The Features of Drone Propellers
The propeller is a rotating airfoil that is suitable for the induced drag of any wing, and the stall and other aerodynamic principles are also applicable to the propeller.
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It provides the necessary pull or thrust to move the drone in the air. The way the propeller produces thrust is very similar to how the wing generates lift.
The amount of lift generated depends on the shape of the blade, the angle of attack of the propeller blade, and the speed of the engine. The propeller blades themselves are torsion, so the blade angle varies from the hub axle to the tip. The maximum mounting angle is at the hub axle and the minimum mounting angle is at the tip of the blade.
Variation of the installation angle of the propeller section
The reason for the twisting of the propeller blades is to produce a consistent lift from the hub axle to the tip of the blade. When the blade rotates, different parts of the blade have different parts with different actual speeds. The tip speed of the blade tip is faster than near the hub axle because the tip of the blade rotates longer than the hub axis at the same time.
A change in the mounting angle from the hub axle to the tip and a corresponding change in line speed can produce a consistent lift over the length of the blade. If the propeller blade is designed to have the same mounting angle over its entire length, then the efficiency will be very low, as-as the airspeed increases, the portion near the axis will have a negative angle of attack and the tip will stall.
Different linear speeds at the same angular velocity of each section of the propeller
Light and microdrones are generally equipped with fixed-range propellers. Large and small drones can be used to improve the dynamic performance by installing variable pitch propellers.
What is fixed pitch propellers
The fixed pitch paddle cannot change the pitch. This type of propeller can only achieve the best efficiency with a combination of airspeed and speed. In addition, fixed pitch propellers can be divided into two types, climbing propellers and cruise propellers. Whether the aircraft is a climbing propeller or a cruise propeller depends on its intended use.
(1) The climbing propeller has a small pitch and therefore has less rotational resistance. Lower drag results in higher speeds and more power capability, which increases performance during takeoff and climb, but reduces performance during cruise flight.
(2) The cruise propeller has a high pitch, so the rotational resistance is more. More drag results in lower speed, and lower power capability, which reduces takeoff and climb performance, but increases the efficiency of high-speed cruise flight.
The propeller is usually mounted on a shaft that may be an extension of the engine crankshaft. In this case, the propeller speed is the same as the crankshaft speed. In some other engines, the propeller is mounted on a shaft that is geared to the crankshaft of the engine. At this time, the rotational speed of the crankshaft is different from the rotational speed of the propeller.
Lightweight and microdrones are commonly used for fixed-range propellers. The size is usually expressed by X×Y, where X is the diameter of the propeller in inches (in) and Y is the pitch, which is the distance the propeller rotates in the air. , in inches (in). For example, a 22 x 10 propeller has a paddle diameter of 22 in, about 55.88 cm, a pitch of 10 in, and about 25.4 cm.
Light and microdrones generally use 2 blade paddles, and a few use 3 blade paddles or 4 blade paddles. According to the habit of the drone industry, it is generally defined that the right-handed propeller is a positive propeller, and the left-handed propeller is a reverse propeller.
Propellers with a diameter of 20 in or less are made of wood, engineering plastics or carbon fiber, and need to be selected according to actual needs. Part of the propeller blades are designed in the shape of a saber, and the tip of the propeller is swept back, which can improve the efficiency to some extent.
What is variable pitch propellers
Some older adjustable pitch propellers can only be adjusted on the ground, and most modern adjustable pitch propellers are designed to adjust the pitch of the propeller in flight. The first generation of adjustable pitch propellers only offered two pitch settings – low pitch setting and high pitch setting.
Today, however, almost all adjustable pitch propeller systems can adjust the pitch within a range. Constant speed propellers are the most common type of adjustable pitch propeller. The main advantage of the constant speed propeller is that it converts most of the engine power into propulsion horsepower over a wide range of airspeed and speed combinations.
Constant speed propellers are more efficient than other propellers because they are able to select the most efficient engine speed under certain conditions.
The UAV equipped with a constant speed propeller has two controls, throttle control, and propeller control, throttle control power output, and propeller control to adjust the engine speed.
Once a specific speed is selected, a regulator automatically adjusts the necessary propeller blade angle. To maintain the selected speed. For example, after the required speed is set during a cruise flight, an increase in airspeed or a decrease in propeller load will cause the propeller to increase the blade angle to maintain the selected speed. A decrease in airspeed or an increase in propeller load results in a decrease in the propeller blade angle.
The blade angle range of the constant speed propeller is determined by the constant speed range of the propeller and the high and low pitch stop. As long as the propeller blade angle is within the constant speed range without exceeding any one of the pitch stops, the engine speed can be maintained constant. However, once the propeller blades reach the stop position, the engine speed will increase or decrease as the airspeed and propeller load change.
For example, a specific speed is selected and the airplane speed is reduced enough to rotate the propeller blades until a low pitch stop is reached. If the airspeed needs to be lowered again, the engine speed must be reduced, just as a fixed pitch propeller is installed.
The same happens when the constant speed propeller airplane accelerates to a faster speed. As the aircraft accelerates, the propeller blade angle increases to maintain the selected speed until a high pitch stop is reached. Once the stop is reached, the blade angle can no longer be increased. If additional acceleration is required, the engine must increase the speed.
On an aircraft equipped with a constant speed propeller, the power output is controlled by the throttle and indicated by the intake pressure gauge. This meter measures the absolute pressure of the hydrocarbon mixture in the inlet manifold. More accurately, it measures the manifold absolute pressure (MAP). At constant speed and altitude conditions, the amount of power produced is directly related to the mixed flow of oil and gas flowing to the combustion chamber.
When you increase the throttle setting, the amount of oil that flows to the engine increases, so the absolute pressure of the manifold increases. The manifold pressure gauge indicates ambient air pressure when the engine is not running. When the engine is pneumatic, the manifold pressure indication will drop to a value below the ambient air pressure.