1465

Item 1465

OTHER: Helicopter - Outside - Coaxial - Sikorsky ~ X2 TD

General Information:

Engine: 1 x LHTEC T800 turboshaft, driving both the coaxial rotor and the six-bladed pusher propeller Power 1000 - 1250 kW (1300 - 1680 shp)

Performance: Max. cruise speed (max.): 460 km/h (250 kts) Range 1300 km These may have changed.

Blades: Eagle Aviation Technologies is to provide the blades. http://www.eagleaviationtech.com/. CEO: Emitt Wallace

Propeller: Aero Composites, Inc. will build the six-bladed propulsor. http://www.aerocomposites.com/

Fly-by-Wire: These technologies include an integrated Fly-by-Wire system that allows the engine/rotor/propulsor system to operate efficiently, with full control of rotor rpm throughout the flight envelope, I.e. Variable speed rotors.

Technical Documents on X2 TD ABC: Have hard copies

1.	Aerodynamic Design of the X2 TD Main Rotor Blade	Ashish Bagai	May 2008
2.	Dynamic Design Characteristics of the Sikorsky X2 TD Aircraft	R. Blackwell	May 2008
Do not have	X2 Technology™ and Emerging Applications	Eadie, Alber & Tinker	May 2008

Comparison between the XH-59A, Proposed future improvements to the XH-59A, and the X2TD:

		XH-59A ABC (S69)	Proposed XH-59A improvements: ⁽¹⁾	X2TD ABC
General:
	Gross Weight:	9,000 lbs \| 13,300 lbs ⁽²⁾	"significant weight reduction"	5,300 lbs ⁽²⁾
	Effective weight: GW * 1.15			6,095 lbs ⁽²⁾
	Number of engines:	4	1	1
	Power - Cruise:	1,500 HP (design)	~	1,450 SHP
	Power - Hover:	1,800 - 1,250 shp	~	above
	Propulsor:	J60 turbo jets	Propeller	Propeller
	Power loading: Using XH-59A cruise power.	8.87 lb/hp	~	3.66 lb/hp
	Projected cruise speed:	300 knot range	~	250 - 265 knots ⁽⁷⁾
Rotor:
	Number of blades:	3	4	4
	Rotor radius:	18 ft	~	13.2 ft ⁽²⁾
	Disk Area - Individual:	1,018 sq-ft	~	548 sq-ft
	Disk Area - Effective: ???	1,453 sq-ft	~
	Disk loading at GW - on single disk: ^{(2) (6)}	13.07 lb/sq-ft	~	11.14 lb/sq-ft
	Blade twist: R is blade radius	-12º	-8º	0.0 R to 0.4 R is +9º, 0.4 R to 1.0 R is -9º
	Taper ratio:	2:1	~	Special blade
	Solidity ratio: - independent disks: [σ]	0.0765	~	⁽⁹⁾
	Solidity ratio - all blades in 1 disk area:	0.153 ~ 0.127 ⁽³⁾	~	0.1441 ⁽⁹⁾
	Thrust weighted solidity: [Nb/σ_TW^(Total)]⁽¹⁰⁾	(3 + 3) / 0.1275 ⁽²⁾		(4 + 4) / 0.1441 ⁽²⁾
	Gap: (rotor separation)	2.5 ft	~	1.5 ft ⁽⁴⁾
	Section profile at root:	Look up later	Modified ellipse	done
	Precone:	3º	~
	Rotor speed - Hover: ⁽²⁾[RRPM_H]	345	~	446
	Rotor speed - Cruise: ⁽²⁾[RRPM_C]	239	~	360
	Speed reduction - Hover to Cruise:	70%	~	80%
	Tip speed - Hover: ⁽²⁾	650 fps	~	620.45 fps ^{(8) (2)}
	Tip speed - Cruise:	450 fps	~	560 fps⁽⁸⁾
	Reverse velocity region: ⁽⁵⁾	93% R	~	80% R was \|90% R ⁽²⁾
	Tip speed - Cruise @ 90 azimuth	0.88 M	~	<= 0.9 M
	Maximum lift offset: [LOF]	20 % R ⁽¹¹⁾	~	30% R
	Rotor longitudinal shaft angle (α_s)	0º		0º
Miscellaneous:
	Figure of Merit: Isolated rotor \| Aircraft	≈ 0.79 \| ≈ 0.68	~	≈ 0.xx \| ≈ 0.xx
	Vibration suppression:	Prevision for only	~	Installed
	Blade spar:	titanium	All composite
	Parasitic Drag - Rotor hubs	50% of total	Fairing of hubs	Done
	Tail surface - horizontal	60 sq-ft	~
	Tail surface - vertical	30 sq-ft	~
	Tail surface - vertical		Invert	Done

_______________________

Sources of Information:

Changes that were recommended at the conclusion of the XH-59 project.

