Introduction to Propeller of Vertical Take-off and Landing Fixed Wing UAV

JOUAV CW-007
JOUAV CW-007 Distinguish the propellers by colour to prevent the wrong insertion

Propeller refers to a device that converts the power of the rotating shaft of an engine or motor into propulsion. In the UAV system, it is part of the power system. The propeller's performance and the propeller's adaptability to the engine or motor directly affect the flight performance of the UAV.
This article will introduce the propeller of vertical take-off and landing(VTOL) fixed-wing UAV from the aspects of the working principle, geometric parameters, classification, and propeller performance indicators.

The working principle of the propeller

1. Analysis from the perspective of momentum

The plane of rotation of the propeller is called the propeller disk surface. The propeller does work on the air flowing through the propeller disk surface. After the air flows through the propeller disk surface, the momentum increases, the accelerated air will produce a reaction force against the propeller. This reaction force is the thrust of the propeller.

propeller disk surface

2. Analysis from the perspective of aerodynamics

The propeller can be regarded as a rotating wing, and the aerodynamic principle is the same as that of the branch. The propeller's section perpendicular to the propeller diameter's direction is an airfoil, called the propeller blade element. Each element produces an aerodynamic force, and the resultant force of all element elements is the aerodynamic force generated by the propeller. The component force of the aerodynamic force along the flight direction is the propeller's thrust. The torque of the component force along the rotation direction to the centre of rotation is the torque's propeller.

propeller blade element

 

the geometric parameters of the propeller

1. Propeller diameter D

The propeller's diameter refers to the diameter of the circle drawn by the tip of the propeller. Generally speaking, the propeller's diameter needs to be determined comprehensively by the engine power, speed, flying speed of the drone, the number of blades, and the pitch. The unit of the propeller diameter is generally inches.

Propeller diameter

2. Number of blades NB

The number of blades is also an essential parameter of the propeller. 2-blade, 3-blade and 6-blade propellers are the most common propellers. The propellers of UAVs are generally 2-blade propellers and 3-blade propellers. For example, the Dapeng series UAVs use 2-blade propellers, and the Rainbow series unmanned in my country use 3-blade propellers. The more the number of blades of a propeller, the greater the maximum power that the propeller can absorb, but the lower the efficiency of the propeller. As the number of blades increases, the weight of the propeller also increases. Therefore, the number of blades needs to be combined with the power of the engine. Under the premise of ensuring that the engine's maximum power and the restriction of the propeller diameter can be absorbed, the number of blades should be reduced as much as possible to improve the efficiency of the propeller.

2-blade propellers

3. Leaf element

The cross-sectional shape of the propeller perpendicular to the radial direction of the blade is called leaf element. This is similar to the wing of an airplane. It can be considered that the propeller is a wing with a large twist angle. The aerodynamic performance of the blade element directly affects the performance of the propeller. The airfoil selected for the same type of propeller is different, and its performance is also different. Generally, a lot of calculations and tests are required to determine the performance of the propeller.

Propeller leaf element
Propeller leaf element

4. Blade width b

The length of the blade element chord line is called the propeller blade width b. Since the efficiency of different propeller positions is different, the efficiency of the root and tip of the propeller is lower than that of the middle region. Therefore, to improve the propeller's overall efficiency, the central area's chord length is generally more significant than the chord length of the tip and root. Commonly, the ratio of chord length to propeller radius is used to express the width distribution, and the typical width distribution is shown in the figure.

Blade width b

5. Thread pitch

The propeller's pitch refers to the distance that the propeller advances in one revolution in a fixed medium. The blade angle of the propeller determines the pitch. The blade angle (β) refers to the angle between the blade element chord and the propeller's rotation plane. At the same speed (V), the linear velocity (ωr) at different radius positions of the propeller is different, resulting in different direction angles (θ) of the airflow. To enable each element to work at a favourable angle of attack (α), the blade The tip is not a fixed value. The typical blade angle distribution is shown in the figure. To meet different speeds of flight, the propeller can be designed as a variable pitch propeller. Under the control of the pitch changing mechanism, the slope of the propeller is adjusted according to the change of the flight speed of the aircraft to improve the efficiency of the propeller.

Thread pitch

The blade angle (β)

 

6. Blade thickness C

The maximum thickness C of the leaf element at any radius is called the blade's thickness at that location. To reduce the weight as much as possible while ensuring the propeller's strength, the thickness of the propeller is monotonously decreasing from the root to the tip of the propeller. The relative thickness (C/b) is generally used to express the thickness distribution of the propeller. The typical thickness distribution is shown in the figure:

Blade thickness C

 

