Airplane Propellers: Principles and Types

e [Music] until the days of supersonic speeds and jet propulsion of rocket ships the propeller is a relatively efficient method of moving our airplanes through the air up to speeds of 5 to 600 milph in general the size of a propeller is dependent on the power of the engine this relatively small engine can turn a small propeller this larger engine needs a larger propeller to absorb its greater power if still more powerful engines were equipped with two bladed propellers they would be so large that the blades would touch the ground or else require a lighting gear of excessive height therefore or three bladed propellers are coupled to medium high horsepower engines while four bladed propellers are usually found on super power engines about 2,000 horsepower most aircraft Mount tractor propellers on the front where they pull the airplane along some however Mount Pusher propellers that push the airplane somewhat in the manner of a steam ship propeller since a col blade is fundamentally an air foil it may be helpful to review Air foil principles an air foil with direction of movement like this produces a lift or Force like this the angle that the cord line of the air foil makes with the direction of movement is called the angle of attack the efficiency curve of any air foil sometimes called the L overd or lift over drag curve varies with the angle of attack from this curve we know that best efficiency is obtained at an angle of attack of 2 to 4° let's see how this affects a propeller as we have said the angle that the blade of a propeller makes With the Wind moving over it is known as the angle of attack the actual Direction the blade moves through the wind which we will will call Blade path is controlled by two factors the speed of the propeller around the Hub known as propeller velocity and the speed of the airplane known as airplane velocity the propeller velocity depends on the propeller RPM and also on the distance from The Hub if should be clear that point C farther out on the blade has a greater linear velocity than points A or B now for any given propeller RPM the propeller velocity will vary with the station or a distance from The Hub the airplane velocity is the same for all parts of the blade at any one moment so to maintain the desired angle of attack of 2 to 4° at all points along the blade it is necessary for the various stations along the blade to have different pitch angles thus we see that a propeller blade has the greatest blade angle near the Hub a and the least blade angle at the tip C this relation remains constant for any propeller regardless of its RPM or the speed of the airplane so far we have shown how propeller velocity influences the the direction of the blade path we've explained how propeller velocity changes with the propeller RPM and with the radius or distance from The Hub here is a cross-section of a fixed pitch propeller at a certain radius of station the engine is running at a constant RPM so the propeller velocity stays constant at this pitch angle the proper angle of attack of 2 to 4° can be obtained only at one certain airplane velocity if airplane velocity is higher the angle of attack will decrease and the propeller efficiency will drop if velocity is lower the angle of attack will increase and the efficiency will drop again this chart has shown that best efficiency is obtained by keeping the propeller blade at such a pitch angle that it is always somewhere near the 2 to 4° point in angle of attack right here let's emphasize the distinction between pitch angle and angle of attack as we have said the angle that the blade path makes with the cord line of the air foil is the angle of attack the angle that the plane of rotation makes with the cord line of the air foil is the pitch angle in other words the pitch is the bite of the propeller because of the twist of the blades a certain radius or station is used to measure this pitch angle this is the so-called 42in station a fixed pitch propeller is more practical on an airplane with a relatively small speed range that is a difference of only about 75 or 100 mph between its maximum and minimum or landing speed its loss of efficiency is somewhat compensated for by its light weight and low cost looking back on the diagram of Blade path and propeller velocity we see that a fixed pitch propeller on an airplane with a relatively small speed range will not get too far from the most efficient angle of attack fixed pitch propellers of wood are carved for a specific make of airplane to provide the best pitch for its speed range ground adjustable propellers with wooden or metal blades and steel hubs can be adjusted while on the ground to the best pitch for any particular airplane according to its speed range basic training type airplanes have a slightly higher speed range in the neighborhood of 100 to 125 mph difference between top speed and landing speed this type of airplane usually mounts a twoos controllable pitch propeller as the name implies the pitch of this propeller can be controlled from the cockpit from the blade path diagram it is clear that a low airplane velocity as at takeoff requires a low pitch for the proper angle of attack at higher airplane speeds after the takeoff the propeller is shifted to a high pitch so the