Induced Flow

What is Induced Flow ?

Induced Flow is the airflow which is forced through a rotor disk perpendicular to the disc.
It is caused by the defllection of air by the passage of a rotor blade.
Aeroplane wings do exactly the same thing, however because a rotor system has a number of blades, which all rotate in the same bit of space very quickly the air at that position is constantly being deflected perpendicular to the disc. In the hover a column of air is deflected through the disc. Induced flow is what is felt if you stand near a hovering helicopter, induced flow is DOWNWASH.

Induced Flow Is Bad

Induced flow reduces the angle of attack by altering the relative airflow
Induced flow causes the total reaction to lean backward in the plane of rotation.
This in turn adds to the Rotor Drag, which requires more power to overcome.
This also reduces the perpendicular component of of the total reaction and reduces Total Rotor Thrust.

The Heavier a helicopter is the greater the induced flow.
The smaller the disc area the greater the induced flow.
The greater the air density the smaller the induced flow, which implies that great induced flow and smaller angles of attack available at higher density altitudes.
The induced flow velocity is a factor in the onset of Vortex Ring State.

Calculating Induced Flow

In The Hover

From the momentum theory the induced flow can be calculated using the formula

V_i = √[ T / 2 ρ A ]
where

V_i: Induced Flow Velocity [m s^-1
T: Thrust (equal to weight in the hover). [N]
ρ: Air Density [kg m^-3]
A: Disc Area [m²]

This is only an approximation, but is a good starting point for calculations involving the rotor system.

In Forward Flight

In forward flight the perpendicular component of induced flow is reduced due to the horizontal nature of the air over the disc.
I'll post the calculations here when I've simplified them.

Web References

Euler Analysis of Rotor System Imperial College Rotor Wake Analysis


© Copyright 2000-2009 Gary Spender