Update aerial_vehicles.md | flapping-wing UAVs extended (#61)

Adding more details about flapping-wing UAVs

Author: matejkarasek

docs/aerial_vehicles.md

@@ -32,7 +32,12 @@ These are the subcategories:
 
 ![flapping](images/flapping.png)
 
-Flapping wing aerial vehicles, also called ornithopters, generate lift through flapping their wings.
+Flapping-wing aerial vehicles generate both lift and thrust by their flapping wings. 
+
+There are two main subcategories:
+
+* **Ornithopters:** These are the traditional bird-like flapping-wing UAVs. The flapping wings generate lift and thrust and an airplane-like tail with traditional control surfaces is used for stbility and flight control. Some minimal forward velocity needs to be maintained such the wings produce sufficient lift. Only some smaller vehicles demonstrate limited VTOL and hovering capability. These vehicles can be passively stabilized by the tail surfaces and do not require active stabilization, similar to fixed-wing UAVs.
+* **Tail-less flapping-wing UAVs:** These vehicles fly like insects and hummingbirds - they can hover, take-off and land vertically, and fly in any direction (forward & backward, sideways, up & down). Lift/thrust is generated by the flapping wings, whose geometry and/or motion is adjustable for flight control. Like multicopters, these vehicles require active stabilization, typically by using an onboard IMU.
 
 ## Blimps