will it take off?

[Paddy]

MossRoad,

Ok I read the site. Two things are kinda funny here. First, your just quoting another message board and second, if you read the post closely, he stating all the No-Fly camp is changing the the problem from "will it Fly" based on the original problem to "can we make a MCB that can counter act the force of the plane" He says , "sure" but that was not the original problem.

Then you go on to say;

((Now I could say "what you fail to understand is..." but I won't. But I will say, open your mind to the possibility that if you could make a conveyor that would move EXACTLY at the same speed as the airplane, but in the opposite direction, it will keep accellerating to counter the planes accelleration in the opposite direction, preventing the plane from moving.))

I say; If they both have the same velocity, they both have the same acceleration. There is no countering here, just matching. You can have very low acceleration and still have very high velicity. not sure your point

But hey let's start with good basic problem solving skills that they taught us in engineering school, sketch/vectors and assumptions. (I hope we don't have to make to many assumptions because that could get us all turned around!)

Problem Statement: (I always hated this part, re-right the problem!)

---------------x-------------x--------------x-------------x--------x----
a plane is standing on a movable runway( something like a conveyor).as the plane moves the conveyor moves but in the opposite direction.the conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction. the question is will it fly?
------x--------x--------x---------x----------x----------x---------x-----
Sketct time;
I'm not very good using sketch tools so I will try to use a mental description I think we can all imagine and work with. let's use some sort of reference like a foot ball field's lines.

Ok, let's start with the first statement about "a plane standing on a movable runway". Ok, we got two objects to place in our sketch. Let's put a dot on the convayer and a dot on the plane and put them on the 50 yard line. I think these dots might help to keep this all straight

Assumption 1; the plane isn't really standing. It doesn't have legs, It's probally was meant to be resting. ie, not moving at T= zero

OK, now let's get that next statemnet we need to get in our sketch. "as the plane moves the conveyor moves but in the opposite direction." We need to place some velocity vectors in our sketch. Let's draw an arrow ponting left on the convayers dot and lable it, Vc (velocity convayer). same for the plane now, but in the oposite direction. This would mean an arrow pointing right labled, Vp.
Assumption 2; We don't need acceleration vectors because of the next statement, "the conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction.". That means If |Vc| = |Vp| then |Ac| = |Ap|. The |Xx| just is way to compare values with out direction

Assumption 3; Speed and velocity can be used interchangabley. The difference between speed and velocity is V has direction and the problem has already stated "in oposite directions"

Last part of the sketch is to make sure the vectors, Vc and Vp are the same length because, ""the conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction.". Let's go a head and note in the sketch, |Vc| = |Vp| just in case someone misses that, Vc is a negitive value because it's going left and Vp is positive...well I think you got it.

Summary; sketch of plane with dot/vector pointing R and lined up on 50 yard line and convayer with dot/vector pointing L

Assuption 4; Plane is a realist type with normal speeds, wheels and bearings ( not rockets or teflon!!)
Assuption 5; Convayer can match the velocity and acceleration of this normal plane (No magic needed here because the planes Vp and Ap are not so impressive. The group of engineers witnessing estimate this weeny plane would require a full min to take off on a good day) As stated in the problem, matches planes speed, carefully note that does not mean match plane thrust or wheel rotational speed or teflon coated rocket do do
Assuption 6; Since bearing friction is such a small percentage compared to wind drag, it can be ignored. (Sea planes endure much higher landing gear drag/resistance than wheeled aircraft and they appear to take off just fine, just with a longer take off distance and time) If you don't belive this take your ele drill and chuck up a 4" dia wheel some or another and spin a small trailer wheel. Once you get her spinning, you won't find it takes much force to hold the drill. If you still don't beleive me ask your self, if I installed two extra wheels on a plane, would it prevent it from rolling? Because that is what the convayer is doing if the friction in porportion to wheel speed, doubling the wheel friction vis double speed. Double speed of the wheels might be diff to understand if you can't get "two cars moving at 30 mi/hr, one heading east and one heading west, How fast are being seperated by?" question. Hint, answer is 60 mi/hr

Time for tables. (I always hated these too)

Time (Sec)----Vc-----Vp

0----------- 0------ 0
1----------- -1----- 1
2---------- -1.1---1.1

Time out here! Now let's go back at look at that sketch and the problem. OK, the speed of the plane is being matched, good. The plane is moving as stated in the problem The dot on the plane is at the 52 yard line and the Dot on the convayer is on the 48 yard line. Ok..please don't make me complete this table! Thanks, I just jump ahead 30 sec

32----------- -42-- 42
42---------- -50-- 50

Stop! we left the foot ball field and were heading for the goal posts at a high rate of speed.

I interviewed the pilot after this crazy stunt and he told me it felt a bit funny. like he had a bit more drag and if wasn't for those darn goal post would reached lift off speed.

 

[RayH]

These are the longest posts Ive ever seen and I dont understand them anymore. I tried to take things back to the basic question without all the anaolgies and formulas. "Does the plane move".
The no flys still say no but they cant explain to me how the conveyor begins moving without the plane moving first. That goes against the original question and according to the question, can not happen. The MCB can not move unless the plane moves and the plane can not move (according to the No flys). I still want to know who thinks the plane will sit in one place, not tied down, with full power on a non moving conveyor and not move?
I think I'll just add to it. The "no flys" say that with the MCB going the same speed as the plane, the plane will stay in one place.
According to that logic, if the MCB was not adjusted correctly and went slower than the plane, the plane would move forward and if the MCB went faster than the plane, the plane would move backwards. Of course, this assumes that the plane and MCB can move at all.

