Within the early years of aviation, propeller-powered plane had been the norm. Nonetheless, within the Nineteen Fifties, with the beginning of the jet age, jet engines grew to become the popular alternative for many massive medium to long-haul plane.
Propeller plane continues to reside on. They’ll nonetheless be present in virtually all common aviation plane and short-range transports. In most transport class propeller plane, the propeller is run by a jet engine, and they’re generally referred to as turboprops. These turboprops are extra environment friendly than pure jet-engined plane on shorter routes.
A propeller is a tool consisting of blades or airfoils which convert engine energy into propeller thrust. The image under exhibits a labeled propeller.
A labeled propeller blade. Photograph: Oxford ATPL
Because the propeller rotates, the blades or the airfoils expertise an angle of assault identical to the wings. This angle of assault generates a raise pressure, perpendicular to the blade chord line. This pressure then splits into vertical and horizontal parts. The horizontal element works within the course of flight and is called propeller thrust, whereas the vertical element acts towards the propeller course of rotation. This element is known as the propeller torque, and this pressure acts towards the propeller rotation.
Propeller angle of assault. Photograph: Oxford ATPL
Because the propeller ideas journey sooner than the foundation, the ideas are likely to generate extra thrust. This may trigger extreme load on the ideas. To unravel this problem, the propeller blades are twisted in a way such that the blade ideas have a decrease blade angle than the roots. This fashion, the same angle of assault is maintained from root to tip, making all elements of the propeller generate a thrust pressure of the identical magnitude.
The propeller blades are designed such that the ideas have a decrease blade angle than the foundation. Photograph: Joe Kunzler | Easy Flying
The propeller blade angle and the angle of assault
The blade angle of a propeller is the angle between the propeller aircraft of rotation and the blade chord line. When the blades are mounted to the propeller hub with a big blade angle, it’s referred to as a rough pitch propeller, and when the blades are mounted to the hub with a small blade angle, the propeller is claimed to be a tremendous pitch propeller.
The angle of assault is the angle between the relative airflow performing on the blade and the chord line. This angle is affected by two foremost components. The RPM (Revolutions Per Minute) of the propeller and the TAS (True Air Velocity) of the plane.
The RPM and TAS varies the angle of assault on the propeller blades. Photograph: FAA
When both the RPM or the TAS is diversified, there’s a change within the angle of assault on the propeller blades. As RPM will increase (TAS mounted), the angle of assault will increase, and as TAS will increase (RPM mounted), the angle of assault reduces. Within the former case, the angle of assault might enhance to the purpose the place the blades stall. Whereas within the latter case, the angle of assault might very effectively lower to zero, lowering propeller thrust to a really low worth.
In some advanced plane, the pilot can management the blade angle, and these propellers are referred to as variable pitch or fixed velocity propellers, whereas in smaller plane, mounted pitch propellers are used.
Mounted pitch propellers
Mounted-pitch propellers are these propellers with a blade angle that’s mounted. That’s, it can’t be diversified or modified in flight.
Within the earlier part, we talked about how TAS and RPM have an effect on the angle of assault on the propeller blades. With a fixed-pitch propeller, the blade angle stays fixed; the pilot can not range it. Thus, in a low TAS, excessive RPM situation (as an example, in a high-power climb out), the angle of assault of the blades might attain a worth so excessive that they stall. Equally, in a excessive TAS, low RPM situation (for instance, in a standard descent), the propeller angle of assault might go to a really low angle, lowering thrust to virtually zero.
A really steep descent may scale back the angle of assault to the purpose the propeller begins to show the engine, and it could result in an engine overspeed situation.
This, nevertheless, doesn’t imply fixed-pitch propellers are dangerous. They’ll nonetheless be present in most common aviation plane, and are chosen for his or her simplicity. Designers select probably the most applicable blade angle for the plane primarily based on its operational necessities. For an plane that’s designed for lengthy cross-country flying, a rough pitch (a propeller with a big blade angle) could also be most well-liked as a result of it’s going to spend most of its time in cruise, flying at excessive speeds.
Variable pitch or fixed velocity propellers
To make propellers extra environment friendly in numerous flight regimes, the propeller blade angle could also be modified by pilot motion. A lot of these propellers are referred to as variable pitch or fixed velocity propellers.
