Moderators Travis In Austin Posted July 10, 2015 Moderators Share Posted July 10, 2015 The thrust of the engine(s) is on the air, the craft moves forward, lift happens and it takes off. Oh ffs. Put the rubber band propeller plane on the treadmill and watch it take off. There is lift because the wheels on the ground versus the treadmill mean nothing. The thrust of the engine(s) is on the air, the craft moves forward, lift happens and it takes off. I'm taking the bait again, and running the line out!!!! Whizzzzzzzzzzzzzzzz!!! Yes, and if the wheels were frictionless and free to spin (thus treadmill has zero effect), the trust generated by the prop would propel the craft forward, which would in turn result in lift being generated by the air flowing across the planes foil. Ah!!!, but friction is real, and the craft will therefore move in a direction opposite to that of it's intended design use. And if memory serves me correctly, this problem stated that the treadmill moves and accelerate with a force that constantly matches the forward thrust (but opposite in direction) generated by the airplanes engine, be it a Lycoming / McDonnell Douglas / Pratt Whittney / Slingshot / Rubberband / Squirrels / Hamsters / Gerbils or anything your imagination can dream up;...... and unless the pilot stands on the brakes, there will always exist zero net movement of the aircraft relative to the air around it. Doesn't matter the engine type, size, or output, forward thrust is always being negated by the rearward movement caused by the treadmill. Put your car on the same self-accelerating treadmill with the skinniest tires money can buy, place the transmission in neutral and kick-on the treadmill. What happens? Bingo!! You either get it, or you keep bumping this very informative and educational tread. cough!, cough! Gilbert, You are thinking with a car analogy. The friction of the wheels is minimal and once you get them rolling it takes very little forward thrust to move. Pretty much every plane I have flown will taxi at the lowest idle speed. The analogy with a vehicle is to put it in meutral on the conveyer belt. Attach a cable to the front bumper of the vehicle beyond the length of the conveyor belt and attach it to a winch. You start winching the vehicle it begins to movw forward, the conveyer belt mives backwards, car keeps moving forward because car is in neutral and free spinning. Which gets faster and faster, car moves forward faster and faster. Put wings on that car and it will fly if pulled fast enoufh, which is one way they launch gliders. The propeller/jet engine on an airplane acts of the airplane the exact same way at a cable pulling on a car. The first question to ask is what would keep the plane from miving forward on the conveyor belt. It would not be the friction if the wheels, it is minimal at test, and way lower still once they begin rotating. Quote Link to comment Share on other sites More sharing options...
moray james Posted July 10, 2015 Share Posted July 10, 2015 gee why do they need a catapult on an aircraft carrier? if all they really need is a conveyor belt? So wind tunnels only really need a conveyor belt not wind. Since when do spinning wheels provide lift? ok so you are flying in space in your coupe de ville at the speed of light and you turn the headlights on, do you see them? 1 Quote Link to comment Share on other sites More sharing options...
cradeldorf Posted July 10, 2015 Share Posted July 10, 2015 Hey I once saw a winnebago flying through space and it had a Mog on board... I think his name was Barf.... Quote Link to comment Share on other sites More sharing options...
Moderators Travis In Austin Posted July 10, 2015 Moderators Share Posted July 10, 2015 gee why do they need a catapult on an aircraft carrier? if all they really need is a conveyor belt? So wind tunnels only really need a conveyor belt not wind. Since when do spinning wheels provide lift? ok so you are flying in space in your coupe de ville at the speed of light and you turn the headlights on, do you see them? The conveyor belt has no effect on being able to fly, one way or the other. It neither aids or hinders the plane's ability to take off. Quote Link to comment Share on other sites More sharing options...
Jim Naseum Posted July 10, 2015 Share Posted July 10, 2015 Heh..heh..make a force diagram.... Picture becomes clear. Thrust is a force directed at the air mass behind the plane.... That air is unrelated to belt!!! The plane is flying with reference to the air mass,NOT GROUND!!! ground references don't matter.... Belt is a ground reference. Quote Link to comment Share on other sites More sharing options...
