Item 1090

DESIGN: UniCopter ~ Rotor - Disk - Large Chord & Low Tip Speed or [High Solidity & Low Tip Speed] ?

Objective:

To evaluate the pros and cons of a large chord and low rotor speed versus a smaller chord and variable rotor speed.

Note:

The Stepniewski ABC Low Tip Speed Design Philosophy has slow constant-speed rotors.

The two Sikorsky ABCs have variable-speed rotors.

The information on this page primarily considers the constant-speed rotor.

If and when Active Blade Twist is implemented in helicopter rotors it will allow the retreating blades to provide lift during high speed cruise and this will mean that the chord can be a little smaller.

Tip Speed:

Stepniewski appears to be considering a constant tip speed of 513 fps.

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

Sikorsky X2 ABC: Design rotor speed = 700 ft/sec. in hover mode; 560 ft/sec. in cruise mode.

Reference:- The tip speed of the S92 is 761 ft/sec.

From Prouty it appears that 400 ft/sec is the minimum tip speed to have stored kinetic energy for autorotation.

My thoughts ~ The above is probably a valid statement for flare but a rotor governor should allow for a slower rotational speed during flight. This is because the rotor governor will assure that the collective is dropped in time. In addition, the high strength of the rotor will withstand slow RRPM. The previous sentence assumes there is sufficient elevation to gain RRPM for the flare.

Tip speeds under 500 ft/sec are quiet. The Hughes "Quiet One was down to 430 fps.

It would appear that 400 to 500 ft/sec is the ideal range.

It appears that the Mach number at the advancing tip during high-speed flight must be below 0.85

The book 'Principals of Helicopter Aerodynamics', section 2.2.11 covers the subjects of Rotor Solidity and Blade Loading Coefficient. However, this section does not discuss the subject of varying the tip speed (ΩR)2. Section 6.3.3 is on Rotor Solidity and it does say "Rotors that are designed for high speeds and/or high maneuverability requirements require a higher solidity for a given diameter and tip speed." For consideration of tip speed [RRPM] see this section below.

Recently Proposed Craft:

Existing/Previous Craft w/ High Forward Speed:

The following two aircraft are (were) attempts to increase the forward speed of a craft with VTOL capability.

Osprey V-22:

The Osprey's major problem is that of the Vortex Ring State. In helicopter mode, the small diameter rotors are operating at a blade pitch that is very close to their maximum. This leaves very little allowance for the unexpected.

Sikorsky XH-59A ABC:

The rotor tip speeds were 650 ft/sec in helicopter mode and only 450 ft/sec. in compound mode. This helicopter also incorporated the Advancing Blade Concept (advancing blades provide more thrust than retreating blades), and a forward thrust device. The combination of these three features allowed the craft to be flown at a high forward speed, until the advancing tips experienced high compression and the craft experienced high vibration.

Sikorsky X2 ABC:

Specifications

BA609 tilt rotor:

xx

There appears to be no reason why an intermeshing helicopter cannot out perform the Osprey in all areas except maximum forward speed, and outperform Sikorsky's ABC in all areas.

This intermeshing helicopter will be similar the ABC in many respects. It will differ from the ABC in that the blades will have a larger chord.. It appears that the vibration caused by lateral dissimitry of lift during forward flight of the S69-ABC can be minimized by; three blades per rotor and a 3P higher harmonic control, or alternatively, by four blades per rotor. At high forward speeds of the intermeshing configuration, the trailing tip vortices from the lower advancing blades should pass below and outside the upper retreating blades.

Wieslaw Z. Stepniewski said; ~ "Very preliminary studies seem to indicate that placing the ABC rotors in the synchropter position should result in a somewhat aerodynamically cleaner than the coaxial ABC, and definitely a much cleaner design than the considered single-rotor types.[Shaft-driven and Cold Jet]"

Horizontal Flight:

The craft can achieve a higher forward speed than current helicopters due to the slower rotational speed of the two rotors. The slower rotors delayed onset of compression and also have less profile drag.

Vertical Flight:

The larger chord blades allow the craft to hover at a slower rotor rpm. The blade pitch will be greater in hover than it will be in forward flight, but this pitch will still be far less that that of the Osprey in hover. An airfoil with a high maximum coefficient of lift will be advantages, not because of retreating blade stall, but because of low rotor rpm and high pitch hover.

