Manish,

Your project looks very interesting. It is quite difficult to accurately calculate/predict the thrust from a model airplane engine driven propeller. Propeller manufacturers do not fabricate xx/yy propellers to the same dimensions (which gives us a good variety to choose from) which makes each manufacturer's propeller react differently even though it is sold as an xx/yy. This of course has an impact on propeller efficiency which is where you are trying to measure how much rubber is going to meet the road. Additionally, the propeller angle of attack varies from the hub to the outer edge and as the engine begins to move through the air the angle of attack on the propeller changes so that even if you maintain a constant engine rpm the thrust generated will vary as a function of airspeed (because the angle of attack is changing) When I was in college (many years ago) I built a frictionless sliding platform and bolted an engine on the front end and strain guages on the back end - to measure thrust. I used different sized propellers running at specific rpm (had to build my own tach) and tried to see if I could develop a relationship between thrust and propeller pitch/diameter at specific engine speeds. My conclusions were that you could but it was pretty loose and only useful for limited applications.

What you might want to try is to take the engine you are going to use and mount it on a platform that can move horizontally but not vertically and use a spring guage to measure the load (in whatever units you prefer) generated by the engine at the engine speeds that you are interested in. These numbers will change when your blimp is moving but they can provide a pretty good frame of reference. There are established empiracal and theoretical ways to calculate the thrust - but it is going to be pretty loose for the size engine you are using and may not be of real value to you.

Hopefully, someone else reading your post has a better idea or suggestion than mine. Enjoyed chatting with you and Happy New Year.

Howard