Chemical
Rain Repellant
Many of
the jet transport aircraft have a chemical rain repellent system that uses a
liquid chemical sprayed on the windshield to prevent the water reaching the
surface of the glass. Since it cannot wet the surface and spread out, the water
will fore beads and the wind can easily carry it away and leave the glass free
of water so the pilot's visibility will not be distorted.
The
repellent is a syrupy liquid that is carried in pressurized cans connected into
the rain repellent system. When the aircraft is flying in rain so heavy that
the windshield wipers cannot keep the windshield clear, the pilot can depress
the Rain Repellent button and a single timed application of the liquid will
then be sprayed out onto the windshield. The windshield wipers will then spread
the liquid out evenly over the wiped surface.
The liquid
should never be sprayed onto the windshield unless the rain is sufficiently
heavy, because too much repellent can smear on the windshield and be difficult
to see through. The repellent is difficult to remove if it is sprayed onto a
dry windshield.
The
operating system shown in Figure 7.4
consists of two pressurized containers of repellent and two DC solenoid valves
that, once actuated, are held open by a time-delay relay. When the Rain
Repellent push-button switch is depressed, the fluid flows for the required
period of time, which is less than a second, and then the valve closes until
the push button is again depressed. The number of times the button is depressed
is determined by the intensity of the rain.
3. Pneumatic Rain Removal Systems
High-pressure compressed air may be ducted from the engine bleed air system
into a plenum chamber and then up against the outside of the windshield in the
form of a high-velocity sheet of air. This air blast effectively prevents the
rain hitting the windshield surface and adhering to it.
