A041

DESIGN: SynchroLite ~ Rotor

Outside Helicopters

Flettner Fl-282:

"A different rotor arrangement - two 3-blade rotors - was tried out on the test bed. It proved to run extraordinarily smoothly, however this was not a consideration for military use."

From 'The Luftwaffe Profile Series N0.9, page 29; "Flights were carried out in wind speeds between 70 and 89 kph; the helicopter's high center of gravity resulted in major speed fluctuations in heavy gusts."

From 'The Luftwaffe Profile Series N0.9, page 29; "The angle between the masts is 24º. The distance between the centers of the hubs is 22.44" (570 mm). The vertical distance between the center of the hubs and the center of gravity is 45.47" (1155 mm)."

Robinson R-22:

References to Information on the R-22 Rotor:

Kaman:

Information and References to Information on the Kaman Rotor:

Kaman - Rotor Hubs

E-mail from E.M. former Huskie pilot: Since, as I recall, the servo flap controlled blade pitch by twisting the flexible blade (which was fixed at the root), the aerodynamic efficiency of the blades must have been very poor. Normally you would want spanwise pitch washout (if I recall the correct term) of approximately 1 degree per foot of blade span, to maximize efficiency, along with blade taper in planform & thickness. My recollection of the HTK is that all control axes had a mushy feel. It did, however, seem to have a stability unlike other configurations, though it's possible that an experienced helicopter pilot might not be the best judge of that.

It would also be nice to get more information on the Kaman synchropter rotors, particularly their lead-lag thinking.
Hummingbird:

The Hummingbird

Chinese Extruded Aluminum Rotor Blade:

http://bbs.chinaflyclub.com/ShowPost.asp?PageIndex=1&ThreadID=6851

Two of many pictures on the above link.

SynchroLite

Characteristics:

OTHER: Helicopter Specification - SynchroLite

Teetering Rotor, Delta3, Knuckle Joint, Coriolis and Phase Lag:

Above is a sketch of 3 rotors. All 3 views are from the tip path axis. All 3 rotors are identical, except for their hub configurations. Assume that all 3 rotor disks have a constant rotational velocity.

Re: Knuckle Joint

If the axes of the 3 masts are inline with the tip path axes, the 3 masts will also have constant velocities.

Should the 3 disks be tipped down at the front (azimuth 180), so that they are no longer horizontal, these 3 rotor disks will still have constant velocities, about their tip path axes.

As for the masts, which remain vertical and are no longer inline with the tip path axes; -

Constant Velocity Joint This mast will have a constant rotational velocity.

Normal teetering Hinge This mast will accelerate and decelerate at 2P, because of the knuckle joint's action. Assume that 2 dots are painted on each side of the mast, at 180-degrees to each other, directly under the 2 blades. The mast (dots) will experience acceleration in the I & III quadrants and deceleration in the II & IV quadrants. The opposite to what the rotor would experience if the mast was rotating with a constant velocity. The maximum teeter of the rotor blades will at azimuths 0 & 180 and the mast will be experiencing its slowest velocity at these two azimuths.

Teetering Hinge w/ Johnson type (a) delta3 ; The rotor disks will rotate at a constant velocity, just like examples [1] & [2]. These blades will also teeter at the same azimuths, just like examples [1] & [2].

Re: Delta3

The difference between [2] and [3] is that because of the Johnson's type (a) delta3 teetering hinge (ie. pivot rotation has an in-plane component), when the blades teeter up or down from the horizontal they will also advance in the direction of rotor rotation. (Since we are assuming that the rotor has a constant rotational velocity, it should be said that the mast will decelerate, when the blades teeter up or down from the horizontal) The blades will:-

in quadrant I ~ be descending to the horizontal and also experiencing in-plane deceleration.

in quadrant II ~ be descending from the horizontal and also experiencing in-plane acceleration.

in quadrant III ~ be ascending to the horizontal and also experiencing in-plane deceleration.

in quadrant IV ~ be ascending from the horizontal and also experiencing in-plane acceleration.

Since the in-plane angular velocity of the rotor cannot change, as indicated in the text above, it forces the angular velocity of the mast to change, at 2P. The mast will experience acceleration in the I & III quadrants and deceleration in the II & IV quadrants.

Result:

If the correct amount of type (a) delta3 is selected, then in each quadrant this changing of the mast's angular velocity because of type (a) delta3 will exactly offset the changing of the mast's angular velocity caused by the knuckle joint's action. In other words; the mast is turning at a constant velocity, irrespective of the amount of flapping.

May 14, 2004 ~ I now believe that the above paragraph is wrong, in that the azimuths of maximum acceleration and maximum deceleration are not the same for [2] and [3]. See the notes and calculations for Lead-flap coupling on OTHER: Flight Dynamics - General - Pitch-Flap Coupling

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Assimilation and Referencing of Information Related to Lead/Lag etc.:

DESIGN: Rotor - Disk - Lead-Lag for Intermeshing Helicopter

OTHER: Flight Dynamics - Rotor Hub - Teetering

OTHER: Flight Dynamics - General - Pitch-Flap Coupling

For doing calculations see: FORM: Rotor - Disk - Delta3

This section has be relocated to its own separate page on the Dragonfly (2-blade Teetering)

Initial Rough Calculations for Rotor: From [Source ~ MDD p.10]

Weight of the blade = W_b = 6.25 + 2.0 = 8.25 [lb]. Includes grip etc.

Rotor rpm = N = 600 [rpm]

Number of blades per rotor = b = 2

Lift of one rotor??? = L =

Radius to the GG of the blade. For blade with tapered spar and plan = r = (D / 2) * 0.45 = (17.33 / 2) * 0.45 = 3.9 [ft]

Normal centrifugal effect = C_F = 0.000341 * W_b * r * N². = 0.000341 * 8.25 * 3.9 * 360000 = 3949 [lbs]

Average coning angle = β₀ = tan^-1 (L / (b * C_F)) = tan^-1 (275 / (2 * 3949)) = 1.99 [degrees].

For the more detailed calculation see; DESIGN: SynchroLite ~ Rotor - Disk - Coning Angle

Normal operating torque per rotor = T = 63,000 * hp / N = 63,000 * (50 / 2) / 600 = 2625 [in-lb] = 219 [ft-lb].

For the more accurate value see; FORM: SynchroLite ~ Power Train (for 2 engines)

Normal operating torque per blade = T_b = T / b = 219 / 2 = 110 [ft-lb]

Response to Cyclic Inputs:

If the thinking is to have heavy tip weights to reduce the coning angle and provide for a slower loss of RRPM then this will result in a slower response to cyclic inputs. Would it be desirable to have offset flapping hinges, in leiu of the teetering hinge, to make the response faster? If this is such a good idea then why has it not already been done? Ground resonance? Another idea might be to give the teetering hinge a type of elastomeric 'spring' centering device.

Scaling Down from 2-blade to 3-blade and taking the GW from 550 lbs Up to 660 lbs:

1 * 1 * 1 = 1

2/3 * 1 = 0.6667 (volume)

0.6667 * (660/550) = 0.8

√³ * 0.8 = 0.9283 (lengths)

Same Page ~ Different Craft: ~ DESIGN: Dragonfly ~ Rotor ~ DESIGN: UniCopter ~ Rotor

Last Revised: July 1, 2008