Most commercially available FLYING WINGS (RC aircraft or UAVs) are battery powered. Why aren't there more with with gasoline engines?
PS: Sorry I edited. The question is specific about flying wings configuration.
Because electric is the best propulsion method available for small commercial UAVs and their mission profiles:
-No smells involved
-No flammable liquid involved
-Way easier to operate: properly tuning an RC gas engine is not for everybody
-Much lower vibration and on frequencies easier to filter out
-More controllability, a BLDC (brushless DC) motor has near-instant response to the throttle and thus enables the actual, simple building scheme of quadcopters
Not so long ago, internal combustion engines were the standard for R/C aircraft. They had some big drawbacks though:
When electric motors + battery packs reached the point where their power/weight ratio was high enough for use in RC aircraft, many hobbyists switched to this (more convenient) option.
For larger aircraft, the square/cube law means higher demands on power/weight ratio, and electric systems still lag behind gas engines in this regard. So large UAVs will have internal combustion engines.
A large variety of R/C engines is still available, from single-cylinder two-stroke glowplug engines with less than 1 cm3 displacement, to 4-strokes, multi-cylinder engines, mini turbines, up to a 1/4 scale Rolls Royce Merlin.
Many UAVs do used internal combustion 2 or 4 stroke engines. Examples include the Scan Eagle, Integrator and RQ-21 Blackjack from Insitu/Boeing. The Scan Eagle has demonstrated flight of 29 hours nonstop. Electric motors are limited by their battery weight and size; internal combustion fuel has far more energy per unit weight or volume.
Of course, these have weights of 30 to 140 pounds (15 to 60 kg) and only need one motor each. For quadrotors and such, the complexity of getting the engine rotation to the multiple rotors and controlling same isn't worth the trouble for most realistic designs.