Run-in of a new aircraft begins long before its chassis first detaches itself from the runway. Modern technologies allow you to try out in the business even what does not exist in nature, with the help of a terrestrial system integrator, or, in air jargon, Iron Bird. The Iron Bird of the project Boeing 787 Dreamliner, a nest in the corner of the hangar of the Boeing research center in Seattle, is functionally analogous to the present aircraft, but outwardly it does not look like it at all.
Here’s how it happens …
Until quite recently, test stands occupied entire hangars and had conditional, but well-recognizable wings, fuselage, tail fins and chassis wells. On the powerful frames of the steel profile of technology installed hydraulic pumps, actuators and controls – ramps, flaps, elevator wheels, chassis mechanisms. All this periphery connected with the “cockpit” bundles of wires and power cables. The need to literally simulate a real airplane was due to the presence of a single hydraulic system, which actuates aerodynamic surfaces, brakes and a number of auxiliary systems. The mutual arrangement of the elements of this network could play a significant role in the machine’s performance.
Even the most advanced computer simulation systems can not replace the tests in the wind tunnel. Typically, in tunnels, “scale” models of aircraft, performed with an amazing level of detail. The model is placed in the working chamber either on a calibrated dynamometer stretch system or on a dynamometer-rack. Typically, passenger airliners are blown in low-speed tunnels, providing a flow rate of 300 km / h. But the network and more powerful subsonic, transonic (they sometimes conduct flutter tests) and even hypersonic tunnels (up to 14 Mach). In some cases, heavy freon R-134a is used instead of air in closed-type tunnels.
Boeing 787 Dreamliner partially switched from the familiar technology fly-by-wire to its new development stage – power-by-wire. The on-board computer now directs the energy through the wires to electric pumps that supply separate, disconnected hydraulic mechanisms. In some nodes of hydraulics it was possible to completely abandon: air and wheel brakes, engine starters, adjustment of the angle of installation of the stabilizer went to the electric drive.
In addition, the developers of Dreamliner abandoned the traditional system of bleeding air from the engines to maintain the preset pressure in the cabin and protect against the icing of the wing. Now the air in the interior is pumped by an electric compressor, and the wing is heated by thermocouples.
Crash tests in civil aviation are not used because of the fantastic high cost and absolute meaninglessness. The only certification crash test of a passenger liner was conducted in December 1984 at the US Air Force Edwards Air Force Base, commissioned by the Federal Civil Aviation Agency. True, the subject of certification in this case was not the plane, but the fuel in its tanks.
Radio-controlled four-engine Boeing 720 with 110 dummies onboard at a speed of 272 km / h crashed into the bottom of dried-up Rogers Lake.
The reduction was intentionally carried out with the chassis locked, yawing and heeling so that, when struck into the ground, the fuel tanks would be destroyed in a guaranteed way. Immediately after contacting the ground, the car drowned in a sea of fire, but 97% of the information from 350 airborne sensors and three TV cameras was safely removed.
Program Controlled Impact Demonstration (CID), costing the treasury of $ 10 million, showed that there is no advantage to the new, allegedly non-evaporating kerosene, is not available.
Therefore, the 75-ton Iron Poultry Project 787 was built quite differently. Suppliers brought their own stands to Seattle, and Boeing engineers only had to combine them with a common electronic nervous system – thin information tires. Engineers from the team of Lena Inderheyes were given the opportunity to perform three flight assignments simultaneously, as the new Bird was equipped with three Honeywell work processors.
Another radical difference between the new system integrator and his colleagues is his role in the process of preparing the machine for certification. In the case of the 787th model, the Ironbird “flew” ahead of the production “locomotive” for six to eight months and the assembly of the test cars began only after Inderhees managed to turn dozens of different systems into a single organism.787 Fatigue Plane Move to Test Fixture at Everett
Ground “flights”, according to Inderhees, are very similar to the real ones. At the same time, engineers are allowed to fool around for a while, for example actively working with flaps at the limiting speeds or provoking stalling in different modes. The data accumulated by the Ironbird, become the basis of “rough” flight instructions, and the team of test pilots of Boeing takes a course of training on it, before taking the helm of this car.
Flights without instructions
Aviation certification bodies do not regulate the amount of flying for the test boards of new passenger airliners, but, as a rule, the average figure fluctuates around 3000. In the case of the Boeing 787 for two incomplete years, the first nine assembled vehicles were in the air for a total of 5357 hours . And this is not counting the thousand that the regular engines Trent 1000 and GEnx-1B have worked onboard the flying laboratory Boeing 757 FTB.
Flight tests are conducted to obtain a permit for operation or to determine the physical limits of the apparatus. The second group of tests is necessary for drawing up a training program for pilots and one hundred percent certainty that the aircraft will not get out of control in any critical situations. For certification, it is enough to meet the standards for 42 speed parameters, and this is not difficult: all modern machines are designed with a huge margin of maneuverability and reliability.
In the test for the maximum bend of the wing, FAA representatives set designers off if the machine can withstand three-second overload. But companies, as a rule, force their new machines to perform this yoga exercise many times. The same Dreamliner hung on the rack for two hours, with the endings of its composite wing bulging up almost 8 m.
The most difficult flight tests in the “mandatory program” – the determination of the minimum takeoff speed during take-off and the flutter test. Flutter is the most dangerous phenomenon of destructive resonance, which caused many air crashes. The key factor in its occurrence is the insufficient rigidity of the structure. Moreover, destructive vibrations can occur not only in the wing, but also in the loosely closed doors, landing gear flaps or cargo ramp.
The test for the minimum breakaway speed is one of the most spectacular in the entire flight program, and for the pilots and the most difficult. Before its execution the crew passes additional instruction, and a safety tail support is attached to the fuselage in the area of the stern. In Airbus ee made of plastic, Boeing prefers a wooden beam. When speed dialing, the pilot gently lifts the nose of the car toward the sky by 10 ° to the touch of the support of the take-off (at that moment, a noticeable vibration spreads through the whole body) and accelerates, waiting for the car to detach. As soon as the speed is fixed, it is immediately entered into the instruction manual and “sewed” into the software.
For research purposes, Americans mercilessly crashed aircraft for 20 years. The experiments were carried out on a special test rig 73 m high at the Langley NASA Center, which was built in 1965 to complete the final phase of the Apollo lunar module landing. To simulate lunar gravity, a copy of the module was fixed on a special suspension, compensating for 5/6 of its mass, and then dropped to the ground from a height of about 40 m. After the Apollo program was rolled up, the structure was decided to be redesigned to study the strength limits of advanced aviation materials. In 1974, the stand was equipped with a pendulum discharge system for crash tests of light-engine aircraft, helicopters, testing of helicopter systems for collision protection with wires, and separable rescue modules of the bomber General Dynamics F-111. Until 2003, more than 40 civil aircrafts were destroyed in Langley, among which there were several innovative composite models, as well as 59 combat, experimental and civil “turntables”. In addition, in the framework of the CID program in 1982 three vertical vertical discharges of the fuselage section of Boeing 707 with mannequins on board from a height of 25 m were carried out three times on the stand.
Another critically important speed parameter is the stalling speed (minimum speed of steady flight). To accurately determine it, the pilot literally stops the car in the air, dropping the thrust of the engines and keeping the flaps in a cruising position. If everything is done correctly, at the time of flow failure, the crew must feel the strongest vibration throughout the body. Each new model of the aircraft passes through hundreds of such takes in different flight modes, after which a new paragraph of text appears in the operation manual, and the next red flags are registered in the control program.
Emergency brakes The
development of take-off and landing techniques in the conditions of a strong side wind manually and on the machine is carried out in the windiest places of the planet. For example, the team Dreamliner went to the wind in Icelandic Keflavik, but was able to test the car only at 50 km / h – seeds for an experienced pilot.
Detachments and landings on wet asphalt 787th tested in Seattle, where, as if on purpose, there was a heat. Therefore, in order not to get out of the tight flight schedule, the runway had to be filled with 200 tons of water from four watering vehicles.
Another very spectacular and unpredictable test in the flight certification program is the determination of the maximum braking energy, or, in translation into the automotive language, the stopping distance. The special interest of the aviation public for this test is due to the fact that Dreamliner is the only liner in the world equipped with innovative electromechanical Messier-Bugatti brakes with discs and overlays of DURACARB, a special type of carbon with high heat absorption.
The first test of the system took place in April 2010 on the 12-kilometer “space” take-off of the Edwards airbase. Mass of the machine with the help of ballast water increased to 250 tons. After the start of emergency braking, Dreamliner already ready to take off, leaving kilograms of burned rubber on the pavement, stopped 220 meters before the FAA control mark.
Special valves immediately blew excess air from the tires, and the approaching fireman left to watch the cracked carbon-heated up to 1400 ° discs, heated for 25 seconds of friction when clamped 16-kW actuators eight brake pairs 787th.
The chassis of this aircraft is also special. The 787 model “stands” on the first in the history of aviation suspension with composite power elements developed for Boeing by the engineers of the French company Messier-Dowty. The testing of this unique site was conducted by seismologists from San Diego on the world’s largest seismic bench Caltrans. The tests in California took six months and confirmed the highest strength of the composite structure. The maximum vertical load, which the chassis managed to withstand without destroying individual elements, exceeded 450 tons.
In addition, the bench simulated vertical, horizontal and lateral vibrations with simultaneous twisting. But the Messier-Dowty engineers decided that this was not enough, and they transported a set of racks to Canada, to the world’s largest test rig Goodrich Super Rig. There they were equipped with wheels with a brake system, and then drove through a series of drop tests, dropping several times the chassis with a 50-ton steel platform from a height of 27 m. To get the cherished red die FAA in the passport it was enough with interest.
Pilot-testers are privileged to sit at the helm of the newest model only when the second assembled car is rolled out of the hangars of the assembly plant. The first, as a rule, goes on passing the longest test in the entire FAA certification program – fatigue failure tests. For three years Dreamliner was tested for endurance in a giant structure, reminiscent of a bridge crane, around the clock. The machine was suspended by stretching, and the wing, tail unit and controls were clamped into a steel hydraulic vise. At this stand, the 787th, under the vigilant control of electronics, made hundreds of thousands of conditional flights, having lived for three years several ordinary lives of the average passenger airliner.
One of the most spectacular tests of the program is a test for breaking the wing and checking the fragment of the fuselage and wing for maximum bending. In the first case, the production wing assembly is installed in a steel Procrustean bed and is entangled by dozens of dynamometric stretch marks and hydraulic stops. Then pumps are turned on, and all this creepy mechanics begins to pull the final section of the wing upwards, until complete destruction. The task of the test is to determine the maximum permissible load on the wing before losing its structural integrity.
A similar test of Dreamliner was successfully completed at the end of March 2010 at the Everett Research Center. During the first test, a detachment of the composite skin from the stringers was revealed, and for more than half a year the engineers were dealing with the elimination of the problem.
In the test for the maximum bend of the wing, nothing is required to break. To conduct it around the “tailless” fragment of the liner, a brutal hydraulic rack is built that can alternately load the wing and fuselage 1.5 times stronger than it is in the most extreme air maneuvers.
All this is only a small part of the checks that fall on the share of each new model of a passenger aircraft, whether it’s Boeing, Airbus, Dry or other winged car. Therefore, safely buy an air ticket, sit back in a chair more comfortably and do not be afraid!