My last column dealt with describing the power required by the rotor to produce optimum results in various stages of flight. This sparked a bunch of interesting conversations that have become the basis for this month’s topic: varying the rotor RPM.
Many of today’s helicopters are operated at a constant rotor RPM, but there are benefits to varying RPM in flight. It allows for a greater portion of the rotor blade to operate at a more optimal angle of attack, maximizing aerodynamic efficiency. Rotor speed adjustment is beneficial to overcoming problems associated with compressibility and stall issues on the advancing and retreating blades, respectively. And rotor speed is a key ingredient in a helicopter’s noise signature. But, as is usually the case in helicopter design, logical answers tend to have more complex caveats.
One of the main reasons NOT to greatly vary the rotor speed is to avoid potential problems with vibrations or resonance with the rotor. Every structural member of the helicopter has its own natural frequency, and if these structural frequencies approach that of the main or tail rotor, then uncomfortable or even catastrophic vibrations could be produced. The helicopters that do have variable RPM only have a small range to work within so that they remain clear of any resonant frequencies.
Another challenge is that of flight control. Since the rotor provides our lift, forward thrust, and control moment, varying rotor RPM has the potential to affect stability and control margins and overall handling qualities.
While large, dynamic changes in rotor RPM to maximize performance in all flight regimes are still on the drawing board, there are helicopters that do employ a varying rotor RPM to a lesser degree, either automatically, or by pilot input. RPM is usually varied within a small defined range, and scheduled depending on airspeed, altitude, temperature and gross weight to provide an increase in aerodynamic performance, a decrease in noise, or to mitigate other design considerations.
In one instance of rotor optimization, the flight manual of a twin-engine helicopter will call for an increase in rotor RPM during certain hover/takeoff/landing phases, especially in an aircraft that boasts “Category A” performance (a topic requiring its own article). Much discussion can be had as to the reason or benefit: Generally speaking, Power = Torque x RPM. Increasing rotor RPM will produce the same power at a lower torque value (i.e., lower collective setting). If you are torque-limited, this will make a few more horsepower available before you hit the transmission-imposed torque limit. But while the power available has increased, the benefit may be minimal. The drawback is that while speeding the rotor up decreases induced drag by operating at a lower angle of attack, it increases profile drag slightly, increasing the power required to hover slightly to begin with. What the increased RPM does get you is more tail rotor authority and more rotor inertia, i.e., more time before RPM decays in a one engine inoperative (OEI) situation, should an engine failure occur. Realize that increasing RPM will also raise your turbine temperature (TOT) and gas producer speed (N1, Ng), so if either of these becomes your first limit during OEI, then “beeping up” the rotor speed is of no help. It’s interesting to note that some helicopter flight manuals call for decreasing rotor RPM for an engine failure. If you are limited by TOT or Ng, then a lower rotor RPM will allow operation at a more efficient Lift/Drag, gaining back a bit of power margin.
The tempting question that might be posed, then, is if these situations provide an increase in performance, why would we not always fly at these higher or lower RPMs? Be aware that this short discussion is just an exercise in the principles involved, and should be taken as generalizations. The deeper implications of flying at higher RPM include much-increased stresses on the blades due to centrifugal and oscillatory forces. In contrast, lower RPMs impose higher torsional forces on the transmission. Any of these conditions, if not designed for as a normal occurrence, will increase component fatigue in unanticipated ways.
The complexity of helicopter flight goes well beyond lift, drag, thrust, and weight. Suffice to say that trying to apply the RPM logic to other phases or durations of flight without the consent of the flight manual is asking for trouble.
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