Gilbert Posted July 10, 2015 Share Posted July 10, 2015 (edited) Gilbert,You are thinking with a car analogy. Nope, not at all. But it is the simplest analogy to use for explaining why it won't fly. The first question to ask is what would keep the plane from miving forward on the conveyor belt. It would not be the friction if the wheels, it is minimal at test, and way lower still once they begin rotating. Why don't you explain how friction decreases with the rotational speed of the wheels. If you do the math, correctly at least, you'll find out that it doesn't. Not-at-all. Questions: For the given problem, please answer the following, 1.) Is the conveyor necessary to support the plane while it remains stationary? 2.) Does the conveyor support the plane during initial roll? 3.) Does the conveyor support the plane at forward airspeeds less than the design takeoff speed? 4.) You're wrong if your response is "No" to any of the above. The conveyor belt has no effect on being able to fly, one way or the other. It neither aids or hinders the plane's ability to take off. You're right, the design of the plane is completely unaffected by the presence of the conveyor. The conveyor only affects the planes ability to gain forward airspeed, relative to the mechanical runway that supports it. Edited July 10, 2015 by Gilbert Quote Link to comment Share on other sites More sharing options...
Fjd Posted July 10, 2015 Share Posted July 10, 2015 Hey I once saw a winnebago flying through space and it had a Mog on board... I think his name was Barf.... I seem to remember a night where two alien “junkies” consume large quantities of plutonium nyborg while piloting a space ship. While trying to land the ship on a space station, one alien says to the other, ““It’s like, you know your perspective is fuc*ed so you just gotta let your hands work the controls as if you’re straight.” https://youtu.be/LzjyPZ76OO4 Quote Link to comment Share on other sites More sharing options...
Fjd Posted July 10, 2015 Share Posted July 10, 2015 ok so you are flying in space in your coupe de ville at the speed of light and you turn the headlights on, do you see them? See the earlier post below as we seem to be too late as it would appear that question was answered last week. Finally, what happens if you drive at the speed of light and turn your head lights on? They project light which, from your perspective, appears to travel at the speed of light. The speed of light is always constant, irrespective of the relative speed of the observer. Weird, but that's what they say. I answered this already in another thread last week. As you approach the speed of light the law of special relativity holds true, but it colapses when it equals C. Quote Link to comment Share on other sites More sharing options...
Moderators Travis In Austin Posted July 11, 2015 Moderators Share Posted July 11, 2015 Gilbert,You are thinking with a car analogy. Nope, not at all. But it is the simplest analogy to use for explaining why it won't fly. The first question to ask is what would keep the plane from miving forward on the conveyor belt. It would not be the friction if the wheels, it is minimal at test, and way lower still once they begin rotating. Why don't you explain how friction decreases with the rotational speed of the wheels. If you do the math, correctly at least, you'll find out that it doesn't. Not-at-all. Questions: For the given problem, please answer the following, 1.) Is the conveyor necessary to support the plane while it remains stationary? 2.) Does the conveyor support the plane during initial roll? 3.) Does the conveyor support the plane at forward airspeeds less than the design takeoff speed? 4.) You're wrong if your response is "No" to any of the above. The conveyor belt has no effect on being able to fly, one way or the other. It neither aids or hinders the plane's ability to take off. You're right, the design of the plane is completely unaffected by the presence of the conveyor. The conveyor only affects the planes ability to gain forward airspeed, relative to the mechanical runway that supports it. It is no different than a car on a conveyor belt and is pulled by a cable while in neutral. The hypothetical assumes it supports the airplane. Quote Link to comment Share on other sites More sharing options...
Moderators Travis In Austin Posted July 11, 2015 Moderators Share Posted July 11, 2015 (edited) Here is a guy who figured out a way to demonstrate the friction of the wheels. As you can see the conveyor belt supports the plane, it supports it on initial roll, and supports it as it accelerates up to take off speed, V1. Edited July 11, 2015 by dwilawyer Quote Link to comment Share on other sites More sharing options...
Moderators Travis In Austin Posted July 11, 2015 Moderators Share Posted July 11, 2015 What is the speed of sound at sea level? https://youtu.be/tIpjgCCZfCc Quote Link to comment Share on other sites More sharing options...
