# Flight and the Conveyor Belt **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [February 3, 2006, 12:25am UTC](https://boards.straightdope.com/t/flight-and-the-conveyor-belt/342759 "2006-02-03T00:25:10Z")\ **Posts on this page:** 20\ **Page:** 7
**Author:** ![DSYoungEsq](https://avatars.discourse-cdn.com/v4/letter/d/c6cbf5/32.png) [@DSYoungEsq](https://boards.straightdope.com/u/DSYoungEsq)\ **Post date:** [February 3, 2006, 11:30pm UTC](https://boards.straightdope.com/t/flight-and-the-conveyor-belt/342759/121 "2006-02-03T23:30:59Z")
> [@SFORCE82](#): > > This scenario can be compared to a jogger running on a treadmill. Imagine, for the instant, that the jogger gets off the treadmill, moves it outside, puts on some rollerskates, and straps on a jetpack. The jogger then gets on the treadmill, starts the treadmill, activates a sensor that determines how much thrust is produced from the jetpack and relays it to the treadmill computer which adjusts the speed of the treadmill to compensate by spinning quickly backwards. Now, since the treadmill can exert a backwards force on the jogger despite the rollerskates, the jogger simply stays in place with respect to a stational observer. Does he feel a strong wind blowing on him from the front? No, because he is not accelerating with respect to the ground. He will probably get blisters on his back and feet from the jetpack and the rollerskates with wheels turning at many, many orders of magnitude higher than intended (the cloth/rubber part of the skates will melt off within the first 20-30 seconds or so), but he will not feel any more wind passing over him than if he got off the treadmill and took off the jetpack and the skates. > > At least that’s what I think. No. No, no, no, no, no. Draw a damned force diagram, instead of thinking about it in your head. It is so damn easy to understand physics once you have to put it on paper and make computations. The wheels can turn any damn speed they want; their speed isn’t in any way important to the velocity of the jogger relative to a position outside the treadmill. The treadmill can only impart force upon the WHEELS; they then turn. The fact they are turning isn’t important to the jogger at all, other than the fact that he hopes to God they don’t stop turning (by, for instance, having the bearings explode), so that the belt suddenly DOES impart force to the jogger. The damn belt can co as fast as it wants, and it’s totally irrelevant to the jogger, once the system overcomes the intitial force of static friction (a very minor issue). I’ll repeat one last time for all readers who fail still to understand the reason the Column got the right answer: **DRAW THE DIAGRAM!!!** ---
**Author:** ![Cervaise](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cervaise/32/16693_2.png) [@Cervaise](https://boards.straightdope.com/u/Cervaise)\ **Post date:** [February 3, 2006, 11:41pm UTC](https://boards.straightdope.com/t/flight-and-the-conveyor-belt/342759/122 "2006-02-03T23:41:36Z")
> [@JBHarrisonIX](#): > > some of you are confused as to what I’m saying I think we are not the ones who are confused. ---
**Author:** ![JKellyMap](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jkellymap/32/15880_2.png) [@JKellyMap](https://boards.straightdope.com/u/JKellyMap)\ **Post date:** [February 3, 2006, 11:47pm UTC](https://boards.straightdope.com/t/flight-and-the-conveyor-belt/342759/123 "2006-02-03T23:47:52Z")
Nothing to add but my disappointment that Unca Cecil picked this relatively silly non-issue when he finally decided to address something that we have hashed out here on the SDMB. Why couldn’t he tackle something _important_ that we’ve discussed here? Even “steaming cups o’ Fido j— in Garfield” seems (IMHO) more worthy of his omniscient attentions. Sorry if this seem like it belongs in the Pit – I didn’t mean it that way. I’m just looking forward to the next time one of our discussions generates a public response from the Perfect Master, and hoping that it will have _nothing_ to do with treadmills and airplanes (at least not in combination). ---
