Author Topic: plane on a conveyor belt? (Read 21049 times)

chancevought · « **Reply #240 on:** January 24, 2007, 06:23:01 AM »

i'm sorry, but i believe you are over thinkin this...but i'll try again.

AIR is at zero, thus AIRspeed=groundspeed, right?

so golfer, I'm not saying it's riding the conveyer....it implies that the conveyer will only match the wheel's SPEED in the opposite direction of the wheels travel...the ground moves which way if the wheel spins forward??? It moves from the front to the back of the plane, right...so if the conveyer tried to reverse that motion, it would move from back to front right??? pretty simple there.

lukster, It says WHEEL SPEED,.....RPM or the wheel's AIRspeed...this is just arguing semantics....i.e. in your car you have a speedometer...it is not telling you wheel speed, but instead speed of the vehicle....these things are measured in two seperate ways, the wheels speed would be measured in RPM's....same here ..

if the conveyer matches wheel's forward speed in the opposite direction the net gain is zero...period..1-1=0 100-100=0 This would be the outcome if your using wheel RPM. This part should not be too confusing...the conveyer IS NOT matching the aircrafts speed in the opposite, jus the wheel's speed........even if you thought the conveyer was moving the opposite of the wheel's AIRspeed thus making them spin forward times two.....the friction on the bearings of a wheel spinning 120 kts would not overcome the thrust of the prop...really, cause thats what bearings are for....so lukster, i believe we are in fact saying the same thing...I just didnt explore the AIRspeed version since wheel speed should mean RPM's

the prop produces enough thrust to pull the aircraft with NO...and I repeat...NO relevance to what the wheels are doin..(unless brakes are applied, or if the nose wheel is turned of course)

here's a simple thought for ya, too...a seaplane takes off into the wind, thus the water at the surface is typically moving in the same direction as the wind, the opposite of the aircraft direction.....The prop of that plane can pull itself against the water (which produces FAR, FAR , FAR more friction than a bearing at any speed) and the aircraft will eventually attain enough lift to fly....in our riddle, U can spin those wheels backward or forward as fast as you want (although we have seen that the same speed opposed equals zero), the prop will still pull the plane's weight forward...please read this thoroughly, and think about what I'm sayin...Read the original post again too..this is really simple if you dont try to overthink it...

Mini D · « **Reply #241 on:** January 24, 2007, 07:43:45 AM »

Quote

Originally posted by lukster
I think they are making a distinction between the force applied from the tire to the axle with a well greased bearing functioning as intended vs a bearing that is inducing drag due to overheating.

What makes you think there has to be overheating to make drag? If there is friction, there is drag. The energy of this friction must be significant to prevent the airfoil from moving forward since THAT IS THE ONLY MEANS OF COUNTERING THE THRUST.

In order to remove that effect, you must remove friction (WHICH IS NOT POSSIBLE). The assumption seems to be that all the energy whill be transferred to the rotation of the wheel and stored via rotational inertia. This can store energy, but it cannot prevent the motion of the aircraft.

Quote

The belt will most definitely apply force to plane in the direction the belt is moving through the tire and axle and as we said previously the only way for this force to be transferred is through friction.

The belt can only apply force to the wheel, not the plane. Of course this HAS TO HAPPEN VIA FRICTION. We're talking infinite rotation, infinite friction here. That means infinite heat. That means disintigration, no matter what it is. Remove the friction... there is absolutely no wheel motion and absolutely nothing to stop the aircraft from moving.

Mini D · « **Reply #242 on:** January 24, 2007, 07:59:54 AM »

PS... the introduction of infinity also introduces every other effect that is normally negligable. Expansion would be infinite resulting in zero contact with the runway after the plane was infinity/2 miles off of the ground. The energy generated by friction would be at infinity causing the formation of a new sun that obliterated the universe.... and so on.

There seems to be a need to eliminate all of these effects to get to a theoretical outcome. Fundamentally, there is only one outcome: the wheels disintegrate. All there is to argue is how fast and how big of a boom.

The problem is that some are ignoring the actually impact of ignoring factors. Eskimo is not realizing that with no friction, the wheel will not move. And that in the presence of friction, the wheel will disintigrate. There is no element that does not absorb heat. There are no materials that move via contact with one another that don't absorb heat. This effect (especially given that a 15 ton aircraft is continually applying 30,000 lbs of thrust AND 15 tons of downward force would be much more significant than inertia at the wheel.