Transition:

This is a non-event. Advancing the cyclic stick adds pitch (thrust) to the propeller, and causes the rotors to give the craft a slight nose down attitude.

Relationship of RRPM and Horsepower during Hover:

The following is a comparison of the UniCopter with NACA 0012 airfoils and a thrust equal to it's GW. The actual values were probably based on the initial lighter UniCopter.

 

RRPM

Collective Pitch:

Horsepower:

 

533

5.6

96

 

450

7.1

82

 

400

8.4

76

 

May 21, 2004 ~ I think that the above values are based on the same chord. If both the chord and the pitch are increased as the RRPM is reduced, it appears that the horsepower increases. In other words, increasing the chord increases the HP during hover.

Considerations re Constant Speed Low RPM Rotors during Cruise:

 

Forward Velocity:

75% of Tip Speed: (1)

Advancing Blade:

Retreating Blade:

Sum: (2)

 

 

540 fps * 0.75 = 405 fps = 240 kts

Lift, as related to the square of the velocity

Lift, as related to the square of the velocity

Lift, as related to the square of the velocity

 

0 kts

240 kts

2402 = 57,600

2402 = 57,600

115,200

 

50 kts

240 kts

2902 = 84,100

1902 = 36,100

120,200

 

100 kts

240 kts

3402 = 115,600

1402 = 19,600

135,200

 

150 kts

240 kts

3902 = 152,100

902 = 8,100

160,200

 

200 kts

240 kts

4402 = 193,600

402 = 1,600

195,200

 

250 kts

240 kts

4902 = 240,100

-102 = -100 (3)

240,000

 

300 kts

240 kts

5402 = 291,600

-602 = -3,600

288,000

Additional Feature:

Low noise due to the elimination of the tail rotor and the low speed of the main rotors.

Aspect Ratio:

My thoughts:

Disk Loading:

If the tip speed of a helicopter is reduced and the disk area is maintained, then the solidity ration must be increased. Taking this tip speed reduction to the extreme and stopping the rotor should logically imply that the Wing Loading of a fixed-wing aircraft should now be equated with the Blade Loading of the helicopter.

In other words, when considering a [Large Chord & Low Tip Speed Helicopter] there might have to be a graph where the Y-axis is between disk loading and blade loading of a conventional helicopter and the X-axis is the reduction of the RRPM below some nominal conventional helicopter rotational speed. In even more other words, a diagonal line which has one end at the intersection of disk-loading and conventional RRPM, and the other end at the intersection of blade-loading and zero RRPM.

____________________________

Blade Loading:

The following is a comparison of The UniCopter to a comparable helicopter and airplane

 

Aircraft:

Disk Loading:

Blade (Wing) Loading:

Velocity Calculated at:

Avg. Velocity of 4 Quadrants of Blade or Wing at Cruise (mph)

Col. 5 / Col. 3

 

Sikorsky ABC(1)

8.84 lb/ft2

57.87 lb/ft2

r/R = 0.75

√((6772 + 3392+ 22+ 3392)/4 = 4152 = 172,044

2,973

 

Stepniewski ABC

 

 

 

 

 

 

Robinson R22

2.76 lb/ft2

90.8 lb/ft2

r/R = 0.75

√((6362 + 4742+ 3132+ 4742)/4 = 4882 = 238,144

2,623

 

UniCopter

3.48 lb/ft2

17.9 lb/ft2

r/R = 0.70 (2)

√((5882 + 3342+ 802+ 3342))/4 = 3792 = 143,641

8,024

 

Midget Mustang

~

14.7 lb/ft2

~

2152 = 46,000

3,129

Notes:

      1. Based on cruise speed of 200 knots and tip speed of 450 fps.
      2. This is less than the R22 because of the UniCopter's large taper and active blade twist.

Outside Information:

A report 'Development of the Autogiro ~ A Technical Perspective' [ http://www.enae.umd.edu/AGRC/Aero/Leishman_giro_paper.pdf ] by J. Gordon Leishman mentions on page 16 the following report which studied the load sharing between the rotor and the wing of an autogiro. ~ Have hard copy

Article by Prouty in Vertiflite, Winter 2004, page 24.

'Wing pressure distribution and rotor-blade motion of an autogiro as determined in flight' ~ Wheatley, John B ~ NACA Report 475 1935 ~ Have hard copy It might give some info on this pages subject. Then again it may not.

What might be more interesting would be the load sharing on a compound helicopter.