**Author:** ![JBHarrisonIX](https://avatars.discourse-cdn.com/v4/letter/j/eb8c5e/32.png) [@JBHarrisonIX](https://boards.straightdope.com/u/JBHarrisonIX)\ **Post date:** [February 3, 2006, 11:55pm UTC](https://boards.straightdope.com/t/flight-and-the-conveyor-belt/342759/124 "2006-02-03T23:55:14Z")
> [@DSYoungEsq](#): > > Overcome if you can the limitation in your mind that persists in assuming that the fact the belt is moving at any speed forces the plane to move at that speed in the same direction, absent its thrust in the opposite direction. All the belt can do is move the wheels; the plane itself is only very barely affected by the force imparted to the wheels. With a frictionless axle, it wouldn’t even move at all, no matter how fast you made the belt go. OK, I understand now. That makes sense. The wheels don’t affect the plane as much (or at least the same ratio) as the treadmill affects the wheels. ---
**Author:** ![JBHarrisonIX](https://avatars.discourse-cdn.com/v4/letter/j/eb8c5e/32.png) [@JBHarrisonIX](https://boards.straightdope.com/u/JBHarrisonIX)\ **Post date:** [February 3, 2006, 11:57pm UTC](https://boards.straightdope.com/t/flight-and-the-conveyor-belt/342759/125 "2006-02-03T23:57:24Z")
> [@Cervaise](#): > > I think we are not the ones who are confused. “I think not that we are the confused who aren’t confused not that confusing isn’t what we are not.” I didn’t use that statement to imply that any of you were stupid. I was saying that you were confused PROBABLY because I worded it very badly, a part of my sentence you so eloguently left out. ---
**Author:** ![zut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zut/32/2875_2.png) [@zut](https://boards.straightdope.com/u/zut)\ **Post date:** [February 4, 2006, 1:18am UTC](https://boards.straightdope.com/t/flight-and-the-conveyor-belt/342759/126 "2006-02-04T01:18:23Z")
There’s nothing _wrong_ with **Cecil** ’s answer, per se, but, unfortunately, it just doesn’t explain things very well. Now, he’s clearly correct when he agrees that “the question is often worded badly, leading to confusion, arguments, etc.” But that’s part of the problem: Cecil himself doesn’t clearly state exactly _what_ question he’s answering. Poor Analogy Case in point: his use of the car analogy. Cecil states: “if the conveyor moves backward _at the same rate that the car’s wheels rotate forward_, the net result is that the car remains stationary (my italics).” Completely true–nothing wrong with that. However, note this very important fact: in the situation described, **the car’s engine is essentially doing no work**. It’s providing only enough power to overcome bearing friction. Then Cecil compares this situation to that of a plane **with the engines at full thrust,** and notes the different result. This shouldn’t be surprising: _Of course_ things are different when you start with different circumstances. If the analogy were truly parallel, he’d compare the case where the car engine provides only enough power to overcome bearing friction with the case where the plane engine provides only enough power to overcome bearing friction. In that case, the plane would remain stationary, as there’s no thrust left over for acceleration. Begging the Question Next, Cecil simply begs the question. Remember the situation is one where “the conveyor moves backward at the same rate that the … wheels rotate forward.” In answer to this situation, Cecil baldly states the following: “Once the pilot fires up the engines, the plane moves forward at pretty much the usual speed relative to the ground–and more importantly the air–regardless of how fast the conveyor belt is moving backward.” Again, not precisely wrong: the answer here depends specifilcally on the assumptions that you make. But Cecil **doesn’t state his assumptions** , and, more tellingly, **violates the condition of the thought experiment.** It’s clear that if you interpret the condition that “the conveyor moves backward at the same rate that the… wheels rotate forward” as meaning that the conveyor speed match the speed of the exterior of the tire, the plane/car will remain stationary (like _the car analogy specifically states_). There are a number of reasons why this condition might not reasonably hold, but Cecil _needs to state those reasons_, not just ignore it with no explanation. Ignoring Physics/Not Stating Assumptions Finally, Cecil serves up this, calling it a paradox: “If the plane moves forward at 5 MPH, then its wheels will do likewise, and the treadmill will go 5 MPH backward. But if the treadmill is going 5 MPH backward, then the wheels are really turning 10 MPH forward. But if the wheels are going 10 MPH forward… Soon the foolish have persuaded themselves that the treadmill must operate at infinite speed.” Again, not wrong, necessarily, _but