Originally Posted by
NickLappos
The original S76A had an rpm arrange in powered flight from 96% to 107%. This allow the pilot to select the RPM based on flight condition and the optimal performance. Some information was provided to show that the rotor was generally more efficient at providing lift for a given horsepower at the lower RPMs, and at high speed cruise efficiency gives better cruise speed the high RPM. Thus it was sometimes better to be at 100% RPM during the steady hover if there was not sufficient engine power to reach transmission limit. In this case then the more limited engine power provide maximum lift at 100% RPM. If however sufficient engine power existed to pull 100% torque, then there was no great advantage to staying at 100%, And in fact the extra rotor energy at 107 percent can produce a better single engine land back if a OEI is experienced.
Note that when power limited in OEI< a droop down to 96% Nr was required, and the climbout acceleration to Vtoss was helped by the lower RPM.
The later model S76 helicopters raised the gross weight, so that generally the 107% RPM was better in cruise on component lives. For this reason on the B, C and D models, only one High RPM was provided. The reason why 107% RPM was favored as the gross weight was increased was because effects of retreating Blade stall create additional rotor loads that stress opponents a bit more and reduce their lives. Use of 107% RPM allow the lives to stay high even up to a 11,700 pounds.
If you fly an old A model, or in A + series, you can try this experiment to see how efficient the rotor is at 100% as compared to 107%. Just taxi out on a very still morning and establish a very low hover at 100% RPM. Adjust the aircraft to just barely bounce the left landing gear, and note the engine temperature required at that very low hover. Now beep the RPM up to 107% and repeat the wheel bounce. Note the engine temperature again. You should note that it takes about eight or 9°more temperature at 107. This is because when the rotor is spinning at 107 it uses more drag to spin the rotor Leaving a little less power to produce the thrust. FWIW, 3.5 degrees of Temp = 1% power.
Now take your aircraft into high-speed cruise, and set 694 degrees engine temperature at 100% rotor and some altitude above about 5000 feet. Take a very careful level flight point and note your cruise air speed. Now beep to the rotor RPM up to 107% and note your cruise speed. Generally at higher altitudes 107% RPM yields 5 to 7 knots more cruise speed, which at the same power means more range for the fuel. This is the because at the higher speed the beginnings of retreating blade stall begin to show and higher RPM has less blade stall effect and more rotor efficiency.
Yes, all works as described. Have used the 107nr in cruise when remaining fuel was on the minimums. easily "made" 50-80 lbs on landing from long sectors. Nick, isn't there also a TAS gain in cruise........107nr for same power setting as 100nr yields "flatter" blade pitch....less drag????
DK