Something to think about

[michal 44]

>It all depends on what>type of hypothetical, nonexistant treadmill you are talking>about. ;)Jeremy,With all due respect it does NOT - so not everyone is right.But I am winding down my participation in this discussion so I am not going to pick apart your application of the Newtonian 3-rd law. Some will never get it no matter what argument is thrown at them. I suggest don't raise this topic with your student pilots ;)Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://www.hifisim.com/images/asv_beta_member.jpg

[Guest Stryker]

Thinking about this some more,I was thinking, "The air is still stationary, and *that's* what the engine pulls it through." So who cares about a treadmill? Sure, it provides more force against the wheels than stationary ground, but there is no extra resistance on the propeller whatsoever.But...You still need enough thrust to overcome whatever force is preventing you from moving forward initially. Stationary ground obviously supplies a significant amount of this, or else we wouldn't need wheels.The thing is that if the treadmill can provide enough *force* to prevent the plane from moving forward. This is starting to go into a little bit higher physics, but I'm beginning to think a simple speed-for-speed match isn't enough to prove the argument either way. There are other things that affect force acting on an object in momentum.What if the treadmill were going backwards at 5x the aircraft's speed, or 100x? What if it were made of adhesive? Or if it had a waxed surface? What if the tires were 36 inches wide? What if they were 3 inches wide? All that stuff affects rolling resistance, which has something to do with all of this, doesn't it?It's not the speed of the treadmill, it's the magnitude of the force applied by it, and that depends on other things too, so matching speed-for-speed may not be enough. After watching the video with the skateboard I was thinking "the paper is too smooth" or "he's not moving it fast enough" and that's what got me thinking about all this.It's kind of like airplane performance at different density altitudes. The speed of the air may be the same, but the density has a large effect on the performance of the airplane.Am I on the right track? I don't care which side I end up on as long as it's the correct one. ;)I probably have better things to do. :(

[Guest enave]

> Everyone>who thinks the plane won't take off is thinking of it as the>powered type of treadmill that "makes the first move" and you>have to move against it to keep stationary. This treadmill is>capable of infinite speed.So? How does infinite speed keep the aircraft in place? Here, maybe this will help:http://www-lag.ensieg.inpg.fr/canudas/Phot...tion_vs_s50.gifThe friction (and therefore the force) that the treadmill can use to hold the plane in place spikes at a low speed, then actually falls off as the treadmill picks up speed. (the spike is called static friction) After that, the friction DOES NOT INCREASE. Meaning, even if the treadmill ran at infinite speed, the force holding the aircraft in place would be exactly the same as the force generated by the treadmill at lower speed.Meanwhile, the thrust available to the plane goes up. The plane always wins and always takes off. End of story.>However, with the powered treadmill, it's actually using your>wheels against you. It is pulling the plane away from you,>and the harder you pull, the faster it goes. It can go as fast as it wants. The force holding the plane in place remains constant. Meanwhile, the force from the engine thrust increases. The treadmill CANNOT win this.

[Guest enave]

>What if the treadmill were going backwards at 5x the>aircraft's speedThen the backwards force on the airplane is X. So long as the airplane's engines generate more than X thrust, the plane moves forward. Coincidentatlly, X equals the amount of friction that the plane would feel rolling down the runway without a treadmill. In other words, the treadmill does not affect the plane at all.> or 100x? Then the backwards force on the airplane is still X. The force on the plane is the same as when the treadmill is going 5x the aircraft speed. The plane still takes off.> What if it were made of adhesive? Now that is a totally different problem.

[mgh 89]

Explain please why it does not.

[Guest Stryker]

This makes a lot of sense and helps very much. But what if you had say the Wright Flyer (with wheels) and a concrete runway moving backwards at 500 knots? You're saying it could overcome that with the same amount of thrust it would require to take off from a stationary runway, given enough distance? Or is there something wrong with that example this I'm missing?

