New Age Spinners
Written by PATRICK CHISHOLM
ROTORCRAFT INNOVATIONS FOR SPEED AND AGILITY.
The speed and range of conventional helicopters has not changed substantially in decades. They only can fly at a maximum speed of about 130 to 150 knots (about 150 to 170 mph) depending on the altitude. High altitude is another challenge. At very high altitudes—such as in Afghanistan where mountains are often thousands of feet above sea level, top speed may be as low as 90 knots.
The thinner air at the higher elevations also may impinge on the craft’s ability to maintain stability while hovering, especially amid wind gusts.
“I flew helicopters in Vietnam at 90 knots. I flew helicopters in Desert Storm at 120 knots. We made 30 knots in 20 years,” said General Doug Brown, USSOCOM commander, speaking at the March 1, 2006, delivery ceremony of the CV-22 Osprey tilt rotor developed by Bell and Boeing. “What is needed is a vehicle with the speed and range to conduct missions in a single period of darkness.” Brown was referring to the new capabilities opened up thanks to the CV-22.
While plenty has been written on the CV- 22, there are other, less well-known vertical take off and landing (VTOL) aircraft currently under development that will open up new possibilities that go beyond even that of the CV-22. They include the vectored thrust ducted propeller (VTDP), the Heliplane, and the XHawk. All are based on concepts that originated decades ago. But technology breakthroughs, along with a revived interest in these concepts as a result of the realities of modern-day warfare, are finally spurring their development.
VECTORED THRUST DUCTED PROPELLER
A compound helicopter is defined as a rotary wing vehicle equipped with auxiliary propulsion and lift systems that unload the rotor in forward flight, resulting in increased forward speeds, range, ceiling, survivability and reduced life cycle cost.
The VTDP compound helicopter is one such concept. Piasecki Aircraft Corporation (PiAC), Essington, Pa., is under an advanced technology demonstration (ATD) contract with the U.S. Army to flight demonstrate the technology. In place of a tail rotor, the VTDP consists of a tail-mounted ducted propeller facing aft to provide thrust and a lifting wing to unload the rotor in forward flight.
A conventional tail rotor provides the necessary force at the tail so that the fuselage can counteract the torque from the main rotor, in order to prevent the fuselage from spinning, and to provide yaw control. But from a speed perspective, the tail rotor of the conventional single rotor helicopter provides no thrust, but rather is a source of significant parasitic drag and vibration.
Like the conventional tail rotor, the VTDP or “ring tail” propeller provides antitorque as well as yaw and pitch control, using a central rudder and thrust vectoring sectors and elevators mounted behind the ducted propeller. In addition to increasing the speed, this significantly increases control over the helicopter’s pitch attitude and trimming capability, according to John Piasecki, company vice president.
The VTDP addresses the age-old problem that prevents conventional helicopters from flying any faster than they do: the phenomenon known as retreating blade stall. This occurs when the helicopter reaches a certain speed, the retreating blade—the cycle of the revolution where the blade moves toward the aft of the helicopter—of the rotor enters blade stall because the “relative wind” of the rotor blade is being reduced by the forward speed of the helicopter. This phenomenon essentially creates a speed barrier for conventional helicopters. And the higher the altitude, the slower the top speed.
With the wing and a thrusting tail, the VTDP compound unloads the lift and propulsion of the rotor. At the high altitudes like Afghanistan, helicopters like the H-60 can be limited to speeds as low as 90 knots. “A compound helicopter’s lifting wing and VTDP thruster unload the rotor, delaying the onset of the retreating blade stall out beyond 200 knots, indicated Piasecki. “Higher speed at higher gross weights required to carry fuel is critical to being able to effectively operate over the extended battlespace where our forces are fighting. Also, if you’re a bad guy and want to shoot down this aircraft, since it’s flying a lot faster through your threat zone, your probability of obtaining a lock is substantially reduced.”