It's like trying to do the experiment that eskimo outlined by ignoring gravity instead of friction.

Waffle · « **Reply #243 on:** January 24, 2007, 08:55:29 AM »

or:

Several cases of scotch are delivered to HTC. The hallway leading down the office magically turns into a conveyer belt running in the opposite direction that the delivery person needs to go to deliver the scotch. The conveyor belt also travels at the same speed as the delivery person and matches his/her speed if he/she trys to speed up/down.

So... How long will it take HT to get the scotch?

(there are several outcomes)

lukster · « **Reply #244 on:** January 24, 2007, 09:05:22 AM »

Quote

Originally posted by Mini D
The belt can only apply force to the wheel, not the plane. Of course this HAS TO HAPPEN VIA FRICTION. We're talking infinite rotation, infinite friction here. That means infinite heat. That means disintigration, no matter what it is. Remove the friction... there is absolutely no wheel motion and absolutely nothing to stop the aircraft from moving.

The belt applies force to the wheel when it moves (through friction of course, this will always be assumed without being stated) the wheel converts some of this this energy into a rotation and some of it is transfrerred to the plane as a force in opposition to the plane's thrust. Of course the discussion is moot when you start talking accelerating mass to the speed of light. It would take all of the energy in the universe to do that.

Mini D · « **Reply #245 on:** January 24, 2007, 09:21:15 AM »

I'm not talking about the friction between the belt and the wheel. The wheel is free-floating on an aircraft. I am talking about the friction between the wheel and the axle. This has to be sufficient enough to prevent the aircraft from moving. This is much more significant that inertia given that all of the energy is transferred to wheel inertia via the same friction.

The only way a belt would stop the plane from moving forward is if the friction generated by the wheel were instantly enough to equal applying brakes to the plane. Applying force to enable rotation of the wheel does not do this... the only thing that does this is the friction between the wheel and the axle. Reducing the heat load means reducing the friction... making it less effective at stopping the forward momentum. Increasing the friction means a greater heat load. These are, by far and away, more critical than inertial energy in this equation.

Casca · « **Reply #246 on:** January 24, 2007, 09:36:26 AM »

Quote

Originally posted by chancevought
it says it tracks WHEEL SPEED, then tunes the speed of the conveyer to be the SAME in the opposite direction. I'm pretty sure if it reached 1 mph forward, the conveyer would go 1mph backward...1-1=0

once the wheel moved the slightest bit..the conveyer would move the slightest bit opposite of that...producing a balance of zero...as the aircraft gained speed the wheel would TRY to accelerate, but wouldn't because the conveyer would COUNTERACT that movement again producing a net gain of zero....
once the wings got enough AIRspeed...it would lift off, it's wheels never having rotated, if they even moved at all

I rolled my eyes when I read this post but after considering it am forced to conclude that it is absolutely correct if one answers the question AS ASKED.

deSelys · « **Reply #247 on:** January 24, 2007, 09:39:50 AM »

I haven't taken the time to read all the posts so this may be a repeat:

What some people seem to be confusing is where the energy is coming from:

energy to move the plane (cinetic energy) is given by the plane engines
energy to make the wheel spin ('rotational energy') is given by THE CONVEYOR BELT

In theory (no friction), the conveyor belt wouldn't be able to stop the plane from moving no matter how powerful it (the conveyor belt) is.

in practice (friction from spinning wheels*), the wheels would disintegrate quickly and you'd loose the perks of the plane.

* edit: as MiniD explained, friction in the ball bearings between the wheels and the axles

Casca · « **Reply #248 on:** January 24, 2007, 09:50:04 AM »

Quote

Originally posted by deSelys

In theory (no friction), the conveyor belt wouldn't be able to stop the plane from moving no matter how powerful it (the conveyor belt) is.

in practice (friction from spinning wheels*), the wheels would disintegrate quickly and you'd loose the perks of the plane.

* edit: as MiniD explained, friction in the ball bearings between the wheels and the axles

It is not friction between the wheels and axles that opposes engine thrust. It is the force component acting in opposition to thrust in the process of overcoming the inertia of the wheel. This is the path of the infinite conveyor speed and is completely theoretical. Eskimo's videos demonstrate the principle.

luckster has the nub of it we are all answering the question as we want it to be, not as it was written.

hitech · « **Reply #249 on:** January 24, 2007, 10:57:57 AM »

Mini D: I tie a rope to the tail of a plane and then to a pole. Put in full thrust. Plane does not move. In this setup there is no friction.

