Last month we asked our Facebook followers a question:
"Imagine a small RC helicopter hovering in a resting train. The doors close and the train starts to move. Will the helicopter stay mid air, move with the train, or smack against the wall in the back?"
Turns out, the question really got people talking – the number of comments surprised us! When we said the helicopter would stay still and whack the back of the train, we started to get death threats from commenters, who said we were idiots with no idea how physics works.
Nearly 10,000 comments were posted on the thread with people adamantly arguing their point of view.
The more people discussed it, the more confused everyone became. We were so intrigued, Alborz and I decided to test the theory in real life. We called a professional drone pilot and got to work.
Unfortunately, we were unable to get hold of a train. Instead, we borrowed a Tesla Model X and a van. Armed with a number of small drones and remote control helicopters, we tried to accurately recreate the scenario pictured online.
First up, we tested a miniature drone in the Model X. The results seemed fairly conclusive, but we wanted to cross-check them nonetheless. This is science, after all.
The miniature drone hit the rear of the vehicle as soon as we accelerated.
Next up, we grabbed a DJI Mavic and switched off all the assistance systems. The results were inconsistent, and confused us rather than helping answer the question.
It looked like the Mavic was going to smash into the rear windscreen of the van, but the drone would jolt forward at the last second, seemingly moving with the van.
Our theory? The DJI is an entry-level hobbyist drone and, as it turns out, can't be put into full manual mode. The obstacle avoidance systems are perfect for amateurs who don't want to crash, but annoying in our science experiments.
Although we had access to a DJI Phantom 4 which would have allowed us to disable all sensors, it was too large to run in a van – and we didn't want to destroy the interior.
Instead, we went to a third brand of mini-drone. With no sensors or intelligent controls, they rely solely on the skill of the pilot.
Our final tests delivered the same results consistently. The drone stayed still in relation to the earth, smacking into the rear of the van as the vehicle drove forward. This is what we expected, which was a real relief.
Another interesting idea was brought up during the debate. Some people suggested the drone would move forward – potentially faster than the van – citing the fact a helium balloon moves forward when a vehicle accelerates.
We wanted to test this out for ourselves, so we threw a few balloons into the Model X and gave it a whirl.
Sure enough, the balloon lurched forward as the car accelerated. This happens because the helium-filled balloon is lighter than air. When the car moves forward, the air within the car actually sits still and, therefore, compresses slightly at the rear of the car – pushing the balloon forward in the process.
It makes sense when you think about it, but seeing it first hand is very cool.
There you have it folks. The helicopter stays still in reference to the earth, and the rear of the vehicle will run into the hovering object.
Acceleration or deceleration of the vehicle will make the hovering helicopter move around the cabin.
Many thanks to the guys from D2 productions, Jay for the Tesla and Option-1 and GT Auto for the loan of their vans.