We detached this subthread from https://news.ycombinator.com/item?id=10826643 and marked it off-topic.

That's not the question I'm asking. Assume perfect adherence on the wall. My question is : to remain stationary, do the propellers need to exert a force equal to the weight of the device?

In other words : if this machine can climb up a wall straight up, does that mean it could also climb up in the air?

Not necessarily. The weight of the vehicle is offset by the friction of the wheels on the wall.

Imagine an ant. It doesn't have an external force such as a propeller. It can climb up walls but it can't fly in any capacity.

> if this machine can climb up a wall straight up, does that mean it could also climb up in the air?

Interesting question! I think the answer is "not necessarily", particularly if the coefficient of friction of the wheels is pretty high.

Think of it this way: the weight of the machine points down. This down force needs to be compensated by an upward force, otherwise we'd have acceleration downwards. This necessary upward force can consist of two components: The upward component of the thrust generated by the propellers, and the friction. Now, the force exerted by friction is the normal force (i.e. perpendicular against the wall) times the coefficient of friction, call it c_f. If I remember correctly, for (macroscopic) things, that coefficient of friction is less than one (i.e., a car cannot decelerate with a 1 g or more, unless there's a spoiler that pushes the car onto the road, thus increasing the normal force to something larger than the weight of the car). Now, the normal force is the component of thrust perpendicular to the wall. (See ASCII "art" below).

Now, suppose the coefficient of friction is very close to one, and thrust is angled 45 degrees down away from the wall. Then:

weight = 1g (down)

upward thrust = 1/2 g + a bit (up)

normal force = 1/2 g + a bit (towards wall)

friction = (1/2 g + a bit) * c_f (up)

total upwards force = Upward force + friction = 1g (up)

Now the system is in equilibrium, and the total thrust is sqrt(2) * (1/2 g + a bit) = 0.75 g, say.

Thus, my hunch is that the thing can go up the wall even if it can't fly by itself, if the friction is big enough.

| weight

| |

| |

| |

| |

| V

|O\

| X <-- Normal = horizontal component of thrust

|O/

| ^ |\

| | . \

| | . . \

| | Thrust

| |

| upward component of thrust + friction

Dammit! Hard to do ASCII art here :-)

According to the paper the wheels are unpowered so wouldn't there only be the rolling friction of the wheel's bearings between the car and the wall? It'd be some but very little.

> The wheels are mounted with a double wishbone suspension based on model-car oil dampers, but they are not propelled in any way.

> According to the paper the wheels are unpowered so wouldn't there only be the rolling friction of the wheel's bearings between the car and the wall? It'd be some but very little.

You're right. I've assumed that it drives horizontally along the wall. As it climbs up the wall, you're absolutely right - the thrust has to overcome the weight AND provide at least a bit of a normal force to keep it pressed to the wall.

Given that the video clearly shows it driving up the wall, this particular vehicle must indeed be able to develop enough thrust to overcome its own weight.

You're probably being downvoted because you come off as incredibly smug.

> 2016 is my year of no longer contributing to HN.

This sucks. I'll miss your comments.