Effect of Rotor Tip Speed and Solidity on Figure of Merit. ~ [Source ~ AH p.62]

Effect of Rotor-Tip Speed on Helicopter Hovering Performance and Maximum Forward Speed ~ NACA ARR L6A16

Variable Speed Rotor and Propulsor:

  Notes Related to Wide Chord and Slow Rotational Speed during Hover:

Segment from PPRuNe thread 'Why are Helicopters with the Flettner-System so slow?'.

My Posting;

Nick & Mart.

Nick, there is no argument with what you say. However, your statement "that the profile power is too high when a big blade is hovered" got the neurons firing.

The theme of this thread relates to forward speed and a craft with twin main-rotors. The three of us, and others on the forum, have an interest in what might be called the next generation rotorcraft. Therefore ...

During cruise, all future rotorcraft will likely slow their rotors. This reduced rotational speed strongly suggests that their chords will be increased over that of current rotorcraft. During hover, a larger chord will increase the profile drag (as you say), however, a slowing of the rotor speed during hover will reduce the profile drag. This raises the question as to which will win the tug-of-war over the profile drag.

The following results are calculated from Prouty's 'Combined Momentum and Blade Element Theory with Empirical Corrections', in the chapter 'Aerodynamics of Hovering Flight'. It consists of incremental changes to the blade's chord and then finding the RRPMs that gives matching thrusts. The constants are: Pitch = 8. Taper = 0. Twist = 0. Blades = 4. Profile = NACA 0012

Thrust:

4972#

4987#

4994#

4994#

4985#

Chord:

0.5'

1.0'

1.5'

2.0'

2.5'

Horsepower:

473

409

411

416

394

RRPM:

370

295

261

242

229

Tip Mach:

0.69

0.55

0.49

0.45

0.43

Aspect Ratio:

40:1

20:1

13.3:1

10:1

8:1

Coefficient of Thrust:

0.0027

0.0042

0.0056

0.0065

0.0073

Sigma:

0.032

0.064

0.096

0.127

0.159

CT/Sigma

0.084

0.069

0.059

0.051

0.046

 

Interestingly, after excluding the first column, the horsepower does not vary much. This appears to imply that wide chord blades on high-speed rotorcraft will not be detrimental to their hover.

From Nick;

Interesting! Can you run the 1' chord blade down in speed, increase collective to keep Thrust constant and see what happens as you make that blade go to .12 Ct/sigma?

My reply;

The following results are calculated from Prouty's 'Combined Momentum and Blade Element Theory with Empirical Corrections', in the chapter 'Aerodynamics of Hovering Flight'. It consists of incremental changes to the blade's pitch and then finding the CT/Sigma for matching thrusts.

The constants are: Chord = 1 ft. Aspect ratio = 20:1. Sigma = 0.064. Taper = 0. Twist = 0. Blades = 4. Profile = NACA 0012

Thrust:

4987#

5014#

4981#

5000#

4986#

4996#

4994#

Blade Pitch:

8

9

10

11

12

13

14

Horsepower:

409

384

367

367

360

358

359

RRPM:

295

275

257

244

232

222

213

Tip Mach:

0.55

0.52

0.48

0.46

0.44

0.42

0.40

Coefficient of Thrust:

0.0042

0.0050

0.0058

0.0066

0.0071

0.0077

0.0084

CT/Sigma

0.069

0.0794

0.090

0.101

0.111

0.121

0.132

 

From Nick;

Dave,
As predicted, the HP drops as Ct/sigma rises to .12 If you keep running the Ct/sigma up, the power will start back up again.
Note that the 1 foot chord at Ct/sigma of .12 uses about 9% less power than any of the other cases use.

My reply;

All very interesting.

Nick, your statement [i]" As predicted, the HP drops as Ct/sigma rises to .12 If you keep running the Ct/sigma up, the power will start back up again.'[/i] is certainly not being questioned. However, you picked a specific chord size and this changed the chord from a variable to a constant.

For the fun and the knowledge, another comparison is made. The turquoise is the [Chord: = 2.5'] column in first chart above. The yellow column is the same as the turquoise except that the Blade Pitch has been changed to 13. This 13 is the pitch in the second chart where the [CT/Sigma] = 0.121.

The constants remain the same.

