B293

DESIGN: UniCopter ~ Control - Flight - Cyclic

Notes:

The Tip-Path-Plane Axis (axis of no flapping) and the Mast Axis (shaft axis, hub axis) will be the same, because there is no flapping with the ARR.

The No Feathering Axis (axis of no variation in cyclic pitch) and the Control Axis (axis of the swashplate, the commanded cyclic pitch plane) will be the same, I think.

Cyclic Stick Movement (Control Sensitivity):

In a conventional helicopter the disk changes orientation within one revolution and the fuselage then follows. If the fuselage takes two seconds to follow and the RRPM is 600 then the rotor will have rotated 20 time. Movement of the cyclic stick with a rigid rotor will cause a very fast response (roughly five times faster). The ratio of cyclic stick grip movement to blade pitch change may be around ten times that of a conventional helicopter.

"It is also observed that a flap frequency above 1/rev significantly reduces the tilt required for a given center-of-gravity offset and hence reduces the cyclic control travel." ~ [Source ~ HT p.248]

Ratio - Stick to Pitch:

My thoughts ~ The rotors" response to cyclic stick changes will probably be quite fast (high control sensitivity) . In addition, there will probably be a need for a fairly range of blade pitch. Would it be advantageous to not have a linear relationship between the cyclic stick movement and the blade pitch change. Consider a large stick:pitch ratio at mean pitch angles. I.e. finer control in the normal operating range, yet a large total range.

Spaced (PPRuNE) also has the same concern and mentioned the same solution.

In addition, he mentioned "The Cheyenne actually had a system where the cyclic control was reduced while the wheels were on the ground."

Pitch-Link Loads:

Prouty [Source ~ RWP4 p.71] states that, contrary to assumption "that because the blade has inertial about its feathering axis, it would mean a resisting load would be produced as the blade pitches up and down each revolution. However ....the cyclic feathering motion with a frequency of once per revolution actually puts the blade into resonance". He says that there are factors that do produce loads, such as aerodynamic damping and pitching moments.

I think that the very high friction of the pitch bearings will significantly effect the loading. Prouty also alludes to the strong possibility that pitch changes at other than one per revolution (HHC & IBC) will produce additional loading.

Cyclic Stick Force:

"Friction prevents accurate positioning of the control because of the extremely nonlinear force gradient it provides for small deflections and because the control tends to jump as the static friction is broken. Friction also prevents self-centering of the control and consequently causes poor follow-up and an increase in the required pilot effort" ~ NACA TN 1799

The friction drag of the pitch bearings and seals will be high, at about 22 in-lbs. See: DESIGN: Rotor - Hub - Pitch Bearings. This increased ratio (mentioned above) will significantly reduce the force that must be applied by the pilot.

Seriously consider Rotor - Hub - Bearings - Pitch - Elastomeric. This assumes that an elastomeric bearing does not have static friction. Or, consider DESIGN: UniCopter ~ Control - Flight - Hydraulic - Schematic

The lateral forces during a longitudinal change may be self-canceling by the two rotors.

In a conventional rotor-control system, oscillating stick forces are due entirely to blade-pitching moments (having both mass and aerodynamic origins). Steady forces may be due to 1/rev. blade pitch moments or to some external source, such as a spring on the control stick. [Source ~ AH p.316]

Rotation of Control Inputs:

The control inputs to both swashplates are rotated 90-degrees, in the direction of rotor rotation, from their location in a conventional (flapping) rotor. This locates the ARR's maximum lift at the azimuth where a flapping rotor would be at its maximum elevation. This is required because the fuselage is being 'levered' into a new attitude, not dragged into it.

Centrifugal Forces:

The result of the centrifugal force is now 90º ahead of the azimuth where the tipping is desired. The centrifugal forces from the two rotors offset each other. Their opposing forces on the frame should be far less than that of the combined aerodynamic forces. This is because, unlike the conventional rotor, the aerodynamic forces (0º phase lag) must work directly against the total inertia of the craft, not just the rotating inertia of rotor.

Amount of Lateral Cyclic:

The inclusion of opposed lateral cyclic will reduce the amount of lateral cyclic that can be applied (i.e. there is a limit on the total lateral cyclic). This should not be a problem the cause the roll moment will be a lot less the pitch moment.

Variation of Forward Velocity:

There will be a need for 'opposing lateral cyclic' to handle the dissymmetry of lift during forward flight. The forward velocity of the craft must be obtained and used to apply a reduction of pitch to the advancing blades and an increase of pitch to the retreating blades. A trim control may also be required, which will set the exact amount of pitch change and thereby remove the indicative vibration.

~ rewritten ~

Dissymmetry of lift will occur during forward flight. It will place heavy oscillatory loading on the drive frame and fuselage. It will probably appear as lateral 3P vibration to the pilot. A mechanical sensor of forward velocity should be considered as a means of applying 'opposing lateral cyclic' to the two rotors. A thumb wheel in the cyclic grip could be considered for fine trimming of this vibration.

There will probably be a need for this sensing of forward velocity to also apply rearward longitudinal cyclic, to handle static stability.

Ref; DESIGN: SynchroLite ~ Rotor - Disk - Opposed (Differential) Lateral Cyclic

Trim Tab for Varying Center of Gravity, Forward Velocity:

The Absolutely Rigid Rotor will provide for a larger range of CG than other conventional rotor heads.
Therefor consider the inclusion of a trim tab (like an airplane). Changing the longitudinal cyclic trim tab to suit a different CG or flight speed (i.e. a different longitudinal location of the center of thrust on the disk) will re-center the cyclic stick, and relieve the force required by the pilot.
In addition, the trim tab will be relocating the above-mentioned 'Ratio - Stick to Pitch' bucket.

Cyclic Stick Lock:

Because of the additional moment caused by the rigid rotor it may be advisable to have a lock to prevent cyclic stick movement when the craft is on the ground.

Comments by Others:

Rigid rotor ships have very small limits on how much cyclic you can put in on the ground, and it's really easy to exceed them inadvertently ~ B.B. on rec.aviation.rotorcraft.

My unqualified thoughts. ~ The rigid rotor will give a faster response to cyclic inputs and this may help. The cyclic stick to blade-pitch ratio may have to be quite high. Will this be detrimental to normal flight?

In fast forward flight it is intended that the rotor disks will have a +/- 4º nose down attitude.

The rotor will be providing +/- 90% of the lift and +/- 10% of the propulsion
The propeller will be providing +/- 90% of the propulsion and +/- 10% of the lift.

Cyclic Control of Pusher Propeller:

Preliminary Conceptualization:

See; DESIGN: UniCopter ~ Pusher Prop - General - Pusher Prop Assist ~ Flight Control

Same Page ~ Different Craft:

B167

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Last Revised: February 13, 2005