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Senior Member
Registered: 10-28-07
Posts: 5022
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quote: ...we are NOT talking airspeed, the plane is still on the ground not in flight.
I know that. But the speed of the plane relative to the belt is still just a construction, based on the belt's absolute speed and the plane's absolute speed. In zero wind, the airspeed will equal the plane's groundspeed. The Mythbusters had to delay a day to wait for zero winds. The airspeed, as indicated in the cockpit *was* the groundspeed of the plane. The airspeed indicator registers whether the plane is on the ground or not. It doesn't matter if we call it "airspeed" or "groundspeed"... in zero wind it is the exact same thing. Airspeed/groundspeed = 25 MPH Beltspeed = 25 MPH Belt matches plane's speed, as per the question. Wheelspeed = 25 - (-25) = 50 MPH. Even if you still insist on objecting to the word "airspeed", let me rephrase... The relative speed of the plane with respect to the belt, is given by the absolute speed of the plane with respect to a chosen fixed axis, minus the absolute speed of the belt with respect to the same chosen fixed axis. Vrelative = Vabsolute - Vbelt. You're trying to set Vbelt = - Vrelative. We end up with Vbelt = Vbelt - Vabsolute. On a stationary belt the wheels will never begin to spin unless Vabsolute is non-zero (because wheels spinning = Vrelative = (Vabsolute-Vbelt) = Vabsolute for the initially stationary belt). So, what is the correct Vbelt to satisfy: Vbelt = Vbelt - 0.000000000000000000000001 ? There! I didn't say "airspeed" at all. The plane needs to move to have the wheels begin to turn, regardless of what name you give to the absolute motion. |
Senior Member
Registered: 01-31-08
Posts: 4354
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For all of you no fliers: The treadmill cannot exert any appreciable force to keep the plane back. There was an excellent youtube video posted earlier, unfortunately the postee misinterpreted the meaning of it and thought it was an illustration of taxiing on a treadmill, wher they demonstrated that doubling the speed of the treadmill had no effect on the plane. It did not increase the backwards force on the plaen. Rolling force is a constant, once you are rolling the force back is the same at 5, 10, 20, 30mph etc.. So once again for you no fliers: What force or forces will hold back the plane on the treadmill?
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Senior Member
Registered: 06-01-06
Posts: 603
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quote: #5 THE TEST: For the test to work properly the plane must stay close as possible to one place. You do this by increasing the treadmill speed to hold the plane in place with resistance of the treadmill and the wheels (see that did come into play) that is matching the forward thrust of the plane. So as the plane is pushing itself forward with thrust you are pulling back on the plane with resistance from the treadmill, and that will make it stay in one place and it will not gain and lift since the air cannot move under the whole width of the wings (this gives the planes it ability to lift off the ground) and it will NOT take flight.
Thing is, that will never happen. It does not matter how fast the conveyer is going, it cannot “match the thrust” of the airplane engine if the pilot is applying enough thrust for the plane to fly. The conveyer could be going 100 times the take off speed of the plane and the thrust required for take off would be virtually the same as a normal takeoff. The only way the plane can be stationary is if the pilot is applying barely enough thrust to taxi or if the plane is tied to a tree. |
Senior Member
Registered: 10-28-07
Posts: 5022
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Let's have a look...
The rolling resistance of the plane's wheels is given by:
F=Fn*b/r
Where Fn is the normal force acting on the wheel, b is the coefficient of rolling resistance, and r is the radius of the wheel.
So, when we plug different velocities in to the equation to see how the spinning wheels affect the drag the belt is offering...
Wait a minute... there's no term for velocity!
Once the wheel begins to roll, it offers a constant resistance. To get the wheel to begin to roll, the net force acting on it *already exceeds* the rolling resistance offered by the wheel. Even if this wasn't a full-throttle situation (although the trying to take off implies it is), once the throttle is in a position to break equilibrium and begin to roll, the belt *can't* offer any additional resistance to motion. Even with such minimal thrust, the plane would continue to accelerate, albeit slowly (until the air resistance countered the thrust).
Once the wheels begin to move, there's nothing the belt can do to affect the plane*. A plane attempting to take off will easily counter both the rolling resistance and the air resistance, and continue accelerating.
* yes, I know, I know...
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Senior Member
Registered: 01-31-08
Posts: 4354
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Excellent point roofinguy, I have been meaning to post something on that order. By the way did you realize that, once he applies enough throttle to get the wheels rolling, he would actually have to throttle back from that position just to keep the plane taxiing in place on the treadmill? So in other words if he does not want to move forward on the treadmill he actually has to take an action to slow down.
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Senior Member
Registered: 10-28-07
Posts: 5022
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How about the friction of the belt on the wheel? Surely *that* can be used to hold the plane back.
Wait a minute... physics says that the frictional forces on a free-rolling wheel are zero!
