Collective PIO
Thread Starter
Joined: Apr 2001
Posts: 113
Likes: 0
From: SoCal
Collective PIO
Nick, if you could, please expand on this subject.
You previously said in another thread: "The collective friction is very important on most helicopters, not just for comfort. It serves as a damper to quell the PIO that can happen. That is also the reason for the friction collar on some Bell rotor heads. Very small helicopters are less likely to get into collective induced PIO because the natural vertical frequency of the airframe is higher than the frequency that a pilot's arm on the collective has. Big helicopters tend to have lower frequencies." This was in relation to the H-53 accident, and the comment that the pilot induced the vibration.
I have never experienced this, other then the usual newbie "I got the spot, no, yes, no.." <img src="eek.gif" border="0">
I am transitioning from 500's to H-3's in February and I am interested on your comments. Of course anyone else is welcomed to dispense their wisdom.
I really hate to add, but Lu, please lets not get into a debate match, I tend to not read any of your comments anyway.
[ 31 December 2001: Message edited by: HeliMark ]</p>
You previously said in another thread: "The collective friction is very important on most helicopters, not just for comfort. It serves as a damper to quell the PIO that can happen. That is also the reason for the friction collar on some Bell rotor heads. Very small helicopters are less likely to get into collective induced PIO because the natural vertical frequency of the airframe is higher than the frequency that a pilot's arm on the collective has. Big helicopters tend to have lower frequencies." This was in relation to the H-53 accident, and the comment that the pilot induced the vibration.
I have never experienced this, other then the usual newbie "I got the spot, no, yes, no.." <img src="eek.gif" border="0">
I am transitioning from 500's to H-3's in February and I am interested on your comments. Of course anyone else is welcomed to dispense their wisdom.
I really hate to add, but Lu, please lets not get into a debate match, I tend to not read any of your comments anyway.
[ 31 December 2001: Message edited by: HeliMark ]</p>
Avoid imitations
Joined: Nov 2000
Aviation Qualifications: ATPL
Posts: 15,110
Likes: 1,083
From: Wandering the FIR and cyberspace often at highly unsociable times
I have experience of this on the S-70 Blackhawk. On that aircraft it occurs at high AUM, and generally at low speed, especially if the aircraft is carrying a heavy USL.
The sequence of events is as follows:
The aircraft begins a gentle vertical vibration at such a frequency as to "bounce" the pilot up and down in hs seat. The vibration is initially quite benign but because the pilot is beginning to bounce, his left arm moves the collective slightly up and down at a resonant frequency.
This unfortunately makes the vibration worse, so the aircraft bounces harder and the pilot bounces more so his arm moves more.... and so on; the phenomena becoming divergent.
The cure is quite easy. The pilot lets go of the collective, as he is inadvertently causing it!
Prevention is simple, be aware of the possibility of the phenomena and apply enough friction to the collective to make it more unlikely for the collective to be "bounced".
The sequence of events is as follows:
The aircraft begins a gentle vertical vibration at such a frequency as to "bounce" the pilot up and down in hs seat. The vibration is initially quite benign but because the pilot is beginning to bounce, his left arm moves the collective slightly up and down at a resonant frequency.
This unfortunately makes the vibration worse, so the aircraft bounces harder and the pilot bounces more so his arm moves more.... and so on; the phenomena becoming divergent.
The cure is quite easy. The pilot lets go of the collective, as he is inadvertently causing it!
Prevention is simple, be aware of the possibility of the phenomena and apply enough friction to the collective to make it more unlikely for the collective to be "bounced".
Guest
Posts: n/a
Shy Torque has it right (except that the gross weight of the machine for PIO varies for each type). Pilot Induced Oscillation -PIO - is not a voluntary thing, it is the interaction of the vertical g force and the pilot's mass, as well as the damping on the collective pitch. Most helicopters have a PIO frequency. The V-22 Osprey and the Bell-222 use a collective pitch that essentially moves fore and aft as a means to prevent this, and the AH-1 Cobra series has a strong coller around the mast to remove this problem.
If too little friction is on the collective, the pilot will note this PIO possibility, so more should be added. Generally, about 3 pounds of friction is just right - enough to prevent PIO and collective creep, but not so much as to interfere with hover comfort.
