Propeller Aerodynamics, I
A propeller is an airfoil and like a wing it will generate an aerodynamic force much the same way. It has a leading and trailing edge, camber and a chord line. The cambered side is called blade back and the flatter side the blade face. The angle which the chord makes to the plane of rotation is the blade angle. The propeller is rotated by the engine and this creates thrust and moves the aircraft forward.
Previous pages discussed the type of propellers and their controls in normal and emergency situations, in this section we will try to describe some of the aerodynamic principles of propellers.
Having a understanding of these principles will really help you with learning to fly and keep your aircraft under control during take-off when these effects are most prevalent.
As the propeller moves through the air the static pressure is reduced ahead of each blade and at the same time at the blade face the flow is retarded resulting in an increase of static pressure. The changes in pressure around the rotating blades causes air to be drawn into the propeller disc and this results in a rearward movement of a column of air. The result being a forward thrust pulling the aircraft. All of this is in accordance with Newton's Third Law.
The amount of thrust generated by a propeller depends on the mass of the air and its acceleration toward the rear. Thus with equal RPM the smaller propeller delivers less thrust than the larger one.
Blade twist and pitch
The need for a changing blade angle from the hub to the tips stems from the fact that angular speed varies also and is greatest at the tips. Combine this with any forward speed the propeller may have, the relative airflow is also different from the hub to the tips. To keep thrust equal along the blade, they have a build in twist. The design is such that the blade is thick at the hub with a large blade angle and thin at the tip with a low blade angle.
During rotation and forward movement the propeller describes a rotational path, called a helix. If the propeller would move forward without giving thrust the distance of one revolution is called experimental mean pitch (angle between plane of rotation and the zero thrust angle of attack). The actual advance is the difference between experimental mean pitch and slip. Slip is the angle between the zero thrust and the actual angle of attack.
If the propeller rotates in a solid medium it would advance according to its pitch (angle between plane of rotation and the blade face, chord line,) also called geometric pitch.
If this was an ideal world the propeller would convert all power to thrust. But as this is not the case, losses occur in the slipstream and aerodynamic drag. Under normal conditions the propeller is able to convert about 85% of the brake horse power from the engine into thrust. Thus propeller efficiency is the ratio between thrust horsepower and brake horsepower.
Remember that Power = Force x Distance / Time (rate of doing work), we can equate propeller efficiency as Thrust x TAS / Brake Horse Power.
It follows that propeller efficiency is zero (0) under two conditions: when there is no forward speed (TAS) or when there is no thrust generated. With the aircraft at standstill (beginning of takeoff roll or taxi) the propeller has zero efficiency until it reaches its optimum forward speed for the propeller where maximum thrust is generated (maximum efficiency), increasing forward speed beyond that point will decrease efficiency (propeller with fixed blade angle). You could say that there is a relation between RPM and airspeed for fixed pitch propellers.
It is obvious that a controllable propeller has a wider range of airspeeds where efficiency is at its maximum, until the governor reaches a position where the blades can no longer be adjusted, which is at the full fine and coarse pitch stops.
During rotation the blades generate lift and drag. But with propellers we talk about thrust (lift) and propeller torque (drag). Another one: with a wing, drag must be overcome thrust to provide lift but with a propeller it is propeller torque that must be overcome by engine torque. Increasing power with the throttle increases engine torque, resulting in a higher RPM until propeller torque is equal to engine torque and RPM stabilizes.
If you would place an aircraft with a fixed pitch propeller into a shallow dive, as forward speed increases the relative airflow changes and the angle of attack is reduced. Resulting in a reduced thrust and propeller torque and as engine torque remained the same (there was no change in throttle setting) engine/propeller RPM will increase. The constant speed propeller would maintain the preset RPM.
Any propeller blade is the most effective between station 60% and 90% with a peak at 75%. It is this point (or station) where blade angle is usually reported in aircraft documentation.
Asymmetric blade effect or p-factor
This is most noticeable during high angles of attack of the airplane, take-off rotation, and for tailwheel aircraft on their take-off run. The main cause is that the propeller disc (normally at a right angle with the airflow) is tilted backwards and the relative airflow between the up going and down going blades are different.
The down going blades have a larger angle of attack (the distance traveled seems greater due to the forward movement of the aircraft) and produce more thrust, pulling the aircraft with a higher force than the up going blades. With an engine rotating clockwise (as seen from the cockpit) the aircraft wants to turn to the left as a result from this.
Twin engine aircraft
On twin engined propeller driven aircraft and with both propellers rotating in the same direction the arm (or distance) of the downward going blades towards the center of gravity are not the same. The engine where the distance is the smallest is called the critical engine. If that one fails the other engine will yaw the aircraft with more force (thrust x arm) than if the critical engine had failed.