it depends on your assumptions_. If you assume massless wheels and no friction, then Cecil’s analysis is correct. However, those are large assumptions, and should be stated. The fact that they’re not stated at all leads me to believe Cecil simply didn’t account for them, in which case his conclusion is misleading at best, and flat-out wrong at worst. If the plane has wheels of any mass, the treadmill will rotationally accelerate them, speeding up at a rate governed by the engine thrust. No infinite speeds required. Conclusion Let me sum up: [ul][li]Not actually wrong, but:[/li][li]Piss-poor analogy[/li][li]Begging the question[/li][li]No statement of assumptions[/li][li]Violation of the conditions of the experiment[/li][li]Ignoring rotational acceleration[/li][/ul] Did I forget anything? ---
**Author:** ![Moe](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/moe/32/2972_2.png) [@Moe](https://boards.straightdope.com/u/Moe)\ **Post date:** [February 4, 2006, 1:23am UTC](https://boards.straightdope.com/t/flight-and-the-conveyor-belt/342759/127 "2006-02-04T01:23:48Z")
> [@guavajuice737](#): > > Why do planes need long runways? The answer is to gain speed, creating sufficient airflow over the wings to take off. I’m a flight instructor and have studied aerodynamics in detail. > > Cecil says, “what gets a plane moving are its propellers or jet turbines.” True, but the engines do not create LIFT. The issue is NOT the thrust of the engines, speed of the wheels, or speed of the plane relative to the ground. The only thing that matters is the relative wind and the wings. Say for example, there is no treadmill, and the engines are off. If there is sufficient wind going straight down the runway, then the plane can theortical take off. It will just hover there, but the wings will think it’s speeding through the air. This is why, at 10,000 feet, if all engines fail, the pilot’s first instinct is to maintain sufficient airflow over the wings. He may have to pitch the nose of the plane down slightly in order to do this, but the plane will glide very nicely. The point: a plane does not take off because of the power of the engines: it takes off due to the aerodynamic effect of wind over the wings (creating lift). > > Scott Yes, but no one is suggesting that the conveyor belt will cause the plane to remain stationary. The question is not “will the plane lift off if caused to remain stationary relative to the air by way of a giant conveyor belt?” but “will the plane thrust forward through the air if on a conveyor belt?” It will (assuming wheel friction is negligible). ---
**Author:** ![DanBlather](https://avatars.discourse-cdn.com/v4/letter/d/f4b2a3/32.png) [@DanBlather](https://boards.straightdope.com/u/DanBlather)\ **Post date:** [February 4, 2006, 1:35am UTC](https://boards.straightdope.com/t/flight-and-the-conveyor-belt/342759/128 "2006-02-04T01:35:50Z")
> [@zut](#): > > Poor Analogy > Case in point: his use of the car analogy. Cecil states: “if the conveyor moves backward _at the same rate that the car’s wheels rotate forward_, the net result is that the car remains stationary (my italics).” Completely true–nothing wrong with that. However, note this very important fact: in the situation described, **the car’s engine is essentially doing no work**. It’s providing only enough power to overcome bearing friction. I believe that is true but misleading. The engine in a car moving at a fixed speed only needs to do enough work to overcome bearing friction, rolling friction, and air resistance. That is true whether it is on a road or a conveyer be. The only difference is a car on a conveyor belt that remains stationary relative to the ground will not have air resistance since the car is not moving relative to the atmosphere. ---