From Sikorsky's Technical Document #1, at the top of this page.

0.153 ~ See elaboration. | 0.127 ~ published by Sikorsky. My thoughts: Perhaps the considering of the disk area as being that of a single rotor is not bad. This is because the two ABC disks have a very small gap between them and thus the rotor assemblage could be equated with a single rotor having 8 blades.

Guestimate. Scaled from picture(s).

Location of circumference of reverse velocity during cruise at 270º azimuth (%R)

Sikorsky bases the disk loading on the area of a single disk

Cruise top Speed: 250-265 (kts) [1 - averaged]

"By the time the X2 reaches 265 knots [305 mph][447 fps], the main rotors will be "80 percent of the original rotor speed [384 RRPM = tip speed of 700 fps], with the retreating blade in 80-percent [0.8 x 17.4 =13.92'] reverse flow, [307 RRPM]" Pino said in above Press Release (Feb. 24, 2008) #1:

Due to the large root cutout and the unique profile perhaps the solidity ratio was obtained by graphical integration.

σ_TW is Rotor thrust weighted solidity.

From Sikorsky's Technical Document #2, at the top of this page.

Lift/Drag:

Lift/Drag comparison between XH-59A and X2TD

From Technical Article #1

IMHO, there is something strange about the advancing blade roots on the two graphs. The XH-59A had a blade twist of -12º and a large root chord. This means that the XH-59A would have a meaningfully better lift than the XHTD, at the advancing root during high-speed, slow-rotation flight, In addition, the X2TD also has a +5º twist from 0.0R to 0.4R to improve the retreating root, and this must be detrimental to its advancing root.

In other words, the two graphs are showing a very large improvement for the lift and drag of the X2TD over the XH-59A at the retreating root, BUT they are showing only a miniscule degradation for the X2TD over the XH-59A at the advancing root.

Perhaps the vertical coordinate is only illustrative of the desired lift and drag comparisons. Particularly when considering the center paragraph in the right hand column this linked document on the XH-59A ABC after a 128 hour flight test program.

Could the graph be representative of autorotative cruise? Is autorotation the optimal arrangement for cruise?

Propulsor (Propeller):

Number of Blades: 6

Radius: 3.33 ft.

Rotational Speed: [PRPM]; 2,529 in hover, 2,041 in high-speed flight. Speed reduction - Hover to Cruise = 80%.

Tip Speed: [ft/sec]; 441 in hover, 356 in high-speed flight.

Pitch Range: -5º to 65º; at 3/4 R.

Designed and built by AeroComposites Inc. Used on Lancair IV-P airplane.

Engine:

One LHTEC T800-LHT-801

Power Train:

" The engine output shaft drives the input of a splitter gearbox mated to the rear of the coaxial gearbox through an overrunning clutch. The splitter gearbox drives the coaxial gearbox and a shaft that drives the auxiliary propulsion gearbox located in the tail. The variable-pitch auxiliary prop is therefore geared to the main rotors at all times."

Vibration:

6 force generators ~ Hamilton Sundstrand. These might be the units.

Blade Profile (re: reverse taper blades): May 11, 2008

The following linked report covers blades that have extreme root cutouts and are used for autorotation. It is a 1939 NACA report and therefore does not even consider the Advancing Blade Concept.

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930091627_1993091627.pdf. Relevant portion of summary: "The elimination of blade area near the hub was found to have a detrimental effect on the rotor L/D." Have hard copy in 'Reverse Velocity' binder.

Four posts on the Rotary Wing Forum with my question and Chuck B's answer; post #24, post #25, post #28 and post #31, add to this consideration of uneven disk loading. The coaxial compounds this problem by having the high-induced velocity areas of both rotors stacked on top of each other.

See sketch below at [Playing with X2 Rotor] for disk loading during hover.

For the future blade profiles that were proposed at the conclusion of the S-69 see; Coaxial - Sikorsky ~ S-69 (XH-59) ABC ~ Profiles. For the actual root end profile of the X2TD see the illustration above.

Rotor Shaft Angle:

Technical Document #1 says;

Page #6; Ref the earlier XH-69 ABC craft "The rotor suffered extremely high profile losses (especially over the retreating side), as well as the advancing side compressibility drag."

Page #7; "The basic premise for the design of the X2TD was based on addressing retreating blade drag losses."

_________________________

IMHO:

The large profile and induced drag on the XH-59X rotors during high-speed cruise resulted in an aerodynamically generated rotor RPM that exceeded the desired RRPM. This increasing RRPM could only be overcome by the increasing compressibility at the tip of the advancing blade, and perhaps a lowering of the collective. The lowering of the collective probably reduced the negative thrust (reduced induced drag) at the root of the retreating blade while increasing the negative thrust (increased induced drag) at the tip of the advancing blade.

