Rain Control

Almost all of the small general aviation aircraft use transparent acrylic plastic windshields, and this soft material is so easy to scratch that windshield wipers are seldom installed. One way to minimize the effect rain has on visibility in flight is to keep the windshield waxed so water will not be able to spread out over the surface, but will bead up so the propeller slip stream or wind can blow it away.

Larger and faster aircraft that routinely operate in rain have rather elaborate rain control systems. There are three methods used to con­trol the effects of rain and they may be used together. These three methods will be discussed separately. They are mechanical windshield wipers, chemical rain repellant, and a pneumatic rain removal systems.

Measures: There are different methods that may be found on aircraft as a measure of  protection against the problem due to rain on windshield:

Pneumatic Rain Removal: High-pressure bleed air from the engine is blown over the face of the windscreen from ducts mounted at the base of the screen. The air blast forms a barrier that prevents the rain spots from striking the screen.

Wind Screen Washing System: A windscreen washing system allows a spray of fluid (usually de-icing fluid e.g. kill frost), to be directed on to be directed on to the windscreens to enable the windscreen wiper to clear dust and dirt from dry windscreens in flight.

The fluid is contained in a reservoir and sprayed on to the screen through nozzles. The fluid may be directed to the nozzles by an electrically driven pump or by pressurizing the top of the reservoir with compressor bleed air via a pressure-reducing valve.

Rain repellant system: When water is poured over clear glass it spreads evenly to form a thin film. Even when the glass is tilted at an angle and subjected to an air-stream, the glass will remain wetted and reduce vision. However, when the glass is treated with certain chemicals (typically silicone based), the water film will break up and form beads of water, leaving the glass dry between the beads. The water can now be readily removed. The principle is used on some aircraft for removing rain from windscreens. The chemical is stored in pressurized, disposable cans and is discharged on to the windscreen through propelling nozzles.

Some of these measures are described in subsequent sections.