Comment on Mythbusters
FuglyDuck@lemmy.world 3 months agoExcept it’s not like attaching an anchor. The plane isn’t physically attached.
The wheels will just roll double whatever the current ground speed is. If the plane has enough thrust to take off with the treadmill moving an inverse of its take off speed, then it has enough force to start rolling, too.
At most, the force applied by the treadmill would be sufficient over enough time to lengthen the take off roll, but given enough space to do so, the plane will take off.
To keep the plane from rolling forward; the treadmill would have to be able to apply an equal force as the engines, it can’t do that through the wheels- the wheels can only apply a force equal to their rolling resistance and friction in its mechanics.
merc@sh.itjust.works 3 months ago
If the conveyor moves at the same speed as the wheels, it is exactly like attaching an anchor. That isn’t the myth they were testing, but it’s a more interesting myth.
It can do that if it can spin the wheels fast enough. Picture the ultra-light airplane from the episode with big, bouncy wheels and a relatively weak propeller. If the treadmill was moving 1000 km/h backwards, that little propeller could never match the force due to rolling resistance from the wheels.
FuglyDuck@lemmy.world 3 months ago
Just to clarify; you understand that because the engines are pushing on the plane itself and not the wheels, by the time the wheels start moving, the plane is already moving relative to ground and air alike.
Which, said another way, this thought problem appears confusing because it’s being considered from otherwise irrelevant reference frames.
An anchor sufficient to keep the plane from rolling forward is different because the force it is apply is significantly greater.
Sure, you can deflate the tires and increase the rate of spin on the wheels. But at that point, you might as well ask “can we creat a scenario where planes can’t take off”
To which the answer is definitely “Yes”,
And as a side note, if we assume the wheels are indestructible, which I’d argue is only fair, then even if what you’re saying is true and we ramp up the drag induced by the wheels sufficient to counter the engines… then the wind generated by the rolling treadmill would be producing a sufficient headwind for the plane to take off. (Remember, the air resistance of the treadmill’s belt moving will accelerate the air some.)
But again, the wheels have almost zero drag to begin with, the speed at which the roll is independent of both the actual groundspeed and the airspeed of the airplane.
If it has the thrust to over come friction at take off speeds, and at standing, then it has enough power to get to take off velocity eventually.
On the other hand, this entire conversation assumes the thrust to weight ratio is less than 1. If it’s more than one, well they just…. Go straight up.
merc@sh.itjust.works 3 months ago
The wheels are attached to the plane so they move at the same time as the plane. But, I get what you’re trying to say, that the wheels are effectively being dragged by the plane, they’re not powering the movement. But, what you need to think about is that if you oppose that dragging by moving the conveyor belt in the opposite direction you can prevent the plane from moving at all. Yes, the wheels are merely dragging and there isn’t a lot of friction there, but if you move the conveyor belt fast enough you can stop the plane from moving relative to the ground, which can stop it from moving relative to the air, which can prevent it from taking off.
No, by definition it’s the same. The conveyor moves with however much speed is necessary to stop the forward motion of the plane. The conveyor would eventually go so fast that it generated enough force to stop the plane from moving, so it’s indistinguishable from an anchor.
You don’t need to deflate the tires, you merely need to increase the speed at which the conveyor moves to match the speed of the wheels.
That seems like an unfair assumption because you’re assuming that the conveyor belt has second-order effects on the air (i.e. generating a “wind” over the wings of the plane), while ignoring the second-order effects the conveyor would have on the wheels (massive heat from friction leading to failure).
I mean, the discussion is of a plane, not a helicopter or a rocket. Even jet fighters with a thrust-to-weight ratio of more than 1 typically have engines that only have that ratio once they’re at high speed, not from a standing start. That’s why even fighter jets on carriers need a catapult-assisted takeoff. A VTOL aircraft like a Harrier wouldn’t need that, but then its takeoff speed is zero, and the myth isn’t very interesting when the conveyor belt doesn’t move.
FuglyDuck@lemmy.world 3 months ago
no. I’m saying that by the time the wheel is rolling, the plane’s is already moving forward, the engines have already overcome the drag in the wheels. the treadmill is locked to the wheels, not the plane. The plane would continue accelerating even as the wheels reported weird rates of turning.
As for the (very brief) time delay, that’s a function of the plane’s gear’s suspension that is quite well sprung.
the rate of roll on the tire is, effectively, decoupled from the airspeed (and groundspeed) of the plane. which makes this:
… entirely different. an affixed anchor does not allow the free motion that a wheel would.
And one of a few things happen. Either the plane has enough engine thrust to overcome the acceleration induced by the wheels, and therefore takes off, or it does not.
In the case that it does not, the wheels would continue spinning in increasing RPM until the plane begins moving backwards. because, again, the airspeed of the airplane is not dependent on the wheel’s RPM. Assuming the airplane doesn’t crash from suddenly becoming incredibly difficult to control… eventually it would take off anyhow. because the airflow over the wings would still generate lift. (though they would become horribly inefficient.) and therefore take off.
this is of course ignoring the whole “can a pilot actually control that and manage a take off like that” thing. If you don’t want to grant godlike piloting skills, we could then just make the treadmill irrelevant and leave the brakes on.