DESIGN: SynchroLite ~ Rotor - Hub - 2-blade Teetering

Outside Helicopters:

Semi-Rigid Rotor

Web page by Paul Cantrell

Rotor Hub Pictures

Pictures of many different helicopter rotor hubs by Burkhard Domke.


Hand-held weight of about 25 lbs. Ultrasport says complete Main Rotor weighs 45.13 pounds; and this would mean that the blades are 10 pounds each.

Preconed at 3 degrees.

The main rotor hub is all aluminum.

CNC machined from 2024-T4 aluminum billet.

Has a rubber shaft bumper that allows startup and shutdown in winds up to 15-20 knots.

See below note on Tension-Torsion Pack.


The Indian hub plate is 2024-T3 aluminum )with alternative of 6061-T6); with the grain running lengthwise.

The Indian rotor hub will allow 19.4 degrees of positive (and negative) pitch. See drawing 0285. 

Mini 500 (Talon) Revolution Helicopter Corp.

Rotor hub is machined from solid billet 6061 T6 aluminum.

Rotor spindles are machined from forged 4130 chromolly steel.

Articulation is teetering, semi-rigid.

Pre-cone angle is 2 degrees.

There appears to be 12 bolts around the hub rim. 


"The lead/lag adjustment is typically Bell Jet Ranger in style. In fact, the Helicycle rotor hub was designed to be a look alike and functional sibling of the classic high quality Jet Ranger hub."

Baby Belle

Titanium main rotor spindles. 

Robinson R-22 

The aerodynamic center is probably at 25-27% chord but it has been mentioned that the attachment point is at 40-45% chord.

The R-22 hub. The blade-feathering axis is 0.28" ahead of the mast's center of rotation, to reduce the moment carried by the pitch change bearings.

The two yokes appear to have a 'flat bar' extension on one side of each that is located under the lower mast bolt and serves as a droop stop. See page 9.14 in Robinson maintenance manual.

It strongly appears that the pitch link on the Robinson R22 is inline with the coning hinge. The delta3 [δ3] angle is 180 and the coning hinge offset is 2'. This will put the pitch link 6.16" off the blade span axis.This means that the delta3 works on teetering hinge but not on the coning hinge. This is by control system geometry not by flapping hinge geometry [See ~ HT p.239].

March 10, 2007 ~ The information in the preceeding paragraph and the layout sketch below may not be correct. The picture Robinson ~ Main rotor hub and swashplate appears to show that the pitch link is inboard of the coning hinge. Since it is on the blade's leading edge, coning will add pitch.

It also looks like the pitch link is vertical and this would mean the phase lag angle is at the 180 delta3 location. This is because the offset flapping hinges give a frequency ratio below 1 and therefore the phase angle will be less than 900. In fact, the pitch link must be vertical so that the 180 delta3 will be matched by the required (90-18)=720 phase angle.

I believe that the side hinges on the R-22 hub are coning hinge, and should not be considered as flapping hinges. This is because the hub yoke assembly teeters and therefore there is no such thing as a flapping hinge offset. These two hinges were probably included to; minimize friction and breakout forces (dynamic & static friction) in the pitch bearings, and to reduce the out-of-plane bending of the blades.

R-22 ROTOR SYSTEM ~ Posting by Frank Robinson on www,pprune.org ~ Nov. 29, 2000.