**Author:** ![cylon](https://avatars.discourse-cdn.com/v4/letter/c/f6c823/32.png) [@cylon](https://boards.straightdope.com/u/cylon)\ **Post date:** [February 4, 2006, 1:48am UTC](https://boards.straightdope.com/t/flight-and-the-conveyor-belt/342759/129 "2006-02-04T01:48:09Z")
> [@Moe](#): > > What gets a plane moving are its propellers or jet turbines, which shove the air backward and thereby impel the plane forward. Propellers shove air backwards in prop planes but how much air is shoved over the wings from a single prop centered on a small plane? Not enough air is going past the wings until the entire plane is shoved through the air. Jet turbines do not exaclty shove air. They shove exhaust which is a compressed and ignited mixture of air and fuel to create thrust. The air is used only for oxygen. Many of these jets shove a small focused blast behind the plane and behind the wings. they shove no air past the wings until the entire plane is shoved through the air. And don’t forget rocket planes. Similar to jets but carrying oxygen rockets need no air and shove no air. They shove the plane through the air. So Cecil how does a rocket shove air? Thrust shoves planes through air. That is what provides lift. Cecil do you understand now that engines only shove air to provide thrust? Imagine a plane with no jets, no props and no rockets, no thrust at all, no air shoved across any part of the plane without airspeed. Now imagine the wheels have enough power to them to move the plane fast enough for take off. Of course it wont get far since the wheels cant provide thrust after takeoff. But this plane would take off from a real runway but never the treadmill. ---
**Author:** ![B.Clifton](https://avatars.discourse-cdn.com/v4/letter/b/3ab097/32.png) [@B.Clifton](https://boards.straightdope.com/u/B.Clifton)\ **Post date:** [February 4, 2006, 1:53am UTC](https://boards.straightdope.com/t/flight-and-the-conveyor-belt/342759/130 "2006-02-04T01:53:02Z")
Nooo-- Strafe, you had it right the first time, but you and everyone else (including Cecil,) are being drawn in by the red herrings of a poorly worded question. Simplify the question to its simplest elements and you’ll realise that you’re describing a wing with zero ground speed. Esentially a wing in a wind tunnel with the fan turned off (Assuming no wind and no humongous front mounted propeller.) No wind and no movement in the Y axis means no lift and no movement in the X axis— the rest is just window dressing. ---
**Author:** ![B.Clifton](https://avatars.discourse-cdn.com/v4/letter/b/3ab097/32.png) [@B.Clifton](https://boards.straightdope.com/u/B.Clifton)\ **Post date:** [February 4, 2006, 1:54am UTC](https://boards.straightdope.com/t/flight-and-the-conveyor-belt/342759/131 "2006-02-04T01:54:15Z")
Nooo-- Strafe, you had it right the first time, but you and everyone else (including Cecil,) are being drawn in by the red herrings of a poorly worded question. Simplify the question to its elements and you’ll realise that you’re describing a wing with zero ground speed. Esentially a wing in a wind tunnel with the fan turned off (Assuming no wind and no humongous front mounted propeller.) No wind and no movement in the Y axis means no lift and no movement in the X axis— the rest is just window dressing. ---
**Author:** ![zut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zut/32/2875_2.png) [@zut](https://boards.straightdope.com/u/zut)\ **Post date:** [February 4, 2006, 2:15am UTC](https://boards.straightdope.com/t/flight-and-the-conveyor-belt/342759/132 "2006-02-04T02:15:23Z")
I see a whole bunch of comments in this thread that look a little like these: > [@kiethwins](#): > > Running the conveyor will not slow the plane down, or keep it from rolling off the end of the conveyor. C’mon, think about it. > [@WindFish](#): > > However fast the conveyor moves, the wheels react to cancel out the force imparted on them. > [@wayward](#): > > Even if the conveyer belt could keep speeding up to infinity (and beyond!) the plane would still move. > [@JustAnotherGeek](#): > > The only thing the conveyor belt can do is to pull back harder on the wheels, making them spin more. So long as the wheels are free to spin, the ground motion makes no difference to a plane. > [@Pushkin](#): > > So they [the wheels] should have no bearing on the forward motion of the plane, as long as there is little friction in the axles right? > [@Telemark](#): > > The converyor belt has no way of applying force to the plane no matter how fast it spins. > [@GlacialMaximum](#): > > Its wheels, which are not connected to any sort of drive shaft, spin freely as the conveyor spins the opposite way, and have no impact on the plane’s acceleration. > [@Biffy the Elephant Shrew](#): > > The faster the conveyor belt goes in reverse, the faster the wheels spin. What you’re doing is creating a feedback loop such that both the belt and the wheels will keep going faster and faster to compensate for each other > [@Thespis306](#): > > all the treadmill does is make the wheels spin faster as the plane moves forward relative to the air around