The above are the jokes we often played when we were undergraduates, especially when the students who studied airplanes and engines were anti-gangsters. In fact, this is really the truth. A plane with dozens or even hundreds of tons was hit by a small sparrow weighing one kilogram, which may be mutually assured destruction. In the final analysis, it is because of relative speed.
The concept of relative speed is easy to understand. For example, a car touches a stationary high-speed guardrail at a speed of 80 km/h, and the speed of the guardrail relative to the car and the speed of the car relative to the guardrail are both 80km/h ... In other words, birds fly very slowly, maybe only 30 to 50 kilometers per hour, but they can't catch the plane flying fast! According to the above statement, a plane hitting a bird can be understood as a stationary plane being hit by a bird (shaped charge missile) with a speed of 8900 kilometers per hour. Even flesh and blood have a great impulse at the moment of impact. According to the impulse theorem,
f = mδv/δt
Where f is the impact force of the bird on the aircraft, m is the mass of the bird, and δv is the speed change of the bird, that is, the speed change from the initial high-speed collision with the aircraft to the zero relative speed after the collision, which is about 800 km/h; Δ t is the duration of the impact (the bird changes from three-dimensional to two-dimensional), which can also be understood as the time required for the plane to fly over the bird's length, which is about milliseconds. It is roughly estimated that if a bird of 0.45 kg collides with an airplane at a speed of 800 km/h, it can cause an impact force of 153 kg on the airplane. A 7 kg bird collided with a plane with a speed of 960 km/h, and the impact force was as high as 144 ton. Such amazing data shows that bird strikes are so destructive to high-speed aircraft that a small house sparrow can crash the engine of the aircraft.
As for why the plane uses such a thin shell, to put it bluntly, it is to save money. For every gram of airplane, the fuel consumption of long-term flight will increase proportionally, so we should reduce the weight of the airplane as much as possible under the condition of meeting all the constraints of strength and stiffness. The main function of the aircraft shell is to maintain the aerodynamic shape of the aircraft, and at the same time, it plays the role of increasing lift and reducing drag, that is, increasing the lift of the aircraft and reducing the drag of the aircraft. So when we design the shell, we only need to ensure that it meets the requirements under the action of pneumatic load. As mentioned above, if the shell thickness of each part is designed according to the maximum impact force of bird impact, and this impact force is hundreds of times of local aerodynamic load, then the design difficulty is no less than letting a tank fly.
In addition to the aerodynamic changes caused by impact (due to the appearance of pits, it is no longer a smooth streamline, local aerodynamic interference and separation will increase, resistance will be greatly improved, and lift will be greatly reduced), and some key parts regard bird impact as natural enemies:
For example, the windshield of the cab, if damaged by birds, may endanger the driver's operation.
For example, in order to maximize aerodynamic efficiency, the blades of the engine are exquisitely designed, and no matter how high the mechanical strength of each blade is, it can't stop the high-speed rotation speed of 500 revolutions per second. A bird, a coin, and a stone thrown in can directly lead to blade fracture, explosion, and fire.
For example, the fuel pipeline, hydraulic oil pipeline and fly-by-wire control system in the wing, the aircraft is an extremely sophisticated integrated system, and the failure of each part may lead to the destruction of the aircraft.
Of course, you don't have to worry too much. Every plane has fully considered the most serious (unlucky) bird impact when designing, and the key parts have been tested by bird impact to ensure the personal safety of every passenger to the greatest extent.
In the impact dynamics laboratory of Xigong University, specimens of different sizes of wings after bird impact are displayed. The largest high-speed air gun in the laboratory has a tube length of 10 m and an inner diameter of 150 mm, as well as test-bed and target frame, data collector, ultra-dynamic strain amplifier, high-speed camera and other equipment. When doing the bird impact test, use an air gun to fire gelatin bullets (simulated birds) to impact the specimen. The specimen is installed on the target frame, and the speed is controlled by air pressure to simulate the situation of being hit by a bird in flight. In the test process, it is necessary to test the speed of the projectile, record the strain and displacement data during the impact process, and take high-speed camera during the test process. "The accuracy requirement of the projectile velocity during the test is plus or minus 3%, but we actually control it at around 65438 0%." Dr Don Zhong Bin of the laboratory said. There is only one such laboratory in colleges and universities across the country.
In the bird impact test of C9 19 parts, live chickens were killed instead of real birds. Considering the large biological pollution, it is not suitable for testing on campus, and some tests will be carried out in the iron anchor laboratory of Xigong University. The experiment was not smooth sailing. In July, 20 14, when doing experiments in the iron anchor laboratory of Xigong University, all the bird bombs were loaded. The air cannon was struck by lightning when it was inflated, which caused the data acquisition board to burn down and forced the experiment to be postponed for one month.
Although it is cruel to experiment with live chickens, in order to ensure the safety of passengers ... let's observe a minute's silence for them before taking off. ...
Even if the engine is hit by a bird and caught fire, the pilot can completely cut off the fuel supply of this engine and land smoothly with one engine. The design of the two engines itself has redundant backups. Of course, if both engines are hit ... the probability is lower than that of aliens invading the earth or monkeys knocking out a complete Shakespeare.
We can't control the birds that come to meet our plane, but we can still do one thing to reduce such accidents: don't use small planes such as fixed-wing and quadrotor near the airport. Once these little things collide with the plane preparing to take off and land, the consequences are unimaginable, and the operator may also bear dozens of lives.