I have read various explanations in this forum attempting to explain the dynamic and aerodynamic characteristics of the R22 rotor system, especially the 18-degree delta-three angle designed into the R22 swashplate and rotor hub. This is a highly technical subject which can only be fully explained using very technical engineering terms. However, since there appear to be a number of misconceptions and a great deal of interest by some pilots and mechanics, the following is a physical explanation of the reasons for the 18 degree delta-three phase angle. First, keep in mind that the 18 degrees is only in the upper rotating half of the swashplate. The lower non-rotating swashplate is aligned with the aircraft centerline and always tilts in the same direction as the cyclic stick. Many helicopter engineers have difficulty understanding how delta-three (pitch-flap coupling) affects the phase relationship between the rotor disc and the swashplate. Delta-three only affects the phasing when the rotor disc is not parallel to the swashplate and there is one-per-rev aerodynamic feathering of the blades. For instance, feathering occurs while the rotor disc is being tilted, because an aerodynamic moment on the rotor disc is required to overcome the gyroscopic inertia of the rotor. But once the rotor disc stops tilting, the rotor disc and swashplate again become parallel and the delta-three has no effect on the phasing. Aerodynamic feathering also occurs in forward flight, because it is necessary to compensate for the difference in airspeed between the advancing and retreating blades. Otherwise the advancing blade would climb, the retreating blade would dive, and the rotor disc would tilt aft. The R22 rotor system was designed with 18 degrees of delta-three to eliminate two minor undesirable characteristics of rotor systems having 90-degree pitch links. In a steady no-wind hover, when forward cyclic pitch is applied, the 90-degree rotor disc will end up tilted in the forward direction, but if no lateral cyclic is applied, the rotor disc will have some lateral tilt while the rotor disc is tilting forward, sometimes referred to as "wee-wa." This occurs because while the rotor disc is tilting, the forward blade has a downward velocity and the aft blade has an upward velocity. This increases the angle-of-attack of the forward blade causing it to climb, and reduces the angle-of-attack of the aft blade causing it to dive. If no lateral cyclic was applied, this would result in a rotor disc tilt to the right while the rotor plane was tilting forward. Pilots subconsciously learn to compensate for this by applying some lateral cyclic as the cyclic is being moved forward. The amount of delta-three required to eliminate "wee-wa" in the R22 rotor system was calculated to be 19 degrees. The other undesirable characteristic in rotor systems having 90-degree pitch links is the lateral stick travel required with airspeed changes during forward flight at higher airspeeds. The ideal rotor control system would require only longitudinal stick travel to increase or decrease the airspeed. This is not possible with a 90-degree pitch link system, because the rotor coning angle causes the rotor disc to roll right as the airspeed increases. This occurs because the up-coning angle of the forward blade increases that blade's angle-of-attack with increased airspeed, while the up-coning angle of the aft blade reduces its angle-of-attack. Consequently, the forward blade then climbs while the aft blade dives, thus causing the rotor disc to roll right with increased airspeed. To compensate for this with a 90-degree pitch link rotor, the pilot must apply some left lateral cyclic as the airspeed increases. The amount of delta-three required to compensate for this effect in the R22 rotor system was calculated to be 17 degrees. A delta three angle of 18 degrees was selected as the best compromise angle to reduce or eliminate the two undesirable characteristics described above, which would have been present in the R22 had a 90-degree pitch link design been used. Subsequent instrumented flight test data confirmed the choice of the 18-degree delta-three angle. Hopefully, this will help clarify a few of the misconceptions concerning the design of the R22.

Layout of Robinson's Rotor:

Lu Zuckerman's Summation of His Position Regarding the R-22:

The other beef with the R22/R44 was the design of the rotorhead required an 18-degree offset when rigging the helicopter which with a 90-degree phase angle would cause the helicopter to fly left. I further contended that with the necessary cyclic input to counter the effects in inflow roll and possible blow back when the cyclic was finally at rest it would be to the right of the rigged neutral setting of the cyclic stick.

I further stated that with the cyclic being offset to compensate for the 18-degree offset at rigging the pilot would be placed in jeopardy if he /she encountered a zero G maneuver and he/she complied with the POH and pulled the cyclic straight back. In doing so the offset cyclic would cause an exacerbation of the right roll component and the pilot could possibly lose the helicopter.

If the Robinson is flown with care it can function as well and in some cases better than any other small helicopters. However, the built in design problems can kill you a lot faster than any other helicopter.


A direct copy of all the postings on a related thread in the Rotorcraft Conference. B185 Supplement.html

A relevant portion of this, and one other thread.

Padfield's book and a referenced report say [i]"... washed-out coupling, which can occur in helicopters with feedback control systems"[/i]. I believe that 'delta-3' is considered as a 'feedback control systems'.

This raises the circular question; was delta-3 incorporated to minimize washed-out coupling, or did the incorporation delta-3 cause washed-out coupling?

The following two quotations are from separate posts by Chuck Beaty, a gentleman with considerable technical knowledge about rotorcraft. They should provide additional information.


"Delta-3 coupling binds the rotor more tightly to the mast position, perhaps a good thing for tail rotors but a bad thing for main rotors."


"Subject: Wee-wa

Bramwell in "Helicopter Aerodynamics" after tossing about some fancy math, derives some fairly simple expressions for cross coupling.

The rotor tilt in the commanded direction is proportional to: 16 * (tilt rate)/(angular velocity) * (1/y)

y is the mass constant of the rotorblade (Lock #), the ratio of aerodynamic force to mass. High inertia blades have lower Lock # than the other way around. Angular velocity is that of the rotor, everything being in radians/sec.

The tilt in the crosswise direction is: (tilt rate)/(angular velocity)

What this says is that cross coupling might not be noticed in a Bell or Hiller but might be a problem with a Robinson.

