If you want to move forward, you need to push backward; that is the fundamental law of physics which was first described in the 18th century by Sir Isaac Newton and it still holds true today. Newton’s third law of motion, is not always obvious, but it’s the essence of anything that moves us through the world. When you’re walking down the street, your feet push back against the sidewalk to move you forward. For a car, it’s the tires that kick back against the road. But what about aeroplanes or vertical rotorcrafts powered by propellers? They too use Newton’s third law, because a propeller pulls or pushes any aeroplane or vertical rotorcraft forward by hurling a mass of air behind or below , respectively. So, how exactly do propellers work? Why do they have a funny twisted shape? Let’s take a closer look!
How does a propeller work?
Propellers, often referred to as “props,” are also called screws — and the reason is quite simple . Let’s consider we need to fit a screw into a wall ; to push the screw into the wall, a clockwise turning force is applied to the head using a screwdriver. The helical thread (spiral groove) on the screw’s surface converts the turning force into a pushing force that drives the screw into the wall.
In Technical terms, a propeller is a machine that moves the vehicle forward through a fluid (a liquid or gas) when it is turned. Though it works the same way as a screw, it looks a bit different: usually it has two, three, or four twisted blades (sometimes more) poking out at angles from a central hub spun around by an engine or motor. The twists and the angles are really important.
Why does a propeller have angled blades?
Propeller blades are fixed to their hub at an angle, just as the thread on a screw makes an angle to the shaft. This is called the pitch (or pitch angle) of a propeller and it determines how quickly it moves the rotorcraft forward when turned, and amount of force used in the process.
The purpose of an airplane propeller is to make more or less thrust (driving force) at different points of a flight (during takeoff, steady cruising, etc.). The angle of a propeller’s blades along with its overall size and shape affect the thrust, and so does the speed of the engine.
Why does a propeller have twisted blades?
The pitch of a propeller blade changes along its length. It’s steepest at the hub (in the center) and shallowest at the tip (outer edge)….Reason — When we take a sideways look at an airplane propeller ,it resembles an aerofoil , a wing that has a curved top and flat bottom. An aerofoil wing produces lift mainly by accelerating air downward and it works most efficiently when it’s tilted slightly backward to. This “tilt” is called angle of attack , with the horizontal. Now suppose we take two aerofoil wings, mount them either side of a wheel and spin it around. When turned fast enough, with the wings at just the right angle, and instead of generating lift it produces a screwing effect and a backward force that pushes us in the forward direction . This is effectively how a propeller works.
Different parts of a propeller move at different speeds: the tips of the blades move faster than the parts nearest the hub. To ensure a propeller produces a constant force (thrust) all along its length, the angle of attack needs to be different at different points along the blade — greater near the hub where the blade is moving slowest and shallower near the tips where the blade is moving fastest — and that’s why propeller blades are twisted. Without the twist, the propeller would be making different amounts of thrust at the hub and the edges, which would put it under great stress.
Variable pitch
Simple propellers on small aircraft (such as light training craft) have their blades permanently fixed at a certain angle to the hub, which never changes; that’s why they’re called fixed-pitch propellers. But the optimum angle of a propeller’s blades varies according to the aircraft’s functionality . Typically, fixed-pitch propellers are optimized either for cruising or climbing. Bigger and more sophisticated planes have variable-pitch propellers, which come in three variations:
- Adjustable-pitch propellers can have their pitch changed by tinkering with the plane when it’s on the ground, though not during flight, which is why they’re sometimes called ground-adjustable propellers.
2 . Controllable-pitch propellers can be adjusted by the pilot during flight, typically through a hydraulic mechanism.
3. Constant-speed propellers have automated hydraulic mechanisms that change the blade pitch as necessary, allowing the propeller always to turn at the same (constant) speed, which helps the engine to generate power efficiently no matter what the plane is doing or how fast it’s going.
Planes with variable-pitch propellers have another useful feature: the ability to feather the propellers if an engine fails. Feathering means turning the propeller blades so they’re edge on, making a very shallow angle to the oncoming air, minimizing drag (air resistance) and allowing the plane either to keep on flying on its remaining engines or glide to a crash landing. On some planes, the pitch of the blades can be reversed so a propeller makes a forward draft of air instead of one moving backward — handy for extra braking , in case of main brake failure.
