Dedman wrote:As a fellow aerospace engineer, I concur with Paul. Whether the plane will fly or not is solely determined by the speed the wing moves relative to the airflow (assuming we are not talking about an aircraft with VTOL capabilities).
The way I read Duper’s question, I see this as a vector problem. Let’s say the plane needs to reach an airspeed of 100 mph to produce enough lift to fly. Now let’s say that you (the observer) are sitting on an air molecule next to the plane. If the throttle is set to provide a ground speed of 100 mph in the forward direction and the surface of the conveyor belt is traveling 100 mph in the opposite direction, the plane stays stationary WRT the observer (100 mph + (-100 mph) =0). Unless the the plane is experiencing a 100 mph headwind, that is you go whizzing past the plane at 100 mph, it will not fly.
An easy way to think of it is this: when you are on a tread mill at the gym and it is set at 4 mph and you are running at 4 mph are you moving WRT to an observer standing next to you but not on the treadmill? Anyone who has been to the gym knows the answer is no. Even though the plane is not propelled forward by its wheels the effect is the same.
No forward motion WRT to the air means no take off.
That's the exact question I had, Dedman. I'm a Physics major, I read the official explanation
here, and I still felt as though I was missing something. I really didn't understand what friction had to do with anything until about 30 or so seconds ago.
Even now, I really don't like the way that aspect of the argument was worded I'm still flipping it back and forth in my mind, but I think I've finally got it down. Let me try rephrasing things here for a second; if there's anything at all I'd consider myself remotely good at, it's laying out arguments in plain language.
You're absolutely right; it really is all about the airspeed. Below a certain speed of air moving relative to the plane, there's insufficient lift produced, and the plane won't get anywhere. (One of the problems I had with the arguments I read is that some make it sound as though the propellor/engine somehow provides the upward thrust, which is dead wrong; it simply provides the necessary motion forward to produce that proper relative airspeed.) Obviously, then, this whole question revolves around whether or not a plane in this situation will be able to generate that relative airflow.
You mentioned a person on a treadmill, which is the same thing as the car example posted before. In both of those cases, the static friction between the person's shoes or the tires is what creates the forward thrust that propels the object forward. (For anyone else following along, yes, I said
static; when your car moves, it's actually the portion of the tire at any given instant that's stationary relative to the road that provides that force.) If the surface you're getting that thrust from is also moving backwards at an equal and opposite rate to the velocity that thrust is generating, then you'll simply stand still, just as that person on the treadmill does. In all of these cases, in a windless environment, there's no airflow relative to the person/car, as well, so a hypothetical plane-car (think Bond
)on a dynamo wouldn't be able to get anywhere. No airflow = no lift.
Now, let's go back to that plane-on-a-treadmill example. As you obviously know, whether in the air or on the ground, a plane generates forward thrust through its propellor/engine pulling at the air in front of it and pushing it out behind. This generates the necessary speed relative to the air molecules that allows the plane to attain sufficient lift to fly. In this vein, a plane that requires 80 mph relative to the air to attain flight would be able to take off in a 70 mph headwind by traveling at a speed of only 10 mph relative to the ground, since the combined vectors produce that 80 mph airspeed. You'd think that this would mean that a plane on a conveyor belt on a calm day would have no chance to take off, since there would be no relative airspeed to produce lift. Obviously, the engines don't push air over the wings to produce that lift (though that would be a pretty cool, if most likely unfeasible, prospect
), so it seems as though there's no way to generate that relative airpseed, just as in the case of the car.
However, there's a fundamental difference between the car and the plane. The car relies on contact with the ground to generate any speed; lift a stationary car up in the air, rev it up, and it'll just freewheel in position. However, a plane requires no such contact to produce forward thrust; obviously enough, if you suspend a plane in midair and rev up its motor, the propellor/engine will produce forward thrust, and the plane will start to move forward, regardless of whether or not it has sufficient airspeed to maintain level flight.
Now, let's look back at that treadmill. When the plane is sitting stationary on the treadmill, there is some static friction between the tires and the belt; there's also some inherent friction in the wheel bearings, but for the sake of argument, let's call that negligible. This friction, though, isn't what produces forward thrust for the plane. It isn't as though a plane's wheels have built-in driveshafts that allow them to produce forward thrust when they taxi; it's really the prop/engines that's producing that thrust. The wheels are really acting as coasters, reducing friction enough for the plane to move forward. That's exactly how seaplanes and ice planes work without wheels; their pontoons/skis reduce friction between the ground and the plane and allow it to move forward.
Here's where we start the treadmill. Say the plane starts moving forward, at 10 mph relative to the ground. At that moment, someone starts the treadmill up; it moves at 10 mph in the opposite direction to the plane's motion. If this were a car, where the motion of the wheels relative to the ground is what drives it forward, it would obviously stop, since the "road" is moving equal and opposite to its forward thrust. However, this isn't a car. The propellor/engine is what produces the thrust that moves the plane forward, not any contact with the road surface. Provided there's no massive amount of friction between the track surface and the plane's wheels, the plane will keep moving forward regardless of what the treadmill is doing, since that prop/engine is still pulling on the air. That pull will drive the plane forward relative to the ground, and it will generate the necessary airspeed to attain lift and take off.
What about the wheels, you may ask? Well, it isn't as though they're not affected. Let's go back to the start, when the plane's moving forward at 10 mph and we switch on the treadmill. Remember, unlike a car, a plane's wheels are no more than freewheeling coasters; they don't have to produce any sort of grip to move the plane forward. When the treadmill starts moving backward relative to the plane's motion, those wheels will remain in contact with the runway. However, since they provide no grip and no forward thrust to the plane, they're able to freewheel at any speed. Since they don't provide any (significant) grip, if the plane is moving at 10 mph forward, and the treadmill starts moving at 10 mph backward, the wheels of the plane will be moving at an equivalent speed to 20 mph. Meanwhile, the plane's engine will continue to move it forward through the air, without being affected at all by what its wheels are doing; the necessary airspeed will be produced, and the plane will attain flight.
It took me a while of racking my brain to come up with this; I kept getting hung up on, "Where the heck is the airspeed coming from?" My brain automatically wanted to leap to that car argument; it took me a while to make the connection that a plane is fundamentally different from a car in the way it produces its forward motion. A plane's wheels have nothing to do with the plane's forward motion, while a car's wheels have everything to do with it; that's the connection I had to make before I understood the answer. At any rate, I hope that I helped you (or anyone else) to wrap your head around this; it's definitely a doozy.
P.S. I think the whole rocket argument really doesn't do much to explain the solution either. While the principle of action-reaction is the same as an airplane's propellor, there's a fundamental difference between them: a plane requires an air medium, while a rocket can work in vacuum; besides, a rocket's thrust is generated straight down, which ignores the whole question of wheels. I know that I kept looking at that and thinking, "But what does a rocket have to do with an airplane?" I see the point now, but I don't think it does much for clearing things up for someone who doesn't.
P.P.S. Wow; this turned out to be a post of Draconian proportions.
