Next generation of V/STOL machines builds on proven tech innovations
Tiltrotor, coaxial, hybrid aircraft, advanced rotor blade configurations and antitorque devices have been adopted and accepted into new designs.
Fenestrons and NOTAR
The fenestron, seen here on the Eurocopter EC120B, has small asymmetrically spaced blades and rotates faster than conventional tail rotor blades. Basically a ducted fan, it provides a safe antitorque system especially for passenger safety.
While some manufacturers stayed with tail rotor blades, others went outside the box and designed new antitorque systems. Eurocopter developed the fenestron which increases the number of blades in the system and allows the blades to be much smaller.
Fenestrons are certainly a safer design because the system is self-contained in a housing and reduces the chance of a tail rotor strike in the trees.
Additional benefits include enhanced safety, separating passengers from the dangers of rotating tail rotor blades, eg, by inadvertently walking into them, since blades rotate so fast they are more or less invisible.
The fenestron system is more streamlined and produces less drag, but creates a signature whine as air passes through the circular opening much like a whistle.
Going in a totally new direction, MD Helicopters eliminated the tail rotor altogether and created a no tail rotor (NOTAR) system in which air from the engine is circulated inside the tail boom, which is hollow, and ejected through 2 airfoil slots.
This accelerates the airflow around the tail boom, changing the release point and crea ting a lateral lift component that is used to counteract the torque of the main rotor. At the end of the tail boom, there is also a rotatable direct jet thruster for hovering flight directional control.
Increased safety is a signature advantage of the NOTAR system where passengers are concerned, but it also provides an increased margin of safety during low-level and nap-of-the-Earth flight. Early advertisements for the MD NOTAR line showed the helicopter with its tail boom stuffed into trees while hovering—a definite advantage over vulnerable tail rotor systems.
The main thing
McDonnell Douglas (later MD Helicopter) developed the NOTAR system using an enclosed articulated fan. Airflow is exhausted through 2 slots to create
lateral lift to counteract torque.
While improvements to the helicopter's antitorque systems were ongoing, changes were made to the main rotors themselves in order to increase lift and reduce noise.
For a full discussion of various rotor systems, see "Advanced rotor designs break conventional helicopter speed restrictions" (Pro Pilot, Sep 2012, pp 80–84).
Main rotor blades in the early 1970s had squared ends with symmetrically shaped blades. These were efficient for slower helicopters but, as most pilots know, the blades produced a loud "whopping" noise which increased proportionately with aircraft speed.
This whopping was often a welcome sound to US troops in the jungles of Vietnam, for they knew their ride was approaching, but by broadcasting the arrival of military forces it also provided significant time for the enemy to prepare for their appearance.
As with the tail rotor theory, increasing the number of blades on the main rotor and reducing RPM decreases the noise level. The Hughes OH6 in Vietnam had a 4-bladed main rotor system with much smaller blades. Given that it was a smaller helicopter, the OH6 created significantly less noise than 2-bladed helicopters.
Today, the 5-bladed MH6 used by the US Army's Task Force 160th SOAR is based on a highly modified MD530F and is a highly maneuverable, but also much quieter, helicopter of comparable size. In the black ops business, stealth is a must—broadcasting your arrival with blade acoustics does not achieve the element of surprise.
Rotor blades themselves have changed tremendously in design too. Early metal main rotor blades were symmetrical airfoils with the chordline dissecting the top and bottom of the blade evenly.
The span of the blade was equal in size at the root of the blade (where it attaches to the rotorhead) and at its tip. This created great variations in lift distribution along the blade as the blade tip (obviously) travels faster than the root. Higher speeds create greater lift differences along the blade and can eventually cause it to fail.
Main rotor blades today are thinner, twisted along the length and often tapered at the tip. The blade tip itself may be swept back and down to reduce drag, blade tip speed and noise. The rear sweep aerodynamics work in a similar way to how swept-wing jets fly faster than straight-wing jets for the same power. Drag is reduced on the swept-tip blade by delaying the onset of the critical Mach number, which would increase drag at the tip if it were not swept back.
These high-tech designs are only possible now due to improvements in composites and in manufacturing techniques over the past 10 years. All newly designed blades prevent the blade tip from entering compressibility at high blade speeds and increase the flight envelope of new-generation helicopters like the AgustaWestland AW101, Eurocopter EC225 and Sikorsky S92.
A jump to warp drive
To make the technological advances required by the US military's FVL criteria, tweaks and block upgrades aren't going to cut it for companies hoping to produce the next generation of vertical lift machines.
The same aerodynamic hurdles must be overcome and maybe a few new ones yet to be discovered will challenge industry giants and newcomers alike. Counter-rotating main rotor systems answer many of the high-speed aerodynamic limitations of conventional rotorcraft—they prevent retreating blade stall, compressibility while increasing fuel range and payload.
For coaxial rotorcraft, the absence of an antitorque system means less aircraft weight and more power available for propulsion.