Tail wobble
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Joined: Feb 2003
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From: ON A HILL
Tail wobble
Is there a technical man out there who can tell me why Robinson tail booms always have that liitle wobble now and again during run up. my guess is, the tail rotor drive tube goes into sympathetic resonance. Id like to know cus its a bit bloody scary
Joined: Apr 2003
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From: USA
bugdevhelo,
The wobble is probably not the shaft, but the actual tail cone,where the natural frequency of the structure is tickled by the rotor rpm at a certian speed on runup. Just walk up to the aircraft while parked (and shutdown, of course!), and shake it by the tail to see if it wiggles that way at that frequency. Most likely, you are seeing what many helos have at odd rotor frequencies. I recall that in Hueys you can hardly fill out the paperwork while at idel rotor, due to the vertical hop in the cockpit, due to the natural fuselage mode.
The wobble is probably not the shaft, but the actual tail cone,where the natural frequency of the structure is tickled by the rotor rpm at a certian speed on runup. Just walk up to the aircraft while parked (and shutdown, of course!), and shake it by the tail to see if it wiggles that way at that frequency. Most likely, you are seeing what many helos have at odd rotor frequencies. I recall that in Hueys you can hardly fill out the paperwork while at idel rotor, due to the vertical hop in the cockpit, due to the natural fuselage mode.
Joined: May 2002
Aviation Qualifications: ATP+Mil
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From: Downeast
The Sikorsky products are definitely more Pilot friendly than the Bell things.....excepting maybe those spongy brakes on the 58T after the tail wheel lock breaks! Why is it....the old girl always wanted to slew around the strong brake no matter which way the wind was blowing? How many times did you hear a "snap", feel the tail start swinging.....cram on the brakes and one of them....and always the one you needed....went to the fire wall while the other one stayed hard?
Iconoclast
Joined: Sep 2000
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From: The home of Dudley Dooright-Where the lead dog is the only one that gets a change of scenery.
Sym Pathetic vibrations
On the S-58 on startup you had to go through 1300 engine RPM as fast as practical or you would tear the cooling fan apart. The Robbie like most helicopters and some airplanes have some point at which vibrations from one element in the power train could excite another part to destruction.
Joined: Feb 2002
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From: N20,W99
Pilot Friendly Sikorsky?
"The Sikorsky products are definitely more Pilot friendly than the Bell things....."
I guess you mean the roof mounted throttles? The ones you have to find while corkscrewing down to the ground at the worst possible time.
I'm sure Sikorskys are great machines but so are Bell's and EC's, they all have their up's and downs.
"The Sikorsky products are definitely more Pilot friendly than the Bell things....."
I guess you mean the roof mounted throttles? The ones you have to find while corkscrewing down to the ground at the worst possible time.
I'm sure Sikorskys are great machines but so are Bell's and EC's, they all have their up's and downs.
Gatvol
Joined: Jun 2000
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From: KLAS/TIST/FAJS/KFAI
Im certainly not going to compete wtih Nick or Lu on Technical reasons. Too me its been sort of simple. Lots of parts at various speeds. Engine, Transmission, Main Rotor, Tail Rotor and tail Rotor Drive train. Has to be Balance and Resonance problem at various speeds.
I do recall picking up a Cobra from Gulfport many years ago for a long cross country. The Tail Rotor was so out of balance I thought the tail boom was going to break off. It took the Mainteneance folks about four sets of blades and a bunch of weights to get it flyable.....
What is it someone recently said. A Helicopter is a massive amount of Metal Fatigue circling an Oil Slick.
I do recall picking up a Cobra from Gulfport many years ago for a long cross country. The Tail Rotor was so out of balance I thought the tail boom was going to break off. It took the Mainteneance folks about four sets of blades and a bunch of weights to get it flyable.....
What is it someone recently said. A Helicopter is a massive amount of Metal Fatigue circling an Oil Slick.
Joined: Jul 2002
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From: Texas
I once had another pilot ask me if my 206L was happy. He said it looked like it was, since it was wagging its tail. Some balance work & new transmission fittings fixed it. 2-bladed designs seem to do this more than others, due to the lower-frequency vibrations caused by fewer blades, plus the vibs are transmitted more directly into the fuselage. I also remember an un-named 4-blade type that had a severe ground-bounce, both at idle & full throttle, that maintenance couldn't fix. It was finally fixed during an inspection, when the tail rotor was checked & found grossly out of balance. Balancing the tail rotor completely cured the ground bounce.
Last edited by GLSNightPilot; 23rd June 2003 at 10:34.
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From: Eastcoast
From memory, tailwag on the B206l is can be eliminated by correct centering of the trunnion within the hub. Correct weights on hub assy. and making sure blades are aligned correctly will make for a smooth ship. Watching Robinsons doing a run-up/maneto check always made me nervous that the tail-cone would crinkle and break off!
Joined: Apr 2003
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From: North of the Border
I've noticed the tail wag on run up as well. I was taught it was resonance, hence the yellow arcs on the tachos.
I had one particular Robby wag its tail in the hover. Not all the time, but every now and again it would shudder. Sent shivers down my spine!
Refused to fly it again until it had been for a thorough check up.
I had one particular Robby wag its tail in the hover. Not all the time, but every now and again it would shudder. Sent shivers down my spine!
Refused to fly it again until it had been for a thorough check up.
Joined: Nov 2000
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From: Pewsey, UK
A student PPL(H) at Booker managed to get the tail boom to vibrate so much in sympathy that it fell off.
He was wearing an ANR headset and wound it up to about 134% if memory serves me right. And I thought MY overspeed was bad !
He was wearing an ANR headset and wound it up to about 134% if memory serves me right. And I thought MY overspeed was bad !