it. > [@control-z](#): > > the very fact that the wheels turn would still let the plane’s engines move the plane forward, regardless of how fast the conveyer or wheels turn > [@Sullivan](#): > > Therefore the treadmill cannot hope to counteract the jetpack’s thrust by moving faster > [@DSYoungEsq](#): > > But the fact the wheels turn is meaningless. Unless they are able to in turn convey some of that force to the plane itself, the plane is unaffected. The gist of these comments being that the conveyor belt **cannot** impart enough force to the plane to counteract the thrust of the plane’s engine. This _could_ be true, if you assume either (a) some real-world limitations on the acceleration of the belt, or (b) that the wheels are massless. Either of these is a perfectly fine assumption. However, _without_ those assumptions, there is a coupling betwee the belt and the plane through the acceleration of the wheels. If the belt accelerates, it will impart a force on the wheel, accelerating the wheel. The wheel will in turn impart this force on the plane. If the acceleration is high enough, **the force will counteract the force of the engine thrust.** It’s perfectly fine to conclude that this force will never, in a real situation, be large enough to equal that from the engine. But you’ve got to _state_ that assumption up front; you can’t just ignore the angular acceleration. ---
**Author:** ![zut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zut/32/2875_2.png) [@zut](https://boards.straightdope.com/u/zut)\ **Post date:** [February 4, 2006, 2:19am UTC](https://boards.straightdope.com/t/flight-and-the-conveyor-belt/342759/133 "2006-02-04T02:19:46Z")
And a few miscellaneous comments: > [@John W. Kennedy](#): > > For the rest, since it is not at all peculiar on this planet to have weather conditions in which the wind speed is negligible compared to airplane rotation speeds, Cecil is quite right not to have introduced this quibble. He’s not talking about external weather conditions. He’s talking about air entrained in a (presumably) fast-moving, mile-long conveyor belt. A quibble, perhaps, but a potentially real effect. > [@frankenberrycat](#): > > See? The treadmill goes one way, the plane goes the opposite, no where in the question does it say the airplane is standing still. Depends. _If_ you interpret “if the conveyor moves backward at the same rate that the car’s wheels rotate forward” (quote from Cecil’s column) as forcing the treadmill to move at the same rate the tire exterior moves, that would force the plane to be stationary. > [@PetW](#): > > The whole point is that lift is aquired because the plane DOES MOVE FORWARD ON THE TREADMILL. Like he said, the wheels just move twice as fast, so the plane still moves forward. Read the damn column again, it’s very obvious. It’s not obvious, since Cecil specifically describes _two different scenarios_ in the final paragraph. > [@Ethilrist](#): > > The conveyor belt is described as running backwards as fast as the plane is moving forwards. The plane is MOVING. It says so, right there in the question. The only thing slowing the plane down is the minimal friction of the wheels, and although the wheels are turning as if the plane were moving twice as fast, that still isn’t enough force to counteract the force of the engines. Sure, but…that’s not a very interesting question. Does _anyone_ really thing that the plane won’t take off if the question is interpreted to allow the _plane_ to move forward at 100mph, while the treadmill goes backwards? I mean, you’re absolutely right, but it’s just not an interesting question. > [@Liberal](#): > > I don’t understand why this issue was ever controversial. Well, seriously, it took me some thought to work through what would happen, taking into account various assumptions/ It’s _not_ obvious, once you lay out all the nitpicky details, exactly what will happen. Ya gotta think about it. > [@zut](#): > > The jet power goes into _accelerating_ the wheels I should point out I’m wrong here–as was pointed out to me in the other thread (and I forgot to correct) the power for accelerating the wheels comes from the _treadmill_, not the _plane_. Sorry. ---
