American B-1 Long Range Military Aircraft Bomber close-up view. Image Credits: Fotoluminate LLC/shutterstock.com
Found at the foremost point of an aircraft, a nose cone must be aerodynamic in order to reduce drag on a plane. On most commercial and military aircraft, the nose cone also houses radar and other instruments that might be used to detect meteorological phenomena, track enemy aircraft or transmit communication signals.
Because they protect sensitive instruments while allowing electronic signals to pass through, nose cones – also known as radomes – must be made from specific materials. These materials often include fiberglass, quartz, honeycomb and foam cores; as well as various chemical resins.
Types of Nose Cones
Aircraft radomes are built in two different styles: sandwich construction and dielectric space frame.
Further divided into A-Sandwich and C-Sandwich, Sandwich radomes tend to offer better performance over narrow frequency bands, making them the preferred option for military and scientific applications. A-Sandwich radomes are comprised of low-dielectric foam or a honeycomb core between two slender laminates.
A C-sandwich radome is made up of three skin layers and two foam layers. The solidity of each foam layer can be tuned for ideal RF performance, allowing for many probable constructions that offer high-quality RF performance and mechanical strength. C-sandwich also provides better performance than A-sandwich radomes. However, the additional complexity boosts material and labor expenses.
Dielectric space frame radomes are supported by a skeleton of inward-turning flanges. Various characteristics of DSF radomes make them ideal for radar systems that operate at frequencies of less than 1 GHz.
Finishing Paint Used to Protect Against the Elements
Aircraft radomes are coated with specially-formulated paint designed to safeguard the structure against harsh conditions or damaging events such as temperature extremes, high-speed impacts with rain, snow and abrasive particles, sunlight and high voltage charges of static electricity. If the coating is compromised, these damaging forces will assault the radome and radar system within.
Polyester, polyurethane, alkyd-enamel and acrylic epoxy finishes are all used to coat radomes. These coatings are formulated to include graphite or carbon particles to prevent static charges from building up. Because these coatings can also disrupt a radar signal, they should be minimally applied to the radome surface.
Also, some anti-static coatings have a greater conductivity than others. Radome manufacturer Saint-Gobain suggests applying a top coat 4 to 6 millimeters thick over a 2-millimeter base of primer.
An airplane cockpit cut away from the fuselage. Image Credits: Ivan Cholakov/hutterstock.com
Combating Static Electricity
When a plane is flying through low-humidity air, the static electricity on a radome exterior can develop significantly. A major spark discharging due to this condition will generate radio interference over a wide band of frequencies. These electrical charge releases can also chip the protective paint and burn small pits or pinholes in the radome exterior.
These small exterior punctures produced by the charges can rapidly grow in size as carbon deposits resulting from the charring promote further strikes. If this condition is left uncontrolled, damage to the radome can come from water breaking through the exterior.
As noted before, an anti-static/anti-corrosion paint can mitigate this problem. Also, lightning diverter strips extending from the front of the radome are designed to continuously gather static build-up from the exterior of the radome and pass it harmlessly to the airframe.
Lightning diverter strips are also, as the name indicates, designed to protect a radome from lightning strikes.
These strips are comprised of reliable metal bars or several closely spaced buttons of conductive material attached to a plastic strip that is glued to the radome. In several ways, diverter strips act similar to a lightning rod on a skyscraper.
In addition to diverter strips, the fuselage of most commercial aircraft are built to withstand a lightning strike. Incidentally, pilots are instructed to avoid electrical storms wherever possible.
Routine Maintenance, Inspection and Care
Radome care is extremely important, even for structures that aren’t exhibiting any apparent damage on their exterior. Inadequate maintenance could cause the electromagnetic integrity of a radome to degrade.
Routine maintenance of a radome should start by inspecting the structure’s surface for pits, cracks, chipped paint or other damage. Also, most protective paints will slowly change from black to brown as they degrade. Lightning diverter strips should also be inspected for solid electrical contact and any damage.
When maintaining the exterior of a radome, avoid using any chemical strippers to remove paint coatings. This is because some chemical strippers can break down resins used in the construction of the radome itself.
Instead of using chemical strippers, maintenance should be performed with fine-grit sandpaper made for removing polyester or catalyzed paints. When using sandpaper, avoid sanding all the way through to the radome’s fiberglass exterior.
Indentations or pin holes in a radome’s surface should be filled with high-quality radome paste or putty applied with a spatula or putty knife. Filler should be allowed to dry and then sanded down until flush with the rest of the surface.
An aircraft’s radome should be regularly inspected to make sure that any external damage hasn’t caused something bad to happen to the radome’s interior. Radomes with neoprene caps are particularly known for being damaged on the interior while seeming fine on the outside.
Maintenance workers should also check for signs of water infiltrating a radome. Water has a high dielectric constant, meaning it can greatly disrupt radio transmissions. Properly built and coated radomes have a hydrophobic surface that prevents this from happening. If water has penetrated into the honeycomb material – it could compromise the entire radome and have to be replaced.
The Compromising Effect of Radome Repairs
Whether it’s due to bird strikes, lightning strikes or corrosion, radome repairs should be dome immediately to keep a small problem from becoming bigger.
Repairs to radomes are similar to those performed on other honeycomb structures; however, those performing the repairs need to be aware of the potential impact their work may having on a plane’s radar systems. Technicians should conduct transmissivity testing to make sure that a repaired radome is allowing its radar system to function properly.
Sources and Further Reading