Rotor Track

[slgrossman]

One of the often repeated premises of rotor tracking is that it's not necessary for the blades to fly perfectly in plane (let's discount those rotor systems constructed with stacked hubs). Rather, all that's important is that the vibration level be brought below the allowable threshold.

I've flown aircraft with the blades noticeably out of track (several inches), without an objectionable ride in cruise flight, but it always seemed that there was a vertical vibration in some other regime (on the ground at flat pitch, in a high power climb, low power descent, or steep turn).

Is it really possible to have a properly adjusted rotor system that gives a smooth ride in all regimes, but flies out of track, or is this merely a myth designed to placate us pilots and defer proper maintenance?

-Stan-

[Lu Zuckerman]

To: SL

When I worked on helicopters many moons ago we busted our humps trying to keep the blades in track on the four types supported on our base. That included the tail rotors. Long after that Sikorsky designed a built in track scan system that would allow you top track blades in flight. Many companies have developed systems that allow you to not only track main and tail blades but to balance them as well. With all of this technology available it is up to the maintenance personnel to use them.

I had a ride in a commercial S-61 that had a blade so far out of track that it would jar your teeth. When I got off at LAX I told the mechanic and he said they would catch it at the next periodic maintenance. Three days later that same helicopter lost a blade in flight and several months later a sister ship threw a blade. The blade loss was not due to the out of track but the condition might have accelerated the situation that eventually caused the blade loss.

My philosophy is to minimize any maintenance and or flight problems keep the blades in track.


:ok:

[Notar fan]

SL,
It always seems me that Chapter 18 (Vibration and Noise Analysis) of most manufacturers maintenance manual is usually the least informative of all the chapters. I think that the reason for this is that track and balancing of any rotor system is a specialised area where manufacturers perfer to leave the science of it to balance equipment maufacturers like Chadwick, Helitune etc.
However, Airframe manufacturers usual do specify a maximum allowable track split. Some as low as .25in. In my experience is possible to have a smooth machine in most flight regimes within that limit. There are so many variables in balancing a rotor system. Factors like individual blade balance, play in the flight control system, blade phasing all have to be taken into account to achieve the acceptable balance. There are times where you have to settle for a slight vibration in one regime, because the flight regime you spend most your time in is smooth.
I don't care how smooth a machine is, I would never ever ever ever settle with any track that is INCHES out, unless allowed by the airframe manufacturer. I doubt that this is the case.

[Lu Zuckerman]

To: Notar fan

Sikorsky developed a system where they would pre track the blades and then they would stencil the pre track information on the blade. When a new blade was installed the mechanic would pre adjust the pitch link, which had a calibrated adjustment nut. The mechanic would adjust the pre track adjustment nut to the setting on the blade. This would bring the blade to within ½ inch of the other blades assuming they were already in track. The ½ inch separation was acceptable regarding any resulting vibration caused by the blade being out of track.

As far as balancing the blades Sikorsky would mass balance the blade to establish the spanwise CG. They would then fly the blade against a master blade that had all of the desirable flight characteristics. They would bend the trailing edge of the blade as necessary to minimize the feedback forces and then they would shift the mass balance weight in the tip to minimize the tendency to climb or dive with pitch input until it matched the master blade. The spanwise balance was to within ¼ inch ounce.

If you are flying a Bell the balancing procedure is extremely complex and involved logic diagram where you would perform a task and the logic diagram asked if this solved the problem. If it didn’t you were sent off in another direction and told to perform another task. This challenge / response logic diagram in some cases had over thirty different things to do to arrive at proper balance. In you exited the logic diagram and the balance had not been achieved you were told to install a new set of blades and start over again.

Chadwick Helmuth and Helitune solved a lot of problems. Now the manufacturer just states in the maintenance manual to use these systems to achieve balance. However under certain conditions you would have to install a new set of blades and start over again. So much for progress.


:sad: