1 horsepower = 7 pounds
I've come across this magic number using different sized rotors, props, and engines in large RC helicopters and planes, and also researching horsepower to lift divided by time in several ultralight and light sport aircraft and rotorcraft, and this is a good rule of thumb when developing something that needs to do more than hover or drift in the wind.
Guidelines for precise calculation of lift or thrust
Comparison information using UAV specialist developer information
Be sure to click on the different links for engine type (4 stroke, wankel, turbine, etc.) In fact, Barnard Microsystems is an excellent reference for those looking to DIY a larger drone, and is highly informative reading material in all aspects of UAVs.