[michal 44]

Jeremy,Perhaps you should start thinking about aircraft with no wheels that is able to levitate (say magnetically) above the runway. Then how does moving runway affect the take-off?And then make a mental transition from such an arrangement to the wheels. Yes, some basic laws on friction (circa 1700) would be helpful.Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://www.hifisim.com/images/asv_beta_member.jpg

[kerosene31 349]

Scroll back a bit to the rollerskating example. The whole airplane idea pushes people in the wrong direction (airspeed, lift, etc). That is the only way I finally got it.

[Guest Stryker]

>Jeremy,>>With all due respect it does NOT - so not everyone is>right.>>But I am winding down my participation in this discussion so I>am not going to pick apart your application of the Newtonian>3-rd law. Some will never get it no matter what argument is>thrown at them. I suggest don't raise this topic with your>student pilots ;)Hey, a good pilot is always learning, right? Some people have put some very informative things on here since my post(instead of just posting the same stuff over and over again) and now I'm pretty much convinced the plane will fly. This has been an educational afternoon for me. ;)

[Guest enave]

>But what if>you had say the Wright Flyer (with wheels) and a concrete>runway moving backwards at 500 knots?I'll do you one better. Imagine the Wright Flyer on its belly. If it could take off like that, then it could take off from a treadmill. In fact, I don't think it could. I don't think it was even powerful enough to take off with wheels. But the point is, if you can overcome the friction (of the wheels, or the friction of your skids or belly or whatever) then you will take off, even if the runway is moving.Now here's something that will really get your goat. For an airplane like the Wright Flyer, which has just barely enough thrust to take off, you could actually limit its top speed (and therefore probably prevent takeoff) by having the treadmill go in the same direction - not the opposite direction.This is true for the same reason that antilock brakes make you stop faster. Haven't you ever wondered about that? Why is it that you stop faster with antilock brakes than by just locking your wheels? The answer is the key to this whole problem. The answer is friction.

[michal 44]

>Hey, a good pilot is always learning, right? Absolutely.Cheers,Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://www.hifisim.com/images/asv_beta_member.jpg

[Guest Charlie]

You

[LAdamson 240]

>>However, with the powered treadmill, it's actually using your>wheels against you. It is pulling the plane away from you,>and the harder you pull, the faster it goes. Even if you were>to yank with all your might, it yanks back the exact instant>you yank forward. In this case, all the pull you provide can>never get the airplane to actually move forward through the>air.>How about........ if the air is considered a large block of wood just above the treadmill and not touching. The prop is now a long woodscrew being twisted into the wood block, with the same corkscrew action that props pull through air. Obviously, the screw is going to pull the vehicle forward, regardless of what speed the wheels are going. When the screw reaches the required airspeed for flight................off she goes!L.Adamson

[Guest Stryker]

That's the kind of reasoning that originally got me thinking that way, but was still unsure because by that logic it seemed that planes wouldn't need wheels at all; they would be able to move with just the prop no matter what was holding them in place. That's obviously not the way it is, so I doubted that line of thinking.I thought a faster treadmill meant more total magnitude acting against the aircraft's desire to move forward. Now that I know (a tiny bit) more about the properties of friction (even if they are 300 years old it doesn't mean they're common knowledge), I get it now. Things are so much clearer when someone puts them in a graph. ;)I have another question then. Does that mean that, in a vacuum, a car would get better gas mileage the faster it went, assuming a direct drive (non-geared) transmission?

[LAdamson 240]

>I thought a faster treadmill meant more total magnitude acting>against the aircraft's desire to move forward. Now that I>know (a tiny bit) more about the properties of friction (even>if they are 300 years old it doesn't mean they're common>knowledge), I get it now. Things are so much clearer when>someone puts them in a graph. ;)>Since I originally figured the plane would not take off. It was my rubber band experiment with a model airplane on my treadmill, that convinced me that friction remained nearly the same. As I got the treadmill "ripping" along at 11 miles an hour, which appeared quite fast for the small size of the model; the rubber bands just wouldn't stretch any farther than at low speed. And they were light weight rubber bands!L.Adamson