Flying a drone in a moving car/airplane

Question

Why does a drone fly with respect to the air in a moving car/airplane, but not in an elevator as shown in this clip: What Happens If You Fly a Drone In An Elevator?

I understand that the drone will move at the same speed as a moving car/airplane. Therefore, you'd have no trouble controlling it. However, according to the clip, in the elevator the drone stays in the same position and not with the elevator. Thus, if you want the drone to fly inside the elevator, you have to fly it upward if the elevator is coming up and vice versa.

Answer 1

Very good question! The point is that when the elevator begins to move (either upwards or downwards), it's accelerating, while the drone -- having no force acting on it directly -- is still moving at a constant velocity.

Imagine you had a drone in a moving car or airplane, and it was "hovering" next to you. This means that its velocity with respect to you (and the car or airplane) is zero. If the car is moving at a constant velocity, the drone will continue to stay where it is, since no external force is acting on it.

On the other hand, in the elevator, the drone is at rest with respect to the "building". But when the elevator "starts", it begins to accelerate. The fact that the elevator has changed its velocity does not reach the drone as it isn't in contact with the elevator like we are. As a result, the drone continues to be at rest with respect to the building while the top of the elevator accelerates towards the drone, whacking it. Something similar occurs when you start from the ground floor and try to move up. Again the drone is at rest with respect to the ground floor, but when the elevator starts to accelerate upwards, the bottom of the elevator rams at the bottom of the drone.

I don't have a drone to test this, but I assume that something similar would happen in a moving vehicle if the vehicle suddenly accelerated (or turned). It's very similar to the feeling you experience when a car suddenly accelerates: you're slammed back because part of your body hasn't got the information yet that the velocity of the vehicle has changed.

If I had a drone (and a long enough elevator), I would try to show that once the elevator begins moving at a constant velocity (which happens after the initial jerk) I could control the motion of the drone without any trouble.

Answer 2

A drone does not always fly with respects to the air in a moving car/airplane. It does so only if they are not accelerating. The elevator is accelerating in your video.

Answer 3

The drone is responding to electronic accelerometer inputs on the device, and is simply a visual demonstration of the feedback loop in action. It is trying to hover in one spot, so when it suddenly moves up/down from that spot when the volume of air it is hovering in gets moved, its on-board controller tries to move it back to that spot.

It has nothing to do with airflow inside the elevator, something you could visually demonstrate by holding a child's toy windmill in the elevator. If the windmill stays still during ascent/descent, you know that the air is not moving (by anything your crude "measuring device" can detect).

Answer 4

The drone flies with respect to the air right there at the drone. The air meanwhile is in whatever state of motion is imposed on it by the forces acting on it. As a car or an elevator starts to move in some new direction, the air inside will not all immediately adjust. Rather, the air near the walls gets pushed at first, and this sends a pressure wave through the air, until eventually all the air is in motion, whether horizontally or vertically. The drone meanwhile 'rides' the air that is near to it and gets pushed around accordingly.

Answer 5

Simply put, if the container (car, boat, plane, OR elevator) you are in is moving at a constant speed, then you won't feel any push and neither will the drone. It will fly nice and stable.

On the other hand, if the container is accelerating (a car speeds up, a boat is on rough seas, a plane does acrobatics, OR an elevator starts rising or falling) then you will feel that acceleration relative to the prior rest state. Likewise, your friendly drone will have a hard time staying in place and is more likely to crash into something.

Drones use a variety of sensors to measure their movement and change in movement relative to their environment. These sensors typically include inertial sensors (accelerometers, gyroscopes), GPS sensors, or magnetic sensors (compasses). They all evaluate the position of the drone with respect to a "fixed" earth reference. If you want/need to keep the drone stable in the elevator (or any of these other environments), it will need sensors that are not focused on stability with respect to earth, but instead with respect to that local environment.

Examples could include ultrasonic or laser rangefinders to measure distances to the walls and floor, video sensors to measure changes with respect to the local environment using some form of machine vision, or possibly transponders in different parts of the environment that would allow time/distance calculations for position determination. It is unlikely that you could keep a drone stable for any length of time in an elevator because they tend to be either slowing down or speeding up for a good part of their trip.

Answer 6

You have some good, correct answers here. So far none of them have addressed the point that when an elevator starts moving upwards you feel heavier, and when it starts moving downwards you feel lighter. This is because the force of its its acceleration is added to (upward acceleration), or subtracted from (downward acceleration) the force of gravity. When you feel heavier as the elevator starts upward, so does the drone. When you feel lighter as the elevator starts downward, so does the drone. So a suddenly "heavier" drone will lower, and a suddenly "lighter" drone will climb. Hope this will give you a better idea of the actions.