ORIGINAL: opjose
Many ARF's have the fuel tanks just aft of the firewall but in front of the C.G.
As the fuel runs down the plane gets heavier towards the tail, but not so much that the plane is uncontrollable.
In turn as the plane slows down, the lift generated by the elevator diminishes, and when coupled with the aft c.g., you get a pitch up of the nose sometimes just before landing...
You could move the C.G. forward to offset this.
When the air is colder the effect is more pronounced as the main wings continue to develop lift in the denser air, but the flat tails stall out earlier.
This leads to what you've seen, particularly on 3D type planes.
I have noticed this on my first real 3d plane myself. I noticed on the first flight that the tail was drooping after about ten minutes of flying. I knew it had to be a change of cg due to the tank emptying out.
Now I just use it to know when its time to bring it in. Built in timer I guess...