# Helicopter downwash force

## Main Question or Discussion Point

Does anyone know how to calculate how much downwards force a helicopter would exert on the roof of a house if it hovered over it at 700ft?

Thanks

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FredGarvin
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?

FredGarvin
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.

FredGarvin