Coriolis or correlator?

[nightsta1ker]

Anyone who has ever flown an S300, or braved flight without a governor in any other helicopter knows that with an increase in pitch comes an initial increase in rotor rpm. I was always taught that this was due to coriolis effect on blade coning (center of mass shifts inwards amd the retention of angular momentum creates a momentary increase in rotation speed). It was brought up that coriolis effect is probably not solely responsible for the increase in rpm, and that the correlator may be causing an increase in throttle which leads to a rise in rrpm. I see a disconnect here though. The correlator rolls the throttle OFF when the collective is raised. This would not be necessary if coriolis force only raised the rrpm a few revolutions per minute (as was suggested in the discussion I had). Anyone up to speed on the math have any commentary? I would love to see the equation for the effect of coriolis on the rrpm. My assumption is that with greater speed, the greater coriolis force is on the rotor system, and that just a few degrees of conin can have a significant effect on rpm change.

[Ascend Charlie]

The increase in pitch, as well as producing lift to make the blades cone up, has a far greater effect on increasing drag, slowing the blades down. No correlator, revs will drop. No conservation going on here.

The correlator adds fuel to the system in a coarse manner, and the RRPM go up because it is a bit too effective.

The coning-causing-RPM-increase really only is apparent when flaring, or in auto.

[Peter3127]

Drawing on my massive 9 hours in the 300CBi ....

The correlator indeed increases throttle when the collective is raised. It is a mechanical mechanism with a fixed arrangement between the collective pull and the throttle. Being a fixed relationship it is not perfect in all conditions hence the twist grip throttle is provided to allow fine tuning of RRPM.

It is worth noting that a correlator differs from a governor in that a correlator uses collective position to control throttle (the control input is the collective lever position), where as a governor uses throttle to control RRPM (the control input is the RRPM).

Increasing the collective will increase TRT and the coning angle as the blades flap up until the increased blade forces come into equilibrium with the centripetal force. Conservation of angular momentum would mean an increase in angular velocity (hence the lead lag hinge). In reality however the increased rotor drag means that more power is required.

I chose the 300CBi to get exactly this experience. It can be a bit overwhelming at first for sure but I think it is a good fundamental skill to have. Others may disagree. Something I worked out that helps a whole lot, particularly in initiating climbs and descents is this - before starting the climb/descent glance at the RRPM to ensure it is where you want it then consciously listen to the RRPM. Then eyes outside and fly the manoeuvre. Your ears will tell you exactly what is required. Otherwise I found the RRPM would be changing and I had to look back inside to the RRPM guage to see what was happening and would end up chasing RRPM, attitude etc. A tip to be considered or discarded.

As you can imagine after 9 hours I am as smooth as silk in all phases of flight .... :}

[topendtorque]

I would put it this way;- If your collective pitch pull is aggressive then it will induce corolois effect, if it is not aggressive then the centrifugal force of the blades mass (as a force in balance against the aircraft weight) will resist upward coning.

Have a look at a fixed wing aircraft;- pull the stick back slowly the aircraft attitude changes and the aircraft climbs gracefully without any perceived wing bending. Pull it back violently and the blessed wings come off upwards. (the strength of the wing structure resists bending up to the engineered design limit)

In reality you should only need to think about Corolois effect as a very useful tool to make it work for you in unpowered flight. (a lot less centrifugal force makes it easier to use)

The drag AC talks about happens after the corolois bending.

Getting a correlator (throttle or cam box) to work well is an art and on some models after a few hours wear you might wonder why they are even there.

[nightsta1ker]

You guys are absolutely right. Thanks for the input. I was getting some bit of info mixed up in my head.

[Shawn Coyle]

A more correct term for the correlator might be 'analog randomizer'.