- What tools and techniques are used to get real data test to confirm the values that are obtained by computational fluid dynamics (CFD) calculations and wind tunnel testing?
- Is this standard procedure for all aircraft before going into production?
What tools and techniques are used to get real data test to confirm the values that are obtained by computational fluid dynamics (CFD) calculations and wind tunnel testing?
Usually one prototype is used for stress testing with special rigs which simulate flight loads. This testing will cover the full life of the aircraft, measured in flight hours, and will accelerate ageing by using higher than usual loads.
Once you have a flying article, test flying is used for validating the design. A new design is first flown by a certified test pilot and normally instrumented with telemetry so engineers on the ground can follow what happens during the test flight. While the first flight is done at low speed, successive flights will expand the flight parameters (speed, load factors, rotation rates) until the whole operational envelope has been covered. This is called envelope expansion and will eventually cover stalls, spins, overspeed and flutter conditions. Military aircraft will also test all planned external loads and their separation in flight.
Is this standard procedure for all aircraft before going into production?
Yes, this is the certification process. Unless a design is certified, it may only be used in a limited way. Commercial use of an aircraft requires a certification.
Pretty much any transport category airplane prototype will be fitted with a complex suit of test equipment as an essential part of the development and certification process. All the test equipment will be painted orange to differentiate it from normal on-board systems. The bigger the airplane, the fancier the test suite.
The main component for measuring aerodynamic data is the Test Boom, or Air Data Boom, installed on the nose or wing, which puts aerodynamic sensors out ahead of the body in relatively undisturbed air (forward of the bow wave, so to speak). The boom will be about 10-15 feet long, with both static and pitot pressure sensors, and vanes to measure airflow direction. The newest booms are called "Smart Booms" and do away with the physical AOA vanes and calculate airflow direction from the pressure distribution about the tip of the boom from a given airflow direction.
The boom supplies data of undisturbed airflow which can be compared with the data coming from the airplane's normal pitot/static/AOA sensors. Among other things, this data is used to generate correction, or calibration, tables that the Air Data Computers and Stall Protection System computers will use to correct for errors induced by changes in flow close to the body where the normal sensor are. The Calibrated Airspeed presented on a primary flight display for example, is derived from calibration tables created using the test aircraft based on test boom vs pitot/static system data.
Tufting is commonly used for specific areas requiring observation of airflow behaviour in real time, with cameras installed to record the results. This is common when actual behaviour during testing differs from predicted behaviour from analysis or the wind tunnel.
The test suite will include lots of other equipment, pretty much whatever the various systems engineering groups need to measure to validate their designs. Load cells or strain gauges to measure structural effects, flow sensors to measure hydraulic or pneumatic system performance, temperature sensors, accelerometers to measure vibrations, etc. The test a/c will be a maze of special wiring runs, in special orange painted wiring trays, going all over the place.
The test a/c will also have things like a variable weight distribution system (a row of tanks with a glycol mixture that can be pumped for and aft to vary the C of G) to allow the center of gravity to be moved around as required.
All of this is managed and recorded by a number of computer consoles on board, manned by Flight Test Engineers, as well as being sent to the ground by telemetry equipment.
There may be more than one test aircraft for the certification process, and second and even third test units may be used, but they usually don't have as extensive a test suite and will be closer to the production configuration as these airplanes will be used for the final certification test flights. First prototype test aircraft are not normally reconfigured to the production configuration and sold off (they're normally kept on for post cert subsequent development work), but second and third test a/c sometimes are, because they are already close to the production configuration, needing little modification.