**Author:** ![zut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zut/32/2875_2.png) [@zut](https://boards.straightdope.com/u/zut)\ **Post date:** [February 4, 2006, 2:24am UTC](https://boards.straightdope.com/t/flight-and-the-conveyor-belt/342759/134 "2006-02-04T02:24:07Z")
> [@DanBlather](#): > > I believe that is true but misleading. The engine in a car moving at a fixed speed only needs to do enough work to overcome bearing friction, rolling friction, and air resistance. That is true whether it is on a road or a conveyer be. The only difference is a car on a conveyor belt that remains stationary relative to the ground will not have air resistance since the car is not moving relative to the atmosphere. I do believe that when a car is on a treadmill _that is synched to the speed of the car_ (which I think is the sense of the analogy in the column), then the treadmill will provide the power to overcome rolling resistance, because the treadmill is continually, actively adjusting to match the tire speed and maintain a car speed of zero. Thus, the car on the treadmill has to overcome neither air resistance nor rolling resistance–only bearing friction. (I admit I’m not completely certain of this, so I’d welcome comment.) ---
**Author:** ![cylon](https://avatars.discourse-cdn.com/v4/letter/c/f6c823/32.png) [@cylon](https://boards.straightdope.com/u/cylon)\ **Post date:** [February 4, 2006, 2:24am UTC](https://boards.straightdope.com/t/flight-and-the-conveyor-belt/342759/135 "2006-02-04T02:24:43Z")
the only reason a real plane would take off from the treadmill is because thrust would move the plane fwd anyway the treadmill would not keep it back unless it was propelled by wheels in which case cecil has the right answer for the wrong reason. not all engines shove air and the ones that do only do it to provide thrust NOT lift ---
**Author:** ![cylon](https://avatars.discourse-cdn.com/v4/letter/c/f6c823/32.png) [@cylon](https://boards.straightdope.com/u/cylon)\ **Post date:** [February 4, 2006, 2:44am UTC](https://boards.straightdope.com/t/flight-and-the-conveyor-belt/342759/136 "2006-02-04T02:44:38Z")
If the plane was propelled by the wheels then and only then the would the treadmill be able to keep it stationary. A treadmill wouldn’t match acceleration from thrust. A wheel propelled plane could be kept stationary by the treadmill and have no airpseed over the wings and therfore no lift. Of course a wheeled propelled plane would stay in the air very long after lauching from a regular airstrip. ---
**Author:** ![elliottw](https://avatars.discourse-cdn.com/v4/letter/e/da6949/32.png) [@elliottw](https://boards.straightdope.com/u/elliottw)\ **Post date:** [February 4, 2006, 3:28am UTC](https://boards.straightdope.com/t/flight-and-the-conveyor-belt/342759/137 "2006-02-04T03:28:12Z")
As with many things, the answer to this came to me as I was explaining something complex to my four-year-old. So in that vein, I will attempt to explain this in as simple terms as I can. Take a plane, any plane, and put it on a conveyor belt. Then start the conveyor belt. Don’t turn on the engine(s) yet, just set the conveyor to any old speed, and watch what happens. The plane doesn’t move. The wheels spin, the plane stays put. If you don’t believe me, try it yourself. Put your bike on your treadmill, and dial in a speed. Any speed. You’ll stay stationary, and the wheels will start turning. Assuming you have a decent bike with well greased bearings, your inertia will keep you basically in place, and the wheels on the bike go round and round, round and round … Sorry bout that. I think I used a “science” word in that last sentence. Still not convinced, Lance? Hit the brake. Now you’ll see the action of the conveyor belt on the bike. Some spackle will take care of the wall, and your co-pay should take care of you. Now, jump back in your plane, and start the conveyor. At first, nothing’s going to happen, just like your bike on the treadmill. But as you rev up your engines, the plane is going to inch forward, just like any other plane taking off from the tarmac. The wheels are going to spin even faster than before, and presumable, by Elfin magic, the conveyor is going to go even faster too. Doesn’t matter. They can go as fast as they want, or need to go. Now assuming you’ve got a couple thousand feet of conveyor belt in front of you, there should be plenty of runway for a beautiful take off … assuming you’ve got nominal weather conditions and a proper certification for that plane you’re flying. From the tower, your takeoff will look just like every other plane, except that a really observant onlooker will notice your wheels turning really, really fast. And of course the giant conveyor belt you’ve left behind in the middle of the airport after your departure. If anything, the conveyor belt itself is the “red herring” here. The speed it rotates, the speed of the plane’s wheels, the magical speed calibration thingy, the math, the aerodynamics, it’s all crap designed to obfuscate the simplicity of the model, and confuse listener. I like it ! Now excuse me while I resume talking to my seven-year-old about how thermodynamics affects the tase of Mommy’s cooking. ---
