Airplane physics question

A minimum of 8 pages, eh??

 

Am I missing something that has resulted in 104 pages? All the surface (conveyor belt, runway, whatever)/wheel contact does is provide retarding resistance to the thrust of the engines. Assuming that this resistance during roll does not exceed the thrust, the plane will take off.

Someone told me that MythBusters showed this .. true?

 

EXACTLY. That's it folks ---- the whole thing boiled down into two succinct statements.

>> Although, it should be qualified that the excess force [thrust (minus) retarding resistance] must be sufficient to accelerate the plane to the minimum air speed required for lift. <<

The rest is (was) just conversation.....

 

So the answer is:
It depends on the plane having enough power to skid the plane up to the required takeoff speed, and the pilots intention to fly including him keeping the brakes on full lock? Sounds like a strange pilot to me. If the question intended to incorporate unlikely acts that are specifically against the original question (A pilot intends to take off). The answer you guys seem to like would also answer the question "Can a plane take off with the brakes locking the wheels from rotating?" It seems like the question went into specifics about the conveyor matching wheel speed with the assumption that the wheels would have some speed, otherwise, why give all that description?

In other words, if I asked you if a car gets pulled forward into a garage and parks on a turntable, can the car pull forward out of the garage without the turntable or car rotating? You might answer "Yes, the house can be lifted off with a helicopter and rotated around so the car will then be facing forward to pull out of the garage." It is about as likely of a scenario as your skidding explanantion!


BT

 

Years later the question or puzzle indicates no movement of air over the wings. If that is true, no flight because lift is generated tat way.

Art

 

Yep & Art's an expert.

Only way that plane will be gone;
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Hi John,

I`m sure you can fly, but you know errare humanum est.

Take your time and set up the principle of linear momentum. There is thrust, inertia and resistance. F_thrust = m * a + F_resistance. Can thrust overcome resistance? Sure it can. As a result you`ll have an acceleration and the plane will finally take off.

And does the propwash over the wing generate additional lift? It certainly does. It`ll increase the airspeed over the wing locally. Therefore there will be additional lift locally. But it is not enough to carry the plane itself.

Think of rudders within and out of the propstream. They will behave differently, because they generate different amount of lift.

Regards Christian

 

Questions says that wheel speed matches belt speed. If so, plane is stationary on the belt. Therefore, it is still relative to the ground, and there is no airflow over the wings. I don't believe airflow from propellor by itself is sufficient, and if it's a jet, the airflow is not over the wings.

That said, thrust from jets or propellors can easily move the plane forward, no matter how fast the belt is going in the opposite direction. The plane would then have airflow, and lift, and take off. However, the moment the plane moves forward on the belt, its wheels are no longer moving at the belt speed and the condition of the question is broken.

So:
1. As long as conditions of the question remain valid, plane does not move, and therefore does not experience lift, and does not take off.

2. The moment thrust is produced, the question's conditions (of wheel speed matching belt speed) are broken, as the plane will move and then the wheel speed will not match the belt speed. Thrust is produced from the moment the prop or jets are started, so the question can never be valid if the engine is running, and the plane will take off.

What do you choose to go with? 1? Or 2? That will dictate your answer.

 

Place a plane on a conveyor belt. Plane engines are OFF.

1. Turn on the motor that operates the conveyor, so that top surface of conveyor moves backward. Does plane move backward? Answer: YES (already discussed)

2. With plane engines still off, continue increasing the conveyor motor speed so that top surface of conveyor continually accelerates backward. Does the plane accelerate backward? Answer: YES (already discussed)

(and yes, these answers are correct even with plane wheels that are free to rotate about their axles)

In this scenario, we need to TURN ON the plane engines to balance the rearward force of the motorized conveyor. In other words, when the plane is on a motorized, backward-accelerating conveyor, we need to TURN ON the plane engines to keep the wings stationary, relative to stationary air.

 

What you are alluding to depends on how much the plane weighs, the coefficient of friction, and how quickly the belt accelerates from rest. You can pull a piece of paper out from underneath a glass of water sitting on a table, without disturbing the glass of water. Same principles apply here.

 

The only assumption i've made is that the force imparted by the engines is not enough to cause slippage at the tire/runway interface, and also (therefore) that the balancing force imparted by the motorized/backward-accelerating conveyor is likewise insufficient to cause slippage at the tire/conveyor interface.

more info in posts #2023 thru #2030 ...

 

I didn't go through 100+ pages but here is the answer:

[ame]http://www.youtube.com/watch?v=S377HwOthjo&feature=results_main&playnext=1&list=PL19D2B874428E6B7E[/ame]

 

not really, the video had an obviously faulty conclusion.

YES, planes and cars have different modes of propulsion (posts 2023 thru 2030 again, and probably several others before).

BUT ... notice what happened when the toy car's motor turned off on the backward-moving conveyor. THE CAR MOVED BACKWARD, just like we knew it would

He needed to apply the force of his hand (simulating the plane engine) to keep the toy car stationary, with the toy car's engine turned off. That's when the faulty conclusion is drawn: he concluded that the force he needed to apply to the toy car, by his hand, was too small ... ???