So, how is the propeller blade angle managed? In plane with constant-speed propellers, a propeller management lever (prop lever) is accessible within the cockpit for the pilot. This lever is separate from the engine management lever or the ability levers. The pilot controls the propeller by various its RPM by shifting the prop levers. When the prop lever is moved ahead, the RPM will increase, and when it’s pulled again, the propeller RPM decreases. A system referred to as a Fixed Velocity Unit (CSU) then maintains the set RPM.
As an example, throughout take-off, the angle of assault reduces because the plane TAS will increase. That is detected by the CSU, and it will increase the blade angle to keep up the pilot set RPM.
How the Fixed Velocity Unit (CSU) works
The Fixed Velocity Unit (CSU) makes use of oil stress to both drive the propeller blade angle to the next angle (coarse pitch) or a decrease angle (tremendous pitch). The CSU is pushed by the engines, and it could detect if the propeller is both in an overspeed or underspeed situation.
The primary parts of the CSU are:
- The speeder spring.
- A verify valve.
Propeller CSU. Photograph: Oxford ATPL
When the pilot strikes the prop lever forwards or backwards, she or he modifies the stress within the speeder spring. When it’s moved backward, the stress is lowered, and when it’s moved ahead, the stress is elevated.
The flyweights rotate with the engine, and it’s the conduct of the flyweight that determines the positioning of the oil management valve.
When the propeller is in an underspeed situation or if its blade angle is simply too excessive, the propeller RPM begins to lower. This will increase the propeller torque, and the speeder spring pressure can overwhelm the flyweights and causes them to break down. This causes the management valve to maneuver down letting oil cross to the fine-pitch aspect of the propeller whereas the coarse aspect is linked to the oil return.
This causes the blade angle to scale back. Because the blade angle reduces the engine can impart the next torque on the propeller, which will increase its RPM. The RPM will increase till the engine torque transferred to the speeder spring by way of the flyweights can not overcome the spring pressure. At this level, the propeller begins spinning on the pilot-set RPM.
CSU conduct when propeller is an underpseed situation. Photograph: Oxford ATPL.
Equally, when the propeller is in an overspeed situation, the blade angle turns into too low or too tremendous, and this causes RPM to exceed the pilot set worth. The RPM will increase as a result of the engine torque is greater than that of the propeller. This causes the flyweights to unfold because of the rise in centrifugal pressure. This makes the management valve transfer up, letting oil cross to the coarse pitch aspect whereas the tremendous pitch is linked to the oil return inflicting the propeller blades to extend their blade angle. This will increase the propeller torque, which exerts a pressure on the speeder spring and thus causes the flyweights to push down till the RPM reaches the pilot set worth.
CSU conduct when propeller is an overspread situation. Photograph: Oxford ATPL.
In bigger turboprops, a situation lever is used as an alternative of a prop lever. The situation lever capabilities like a prop lever in that it controls the velocity of the propeller. Along with this, the situation lever additionally controls the gas provide to the engines in the course of the engine begin up. It is usually used to chop off the gas in the course of the engine shutdown.
Situation levers of an ATR 76. Photograph: ATR
One of the vital options of a propeller is its potential to feather. When the propeller is feathered, its blade angle is sort of 90 levels. This angle is known as the zero-lift angle of assault. On this place, the propeller can not generate any thrust.
A feathered prop lies 90 levels to the relative airflow. Photograph: FAA
This is a vital function in an engine failure scenario. In a variable-pitch propeller, if an engine loses its energy, the propeller’s RPM naturally reduces. This causes the CSU to tremendous off the propeller to the purpose the place the blade angle turns into too tremendous, and it begins to windmill. This causes the air to assault the blades from the entrance, producing a damaging thrust pressure. This thrust acts towards the course of flight and provides to the plane’s drag. In an engine-out situation, the drag from a windmilling propeller will be extremely detrimental to the general efficiency and management of the plane.
To forestall this, the feathering mechanism exists. When the propeller is feathered, the airflow can not work together with it, and there’s no danger of windmilling. In massive plane, the feathering of the propeller is essential to satisfy the takeoff efficiency if an engine failure happens in the course of the roll. Thus, in such plane, an autofeather system exists. The pilots “arm” the system for take-off, and if an engine failure happens on the take-off run, the propeller mechanically goes into feathered place, retaining the plane efficiency intact.
Most turboprops have their propellers in a feathered place when engines are shut down. This fashion, they seem pointed forwards. Because the propellers come out of feather, they change into flatter.
Within the left image, the blades are flatter, and thus the propeller is unfeathered whereas the fitting image exhibits a propeller that’s feathered. Photograph: ATR