Ok, that's an idealized situation, and there will be very small frictional forces acting on the wheels in real life, but again they are too small to oppose the plane's thrust. Or else a plane couldn't roll on a normal runway!
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Senior Member
Registered: 10-26-07
Posts: 1448
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Your forgetting about all the no-flies piled up in front of the plane.
Also the pilot might be related to graham. He was saying some weird things as he was heading for his seat.
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Senior Member
Registered: 07-03-07
Posts: 1012
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quote: Originally posted by codeman2925:
Hey it's Codeman from a few days ago.
Now I have been reading some of the messages and the correct answer has already been posted by many people.
I went and found out where this all started and that is with remote control planes.
Now the concept of the myth (really question in this case) was correct but the test failed. And I will tell you why.
Now as I am reading the messages there are allot of pieces to the puzzle but no one gets the whole picture.
Ok now #1 the ORIGINAL QUESTION WAS: Hi there is a question going around on a remote control plane forum that goes like this:
"Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation. There is no wind. Can the plane take off?"
The Quick answer is no. The propeller cannot provide enough lift on its own it provide the lift needed under the wings for the plane to lift off the ground.
NOTE: Now before you start talking wind speed vs. wheel speed you must realize that the plane is still on the ground, so you will have wheel speed. This does not mean that the wheels are powered by an engine at all, because we all know that the wheels are free spinning.
#2 THE PLANES MOVMENT: Now the plane is sitting on the ground and in order for it to move under its own power it must use the propeller to push itself using air thrusted thru the propeller (hence the word "Thrust") and then it will start to move on the ground on its wheels, but thrust must be powerful enough to overcome the resistance of ground and the wheels.(This comes into play later) This is would be the planes groundspeed since it is still on the ground at this point.
#3 WHERE THE "Conveyor Belt" COMES INTO PLAY: Moving the conveyor belt in the opposite direction will act as resistance to the plane though the wheels holding it back or pulling it backwards.(as seen on the small scale test they did)
OK now have the basics for the question of "Plane on a Conveyor Belt/Treadmill"
#4 THE PLANE AND BELT TOGETHER: Ok now with the plane on the conveyor you will still have the principles of resistance, but you can now control the resistance of the wheels on the plane by the speed of the treadmill.
#5 THE TEST: For the test to work properly the plane must stay close as possible to one place. You do this by increasing the treadmill speed to hold the plane in place with resistance of the treadmill and the wheels (see that did come into play) that is matching the forward thrust of the plane. So as the plane is pushing itself forward with thrust you are pulling back on the plane with resistance from the treadmill, and that will make it stay in one place and it will not gain and lift since the air cannot move under the whole width of the wings (this gives the planes it ability to lift off the ground) and it will NOT take flight.
CONFIRMED!!
Thanks and have a great day!
-Codeman
You utter utter plonker. You are busy pontificating about how the correct answer has been posted when in fact you offer the WRONG answer!!! And you have the audacity to claim "no one gets the whole picture." Well how the hell would you know as you don't even understand the basic "picture"? "Can the plane take off?". The quick answer is YES not "no". You are completely and utterly wrong. The wheels cannot provide a force to oppose the thrust as you suggest except in very special circumstances. Imagine that the belt was covered in ball bearings or some frictionless ice, what do you think would happen then? Well the belt would not provide any force to oppose the thrust and that is the equivalent situation for the plane's wheels, they effectively act like a big ball bearing. You are busted for posting this drivel. |
Senior Member
Registered: 01-31-08
Posts: 4354
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Actually right now I am envisioning a belt covered with no fliers, everytime the plane starts to speed up it runs over one of them, accompanied by a large Ooof! And they are right! If you do cover the belt with no fliers the plane does not fly. There are too many of them for the plane to build up the right speed.
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Senior Member
Registered: 10-26-07
Posts: 1448
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quote: There are too many of them for the plane to build up the right speed.
That's why you have the steam roller to smooth out the treadmill. |
Senior Member
Registered: 01-31-08
Posts: 4354
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Naw, we don't have to go that far. Even though the plane doesn't fly now, there is something stangely satisfying about running over all of these no fliers.
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Senior Member
Registered: 02-04-08
Posts: 418
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Careful so you don’t hit them with the propeller. That might hurt the prop. Oh no the prop, its come for me, ahhhhhhhhhhh!!!!!!
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Member
Registered: 10-22-05
Posts: 6
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Well I have read some of your responses and I will have to say no I don't have a pilot’s license which would make me a “no flier” as you call it but my brother-in-law is a pilot. Now I have pushed a few planes out of a hanger and it you have to put some weight behind it to push it. That is ground to wheel resistance, and yes as the wheels get to spinning the resistance is reduced but as long as the plane is on the ground there is resistance from the wheels.
