0891

Item 0891

OTHER: Helicopter - Outside - Coaxial - Sikorsky ~ S-69 (XH-59) ABC

Picture:

Picture from: http://www.russian.ee/~star/vertigo/sik_s-69-r.html

General Information:

From document 5. Below; "The ABC rotor system was devised as a means for eliminating rotor blade stall and providing greater speed and agility in helicopters"

From the American Helicopter Society, Annual Forum, 1981, "The Advancing Blade Concept (ABC) uses two rigid counterrotating rotors in a coaxial arrangement to provide advancing blades on both sides of the aircraft. This makes use of the high dynamic pressure on the advancing side of the rotors at high forward speed, virtually ignoring the low dynamic pressure on the retreating side, while still keeping the rotor system in roll trim."

From R.W. Prouty "The two very rigid rotors on the Sikorsky Advancing Blade Concept (ABC) coaxial aircraft. The key to hingeless rotors is that the cyclic pitch can be used to trim out almost all of the flapping, leaving flexibility to do the rest. ". [Source ~ RWP2 p.101]

Technical Documents on S-69 (XH-59) ABC: (that I have hard copies of)

12.	The ABC Helicopter	M.C. Cheney Jr.	Feb 1969
13.	Development of the ABC Rotor	R.K. Burgess	May 1971
15.	Forward Flight Performance of a Coaxial Rigid Rotor	V. M. Paglino	May 1971
8.	Navy/Marine 1980 Rotary Wing Candidates	G. Unger	1975
1.	Advancing Blade Concept (ABC) Development	A.J Ruddell	May 1976
4.	Rigid Coaxial (ABC) Rotor System Stability and Control Characteristics	R.F. Klingloff	May 1976
5.	Advancing Blade Concept (ABC) Dynamics	Abbe & Blackwell	May 1977
7.	Review of Advanced Rotor Research	Kelly & Rabbott	May 1977
3.	Advancing Blade Concept (ABC) High Speed Development	A.J Ruddell	May 1980
6.	ABC Development Status and Design Considerations for Several Military Applications	Halley & Knapp	May 1980
14.	The Impact of Missions on the Preliminary Design of An ABC Rotor	Gary de Simone, R.S. Blauch & R.A. Fisher	Oct 1980
9.	Reliability and Maintainability Evaluation of Rotor Systems	Robert A. Hall	1981
2.	An ABC Status Report	Linden & Ruddell	After Jan 1981
16	Advancing Blade Concept (ABC) Technology Demonstrator	J. Ruddell	April 1981
10.	Convertible Engines For Fold Tilt Rotor Aircraft and ABC Rotorcraft	J.C Gill	1983
11.	An Assessment of the Impact of Technology on VTOL Weight Prediction	David K. Unsworth	1984

Miscellaneous Information from ABC documents above: (number at start is the document number)

Control - Flight:

1. Push rod loads in the absence of blade stall appear to be primarily related to blade bending.

2. The XH-59A rotor has the capability of other hingeless, or "rigid", rotors to produce substantially higher control power than conventional zero or low hinge offset rotors.

2. The result is that the aircraft is easy to fly, giving the pilot purer responses to his inputs.

5. As the phase angle is decreased, the clearance between the blade tips of the two rotors decreases. Minimum clearance occurs at the crossover of a retreating upper blade and the advancing lower blade on the left-hand side of the rotor disk. (Ref. Rotors articulated at the center of the mast have a phase angle of roughly 90º.)

1. The swashplate has an adjustable phase angle of 0º to 70º.

Opposed Lateral Cyclic:

1. Lift Offset: Differential displacement of the rotor thrust vectors can be controlled in two ways on the demonstrator aircraft: by introducing differential lateral cyclic (B₁') through an independent control or by the variable phasing of the pilot inputs to the rotor (Г). Testing to date has concentrated on the use of variable phase angle.

