pylon rock oscillation
Checking the Tranny mounts can be done on the ground during the preflight as well....takes two folks to do it properly. One makes like King Kong and shakes the head fore/aft and left/right...while the other guy feels for movement within the mount/bolts and gearbox flanges. Doing a climbing turn will also point out any mount problems by generating a shuffle sometimes.
As previously described....stirring the stick at a hover or in slow forward flight works as well.
A loose lift link caused by worn bearings will cause some problems as well. Have seen some bearings fall slap out of the link when the bolts were removed. The problem disappeared with the installation of new bearings.
(Bell 204/205/212/412/UH-1A,B,C,D,H,K,L,M,N models)
Question:
Why do most "Huey" pilots fly with all of the cyclic friction removed....but Sperry requires a "pre-set" minimum friction when their AFCS is installed?
Does it not make sense for some friction to be left applied ala Sperry.....and thus dampen out some of the oscillations that will be caused by "feedback" from out of balance condidtions and PIO?
As previously described....stirring the stick at a hover or in slow forward flight works as well.
A loose lift link caused by worn bearings will cause some problems as well. Have seen some bearings fall slap out of the link when the bolts were removed. The problem disappeared with the installation of new bearings.
(Bell 204/205/212/412/UH-1A,B,C,D,H,K,L,M,N models)
Question:
Why do most "Huey" pilots fly with all of the cyclic friction removed....but Sperry requires a "pre-set" minimum friction when their AFCS is installed?
Does it not make sense for some friction to be left applied ala Sperry.....and thus dampen out some of the oscillations that will be caused by "feedback" from out of balance condidtions and PIO?
Join Date: Nov 2005
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pylon rock
Thanks 212man. Had a pilot a few year ago who thought he could teach a huey to skate. Came in on a running landing on wet grass and didn't want to slow down real well. Pulled a little collective back on the cyclic to slow down just as one skid sank into soft ground causing the aircraft to start to go sideways. The aircraft continued for a little bit more before tilting up on one skid and falling down (at least he stayed upwright). The crewman in the back seat had a good view of the event. Anyway, he called it a heavy landing and we started on the inspections. First thing we found (very noticeable) was that every txmn mount had been crushed to such a degree that all the big disks (part of the mounts ) had been concaved like saucers, we gave the pilot one for his effort! Another situation we had was that one pilot got into such trouble that when he returned the aircraft to us, the upper wire strike had actually cut the bottom of both blades! Don't try this at home! The same pilot was going to get an award for a rescue but was denied because after the rescue (not the same day!) he suffered a blade strike on trees while recovering crab pots with the winch.
Join Date: Apr 2003
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SASLess,
The friction is needed whan the sas is on (when you are not sasless....) because the sas actuators have to push against the friction to work the servos. If you have sas on with a fully limp noodle cyclic, some portion of the sas actuator motion will feedback toward the pilot, and that motion will be lost. This makes the sas very much less effective (it throws away about 20% of the total sas motion) and it also produces a slight and objectionable stick throbbing (the only throbbing folks of your age will ever again see). We call that "stick nibble".
Pylon rock is an artifact of the Huey family, as is collective bounce, both more prevelent in some models than in others. Pylon rock is basically a matching of the looseness of the transmission structure with a natural rotor in-plane motion. The transmission makes a slight fore and aft rocking motion that the rotor feels and amplifies, a sort of "air resonance". It is a whirl mode of the rotor that pumps the structure, and it nags the 540 system especially. It is often called the 'Huey Shuffle" and has a distinct low frequency beat as it pumps the pilots fore and aft at about 3 times per second.
Loose components can help make it big enough to be objectionable, so the advice above about mounts and lift links makes sence. In the Snake, the transmission had dampers built into its aft mounts, so the rocking of the transmission was absorbed (like the lag dampers on an articulated rotor blade). Also, the sas on a cobra had an input that came from the transmission (there were electronic transducers that read the pylon rock motion, and inserted that signal into the pitch sas so the sas would wipe the motion out by rocking the cyclic to cancel it.)
Why does the Huey have pylon rock and few other machines? Because it has no lag damping at all, so there is no resistance to the whirl mode at all, and that big heavy rotor system on the high mast has a ton of inertia that makes the mode frequency so low that it is easily felt by the crew. Also, the Huey is designed to ancient, very poor structural requirements, so that transmission is held in by very little structure. This makes it rock fairly easily, on a loose mounting structure. Before all the Huey-lovers jump on me, please recall what happens in a Huey if the rotor hits something solid - i. e. which crew member gets the transmission in his lap. Modern helos have much stronger transmissions, which stay in place when the rotor has the misfortune to hit something.
The friction is needed whan the sas is on (when you are not sasless....) because the sas actuators have to push against the friction to work the servos. If you have sas on with a fully limp noodle cyclic, some portion of the sas actuator motion will feedback toward the pilot, and that motion will be lost. This makes the sas very much less effective (it throws away about 20% of the total sas motion) and it also produces a slight and objectionable stick throbbing (the only throbbing folks of your age will ever again see). We call that "stick nibble".
Pylon rock is an artifact of the Huey family, as is collective bounce, both more prevelent in some models than in others. Pylon rock is basically a matching of the looseness of the transmission structure with a natural rotor in-plane motion. The transmission makes a slight fore and aft rocking motion that the rotor feels and amplifies, a sort of "air resonance". It is a whirl mode of the rotor that pumps the structure, and it nags the 540 system especially. It is often called the 'Huey Shuffle" and has a distinct low frequency beat as it pumps the pilots fore and aft at about 3 times per second.
Loose components can help make it big enough to be objectionable, so the advice above about mounts and lift links makes sence. In the Snake, the transmission had dampers built into its aft mounts, so the rocking of the transmission was absorbed (like the lag dampers on an articulated rotor blade). Also, the sas on a cobra had an input that came from the transmission (there were electronic transducers that read the pylon rock motion, and inserted that signal into the pitch sas so the sas would wipe the motion out by rocking the cyclic to cancel it.)
Why does the Huey have pylon rock and few other machines? Because it has no lag damping at all, so there is no resistance to the whirl mode at all, and that big heavy rotor system on the high mast has a ton of inertia that makes the mode frequency so low that it is easily felt by the crew. Also, the Huey is designed to ancient, very poor structural requirements, so that transmission is held in by very little structure. This makes it rock fairly easily, on a loose mounting structure. Before all the Huey-lovers jump on me, please recall what happens in a Huey if the rotor hits something solid - i. e. which crew member gets the transmission in his lap. Modern helos have much stronger transmissions, which stay in place when the rotor has the misfortune to hit something.
