Well kinda....... a RWD car with the rear wheels on a dyno will sit still because the rear wheels are what pushes it forward. A plane is driven forward by the prop/jet so it is more like putting the rear wheels of a FWD car on a dyno. Sure the rear wheels will just spin but the front wheels will pull the car forward.
OHMYGOD!!! What the hell is this ridiculous thread?? (I only read pages 1,2 and 6- so if I missed something important let me know...) Groundspeed=Time/Distance. Nothing more, nothing less. Conveyor belts, treadmills, nothing else matters. From here to there in x seconds/minutes/hours. Treadmills need not apply. Groundspeed has ABSOLUTELY nothing to do with whether or not an airplane flies. Airspeed on the other hand... I have personally flown a J-3 Cub from liftoff, through an entire pattern and to landing without changing heading. Why?? Because the winds on the ground at that time were in excess of 35MPH straight down the runway. In short: Airspeed (Loosely defined as the speed of the relative wind over the wing.) is not related to groundspeed in any way. At the time I landed the plane I was not moving forward at all; the wheels weren't moving a bit when they came in contact with the ground. If wheelsped is related to the ability to fly then how do you explain my flying with no wheelspin at all?? Newton's Third Law: For every reaction there is an equal and opposite reaction. The thrust generated by the engines (In whatever form.) Does NOT act on the wheels- it acts in the opposite direction of whatever the engine is attached to. (Ummm the plane maybe??!!) Ever see a plane on skis?? What would the addition of skis do to this argument?? NOTHING!! They would slide along the ground faster and faster- actually decreasing their overall drag coefficient as they went. Substitute skis for wheels and what happens to this question?? The really really simple answer: As the plane accelerated to V1 (Around 140 Knots in a 747, as opposed to the 250 MPH previously offered...Or 35MPH in a J3 Cub, Maybe 70 MPH in a Pitts, or perhaps 220 Knots in an F-104..) the wheels would spin faster and the treadmill would speed up. and they would continue to go faster and faster until... Either the plane passed V1, rotated and flew, or the wheel bearings failed and the plane probably still passed V1 and flew. (But with a fire in the wheelwell. Ouch!!) As long as thrust is greater than drag the airplane is going forward. The treadmill is not adding any drag- in fact it is actually decreasing it!! (Yes decreasing it!! Think about that please!!) As long as thrust is greater than drag the airplane will be accelerating. PERIOD. My suggestion: read Stick and Rudder by Wolfgang Langeweische- it will answer all the questions you could possibly create from this thread. (Not to mention being written around 50 years ago...) Cheers, dce
That is the only bit I would disagree with....... The aerodynamic drag would stay the same but because the wheels would spin twice as fast there would be would be a slight bit more "mechanical" drag from the additional friction in the wheel bearings...... I'm open to change my view if you can give me a new way to think about it
Just let this thread die already, it's pointless to argue it any further, and we're all getting dumber by the minute reading some of this crap.
Oh Yeah.... well what if it were George W flying an F-14 off the USS Ronald Reagan with a treadmill? How would it change if John Kerry was the Captain of the ship and Al Gore was the Catapult Officer?
If GW flew an airplane off a carrier that didn't have dual controls well... That's why they have a rescue helo always in the air isn't it?? If Kerry was the Captain of the RR then who the heck would know where the ship was going?? And if Al Gore was the catapult officer then no airplanes would be launched since the entire catapult crew would be engaged in a discussion about the merits of steam as a renewable energy source!! Cheers, dce
No, just the intarweb. If this kind of talk keeps up I'm gonna have to move this thread to the P&R section.
Crikey, it goes on. Teak you are right the conveyor makes little difference. I was wrong with the opposite direction thing, it still would not cause a catapult effect what so-ever because as you say the wheels spin freely. I think the issue here that I struggle with is the wheels turning at the same speed as the conveyor. One minute I think that they have to turn slightly faster, due to acceleration than the conveyor, but of course as the conveyor is programmed to match the speed of rotation then it increases to equal the speed. I still think the question is valid though whichever piggin way the conveyor goes Art, I am not sure where your evidence lies that would prevent a plane taking off on a conveyor going any direction. I was getting tied up in "problem constraints", but really it is much simpler than we think. As I think you mentioned you could chock the wheels but that would not hold an aircraft for long.
ummm,... how is the air moving if the plane is static? The only thing to cause a plane to fly is lift, from air. Try to fly a kit on a calm day without running.
Newton's third law again: With no treadmill the wheels have to overcome the friction of their contact point with the runway. (Not aerodynamic drag, just plain old friction.) With the treadmill the only thing that has to be overcome is the friction of the wheel bearings, as the treadmill effectively zeros out the contact point friction. Thus whatever drag had been induced by the friction at the contact point of the tire is gone- driven away as it were by the energy addition made by the treadmill. We're discussing trivial points- my only reason for suggesting that people think about the opposite trend of this entire thread was to get them off this silly idea that wheels have anything whatsoever to do with whether or not an airplane flies. Cheers, dce
Wonkazoo, you are correct, with the correct wind speed, the wheels have nothing to do whether the plane can fly or not. If the forward belt speed matches the rotation speed the plane will lift. Even if the plane is stationary, facing into the wind and the wind is howling at rotaion speed, the plane will lift off. However, if you read the partial quote above by Mr. Henson, the belt is moving in the opposite direction of rotation. The plane is moving backwards? Guys, there are days when I am dumber than other days. This is one of those physics questions in 8th grade where you wanted to break the pencil off in the teacher's azz becasue there is not enough clarification or information to make a decision. The only thing stupid about this is we keep posting.
