I have had this question for ages, nearly impossible to find anything on the web. My experiments confused me even further! Its concerning the blockage effects of surfaces ahead of a propeller. For example lets consider the usual dual vectoring propellers on the sides of airshipcars like in the skyship 600 for instance.The gap between the propeller disc and the underside of blimp is 1diameter.
In vertical flight the propeller's intake are faced by the underside of the blimp body itself. How are the propellers still able to generate thrust when the propellers' intakes are "somewhat blocked" due the very close surface of the blimp?
As An experiment I attached a foam sheet about 1.5 diameters above a big 28 lbs octocopter in hopes to see that the multirotor would still lift. At full throttle the multirotor didnt even move or lift. Obviously it lost almost all of its thrust due to the the sheet blocking the airflow above.. even though the foam sheet was high above the props, about 1.5 diameters.
Can someone help me on what could be going on here? does having a surface above a propeller decrease its thrust? Why did my experiment showed different results than the blimp. Are there other factors involved? if so.. what is the relation between distance, disc loading, surface shape, other factors etc and thrust decrease?