In the early years of aviation, propeller planes were the norm. However, in the 1950s, with the onset of the jet aircraft era, jet engines became the preferred choice for most large medium to long-range aircraft.
Propeller planes live on. They are still found in almost all general aviation aircraft and short-range transport. In most transport-category propeller-driven aircraft, the propeller is driven by a jet engine, and they are commonly referred to as turboprops. These turboprops are more efficient than pure jet aircraft on shorter routes.
A propeller is a device made up of blades or airfoils that convert engine power into propeller thrust. The image below shows a labeled propeller.
A labeled propeller blade. Photo: Oxford ATPL
As the propeller spins, the blades or airfoils experience an angle of attack just like the wings. This angle of attack generates a lift force, perpendicular to the chord of the blade. This force then splits into vertical and horizontal components. The horizontal component works in the direction of flight and is known as propeller thrust, while the vertical component works against the direction of propeller rotation. This component is called propeller torque and this force acts against the rotation of the propeller.
Propeller angle of attack. Photo: Oxford ATPL
As the propeller tips move faster than the root, the tips tend to generate more thrust. This can lead to excessive load on the tips. To solve this problem, the propeller blades are twisted so that the tips of the blades have a lower blade angle than the roots. This way, a similar angle of attack is maintained from root to tip, causing all parts of the propeller to generate thrust force of the same magnitude.
Propeller blades are designed so that the tips have a lower blade angle than the root. Photo: Joe Kunzler | single flight
The angle of the propeller blades and the angle of attack
The blade angle of a propeller is the angle between the plane of rotation of the propeller and the chord line of the blades. When the blades are attached to the hub of the propeller with a large blade angle, it is called a coarse pitch propeller, and when the blades are attached to the hub with a small blade angle, the propeller is said to be a pitch propeller. not end.
The angle of attack is the angle between the relative airflow acting on the blade and the chord line. This angle is affected by two main factors. The RPM (Revolutions Per Minute) of the propeller and the TAS (True Air Speed) of the aircraft.
RPM and TAS vary the angle of attack of the propeller blades. Photo: FAA
When the RPM or TAS is changed, there is a change in the angle of attack on the propeller blades. When the RPM increases (fixed TAS), the angle of attack increases and when the TAS increases (fixed RPM), the angle of attack decreases. In the first case, the angle of attack could increase to the point where the blades stall. While in the latter case, the angle of attack could very well decrease to zero, reducing the thrust of the propeller to a very low value.
In some complex aircraft, the pilot can control the angle of the blades, and these propellers are called variable-pitch or constant-speed propellers, while in small aircraft, fixed-pitch propellers are used.
Fixed pitch propellers
Fixed-pitch propellers are propellers with a fixed blade angle. That is, it cannot be varied or changed in flight.
In the previous section, we talked about how TAS and RPM affect the angle of attack on the propeller blades. With a fixed-pitch propeller, the angle of the blades remains constant; the driver cannot modify it. Thus, in a low TAS and high rpm condition (for example, during a high power climb), the angle of attack of the blades can reach such a high value that they stall. Similarly, in a high TAS, low RPM condition (e.g., during a normal descent), the propeller’s angle of attack can reach a very low angle, reducing thrust to almost zero.
A very steep descent can reduce the angle of attack to the point where the propeller begins to spin the engine, and this can lead to an engine overspeed condition.
This, however, does not mean that fixed pitch propellers are bad. They can still be found in most general aviation aircraft and are chosen for their simplicity. Designers choose the most appropriate blade angle for the aircraft based on its operational requirements. For an aircraft designed for long range flight, a coarse pitch (a propeller with a large blade angle) may be preferred as it will spend most of its time cruising, flying at high speeds.
Variable-pitch or constant-speed propellers
To make propellers more efficient in different flight regimes, the angle of the propeller blades can be changed by pilot action. These types of propellers are known as variable pitch or constant speed propellers.
