Wednesday, August 1, 2012
Feedback August 2012
It was noted in the CHC Safety & Quality Summit article in the May 2012 issue (page 40) that the Sikorsky S-76 was specifically cited as “lacking in safety design requirements” in part because it does not have a crashworthy fuel system. I’m sure we all prefer to base our beliefs on facts instead of rules, so let’s look at the numbers. About 800 S-76 helicopters have been delivered since 1978 and these aircraft have accumulated about six million flight hours.
During that period, the S-76 has experienced four post-crash fires in survivable accidents. In those four accidents no person was injured by fire. In other words, if the S-76 had been equipped with a heavy, expensive crashworthy fuel system since the first delivery, not one person would have been saved from being burned. The reason that the S-76 has had such a good track record is the use of a suction fuel system, something that has been standard practice at Sikorsky since the early 1970s. Suction fuel systems are also rare even in current helicopter designs. Why they are not mandated by rule and by the customer is beyond me. They work.
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Frank Lombardi’s June column (“A Couple of Things,” page 44), credits gyroscopic precession as one of the phenomena positioning the rotor disc in cyclic control. The rotor system is a rotating system, so gyroscopic tendencies are present; but only to a very minor extent, existing hinge to hinge and insufficient to move the rotor disc.
The rotor system is not a rigid body due to hinging and/or blade flexibility so gyroscopic moments sufficient to move the disc can not be transmitted across the hub. Because such moments can not be transmitted across the hub, the rotor system can not react as a gyroscope.
Rotor disc attitude control is achieved through blade aerodynamics and flapping, and not gyroscopic precession.
Thanks for your great magazine. It contains a wealth of rotary wing info and news. I always read it and leave it in the ready room for other pilots and have done so for more than 30 years!
Commercial Pilot and AGI
MS Aeronautical Engineering
Flight Simulator Instructor
San Diego, Calif.
Chip, you are correct. I inaccurately credited gyroscopic precession when I described how the blades flap to a new position relative to the mast. As you said, it is aerodynamics that provide blade flapping. The phase lag, which most people attribute to gyroscopic precession, can be more correctly referred to as “aerodynamic precession.”
It is the angular difference between the input lift force location and its perceived reaction, and the reason I mentioned as a source of acceleration cross-coupling. The lag angle is dictated by the ratio of how quickly the blade flaps up and down to how quickly it rotates.
This ratio is a function of hinge offset and air density, not angular momentum (i.e., gyro effects). The reference to the rotor acting like a gyro occurs quite often in text I believe, due to the fact that it is a rotating system that seems to exhibit the physical traits of one, and it is simpler for people to grasp the resulting motions. To be a purist, I must stand corrected.
I’m glad to see that Rotor & Wing has insightful readers who take time to provide us all with valuable feedback. It is always welcomed!
Rotor & Wing contributor
Leading Edge columnist
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