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LAPTOP HELPS IN AIRCRAFT TESTS

Finally there is professional flight test equipment available for testing and evaluating light aircraft. Up to now only relatively simple aids to measure flight characteristics and performance were available when it came to the testing of gliders, motorised gliders and light motor aircraft in the category up to 2 t. For decades the members of the academic flying groups, who examine flight characteristics of planes regularly during the annual summer meetings of their international association "Idaflieg", have been using the so-called "Zachern".

The Zacher protocol, which was developed by Hans Zacher a few decades ago, is a standardised program consisting of flight manoeuvres during which measurements are taking place. One uses simple but effective aids, the so-called "Zacher instruments". During these trial various planes are being flown by different pilots. The disadvantage of this traditional test method: the results often depend on subjective influences of the individual pilots, since, as experience has shown, every pilot carries out the test maneuvers slightly differently. What is more, the technically simple aids also restrict the accuracy and meaning of the established readings.

Unlike the "old" Zacher instruments the new FLITEDAC sytem, (Flight Test Data Acquisition), uses state of the art technology. Various sensor packets are connected to memory card (PCMCIA), and an elaborate electronic unit to interpret signals, (amplification and filtering). These sensors make it possible to record all flight mechanical quantities, which are necessary to describe the state of flight.

Among the sensors are an air data sensor, which measures air temperature, angle of attack and drift angle as well as the complete pressure and static pressure. In order to measure these quantities without disruption, the sensor is fitted to a carbon-fibre composite boom on the tip of the fuselage or to the wing tip of the respective test aircraft. From these measurements the altitude, vertical speed and airspeed can be calculated.

The so-called nose-boom sensor was verified inflight beforehand on Akaflieg's fs 32 "Aguila", an early glider prototype. Trial flights at maximum speed of fs 32 were carried out, (250km/h IAS, indicated air speed). One paid especially attention to swinging and flapping at high speeds. Up to date no problems of this kind have been experienced.

An inertia platform serves to measure the aircraft's acceleration and rotation speeds. Additionally the flight position is calculated in the notebook with the help of a so-called strap-down algorithm from the available acceleration data. This is similar to procedures in modern inertia navigation systems. The development of the necessary hard and software is taking up most of the time and effort during the set up of the facility. Newly developed piezo swing gyroscopes are used as rotation rate sensors. These are markedly cheaper than comparable mechanical or optical sensors. These piezo gyroscopes are also used in civil airliners.

In order to find a coherence between the pilot's steering data and the aircraft's reaction, the angles of aileron, elevator, rudder and the angle of the spoiler are measured with special protractors at the relevant flaps. The instruments with their four joint kinematics are simply stuck to fin and rudder. The necessary signal leads can be laid and secured with fabric tape along the wings and fuselage. Their negative effect on aerodynamics is negligible, since their diameter is only small, if they are fixed to the back edge of the wing and to the fuselage in the direction of the air stream.

Since the direct use of the equipment during flight with the help of the Notebook is not practical, especially with single seat aircraft, a special knee board is installed as pilot interface. Switches and control lamps are fixed to it, with which the equipment is operated and checked. During flight the construction can be calibrated and the collection of data switched on and off. Apart from this the relevant measuring protocol and the test program can be attached to the board.

The Notebook PC takes on the task of collection of data and the conversion of the physical quantities into the relevant units. Furthermore it calculates the true altitude as well as the true airspeed TAS based on the ICAO standard atmosphere.

Determining the flight attitude puts great demands on the computer's ability, because this program has to run in real time. Since 50 data are being read on each channel per second, only 20 milliseconds remain to carry out calculations in each cycle. This high data acquisition rate allows for the evaluation even of very quick movements.

The data is stored as ASCII data on the hard disc, which makes it possible to evaluate the data after the flight by using a variety of software. The dataset is compatible with most programs used for the evaluation of data.

The equipment is built so that it can be installed into any aircraft without modification. Only the support for the air data probe and the inertia platform have to be adjusted to the respective aircraft. Important prerequisites in constructing the equipment were a low power consumption and, more significantly, low mass and volume. The equipment was intended to have as little influence on flight characteristics and performance as possible in order not to falsify readings.

The most limiting factor in the program were the budget constraints. There was only a small financial support available. The construction was to be copied by other groups of the "Idaflieg". The ultimate aim was to make the procedures of professional flight-testing available to small general aviation aircraft. This explains why only relatively cheap sensors and components could be used. All the hardware and software were built by the team, and the Notebook and measuring card were donated by industry.

The FLITEDAC equipment was used for the first time during fs 33's test flights. It is the new prototype of the Academic Flying Group Stuttgart, which only took its maiden flight in May last year. Gliders put most demands on compactness, weight and electricity consumption. The equipment could prove its good performance in the fs 33. With its help the necessary manoeuvres could be documented in detail. Far more data could be collected per flight than it has ever been possible with the old Zacher method.

A lot less time is used now for flight proving. The documented data could easily be passed as documentation to the German aviation authorities. Not only the quality of flight characteristics could be shown, but prove could be given with the various diagrams and data graphs. Most of these test flights took place during the Idaflieg summer meeting in 1998.

The FLITEDAC equipment will not substitute the classic Zachern. Zachern will still be used in order to train pilots for the test flights. It is still a fast way to get a feel for the flying characteristics of an aircraft. Since this system is so easy to use, it can be applied universally. However, the time had come to create a state of the art tool for more precise tests.

The area in which the FLITEDAC can be used reaches beyond flight testing. Further studies can be carried out in order to determine mechanical properties during flight. The data may then be used for exact simulations. It might also be conceivable to systematically determine the reaction of different pilots in special flight situations. This will help in understanding mistakes in contol techniques. The equipment would then be a tool for training and would also help to establish flight characteristics for smaller aircraft.

From page 82 of FLUG REVUE 5/99