glider UAV

I’ve been looking at RC gliders of late, and their potential for UAV missions.

To get a feel of the potential, here’s an RC glider going 505 mph:

This NASA project studied if you can use an autopilot on a powered glider to extend its range by riding thermal lift:

The Airbus Perlan project to fly a manned glider at 90,000 ft:

Also from Airbus is the Zephyr 7, a motorised glider using solar power, flew for two weeks, up to 70,000 ft:

So I was thinking, a glider would make an interesting UAV: it can fly for an extended period of time, it has no motor or fuel problems to worry about, and it can get very high.

The main issues I see are power for sensors and the operating system, and payload limitations.
The Zephyr solves these issues by using very light sensors, solar panels and a number of electric engines.
But I was wondering, would it be possible to equip the glider with rotor at the front, that would be used to generate energy from wind resistance? Alternatively it could be a small engine, used to get to altitude, and once there the engine is turned off and the rotation works like a windmill, generating electricity.
The advantage compared to the Zephyr is that you can do with less engine power, so you need less engines, no solar panels… All of which allows for a lighter and cheaper deisgn.