angle of attack will remain close to the proper figure the two- position controllable pitch propeller therefore is satisfactory on airplanes of medium speed range this propeller is hydraulically operated by engine lubricating oil by forcing the oil into the hollow Hub the pitch of the propeller is changed as the cylinder moves forward away from the Hub a propeller with a cylinder of somewhat flat-end shape blades counterweights and a small manually operated valve located on the crank case nose can be identified as a Hamilton standard twoos controllable pitch propeller airplanes with still higher speed ranges need greater propeller pitch ranges in addition they require intermediate pitch positions between the two extremes this need is filled by the Hamilton standard constant speed propeller a governor usually mounted on the nose of the crank case maintains the engine speed at a constant RPM as set by the cockpit control should the engine speed drop the governor notes the change it operates a valve sending oil out to the propeller this flattens out the propeller pitch so the engine can come up to proper speed just how this is done will be explained in subsequent training films dealing specifically with the Hamilton standard constant speed propeller many mediumsized and large multi-engine aircraft are fitted with Hamilton hydromatic propellers which have the additional property of Feathering in case one engine fails that propeller must be capable of Feathering or turning its blade straight into the wind for clarity Feathering is shown with the engine stationary the blades off or less airplane resistance and act as a brake to prevent windmilling when Flight is continued on the remaining engine in some instances windmilling has caused vibration sufficient to actually tear the engine from its mouth Feathering also greatly improves the ability of the airplane to hold or gain altitude on its remaining operating engines this propeller is capable of Feathering in flight and in addition has the constant speed advantages and the high pitch range of the Hamilton standard constant speed propellers counterweight type the hydromatic propeller can be identified by its large oil Dome with nut or breather cap on the end and by its governor on the nose of the crank case section some aircraft Mount still another type of controllable pitch propeller the Curtis electric propeller changes its pitch with a small electric motor mounted ahead of the Hub constant speed operation is provided by a cockpit control which sets a governor in the automatic position this Governor operates the electric switches to the pitch changing motor on multi-engine aircraft a small switch in the cockpit allows Feathering of the Curtis propeller in flight to stop the engine and prevent damage when the Curtis propeller Hub is concealed beneath a spinner the propeller can be identified by the name plate on the blades or by the control switches in the cockpit recently adopted by the air core is the new AOW products propeller a hollow Hub design permitting Cannon installation each blade has its own pitch changing mechanism in the hollow blade chain a cylinder fixed to the blade and a piston which through helical splines rot rotates blade and cylinder action of all three blades are synchronized by the coordinator gear in the hub which meshes with a gear segment on the base of each blade the propeller is entirely self-contained with Governor oil pump and pressure relief valve in one regulator unit just behind the Hub the AO products hydraulic propeller is found on later model Bell P39 era cobras where its Hollow Hub construction makes it ideal for the P39 Cannon because it is self-contained the AO product propeller has no external identifying features it has a propeller Governor control quadrant in the cockpit but no electric switches as in the Curtis electric propeller under the spinner is the regulator unit The only positive identification of the arrow product propeller in summarizing the principles of propellers keep in mind the distinction between angle of attack and pitch angle pitch angle is varied to keep the angle of attack constant as the direction of the blade path varies the direction of the blade path is the resultant of two vectors the propeller velocity and the airplane velocity the high propeller velocity of the blade tip C produces a blade path at the smallest angle so the pitch angle is smaller at the blade tip than at points b or a closer to the hub that's why a propeller blade is Twisted along its length likewise when airplane velocity changes the direction of the blade path varies to allow for this two position controllable pitch propellers give the pilot a choice between low pitch or small bite for low speeds and high pitch or Big Bite for higher speeds an automatic speed Governor controlled by the pilot transforms a controllable pitch propeller into a constant speed model by automatically varying the pitch of the propeller it keeps the engine running at the specified RPM a desirable feature from the standpoint of engine operation today the aircore has many types of propellers for many types of airplanes the highly complex propellers developed by recent engineering research have made possible more powerful engines in the faster airplanes we must keep for aerial Supremacy [Music]