Item 1465

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

General Information:

Technical Documents on X2 TD ABC: Have hard copies



Aerodynamic Design of the X2 TD Main Rotor Blade

Ashish Bagai

May 2008



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)









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:






Power - Cruise:

1,500 HP (design)


1,450 SHP



Power - Hover:

1,800 - 1,250 shp






J60 turbo jets





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)








Number of blades:






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



0.0 R to 0.4 R is +9, 0.4 R to 1.0 R is -9



Taper ratio:



Special blade



Solidity ratio: - independent disks: [σ]






Solidity ratio - all blades in 1 disk area:

0.153 ~ 0.127 (3)


0.1441  (9)



Thrust weighted solidity: [Nb/σTW(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









Rotor speed - Hover: (2) [RRPMH]






Rotor speed - Cruise: (2) [RRPMC]






Speed reduction - Hover to Cruise:






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 (αs)











Figure of Merit: Isolated rotor | Aircraft

≈ 0.79 | ≈ 0.68


≈ 0.xx | ≈ 0.xx



Vibration suppression:

Prevision for only





Blade spar:


All composite




Parasitic Drag - Rotor hubs

50% of total

Fairing of hubs




Tail surface - horizontal

60 sq-ft




Tail surface - vertical

30 sq-ft




Tail surface - vertical





Sources of Information:

      1. Changes that were recommended at the conclusion of the XH-59 project.
      2. From Sikorsky's Technical Document #1, at the top of this page.
      3. 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.
      4. Guestimate. Scaled from picture(s).
      5. Location of circumference of reverse velocity during cruise at 270 azimuth (%R)
      6. Sikorsky bases the disk loading on the area of a single disk
      7. Cruise top Speed: 250-265 (kts) [1 - averaged]
      8. "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:
      9. Due to the large root cutout and the unique profile perhaps the solidity ratio was obtained by graphical integration.
      10. σTW is Rotor thrust weighted solidity.
      11. From Sikorsky's Technical Document #2, at the top of this page.


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 -12 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 +5 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):


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."


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

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

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."



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 [αs] will result in the propeller's thrust having a negative lift component.

Playing with X2TD Rotor Disk:



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

However, some quick and crude impressions are;


The following may eventually end up elsewhere.

Aircraft's Lift/Drag Ratio:



    1. 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.
    2. The portion of the blade that is subjected to reverse airflow must be able to present an effective aerodynamic profile to this airflow.

Patents Related to This Craft:

US Design Patent:

US Patent Applications:

US Patents:

Related Pages; at This Site:



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Last Revised: September 18, 2010