Rotor - Blade:

1. The full benefit of the coax effect is not realized in the present ABC rotor system as some penalty is paid due to the thicker inboard airfoil sections required to carry rotor moments.

5. The high ABC blade bending and torsional stiffness permit such small elastic deflections that potentially unstable pitch-flap-lag couplings are precluded.

1. Blade first flatwise and edgewise mode natural frequencies are both at approximately 1.4 per rev. The frequency ratio is 0.71. One is the inverse of the other.

14. Profiles; Existing and proposed for future ABC.

Miscellaneous:

2. The presently configured rotor hub makes up one-half the total drag. It is this drag which the fairings are attempting to reduce.

Vibration Analysis:

See dedicated page below.

5. The two rotors in their present spacing were therefore expected to generate low levels of vertical and longitudinal excitation and relatively high lateral excitation.

5. Preliminary evaluations of vibration treatment needed for ABC in the helicopter mode show that a weight allowance of about 2% of gross weight for absorbers and transmission tuning can be expected to get the vibration levels down below 0.1g in the cockpit.

Trim:

6. A nose down attitude of about 12 degrees is required at 156 knots airspeed. (Note that this is not in compound mode.)

Re: Horizontal Stabilizer ~ DESIGN: UniCopter ~ Fuselage - Tail - Stabilizer - Horizontal

Lift offset:

1. Differential displacement of the rotor thrust vector can be controlled in two ways on the demonstrator aircraft: by introducing differential lateral cyclic ( B₁') through an independent control, or by variable phasing of the pilot inputs to the rotor ( Γ ).

Shaft Tilt:

1. Performance improvements would be realized in both configurations by providing a forward shaft tilt in the order of 4º for the helicopter and an aft tilt of approximately 2º for the auxiliary propulsion configuration.

My thoughts:

The aft tilt of 2º implies an autorotative mode (gyrocopter). Why not apply a little more power to the rotors and have the disk normal to a vertical shaft?
The proposed propeller for the UniCopter is to have a variable pitch and the pitch change is linked to changes in the forward longitudinal cyclic then a vertical mast may work well.

Cruise and Rotor RPM:

Page 83 from technical document #2 above; http://www.unicopter.com/ABC_Status_Report.jpg

Locations of Additional Information on Sikorsky ABC:

This Web Site:

OTHER: Aerodynamics - Vibration - Rotor Induced - Analysis re: Sikorsky

DESIGN: UniCopter ~ Rotor ~ ABC

DESIGN: UniCopter ~ Rotor - Hub - Pitch Bearings - ABC

DESIGN: UniCopter ~ Rotor - Hub - Absolutely Rigid Rotor ~ An Idea

DESIGN: UniCopter ~ Rotor - Disk - Advancing Blade Concept

DESIGN: UniCopter ~ Rotor - Disk - Absolutely Rigid Rotor

DESIGN: UniCopter ~ Rotor - Blade - General

DESIGN: UniCopter ~ Pusher Prop - General - Angle of Incidence - Rotor & Propeller ~ Sikorsky

DESIGN: UniCopter ~ Control - Flight

OTHER: Aerodynamics - General - Phase Angle

Other Web Sites:

http://www.russian.ee/~star/vertigo/sik_s-69-r.html

NACA Technical Paper 3675 ~ A Survey of Theoretical and Experimental Coaxial Rotor Aerodynamic Research ~ March 1997

Performance and loads data from a wind tunnel test of a full-scale, coaxial, hingeless rotor helicopter - A full-scale XH-59A advancing blade concept helicopter was tested in Ames Research Center's 40 by 80 foot wind tunnel. I have copy of it on removable drive under the file name;- 19820004167_1982004167.pdf

"The ABC had independent control for each disk, and very stiff blades (over 25% hinge offset) so tip clearance was not as big an issue as it is on other co-axials. This meant that the swashplates could be tilted to wash out lift on the retreating sides, but keeping it up on the advancing side. The mast was very strong, so the moment battle between the rotors (one "leaning" to the right, one to the left) was cancelled at the transmission.

