Are there any data regarding the helicopter rotor that pushes down on the cabin and tail rotor beam? You can clearly see that a large portion of the rotor actually sweeps over the fuselage pushing it downward.
It's not called or considered 'downwash' (since something else is referred to by that). It's called vertical drag. Can be as much as 5%. Dependent on design even higher.
That’s correct, the term is “Vertical Drag” i.e. the download penalty. We assume the thrust required by the main rotor is equal to the weight of the helicopter; however, there is an additional degree of power required due to the download, vertical drag on the helicopter fuselage due to the rotors induced slipstream velocity, especially during low-speed hovering and vertical flight.
The drag effect is often express as a fraction of the gross weight. Because of the download produced on the fuselage by the rotor wake, the required rotor thrust is 4% to 7% larger than the gross weight, producing a corresponding increase in required power, that’s what your 5% term relates to.
There’s data available along with a number of technical papers and references. Below are two equations that yield an approximation of the vertical drag. Also, four technical references.
Estimating Vertical Drag on Helicopter Fuselage during Hovering
See pg. 37 - Rotary- Wing Aerodynamics Volume II - Performance Prediction of Helicopters
Rotorcraft low-speed download drag definition and its reduction
Principles of Helicopter Aerodynamics 2nd Edition by Gordon J. Leishman D.Sc.(Eng.) Ph.D. F.R.Ae.S.
Edited by iChris, 30 May 2017 - 04:58.