the air flow during step d pivoting 106 each shutter pivots towards its closed position this movement is driven by the return means gravity can allow the shutters to come down they are then essentially mounted to rotate freely as a complement or as an alternative permanent magnets and or springs can equip the shutters so as to lower the flaps towards the closed position because of the persisting air flow the closing movement of the shutters is limited the movement is constrained and so the shutters remain balanced at a distance from their closed position for as long as the air flow is greater than a threshold s during step d pivoting the shutter remains partially open for at least 1 second for example for at least 5 seconds for example at least 20 seconds in various instances for at least 1 minute at step e shutter s in the closed position 108 the air flow has become lower than or equal to the threshold s from then on the mechanical action of the return means is sufficiently high to bring each shutter to the closed position the flow pushed out by the engine is no longer sufficient to move the.