Blade Pitch:

8

13

Thrust:

4985#

4972#

Chord:

2.5'

2.5'

Horsepower:

394

260

RRPM:

229

150

Tip Mach:

0.43

0.28

Aspect Ratio:

8:1

8:1

Coefficient of Thrust:

0.0073

0.0135

Sigma:

0.159

0.159

CT/Sigma

0.046

0.0847

This third chart shows that changing the chord from 1' to 2.5' drops the horsepower further, from the 358 hp in the second chart, to 260 hp.

This suggests that a low solidity ratio is good for existing helicopters, for a number of reasons. One is the utilization of centrifugal force.

However very, very early attempts at hovering, were best served by a very high solidity ratio, to work with their slower rotational speed. Of course, to keep the weight down they used cloth for the airfoils and guy wires for the strength.

It appears that future helicopters will move back toward the earlier ones in that they will have large chords and slower speeds. Of course, the cloth and guy wires will be replaced by composite construction.

Any thoughts?

Calculations re Constant Speed Slowed Rotors

The following uses the Sikorsky XH-59A ABC specifications, modified for simplification;

Summation;

      1. The utilization of single speed slowed rotors appear to be slightly more efficient in hover than they are in forward flight. Assuming that this is correct, there appears to be no reason to have rotors turn faster during hover.
      2. Line 1 may not be exactly true for the Sikorsky XH-59A ABC since the blades on this craft had taper and twist.
      3. It will be interesting to hear what the rotor characteristics of the Sikorsky X2 ABC will be.

Addendum;

      1. It appears that the efficiency in hover will probably be less than that of cruise when reverse velocity utilization is implemented on future rotorcraft.
      2. However, the use of blades with an inverse taper may improve the lift in hover visa vie the lift in cruise because in hover the speed differential between the tip and the root will be greater; at least at 90 azimuth it will, at 180 and 360 it won't make any difference and at 270 it will ????????
      3. In addition, the use of blades with positive twist may provide an improvement similar to that of inverse taper. A small positive twist should also improve autorotation. This positive twist would be implemented by the flight-control 'mixer box' that is controlling the active twist blades.
      4. 3 and probably 2 will not be valid. In hover a large negative twist will be preferred. In cruise it will be reduced on the advancing side and will be positive on the retreating side.

PPRuNe ~ December 6, 2006

It was mentioned by IFMU, in a different thread, that Sikorsky's new X2 coaxial ABC will have multiple-speed rotors, as did its predecessor, the XH-59A ABC.

However, the possibility of using single-speed, low-rpm rotors with wide chord blades (large
solidity ratio) for tomorrow's high speed rotorcraft appears to have significant advantages.

Two bits of information have come to light while digging deeper into this subject.

1/ A cursory review of the technical papers on the XH-59A ABC show that the 'solidity ratio' is much used in the evaluation of many variables. However, I cannot find any place where the solidity ratio, itself, was evaluated as a variable.

2/ Stepniewski considers the rpm of the rotors as a variable in his conceptual Intermeshing ABC, but it appears that this evaluation was only to pick the optimum single-speed RPM.


Can anyone suggest one of more potentially viable reasons for using multiple-speed main rotors?

Autorotation:

Consideration of the Two-speed Rotor on the Sikorsky XH-59A ABC and its effect on opposing gyroscopic precession moments of the two rotors:

For blade element method, the centripetal force F acting on a body of mass M is given by the equation F = (Mv2)/r, where v is its velocity and r is the radius of its path. [F is in pounds], [M is in slugs (1 slug = 32.175 lbs)], [v2 is in ft/sec.], [r is in feet].

The square of the velocities at 450 ft/sec is 48% of what it is at 650 ft/sec Without looking furthere into gyroscopic precession, the preceeding probably means that the 450 ft/sec rotor will probably/possibly produce moments that are half of what they would be at 650 ft/sec. Look further into themoments created by the gyroscopic precession at some point in time.

Perceived Reasons for Current High RRPM and Narrow Chord:

Related Web Page - At This Site:

OTHER: Aerodynamics - General - Blade Loading Coefficient

OTHER: Aerodynamics - General - Solidity

Related Web Page - At Outside Site:

Experimental Investigation and Fundamental Understanding of a Slowed UH-60A Rotor at High Advance Ratios Have hard copy.

Introduction Page | SynchroLite Home Page | Electrotor Home Page | UniCopter Home Page | Nemesis Home Page | AeroVantage Home Page

Initially displayed with section on Calculations and Addendum: November 27, 2006 ~ Last Revised: December 19, 2011

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