First of all, you have a really awesome hobby! I am planning to buy a simple coaxial RC-heli and join.
Recently I saw a bunch of videos on Youtube with amazing stunts performed by RC helicopters pilots, and my interest grew rapidly. That's why I showed one of them to a friend.
To my big surprise he said that it must be fake and was absolutely sure about that. Then I've shown him many other videos with similar stunts (I think it is called 3d aerobatics?), but this didn't convince him at all. He is sure that this type of flying is physically impossible.
As for me, I am sure it is all real, since there are thousands of videos (also in HD), websites, and forums (like this one) on the topic. It would cost billions of dollars to create all this 'illusion', and there is no sense to do it.
He is especially skeptic about one stunt - when the helicopter hovers with its rotor being perpendicular to the ground. I attached a screenshot to the thread.
He can't understand which forces hold heli still in this moment.
Initial googling helped me to find out that this can be done only with collective pitch models.
I understand that changing pitch you can stabilize the heli, but where does the thrust, which prevents it from falling down comes from?
I need your help to get physics behind this stunt and finally convince my friend.