Advance ratio

Diameter of the propeller.

In aeronautics and marine hydrodynamics, the advance ratio at which a propeller is operating is the ratio between the distance the propeller moves forward through the fluid during one revolution, and the diameter of the propeller. When a propeller-driven vehicle is moving at high speed relative to the fluid the advance ratio of its propeller(s) is a high number; and when it is moving at low speed the advance ratio is a low number. The advance ratio of a propeller is analogous to the angle of attack of an airfoil.

Advance ratio

The advance ratio J is given by:^[1]

$J = \frac{V_a}{nD}$

where

Va	is the speed of advance, typically the true airspeed of the aircraft or the water speed of the vessel
n	is the propeller's rotational speed in revolutions per unit of time
D	is the propeller's diameter

The advance ratio is a means of describing the incoming angle of the fluid relative to the propeller blade.

Fluid velocity

The fluid velocity relative to the propeller blade increases with distance from the hub, due to the rotational velocity of the propeller. Therefore, the direction of the flow relative to the propeller blade is equal to the vector sum of the speed of advance and the rotational speed of the propeller.^[2] In mathematical terms,

$\overrightarrow{Vr} = \overrightarrow{\omega}\times\overrightarrow{r}$

$n = \frac{\omega}{(2 \pi)}$

$\overrightarrow{V} = \overrightarrow{Va} + \overrightarrow{Vr}$

where

Vr	is the linear velocity vector of the propeller blade at distance r from its center.
ω	is the propeller's rotational speed in radians per unit of time
V	is the vector of the incoming fluid flow relative to the propeller blade at distance r from its center
r	is the distance from the center of the propeller to the radial point of evaluation, 0<=r<=R
R	is the radius of the propeller

Any propeller, be it for marine or aeronatical use, has blades shaped like airfoils, which twist towards the direction of rotation with increasing distance from the hub. This is to produce a relatively constant "angle of attack" for the blades at the speed of advance for which that particular propeller was designed.^[3]

It is worth noting that the formula for the advance ratio is remarkably similar to the ratio of V_a/V_r, the difference being a constant of value r/(D*2π), considering that V_a and V_r are orthogonal by design, V_a/V_r is the tangent of the incoming angle of the fluid relative to the propeller blade. It is for that reason that the advance ratio is in fact analogous to angle of attack for propeller blades.^[4]

It is also important to note that the advance ratio J is a non-dimensional parameter, and as such all units on the right side of the equation should cancel out. If the speed of advance is measured in feet per second, for example, the rotational speed must be measured in revolutions per second and the diameter must be measured in feet.^[5] The performance of different propellers should be compared for the same advance ratio.

Notes

1. ^ Clancy, L.J. (1975), Aerodynamics, Section 17.2, Pitman Publishing Limited, London. ISBN 0 273 01120 0
2. ^ Wound, Hans Klein; Douwe, Stapersma (2002), Design of Propulsion and Electric Generation Systems Chapter 10: Propellers, The Institute of Marine Engineering, Science and Technology, London. ISBN 1 902536 47 9
3. ^ Wound, Hans Klein; Douwe, Stapersma (2002), Design of Propulsion and Electric Generation Systems Chapter 10: Propellers, The Institute of Marine Engineering, Science and Technology, London. ISBN 1 902536 47 9
4. ^ Wound, Hans Klein; Douwe, Stapersma (2002), Design of Propulsion and Electric Generation Systems Chapter 10: Propellers, The Institute of Marine Engineering, Science and Technology, London. ISBN 1 902536 47 9
5. ^ Wound, Hans Klein; Douwe, Stapersma (2002), Design of Propulsion and Electric Generation Systems Chapter 10: Propellers, The Institute of Marine Engineering, Science and Technology, London. ISBN 1 902536 47 9

External links

• Propeller Aircraft Performance and The Bootstrap Approach