Unlocking the Mysteries of Robinson’s Derated Engines
One of the most misunderstood aspects of the R22 and R44 continues to be the “why” and “how” the engines are derated. Since the very first R22 entered the marketplace, the derating of these engines has, for some reason, confused pilots. I’ve even been involved in court cases where the opposing “experts” on the other side completely misunderstood Robinson’s purpose and method (I use the term “experts” loosely). Everything from main rotor gearbox limitations to small carburetors to hidden black boxes have been offered as reasons for the “why” and “how” of derating. Let me set the record straight.
Robinson has used five engines in their piston helicopters, three in the R22 and two in the R44. All five engines have been “derated” in that Robinson limits the amount of horsepower the pilot should use. Rather than confuse things with all the different numbers (normal ratings, five-minute ratings and max continuous ratings) from all five engines, I’ll use the R22 Beta engine for this discussion, understanding that everything I mention about the Beta is true for all the other engines except the numbers change. The Beta uses the Lycoming O-320-B2C. The basic O-320 has been around since 1953 and various models have been used in numerous aircraft including the Cessna 172, Piper Cherokee, Mooney M20 and the Super Cub. The B2C model has a normal brake horsepower rating of 160 hp at 2700 rpm; however, Robinson limits the pilot to 131 hp (at 2652 rpm) for five minutes and 124 hp for maximum continuous power (MCP). The obvious question is why? If Robinson only wants the pilot to use 131 hp why not put in an engine that that can only produce 131 hp? It would be lighter and probably a lot less expensive. The primary reason for the derating is to increase the altitude capability of the helicopter. As the density altitude increases the engine’s ability to produce horsepower decreases. So, if we started at sea level with an engine that can only produce 131 hp, how much horsepower would be available as we climbed up to perhaps three, four, or five thousand feet? Obviously, much less than 131. If we start at sea level with an engine that, in the case of the Beta, is capable of producing 160 hp but only let the pilot use 131 hp, then the engine will be able to maintain that 131 hp all the way up to what is called the critical altitude. The altitude at which the engine is only capable of producing 131 hp. For the Beta on a standard day this is a little above 5000 ft. Every engine change in both the R22 and R44 was made primarily to increase the helicopter’s performance at higher altitudes. Many of the early low RPM rotor stall fatal accidents were caused by overpitching the collective (increasing the collective when operating at full throttle). Increasing the altitude capability increases the altitude at which the pilot will be operating at full throttle, thereby reducing the pilot’s exposure to a possible overpitching situation. For example, at +20ºC a pilot in the Standard R22 (O-320-A2B engine) will be at full throttle at approximately 3700 ft pressure altitude. While a pilot in an R22 Beta II (O-360-J2A engine) will not experience full throttle until 6500 ft.
A secondary reason or benefit of derating the engine is higher reliability and longer engine life. It stands to reason that if the engine doesn’t work as hard it will last longer and be more reliable. If a pilot were to fly the Beta at maximum continuous power (124 hp) all the time, the engine would only be working at 77% of its normal horsepower rating, a cruise power setting for an airplane. As a result of this engine derating, Lycoming established to a 2000 hour TBO (time before overhaul) on the engine from the very first R22. To my knowledge, this was the first time a piston engine in a helicopter was given the same TBO as in its airplane counterpart. The R44 is the second.
For years many pilots incorrectly thought the R22 and R44 had some black box or limiting device that prevented the pilot from asking the engine for more than the five-minute takeoff limit. Of course, this is not the case. The only thing that limits the pilot is the Limit Manifold Pressure Chart found in the Limitations Section of the Pilot’s Operating Handbook and placarded in the cockpit. Using the chart, the pilot should determine the manifold pressure limit for the five-minute takeoff rating (131 hp) and maximum continuous power (124 hp), then it is up to the pilot to keep within these limits. If the pilot exceeds the limit, the engine is being asked to provide more than 131 hp and it will continue to provide more horsepower until full throttle is reached. Once the throttle is full open, the engine is providing all the horsepower it is capable of and any increase of the collective will lead to a decrease or decay in RPM which we call “overpitching”. So, the R22 & R44 engines are derated to increase the density altitude at which the pilot will reach full throttle, thereby reducing the pilot’s exposure to a possible overpitching situation.
A couple of footnotes should be noted about Robinson’s derated engines. First is with the R22 Beta II engine. The Beta II uses a variant of the Lycoming O-360 engine, which like its O-320 brother has been around since the early 50s and typically has a normal brake horsepower rating of 180 hp at 2700 RPM. However, the J2A model used in the Beta II has a normal rating from Lycoming of only 145 hp at 2700 RPM (page 1-5 of the R22 POH). The reason is weight. The J2A has lightweight cylinders and saves approximately six pounds in overall engine weight. Six pounds is significant–that’s a gallon of gas. What is important to understand is the engine is still capable of producing 180 hp. It has the same displacement, bore, stroke and compression ratio as other O-360 models, just made out of lighter weight materials. So, Robinson got the best of both worlds; they saved six pounds and have an engine capable of producing 180 hp. Remember, Robinson doesn’t want to use 180 hp in the Beta II, only have that capability so that the engine can maintain 131 hp to a much higher altitude.
The second footnote is with the R44 Raven II engine. Notice that with all the other engines the POH states the normal brake horsepower rating from Lycoming, but not in the Raven II POH. What is the normal horsepower rating of the Raven II? Well, it depends on who you ask. On the Raven II engine data plate Lycoming states 260 hp for takeoff and a normal horsepower of only 235 which is confirmed on the engine’s type certificate data sheet (number 1E4, revision 25, dated 7/2015). If this were true why would Robinson change from the 0-540 engine in the Raven I which has a normal rating of 260 hp to a version of the fuel injected IO-540 with the same horsepower rating? The fact is the Raven II’s AE1A5 model of the IO-540 can produce much more horsepower, up to 300 hp at full throttle. Understand most of the developmental work on the Raven II engine was done by Robinson, at Robinson in their engine test cells equipped with two engine dynamometers. Robinson engineers investigated turbochargers, dual turbochargers even evaluated the Lycoming IO-580 engine in an effort to get more horsepower to increase the R44’s altitude capability. It was decided to modify the IO-540 to optimize horsepower output.
Angle valves replaced the parallel valves, increasing the efficiency of each cylinder thereby creating an additional 20 hp. A tuned fuel induction system was developed which adds another 20 hp and lightweight cylinders, similar to the Beta II, are used to decrease overall engine weight. So, for much the same reason as with the Beta II engine, Lycoming does not want to rate the engine at 300 hp but, also like the Beta II, Robinson just wants that capability to maintain max continuous power (205 hp) and takeoff power (245 hp) to a higher altitude.
Hopefully, some of those “experts” get the chance to read this explanation.