This let the rotors blast on out to high speed, and just as importantly, let the ABC develop very high load factor. It pulled 2.5 G's at 25,000 feet and cruised at over 250 mph, as the thrust engines pushed it along in autorotation (it was an autogyro at high speed!) Not too shabby. Of course the speed was driven by two jets that ate gas like there was no tomorrow, so the speed was really only good for short bursts. In a production configuration, the lift engines would drive the prop shafts, so the cruise efficiency would be much better.

One really unique benefit of the ABC was the purity of the controls. The high offset made the aircraft snap to pitch and roll commands like a fighter, but with no cross coupling like the Boelkow family. The couplings were generated by each rotor, of course, but the rotors developed them in opposite directions so they cancelled at the head, and the pilot was left with a very crisp clean feel.

I flew it to about 250 MPH and was really impressed." ~ Nick Lappos, PPRuNe, August 27, 2001

Nice round cylinders in the downwash of rotor blades are bad for stability. The XH-59A was a full scale coax that had that problem due to a round cross section. They used to hover it with the side doors open to keep it stable. If you want to fly with those cigar tubes, put some spoilers on the sides of them.

-- Bryan RC Groups com

Recommended Improvements to the Coaxial ABC; from Technical Document #2 Above:

More composite construction in the rotor; such as replacing the titanium spar with a composite one.

Two engines in lieu of four, to power the rotor system auxiliary propulsion.

Faring on the rotor hubs.

Reduced rotor twist; from -10º to -4º.

Increase taper; from 2:1 to 3.6:1.

Weight reduction.

Improved airfoil.

It will be interesting to see which of the above and which new ones are implemented in the X2:

Patents:

Note: Igor Sikorsky died on October 26, 1972.

Coaxial Rigid Rotor Helicopter and Method of Flying Same ~ US 3,409,249 ~ Nov 5, 1968

The assignee is United Technologies but the inventor is not Igor Sikorsky.

Abstract ~

_______________

References Cited by US 3,409,249

	2,473,331	6/1949
	3,126,966	3/1964

_______________

Patents Referencing US 3,409,249

	5,351,913	Coaxial transmission/center hub subassembly for a rotor assembly having ducted, coaxial counter-rotating rotors
	5,152,478	Unmanned flight vehicle including counter rotating rotors positioned within a toroidal shroud and operable to provide all required vehicle flight controls
	4,367,063	Pitch control mechanism for coaxial helicopter steering
	4,027,999	Analog mixer to vary helicopter rotor phase angle in flight
	4,008,979	Control for helicopter having dual rigid rotors

Control for helicopter having dual rigid rotors ~ US 4,008,979 ~ February 22, 1977

The assignee is United Technologies but the inventor is not Igor Sikorsky. Note ~ A quick overview suggests that this is for ABC.

Abstract ~

A control for a helicopter having dual, coaxial, counterrotating rigid rotors which varies the cyclic control phase angle of each rotor in flight as a function of vehicle forward speed to thereby control the coupling of lateral cyclic pitch with longitudinal cyclic pitch so as to introduce differential cyclic pitch inputs which automatically produce aerodynamic moments in each rotor to minimize maneuver generated gyroscopic precession moments, which also produce optimum lateral lift vector displacement for all flight speeds.

	Inventors:	Cooper; Dean Earl (Trumbull, CT), Klingloff; Robert Fuller (Huntington, CT)
	Assignee:	United Technologies Corporation (Hartford, CT)
	Appl. No.:	05/631,600
	Filed:	November 13, 1975

____________________

References Cited by US 4,008,979:;

2546881 Mar., 1951 Avery 416/114.

2748876 Jun., 1956 Daland et al. 416/33.

3120276 Feb., 1964 Culver et al. 416/114.

3146970 Sep., 1964 Girard 416/114.

3199601 Aug., 1965 Dean et al. 416/130.

3409249 Nov., 1968 Bergquist et al. 416/129.

3521971 Jul., 1970 Cheney 416/33.

3570786 Mar., 1971 Lewis 416/115.

3620488 Nov., 1971 Miller 244/17.

3799695 Mar., 1974 Yamakawa 416/40

__________________

Patents Referencing US 4,008,979;