If you don't like this thread, don't post in it. I am enjoying the discussion. What's the problem? The plane does not take off. It really is very simple. Wonkazoo, you are right, there is virtually no friction force except that in the bearings. That's why the tires accelerate so quickly. If you push something at rest and its well-greased, it accelerates quickly. If its a wheel, it rolls. If the ground on which it is rolling forwards is rolling backwrds at the same speed, then the wheel will appear to be still even though it is rolling. That's because it is rolling in place. If the wheel is rolling in place, the plane does not move forward. The question says that the the treadmill speed is the same as the wheel speed at all times, but the movement is in the opposite direction. So the tires rotate, faster adn faster, but they stay in place. The plane does not move forward. No forward motion, no airflow, no lift, no take off. Done.
Yup, it's a valid question. Not well worded, but valid. Strap a battery powered fan (decent power) to a freewheeling model car or make a wheeled platform out of lego and attach a fan to it. See if it will move forward on level ground. You have approximated a propellor driven airplane on the ground. Now put it on a moving treadmill. Start up the fan and do a run in each direction. You will find that it goes much faster in one direction than the other. This will prove to you that the relative direction of the treadmill does matter, even though the wheels are not directly powered. One direction acts like a catapult, the other like a brake. Now block more and more of the fan's intake until it produces just enough thrust to make the the car pretty much sits still on the treadmill. So, varying the amount thrust does affect the thrust at the wheels. More thrust means higher wheel speed, until you can overcome the treadmill's belt speed and your car will move forward against the treadmill. Finally, increase the treadmill speed to a higher level. The car should move backwards. Again, if you remove more of the fan's blockage, you will be able to get the car to stay still on the treadmill at the higher speed. Isn't it reasonable that at all points between the lower and higher speeds of the car, the treadmill also has a corresponding matching speed that will make the car sit still in relation to the ground? This is why the plane will sit still even as the engine provides more thrust and the wheels spin faster--because the treadmill also accelerates at the same rate in the opposite direction at each moment, matching the wheel speed at any given moment. The net result is that even though the wheels and treadmill speed up, the plane does not move. What have we proven? A. The engine's thrust does cause the wheels to rotate even though they are not linked. B. Engine's thrust causes the wheels to rotate on the ground OR on the moving treadmill. C. Direction of the treadmill surface rotation matters--same direction would result in a catapult effect. The plane would accelerate quicker, adn get more lift earlier. D. The tires rotating forward and the treadmill moving the same amount in the opposite direction result in a net movement of ZERO relative to the ground. Zero travel means zero wind over the wings. No take off. It really is that simple. Case closed.
Did you take your crabby pills this morning? Or are you the teacher whose azz I was determined to deposit the pencil?
Exactly. For all those saying the plane takes off, what is happening at the wheels as it is moving forward? Tehy have to be going faster than the treadmill going in the other direction in order to move forward over the treadmill. The question says their speed is matched at all times. How can the plane move forward if it is attached to the wheels? Answer, it doesn't. It is a case of dynamic equilibrium.
Art, think about this again here we go:- The aircraft is on the runway no thrust no forward movement. The conveyor belt is stationary as the wheels on the aircraft are stationary. The pilot applies a little thrust, the aircraft will move forward maybe only a small distance. This causes the wheels to turn (what else can they do?). The conveyor reacts immediately but not until the wheel turns it cannot guess the speed the wheel is going to turn. So at this point the aircraft must of moved at least a fraction. The conveyor cannot accelerate any faster until the wheels do. The pilot applies a little more or alot more thrust again the plane moves forward etc etc. This would all happen rather quickly, the plane would accelerate causing air to pass over the wings and generate lift at the aircrafts normal airspeed lift off. Of course you could argue that it is breaking the constraints at the initial movement of the a/craft wheel (i.e it is moving faster than the conveyor). In this case the problem is unsolveable, as neither are allowed to move. This is where a few are saying the question is invalid.
The question assumes that there is no lag and that the conveyor reacts instantly. It's a theoretical problem. In the real world, you would be right, although if you hook the plane and belt up to the appropriate sensors and power them independently, you could probably closely match the two speeds. You could certainly have sensors that trigger an increase or decrease in the belt speed depending on the plane's position, so as you applied more throttle to the plane engine you could get the belt to speed up quickly. It would result in the plane rocking back and forth just a few inches. In no wind, it would sit there rocking back and forth. It would not take off, if the belt speed could match the tire speed at full throttle and the tires and belt were moving in opposite directions.
OK. I will restrain from inserting the pencil. I appreciate your approach of logistically supporting your answer to this question. I am jealous that I was not able to make such an eloquent presentation as yours. However, like in school, I took all the short cuts but still came up with the right answer.
Quick physics question for you expresscat39. If you drop an elephant and a baby from a 100 storey building, letting them go at the same instant, which would hit the ground first? Let's say the elephant weighs 10,000 lbs and the baby weighs 10 pounds. Ignore air resistance for this question ie. they are being dropped inside a 100 storey tall tube that has a vacuum inside.