Irrelevant sentence snipped

I've met Frank Robinson and remember having a discussion with him about his use of delta-3 coupling back when he was still working on the R-22 prototype. He's a very competent engineer and wrote a number of papers about tail rotors while he was a project engineer at Bell-Textron but I suspect tail rotors gave him his fixation about delta-3 coupling where suppression of cyclic flapping is considered desirable.

Mr. Robinson is not only president and chief engineer of Robinson Helicopter but is also chief promoter and head salesman. When forging dies for blade grips and pitch arms are bought and paid for, it's sometimes expedient to make theory fit practice.

With a model rotor at least, if one wants to see some really nasty "wee-wa," skew the teeter hinge and observe the nutating behavior. It will go crazy with a 45 degree skew angle. Model rotors don't exhibit any detectable cross coupling because the ratio of rotor rpm to tilt rate is so high."

The Robinson's tri-hinge hub is functionally similar to the Bell's AH-1G and 222 Flex-beam hubs.


A Belgian kit helicopter. The rotorhub has teetering and flapping, similar to the Robinson. I wonder if it has delta3.

Angle CH-7

Has a tip weight of 3.3 lbs.

Bell 206

Has a precone angle of 2.5 degrees.

Has flapping static stops and centrifugal flap restraint. 

Hiller FH-1100

Has flapping static stops and centrifugal flap restraint (droop stop). 

Scorpion 331

It uses 5 only 5/16" diameter bolts to hold a blade on.

ref. It is a 2 seater and a 130 HP engine.

I could find out sheer load on bolts and centrifugal load on blade and then calculate safety factor.

Dick DeGraw's Hummingbird

His rotor head components are fabricated from 2024-T3 aluminum plate.

Pitch-Cone Coupling re: R22 & Groen Gyroplane

PRA posting; April 14, 2000; CA BEATY

The rotor employs pitch-cone coupling in the ratio of 1:1, meaning that as the coning angle increases by 1 degree, collective pitch is automatically reduced by 1 degree. Note that pitch-cone coupling is not the same thing as pitch-flap (delta-3) coupling which should be avoided if possible.

The rotorhead has the same general layout as the Robinson R-22 helicopter which makes the application of P/C coupling very simple. There is a hub with a central teeter hinge and separate coning hinges for each blade 6 or so inches outboard from the teeter hinge. The pitch control arms of the blade grips are on the trailing edge side and are picked up on the centerline of the teeter hinge. Teetering motion produces no coupling into blade pitch; thus no delta-3 coupling but as the blades cone up about the outboard coning hinges, the pitch control arms would swing downward if not restrained by the push rods from the swash plate. Instead, motion is forced about the feathering hinge, pulling pitch out of the blades.

There are several reasons for pitch-cone coupling; first, even the most competent pilot can be distracted and fail to lower the collective pitch following a jump and second, P/C coupling improves rotor angle of attack stability. An upward gust causes a greater incremental increase of lift on the advancing blade than on the retreating blade, causing a nose up reaction of the rotor, an unstable response. As most helicopter pilots know, raising collective causes a nose up response and lowering collective causes a nose down response. With P/C coupling, an upward gust reduces collective and with high enough coupling ratios, a rotor can become stable Vs. angle of attack. I'm happy to report that Jim first learned of P/C coupling from reading some of the postings on this conference.

Chesapeake (University of Maryland 1988 Proposal)

Material for rotor hub to be titanium alloy, TIMETAL 6-4 (Ti - ^%Al - $%V) 

CalVert (University of Maryland 1989 Proposal)

The hub will be a forged assembly made out of titanium. There is no need for machining the finished part: modern forgings maintain dimensional accuracy without rework.





Baby Belle


Mini 500 (Talon)


2024-T4 aluminum billet

2024-T3 aluminum alt. 6061-T6



solid billet 6061 T6 aluminum





Steel (double vacuum melt, high Vanadium, heat treated)

forged 4130 chromolly steel




Pre-cone: See DESIGN: Rotor - Coning Angle 0735

Undersling: See DESIGN: Rotor - Disk - Undersling 0734

Lead-Lag: See DESIGN: Rotor - Disk - Lead/Lag for Intermeshing Helicopter 0433

Coning Hinge: See DESIGN: Rotor - Hub - w/ Coning Hinges 0813








Maximum Up






Maximum Down






The swashplate phasing required is just equal to the delta3 angle.