 

[SPYDERLK]

Original problem statement: "a plane is standing on a movable runway( something like a conveyor).as the plane moves the conveyor moves but in the opposite direction.the conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction."

jk96 The two conceptual interpretations of the problem follow directly from its use of the term "match". One of its definitions is "to compete with successfully".

I think consideration of both interpretations is necessary in order to formulate an answer deserving of full credit. I see that we have done this in this thread.
Larry

However, Im sure this will be refuted indefinitely.

 

[RayH]

I guess Im close minded because Ive only seen the question in one way since it was first posted. I can still only see one version of it. Ive considered the meanings of every word of the question and I still only get one meaning. You guys are seeing something in the question Im not seeing.

 

[jk96]

Ray,

Here's were we ended up.

1. The "will take off" crowd has proven that if the MCB cannot move faster than the airplane is capable of flying, then it will take off. Even the no fly's have acknowledged this.

2. The "will not take off" crowd has proved that the airplane can be held in place on the MCB, but only by getting creative with the word "match". By doing this, they are allowing the MCB to operate without any restrictions on acceleration and speed. Here's the theory. Because there is some resistance pushing back against the airplane from the contact with the MCB and the wheels, and this resistance increases as the MCB speed increases, even if it's very minute, the MCB can stop it. At the very fraction of a second the plane starts to move, the MCB reacts with enough reverse speed to equal the thrust. It continues to increase this speed to the point that it creates the same force pushing back on the plane as the engine produces thrust. On paper, in theory, it might work. I don't know. I'm not smart enough to figure it out but I understand how their justifying it.

Now, we have one finale chance to prove that even this theory is wrong. We need to prove one of two things.

1. At 500,000 mph or whatever crazy speed the conveyor would have to move to equal the thrust of the engine, enough wind would be created to lift the plane of of the ground, even for a fraction of a second. Remember the question was "Will it lift off"

or

2. The conveyor would have to move faster than the speed of light to create force equal to the thrust of the engines. Because we know nothing can move faster than the speed of light.

What do you think. Anyone up for another thread?

jk

 

[SPYDERLK]

jk. Rotational acceleration of the wheels, not speed of the wheels. Yes, since speed develops over time with acceleration, the speed will eventually become huge. Speed does not occur instantly -Speed is aceleration [A] times the amount of time that A lasts. The wheels rotational speed starts out at zero and increases continuouslly, but steadily, to hold against thrust. As a ballpark example, the acceleration, A, may be 200rpm per second. This means that the wheel will be turning 200rpm after the 1st second, 400 after 2 seconds, 1100 after 5.5 seconds, etc. Even at this relatively conservative A rate the wheels could be expected to explode within 20 or 30 seconds. This would end the experiment at a wheel surface speed of maybe 400mph. By contrast, the speed of light is 186,000 miles per second. The experimenters could be expected to reach their conclusions long before this speed was reached. Seriously tho, the idea is that the plane only has to be held still for a short time to show that it can be, and wheel speeds during proof of this feasability will not climb very high.
Larry

 

[TomKioti]

I still say send the whole thing to the folks at Mythbusters and see if they'll put all this to rest. I'm still with Pat on this. He has a crystal clear understanding of physics IMHO.

 

[jk96]

Spyder,

I understand. The last part of my post was just a joke. You guys are really taking this serious. My only point over the last few posts is that the "movable" runway had to be changed to a "magical" runway and the phrase

"conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction"

had to be manipulated or very creatively interepreted to mean "the runway turns at whatever rate it darn well pleases" in order to keep the plane from moving. I think its safe to say that there are very few people who when presented with the question would interepret the action of the runway the way you, Pat, and Tom are.

jk

 

[turnkey4099]

to resurrect a way back post #462 I think it was about the plane on floats with the current running backwards to the direction of the plane. It is not the same problem as the floats are fastened to the current and the plane is being drug backwards due to that.

For the no-fly due to friction crowd. Picture your plane as at taxi speed 10mph and stationary with respect to the ground per your theory. Pilot wants to speed up to 11 mph and keeps advancing throttle until he does. How do you even begin to postulate enough friction to conteract the force of the engine. The wheels will be rotating at 10 mph and only need a slight increase in speed to reach 11 mph. By extention, the MCB

 

[turnkey4099]

to resurrect a way back post #462 I think it was about the plane on floats with the current running backwards to the direction of the plane. It is not the same problem as the floats are fastened to the current and the plane is being drug backwards due to that.

For the no-fly due to friction crowd. Picture your plane as at taxi speed 10mph and stationary with respect to the ground per your theory. Pilot wants to speed up to 11 mph and keeps advancing throttle until he does. How do you even begin to postulate enough friction to conteract the force of the engine. The wheels will be rotating at 10 mph and only need a slight increase in speed to reach 11 mph. By extention, the MCB

Hit the wrong button. Continueing:

By extention, the MCB wheels cannot cause enough friction to account for all the engine thrust. As I said before. The only way the plane won't fly even in theory is to postulate a huge amount of axle friction.

Harry K