1281

Item 1281

OTHER: Helicopter - Outside - Single (large) - Sikorsky's Reverse Velocity Rotorcraft Proposal

Sketch:

The above sketch was in Vertiflite Fall 2002.

For clearer sketch see; http://rotored.arc.nasa.gov/activity/HiSpeedRotor.html

Information from AHS International 58th Annual Forum ~ 2002

High Speed Rotorcraft Concepts:

by Dale Ashby & William Eadie ~ Sikorsky Aircraft Corporation

Double ended airfoil.

Forward velocity: 300 kts (507 ft/s)

Slow tip speed (at 50% Nr): 275 ft/s

Tip speed ratio (mu) μ = V_fwd / ΩR = 507 / 275 = 1.84 ~ Calculated from above.

Variable-speed transmission.

2-P flight-control for transition

Auxiliary propulsion.

Rotor solidity: 0,255

Blade aspect ratio: 10

C_T/σ design: 0.087

For slide presentation of Sikorsky's Reverse Velocity Rotor concept, go to; Highspeed Rotorcraft Concept by Sikorsky (2002) and select second file on the list. It is a large 9M .pdf file. I have printouts of informative pages. The original site and the Russian mirror site are no longer there but it appears that the document may be purchased at http://www.aiaa.org/content.cfm?pageid=406&gTable=Paper&gID=6962

It's an interesting, but problematic, concept. In fast forward flight, the forward speed of the craft will be slightly less then the rotational tip speed of the rotors. This means that the span of the retreating blades will never be totally in reverse airflow.

One concern, which they have presumably considered, is that of maintain lift on the retreating side during transition, when the blades on this side are subjected partial reverse velocity. Active blade twist would probably allow this transition, but they do not seem to be considering it. Instead, It appears that they are proposing to use 2/rev Higher Harmonic Control.

Paper on this craft was presented at the AIAA Powered Lift Conference by Mr. Ashby.

Session 17- IPLC-17	RIA (Runway Independent Aircraft) Vehicles I	Room 3
Chaired by: C. HANGE, NASA Ames Research Center, Moffett Field, CA
0800 AIAA-2002-5991 An Investigation of the Reverse Velocity Rotor Concept and Its Application to High Speed Rotorcraft D. Ashby, W. Eadie and G. Montoro, Sikorsky Aircraft Corporation, Stratford, CT

Initial Thoughts:

One of the major advantages of this concept, in my opinion, is the elimination of the requirement for both rotor(s) and wings. All compound helicopters, to my knowledge, require a wing. This has meant that the wing is excess baggage in hover and the rotor is excess baggage in fast forward flight. This concept uses the rotor for lift in all modes of flight.

IMO, rotorcraft with laterally located twin main rotor configurations will achieve better results. In fact, this single rotor configuration appears to have a fatal flaw. See serious concern below at [Autorotation].

The rotor rpm will be significantly lower than that of conventional helicopters and this will postpone the advancing blade compressibility until a much higher air speed has been reached.

This concept is developed for fast flight, therefore the whole rotor assembly, including blades, will require a high level of rigidity.

As μ [forward velocity / tip velocity] increases, the location on the retreating blade, which experiences no airflow, will move outward towards the tip. At μ => 1 the whole retreating blade, at 270-degrees azimuth, will be in reverse airflow. Therefor at μ =>1 the whole blade, at 270-degrees azimuth, will have negative pitch.

The key to this concept is its name [Reverse Velocity Rotorcraft Concept]. This concept very interesting, for me at least, since I started to work and post Internet pages on the same idea, [Negative Pitch], about four months earlier. For information on this concept see; OTHER: Rotor Concept - Reverse Velocity Utilization

Potential Problems with Sikorsky's Reverse Velocity Rotorcraft Concept using HHC

Hover:

The profile(s) of the blade's airfoil is designed for bi-directional airflow. Therefore, the blades cannot be optimized for forward velocity airflow.
The proposed arraignment would appear to be incapable of taking advantage of any conventional 'fixed' blade twist. This is because any negative twist on the advancing side will result in positive twist on the retreating side, when it is in reverse velocity.
A larger than normal anti-torque force will be required from the thrusters to compensate for the above limitations. In addition, a thruster with deflectors will be less efficient than a conventional tail-rotor.