Gilbert Posted July 11, 2015 Share Posted July 11, 2015 (edited) It is no different than a car on a conveyor belt and is pulled by a cable while in neutral. The hypothetical assumes it supports the airplane. I hope you're not assuming the force generated by the a fixed position winch system is in anyway similar to the force exerted by the prop / turbine / squirrel powered engine. The plane's engine is directly connected to, and part of the plane. The winch is not part of the plane, since one end of that system is (either the winches motor or the cable end) and must be fixed or stationary...... UNLESS the force acting on your cable is due to the conveyors' movement (this concept might be interesting, but I doubt it's where you were going). And I'm sorry to inform you that there's nothing hypothetical about the conveyor supporting the plane. The weight of the plane is directly supported by the conveyor, same your house is directly supported by the foundation system. There's nothing remotely complicated about the concept of friction for this problem, and it is a constant value for both conditions, i.e. at the contact patch and at the bearings. It could be argued that friction acting on the bearings is not constant, but I don't think anyone is interested in getting that deep into the subject, besides, the change would be marginal. There are other forces at work, but I think we can ignore all of them since they are independent and of the conveyor system. Good luck with the youtube videos. Edited July 11, 2015 by Gilbert Quote Link to comment Share on other sites More sharing options...
DizRotus Posted July 11, 2015 Share Posted July 11, 2015 The original hypothetical is flawed, probably intentionally. Discussing whether a plane will take off on the hypothetical magical reverse speed matching conveyor belt makes as much sense as if the belt were replaced by magic anti-gravity pixie dust and then arguing about the effect of the dust. Notwithstanding the above opinion regarding the defective hypothetical, for the reasons stated by dwilawyer, the wheels, wheel speed, conveyor speed, friction, etc. are irrelevant. The fact that the runway surface is moving in the opposite direction of the plane (irrespective of the speed or the magic required to "match" in reverse the forward wheel speed) will not prevent the plane from moving forward and achieving flight. Quote Link to comment Share on other sites More sharing options...
Fjd Posted July 11, 2015 Share Posted July 11, 2015 (edited) The question is actually an interpretive logic problem more than a physics issue. We all agree on the physics required. I think. I agree with Oldtimer’s post on the prior page in that “the question is actually an interpretive logic problem more than a physics issue.” For example, if the question stated that the conveyer belt would exactly match the speed of the “tires” at any given time, moving in the opposite direction of rotation rather than the “plane” itself, we would have a paradox and a big problem. However, the version here states that the conveyor moves backward at the same rate that the plane moves forward. Therefore, if the plane's forward speed is 50 miles per hour, the conveyor must roll 50 miles per hour backward, hence, the tires rotate at 100 miles per hour. I attempted to explain propulsion and air speed on the prior page, then compare that to a gull on the beach. Think about how a gull can take off by only jumping up into the wind as another way to look at the “effect” of the conveyor belt. The situation would be where a gull is standing still and the wind is blowing and the ground is still in relation to the bird, which would seem to be analogous to the conveyor belt moving at the same speed as the plane. The gull will still fly because the air is not still in relation to the bird itself just like the air is not still in relation to the propulsion effects on the wings of the plane. Here is a link to a clip from the Mythbusters episode. An interesting aspect is that the pilot did not think the plane would lift. https://youtu.be/0ul_5DtMLhc Edited July 11, 2015 by Fjd Quote Link to comment Share on other sites More sharing options...
Moderators Travis In Austin Posted July 11, 2015 Moderators Share Posted July 11, 2015 It is no different than a car on a conveyor belt and is pulled by a cable while in neutral. The hypothetical assumes it supports the airplane. I hope you're not assuming the force generated by the a fixed position winch system is in anyway similar to the force exerted by the prop / turbine / squirrel powered engine. The plane's engine is directly connected to, and part of the plane. The winch is not part of the plane, since one end of that system is (either the winches motor or the cable end) and must be fixed or stationary...... UNLESS the force acting on your cable is due to the conveyors' movement (this concept might be interesting, but I doubt it's where you were going). And I'm sorry to inform you that there's nothing hypothetical about the conveyor supporting the plane. The weight of the plane is directly supported by the conveyor, same your house is directly supported by the foundation system. There's nothing remotely complicated about the concept of friction for this problem, and it is a constant value for both conditions, i.e. at the contact patch and at the bearings. It could be argued that friction acting on the bearings is not constant, but I don't think anyone is interested in getting that deep into the subject, besides, the change would be marginal. There are other forces at work, but I think we can ignore all of them since they are independent and of the conveyor system. Good luck with the youtube videos. Ditch the cable and winch from the car. Disconnect driveshaft from rear differential. Mount propeller on front of car, get power to it from engine in whichever way you chose, and vehicle will move forward from fixed point as you apply power. The belt can go in reverse at the same speed as the car, twice the speed of the car, whatever. The thrust of the propeller moves the car forward towards the end of the conveyor belt because the wheels are free spinning. Put a skateboard on the same conveyor belt, stand on skateboard, strap a large industrial fan on your back, flip it on and you will move forward regardless of the rearward speed of the conveyor belt/treadmill. Get a swamp boat, air driven, mount 4 free spinning wheels on the bottom of it, put it on conveyor belt or giant treadmill, it will move forward regardless of the rearward speed of the tread, same speed as forward movement, 2X, 3X. It will move forward and accelerate as if no treadmill is there. If the vehicle happens to have wings, and the treadmill is longer than the minimum take off length for that particular configuration, it will, in fact, fly. Travis Quote Link to comment Share on other sites More sharing options...
