Helicopter downwash force

Depends, depends, depends.

There are way too many variables that need to be nailed down. Helicopter type. Gross weight. Altitude. Temperature.

 

assume that we have all the parameters, which theory will be used for the calculation?

 

Basic momentum theory in a hover which ties into Bernoulli would be the place to start. Essentially it is calculating the change in momentum of a column of air equivalent in area to the rotor disk.

http://www.public.iastate.edu/~aero442/unit2.pdf

 

apply bernoulli.
basically its the same problem as the old trick. would your truck go light if all the birds inside it took flight at the same time.
and 700 ft a lot of distance

 

Momentum theory is very good to calculate the induced velocity at rotor dics as well as at distance very below the rotor. Unforturnately, it does not have any information for determining the induced velocity at these points which are between the rotor dics and infinte distance, for example at 3 or 4 radiaus below the rotor.

 

That is very true. Usually something along these lines would be calculated via fluid dynamics package or simply measured in the field. There are so many things that could have an effect on the outcome.