Transition:

The inability to provide lift on the retreating side, when the blades on this side are transitioning from conventional flow to reverse flow. In addition, the need to have bilateral symmetry of lift will require that the lift on the advancing side be removed, as well. Probably the "2-P control for transition" means that the lift must be provided solely by the forward and the rear quadrants during transition.
The greatest thrust in these two quadrants will be when the blades are at 0º and 180º azimuths. This is because the thrust must diminish to zero as the blades rotate to 90º and 270º azimuths. The large long fuselage is located directly under this large downwash.

Fast Forward Flight:

The airfoil cannot be optimized for airflow in one direction.
The slow turning rotor will require high structural strength in the blades, hub and frame. (Similar to the UniCopter). This is of particular concern with the blades, since this high strength must be integrated with the special reverse velocity blade profile.
The pusher prop(s) must still offset the torque generated by the main rotor. An advantage is that the slow turning rotor may reduce this torque significantly.

Autorotation:

To make the transition from cruise to autorotation may be impossible. The retreating blades must make the transition from 100% reverse velocity to 100% conventional velocity. At some point during the transition, the positive lift on the retreating side will be offset by an equal amount of negative lift on the same side. Since there must be bilateral symmetry of lift, all lift must be removed from the advancing side. During this time, only the front and rear quadrants will be contributing to the lift and to the autorotation of the rotor. In addition, the speed of the rotor must be increased from 50% N_Rto 100% N_R.

The rotor will probably be unable to support the craft from only two quadrants, while at the same increasing its rotational speed.

Perhaps it might be possible to 1/ Reduce the forward velocity. /2 Put the advancing blades in to negative pitch; to give lateral equality of lift, and to accelerate the RRPM, /3 Then perform a normal entry into autorotation. Just speculation.

I notice that the wings were added on later sketches.

Miscellaneous:

Exceptionally high engine power will be required to overcome these limitations. The current specification shows 330 horsepower per passenger seat. In addition, It is questionable if autorotation can provide this power during the transition phase.
The craft will also require high power for very fast forward flight. Therefore, this power is also available for, PERHAPS, overcoming the high hover and transition demands.

IMNSHO, dual 1-P cyclics (Independent Root & Tip Control) will offer far better performance than a single 2-P HHC cyclic can. Independent Root & Tip Control will allow all four quadrants to product thrust during transition. In addition, it will result in much more efficiency during hover and fast forward flight.

Why doesn't Sikorsky propose a twin bilaterally displaced main rotor configuration. Boeing has proposed a Side-by-side Configuration; but the Interleaving Configuration and the Intermeshing Configuration are still available.

Proposal Related to Reverse Velocity Rotor ~ CH-83 Condor:

http://web.nps.navy.mil/~brutzman/Savage/AircraftHelicopters/JhlHeavyLift-NPS/JointHeavyLiftHelicopterDesignReportMarch2003.pdf Have hard copy.

The selected method in this proposal is a modified version of Sikorsky's proposal above, to utilize the reverse velocity. This proposal has eliminated the ducted fan and added wings. The wings are probably included to overcome the shortcoming during transition in the above proposal. Unfortunately, this craft is now a compound helicopter and is subject to a new, but perhaps less fatal, set of problems.

The graph on this report shows the S-67 having an equivalent L/D of 5.2 at an advance ratio of 0.3. It also shows the AH56-A as having a very similar spec.

The proposed craft has a predicted L/D of 9.1 at an advance ratio of 0.4. The value probable takes into account the compound wings at higher velocities since the L/D ratio decreases until an advance ratio of 0.8 and then it climbs to 10.0 at 1.8.

Additional Information:

This proposed craft was much referenced by Steven Riper in his thesis 'Investigation of Increased Forward Flight Velocities of Helicopters Using Higher Harmonic Stall Control and Reverse Velocity Rotor Concept'. I have hard copy of thesis.

Related Information:

US patent ~ Optimum speed rotor ~ 6,007,298

The RAH-66 Comanche has a multiple speed rotor for noise suppression.

DESIGN: UniCopter ~ Rotor - Disk - Variable RRPM

Related Information at This Site:

OTHER: Helicopter - Inside - Interleaved - w/ Dihedral Rotors

Later Modification:

Short wings have been added. Perhaps this is to overcome the difficult, if not impossible, transition between hover and cruise, and particularly the entry into autorotation (see; Transition and Autorotation above).

Last Revised: January 12, 2008