This being the case, the X2TD must significantly reduced the excessive autorotative torque if the ABC-rotorcraft is to increase its forward velocity.

The above statement "Tilting the rotor forward provides a propulsive force component, but results in a reduction of rotor efficiency" may be very true. However, tilting the rotor forward may very likely result in a increase the craft's efficiency. This is because the forward tilt will significantly reduce the propulsive force demanded from the propeller. The efficiency of aerodynamic power transmission is approximately 0.85. This is the position that Stepniewski appears to advocate ~ here.

Should the drag on the retreating side of the X2TD rotors be significantly reduced, even at the expense of a possible drag increase on the advancing side, there is still a problem. For the X2TD to test the use of a negative rotor longitudinal shaft angle [α_s] will result in the propeller's thrust having a negative lift component.

Playing with X2TD Rotor Disk:

Values based on Patent Application 20060269418, the currently limited known information on the X2, plus information from the Sikorsky ~ XH-59A ABC

Targeted forward speed is >250kts. = 422 fps Check

Tip Mach Boundary in Hover is .661 Check

Fig. 17 shows a maximum tip speed ratio of mu = 0.8, which means that 80% of the retreating blade is in reverse velocity. This may be that the cruise tip-speed or maximum tip-speed = 338 fps.

Reference; Sikorsky S-69 (XH-59A) ABC: Design rotor tip-speed = 650 ft/sec. in helicopter mode and 450 ft/sec. in cruise mode.

080R is the location of the widest chord.

In addition, the pusher propeller is 'stripping' some of the lift out from under the coaxial rotor disks.

Concerns:
Maybe seriously reassess Technical Documents #1 & 2 in the future.

However, some quick and crude impressions are;

The high power/weigh ratio of this craft and the incremental improvements should allow it to obtain the targeted cruise speed, but will it be able to do so efficiently?

The ramifications on hovering efficiency (Figure of Merit) were not presented.

It appears, from the description of the power-train, that the propeller will be rotating on the ground when the rotors are rotating. This represents a danger, plus the following;

On page 54 of 'Quiet Rotorcraft' Stepniewski considers the power consumption of the propeller when he writes, "a noticeable fraction of the total power required in hover may be lost."

Will the large amount of aerodynamic interaction between the blades of the two rotors significantly reduce the lift/power ratio of the craft?

Reference the link to the four posts here.

Will the far aft propeller create vibrations; due to its aerodynamic interaction with streamtubes of the main rotors and the long moment arm from the propeller to the craft's CG?

Will pusher propeller (as compared to a tractor propeller) 'strip away' some of the thrust of the rotors? See; http://www.unicopter.com/1538.html.

It is known that contra-rotating propellers produce vibrational effects due to the blades passing each other. This vibration increases as the distance between the blades decreases. The close proximity of the coaxial rotors may be the reason for installing the 6 force generators in the fuselage.

Musings:
The following may eventually end up elsewhere.
Aircraft's Lift/Drag Ratio:

The earlier XH-59A and the modifications to the current X2TD make no attempt to utilize the reverse airflow.
The X2TD is simply reducing the negative lift and the induced and profile drag that was experienced by the XH-59A in the reverse velocity region.
The obtainable drag reduction will significantly help, however IMO, any meaningfully higher forward speeds will come at a large power/weigh penalty.
In addition, the planform of the X2TD blade and the desired forward speed increase has resulted in a thickening of the ellipse at the blade root, over that of the XH-59A.

_______________________

Ever increasing forward speeds will require that the RPM of the rotor be decreased; due to compressibility at the advancing tip.

This means that the spread between the air velocity at the root of the advancing blade will get closer and closer to the air velocity at it's tip.[(V + Ωr₀) / (V + ΩR)].

Airfoils (wings and rotorblades) normally have a greater surface area at their roots.

In addition, the length of the moment arm is less to the root sectors than to the tip sectors.

This STRONGLY suggests that the most valuable zone for lift is next to the fuselage. Therefore, this is where most of the lift should be generated.

So much for the advancing side.

_______________________

For the retreating side only two requirements must be met;

The portion of the blade that is in reversing airflow must have a negative pitch, thereby giving it a positive angle of attack. Note that for very fast cruise the whole blade will be in reverse airflow for a portion of its rotation.
The portion of the blade that is subjected to reverse airflow must be able to present an effective aerodynamic profile to this airflow.