While the VTDP compound provides superior forward flight performance, there is a cost in weight, empty and downloaded. The lifting wing, being underneath the downwash of the rotor, interferes with some of that downwash. Because of the wing and more robust drive system, the aircraft has a higher empty weight, so the power required to hover is slightly higher at normal altitude and normal operating environments. However, this increase in hover power required is more than offset by the additional power installed with Piasecki’s patented Dual Path Powertrain system that has been fully flight qualified at the Navy’s helicopter transmission test facility. Hover capability is actually enhanced at very high altitudes by the VTDP because it is designed with greater prop solidity, whereas the conventional tail rotor begins to stall and lose control.
“The bottom line is, you don’t get something for nothing,” he pointed out. “While offsetting the wing and the tail device weight requires some additional power, you have to have more power installed if you want to increase the helicopter’s ability to hover in super high heat environments like Afghanistan. The most important thing to remember about the VTDP compound helicopter technology is that it increases the helicopter’s ability to productively use additional power, not just for hover, but for significantly increased forward flight performance, leading to a more balanced design for these more demanding operating environments, whereas conventional helicopter, which can’t use additional power to overcome retreating blade stall and its many other aeromechanical limits.”
The wing could be made to retract, but the retracting mechanisms would add more weight to the craft. The way PiAC has approached that problem is with full span flaps on the wing to minimize the download. These flaps in combination with the elevators in the VTDP provide control redundancy with rotor controls in the roll and pitch axis. This means that if you do get hit, the VTDP compound has alternate means of controlling the aircraft and increasing survivability. This makes it is much more likely to be able to withstand the ballistic impact of small arms fire. “If your longitudinal cyclic control on the rotor gets shot out, you have a control device in the tail in the form of the elevator to provide for the pitch attitude control, and roll control with the flaperons, which allows you to still fly the aircraft back and land it like a conventional winged aircraft,” said Piasecki
The VTDP is designed to be retrofitted onto a conventional helicopter. The test platform is an H-60 Black Hawk, but it can be applied to many other platforms as well. The objective is to demonstrate the ability of the VTDP to expand the conventional helicopter’s performance in terms of speed, range, ceiling, high-altitude hover capability and survivability—which includes both maneuverability and redundancy. A further aim is to reduce the fundamental drivers of helicopter life cycle costs, specifically, vibration and fatigue loads.
The aircraft currently is in final assembly and being prepared for the flight test phase, according to Piasecki. “This demonstration aims to verify the ability of this technology to expand legacy helicopter capability so that they can operate in the extreme environments in which our forces are now fighting,” referring to areas such as Afghanistan. “If we are successful, we might be able to address the helicopter’s significant shortfalls now. The replacement for the H-60 and many of the other DoD helicopters are not contemplated to be fielded until after 2025. I do not think our soldiers can wait.”
HELIPLANE
Another hybrid aircraft under development is the heliplane. Last year, the Defense Advanced Research Projects Agency (DARPA) selected a Groen Brothers Aviation (GBA), Salt Lake City, Utah-led team to design a proof-of-concept for the heliplane for use in combat search and rescue roles. The rotorcraft is designed to exploit GBA’s gyrodyne technology.
The heliplane is an aircraft that has both helicopter-style rotor blades that produce lift, and fixed wing-style turbofan engines that produce thrust. A heliplane can fly much more slowly than airplanes and much faster than helicopters. They can hover and they can take off and land vertically, without a runway. Because of their simplicity, they have the potential to be easier to operate and less expensive to maintain than helicopters, according to a company spokesperson. They are also safer, as they can land even in the event of a power failure.
“The heliplane offers the military the potential to have a runway-independent VTOL aircraft that can operate from unimproved terrain and that can provide a cruise speed and range that is competitive with fixed wing aircraft,” observed Don Woodbury, heliplane program manager in DARPA’s Tactical Technology Office. He said the heliplane demonstrator aircraft will prove that it is possible for a VTOL aircraft to cruise at speeds up to 400 mph and to carry a 1,000 pound payload 1,000 nautical miles (about 1,150 statute miles).