	6,254,037	Variable gradient control stick force feel adjustment system
	6,089,501	Tandem-rotor gyroplane
	5,765,783	Vertically launchable and recoverable winged aircraft
	5,370,341	Ultralight helicopter and control system

Analog mixer to vary helicopter rotor phase angle in flight ~ US 4,027,999 ~ June 7, 1977

Coaxial ABC ~The assignee is United Technologies but the inventor is not Igor Sikorsky.

Blade pitch control mechanism for helicopters ~ US 4,047,838 ~ September 13, 1977

The assignee is United Technologies but the inventor is not Igor Sikorsky.

Postings re why the ABC did not Continue:

Advancing Blade Concept (ABC) was a serious contender for the RAH-66. However, the transmission weight penalties were too high and so the trade study decided that the advantages were outweighed (no pun intended although I will gladly take credit for it) by the weight disadvantage. In a zero sum game, weight for the ABC xmsn would come out of avionics or armor. ABC is in mothballs as it outlived the design life of a number of critical components. All who flew it were impressed with its capability. We had also envisioned using it in conjunction with a pusher prop ala the Cheyenne. The Helix is a form of ABC being coaxial but you can see by the mast its not weight or design efficient. As we put more effort into conventional rotors they get better and lighter. It's going to take a lot of money to catch up the ABC given the lack of work in the last 20 years.

Regards, Bruce

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I worked on the ABC late in its life, reducing flight test and wind tunnel data and simulating it in the aero group's codes. The ABC had a lot of advantages. It had a higher hover Figure of Merit than the average helicopter, due to its co-axial arrangement. With the J-60s installed, it had the ability to accelerate from hover without changing body attitude, which helped in combat tasks. By changing the phasing of the two rotors slightly, rotor-induced vibration could be controlled. The counter rotating rotors balanced out the moments resulting from retreating blade stall and eliminated the need for a tail rotor. Lastly, it was fast and manouverable. The manouverability came from the high effective hinge offset of the rigid rotors. The downsides were that it sucked fuel like a pig, primarily due to the J-60s, which weren't intended for flying on a slow helicopter (compared to a business jet). It was difficult to use the ABC arrangement for anything but an attack helicopter, due to the big, tall gearbox assembly. The rotors had to be well separated and very stiff so that they wouldn't strike at high blade deflections. This resulted in a tall mast, which was very draggy. Inter-rotor fairings were designed and wind tunnel tested, but it was difficult to design a fairing that really resulted in a drag decrease. Initially, there were problems with control, which led to the crash of the first prototype, but these were later fixed. There was a proposal for a XH-59B follow-on, which would have had a ducted prop replacing the two J-60 jet engines. The PT6s would have powered the rotor in hover and the prop in high speed cruise. Unfortunately, the project didn't go ahead. The tilt-rotor killed the ABC concept. > Questions: > > 1. Since an ABC helicopter would have two sets of rotors dedicated > for lifting thrust, would this mean that the rotors would not have to > spin as fast to create the required lift for flight/hover as those of > a standard, single main-rotor helo? Would this make the helicopter > even quieter, given that it does not have a tail rotor either? The tip speed on the S-69's rotors was a little lower than the average Sikorsky helicopter. It was a little quieter than a S-76, due to the lack of a tail rotor, but the aux propulsion J-60s were VERY noisy. > 2. Are there any coaxial, contra-rotating helicopters with more than > three blades to each rotor? I have seen coaxials with two bladed rotors, but this isn't common. A note: a coaxial is not necessarily an ABC. Kamov helicopters are not ABCs. > 3. Is yaw control sacrificed at all in an ABC helo? No, not at all. If I remember right, in hover, yaw control is achieved by differential collective and in high speed forward flight, there were rudders. > 4. Why was the Advancing Blade Concept not pursued further? The US Army decided they didn't want it. The tilt rotor helped bring about its demise.