Δψ = 90 - tan-1 * Kp = 90 - Δ3

Change in the azimuth = 90 - tan-1 * [pitch-flap coupling] = 90 - [delta3 angle]

Possible Locations of Swashplate: 

  1. Controls originating below Gearbox 
  2. In Hub - 3 non-rotating rods in mast. - Disadvantage - Mast rotates and rods do not. 
  3. Below Hub at top of Mast - The conventional location.
  4. At bottom of Mast - 2 rotating rods in mast (like Mini 500) - Disadvantage - Oscillating of rod masses at rotor rpm. 
  5. Controls originating above Gearbox 

Strength of Hub: (as related to Vortech blade)

The weight of 8-H-12 8" chord blade is 0.1176 lb/in.

The weight of 8-H-12 8" chord nose wt. is 0.0673 lb/in.

Total weight is 0.1849 lb/in.

The multiplier for 4.75" chord is 0.34375

There for the weight is 0.06356 lb/in

Aluminum weights 0.1 lb / cu-in

There for the X-sectional area of blade must be .6356 sq-in.

The X-sectional area at every location on the grip and yoke must be at least this (more or less as adjusted by strength of grade of aluminum).

The aluminuim in the Vortech blade is 6063, which is very weak. 

From Practical Theories; by Colin Mill; Parts 9, 10 and 11; off of Internet

Part 11  

In the last few articles we have seen how cyclic control of the pitching and rolling motion of the helicopter is accomplished and how Hiller and Bell-Hiller control systems allow the response of the helicopter to cyclic control to be tailored. We saw that, for freely flapping blades the maximum cyclic pitch must be applied 90 degrees before the required high point of the flapping. In other words, for a clockwise rotor a right roll is caused by having maximum cyclic pitch as the blades cross the boom, and nose-down pitching is caused by having maximum cyclic pitch on the retreating side of the rotor.  

In the May issue I mentioned (when considering forward flight) that offset flapping hinges and damper rubber stiffness change the way in which the blades flap, and that they could cause a roll tendency in forward flight. Cyclic control response is also changed by flapping hinge offset and damper stiffness. They cause the high point of the flap to occur earlier, that is, less than 90 degrees after the maximum cyclic pitch.  


In this illustration we see what happens if we continue to apply the cyclic controls 90 degrees ahead of the required motion. Notice how the right roll command now additionally causes a nose down pitching motion. To combat this the control system needs to feed the cyclic inputs in later in the rotation. This can be done by rotating the swash plate in the direction of rotation. With the cyclic controls retarded in this way the correct response to cyclic inputs is obtained as seen below.  


Phase errors and the control system  

The Hiller and Bell-Hiller control systems have the effect of reducing these cyclic control phase shifts and so many model helicopters make no provision for rotating the swashplate. The flybar in the control system is freely rocking and pivoted on the axis of the main shaft (the equivalent of having zero flapping hinge offset) so it does not suffer the sort of phase errors just described. In the Hiller control system the cyclic control to the main blades comes from the flybar and any tendency for the blades to misbehave (pitch up or down during a roll command for example) is suppressed. This happens because any angle between the plane of the main blades and the flybar causes a correcting cyclic control to be fed to the main blades to make them follow the flybar. If in a roll the main blades start to pitch nose down the nose down attitude of the main blades relative to the flybar will cause some nose up cyclic control to be fed to the main blades opposing further nose down movement. The same happens with the Bell-Hiller system but because a proportion of the cyclic control of the main blades taken directly from the swashplate, the degree of phase error suppression is lower. Where the mechanics provides the facility, residual phase errors can of course be removed by rotating the swashplate as mentioned before. 


See; Outside Helicopters above.  

Flapping Static Stop:

The Ultrasport 254's hub is equipped with a rubber shaft bumper that allows startup and shutdown in winds up to 15-20 knots. Could this bumper be incorporated into the flap resistor device?

The Robinson R-22 has tabs on the inside ends of yoke spindles. Very simple.

The SynchroLite's maximum cyclic pitch angle is 13 degrees. This probably means that the flapping static stops should be set at this angle and this also means that for azimuths of 90 degrees it will be too much and the flapping will have to be limited aerodynamically at this azimuth. 

RRPM: ~ Author: CA BEATY ~ Date: February 10, 1999

A common misconception is coning angle changes with load in a gyro. It doesn't. An increase in load increases RPM in the correct proportion to keep coning angle constant.  

A helicopter, which runs at constant rotor speed, does have a change of coning angle with load change. 

Pitch-Cone Coupling: ~ Author: F.D. on rec.aviation.rotorcraft

Pitch-cone coupling is a mechanical method of maintaining rotor rpm used on some designs.

Bearingless Hub:

One of the advantages of a bearingless hub is lower weight.

Same Page ~ Different Craft: ~ SynchroLite ~ Rotor - Hub - 3-blade Constant Velocity Joint ~ Dragonfly ~ UniCopter

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Last Revised: March 10, 2007