Stick Migration

[R1Tamer]

Looking at 'a book' to research some homework.

I note an interesting diagram plotting cyclic left/right displacement against airpseed.

It puts the cyclic to the left of centre at the hover.
It then moves right of centre at approx ten knots until approx 55 knots where it becomes approximately centred.
It then returns to left of centre from 55 knots up to presumably Vne

I'm pretty certain the diagram refers to a counter clockwise rotating helicopters

In my very limited understanding a cc rotating rotor will tend to tilt right at lower airspeeds i.e. 20 kts due to transverse flow effect requiring a left cyclic control pressure. As airpseed increases dysymmetry of lift becomes more prevalent but is dealt with by flapping and some fore/aft cyclic to control flapback but no lateral cyclic input to neutralise.

In other words I expected the cyclic to remain left of centre!

In brief terms can anyone elaborate on what other phenomena act on the rotor disc which would cause this cyclic displacement from left to right to left

Am I missing something obvious?

R1tamer

[Matthew Parsons]

There's much more than just a main rotor affecting the balance of forces. Consider also what the fuselage, horizontal stab, vertical stab, etc. are doing to the aerodynamics.

Typically at hover/low speed cyclic is needed to counter roll due to tail rotor, as speed increases, power decreases so less tail rotor power, less roll due to tail rotor and less opposing cyclic. Increase speed even further and antitorque is provided from the vertical stab as it now has enough relative airflow to be effective.

In the end, trying to explain it can be a dice toss. See what the control positions are in trimmed flight and don't worry about them if there's no irregularities. (Even if there are pilots would accomodate small irregularities without noticing).

[Farmer 1]

I wonder how old your book is.

Was it written before we had these modern gismos like vertical stabs and fuselages (not counting Meccano-type jobs), as mentioned by Matthew?

If it is, then it might be about right. It's a long time since I've done this sort of stuff, but some of it rings a very tiny bell. The big crunch comes when you gain translational lift, which happens at 12 knots ("approx ten knots"?). It's a rite of passage to learn exactly when that is about to happen, and act accordingly in order to effect a smooth transition.

As to 55 knots, I think that bit was superseded by more modern and more powerful machines.

Even with an ancient, unsophisticated machine there are many, many variables, and, as already mentioned, we accommodate these irregularities, large and small. Which could in itself answer a lot of other questions you were too polite to ask.

[sycamore]

As has been mentioned by MP andF1, there are a multitude of effects that determine the stick position, some aerodynamic, and others mechanical.
When looking at a `new` helo to determine the `apparent` longitudinal static stability ( ie does stick movement follow convention ie stick fwd-go faster ,etc) one would start at a mid-Cof G,at low speed and fixed collective, and open the envelope by 5-10 kt, over maybe +- 30 kts. Obviously, the a/c will descend as the power is fixed, at the higher speeds, so now one moves up the range and starts at about 40 kts, repeating the speed changes up to Vne, and then do it all again throughout the C of G range.This will then give one a total stick-plot- one would ,with a fully instrumented a/c, also have the lateral stick and yaw positions,TQ, engine parameters, attitudes etc,etc.
If during these tests it becomes apparent that for an increase in speed, the stick is/has stabilised in a reversed position, ie moving in the opposite direction(back) then it is time to quit and have a few beers , get the designer chappie in,and ask for an explanation !!
Of course, all this work cannot be done on a couple of sorties, and with modern simulators, this would have been caught early in the programme, but this was the way we did it when old Nick L, was in short pants, and couldn`t spell hicopleter...
It is also the way to do a quick-assessment, if one does not have access to an instrumented helo and limited time available.

[ShyTorque]

I saw a big pile of sticks at the bottom of my neighbour's garden last weekend.

Now they have all gone, I think they might have migrated, a true sign that autumn is well and truly here.

Round these parts most of them usually stay around till November 5th, so I think it might be a long hard winter this year.

:E

[Matthew Parsons]

subtle....very subtle.

[R1Tamer]

ShyTorque,

Now that ain't too far from the truth. In the space of about 4 hours yesterday Wilma blew threw here and replaced summer with autumnal air temperatures nearly 50 degrees lower.

There was plenty of stick's migrating then for sure. It's the most remarkably quick change of season i've ever witnessed

R1tamer

[Whirlygig]

R1tamer,

hear what you say but in Sheringham, we live with wind every day (and I ain't talking about the other 'arf!). There's nothing stopping this wind from the Urals and head due north, the first land mass you'd hit would be the polar ice-cap!

Sticks migrating? My Triumph Bonneville nearly migrated to Cromer!

Cheers

Whirls

[delta3]

This topic has been discussed indirectly in a number of discussions on rotor flapping, in particular the R22-R44 flapping (search this forum)

One should make a distinction between static equilibriums ie what happens at steady state 0, 5, 10, 20knts etc, and what happens when accelerating from 0 to 20 knts.
'Controllability' means that the steering, in particular the cyclic in this case, correlates well (if not 100%) with the intended control action (forward is forward etc). As part of certification Controllability needs to be good enough.
I showed in earlier discussion that in the case of the R44, the dynamic (side-) effect of the rotor plane tilting forward to create forward trust (whee whaa as Frank Robinson calls this) is pretty well compensated, but not a 100%.

As been stated by Matthew P, it depends on many things. For the part of the dynamics I describe above the precise rotor geometry (coning, stiffness, and compensating controles such as delta3 to name one...) Without the active (see also the piezo flap discussion) controls, it is probabily impossible to design the mechanical control systems in such a way that all static and dynamic disturbances are compensated 100%.

The example you quote could indicate indeed a clockwise rotating rotor, but data on the compensating control mechanisme would be required, because it could also be an over controlled anti-clockwise rotor.....

d3

[R1Tamer]

Thank to all who offered insights as to the possible causes of this observation. Without diving headlong into the text books again some of the concepts talked about here are beyond me.

Suffice to say it seems there's no single obvious factor i'm missing which i'm pleased about. As MP stated - it's probably better just to accept it without wasting too much time on mindmelting theories

Thanks,

R1tamer