (so, same as an aircraft wheel), the rotating force of the road beneath the car would spin the wheels but not move the vehicle.

Another example.

Car is on a flatbed tow truck, in park with parking brake on.

Drive away quickly and the car will stay on the truck as long as it's not so fast as to break traction between the locked tires and flatbed.

Now put the same car on a flatbed and leave it in neutral, parking brake off. Free to roll like an aircraft.

Drive away quickly and watch the car remain in place and roll off the truck.

Same tire to surface interface, loss of mechanical link to transfer the force.

 

Thanks again. The correct answer is, as you have noted, YES ... i can "create" the same friction force by rotating the road instead of the tire.

So we would agree, then, that an accelerating conveyor can "create" thousands of pounds of friction force (if not more) at the tire/conveyor interface

And the only question that remains, is this: is that friction force at the plane's tire (created by a conveyor that must be accelerating) "available" as a translational force on the plane's center of mass?

Right?

 

As mentioned, any force applied to the tire would not be transferred to the aircraft due to the wheel bearings.

(so, force transmitted would be limited by how much drag the wheel bearings have, which is of course minimal by design)

 

So the remaining question is: does the force applied to a freely rotating wheel, become "realized" as a translational force on the axle (for example)?

One way to answer: let's say i have a freely spinning wheel, and i apply 500 pounds of force on one edge, and 500 pounds of force on the opposite edge (so that both forces tend to rotate the tire in the same direction). I think we agree that there's zero net "translational" force on the axle. Agreed?

Thanks again ...

 

Car in flatbed, brakes off, will stay there if you drive away with minimal acceleration. Reach 200 mph slowly enough and the car will stay. But will roll off the truck as soon as acceleration is applied, either driving away from a stop or from any constant speed. Acceleration is key because imparts force (did I read this before anywhere?).