 

the classification of propellers

 the classification of propellers

There are many ways to classify UAV propellers. According to the classification, they can be divided into propulsion propeller and rotor propeller. The propulsion propeller is the propeller that provides the thrust required for level flight, and the rotor propeller is the propeller that directly provides a lift for the aeroplane. Compared with the flat propeller, the rotor blade has a smaller pitch.
According to the static method, it can be divided into the fixed-pitch propeller and variable pitch propeller. A fixed-pitch propeller's advantage is that it does not need a pitch changing mechanism, the structure is light, and the system reliability is high. The disadvantage is that the engine can only achieve maximum efficiency at a certain rated flight speed. The biggest problem with fixed-pitch propellers is that they cannot take into account the power requirements of both climb and cruise at the same time. The advantages and disadvantages of variable pitch propellers are opposite to those of fixed pitch propellers. Currently, fixed-pitch propellers are widely used in UAVs.
According to the power mode, it can be divided into oil-driven propellers and electric propellers. Oil-driven propellers are propellers used for oil-powered engines, and electric propellers are propellers used for electric motors. The main difference between the two is the thickness of the blades. Compared with electric propellers, oil-powered propellers have thicker blades. Our attention should be paid to distinguish between oil-powered propellers and electric propellers in the process of propeller selection.

The rotation direction of the engine (motor) is clockwise and counterclockwise. To match the different rotation directions, the propeller has a distinction between forward and reverse propellers. Take the propeller's windward side as the front, the counterclockwise rotation is the forward propeller, and the clockwise rotation is the reverse propeller. In the process of propeller selection, the forward and reverse propellers should be selected according to the engine's rotation direction (motor).

Positive paddle &Reverse paddle

According to their materials, propellers can be classified into aluminium alloy propellers, wooden propellers, carbon fibre propellers, nylon propellers, etc. Among them, aluminium alloy propellers are mainly used for human-crewed aircraft and rarely used for uncrewed aerial vehicles. Wooden propellers are generally made of beech wood or laminates, which are more potent than nylon propellers and lower in price than carbon fibre propellers, but they are heavier. The carbon fibre paddle is made of composite carbon fibre, which has high strength and lightweight advantages, but the price is relatively high. Nylon propellers have weaker strength and large deformation, but the price is lower. At present, wooden propellers and carbon fibre propellers are widely used in UAVs, and nylon propellers are mainly used for light and small UAVs.

carbon fibre propellers

 

Propeller performance indicators

There are three leading performance indicators of the propeller: the pulling force, the absorbed power, and efficiency. The efficiency is determined by the pulling force and the absorbed energy. Generally, the tension coefficient and the power coefficient are used to express the tension and energy, and the calculation method is as follows:

propeller performance indicators

T is the pulling force, P is the propeller absorption power, ρ is the air density, n is the propeller speed, D is the propeller diameter, J is the propeller advance ratio, and V is the incoming flow velocity.
The typical aerodynamic performance curve of the propeller is shown in the figure:

 the typical aerodynamic performance curve of the propeller

There are three methods to obtain propeller aerodynamic performance data: theoretical calculation, experimental measurement, and CFD calculation.

1. Theoretical calculation

There are many theoretical calculation methods, such as momentum theory, leaf element theory, etc. The most accurate calculation is piece theory, which is developed based on leaf element theory. First, calculate the aerodynamic force of the leaf element at different radius positions and then integrate the blade's aerodynamic point along the radial direction. The typical calculation process is shown in the figure:

blade element loading distribution plot

 

2. Experimental measurement

Experimental measurement is a method of directly measuring the propeller's aerodynamic force in different states through a force/moment sensor in a wind tunnel. An empirical test is the most accurate method to obtain the propeller's aerodynamic performance, but the cost is relatively high.

Experimental measurement

 

3. CFD calculation

CFD calculation is also one of the most commonly used methods. The aerodynamic force of the propeller is calculated through numerical simulation. The CFD calculation method can obtain the propeller's aerodynamic performance and analyze the interference between the propeller and the airframe through the flow field information. The calculation cost is lower than the experimental measurement, but the accuracy needs to be further improved.

CFD calculation

 

The selection of the propeller

The propeller's selection needs to combine the aerodynamic performance of the aircraft, the aerodynamic performance of the propeller and the engine's load characteristics so that the propeller and the aircraft are matched, and the propeller and the engine are matched. Firstly, through geometric constraints, initially select a propeller diameter range, and then select appropriate parameters such as pitch and number of blades; secondly, evaluate the aerodynamic performance of the propeller according to the propeller parameters; finally, according to the aircraft, aerodynamic performance data, propeller aerodynamic performance data and engine load Performance data is used to evaluate whether the propeller meets the design requirements.

1. Matching propeller and aircraft

Propeller and aircraft matching means that in a given flight state, the propeller can provide the aircraft's thrust to fly. The specific process is as follows: (1) Determine the required thrust of each flight stage by the plane's aerodynamic performance. (2) Determine all candidate propellers and their aerodynamic performance, including the pull and power coefficient at different forward ratios. (3) Use the speed of varying flight states to determine the propeller's pulling force and management to varying rates at that speed. (4) Determine the feasible region of other flight states through the engine or motor's maximum speed and maximum capacity.
For vertical take-off and landing fixed-wing UAVs, four types of flight states can be considered when selecting the propeller, namely cruise, climb, maximum level flight speed and practical ceiling flight. The following formula can calculate the required thrust for the four flight states:

required thrust
W is gravity, γ is the climb angle, and K is the lift-to-drag ratio.

