This is completely normal. When a load is applied to the battery, the voltage will drop. Higher quality and higher C-rated batteries should drop less when the load is applied, but they all will drop.
On my Hexacopter my 4S battery reads 16.8V, and drops to about 15.8-9 as soon as I take off. Then it slowly drops as the capacity is depleted. I land at between 14.0-14.2V. I have a few lower-quality batteries that will drop to 15.2-3 right after takeoff when fully charged.
For my racing drones, the 4S batteries drop from 16.8 to about 16 on takeoff, but on really hard punch outs or full throttle runs the battery will drop as low as the high 13V range and then bounce back to 15V when the throttle returns to the middle.
Also, after landing and letting the battery rest, the voltage will "rebound" and come up a bit. I just have all of my timers and battery warnings setup so that I land when the resting voltage of the battery returns to about 3.6-3.8 volts per cell after landing, which on my racers is about 14.5 under load and the hexa is about 14V.