So how is the angle of the propeller blades controlled? In aircraft with constant speed propellers, a propeller control lever (prop lever) is available in the cockpit for the pilot. This lever is separate from the engine control lever or power levers. The pilot controls the propeller by varying its rpm by moving the propeller levers. When the propeller lever is moved forward, the rpm increases and when it is pulled back, the propeller rpm decreases. A system called the Constant Speed Unit (CSU) then maintains the set rpm.
For example, during takeoff, the angle of attack decreases as the aircraft’s TAS increases. This is detected by the CSU and increases the angle of the blades to maintain the set pilot RPM.
Constant Speed Unit (CSU) Operation
The Constant Speed Unit (CSU) uses oil pressure to drive the angle of the propeller blades to a higher angle (coarse pitch) or a lower angle (fine pitch). The CSU is driven by the motors and can detect if the propeller is overspeeding or underspeeding.
The main components of the CSU are:
- The speeder spring.
- A control valve.
CSU propeller. Photo: Oxford ATPL
As the pilot moves the propeller lever forward or backward, it changes the spring tension of the speeder. When moved backward, the tension is lowered and when moved forward, the tension is increased.
The flyweights rotate with the engine and it is the behavior of the flyweight that determines the positioning of the oil control valve.
When the propeller is underspeed or if the angle of its blades is too high, the propeller rpm begins to decrease. This increases prop torque and the speeder spring tension can overcome the flyweights and cause them to collapse. This causes the control valve to drop, passing oil from the fine pitch side of the propeller while the coarse side is connected to the oil return.
This results in a reduction in the angle of the blade. When the angle of the blades decreases, the engine can transmit more torque to the propeller, which increases its speed. The rpm increases until the engine torque transmitted to the speeder spring via the flyweights can no longer overcome the spring tension. At this point, the propeller begins to spin at the rpm set by the pilot.
Behavior of the CSU when the propeller is underpowered. Photo: ATPL of Oxford.
Similarly, when the propeller is in an overspeed condition, the angle of the blades becomes too low or too fine, which causes the rpm to exceed the pilot’s setpoint. The RPM increases because the engine torque is greater than that of the propeller. This causes the flyweights to spread due to the increased centrifugal force. This causes the control valve to rise, passing oil on the coarse pitch side while the fine pitch is tied to the oil return, causing the propeller blades to increase their blade angle. This increases the torque of the propeller, which puts a force on the speeder spring and thus pushes the flyweights down until the rpm reaches the pilot’s set point.
Behavior of the CSU when the propeller is in boost condition. Photo: ATPL of Oxford.
In large turboprops, a condition lever is used instead of a propeller lever. The condition lever works like a propeller lever in that it controls the speed of the propeller. In addition to this, the condition lever also controls the fuel supply to the engines during engine start. It is also used to shut off fuel when stopping the engine.
Condition levers of an ATR 76. Photo: ATR
One of the most important characteristics of a propeller is its ability to feather. When the propeller is feathered, its blade angle is nearly 90 degrees. This angle is called the angle of attack at zero lift. In this position, the propeller can no longer generate thrust.
A feathered accessory sits at 90 degrees to the relative airflow. Photo: FAA
This is an important feature in an engine failure situation. In a variable pitch propeller, if one motor loses power, the propeller rpm naturally decreases. This causes the CSU to thin the propeller to the point where the blade angle becomes too thin and it begins to windmill. This causes the air to attack the blades from the front, generating a negative thrust force. This thrust acts against the direction of flight and adds to the aircraft’s drag. In an engine failure condition, the drag of a windmilling propeller can be very detrimental to the overall performance and control of the aircraft.
To avoid this, the feathering mechanism exists. When the propeller is feathered, the airflow cannot interact with it and there is no risk of spinning. In large aircraft, propeller feathering is very important to meet takeoff performance if an engine failure occurs during roll. Thus, in such aircraft, an automatic feathering system exists. Pilots “arm” the system for takeoff, and if an engine failure occurs during the takeoff roll, the propeller will automatically feather, preserving aircraft performance.
Most turboprops have their propellers feathered when the engines are shut down. This way they appear pointed forward. As the helices come out of the feather, they become flatter.
In the left image, the blades are flatter, and therefore the propeller is not feathered while the right image shows a feathered propeller. Photo: ATR