The apparent speed, and apparent angle determine the twist of blade propellers

## Understand and master the apparent velocity and apparent angle:

When a cyclist moves on a road 30 m / sec by a windless day, the relative velocity of wind for him to 30 m / sec. This bike can say that the apparent wind of 30 m / sec and it comes from the front. If the wind starts blowing facing him, the apparent wind speed will be composed of the speed of the cyclist and the wind speed.

- If the wind blows perpendicularly with a speed of 1 m / sec, the apparent wind will be made primarily on the speed of the cyclist and will almost face
- if perpendicular wind blowing at 30 m / sec, the apparent wind will all face perpendicular and that will be felt by the rider at 45 ° off course
- If the wind starts to blow at 800 m / sec the rider feel the wind almost perpendicular to the road is 90 °. Note also that he risk of permanently stop the bike with such a wind...

For a propeller blade element located at radius

**r**, the relative speed is composed of the peripheral speed (m / sec) and the velocity

**V**of the fluid upstream of the propeller. The peripheral speed

**Vp**is related to the radius

**r**and the rotation speed

**w**(in rotation per second) as follows:

**Vp**(m/sec)=

**w**X 3.14 X

**r**X 2 It is therefore more important at the blade tip that near the center....

The apparent angle received by a profile placed on radius r is thus the sum of the peripheral speed and the speed of the fluid upstream of the propeller:

**the apparent wind speed (m / sec) is therefore :**

**(pythagore)=sqrt(Vp²+V ²)**

**Vp**(m/sec) is the peripheral speed=(

**w**X 3.14 X

**r**X 2) and

**v**is the speed of the fluid upstream (m/sec)

- tangent(apparent angle)=Vp/V
**The apparent angle is therefore = atan (Vp/V)**

- propeller's blades's twist compensates the variation of the apparent angle:

**Warning: The speed actually received by the profile of the blade is slightly different from the apparent speed, because the profile acts on the direction and velocity of the fluid and induces a change**:

- The air just front of an airplane propeller that rotates while the aircraft is stopped, is drawn through the propeller !
- The air just in front of a wind turbine turning is braked !

- Calculation methods and theories will inform you on the induced velocity and its calculation. Of course, the software tool Heliciel calculating these speeds to create the optimum twist in your propeller and produce plans 3D to create the prototype.