David Lednicer | "Applied Computational Fluid Dynamics" Analytical Methods, Inc. | email: dave@amiwest.com 2133 152nd Ave NE | tel: (206) 643-9090 Redmond, WA 98052 USA

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> Odd in retrospect, considering it still hasn't _really_ gone > anywhere.

True, but remember - Sikorsky initially pitched the ABC against the tilt rotor for JVX. In the end, the DoD specified that the JVX had to be a tilt rotor. Additionally, the ABC was pitched for LHX (admittedly a better mission for it) and Bell initially pitched a tilt rotor. Eventually, the US Army decided that they wanted a conventional helicopter, and if they ever wanted speed, they would buy a tilt rotor.

rec.aviation.rotorcraft

The XV-15 kicked the ABC's a$$ to the curb. That is why there is a tiltrotor and the ABC has been sent to the scrap bin of bad ideas money has been wasted on. Besides being noisy, requiring twice as many engines as a tiltrotor, having a range measured in 10's of miles, and shaking like a dog it was a good idea. (If one forgets the nasty habit of intermeshing the upper and lower rotors.)

Considerations Related to These Points:

OTHER: Aerodynamics - Vibration - Rotor Induced - Analysis of Coaxial

Vibration - may have resulted from a lateral dissymmetry of lift due to 3-blade rotors. ~ DJ

See DESIGN: UniCopter ~ Rotor - Disk - Lateral Dissymmetry of Lift and Drag? - (3-blades)

And DESIGN: UniCopter ~ Rotor - Disk - Lateral Dissymmetry of Lift (Drag?) - 4-blade Rotors

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Advancing Blade Concept was a contender for the RAH-66, but the transmission weight penalties were too high and so the trade study decided that the advantages were outweighed by the weight disadvantage.

XH-59A Accident:

"The aircraft basically ran out of longitudinal control due to very conservative rigging based on simulation data that was seriously in error. To the best of my knowledge, blade clearance was monitored in flight test, and no close calls were ever recorded on either ABC demonstrator." ~ Nick Lappos, PPRuNe, August 27, 2001

"Also came across some old footage of the crash of the original XH59A (71-1472, pre J60 days), and while the TP was literally able to walk away from what looked like a horrendous accident, it makes one understand why we are not rushing things with X2. Nicks quote is spot on, the descent definitely looks to be a pitch limitation. The helo ends up landing on its tail at about a 50 degree nose-up attitude from horizontal, and after the hard contact with the ground the rotors contact. It then rolls over and essentially self destructs after blade-ground 'interaction'." ~ CEFOSKEY, PPRuNe March 8 & 9, 2010

The ABC lost out to:

JVX became the V-22 Osprey.

LHX became the RAH-66 Comanche.

From; Department of the Army Historical Summary: FY 1976

Flight testing of the XH-59, advancing blade aircraft, resumed during the year. The tests confirmed certain advantages and shortcomings of the advancing blade concept, which is a coaxial counterrotating, hinge-less rotor system. The aircraft control system was quite responsive, noise levels were low, retreating blade stall was virtually eliminated, and hover performance was improved by the absence of a tail rotor. Structural loads in the rotor and control system ranged from low to moderate and indicated potential for substantial weight reduction. Weak directional control power in partial power descents and autorotation was observed. In future testing, improvements in autorotation directional control at low collective settings and high flare angles will be emphasized.

Perhaps the Hiller_X-2-235 was the forerunner of the XH-59A ABC.

"As a result of the NACA tests, Hiller initiated redesign of the X-2-235 to incorporate three-bladed coaxial rotors augmented by an aft-thrusting rear propeller. The reworked craft was obviously geared to flight at speeds substantially higher than those attained by existing helicopters."

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Last Revised: March 9, 2012