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.
By Jay Chandler
ATP/Helo. Learjet, Shorts 360, Sikorsky CH54, Boeing Vertol 234
Tiltrotor technology is one way manufacturers will meet FVL requirements head on. The Bell/Boeing V22 (L) has logged over 170,000 hrs and the Bell V280 Valor (above) promises to cover 5 times the area of conventional medevac helicopters at twice the speed and range.
Despite world economic woes and the effects of sequestration in the US, there is no shortage of new helicopters from manufacturers boasting modern technology and advances. A quick review of where we came from, where we are and where we hope to find ourselves in the rotorcraft future, may help us prepare for the next warp-drive solution in the vertical lift world—Future Vertical Lift (FVL), the DARPA VTOL X-Plane or AgustaWestland's Zero project.
The word helicopter has its roots from the Greek words helix and pteron, which translate roughly as "twisted or spiral" and "wing," respectively.
Most readers will be familiar with Leonardo da Vinci's design of 1480 which imagined a "twisted screw" machine designed for vertical flight. Since then, inventors and scientists have created progressive designs like Mikhail Lomonosov's in 1754, when he demonstrated a tandem rotor design to the Russian Academy of Science.
In 1861, Gustave d'Amecourt created a steam-powered "helicopter" which never left the ground due to the weight of the steam engine.
Slow successes began to emerge when in Nov 1907 the first untethered helicopter with counter-rotating rotors reached an astounding height of 1 ft and stayed aloft for nearly 20 sec. Development of the internal combustion engine certainly accelerated the helicopter's development by providing a viable lightweight power source.
Any accomplishment may seem insignificant individually, but each may be a stepping stone to the next breakthrough, which in the case of helicopters was Russian-born Igor Sikorsky's development of a single-rotor design—the VS300. The first real helicopter had one main rotor and a small vertically mounted rotor on the tail boom to counteract torque.
From the VS300, Sikorsky went on to design and produce the R4—the first mass produced helicopter, with more than 100 aircraft ordered. The rest, as they say, is history. Sikorsky did have to overcome several hurdles before deciding on a final design—mainly, the antitorque system for a single-main-rotor helicopter.
Newton's 3rd law and antitorque improvements
The Sikorsky-Boeing S97 proposal for the US Army's Joint Multi-Role (JMR) aircraft uses proven Sikorsky X2 technology with counter-rotating coaxial main rotors and pusher propeller to achieve a 230-KIAS cruise speed.
When designers select a single-rotor system, they have to overcome Newton's 3rd law of motion, which states, "For every action there is an equal and opposite reaction." In other words, if the rotor turns to the right, the fuselage is going to rotate to the left unless you control it—hence the antitorque system or tail rotor.
The vertically mounted tail rotor was originally a 2-bladed system operated by the pilot, by means of foot pedals, to increase and decrease horizontal thrust to turn left or right, or to compensate for torque variations due to power changes.
As time went on, additional improvements were made to the early 2-bladed tail rotor. For this discussion, if we take a well-known rotorcraft—the Bell UH1 "Huey"—the main rotor blade turns counterclockwise when viewed from the top.
The tail rotor was a 2-bladed system mounted on the left side of the tail boom to counteract torque. Through the years it was determined that, if the tail rotor were mounted on the right side, the thrust it produced would not be blocked by the vertical fin on which it was mounted.
Continued aerodynamic gains were found by having the tail rotor blade rotating in the opposite direction and traveling up into the downward rotorwash of the main rotor at a hover. This increased efficiencies by means of a higher relative wind passing over the blades.
In civilian and military applications alike, the byproduct of rotating airfoils is noise and blade slap—the unwanted enemy for people on the ground. Sometimes it's not the level of noise but the character of the sound that makes it stand out, like the characteristic "whop, whop, whop" of an approaching Huey.
Eurocopter's X3 hybrid helicopter made aviation history recently by achieving a speed of 255 kts during level flight. No antitorque system is required due to the wing mounted dual propellers.
Hence the development of noise abatement plans like the "Fly Neighborly Program" for helicopters in an attempt to pacify noise-averse residents on the ground.
Given the fact that the tail rotor rotates at a much higher speed then the main rotor, the tail rotor system creates a significant amount of noise. To address this problem some manufacturers use a 4-bladed tail rotor system and reduce the RPM needed.
Sikorsky went further—first with the CH53, then with the UH60 Black Hawk—and tilted the tail rotor by 20° so that it no longer rotates perpendicular to the ground. In this configuration, the tail rotor actually provides 2.5% (400 lb) of the overall lift produced at a hover in addition to reducing the noise signature of the antitorque system.
The Boeing AH64 Apache has a double 2-bladed tail rotor system, but instead of being perpendicular to each other they are offset 30° to further reduce noise propagation. Original designs found 45° to be the ultimate offset angle for noise reduction, but to meet US Army design criteria for length and transportability, the blades were offset only 30° to shorten the overall length of the helicopter while still reducing the noise signature of the tail rotor system.