Tail wobble

[bugdevheli]

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

[Hilico]

Is this to do with the yellow bits on the tach that you're supposed to wind through as quickly as possible?

[SASless]

Some Tail Wobbling is very enticing.....certainly not on a Robbie though....on a Robbette might be fine however.

[NickLappos]

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.

[SASless]

Nick,

It seems the Huey jumps worse with the wind from the tail as well.....can get downright brutal with a full bladder (pilot's not fuel).

[NickLappos]

Don't I remember, Sasless. That's when the blades flop against the head stops and help drive the vibration even worse!

[SASless]

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?

[NickLappos]

I dunno, Sasless, its been a while since anything went to the firewall or stayed hard....;)

[Lu Zuckerman]

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.

:uhoh:

[BlenderPilot]

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.

[C4]

Hey Blender,
There are more products out there with throttles in the roof than on the collective.. You explain that.. (we are talking turbines here!!!)
:ok:

[B Sousa]

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.

[GLSNightPilot]

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.

[M/V]

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!

[Crashondeck]

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!:ooh: Refused to fly it again until it had been for a thorough check up.

[The Nr Fairy]

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 !