# What happens to the helicopter when

What happens to the helicopter when....

Hello all, I recently came up with an interesting situation in which I cannot predict the outcome. This is purely an item of interest, not an experiment I intend to try.

After flying my ultra-micro rc helicopter today, I thought of a strange scenario. The product is advertised as being able to fly anywhere, with regards to space limitations, like in the house or garage.

The scenario is this; a micro helicopter, hovering inside the the back of a van or box truck. If the truck is driving down the road at any given velocity, say 60 miles per hour, and smashes into a wall, what happens to the helicopter?

Does it crash into the windshield, or simply remain hovering in its restricted airspace?

This situation has me baffled, as it is a stiuation of zero airspeed, yet the airspace is travelling at speed. Like I stated, this is just an item of interest, please do not attempt to test this "at home".

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HallsofIvy
Homework Helper

The helicopter will maintain it forward velocity even after the truck stops. It would move forward. Whether or not it would "crash into the windshield" depends upon the speed. This is a situation of zero airspeed relative to the truck. It has speed relative to the ground.

That was my initial impression as well, but why is the helicopters velocity relative to the truck? The vehicle is only surounding the helicopter; it is not in contact with it. I understand the concept that when a vehicle accelerates, stationary objects move the opposite direction, but why is this velocity relative to the ground?

This could be considered in larger scale by supposing the earth stopped spinning; would a full size aircrafts' velocity be altered? It was travelling at V velocity, so what impact would the movement of the ground below it have?

Rob D
Gold Member

A very simple experiment should answer your question. Take a ping-pong ball or any small object and hang it by a thread a foot or so below the head liner of your car.

Now drive and you will see that when you accelerate or decelerate, the ball will swing aft or forward as is appropriate. It does so because gravity and inertia act seperately on each part of the vehicle but those that are not rigidly attached to the car and are free to move, do.
Likewise, if you turn a corner hard the ball will move to the outside of the turn driven by centrifugal force which is acting on all parts of the vehicle system. But, as in the previous example, the ball is free to move where other parts of the car, such as the rear view mirror are not.

In the case raised in your question, should the van violently crash into a wall, the helicopter will smash into the front of the van's interior, probably the windshield.

There's math that goes along with this if you need it.

Best,
Rob

K^2

A very simple experiment should answer your question. Take a ping-pong ball or any small object and hang it by a thread a foot or so below the head liner of your car.

Now drive and you will see that when you accelerate or decelerate, the ball will swing aft or forward as is appropriate.
And if he uses a helium balloon, he will get an opposite effect. Sometimes, you shouldn't oversimplify.

W201028 said:
That was my initial impression as well, but why is the helicopters velocity relative to the truck? The vehicle is only surounding the helicopter; it is not in contact with it. I understand the concept that when a vehicle accelerates, stationary objects move the opposite direction, but why is this velocity relative to the ground?
Newton's first law of motion applies to inertial frames of reference. In a collision, truck accelerates, so "relative to the truck" isn't an inertial frame of reference. If you do describe the situation relative to the truck, because your frame of reference is accelerated, you pick up a fictitious force that acts on the helicopter and pushes it forward relative to the truck. But that's a bit complicated. Your best bet is to pick an inertial frame of reference. Relative to ground works fine for that, because ground isn't going to suddenly accelerate in collision. So relative to ground, there will be a tendency to maintain velocity as per Newton's first law.

CWatters
Homework Helper
Gold Member

Does it crash into the windshield, or simply remain hovering in its restricted airspace?
It crashes into the windscreen. Just as an unrestrained passenger would.

The truck is irrelevant to the problem. Even the speed relative to the road is irrelevant. What matters is the speed of the helicopter relative to the wall. It's easy to see that's the case if you replace the wall with a very slow moving truck.

Nugatory
Mentor

The scenario is this; a micro helicopter, hovering inside the the back of a van or box truck. If the truck is driving down the road at any given velocity, say 60 miles per hour, and smashes into a wall, what happens to the helicopter?
The truck, the air in the truck, and the helicopter are all moving at 60 mph. The truck hits something and stops. The air and the helicopter flying in that air keep moving forward at 60 mph.

This forward-moving air is slowed as it rams into the windshield, while the helicopter is still moving at 60 mph. Thus, the helicopter experiences a headwind that tends to slow it down as well.

Whether the helicopter hits the windshield or not depends on whether the helicopter slows from 60 mph to 0 mph quickly enough to avoid the windshield, and that depends on a lot of things:
- how quickly does the truck stop? If instead of slamming into a wall, it gently brakes to a stop, then there is plenty of time for the air and the helicopter to slow down as well.
- how close to the windshield is the helicopter? If it's one inch away, it's going to hit the windshield in response to even a quick stab at the brakes; if it's all the way at the back of a 60-foot trailer there's much more time for it to slow before it hits the front.
- how aerodynamic is the helicopter? If had zero aerodynamic drag (impossible) then the air wouldn't slow it at all, and it would always hit the windshield at 60 mph no matter how gently we stopped. Of course it would also hit the rear wall any time the truck speeded up.

Rob D
Gold Member

This reminds me of the thought experiment involving a cargo plane carrying 2,000 pounds of pigeons in light cages. It's a big cargo plane with a large cabin.

Suddenly the pigeons escape from their cages and begin flying wildly about the cabin.

Question: With all the birds in flight inside the cargo cabin, what happens to the airplane? Is it now lighter or heavier than when the birds were in their cages?

Mods: Hope this isn't too far off topic.

Drakkith
Staff Emeritus

I believe the air is supporting the weight of the birds and the aircraft is sufficiently airtight so that the air cannot escape very easily, leading to the plane staying the same weight.

Your question reminds me of the classic question, "a fly is hovering next to a driver, if the car crashes will the fly crash into the windshield? yes it will. Because the only thing that matters is that a fly is moving with the same velocity V as that of the car with respect to ground. So EVERYTHING ELSE is immaterial. Simply discard the driver and the car, and think what would happen if the fly were travelling at velocity V and smash into the wall which is stationary with respect to ground.

sophiecentaur
Gold Member

The fly won't hit the windscreen because its mass is so small and the viscosity of the air is the most relevant force here. To the fly, the air is more like treacle and the fly will only keep moving forward relative to the ground for a short distance before it stops.

Yes i was thinking on theoretical terms. Just to point out that even though the fly looks stationary, with respect to car, it is still moving with respect to the ground. So that is the reason it will crash into the wall, or windshield. Neglecting what all forces will impede its motion.
Is not that true?

sophiecentaur