CU Skunks at the University of Colorado Boulder has designed a Vertical Take-Off and Landing (VTOL) aircraft and aims to fabricate this aircraft with a special carbon nanofiber (CNF) reinforced composite to achieve near-zero radar cross-section.

Technology
VTOL capability combined with stealth technology gives aircraft the features of hovering and reduced radar cross-section, i.e. low observability. The transitional motors on the wing in vertical configuration accomplish the vertical take-off and landing and in horizontal configuration accomplishes the cruise phase.

Typically, 65% of stealth is achieved through the design of the aircraft and 35% by radar-absorbing material (RAM). Achieving stealth through aircraft design calls for major aerodynamic compromises in terms of stability and control. The F-117A Nighthawk, with its angular body, is a good example of this compromise. The special nano-composite developed has a high absorption rate of radar waves. CNF can be synthesized from cotton fiber and activated with a mixture of Ni+Co oxide. The cylindrical structure of CNF acts as an electrical wire and the entire sample forms an electrical network, thus absorbing the microwaves at a greater strength due to many points of contacts. Nickel has a multi-domain structure and absorbs magnetic component of microwave by displacing the domain-walls while the dielectric loss is due to lags in polarization between the core/shell interfaces with frequency variation. Pure melamine-formaldehyde (MF) composite is a poor microwave absorber due to cross-linking of melamine and formaldehyde. MF composites activated with CNFs have shown good absorption. Among variants of MF composites, absorption by MF-CNF bilayer composites was greatest with a slight trade off with thickness. MF-CNF bilayer composites are economic as they require less quantity of CNF, thus, making it most suitable for absorption in the C band. In hindsight, a relation between thickness and frequency variation has been observed.

Picture 1 below shows a sample of this CNF reinforced composite with a special architecture and layer thickness configuration capable of absorbing 99.88% of waves in the frequency range 6-8 GHz. This is a worst-case scenario when the waves are normal to the surface. When the waves hit the surface at an angle, the percent absorption will be higher. This composite can be used to coat the aircraft, rendering it practically invisible. Because the nano-composite can be used as a coating, there would be no need to design the aircraft specifically for stealth, thus limiting the compromises that need to be made on aerodynamic performance, speed, control capability, and maneuverability.

Benefits
1. No need of support equipment for launch or land
2. No need of prepared runways
3. No compromise on aerodynamic performance to achieve stealth
4. Easy to access remote areas for rescue and/or medicine delivery

Applications
1. Military - for stealth fighters, reconnaissance planes, and bombers that maintain high aerodynamic performance
2. Commercial - product deliveries to remote areas; immediate response to adversary or threat; atmospheric sampling - UAV based investigation of thunderstorms (like the ones done by RECUV at CU Boulder); can be scaled to passenger aircraft.