1020

Item 1020

OTHER: Aerodynamics - Vibration - Rotor Induced - Analysis of Single Rotor Helicopters

Vibration encountered by helicopters with a single main rotor. For helicopters with twin rotors see 1089.html

Source - Downwash on Fuselage: Aerodynamic forces acting on the fuselage and other non-rotating parts of the craft.

N per revolution [nP] vibrations

ID	Possible Source of nP Vibration:	Hover:	Transition: μ ≈ 0.1?	High Speed
1/	The downwash from the main rotor striking the cockpit. ⁽¹⁾	Yes	Yes	Yes
2/	The downwash from the main rotor striking the tail boom. ^{(1) (2) (3)}	Yes	Yes	Yes
3/	The downwash from the main rotor striking the horizontal stabilizer. ^{(1) (2) (3) (4)}	^{(3) (4)}	^{(3) (4)}	^{(3) (4)}
4/	The swirl of the main rotor's downwash striking the vertical stabilizer. ^{(1) (2)}	^{(3) (4)}	^{(3) (4)}	^{(3) (4)}

Notes re Above:

(1) It would appear that 2/ and 3/ should be most pronounced during hover. One gentleman states that there is very little movement the cabin absorbers during hover and therefor the tail's contribution would so negligible as to be insignificant. CTD mentions that on the Bell 412 and 407 it is noticeable.

(2) On the SynchroLite configuration, consider the alternating swirl from the two counter-rotating rotors.

(3) It would appear that 2/ and 3/ could be most pronounced during hover, depending on the location of the stabilizers.

(4) A 'T' tail combined with RRR (UniCopter) should negate any chance of vibration.

For specific information see: DESIGN: UniCopter ~ Vibration Analysis - Rotor Induced - Downwash on Fuselage

Source - From Individual Rotor via Hub: Aerodynamic forces acting on a (single) rotor and transmitted via the hub.

1P vibration:

ID	Possible Source of 1P Vibration:	Hover:	Transition: μ ≈ 0.1?	High Speed
5/	Mass imbalances. ⁽⁶⁾	Yes	Yes	Yes
6/	Trim tab faults. ⁽⁶⁾	Yes	Yes	Yes
7/	Pitch-link miss-adjustments.	Yes	Yes	Yes

N per revolution [nP] vibrations are most noticeable in transition and at high-speed flight regimes.

ID	Possible Source of nP Vibration:	Hover:	Transition: μ ≈ 0.1?	High Speed
8/	The lift may be equal between the advancing and the retreating sides but the drag will not, because changes in blade pitch do not result in an equality between changes to the coefficient of lift and the coefficient of drag.~[my thought]. The asymmetry of the rotor system in forward flight.		?	Yes
17/	2P variation in H-force when the blades on a 2-blade rotor are at azimuths of 0º & 180º vs. 90º & 270º.	No	?	Yes
9/	If the blade has twist, then the spanwise center of lift will be further outboard on the retreating blade then it will on the advancing blade. [Source ~ RWP5 p.52] ⁽³⁾	?	?	Yes
10/	Retreating blade stall [Source ~ NASA/TP below]	No	No	Yes
11/	Compressibility effects on advancing tip. [Source ~ NASA/TP below]	No	No	Yes
12/	The blades striking the trailed vortices of other blades (Blade Vortex Interaction) [Source ~ NASA/TP below] ⁽⁵⁾	Yes (2 rotors)?	Yes	Yes (2 rotors)?
13/	When the axis of the tip path plane is not inline with the mast's axis, an oscillating dynamic load is transferred to the mast [Source ~ XXX p.xx] (ref: Cyclic Coriolis and Hooke's Joint). ⁽¹⁾ Delta3 may eliminate this, when the rotor is at mean thrust. At other than mean thrust, the varying radius of gyration may cause vibration.	?	Yes	Yes
14/	Atmospheric turbulence. Gust. ⁽⁸⁾	Yes?	Yes	Yes

From Introduction of NASA/TP-1998-207687

In forward flight, asymmetrical airflow through the helicopter rotor causes large vibratory forces and moments [8/] to be generated on the rotor blades. The blade loads may be additionally affected by the presence of rotor blade stall [10/], shock waves (compressibility effects) [11/], or by the blades striking the trailed vortices [12/] of other blades at one or more places around the rotor azimuth. For an N-bladed rotor, these harmonic air loads produce large oscillatory blade-root shear forces and bending moments which are experienced as N-per-revolution (N/rev) vibration in the fuselage reference frame. This vibration degrades the ride quality of the helicopter and shortens the life of critical rotor hardware.

Notes re Above:

(1) The UniCopter's rigid rotor system should eliminate this source of vibration (If it is a source of vibration. Maybe only for teetering rotor). N.L.says that changes in the longitudinal CG have little effect on this vibration and he therefor feels that this is not a significant source of the vibration.

(3) This may work well for the ABC concept since a small amount of ABC may result in greater lift on the advancing side but an equality of rotation about the longitudinal axis. (ie. heavy daddy and light son on a see-saw)

I think that [12/] is localized at one or two of the blade's segments and therefor a stiff blade may cause the aerodynamic dampening of other blade elements to reduce the amplitude of the vibration, which reaches the hub.

(5) N.L. states that " I believe that translational vibration is mostly due to aerodynamic disturbance of the blade by the previous blade wake, and is especially a problem on some helos during approach when the descent allows more BVI (Blade Vortex Interaction). [12/]"

(6) With the ARR, the static balancing of the blades and the hubs, plus the dynamic balancing of the rotor assemblies, should eliminate vibration caused by inertial dissimilarities.

(8) The more rigid the rotor, the greater the disturbance. The greater the forward speed, the greater the disturbance.

Source - From Individual Rotor via Pitch Links: Aerodynamic forces acting about the blade's feathering axis

ID	Possible Source of Vibration:	Hover:	Transition: μ ≈ 0.1?	High Speed
15/	For any number of blades 1/rev. pitching moments produce steady stick forces.
16/	For a rotor with n blades, periodic stick forces of frequencies n /rev., 2 n /rev., 3 n /rev. etc, may occur due to blade pitching moments which vary with frequencies (n+1)/rev., (n -1)/rev., (2 n +1)/rev., (2 n -1)/rev., etc.

For a concise overview see: [Source ~ AH p.316]

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Last Revised: February 3, 2007