Now it is not a 1 to 1 ratio for resistance so in order for you to keep the plane in a stationary position you would need to find out what the ratio is. And no I don’t know what that is, it depends on the conditions of the ground surface and the plane.
Now I have flown over the last weekend and the ground offers plenty of resistance.
In fact the rougher the surface, the greater the resistance for the plane.
And for the 0 friction thing… get on a bike and petal on a flat surface and then stop pedaling… if you stop …. That is call resistance and the same principle applies to the planes wheels. There will always be friction on the wheel somewhere, now the ball bearings reduce that by a lot but there is always some friction somewhere in a wheel and bearings.
Roofingguy, you are right that there no friction on the surface between the ground and the wheel while it is rolling, the bearings are the point where the friction is taking place, but there is resistance and that is why the rougher the surface the greater the resistance. Like going from a hard surface to a soft surface like dirt, it will offer the tire greater resistance.
Thanks again,
Codeman
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Senior Member
Registered: 01-31-08
Posts: 4354
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Codeman, yes the rougher the surface that you are rolling on the higher the rolling resistance. But rolling resistance is relatively constant. Here is a video that shows that rolling resistance does not increase with increased speed. It is very hard to judge the constancy of rolling resistance when you are pushing something by hand. |
Senior Member
Registered: 03-22-07
Posts: 877
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quote: Originally posted by feldspar99:
You still have not answered the rather simple question – is the pilot allowed to try to take off. Nor have you suggested a test design that would fit your interpretation.
the questions have been answered in detail. I will forgive for not knowing that as there are 214+ pages here and I have no idea where the answers were posted. |
Senior Member
Registered: 10-28-07
Posts: 5022
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quote: And for the 0 friction thing… get on a bike and petal on a flat surface and then stop pedaling… if you stop …. That is call resistance and the same principle applies to the planes wheels. There will always be friction on the wheel somewhere...
No. Even if you remove the wheel completely. The only forces acting on it as it rolls along (neglecting air) are the wheel's weight and the ground reaction. The actual friction forces on the wheel are zero. The rolling resistance is what stops the wheel from going forever and is essentially due to the ground reaction not quite being vertical. A steel wheel on a steel rail has a very low rolling resistance. A wheel on very soft ground has a higher rolling resistance. This is why it's harder to pedal your bike, the softer the surface gets. The tarp in this instance offers no real extra resistance for the wheel's rolling than what the bare asphalt does. The friction between the ground and the rolling wheel we can both agree is zero. The rolling resistance of the wheel and any bearing friction (which is mostly rolling resistance of the axle) are the exact same for the plane on bare asphalt and on the tarp. Yes, it would be higher on soft ground. But planes take off of concrete, asphalt and turf all the time. The thrust greatly outweighs any resistance the wheels/bearings offer. There is nothing "extra" in the belt scenario that isn't in the normal takeoff, other than the belt spinning the wheels twice as fast. The same minor throttle that gets the wheels spinning on solid asphalt will get them spinning on the belt. |
Senior Member
Registered: 06-01-06
Posts: 603
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quote: the questions have been answered in detail.
I will forgive for not knowing that as there are 214+ pages here and I have no idea where the answers were posted
Seems like in the amount of time you spent telling me you already answered the questions it wouldn’t have been that tough to type a simple yes or no and a couple of sentences to describe how you would do the test. |
Senior Member
Registered: 10-28-07
Posts: 5022
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Yes, but feldspar, the answers have been sparse and varied.
At one point in a long-closed thread when I was talking about the pilot and the throttle, graham75 asked "What pilot? What throttle?" and said to only go by what was specified in the question. So under that answer, there is no pilot to worry whether he's allowed to try and take off or not. But in other responses, graham75 seems to have referred to the pilot and what his actions might be, so who knows if the pilot exists or not.
A good starting question would probably be "Is there a pilot?" followed by "Is the plane serviceable and fueled?".
Once we're sure we're actually allowed to use common sense and read those into the question, then we can start to sort out the actual question itself.
The answers are more likely just "I've answered that before", though, which is no help in terms of figuring out *which* previous answer we should be going by. But woe to the one who chooses the *wrong* previous answer.
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Member
Registered: 10-22-05
Posts: 6
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Subductionzone, Yes you are correct (and the video shows) that at a certain point the treadmill will not offer any extra resistance after a certian point, and at that piont as you speed up the treadmill it will not effect the resistance on the plane, and the video clearly shows that.
And I knew that but in explaining it it came out on the message boards wrong. I was a work and trying to write that and work at the same time lost alittle inbetween. I apoligize to anyone I offened.
-Codeman
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Senior Member
Registered: 01-31-08
Posts: 4354
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I don't know if it was roofinguy or rational that was calling graham "cracker". I propose a new nickname "monkeyboy" just read the first three posts on this thread Here.  |
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