Parabolic flights: fluctuations of g level and mechanical vibrations
Among the advantages of parabolic flights over other platforms are:
- Low cost of design and production of an experiment.
- Possibility to carry out several series of experiments during three flights on consecutive days, each flight yielding about 30 parabolas, for a total of about 10 minutes of microgravity per flight.
- Possibility of interaction of the experimenters with their experiment.
- Possibility to conduct biomedical research on human subjects in microgravity (under certain conditions).
During a parabolic flight not only microgravity and normal gravity conditions are experienced: each parabola consists of three phases, the microgravity phase being in the middle. During the two other phases (i.e., the pull-up and the pull-out) the occupants are subjected to around 2 times the force of gravity. Experiments can therefore be designed to take advantage of these 2g phases, or at least they should not be negatively influenced by them.
It is important to note that the transition between the 2g and the μg phases is not instantaneous. At the end of the pull-up phase, when the plane is flying at a pitch-angle of around 47º, the pilots manoeuvre to cancel the lift generated by the wing (angle of attack of the wing equals zero) while the flight engineer reduces the thrust from the engines to compensate the small drag generated in this configuration. This procedure is carried out as quickly as possible and, as a result, short phases of negative g (gravity pulling upwards) may be experienced.
In addition, as explained in the previous section, weightlessness cannot be perfectly attained, because of the presence of other forces besides gravity. The plane is still surrounded by air and thus aero-dynamic lift, drag, and thrust will be present to some extent. The pilots are then always trying to compensate these disturbing forces and as a result, the microgravity level attained in a parabolic flight has some variations on the order of +/-0,05g at a frequency of around 1 Hz.
As in any type of flight, mechanical vibrations caused by engines and shocks in the plane are also present, but in this case they are noticeably higher due to the performance of the plane and the nature of the flight. Consequently, experimenters must pay special attention to the design of the experimental set-up in order to prevent vibrations from, for example, distorting the results or spoiling the samples (e.g. sensitive optical devices).
As a final remark, experiments involving free-floating objects do not enjoy the whole 20s period of microgravity. There are some safety issues associated with this type of experiment and enough time must be left to release the sample and to secure it before the microgravity phase ends. As a result of this, the actual test time frame available for this kind of experiment should be considered to be around 5 seconds.