Some helos have a maintenance setting for collective friction (S-76 and H-53 come immediately to mind) and others rely on the pilot. It is a good idea to train all newbies to respect the collective friction setting as an important thing, and to understand PIO so that if they cross into the PIO region, they know to relax on the control to sort it out (some helos will PIO in cyclic, as well, especially at high speed, where the longitudinal cyclic creates g.)
Sling loads are their own can of worms, as an elastic cable or sling, with a large mass below, can tune to an airframe frequency if the length is exactly right. Usually not an issue for short slings, where the sling frequency is quite high, but as the sling gets longer, it could become a problem. When trying a new sling/weight combination in your type helicopter, check the vertical vibration sensitivity as you first lift the load, and if an undamped vibration seems to exist that doesn't go away, put the load down or pickle it and shorten or double up the sling (this will retune the sling).
[ 31 December 2001: Message edited by: Nick Lappos ]</p>
If too little friction is on the collective, the pilot will note this PIO possibility, so more should be added. Generally, about 3 pounds of friction is just right - enough to prevent PIO and collective creep, but not so much as to interfere with hover comfort.
Some helos have a maintenance setting for collective friction (S-76 and H-53 come immediately to mind) and others rely on the pilot. It is a good idea to train all newbies to respect the collective friction setting as an important thing, and to understand PIO so that if they cross into the PIO region, they know to relax on the control to sort it out (some helos will PIO in cyclic, as well, especially at high speed, where the longitudinal cyclic creates g.)
Sling loads are their own can of worms, as an elastic cable or sling, with a large mass below, can tune to an airframe frequency if the length is exactly right. Usually not an issue for short slings, where the sling frequency is quite high, but as the sling gets longer, it could become a problem. When trying a new sling/weight combination in your type helicopter, check the vertical vibration sensitivity as you first lift the load, and if an undamped vibration seems to exist that doesn't go away, put the load down or pickle it and shorten or double up the sling (this will retune the sling).
[ 31 December 2001: Message edited by: Nick Lappos ]</p>
Iconoclast
Joined: Sep 2000
Posts: 2,132
Likes: 0
From: The home of Dudley Dooright-Where the lead dog is the only one that gets a change of scenery.
On early Sikorsky helicopters (S-55 series) they had problems of vertical oscillations but it was not pilot induced. It was determined that if the pilot did not have sufficient friction on the collective the weight of the flight control system would tune into the frequency of the helicopter movement moving up and down causing the servo valves to displace either in or out depending on the direction of movement of the control linkage. It only took 1-2 thousandths of an inch displacement to get the servo to move. They solved the problem by placing short but very strong springs across the sloppy link and the combined strength of the two springs on the servo(s) was sufficient to offset the mass of the flight control system so the pilot could fly hands off on the collective with minimal friction. A similar problem existed on the S-58 series and this problem was solved by the inclusion of a small force gradient spring in the collective linkage. The spring could be adjusted to support the weight of the collective and the interconnecting linkage to the auxiliary servo pack. The aux servo was fairly close coupled to the upper linkage and the primary servos. If the spring were adjusted too much in one direction the collective would rise and if adjusted in the opposite direction the collective would fall.
On later Sikorsky designs (S-61) they relocated the (Aux) servo system relative to the primary system and I do not know if they maintained the collective adjustment spring.
On the S-55 series they had another control related problem. When the helicopter was flown with the boost off the vertical feedback forces on the lateral linkages was excessive which made for difficult precision control. They installed an aux servo that was powered from the engine oil system. Upon the loss of boost pressure the engine oil powered servo was activated and it dampened out the lateral feedback.
On later Sikorsky designs (S-61) they relocated the (Aux) servo system relative to the primary system and I do not know if they maintained the collective adjustment spring.
On the S-55 series they had another control related problem. When the helicopter was flown with the boost off the vertical feedback forces on the lateral linkages was excessive which made for difficult precision control. They installed an aux servo that was powered from the engine oil system. Upon the loss of boost pressure the engine oil powered servo was activated and it dampened out the lateral feedback.
