<p>An airplane is sitting on an enormous treadmill. As the plane starts its engines, the treadmill runs in the opposite direction at the same speed the plane is moving. Can the plane take off?</p>
<p>Well I don’t know. </p>
<p>If the treadmill always matched whatever force the jet engines applied (i.e., it accelerates as the engine runs for longer), and assuming eveything’s perfectly still in terms of wind, the plane itself would not be moving forward, and thus the air over the wing wouldn’t be at non-zero velocity relative to the airplane. Of course, you need air running at a great deal of speed over the wing to generate lift. If you were standing on solid ground, relative to you, the airplane would not be moving. Since it’s not moving, it’s wings aren’t cutting through the air, then I guess the air ain’t flowing, no lift, no flight. So I would say, no.</p>
<p>At least I think that’s what you’re asking.</p>
<p>“at the same speed the plane is moving”? I assume the plane is stationary, the ENGINES are running. No groundspeed = no airspeed = no lift = no flight.</p>
<p>How 'bout this? An airplane is loaded to gross weight which includes 1000 pigeons that are happily flying around inside the fuselage. The plane begins take-off roll and all the pigeons decide to land on the floor of the fuselage. Does the plane now exceed gross weight?</p>
<p>Sorry for the hijacking, I’m just a lame parent.</p>
<p>Lowofo has it right… but this riddle is not very exciting. All it requires knowing is that lift generated by movement of air is what makes airplanes fly.</p>
<p>If the plane pulled air (in most common planes) then it would not matter if it were moving or not. Planes’ wheels are free-rolling.</p>
<p>Who says the airplane has to be a jet?</p>
<p>If it is a propeller airplane, where the air from the thrust of the propeller flows over the wings, the plane will take off.</p>
<p>56forceout:</p>
<p>Well if the weight of the pigeons were already accounted for, it wouldn’t really matter if they were flying or standing?</p>
<p>If you mean the plane was loaded to gross weight w/o considering the pigeons, then yes, it would exceed max takeoff weight. Prior to the pigeons landing, their weight is supported by lift, generated by their wings. When they land, their weight is opposed by the normal force; in other words, it actually contributes to the plane’s total weight.</p>
<p>I guess an analogy would be if you placed a weight on a scale, and then held another weight over that weight. Though the mass of the entire two-weight system is the sum of both masses, the apparent weight of the system is still of that single weight on the scale; the force you apply with your arm to hold the second weight above is analogous to the lift generated by the pigeon’s wings.</p>
<p>bmanbs2:</p>
<p>If that were true, why would propellor aircraft need runways? Why don’t they just turn on their engines and begin levitating?</p>
<p>The propellors alone do not generate enough “airspeed” and do not allow air to flow over enough of the wing’s surface area. Their primary purpose is to produce thrust, thus allowing the entire airplane to move forward and thus generate current over the wing.</p>
<p>bmanbs2 — hahahaha. Try this sometime: tie two strong ropes that are 10 yards long to the wheels of a prop plane and max out the throttle. If your theory is correct, the plane would (as lowofo said) levitate to full extension of the ropes. In fact, no such thing would happen. You can verify this by computing the airflow that the props generate versus the airflow generated by moving through the air at 100 knots.</p>
<p>So then how, in a strong wing, can a propeller airplane take off straight up or even backwards? </p>
<p>I’m assuming that this is a theoretical question, so take a Beach Barron twin with infinite horsepower and propellers large enough to have air flow over the entire wing (sure they would hit the ground, but this is theroy, not practice), there would be adequite airflow over the wings, and the plane would take off.</p>
<p>The wind would have to be very strong (60 knots would be enough for a Cessna 172). How often have you seen this actually happen? Note that 60 or 70 knots flowing over the entire wing is a lot more than the prop of that airplane (or those of a Beech Baron) generate.</p>
<p>While this is a theoretical question, the airplanes involved should have some resemblance to actual airplanes. Otherwise, I would say, sure it would take off. Just assume infinitely powerful jet engines that are aimed downward.</p>
<p><a href=“http://www.aeronautics.ru/img003/v-22-osprey-01.jpg[/url]”>http://www.aeronautics.ru/img003/v-22-osprey-01.jpg</a></p>
![http://www.aeronautics.ru/img003/v-22-osprey-01.jpg[/url]](http://www.aeronautics.ru/img003/v-22-osprey-01.jpg%5B/url%5D){kind=link}
<p>Haha. Doesn’t count. ;-)</p>
<p>Actually Ben Golub, I have seen it done. I also would bet that I am the only person on this thread that has actually flown an airplane.</p>
<p>and 60 knot winds do happen, believe it or not, in cases other than hurricanes and tornadoes. They can occur on crystal clear blue days.</p>
<p>The point of this question is that if the treadmill is always the negative force of the airplanes thrust, in a pusher airplane, no air would flow over the wings. In a puller airplane, air would flow over the wings. And air flowing over the wings is what causes lift.</p>
<p>And don’t try the taking off straight up/backwards unless you have thousands of hours of flight time and at least a commercial liscense, though and ATP liscense is preferrable. But to see it is really cool, as long as you have a windbreaker on or something.</p>
<p>The weight of the plane filled with pigeons doesn’t depend on whether they are standing or flying. If they are flying, they exert a force on the air, which in turn exerts a force on the floor. If the air didn’t exert a force on the floor, the floor would not exert one on the air. This would mean that the pigeons would be pushing the center of mass of the air downwards, but this doesn’t happen. So the force from the floor must balance that from the pigeons.</p>
<p>durt:</p>
<p>I don’t know. Imagine a hollow sphere on a scale with the top cleaved off. You hold a mass in the middle. Does the scale reading go up by the weight of the mass and whatever portion of your arm is inside the sphere? Sure, air pressure changes, I don’t know what type of effect that might have.</p>
<p>You’re probably right, I never even thought of that. That pigeons displace air, which in turn put force on the walls of the airplane. Beyond that, I don’t know. My answer above is considering air to be massless, which is ideal, but not realistic at all (after all, air needs to have mass to exist, air has to exist for lift). I’ve never considered air to be a fluid, or have mass; then again, I’ve never taken aerodynamics or any of those fancy classes.</p>
<p>bmanbs2:</p>
<p>Well, okay, planes can take off backwards. So what do they do when they get in the air? They have no forward momentum, as the wind is pushing them back. What happens when the wind dies?</p>
<p>bmanbs2 – (a) don’t assume; (b) the fact that you have flown an airplane apparently has no bearing on your knowledge of physics.</p>
<p>Note that I did not claim that taking off backwards didn’t happen – just that it requires high winds. In particular, I am saying that this has nothing to do with the question. Propellers would never generate enough flow to match a 60 knot wind (either a real wind or the flow generated by the motion of the airplane.)</p>
<p>You stay very close to the ground, put on the parking brakes, keep your hand on the throttle to kill the power, and pray to God nothing like that happens.</p>
<p>theoritical Ben, theoretical.</p>
<p>Theoretical discussions don’t mean being completely unrealistic. The treadmill we are discussing could in principle be built; the airplane you are discussing could not. That is all.</p>
<p>Anyway Ben, we’re both wrong.</p>
<p>Sky, on the Egineering major forum, answered this question. The wheels spin independently of the planes thrust, so the full force of the treadmill will not act on the plane. Only the frictional force of the bearings will be transferred to the landing gear. So the thrust force on the plane will be greater than the force from the treadmill. Athough more slowly, the airplane will accellerate and take off.</p>