I believe that #1 can be done - TODAY. Independent Root & Tip - Torque Tube Method

#2 will require 'a wing and a prayer'. Alternatively, perhaps there is a answer hidden in here; Rotor Concept - Reverse Velocity Utilization

Patents Related to This Craft:

US Design Patent:

Sikorsky's six patents [D524,227, D524,228, D524,229, D524,230, D524,718 and D526,269] are Design Patents, not the normal Utility Patents. They are simply a collection of sketches of five coaxial helicopter arraignments.

What Are Design Patents and When Are They Useful? http://www.tms.org/pubs/journals/JOM/matters/matters-9303.html

More about Design Patents; http://www.inventorshq.com/mboard/displayMessage.cfm?messageID=4208

Sikorsky has one conventional patent 7,083,142, Compact co-axial rotor system for a rotary wing aircraft and a control system thereof. However it is all fluff and no substance.

October 10, 2006, 7,118,340, Swashplate and pitch link arrangement for a coaxial counter rotating rotor system.

June 12, 2007 And another patent; 7,229,251 ~ Rotor hub fairing system for a counter-rotating, coaxial rotor system.

US Patent Applications:

20060269418 ~ Rotor blade for a high speed rotary-wing aircraft ~ November 30, 2006 ~ Filed: May 31, 2005

Claims do not mention 'coaxial' rotor system, However it looks like they are attempting to patent a bunch of blade profiles that were used 70 years ago. The word 'twist' does not appear until the 19th claim.

20070110582 ~ Rotor blade twist distribution for a high speed rotary-wing aircraft ~ May 17, 2007 ~ Filed: August 23, 2006. Have hard copy.

Similar to 20060269418 except it states 'dual, counter-rotating, rigid coaxial rotor system'

US Patents:

7,296,767 ~ Variable speed transmission for a rotary wing aircraft

Was filed May 31, 2005. It describes a means of providing variable speed to the main rotor but not to the tail rotor.

7,434,764 ~ Variable speed gearbox with an independently variable speed tail rotor system for a rotary wing aircraft

Was filed December 2, 2005. It mentions 'variable speed' 25 times and yet it does not give any information on a variable speed unit.

7,607,607 ~ De-rotation system suitable for use with a shaft fairing system

A de-rotation system includes a first ring gear defined about an axis of rotation, a second ring gear defined about the axis of rotation and a gear set in meshing engagement with the first ring gear and the second ring gear to control a position of a housing about the axis of rotation.

7,621,480 ~ De-rotation system for a counter-rotating, coaxial rotor hub shaft fairing Have hard copy.

A rotor hub fairing system includes an upper hub fairing ..................

7,628,355 ~ Variable speed transmission for a rotary wing aircraft

A transmission gearbox for a rotary-wing aircraft includes a main gearbox and a variable speed gearbox in meshing engagement with the main rotor gearbox. The variable speed gearbox permits at least two different RPMs for the main rotor system without disengaging the engine(s) or changing engine RPMs. The variable speed gearbox includes a clutch, preferably a multi-plate clutch, and a freewheel unit for each engine. A gear path drives the main gearbox in a "high rotor speed mode" when the clutch is engaged to drive the main rotor system at high rotor rpm for hover flight profile. A reduced gear path drives the main gearbox in a "low rotor speed mode" when the clutch is disengaged and power is transferred through the freewheel unit, to drive the main rotor system at lower rotor rpm for high speed flight. The variable speed gearbox may be configured for a tail drive system that operates at a continuous speed, a tail drive system that changes speed with the main rotor shaft or for no tail drive system.

7,648,338 ~ Dual higher harmonic control (HHC) for a counter-rotating, coaxial rotor system

7,651,050 ~ Variable speed gearbox with an independently variable speed tail rotor system for a rotary wing aircraft

7,695,249 Bearingless rotor blade assembly for a high speed rotary-wing aircraft ~ April 13, 2010

Related Pages; at This Site:

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

OTHER: Helicopter - Outside - Intermeshing - Stepniewski ABC (concept) Stepniewski's proposal for an Intermeshing-ABC.

DESIGN: UniCopter ~ Rotor - Disk - Large Chord & Low Tip Speed Stepniewski proposes a slow fixed-speed rotor (tip speed of 513 fps), whereas Sikorsky is building a variable-speed rotor.

DESIGN: UniCopter ~ Pusher Prop - General - Angle of Incidence - Rotor & Propeller Stepniewski proposes that a portion of the thrust of the rotors contribute to forward propulsion during cruise, whereas Sikorsky, at least on the XH-59A-ABC, had the rotors in and near autorotation during cruise.

DESIGN: UniCopter ~ Pusher Prop - General - Tractor vs. Pusher
Why the Coaxial-ABC Cannot Compete with the Intermeshing-ABC

Miscellaneous:

Last Revised: September 18, 2010