Do surfaces ahead of propellers decrease thrust? 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 propeller disc and 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 8-rotored octo- multirotor 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? 
Thanks!
Best Regards
/Eric
does having a surface above a propeller decrease its thrust?


Anything that can prevent or reduce the acceleration of air will reduce the effective thrust of a propeller.  That might include obstructions in front of or behind the propeller.


  Why did my experiment showed different results than the blimp.


First of all, I don't see any information about the blimp.  Perhaps it too is greatly affected.  I don't know how much thrust is required in this configuration.  It may be tiny compared to the thrust available when pointing fore-aft.  If true, both propellers have reduced efficiency.

Second, I would think there's a big difference between a flat obstruction and a smoothly curving one.  Your experiment would be closer to the blimp situation if you put a large blimp shape over the propellers rather than a flat sheet.