The aircraft flight manual for my light, twin-turbine helicopter recommends 100-percent rotor rpm for flight with a continuous range of 98-102 percent. Some fellow pilots suggest using 102 percent when taking off from a confined area for better performance. If you actually get better performance at 102 percent, why doesn't the AFM state this?
Name Withheld By Request
If "performance" near hover can be related to the maximum vertical rate of climb at the transmission limit, then increasing rpm will improve performance. I have done the calculations for the example helicopter that I invented for my textbook ("Helicopter Performance, Stability and Control," Krieger Publishing). Its transmission torque limit corresponds to 4,000 hp. at 100-percent rpm. At this limit, the vertical rate of climb is 2,770 fpm. At 102-percent rpm, an additional 80 hp. can be put into the transmission before reaching its torque limit and the rate of climb increases to 2,850 fpm.
Although engineers have convinced the FAA or the military customer that the helicopter is safe at either rotor speed, there are a couple of small "Yes, buts." One is that the centrifugal loading in the blades and hub goes up 4 percent when the rotor speed is increased by 2 percent. This doesn't sound like much, but with many start-stop cycles, it might decrease the allowable fatigue life.
Another has to do with the oscillating stresses in the blades. If the natural frequencies of the bending and torsion modes are sufficiently separated from the blade passage frequency (or its multiples) at 100-percent rpm, resonance is not a problem. But increasing (or decreasing) rotor speed might make a less comfortable match and result in somewhat higher stresses and vibration.
The response of other parts of the helicopter is also a concern. Consider a horizontal stabilizer next to a two-bladed tail rotor. If its bending frequency is well above 12 times the tail rotor blade passage frequency, then resonance is not a problem. However, with a 2 percent increase in rotor speed, 12 percent of that margin has been lost. Other components have similar situations. Shake test of the Apache tail boom and empennage found 11 different modes of bending or twisting that could be excited by main- or tail-rotor oscillating forces. This resulted in a least one structural modification before the helicopter could be delivered to the U.S. Army.
I am in an argument concerning the proper design of tail rotors, specifically the power required to produce tail-rotor thrust at a hover. I contend that, all things being equal, simply increasing blade chord will not necessarily give more thrust if the tail rotor is limited by gearbox power. Others seem to think that increasing blade chord will always give more thrust regardless of power limits. Who's right?
As with most helicopter questions, the not-so-satisfactory answer is: "It all depends." In this case, it depends on how close to stall the tail-rotor blades are when the tail-rotor gearbox limit is reached. If they are well below stall, then increasing chord is the wrong thing to do. Adding that extra area would increase the profile power due to skin friction and the power limit would be reached at a lower thrust than before. On the other hand, if the limit was reached because of blade stall, adding chord is the correct thing to do. For a given hover condition, the optimum chord would be the one that makes the blades operate at just under stall as the power reaches the gearbox limit. A rotor aerodynamicist would work with the maximum "blade-loading coefficient" as determined by characteristics of the airfoil being used.
Torque in a Turn
In a U.S. Army helicopter class, we were told that, in a 60-deg. banked, coordinated turn, torque would have to double to maintain airspeed as load factor doubles. This didn't seem right; it does not consider profile and parasite power, which don't change significantly. I know that in fixed-wing you add power in this turn, but not double it. Yet we were told even airplanes need double power to maintain airspeed.
Schofield Barracks, Hawaii
The power required to maintain a constant speed and altitude in a turn is a combination of the induced, profile, and parasite components just as in level flight. For moderate load factors, the primary change from level flight is in the induced power which is a function of the increased rotor thrust. For high load factors, however, as in a 2g turn, the profile power increases significantly as some of the blade elements approach stall at thrust levels for which the rotor (unlike a wing) had not been designed to operate comfortably.
The induced power will increase as a function of the square of the rotor thrust. In a steady 60-deg bank, the induced power increases four times. The total effect on engine power will depend on speed; large at low speeds, but not so much at high speeds where the induced power is low to start with. I've used my example helicopter to calculate the engine power at 80 kt. in level (1g), and in a 60-deg bank (2g). At level flight, the engine horsepower is 893, of which 355 is induced. In the turn, the induced power is 1,266 hp., up by a factor of almost four, but the total engine power is 2,304, which is more than double. This is due to a large increase in profile power. This shows that doubling power in a 60-deg turn is a possibility, but it is due to a combination of effects rather than just one.
If you have questions for Ray Prouty, send them to email@example.com, 301-354-1809 or Rotor & Wing, 4 Choke Cherry Road, Second Floor, Rockville, MD 20854.