Robinson Tail Rotor Design and History

When Frank Robinson designed the tail rotor system for, initially, the R22 then subsequently the R44 and R66, he borrowed from some of his design work at Bell and Hughes Helicopters, added a few twists of his own and created what today is one of the strongest two-bladed tail rotor systems.

In 1969 Frank Robinson left Bell Helicopters where he had worked for 2 years developing a reputation as a tail rotor authority and moved to California to work for Hughes Tool Company’s Aircraft Division in Culver City (manufacturer of the HU-269 and 369 helicopters). He was given the task of trying to solve a problem Hughes was having with the US Army’s OH-6 Light Observation Helicopter (LOH, which is the 500C or HU-369 in the civilian world). The Army’s problem was what today we would refer to as loss of tail rotor effectiveness or LTE but the phenomenon of LTE was not really understood in the late1960s so Army pilots in Vietnam just called it the “Hughes tail spin”. It occurred because of a special mission the OH-6 had in the Vietnam War called a Hunter-Killer team. This team comprised a single OH-6 and, depending when during the war, one or two UH-1 (Huey) or AH-1 (Cobra) gunships. The OH-6, called “the little bird”, would meander along at low altitudes and slow airspeeds trying to find bad guys. The thinking was if you exposed yourself as an easy target and someone shot at you that was a good sign he was a bad guy. The little bird’s observer would pop a smoke grenade, toss it in the incoming fire’s direction and the ship would quickly accelerate, leaving the immediate area. Unbeknownst to the bad guy, the gunships loitering up at 2500 feet would roll in on the smoke and obliterate the area with rockets. Since the mission called for the OH-6 to hover OGE for extended periods of time at less than ETL in varying wing conditions, it constantly exposed the helicopter to an LTE environment resulting in the “Hughes tail spin”

Frank was given quite a bit of leeway by Hughes to investigate and solve this “Hughes tail spin” issue. He conducted one of the most thorough investigations into helicopter tail rotor systems and, more specifically, tail rotor airfoils that had been done up to this point in helicopter development. Understand that during the Vietnam War (the 1960s and early 70s) most of the engineering man-hour at all the big helicopter companies (Bell, Boeing, Skorisky) was being spent on main rotor design–trying to get helicopter main rotor systems to fly faster and carry more weight. Most helicopter tail rotors at the time used a standard 4-digit symmetrical airfoil, such as the NACA 0015 airfoil. Through Frank’s extensive mathematical modeling and wind tunnel tests, he found one airfoil, a 63-415 NACA airfoil, that showed enhanced aerodynamic characteristics when operating at the high Mach numbers (the ratio of airfoil speed to the speed of sound) and low Reynolds numbers (a dimensionless number used to predict airflow characteristics over an airfoil.) that tail rotors are forced to operate at. There are several competing factors when it comes to tail rotor design especially on a small helicopter like the Army’s OH-6 or, for that matter an R22. Since the tail rotor has such a long center-of-gravity arm, it has to be as light as possible but since the speed it turns at produces high centrifugal loads, it has to be strong. Of course, it has a job to do back there, so it has to produce the appropriate amount of thrust. The 63-415 NACA airfoil is an asymmetrical airfoil because it has a “full camber” that extends the full length of the blade but is most notable towards the trailing edge and produces a large amount of thrust given its length and cord. The main disadvantage of this airfoil is the camber produces a negative or nose down pitching moment which can produce undesirable control loads. Robinson compensated for these loads by incorporating a built-in conning or “precone” angle into the tail rotor system. It’s my understanding that when Hughes put this airfoil on the Army’s OH-6, it was one of the first instances, if not the first, of an asymmetrical airfoil being used on a production tail rotor.

Years later when Frank was designing the R22 he resurrected his investigation from Hughes and used the 63-415 NACA airfoil on the R22. This airfoil helped Frank remain consistent with his low weight philosophy and solve another nagging problem he was having with the R22 design–the tail rotor direction of rotation. Let me state right here that Frank Robinson is a weight fanatic. I’ve seen him make numerous design decisions or reject various modifications because of just a few pounds and sometimes even ounces. Whenever anyone suggests a new modification, Frank’s first question is “What will it weigh?” With the R22, reducing the weight in the tail section was important. Additionally, in order to have side-by-side seating in a small helicopter, like the R22, the empty weight longitudinal C of G needs to be as far forward as possible.

Let’s talk tail rotors in general for a moment. When the tail rotor is needed most is at speeds less than ETL or at a hover. Sure, high speed flight is a high-power environment hence the need for a lot of anti-torque, however, not all the anti-torque has to come from the tail rotor. Some of it comes from the streamlining effect over and around the fuselage and the airflow over the vertical stabilizer. Consequently, most helicopter manufacture’s (not all but most) will turn their tail rotor in a direction that takes advantage of the tip or shed vortices coming off the main rotor, or clockwise when viewed from the helicopter’s left side. However, this caused Frank a problem, the source of which is how the engine is installed in the R22. The engine is mounted backwards (compared to airplanes) or has been turned 180∞ in the R22. For example, if the R22 engine were installed in an airplane the propeller would be back where the R22’s cooling fan is located. That’s why when you turn to key to “R” during the magneto check in an R22 it’s the mag on the left side of the engine that’s being tested. The natural direction of rotation coming out of the engine lends itself to a simple and, more importantly, lightweight set of gears in the tail rotor gearbox but would turn the tail rotor in the least efficient direction or counterclockwise as viewed from the left. Frank felt he could get away with this least efficient direction of rotation, thereby saving quite a bit of weight in the tail rotor system, yet still produce an appropriate amount of thrust because of the airfoil he was using. This is in fact how it has worked out. The R22 has a very strong tail rotor given its short length and cord. I’m not saying you will never run out of left pedal in an R22 but it will be difficult.