Well, what if the conveyor moved faster ... indeed, what if it ACCELERATED backward, to apply a constant force to his toy car? RESULT : force required by his hand to keep car still would be GREATER

Why can't the conveyor move FAST enough, and ACCELERATE at a high-enough constant rate, to balance the thrust of the plane's engine? ANSWER : there's no reason why not. The backward-accelerating conveyor can match the engine thrust, keeping the plane stationary relative to stationary air. You just need to match the conveyor's backward force to the forward force of the engines.

In other words, a conveyor that moves backward at any old speed won't do it

EDIT: Please note a point i've made before : a motorized conveyor moving backward at a constant velocity, and one moving backward at a constant acceleration, are two very, VERY different things.

 

The question asked was..."CAN the plane take off?" (on a conveyor belt).



The answer is "yes"...the plane CAN take off.

If the motor is running and the throttle is applied, the airplane will take off.

That's the cool thing about an airplane. It doesn't care about what the ground is doing under it's feet because it's not dependent on the ground for flight. It's only dependent on air for life. Period.

the only way a plane would not take off is....

If the brakes were on...or...

If it's in a vacuum.

otherwise....the plane must fly!!! That plane WILL fly!!! YEAH AIR-PLANES!!

 

Nah.

Airplanes don't care 'bout dat crap.

 

or ... if a backward-accelerating conveyor provides a backward force to balance the forward thrust of the engines.

If a rope ties the axles of the wheels to the ground (which is essentially identical to what a backward-accelerating conveyor can do ... if the acceleration, and therefore force, are chosen correctly), will the plane then care about what's happening on the ground?

 

An airplane tied to the ground is not the same thing as an airplane sitting on a movable surface.
"
An airplane tied to the ground will ask..."Why can't I take off?"

An airplane on a moving surface will say...."Yeah...I'm FREE and I'm going home where I belong!!" The other stuff is just over thinking. Airplanes don't think. They feel.

 

The prop cutting through the atmosphere is all that matters.
Again....this has nothing to do with the ground moving under it's wheels.
Two unrelated events


If the conveyor belt is motorized to make it move as fast as the wheels are turning, then that only means that the wheels will be moving twice as fast as they would be otherwise during the take=off roll.

If the conveyor belt is not motorized, it probably will not even turn at all during the take off roll.

the ground and the air....two different topics completely

 

If the "movable surface" is motorized so that it accelerates backward while the plane's engines are trying to accelerate the plane forward ... then the situation IS the same thing as tying the plane to a stationary ground. The plane won't know the difference.

 

nope.

The friction force applied to the tires by the motorized conveyor is identical to the force applied to the tire's axle (resolution of forces, according to kinematics). The force also happens to rotate the wheels.

Answer me these couple :

Turn plane engines off. Does a backward-moving conveyor move the plane backward? Does a backward-accelerating conveyor accelerate the plane backward? If the acceleration rate of the conveyor increases, will the backward acceleration of the plane increase?

Congrats ... you just found a variable, backward-directed force that can be chosen to balance the forward thrust of the engines (just like the tension in a rope tied to the ground)

 

You have to forcibly separate the ground from the air in your mind.

The prop, wings, and atmosphere is all that matters to an airplane. NOT what the ground is doing.

That is what's cool 'bout those planes....they DON'T know the difference....the difference in whether the ground is moving, not moving, shaking, not shaking, rising, falling, etc....an airplane does NOT CARE what the ground is doing....


(Unless it is physically tied to the ground...which would be sad )

 

I don't know what happens to the plane if it's motor is turned off. But...that's not the question. The question is...."Can the plane take-off?"

As far as the prop, wings, and atmosphere are concerned, there are no backward acting forces. And they are all that matters to whether an airplane flies or not.

 

Look at it this way....

If an airplane is already flying...and it does a "touch n go"....it will not suddenly STOP flying because it's wheels touch something that is turning as fast as the plane is flying.

The same holds true on landing. It will not suddenly STOP. Again...that's because flight is independent of the ground.

 

dcate, agree with you on everything except the plane's wheels moving twice as fast if the ground is moving backwards. Don't know how you get that. If the plane wheels are moving, and the belt is moving backwards to match their speed, then as the plane moves forward and accelerates, the wheels will accelerate. As they accelerate, the belt will accelerate to match them. So far so good.

How do you come up with their speed being twice the normal takeoff speed by the time the plane takes off?

 

Superb. But the clip doesn't show what happens. Does the real plane take off (as physics says it should)?

werewolf, you are overthinking it. Plane's thrust force has no opposition. Therefore thrust wins. Plane moves forward. This results in airflow past wings. This creates lift. Lift will cause the plane to take off. The wheels rolling or not rolling have no effect. Either they roll, and the plane moves forward easily, or they don't roll, and the plane will drag them along, no matter which they are or are not rolling. Simple.