The principle behind a propeller is just like the forces that work in an airfoil. The plane undergoes different aerodynamic forces that directly act on it including lift, drag, weight and thrust. Thrusting is mainly provided by the propeller’s action that is also affected by the same aerodynamic forces. The efficiency of propellers can be obtained by making sure that it is positioned in its optimal blade angle of attack.
Propellers can be affected by the blade angle or the pitch angle such that at low pitch, the number of revolutions per minute is increased, thus would be beneficial during takeoff. Propeller design also affect its efficiency that is why a twist in the blade is usually created to provide a more balanced angle of attack or optimal lift to drag ratio on each blade assembly.
Propeller’s thrusting motion
One general function of propellers is that it can either slow down or increase thrusts of an airplane through the change in the angle of pitch of the blades. As air particles hit the propellers, they will be accelerated and will be pushed opposite the direction of flight. This in turn would induce a force to create forward movement of the plane, thus, thrusting occurs.
In aircrafts, thrust would be dependent on the volume and density of air that is accelerated as it comes in contact with the propellers at a given time.
How the types of propellers work
The functions of propellers would also depend on their types. Fixed pitch propellers, for example, have only one pitch settings. They are widely used in early aircrafts and are usually composed of two blades which can be made from wood or metal. The metal propellers, which was intended in the military aircrafts, is now widely used in aircraft propeller construction since it can be made thinner with the use of high end materials with optimum mechanical properties such as aluminum alloys.
However, variable pitch propellers, which can either be adjustable or controllable, would work depending on the need for a situation such that a pilot has control on them. These types of propellers will be discussed below.
Ground adjustable pitch propellers, as the name implies, can be manipulated only before flight when the aircraft is still in the ground. This type has a split hub and the advantage in using this type is that it can be adjusted depending on the field of flight, altitude as well as airplane characteristics.
Two-position propellers work even in flight unlike the ground adjustable ones. The pilot can change the pitch angle depending on the need of the aircraft in a particular flight situation.
Constant speed propellers maintain the engine as well as the propeller to work at constant rpm. It happens because the pitch of propeller would comply with the power requirement of the engine during flights. The power of the engine is directly proportional to blade angle such that a decrease in one would also mean a decrease in the other for them to be able to compensate with the amount of force of air during thrusts so as to maintain constant rpm.
Full feathering propellers work especially during engine failures. Full feathering is actually not a separate type of propeller than constant speed propellers because the latter works this way in engine failures. Propellers in this situation are in parallel to the airflow that prevents the dragging produced by windmilling. The propellers are set whit high pitch angle with the aim of equalizing the amount of air pressure on the front and on the back parts of the blade, thus, stopping the propeller from rotating during failure of engines.
Another function of constant speed propellers is a reverse function that happens by rotating to its negative angle to create a backward thrust which is opposite to the propeller’s function in flight. This is helpful during landing because it would lessen the distance needed to be traveled by the reverse motion of propellers that functions to push the air forward instead of backwards, thus, it helps the function of the break as well as it lessens the wear of tires.
The last types of propellers are the beta control propellers. This type allows the pilot to manually change the propeller blade even beyond the normal low pitch stop. It is usually used when planes taxi on taxiways of an airport like when it moves from the terminal to the runway.
In conclusion, a propeller is an important component of the airplane. What should be considered in choosing an appropriate propeller to maximize its function would include the type of aircraft, the size of the aircraft and the function intended for the aircraft. Even if different forces could affect the efficiency of propellers, there are a lot of actions that a pilot can do to counteract such forces depending on what aerodynamic functions are needed in a particular situation.
References:
http://www.pilotfriend.com/training/flight_training/fxd_wing/props.htm
http://www.thaitechnics.com/propeller/prop_type.html
http://travel.howstuffworks.com/airplane2.htm
http://www.mh-aerotools.de/airfoils/propuls4.htm
http://www.centennialofflight.gov/essay/Theories_of_Flight/props/TH18.htm