Gilbert Posted July 11, 2015 Share Posted July 11, 2015 (edited) When a person runs on a stationary treadmill, does that person feel the air blowing on their face? I don't think so, but then again, I've never run on a treadmill. The youtube video, even though the guy can't demonstrate that the plane lifts off, is a poor example for several reasons. Most significantly the model he chose to represent a real plane, the scales are completely wacked. Size versus weight, but more significantly, the ratios for the prop dia. to wing span (and depth) aren't remotely close to emulating the real condition for any plane. The ratio for prop to wing span is too large, indicating the prop alone is likely capable of moving enough air across the wing, (but none of that matters since the experiment wasn't setup to show lift) to generate lift. But what he did accomplish was to demonstrate that at times he was able to match the speed of the treadmill by playing with the throttle, and that the planes throttle control of his toy plane was significantly more responsive than the speed control of his treadmill. Sorry, but the myth-busters video is even more lame, though I'm sure they easily convinced the producer to air their $how. In their video the ultra lightweight plane clearly out-accelerates the small pick-up truck, who'd a thought? They didn't attempted to discuss the acceleration discrepancies between the vehicles (no small wonder why). And they didn't address the subject of friction between their simulated magical conveyor and the concrete (which is why the material tore from the fella prancing around on top of it). Don't know how anyone could watch the myth-buster example and walk away thinking their conveyor was even remotely close to what is interpreted for the problem........ Maybe OT is right,.... it is a poorly explained problem. Either way, I remain as unconvinced and hard headed as ever..... "No Fly" EDIT: Travis, it's time for us to take a break from this BS. Don't know about you, but I'm gonna take my girlies to canyon lake. Gonna do a experiment and test the change in acceleration while their swimming and I yell "Gar!" or "Snake!". Then I'll let'em practice falling down with a wake board on their feet. Actually, they're pretty good at it, but I need a bigger boat to generate a good wake. Cheers!! Edited July 11, 2015 by Gilbert 1 Quote Link to comment Share on other sites More sharing options...
DizRotus Posted July 11, 2015 Share Posted July 11, 2015 (edited) Now playing, The Great Conveyor Debate. See Galileo, as portrayed by the indefatigable Travis, match witticisms with the infallible Pope Paul V, brought to life through the thespian skills of Gilbert. The production is courtesy of the irascible Coytee, who shamelessly bumped this dog back to life. Edited July 11, 2015 by DizRotus Quote Link to comment Share on other sites More sharing options...
Jeff Matthews Posted July 11, 2015 Share Posted July 11, 2015 (edited) When a person runs on a stationary treadmill, does that person feel the air blowing on their face? I don't think so, but then again, I've never run on a treadmill. Correct. Now, however, add a jet-pack to your back and give it some thrust. Then, what happens? Edited July 11, 2015 by Jeff Matthews Quote Link to comment Share on other sites More sharing options...
Sancho Panza Posted July 11, 2015 Share Posted July 11, 2015 ^^^ Hold my beer & watch this... 1 Quote Link to comment Share on other sites More sharing options...
Fjd Posted July 11, 2015 Share Posted July 11, 2015 Put a skateboard on the same conveyor belt, stand on skateboard, strap a large industrial fan on your back, flip it on and you will move forward regardless of the rearward speed of the conveyor belt/treadmill. You could also attach a rope to the wall in front of a conveyor belt. While standing on the skateboard and holding the rope, pull the rope toward you as the conveyor belt starts. Granted, there will be initial friction pulling you backward that must be overcome; however, with a "propulsion mechanism" (pulling the rope) independent of the conveyor you should be able to pull yourself forward. 1 Quote Link to comment Share on other sites More sharing options...
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