Item 0842
OTHER:
Flight Dynamics - General - Cross-Coupling
The off-axis cross-coupled response to an on-axis action.
See Cross-Coupling information at
[Source ~ RWP1 p.459]
"The ubiquitous nature of cross-coupling constitutes one of the chief reasons why piloting this type of aircraft [helicopter] requires such high skill levels developed through long training programmes" ~ Gareth D. Padfield - 1996
"In more distracting circumstances, however, he [the pilot] would benefit if the helicopter control system would make the aircraft quickly do what the pilot wants it to do, and nothing more." ~ R.W. Prouty
Types of Cross-coupling:
[Source ~ RWP1 p.460]Acceleration Cross-coupling: Control Cross-coupling:
Associated with rotor moments.
Rate Cross-coupling:
Associated with steady rates in a maneuver.
Washed-out Cross-coupling:
Appears to be frequency dependent. More typical of augmented rotorcraft. [Source ~ HFD p.419]
"On the other hand, the fixed-base simulation results for the washed-out coupling cases did not match the flight test results for the same configurations. It is speculated that the washed-out coupling configurations were a higher frequency and lower amplitude coupling than the control and rate coupling, and hence the handling qualities ratings would be more influenced by acceleration cues or the lack of these cues. This is corroborated by the fact that the fixed-base simulation allowed higher amounts of coupling than the flight tests results allowed for the same ratings." Source ~ NASA Annual Report FY 1993
The Robinson "
wee-wa" (small washout?) may be a Pitch to Roll instances of this type of cross-coupling.
Instances of Cross-coupling:
Pitch ~ Flap:
delta-3
mean lag angle
(ζ0), due to the rotor torque. [Source ~ HT p.239]Corollas effect is a type of this cross-coupling.
?? Wouldn't this be Flap - Lag coupling?Pitch ~ Lag:
(kpζ)Information;
[Source ~ HT p.xvii] and [Source ~ HT p.658]Flap ~ Lag:
Information; [Source ~ HT p.xvii] and [Source ~ HT p.653], [Source ~ PHA section 4.11]
See:
OTHER: Flight Dynamics - General - Flap-Lead for Intermeshing RotorsPitch ~ Roll:
Generally, the magnitude of the pitch to roll couplings are more severe than roll to pitch. [Source ~ HFD p.417] The symmetry of the rotors should minimize the transmission of roll to the fuselage, BUT may cause serious problems with rotor-rotor interaction. The problem will be that of the advancing blade being lower than normal while, at the same time, the retreating blade is higher than normal. Look into and see how well delta3 might handle this.
Roll ~ Pitch:
[not applicable to UniCopter] The symmetry of the rotors might minimize the transmission of pitch to the fuselage, BUT may cause serious problems with rotor-rotor interaction. The problem will be that of the one blade being tipped forward more than normal while, at the same time, the other blade is tipped back more than normal. Look into and see how well delta3 might handle this.
Collective ~ Pitch:
A sudden increase of collective causes a helicopter (with single rotor; and probably twin lateral rotors) to pitch nose up; a sudden reduction causes it to pitch nose down. [Source ~ CAB] The faster the forward flight the more significant the pitch.. [Source ~ HFD p.419]
See also Torque ~ Pitch below.
Torque ~ Pitch: (aircraft pitch not blade pitch)
All intermeshing helicopters (synchropters) have a torque-pitch coupling. This is due to the lateral axis component of the rotors' torque. An increase in torque will cause the nose to pitch up on a craft, where the rotors are turning inside-forward (breaststroke) [SynchroLite]. Conversely, an increase in torque should cause the nose to pitch down on a craft where the rotors are turning inside-aft [UniCopter].
On the intermeshing Huskie HH-43 there was a nose rise or nose tuck during take off, caused by adding collective or upon unloading collective.
My notes ~ The
obliqueness of the FL-282 is 24º. The obliqueness in the HH-43 is 25º or 26º. The obliqueness of the CVJ+HS rotor is 22º and the obliqueness of the ABC rotor is 18º. Sin 12.5º is 0.216, Sin 11.0º is 0.191, and Sin 9.0º is 0.156. Therefore the nose tuck and rise on the CVJ+HS rotor should be 0.191/0.216 = 88% of that of the Huskie, and the nose tuck and rise on the ARR rotor should be 0.156/0.216 = 72% of that of the Huskie.The location of the HS, which is under the rotor disk on the Huskie, K-Max and SynchroLite, but aft of and above the disk on the UniCopter, will probably have an effect on this pitching. On the UniCopter, the HS might have to be linked to the collective.