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [February 4, 2006, 3:30am UTC](https://boards.straightdope.com/t/flight-and-the-conveyor-belt/342759/138 "2006-02-04T03:30:34Z")
> [@cylon](#): > > Propellers shove air backwards in prop planes but how much air is shoved over the wings from a single prop centered on a small plane? Not enough air is going past the wings until the entire plane is shoved through the air. > > Jet turbines do not exaclty shove air. They shove exhaust which is a compressed and ignited mixture of air and fuel to create thrust. The air is used only for oxygen. Many of these jets shove a small focused blast behind the plane and behind the wings. they shove no air past the wings until the entire plane is shoved through the air. > > And don’t forget rocket planes. Similar to jets but carrying oxygen rockets need no air and shove no air. They shove the plane through the air. > > So Cecil how does a rocket shove air? Thrust shoves planes through air. That is what provides lift. Cecil do you understand now that engines only shove air to provide thrust? > [@cylon](#): > > the only reason a real plane would take off from the treadmill is because thrust would move the plane fwd anyway > > the treadmill would not keep it back unless it was propelled by wheels in which case cecil has the right answer for the wrong reason. > > not all engines shove air > and the ones that do only do it to provide thrust NOT lift I’m not really sure what point you’re trying to make here. No one here is suggesting that the plane’s engines provide lift directly (as opposed to thrust). Engine thrust is engine thrust. Period. No airplane engine operates by directing air over the wings of the plane. All of the engines you have described (props, jet engines, and rockets) operate by reaction forces in accordance with Newton’s Third Law of Motion. Bringing in a “wheel-driven plane” is a red herring, as no airplane has powered wheels. ---
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [February 4, 2006, 3:36am UTC](https://boards.straightdope.com/t/flight-and-the-conveyor-belt/342759/139 "2006-02-04T03:36:58Z")
> [@elliottw](#): > > As with many things, the answer to this came to me as I was explaining something complex to my four-year-old. So in that vein, I will attempt to explain this in as simple terms as I can. > > Take a plane, any plane, and put it on a conveyor belt. Then start the conveyor belt. Don’t turn on the engine(s) yet, just set the conveyor to any old speed, and watch what happens. The plane doesn’t move. The wheels spin, the plane stays put… I used the exact same analogy as you in the December thread. Unfortunately, I was wrong, and so are you. It takes a force (actually a torque) to make the wheels spin, assuming your wheels have mass. This force is transmitted to the plane via the axles. Your plane therefore does not stay put. It accelerates backwards. This happens even if the wheel bearings are frictionless. ---
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [February 4, 2006, 3:49am UTC](https://boards.straightdope.com/t/flight-and-the-conveyor-belt/342759/140 "2006-02-04T03:49:20Z")
> [@elliottw](#): > > …If you don’t believe me, try it yourself. Put your bike on your treadmill, and dial in a speed. Any speed. You’ll stay stationary, and the wheels will start turning… Actually, as the treadmill starts up and accelerates backwards, you and your bike will also go backwards unless you hang on to an external fixed object (like a wall). That will produce a force in the forward direction to counterbalance the backward force exerted on your bike by the treadmill. And as I stated above, there must be a force exerted on your bicycle’s wheels to make them turn in the first place. [Previous page](https://boards.straightdope.com/t/flight-and-the-conveyor-belt/342759.md?page=6) [Next page](https://boards.straightdope.com/t/flight-and-the-conveyor-belt/342759.md?page=8)