You seem to be confusing rolling friction (force tramsited to the plane) vs wheel sliding static friction (the friction that alows the belt conveyor to turn the wheel). Yes there has to be enough weight on wheels to allow the wheels to grip the belt. But this is not transmitted as a force counter to thrust of the airplane. If the wheel had zero mass when speeding up the belt conveyor, no change in force would be transmited to the plane.

eskimo2:
Not a bad analogy eskimo. And obviously they would accelerate at different speeds do to the rotational inertia stored in the wheels. Since 1 has 0 mass, 1 has 1/2 mass, the 0 would be in front , followed by the 1/2 mass. Followed by the original plane.

Eskimo it might be a good idea to do some quick calculations on what force from belt (translated to acceleration) vs what moment of inertia the wheel needs per lb of thrust from the plane.

These problems are so much easier to show with a white board and a few equations.

JB88 · « **Reply #250 on:** January 24, 2007, 11:05:00 AM »

Mini D · « **Reply #251 on:** January 24, 2007, 11:22:01 AM »

Sigh..

No.. I'm not confusing the frictions Hitech. Try to stop that plane from rolling with a rope that isn't actually contacting it. That is the scenario you are trying to create. That rope is seeing the full force of the engines and is pulling with equal force against it. The wheel, in turn, must pull AGAINST THE AIRCRAFT with enough energy to counter the thrust.

hitech · « **Reply #252 on:** January 24, 2007, 11:33:06 AM »

Mini D: In agreement so far.

And I also agree the tire static surface friction (I.E. Brake's locked how much force can the tire hold back) has to be as great as the total thrust, or the wheel would start to slide on the belt.

But once again, that is not what is normally considered as the rolling friction force transmitted to the plane. The rolling friction force is a constant at any speed the wheel turns.

As you accelerate the wheel a force is needed for this acceleration (energy is stored in the rotation of the wheel). That force can not be larger than the STATIC friction of the tire to the belt or the wheel would begin to slide. If it is not greater the force is transmitted to the belt and the plane. But note, that force is not what eskimo and I are considering friction.

HiTech

SteveBailey · « **Reply #253 on:** January 24, 2007, 11:55:43 AM »

I just read this whole thread and it is unbelievable that some of you don't get it.

The initial question is a bit oddly worded. The belt is spinning in the opposite direction of the wheels.

The plane would move forward at a normal take off as thrust is applied(to the air, not to the belt). The conveyer wouldn't double the speed of the wheels.. it would bring the speed of the wheels to 0. The plane would move down the conveyer at normal acceleration and take off departure behaviors, the speed of the wheels would be 0.

The only way the wheels would accelerate is if the converyer was spinning the SAME way as the wheels.

Basically, the plane would move along the conveyer at 0 ground speed until it achieved enough air speed to lift off.

The fact that you guys don't get this is staggering.

lukster · « **Reply #254 on:** January 24, 2007, 12:03:38 PM »

Quote

Originally posted by SteveBailey
I just read this whole thread and it is unbelievable that some of you don't get it.

The initial question is a bit oddly worded. The belt is spinning in the opposite direction of the wheels.

The plane would move forward at a normal take off as thrust is applied(to the air, not to the belt). The conveyer wouldn't double the speed of the wheels.. it would bring the speed of the wheels to 0. The plane would move down the conveyer at normal acceleration and take off departure behaviors, the speed of the wheels would be 0.

The only way the wheels would accelerate is if the converyer was spinning the SAME way as the wheels.

Basically, the plane would move along the conveyer at 0 ground speed until it achieved enough air speed to lift off.

The fact that you guys don't get this is staggering.

Ok, I'm glad you used the word spinning in regards to the conveyor belt. For the purpose of clarity take the belt out of the picture and just set the plane on the convey belt's wheel. This is the part of the belt that "spins", the belt itself only moves horizontally. If the belt spins in the opposite direction but at the same speed from the plane's wheel then plane's wheel will stay in place. Spin the belt wheel in the same direction and the plane's wheel and you will get the effect you described.

I didn't say that well. You have to envision which way the conveyor belt's wheel is spinning with regard to the plane's wheel. In order for the belt to move forward causing the plane's wheel not to spin as the plane moves forward the belt's wheel will have to spin in the same direction as the would the plane's wheel if it were not on a moving belt. The original question proposed a belt moving in the opposite direction.