Potential Solutions:
Notes: Item 4. and 5. might have the deflector rotateable so that it can assist with yaw.
Both items 5. must consider the results of an engine failure.
The textbook recover from vortex ring state is to reduce collective and add forward cyclic, therefor a small amount of nose down upon the loss of power or VRS should be a good thing.
Torque ~ Roll: (Horizontal Thruster)
May result from the torque of a single pusher propeller.
Aerodynamic modifications such as contouring the fuselage, warping the stabilizers or adding fins might reduce this tendency to roll when the prop torque is increased. It should be noted that the amount of resistance to roll will be proportional to the velocity of the air passing over these 'surfaces. The more that these 'surfaces' are directly in the prop induce flow, the sooner they will react the change in prop pitch and not have to weight for the craft to build forward velocity.
Torque ~ Yaw: (Horizontal Thruster)
The tendency to yaw on takeoff and climb is due almost entirely to the '
Also see second paragraph in Torque ~ Roll: (Horizontal Thruster).
Collective ~ Yaw:
This should not be a problem for intermeshing helicopters. There are two counter-rotating rotors and with the SynchroLite there are two counter-rotating engines. Any change in rotor collective pitch or engine torque should not cause yaw.
Sideslip ~ Pitch and Roll:
If the helicopter tends to roll away from the sideward velocity it is similar to dihedral and it is a positive effect for stability. This sideslip, to the right and to the left, will also cause the nose to pitch down on a conventional flapping helicopter, which May not be the case on an absolutely rigid rotor.
One concern here is the wake of the rotor washing of to one side of the horizontal stabilizer and thereby eliminating the nose up pitching moment. This can probably be handled by aft cyclic stick BUT here again it may cause reduced rotor-rotor clearance.
Yaw ~ Roll:
On single rotor helicopters, a change in the tail rotor's thrust will usually cause a roll since the tail rotor is normally above the helicopter's CG.
On intermeshing helicopters, yaw will be achieved by applying longitudinally opposed cyclic between the two main rotors. Should additional yaw be required, then dissymitry of lift (and drag) between the two angled rotors will be used and this will cause the mast with the greater thrust will assume a more vertical position.
Centrifugal ~ Pitch:
A centrifugal-pitching-couple from the rotation of the rotor that tends to force everything into a flat pitch.
For more see Tennis Racket Effect.
Pitch ~ Plunge:
xxx.
Pitch ~ Cone:
Pitch cone coupling is the characteristic of the rotor to inherently reduce blade pitch with increasing coning under loading which aids to maintain rpm and retard blade stall. With severe rotor loading, the rotor rpm may overspeed above the red line unless collective pitch is increased.
My though ~ When used on a main rotor, as a rotor governor, would the pitch-cone coupling be detrimental to the consuming of rotor inertia when the collective is pulled during an autorotative landing?
Nick Lappos, PPRuNe, 16th November 2004
That coupling (also called delta 3) is very much a ride quality and rotor thrust stability improvement. It basically flattens out the rotor's thrust changes as affected by gusts. This also makes the aircraft seem more dynamically stable in turbulence. It does cause rotor speed increases when the plot commands thrust changes with cyclic, as well. It would have a secondary effect of reducing collective as the rotor coned, but to cone the rpm would have to decay first. I'll bet the effect is secondary.
At the bottom of an auto, the delta 3 is a bit harmful, because it slightly reduces the thrust in the flare, making the pilot need to flare more quickly and to a higher nose angle to extract the energy to stop the descent. This effect is fairly small, however, so it should not dominate.
Related to tail rotors. [Source ~ HT p.786]
Of interest is the Robinson R-44 rotor. On this rotor, increasing the coning increases the pitch.
Picture of R-44 Rotor head
Additional information on Cross-Coupling is available at
[Source ~ RWP4 ch.11] & [Source ~ HFD p.416]
SynchroLite
UniCopter
Related Coupling:
OTHER: Miscellaneous - Thoughtless Ideas - Torque/Pitch Collective Rotor Hub
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