Bill.
The formula from Newtonian physics for a mass swinging around on a string is F=Mv^2/r where F is force in pounds, M is mass in slugs, v is velocity in feet per second, and r is the radius in feet. To convert weight in pounds to slugs divide by 32.2 feet per second squared. As an example consider a sport model at 3 pounds going 60 mph on 60 foot lines: A useful conversion factor is that 60mph equals 88 feet per second. 3# divided by 32.2 is 0.093 slugs. So force = 0.093x88x88/60=12# of force. This calculation does not consider any of the aerodynamic forces such as engine offset or rudder offset on the model. Note that the pull increases with the square of the speed; twice as fast is 4 times the pull.
Chuck