DESIGN: UniCopter ~ Rotor

Outside Helicopters

Sikorsky S-69 (XH-59A) ABC:

Rotor Specifications:

Upper Rotor turns CCW & lower rotor turns CW

Rotor diameter = 36 ft.

Blade area = 155.5 sq-ft. From Access. GW / Blade loading.

Individual disk area = 1.018 sq-ft

Effective disk area = 1,453 sq-ft

Disk loading = 6.2 lb/sq-ft; at its gross weight of 9,000 lbs. [Published by Sikorsky ~ 13.07 lb/ft2] [Published by Sikorsky ~ GW = 13,300 lbs]

Rotor solidity ratio = 0.153 Calculated in Access, based on the area of 6 blades and a single disk. 0.0765 for each rotor.

Total rotor solidity = 0.1275 Published by Sikorsky. What is this value based on? Could it be an adjustment for the Twin Rotor Factor (interference-induced power factor) [Source ~ PHA p.70]; with some slight modification? It is probably based on combining the blades of the two rotors into one disk and the chord being smaller than that which I used.

For more see: FORM: Helicopter Specifications. Put it out on a web page.

Has 3 degrees of precone.

Has 1.4 degrees of prelag.

Flapping hinge offset: Flapping frequency ratio (v/n) ≈ 1.4 [Source ~ RWA Book I p.39]. From the graph on [Source ~ RWA Book I p.18] it appears that the equivalent (virtual) flapping hinge offset will be approximately at 50% of rotor radius.

Design rotor tip-speed = 650 ft/sec. in helicopter mode; 450 ft/sec. in compound mode.

Upper rotor advances on the right (starboard) side. CCW

Stick dampening would be required to maintain good handling.

Standard aircraft cable systems leading to the rudder and elevator.

Thicker inboard airfoil sections required to carry rotor moments.


Calculations re S-69 and Reverse Velocity:


Calculations re S-69 and Blade Loading Coefficient [CT]:


This little section has to be wrong:

Calculations re S-69 and Blade Loading Coefficient at Maximum Forward Speed [CT]:

The following is my thinking re Advance Ratio Disk Area Modifier: ?? To take into account the loss of lift due to the reverse velocity

To eliminate the area of reverse velocity and an equal area of opposing positive velocity.


For Sikorsky ABC Blade Specifications see; DESIGN: UniCopter ~ Rotor - Blade - General ~ Sikorsky ABC

For Sikorsky ABC General Information see; OTHER: Helicopter - Outside - Coaxial - Sikorsky ~ XH-59A ABC

Sikorsky X2 ABC:

Rotor Specifications:

To come. For now see; 1465.html

Stepniewski Intermeshing ABC:

Rotor Specifications:


The BO-105 helicopter rotor system is a four-bladed, soft inplane hingeless rotor with constant chord (0.886 ft), -8 linear twist, and a NACA 23012 cambered airfoil. The rotor radius is 16.11 ft; rotor solidity is 0.07. The rotor hub has 2.5 of built-in coning and zero droop or sweep of the blade outboard of the pitch bearing.

Picture of B0-105 Rotor Head Caption: "Main rotor hub detail. Rigid rotor with GFRP blades and a forged titanium hub is hingeless (except for the feathering hinge), giving full aerobatic capability."

Hiller Rigid Rotors:


Hiller X-2-235

Hiller XH-44

"... the extremely rigid rotor of the Hiller lead to "Vibration problems curtailed these tests for fear of damaging the wind tunnel" When you worry about the aircraft wiping out the tunnel, you have problems! ~ Nick Lappos on RAR

Dual rotors with two blades each will result in a lateral vibration when longitudinal cyclic is applied. See: DESIGN: SynchroLite ~ Rotor - Disk - Lift Distribution re: Vibration ~ DBJ

RSRA/X-Wing Project (Sikorsky):

Reports from NASA:


Re twisting: Model rotors designed for wind tunnels can be built so stiff that they are virtually "stall proof." This degree of stiffness is usually not built into actual helicopter rotors because of the extra weight required. [Source ~ RWP2 p.26]

Optimum speed rotor. US patent application 20010001033 Interesting patent related to optimizing the UniCopter for fast forward flight and long hover time.



Comparison of the UniCopter's CVJ+HS Rotor and the UniCopter's Absolutely Rigid Rotor:

Objectives of Absolutely Rigid Rotor:


UniCopter Characteristics ~ Rotor


OTHER: Helicopter Specification - UniCopter ~ for rotor with 3 blades

OTHER: Helicopter Specification - UniCopter ~ for rotor with 4 blades

Direction of Rotation:

All previous intermeshing helicopters used the inside-forward (breaststroke) direction of rotation. They all had articulated heads. Stepniewski intermeshing ABC proposal is for the inside-backward. Some advantages for the inside-backward, when applied to the intermeshing ABC are;

Slow Speed Rotor:

A slower rotor speed will reduced the profile drag in forward flight while not being detrimental in hover. For information related to a larger chord and slower RRPM see, DESIGN: UniCopter ~ Rotor - Disk - Large Chord & Low Tip Speed.

Greater Solidity:

Greater solidity should improve hover while also providing sufficient blade area in ABC fast forward flight when the craft is being primarily supported by only the advancing blades. See, DESIGN: UniCopter ~ Rotor - Disk - Large Chord & Low Tip Speed.

Size Consideration:

Consider taking the GW up to just under 1320 lbs. and increase the Radius to say 10 feet. This will be better for having a 2-seat craft and put it at the top of the VLH category.

