How do the blades on a propeller generate thrust ?

How do the blades on a propeller generate thrust ?

The cross section of propeller blade is an airfoil. As the propeller rotates through the air, it generates lift, which becomes thrust as the it is directed forwards rather than vertically upwards as in aircraft wing.


The relative airflow on the blade section (airfoil) is the resultant vector of two things - the forward motion of the propeller (the aircraft) and the rotational velocity of the propeller blade itself. Most of the propellers in use today are fixed speed variable pitch propellers. So, velocity experienced by the propeller blade is maximum at the tip (where the rotational speed is maximum) and minimum at the hub.


So, at any RPM, different parts of the propeller are at different angles of attack. To remedy this, the airfoil sections are varied along the propeller. So the propeller is practically twisted along its length, with the root at higher angle of attack. Also, in some cases, the airfoils are changed along the length (thinner ones are near tip).


As already noted, the pitch of the propeller blades can be varied in flight. What the photo shows is afeathered propeller i.e. a propeller that is set parallel to the airflow. This is done usually in order to reduce drag in case of engine failure. During normal operation, the blades will not be in feathered position. For example, in the figure below, the propeller of engine 1 feathered while the next one is not.

From wikimedia commons, work by Julian Herzog

In aircraft, propeller pitch is changed with speed, with fine pitch at low speeds and strong acceleration (like takeoff) and a coarse pitch for high speed (cruise).

In most aircraft, the propeller pitch settings are adjusted automatically by a governor according to requirements. The following figure shows the pitch variation in flight for a similar propeller.

"Propeller pitch on an ATR 72" by Olivier Cleynen - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons.

Thrust The thrust in both turboprop and turbofan is provided by accelerating air through fans. In a turboprop, it the propeller, while it is the bypass fan in case of the turbofan.

The thrust produced, T is given by,


where m˙m˙ the mass flow rate of air and vfvf and v0v0 are the final and initial (unaccelerated) velocities of the air respectively. Usually case of propellers, the mass flow rate m˙m˙ is higher, while in case of turbofans, the variation in velocity is comparatively higher.
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