When I first started learning about helicopters, the one thing that I remember being taught was that verticals were bad.
Over time, as stated above, I learned that a vertical was really a tracking problem in the normal course of engineering. The worry about verticals came from a couple of problems with the masts of our KH4's. On one memorable occasion one of our pilots took off from a property and shortly there after started getting a 'bounce'. This bounce was described by the pilot as a short sharp bounce that was not a resonant type of movement (hard to describe I guess). He landed the aircraft and inspected it looking for what ever he could. Nothing was found so he took off again. Now this is where the crash comics would of had a field day, but fortunately for him he got it back down. When he took off the collective jammed. For about 1.5 minutes he tried to get the collective down while the bounce continued to manifest itself.
When the engineers flew out to the aircraft they found that the mast had a crack that covered 80% of the diameter. The only reason that he is alive today is that the crack was situated under the collective sleeve. When it finaly gave way the mast actualy twisted to a figure eight shape, when viewed from above, and jammed against the sides of the collective sleeve. Lucky for him it held till he got it back down.
Anyway it spooked a few people. But it got us all talking about the types of bounces etc that you can get in these types of helicopters. Because of this discussion, I believe it saved the life of another pilot.
He was flying a B47 on stream sampling work when he felt a slight lateral. It was the first flight of the day so he assumed that over night condensation had acumalated in the blade that was tied to the tailboom. (As Lu suggested, this does happen. In the dusty parts of Oz, it can be quite dangerous as the dust accumalates in the blades throught the tip cap. Mixed with water it runs down to the lowest point usually at night with condensation. If it dries and solidifies the machine slowly goes out of balance. Worst case scenario, it can move as a clod in the blade.) Over about 25 minutes the bounce became worse. At this point he bailed out of the job and called the engineers.
To get an increasing bounce means trouble - if you get a bounce for no reason - land.
The young engineer at the time didn't seem overly concerned as the bounce was not that big, he even went on to say fly it back to base as it was due a service anyway. This is where the pilot should probably be applauded cos he said no (a couple of times).
An engineer was sent to the machine. The pilot had a mind set that there was a crack in the mast. The stab bar was changed, the bounce got worse. The mast was changed and the bounce got worse. The swashplate was changed and the bounce got worse. Finally the head was changed and all was fine.
On inspection it was found that the yoke where the blade grip is attached had a crack that ran across two threads and around the yoke covering over two thirds of the radius. When he raised the collective it was literally opening up the crack and allowing one blade to rise. Now this sounds like a tracking problem but it was not felt as a vertical - it was a lateral. I can't help but think that if they had of ran the machine for another 10 minutes the blade would have been seperated as there was absolutely nothing else holding that blade on but an inch of metal. Now that is scary 5hit !!
Long winded I know but it proves the worth of other peoples experiences. The moral I guess is that if you get a bounce in flight there has to be a reason. It could be just blade tape moving or moisture etc, but even so land the machine to find out. You must then be able to tell an engineer (usualy over the phone) what the bounce felt like - either a lateral or a vertical.
You gotta love stories.
cheers.