Four Blades per Rotor (vs. three per rotor):


3-blade rotors, such as used on the Sikorsky ~ XH-59A ABC 3-bladed coaxial ABC generated high vibration at fast forward speeds. It appears (to me) that this vibration is one of 3P and is about the x-axis. It also appears (to me) that this vibration is due to a lateral disymmetry of lift resulting from the blade at azimuth 90 producing more lift than the blades on the other rotor, which are at azimuths 30 and 150. Ref; DESIGN: UniCopter ~ Rotor - Disk - Lateral Dissymmetry of Lift and Drag

It appears (to me) that this specific major source of vibration can be removed by; A/ providing the 3-blade rotors with a 3P Higher Harmonic Control (swishring) or B/ using 4-blade rotors with a conventional 1P control.

In addition, Stepniewski, in his ABC synchropter, mentions the possibility of both 3-blade and 4-blade rotors.

Advantages of 4-blade Rotor:

Disadvantages of 4-blade Rotor:

Reverse Velocity Region:

With slow speed rotor at 275 RPM and at 150 knots zero velocity occurs at 8.75' radius on the retreating rotor at 270 azimuth.

Rotor Flight Profile:

 Design considerations for a rigid rotor assembly:

Additional Thoughts:

Because there is minimal coning, beyond pre-cone, and no flapback, the craft may be lacking lateral stability. By applying more lift to the advancing blade then the retreating blade (ABC) and in consideration of the fact that in the intermeshing configuration both the advancing blades are high, this should/may create a dihedral effect.

The ratio of lift on the advancing side to the amount of lift on the retreating side will probably be determined by the static stability, vibration and achievable to speed.

Consider a small upward pointing winglet at the tip of the blades. On the advancing side it will provide a little additional lift and it may reduce the magnitude of the tip vortices. At the front and the back, during forward flight, it should provide some positive pitch stability and this might result in a smaller horizontal stabilizer and slightly lighter boom. The winglet may also provide a small amount of dihedral and thus static stability. Then again, it might open up the 'centers' and/or cause a vibratory moment.

Having dihedral at the wing tips may also provide an additional level of safety when the blades are crossing at the front of the craft. This is because the dihedral will help assure that the blades cross properly during a server perturbation.

The above three paragraphs should be thought out then fleshed out.

Airfoil can be selected for high speed advancing blade not for retreating blade stall.

A small amount of twist (Sikorsky proposed reducing from 12 to 8 for the next generation of ABC) may work well for the ABC. Twist causes the blade's moment arm, between the center of lift and mast, to be longer on the retreating side. [Source ~ RWP5 p.52] 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 seesaw) and therefor less vibration. This advantage is in addition to the reduction in retreating blade stall and dihedral achieved from ABC.

Consider filling the area of the blade in frount of the spar with unidirectional roving, epoxy, and glass beads if the weigh is excessive. This may balance the blade chordwise and provide additional strength and passing the loading to the very root of the blade.

Consider having the feathering axis at 35% of chord (thickest location) at root to 27% at tip. This might be ideal for root hydraulic collective control and un-powered tip cyclic control.


Possibly, consider producing a symmetrical blade with taper but no twist. This one blade can be used on both rotors. If the blade is experiencing some flapwise deflection when under a loading of 1G, then instead of the necessity of two molds to put in anhedral consider conning the pitch bearings in the hub downward slightly.


A stall resulting from excessive cyclic or collective might quite benign. This is because the blades are spending a fair portion of their time operating in the downwash and downdraft of the blades on the other rotor. Therefor the blades and hence the pilot will be subjected to quit a bit of 'buffeting' as the blade pass in and out of stall before a sector or the total rotors stall. Just a thought.


Consider the inclusion of a type of led/lag hinge just outboard of the feathering hinge and within 1/2 the rotors to rotor distance. This hinge (6 hinges in total) will only be used for grouping the blades for ground transportation. This might work because there should be a reduction of forces in the in-plane direction. See: 1004. The hinge will have to have a long axle (vertical) to handle the out-of-plane loads. This axle cannot extend up into the path of the blades on the other rotor, but it can extend downward as far as it wants. In other words the center of the axle (lengthwise) may not be, nor need to be, on the centerline of the feathering axis.

 Potential Problems:

Cyclic Stick Loading & Vibration Induced by Friction in Pitch Bearings:

Revolution literature states 'During flight, the centrifugal force on each blade is 8,810 lbs'. The two blades will have a combined load of 17,620 pounds.

For the UniCopter at 100% RRPM, the centrifugal force on each blade is 7,510 lbs'. The six blades will have a combined load of 45,060 pounds. This is 2.5 times the Mini 500 and the mini 500 was having problems. Note that with a variable speed rotor and Propulsor, the rotational speed will be significantly lower during forward flight.

The radial force

See DESIGN: UniCopter ~ Rotor - Hub - Pitch Bearings - Roller & Ball re the force required on the cyclic stick.

Gyroscopic Precession:

The UniCopter's rotor disk is completely rigid, with the exception of the pitch bearings, which are irrelevant in this discussion. When the pilot starts to rotate the helicopter about the X or Y-axis the rotors will exhibit gyroscopic precession. The good news is that the gyroscopic precession from the two rotors will be self-canceling. The bad news is that the power-train's frame must be strong enough to handle this loading.


Matrix method for obtaining spanwise moments and deflections of torsionally rigid rotor blades with arbitrary loadings

Alton P. Mayo, NACA TN 4304, Aug 1958, ~ http://naca.larc.nasa.gov/reports/1958/naca-tn-4304/naca-tn-4304.pdf

Patents, which may be Relevant:

5,620,305 Hub for rotary wing aircraft

Same Page ~ Different Craft: ~ SynchroLite ~ Dragonfly

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Last Revised: August 15, 2008