The heliplane, which is still in the design stage, builds upon the Fairey Rotodyne, an experimental aircraft with a rotor system similar to that of the heliplane and that first flew in 1957. It flew several hundred flights and reached 191 mph. The heliplane has the objective of flying twice as fast as the Rotodyne.
Added Woodbury, “If the heliplane is successful, it has the potential to revolutionize VTOL systems. Not every mission performed by a helicopter today can be performed better by a heliplane, but a great many of today’s helicopters, both military and commercial, could be replaced by heliplanes.”
XHAWK FANCRAFT
“Get my people off the streets!” is a frequent plea of military commanders operating in urban environments. Helicopter rotors often make flying through or landing in urban canyons unfeasible. A potential solution is fancraft technology. In the form of the XHawk in collaborative development by Bell Helicopter, Fort Worth, Texas, and Israelbased Urban Aeronautics, this type of craft has an enclosed rotor or “fan” at both ends of the vehicle. The highly compact footprint and lack of exposed rotors enable it to reach tight locations no conventional helicopter could reach.
The concept has been around for a half century, dating back to the Airjeep, two prototypes of which were built by Piasecki Corp.
Two principal technology breakthroughs have helped revive the concept. Adaptable ducts permit high-speed operations of up to 140 knots, and a vane control system provides lateral stability and control.
“One of the great innovations of this configuration is to put control vanes also on the intake side of the duct,” pointed out Jon Tatro, director of advanced concept development with Bell Helicopter. “That almost doubles the lateral control power of the vehicle. It can be dedicated to what we’re calling a semi-autonomous automatic flight control system, or stabilization system, which allows you to give the pilot independent access to all six degrees of freedom.”
The Bell team is exploring the potential of adding wings that fold out from underneath the body of the aircraft in forward flight, thus offloading the main lift fans further. Such wings could potentially enable speeds of 20-40 knots above the 140 knots being predicted now. But they add extra weight. On the other hand, faster speeds would enable the craft to carry less fuel. “All of those different engineering trades need to be done. Much of that, of course, will depend on what the services want,” remarked Tatro.
Unlike conventional helicopters, which can be heard from long distances, the XHawk would be relatively quiet. “We anticipate that we will not project sound anything like what helicopters do today,” explained Tatro. “It means we can get a lot closer to where we need to go without announcing our arrival. That improves our survivability. When you couple that with having such a small footprint, you have a great deal of flexibility in terms of where your ultimate destination is going to be.”
The XHawk would be able to pull up alongside of buildings. Among other things, that would aid in confronting insurgents inside. Instead of doing what’s done today, which is either fast-roping to the roof and fighting your way down through the structure, or coming in at street level and fighting your way up, “with the XHawk you could pick the level and infill the team,” noted Tatro. “Also, if you get a team pinned in a structure, where there are enemy forces above or below you, you could bypass them. And it would give you the ability to medevac and extract team members from any level, not force them to fight their way up or down through the enemy.”
A possible configuration is to have a more open cabin, so that shooters can be shooting on the way in and on the way out.
Regarding medevac, Tatro said the aircraft and its configuration would enable the capability to extract wounded from places that helicopters can’t go, boosting the chances of saving them by treating them within the “golden hour”—and in a ride that’s much more comfortable than a Humvee.
A flight demonstration program for the Office of Naval Research is expected to begin in FY2010, possibly sooner. “I wouldn’t think that an actual program would be launched any sooner than 2014, unless this thing gets fast tracked and someone like DARPA jumps on board,” speculated Tatro. “And then all of those timetables would get accelerated.”
He added, “So far, nobody has done anything to fundamentally address the need for the urban warfighter when it comes to vertical lift since the tragic lessons learned of Mogadishu, Somalia, in 1993. We’ve toughened up our helicopters. We’ve hung a little armor on them to the extent we can afford to do so, and have improved the performance. But there’s no fundamental change that’s been accomplished—innovation-wise—to improve our capability for vertical lift in the urban war fight.” The XHawk, he said, will change all that. ♦