To understand the flight principle of aircraft, we must first know the composition and function of aircraft and how the lift of aircraft is generated. These problems will be briefly explained in several parts.
I. Main components and functions of flight
Up to now, except for a few special types of aircraft, most aircraft consist of five main parts: wing, fuselage, tail, landing gear and power plant:
1. Wing-The main function of the wing is to generate lift to support the flight of the aircraft in the air, and at the same time, it also plays a certain role in stability and operation. Generally, ailerons and flaps are installed on the wing. The aileron can make the plane roll, and lowering the flap can increase the lift. The engine, landing gear and fuel tank can also be installed on the wing. The shape and size of airplane wings are different for different purposes.
2. fuselage-the main function of fuselage is to load passengers, crew, weapons, goods and various equipment, and connect other parts of the aircraft, such as wings, tails and engines into a whole.
3. Tail wing-Tail wing includes horizontal tail wing and vertical tail wing. The horizontal tail consists of a fixed horizontal stabilizer and a movable elevator. Some high-speed aircraft combine horizontal stabilizer and elevator with full-motion horizontal tail. The vertical tail consists of a fixed vertical stabilizer and a movable rudder. The function of the tail wing is to control the pitch and deflection of the aircraft and ensure that the aircraft can fly smoothly.
4. Landing gear-The landing gear of an aircraft is mostly composed of shock-absorbing struts and wheels, which are used to support the aircraft during takeoff and landing, ground taxiing and parking.
5. Power plant-The power plant is mainly used to generate tension and thrust to make the plane move forward. Secondly, it can also provide power for other electrical equipment on the plane. At present, the widely used aircraft power devices are: aviation piston engine plus propeller, turbojet engine, turboprop engine and turbofan engine. In addition to the engine itself, the power plant also includes a series of systems to ensure the normal operation of the engine.
In addition to these five main parts, the aircraft is also equipped with various instruments, communication equipment, pilot equipment, safety equipment and other equipment according to the needs of aircraft operation and mission execution.
Second, the lift and drag of the aircraft.
An airplane is a kind of airplane heavier than air. When an airplane flies in the air, it will generate aerodynamic forces acting on the airplane, and the airplane flies by aerodynamic forces. Before understanding the generation of aircraft lift and drag, we should also understand the characteristics of airflow, that is, the basic laws of airflow. The flowing air is the airflow, which is a fluid. Here we will quote two fluid theorems: continuity theorem and Bernoulli theorem:
Theorem of fluid continuity: When the fluid continuously and stably flows through pipes with different thicknesses, the mass of the fluid flowing into any tangent plane is equal to the mass of the fluid flowing out from another tangent plane at the same time because no part of the fluid in the pipe can be interrupted or squeezed.
The continuity theorem expounds the relationship between the velocity of fluid in flow and the cross section of pipeline. In the flow of fluid, not only the velocity and pipe section are interrelated, but also the velocity and pressure are interrelated. Bernoulli theorem is to explain the relationship between velocity and pressure of fluid in flow.
The basic content of Bernoulli's theorem: when the fluid flows in the pipeline, the pressure at the high speed is low, and the pressure at the low speed is high.
Most of the lift of an aircraft is generated by the wings, and the tail usually generates negative lift, while the lift generated by other parts of the aircraft is very small, so it is generally not considered. From the above picture, we can see that the air flows to the leading edge of the wing and is divided into two streams, which flow along the upper and lower surfaces of the wing respectively and rejoin at the trailing edge of the wing to flow backward. The upper surface of the wing is relatively convex and the flow tube is relatively thin, indicating that the flow velocity is accelerated and the pressure is reduced. On the lower surface of the wing, the airflow is blocked, the flow tube becomes thicker, the flow velocity slows down and the pressure increases. Here we refer to the above two theorems. Therefore, there is a pressure difference between the upper and lower surfaces of the wing, and the sum of the pressure differences perpendicular to the relative airflow direction is the lift of the wing. In this way, the aircraft heavier than air overcomes its own gravity caused by the gravity of the earth by the lift force obtained from the wings, thus soaring in the blue sky.
The lift of the wing mainly depends on the suction of the upper surface, not the positive pressure of the lower surface. Generally, the suction formed on the upper surface of the wing accounts for about 60-80% of the total lift, and the lift formed by the positive pressure on the lower surface only accounts for about 20-40% of the total lift.
There will be all kinds of resistance when the plane flies in the air. Resistance is the aerodynamic force opposite to the direction of aircraft movement, which hinders the aircraft from moving forward. We also need to know about it here. According to the causes of resistance, it can be divided into friction resistance, pressure resistance, induced resistance and interference resistance.
