When Robinson Helicopter Company unveiled its new R88 model at Verticon in Dallas in March, the reception was not universally positive.
The 10-seat R88 will be Robinson’s largest helicopter, its first new type since the R66 was certified in 2010, and its first collaboration with Safran, which will supply the aircraft’s single Arriel 2W engine.
Yet, this otherwise trailblazing product for Robinson retains the most controversial feature of its existing product line: a teetering two-bladed main rotor system that, like all such rotor systems, is susceptible to the hazardous phenomenon of mast bumping. To critics, the decision to stick with two blades was a missed opportunity to improve on safety as well as maneuverability, ride quality, and noise.
The overall dynamics of the rotor system are likely to be more reminiscent of the Huey than of Robinson’s smaller helicopters, according to company CEO David Smith. Robinson Image
Robinson CEO David Smith has been outspoken about the advantages of a simple two-bladed rotor system, including affordability and ease of maintenance. However, he is not oblivious to the drawbacks, and recently connected with Vertical for a more detailed discussion of the tradeoffs associated with the R88’s rotor design.
What many people may not realize is that the design process is still under way. While the R88’s rotor system will share some similarities with those on the R22, R44, and R66, it will not simply be a scaled-up version of them.
“It’s nothing like an R22; it is only a little heritage at all, really,” Smith said during an interview in April. “So the future is derived from that past, but not defined by it.”
As Robinson works towards first flight by next year, here’s how its engineers are thinking about the R88’s handling and autorotation characteristics, rotor hub and blade design, stability, vibration, and noise — plus the additional steps the company is taking to mitigate the risk of mast bumping.
Harking back to the Huey
In announcing the R88 at Verticon, Smith presented it as a “pickup truck” for the utility sector, drawing comparisons to the venerable Bell Huey helicopters that for decades dominated external load missions such as firefighting. And in fact, the overall dynamics of the rotor system are likely to be more reminiscent of the Huey than of Robinson’s smaller helicopters, Smith told Vertical.
“It’s a broader-chord rotor — that will change the blade loading characteristics and the fundamental performance of the rotor,” he said. “We’re not designing it to be a snappy, quick-flying nimble thing. It’s a workhorse, and it’s going to be lifting a lot of people, or lifting a lot of stuff and water, or things from the hook.”
Hueys are renowned for their high-inertia rotor systems that are particularly forgiving in autorotation, and Smith suggested that the R88’s rotor system will have similar characteristics, with proportionally heavier blades than the ones on Robinson’s existing product line.
“The system design will be, from a Lock number perspective, more in line with the autorotation characteristics of aircraft like the Huey, which is a lot of energy in those blades,” Smith said, referring to the ratio of lifting forces to inertial forces in a rotor system.
He said he expects the R88’s main rotor blade Lock number to be in the “low Lock number” grouping along with helicopters like the Bell LongRanger, Cobra, and R66, which have Lock numbers of around five to six. This is in contrast to helicopters with less favorable autorotation characteristics including the Bell 412 (with a Lock number greater than seven) and Sikorsky S-76 (around nine).
The case for coning hinges
One key similarity between the R88’s rotor system and Robinson’s traditional main rotor system design is the use of coning hinges, which are individual blade flap hinges mounted below the central teetering hinge in Robinson’s tri-hinge rotor system. Over the years, some people have claimed that coning hinges account for the relatively high incidence of mast bumping in Robinson Helicopters, despite the company’s own conclusion that its original asymmetrical horizontal stabilizer was the more relevant design factor.
The R88 will have a symmetrical horizontal stabilizer designed in from the beginning, so it shouldn’t experience the aggravated roll in low-G conditions that Robinson’s original asymmetrical design induced. As a further safeguard against mast bumping, the R88 will come equipped with a four-axis advanced flight control system (AFCS) with low-G protections similar to those recently certified in the Garmin GFC 600H autopilot for the R66.
In the R66 with GFC 600H, if the aircraft experiences a low-G condition while the pilot is hands-off, the AFCS will automatically reload the rotor and prevent any rolling movement. (In a mast bumping scenario, this roll is the action that leads to contact between the root of the down-flapping blade and the rotor mast, generally with catastrophic results.) If the pilot is hands-on during a low-G condition, the AFCS will attempt to reload the rotor and make it more difficult for the pilot to command a roll.
“From all of our study, all of our simulation, all of our flight testing, we are confident that will mitigate low-G mast bumping as a cause for accidents in our fleet,” said Smith. Yet, he acknowledged that there is lingering suspicion in the industry around the coning hinge: “People ask about the coning hinge because there have been a lot of folks that have misrepresented what the coning hinge does and how it interacts with the rotor performance,” he said.
Coning is the tendency of helicopter rotor blades to describe a cone shape as they rotate due to the balance of centrifugal and lifting forces. As Robinson previously explained to Vertical, teetering rotor systems without coning hinges minimize bending stresses near the root of the blade by incorporating a fixed pre-cone angle that represents the coning angle for typical flight. However, some blade bending occurs whenever there is a deviation from the typical condition.
Incorporating a coning hinge allows the rotor system to achieve the required coning angle without bending the blades. A 2018 study by researchers at the University of Maryland found that Robinson’s tri-hinge design results in reduced loads at the blade root as well as reduced bending loads in the rotor mast.