In the R44, the weight of the tail rotor gearbox is not near as critical as it is in the R22 so Robinson changed the gear set in the tail rotor gearbox making the direction of rotation a more conventional clockwise direction. Without question this is the biggest difference from a design standpoint between the two helicopters–the direction of rotation of the tail rotor. The R44 also uses the same 63-415 NACA airfoil which helped Frank address another issue, noise. After the R22 was certified but before the R44 was approved, 14 CFR § 27 changed to include noise standards. Essentially, the higher the gross weight the higher the acceptable noise level. The tail rotor can be one of a helicopter’s main sources of outside noise, so the speed of the R44 tail rotor was reduced to reduce the noise level without a big loss of thrust due to the airfoil and efficient direction of rotation. The R44 has one of the strongest tail rotors I have flown. To demonstrate this, I’ll roll the RPM down to 85% (almost 15% below the green) while at a hover (two people on board, full fuel, don’t really care where the wind is from). Not only can the helicopter make a 360º left pedal turn but you can start an aggressive right pedal turn and stop it with left pedal. That impresses most pilots, especially if they come from a Jet Ranger background, so then I’ll further decrease the RPM down to 75% (25% below the green) and repeat the demonstration. In fact, if I continue to decrease the RPM I’ll run out of engine power and will be unable to sustain a hover before I hit the left pedal stop. An incredible tail rotor!

But it seems the way life works there are always unintended consequences. In this case the unintended consequence in the R44 is too much tail rotor which has manifested itself quite a few times around the world. It even happened at the factory a few years ago. There is an area on Robinson’s flight test ramp where tail rotor track & balance and fan balance is done to each new helicopter. Since there is no intention for the helicopter to fly, a mechanic not a pilot runs up the aircraft. In this instance, a mechanic was sitting in the helicopter at 102% RPM, collective full down and one foot, his left foot, was resting on the junction of the two pedals (something I’ve seen many pilots do for comfort). When the track & balance was completed, the mechanic at the controls was given the signal to shut the helicopter down. As he repositioned his body to go through the shutdown procedure, his foot slipped off the junction of the two pedals and full left pedal was applied. That tail rotor is so strong that even with the collective full down the helicopter spun 26 times to the left, down the Robinson ramp, across the airport tarmac into the grass until finally the mechanic had the presence of mind pull the mixture. Were we ever lucky! It did not spin into a person, another aircraft, a building or roll over. In fact, the only component that had to be replaced were the skid shoes from scraping across the asphalt–oh, and the seat cushion. It had a large stain on it. Variables such as aircraft weight and type of surface will certainly affect the tendency to spin to the left but it can take as little as ½ to 1 inch of left pedal to start the spin. There have also been instances where the pilot has inadvertently adjusted the left pedal more forward than the right pedal so when both pedals are seemingly aligned, there is actually enough left pedal applied to spin left. The lesson here is: when that R44 is at 102% it's ready to fly, so you better be ready to fly–both feet on the pedals and hands on the other controls. If you’re still adjusting the GPS, listening to ATIS, briefing a passenger etc. reduce the RPM down below 80% where the helicopter can’t fly.

The R66 uses the same airfoil as the R22 and R44 but is two inches longer than the R44 tail rotor blade and has a 0.4 inch wider cord. This is because of the R66’s excellent performance at high density altitudes. The R66 can hover OGE at max gross weight at 11,000 feet DA but tail rotor authority was becoming an issue. By increasing the length and cord additional thrust can be generated without increasing the blade’s angle of attack. Down at sea level the R66’s larger blade results in the same unintended consequence as the R44, so again, when you’re up at operating RPM, be ready to fly.

One important factor pilots should be aware of regarding Robinson tail rotor blades is the construction. On all three models the blade skins are aluminum with a thin strip of aluminum honeycomb running the length of the center portion of the blade (The R44 and R66 have a Kevlar strip over the honeycomb to help stiffen the blade.). There is no stainless-steel D-spar on the leading edge as there is on the main rotor blade. So, when hovering, if the tail rotor were to strike any debris kicked up by the rotorwash, damage to the blades is a distinct possibility and they are not cheap.

There is one footnote I would like to make to this discussion of the Robinson tail rotor system. As I’ve explained, Frank Robinson’s research and testing has made him an accepted authority on tail rotor design. He’s done consulting work for other manufactures and is, as I said, an expert. So, it stands to reason that the length of this somewhat unique R22 tail rotor blade would have been determined by precise aerodynamic concerns, meticulous calculations or innovative engineering. Not so, the length of the R22 tail rotor blade was determined by nothing more than the diagonal distance in the oven in Frank’s Palos Verdes, Ca home where the very first tail rotor blade was bonded in 1974. It has not changed since.

Tim Tucker

March 2020.