According to the size restrictions of the aircraft's overall layout and the specifications of the finished propeller, select the appropriate diameter range and pitch range. Then, through CFD calculation or strip theory, aerodynamic calculation program, and wind tunnel experiment, all candidate blades' pull coefficient and power coefficient at different, forward ratios are obtained. Then the efficiency is calculated by the pull coefficient and power coefficient.

the pull coefficient and power coefficient.

According to the aircraft's flight speed, combined with the propeller's pulling force coefficient and power coefficient, calculate the propeller's pulling force and required power at different rates at that speed.

the propeller's pulling force

According to the required thrust in different states, the maximum engine speed and the engine's ultimate power, the feasible region of the propeller diameter and pitch in other flight states is determined.

the feasible region of the propeller diameter

2. The propeller is matched with the engine

The matching of the propeller and the engine mainly refers to matching the aerodynamic force of the propeller and the load characteristics of the machine. The propeller's efficiency points and the engine or the motor are checked, and both are at the optimal operating point.

3. Selection target and evaluation method of propeller

The primary goal of propeller selection is that the propeller can be matched with the aircraft and engine simultaneously under the corresponding conditions of various flight performance requirements.
The propeller evaluation method is: according to the weight of each flight performance, score based on the hourly fuel consumption of the whole machine, and select the best propeller.
Different types of aircraft have other main flight conditions in the flight profile and various performance concerns. Here, the propellers are scored by selecting appropriate weight coefficients for flight performance such as cruise, maximum level flight speed, climb, practical ceiling, etc., as follows:

selecting appropriate weight coefficients
Among them, Score is the total score of all flight states, wi is the weight corresponding to each flight state, T is the pulling force, V is the speed, η is the efficiency of the power system, Pgen is the power extracted by the generator, and SFC is the fuel consumption rate.

 

propeller diameter

The selection process of the propeller is as follows:
1. Determine the diameter range of the propeller according to the overall layout design of the UAV, and initially calculate the aerodynamic performance of propellers with different diameters and pitches at different, forward ratios;
2. Determine the thrust required by the drone in different flight stages, and select propellers that can meet all flight conditions at the same time;
3. Calculate and test the performance of the propeller to be selected, score the propeller according to the evaluation criteria, and select the best propeller;
4. Pass the flight test to inspect the performance of the propeller.

 

Vertical take-off and landing fixed-wing(VTOL) UAV propeller

There are many layouts for vertical take-off and landing fixed-wing UAVs. The three most common layouts are compound wing layout, tail-mounted layout, and tilt-rotor layout. Regardless of the layout, the vertical take-off and landing fixed-wing UAV have two states: the rotor state during take-off and landing and the fixed-wing state during flight. The tail-mounted layout and the tilt-rotor layout UAVs share a set of propeller power during take-off, landing and flat flight. Simultaneously, the compound wing UAV uses two independent propeller power groups during take-off, landing and balanced flight.

vtol fixed-wing UAV propeller

In the rotor state, the downflow velocity is almost zero, and the propeller is required to generate a sizeable pulling force to balance the gravity of the UAV; in the horizontal flight state, it has a considerable incoming flow velocity, and only the propeller is required to overcome aerodynamic drag (generally no The thrust of the human-machines own gravity is about 1/10). Therefore, in the rotor state, the drone needs a small pitch propeller to generate a large static pulling force; in the horizontal flight state, the required thrust force is small, and a large pitch propeller is required to ensure that sufficient thrust is generated under the condition of the incoming flow. The propeller always works at the best efficiency point. Compared with a large pitch propeller, a small pitch propeller has greater static pulling force at the same input power, so a small pitch propeller should be used in the vertical take-off and landing phase; a large pitch propeller has greater maximum efficiency, so a large pitch propeller should be used in the level flight phase.

pitch propeller

 

The advantage of the tail-sit layout and the tilt-rotor layout is that only one set of propeller power is needed to meet both takeoff, landing and flat flight conditions. The disadvantage is that the propeller is not at the optimal operating point in both flight conditions. The overall efficiency is low. Although the need for propeller capabilities and efficiency of drones can be met by adding a variable pitch mechanism, the addition of a variable pitch mechanism to light and the small drone will increase the structural weight and seriously reduce the structure control reliability of the power system.
The compound wing layout uses two independent power systems. The disadvantage is that it will increase a certain amount of dead weight. The advantage is that the propeller can work at the best efficiency point in both takeoff, landing and flat flight conditions; besides, the two sets of power are independent Control, both in terms of structure and Control, has high reliability, and the two sets of power are redundant with each other, which can improve the overall safety of the UAV.
Judging from the current UAV's actual application situation, the composite wing layout has higher reliability and broader application than the tail seat layout and the tilt-rotor layout. Although the composite wing layout will have a certain dead weight, the practical structural design can effectively reduce the proportion of dead weight; and the aerodynamic benefits of the high-efficiency propeller can offset the impact of the structural dead weight, and the composite wing layout UAVs have more significant advantages in terms of reliability and safety.
Therefore, from aerodynamic efficiency and reliability, the UAV with compound wings has more significant advantages.