Consider a toy helicopter that's floating(battery on) in the air. What's interesting was that it cost more energy for it to suspend at higher places. However, we knew that $F=mg$ thus the "thrust" or the "wind force" must be same since the only force acted on the toy helicopter was the gravity and the force from the air.
When it's very close to the ground, this was making a little bit sense since the air was kind of "compressed" by the ground. However, even when the toy helicopter was at say 2 or 3 meters high, or even 10 or 20 meters, this still appeared to be true. Compare to the size of the toy helicopter, for example, 0.1 m or 0.2 m, it doesn't quite make sense then, as the helicopter appeared to be high enough above the ground. However, the air pressure was obvious not a big influence at such occasion.
Question: Why it cost toy helicopter more energy to stay at higher place? and does this continuous forever?(in the case that the air pressure and gravity would not be taken into account, say a mile or so.)