Obviously, the main technical limitation would be devising a practical conveyor system that could go fast enough for takeoff (maybe impossible?), but if you could get past that, it seems like it could free up a lot of real estate for takeoff runways. Maybe could have some use on aircraft carriers, and could give some aircraft essentially VTOL capabilities (for takeoff, anyway), without doing retrofits.
I’m still not buying that answer. If I’m standing next to this "motionless relative to plane, then the plane is also motionless relative to the air. Or think of it this way. I feel no breeze at all. A crane picks me up and rotates and deposits me on the wing. Where is this magical airflow over the wings supposed to be coming from?
Assuming you mean a conveyor belt going backwards, you’re thinking about it wrong. The plane still moves forwards because unlike a car, it’s not being propelled forward by it’s wheels. You would still need a conveyor belt as long as a runway.
Seems like safety would be a consideration. There are many things that can go dangerously wrong with having a conveyor belt going 100-200 mph in this fashion. It is easier to correct things with a high-speed airplane running along a stationary, motionless runway, than with an airplane on a high-speed conveyor belt that has gone wrong.
Also, there’s industrial inertia; airplanes have used runways for over a century, and it would be near impossible to persuade any airport or airplane manufacturer to do things differently.
Plus, surely such a conveyor belt would require an immense amount of energy to operate.
A plane on a conveyor belt doesn’t just stand still and then take off. It basically ignores the conveyor belt, because the plane’s wheels are free-spinning. It won’t save any space. If you want to do that you need a catapult, which aircraft carriers have.
The plane doesn’t care about the ground, it cares about the air speed over the wings. A conveyor moving in the direction of takeoff would help somewhat, but likely only marginally, it would just be using the propulsion normally provided by the engines and replacing it with propulsion provided by the conveyor. It’d be a waste of energy. A better solution might be a giant fan pointed at the plane, but the danger there is that you’re taking a plane moving through a quick air column and when it rises above the fan it would immediately hit a column of slowly moving air. A recipe for disaster.
I’m assuming he meant a belt running in the same direction the plane is taking off, not the opposite. Thing is, it would take one helluva motor to appreciably accelerate a plane that can weigh as much as 200,000 pounds plus whatever the weight of a belt big and sturdy enough to carry it to say nothing of the inertia in all those rollers. A catapult would be a lot easier to implement but you still have the issue of building the plane sturdy enough to say nothing of passengers objecting to four or five Gs no matter how momentary.
people misunderstand this situation because they’re thinking of the airplane as though it’s a car. a car moves by applying a torque to its drive axle(s,) which then uses the tires to convert that torque to a linear force pushing against the ground to propel itself forward. if it was driving on a conveyor belt then yes, the conveyor would affect its speed. if the conveyor could theoretically “keep up” with the car’s wheels, the car would stay in place.
an airplane uses a propeller or jet engine to generate thrust. Thrust does not act against the ground. the plane does not need to do anything to nor get anything from the ground in order to move forward. the conveyor wouldn’t even be an annoyance to the plane.
aircraft carriers do this, but obviously because of the limited distance to reach takeoff speed.
Right, if a plane ordinarily needs 100 meters of runway to take off, then if you put it on a conveyor belt, then the belt would also need to be 100 meters long. Making a 100-m long conveyor belt would be really difficult and expensive, and would have no benefit whatsoever over a concrete runway.
Not to mention that, in any case, the airplane would have to have its engines running at power anyway, because once it took off, it would have to be propelling itself in flight. So since it has to spool up the engines to high thrust anyway, it might as well use a conventional runway. It would have to have its engines going strong regardless of whether it was runway or conveyor, so it’s not like the airplane is saving any energy.
They sort of do use conveyor belts - except they’re called steam catapults. They make the aircraft go fast without waiting for the engines to do the work… that way it can safely fling a small jet off a 1000-foot aircraft carrier. Of course, without the jet to keep the flight going, it wouldn’t. (I suspect there are videos of this problem…)
The thing that generates lift is airspeed over the wing. An aircraft could theoretically take off with zero ground speed if the wind were strong enough - but I don’t really want to be the guy trying to get the aircraft into position on the ground. As a crosswind, it would probably flip the plane, in that weather they’d all be tied down… (Planes take off and land into the wind to make the ground speed lower). Similarly, you can put a small plane on a flatbed truck and drive down the road. At a certain speed it will take off. But again, without and engine running, it can only last a short while in the air, just like launching a paper airplane…
I suppose that, in principle, you could cut down on needed runway length by putting a really big fan at one end of the runway. But a fan big enough would be extremely impractical.
I used to love flying radio controlled airplanes. Still would if there weren’t so few appropriate areas to fly them. Because they are small and light, some models could fly at 15 to 20 miles per hour. On a windy day, flying into the wind at low power, the plane might stay motionless relative to the ground, or even fly backward. No problem for the plane, but it tended to screw up my ability to control it, as I was not used to how it reacted flying backward.
You want a really huge fan that can blow a stable 200-mph wind over all surfaces of the airplane from the front.
Plus something else to instantaneously provide a 200-mph airspeed relative to the airplane once it flies out of the fan’s wind stream. Maybe more fans.