1. Friction resistance-One of the physical characteristics of air is viscosity. When air flows over the plane surface, due to its viscosity, the air will rub against the plane surface, thus generating a force to stop the plane from moving forward. This force is frictional resistance. The magnitude of friction resistance depends on the viscosity of air, the surface condition of aircraft and the surface area of aircraft in contact with air. The greater the air viscosity, the rougher the aircraft surface, the larger the aircraft surface area and the greater the friction resistance.
2. Piezoresistance-When people walk against the wind, they will feel the effect of resistance, which is a kind of piezoresistance. The resistance formed by the pressure difference before and after is called piezoresistance. The fuselage, tail wing and other parts of the aircraft will produce pressure resistance.
3. Induced drag-When the lift force is generated, the aircraft is subjected to a drag force. This kind of resistance caused by lift is called induced resistance, which is a kind of "price" paid by aircraft to produce lift. Its production process is complicated, so I won't go into details here.
4. Interference resistance-it is the additional resistance caused by airflow interference between different parts of the aircraft. This kind of resistance is easily generated between fuselage and wing, fuselage and tail, wing and engine nacelle, wing and auxiliary fuel tank.
The above four kinds of resistance are aimed at low-speed aircraft. As for high-speed aircraft, in addition to these resistances, there will be other resistances such as wave resistance.
Three, the factors affecting the lift and drag
Lift and drag are produced in the relative motion between air (relative airflow). The basic factors that affect the lift and drag are: the relative position of the wing in the airflow (angle of attack), the speed and air density of the airflow, and the characteristics of the aircraft itself (surface quality of the aircraft, wing shape, wing area, whether the flap is used, whether the leading edge seam is opened, etc.). ).
1. The influence of angle of attack on lift and drag-the angle between the relative airflow direction and chord length is called angle of attack. The angle of attack at which the maximum lift is obtained at the same flight speed and other conditions is called critical angle of attack. When the angle of attack is less than the critical angle of attack, the lift increases; When the angle of attack is greater than the critical angle of attack, the lift decreases. When the angle of attack increases, the resistance increases, the angle of attack increases and the resistance increases. When the critical angle of attack is exceeded, the resistance increases sharply.
2. The influence of flight speed and air density on lift resistance-the higher the flight speed, the greater the lift and resistance. Lift and drag are proportional to the square of flight speed, that is, the speed is doubled and the lift and drag are quadrupled; When the speed is tripled, the victory and resistance will increase ninefold. When the air density is high and the aerodynamic force is high, the lift and resistance are naturally high. When the air density is doubled, the lift and drag are also doubled, that is, the lift and drag are directly proportional to the air density.
3. The influence of wing area, shape and surface quality on lift and drag-large wing area, large lift and large drag. Lift and drag are proportional to the wing area. The wing shape has a great influence on the lift and drag, from the relative thickness of the wing section shape, the position of the maximum thickness, the plane shape of the wing, the position of the flap and the leading edge seam to the icing of the wing. Also, whether the plane surface is smooth or not will also affect the friction resistance. If the plane surface is smooth, the drag will be small, and vice versa.
(2) The principle of helicopter flight
Helicopter is mainly composed of airframe, lift (including rotor and tail rotor), power and transmission system and airborne flight equipment. The rotor is generally driven by a turboshaft engine or a piston engine through a mechanical transmission system consisting of a transmission shaft and a reducer, and can also be driven by the reaction force generated by the jet from the tip of the blade. At present, there are mechanically driven single-rotor helicopters and double-rotor helicopters in actual use, of which single-rotor helicopters have the largest number.
The maximum speed of a helicopter can reach more than 300km/h, the dive limit speed is nearly 400km/h, the use ceiling can reach 6000m (the world record is 12450m), and the general range can reach about 600 ~ 800 km. The range of internal and external auxiliary fuel tanks can reach more than 2000 kilometers. Helicopters have different takeoff weights according to different needs. At present, the largest heavy helicopter put into use in the world is the Russian Mi -26 (with a maximum takeoff weight of 56t and a payload of 20t).
The outstanding feature of helicopter is that it can do low altitude (a few meters above the ground), low speed (starting from hovering) and the nose direction remains unchanged, especially suitable for small-area vertical take-off and landing. Because of these characteristics, it has broad uses and development prospects. In the military field, it has been widely used in ground attack, aircraft take-off and landing, weapons delivery, logistics support, battlefield rescue, reconnaissance patrol, command and control, communication, anti-submarine mine clearance, electronic countermeasures and so on. Used for short-distance transportation, medical rescue, disaster relief and lifesaving, emergency rescue, lifting equipment, geological exploration, forest protection and fire fighting, aerial photography, etc. The transportation of people and materials between offshore oil wells and bases is an important aspect of civil use.
At present, compared with airplanes, helicopters have higher vibration and noise levels, greater maintenance workload, higher use cost, lower speed and shorter flight range. The future development direction of helicopters is to improve these aspects.