Because Robinson is confident that the use of coning hinges does not make its helicopters more susceptible to mast bumping, the company has chosen to also use this design feature on the R88.
“The reason why that’s in place is blade longevity — to give [the blade] a stronger and longer life,” Smith said. “It doesn’t have blade flexure. It doesn’t have pre-coned bent structural elements in it. So it is a simpler blade to construct, simpler blade to inspect, simpler life for the blade to live through.”
Better metal blades
Simplicity is also a primary reason why Robinson is planning on using metal rotor blades for the R88, although the company has been exploring composite rotor blades over the past several years. According to Smith, if a composite rotor blade “doesn’t last for a significant multiple of the life of a metal blade, the economics don’t make sense,” and Robinson has not yet been able to achieve a life extension that justifies the expected production cost.
“In the end, the thing that really sells composites in some of our competitors’ products is they spend a lot of time tuning shapes and forms of the blade for hover performance, cruise performance, or noise,” Smith added. But Robinson believes that the anticipated low empty weight fraction of the R88 means it shouldn’t be hurting for performance, so composite blades are “one of those areas where the juice hasn’t quite been worth the squeeze in getting our customers what they need,” he said.
Robinson is however planning on making improvements to its basic rotor blade design and manufacturing processes to extend the life of the blades on the R88 to 5,000 hours, in line with the times between overhaul of the engine and other components. That represents a significant increase from the 2,200 hours for its current blades (or 2,400 hours in the case of the R44 Cadet).
“Environmentally, where we see our biggest threat for blade integrity in the fleet has been corrosion, erosion — high salt, high sand. That stuff tends to be the thing that ages our blades faster than we’d like,” Smith said. “We want to improve that with a combination of better materials [and] better processes in the bonding and the manufacture of blades and painting.”
Tail to be determined
While Robinson is using various modeling and simulation tools to design the R88’s rotor system, Smith said that physical testing generally “produces superior results even to the best simulation tools.”
“We’re excited to build the parts, get the ground test vehicle going, and that we’d expect to be sort of late this year, early next,” he said. Flight testing will follow in due course, and among other things will be critical for refining the design of the helicopter’s empennage, or tail section.
Besides incorporating a symmetrical horizontal stabilizer from the beginning, Robinson is also aiming to achieve improved directional stability through the R88’s empennage design.
“You’ll notice that the renderings that we’ve shown of the tail — we’ve not produced the tail for the world to see yet, but the empennage shows a set of end plates on the tail that provides . . . across the flight conditions a better directional stability,” Smith said. “When we fly, we’ll know if it’s too big or too small, or if it needs to be refined.”
Two-bladed tradeoffs
Physical testing will also be key for assessing the design’s vibration characteristics and developing any necessary mitigations. Smith acknowledged that the R88 will likely have a noticeable two-per-rev hop that would not be present with a multi-bladed rotor system, which is “not a surprise to anyone flying a Huey or flying a Bell 212, for example.” However, he is optimistic that modern analytical tools will help Robinson identify where best to place dynamic vibration absorbers or perform localized stiffening of the airframe to reduce more troublesome vibrations.
Smith added that Robinson’s high level of vertical integration should also be an advantage in addressing any localized vibrations that emerge during flight testing. “That can be difficult to identify and change when the structure or the components are sourced through three layers of manufacturing supply chain,” he said. “For us to make changes to the structure to better mitigate vibration, it’s all here in the factory.”
Besides their higher vibration levels, two-bladed rotor systems also tend to be noisier than multi-bladed rotor systems, which Smith does not deny. He said that Robinson is investigating multiple RPM setpoints using the Arriel 2W’s full authority digital engine control (FADEC) system to achieve better noise performance than larger two-bladed helicopters.
“Provided that no objectional vibration, autorotation, or fatigue/dynamic load conditions materialize, this will provide significant noise reductions for noise-sensitive flight conditions as has been used in other helicopters in the past,” Smith elaborated in a follow-up email, adding that Robinson is also exploring the use of different blade tip shapes to potentially reduce noise. “We expect to evaluate several shapes to reduce noise with acceptable performance tradeoffs. Both of these efforts will be confirmed through flight testing.”
That said, a core element of Robinson’s strategy is simply to target markets that are less noise-sensitive, whether due to mission set or geography.
“A well thought-out and designed and supported single [engine helicopter] and a two-bladed rotor system are robust for missions like utility lifting, firefighting — so cases where people don’t mind the noise,” Smith said. “A reality of this is that we’ll have a higher noise signature than a multi-bladed [aircraft] designed for noise, so we expect utility and firefighting to be significant opportunities.”
He suggested that many developing markets may also be less noise-sensitive than established markets in the U.S. and Europe, particularly when noise is traded against other attributes like ease of maintenance.
“Three-quarters of our aircraft are operated overseas. Increasingly, the challenge that we, I think, as an industry face is around the availability of good quality service and maintenance, and I think that is going to continue to be a challenge,” he said.
“One of the things that is at the core of Robinson is, simple is more supportable. . . . Trying to keep the aircraft as simple as possible is an essential ingredient to what we feel is an existential risk around industry availability of technical talent.”
