0815

Item 0815

DESIGN: UniCopter ~ Rotor - Disk - 'Absolutely' Rigid Rotor (ARR)

There's no such thing as an 'Absolutely Rigid Rotor' but the phrase 'Rigid Rotor' was already taken.

The expression "Absolutely Rigid Rotor" is used in the context that 'absolute' is the optimal, albeit impossible, objective.

Perceived Advantages:

If all the forces within a rotor disk are concentric and if they are also on the centerline of the mast at all times, then there should be a considerable reduction of cross-coupling.

There should be a considerable reduction of rotor-induced oscillations, because the centers of forces will be concentric and on the centerline of the mast.

There will be instantaneous response to control inputs.

The allowable center of gravity will have a greater range.

There will be very little off-axes response to control inputs. See: OTHER: Flight Dynamics - General -Cross-coupling.

Reduction of rotor head complexity due to elimination of flapping and lagging hinges.

The high rigidity should result in vibrations only at very high frequencies. If so, this may allow for having variable rotor RPM without exciting the rotor at specific rotational speeds.

The stiffer blades may allow for the use of more radical asymmetrical airfoil profiles.

No Coriolis effect.

The strong coupling between the rotors and the fuselage may allow the front of the disks to act similar to the canard on a fixed wing craft. This might be particularly true if the coning angle (actually the pre-cone) was slightly greater than convention would dictate. This would reduce the required downward thrust of the HS and thereby slightly improve the craft's lift/drag ratio.

There may be little or no need for a horizontal stabilizer; thereby reducing weight and drag.

From the report; THE ABC HELICOPTER by M.C. Cheney, Jr., United Aircraft Corporation ~ Feb 1969

"It is evident that an infinitely rigid rotor not burdened with roll trim requirement [i.e. twin rotors] could maintain constant dynamic pressure on the advancing blade by reducing rpm and thus avoiding exceeding the drag divergence Mack number until the rotor was stopped. This, of course, implies potential helicopter rotor forward speed capabilities exceeding 500 knots."

From the report; FORWARD FLIGHT PERFORMANCE OF A COAXIAL RIGID ROTOR by V.M. Paglino, United Aircraft Corporation ~ May 1971

"The rigid retention gives rise to increased control power, increased center of gravity travel and reduction of rotor head complexity through elimination of flapping and lagging hinges."

Component Construction:

Hub:

No flapping hinge.
No lead-lag hinge.
Four-degree pre-cone.

Blades:

Composite construction with very high strength and stiffness. Blades constructed with a downward bow (anhedral). See: DESIGN: UniCopter ~ Rotor - Disk - Anhedral. Preferably more stiffness and less anhedral. Blades to be horizontal when hovering out of ground effect at gross weight.

Rework the following. The gross weight is 750 pounds and there are 6 blades therefor each blade is required to support a mean weight of 125 pounds. If the disk radius is 8'-0" and the lift is concentrated at 70% of radius then it is located at 67.2" radius. Assuming a 3G maximum loading and a 1.5 safety factor then the load at 72.8" will be 562 pounds. = 37,800 in-lb.

Mast:

Static Mast

Rotor Flight Profile:

The blade stiffness combined with the manufactured-in anhedral are such that at mean thrust (for example - in hover, out of ground effect and at gross weight) the blades are straight and horizontal. All the force-centers are concentric, are in the plane of the blade tips and also on the axis of the mast.

In forward flight and during maneuvers the strength and stiffness of the rotor will minimize the divergence of 'moment centers' from each other, the tip plane and the mast centerline.

Disadvantages:

Real:

The large forces created by the rigid rotor will be transmitted on through to the fuselage and this will necessitate an increased strength. Additional weight is the penalty for this increased strength.

Possible:

High loading on feathering bearings. Possible solutions: The friction may be lower in elastomeric bearings.

There maybe reduced pitch and roll static stability because there is no flapback. Possible solutions: A precone angle of approximately 4º should overcome this problem.

Will the rotational inertia be high enough to detrimentally effect control by providing too much stability?

Notes:

Posting on rec.aviation.rotorcraft by NL ~ March 29, 2002

To answer the basic question, "why not rigid?":

the stiffness of a "rigid" rotor brings goodies like sweet handling, large CG range and simplicity, but they bring costs, like higher vibration, heavier rotors and masts, more cross coupling (roll control is mixed with longitudinal). It is up to the designer to balance all the needs against the costs. Generally, weight wins and rigid rotors are not favored. For Comanche, the handling and maneuvering quickness was required, so it has a "rigid" rotor. I have banked from 60 degrees left to 60 degrees right bank, and touched 100 degrees per second in the maneuver. It took the blink of an eye to accomplish, because the Comanche is a fighter helicopter.

Posting on rec.aviation.rotorcraft by RC ~ March 29, 2002

re: Cross-Coupling:

Yes and no. A single main rotor helicopter with a rotor so rigid that it won't be able to flap won't be able to fly, except perhaps at hover, because there will not be anything to compensate for the dissymmetry of lift between advancing and retreating side. A co-axial rotor will be able to fly, but the reason for the absence of cross-couplings is not the rotor stiffness, but the presence of the two counterrotating rotors.

A stiffer rotor has *higher* cross-couplings, unless the root section is so flexible in flap that the rotor behaves like an articulated rotor. The reason is that a stiffer rotor has a higher natural frequency in flap (about 1.15/rev for a BO-105, compare with about 1.03/rev for a UH-60, for the Comanche Nick will tell you, but then he will have to kill you), so it will respond to the usual 1/rev excitation in forward flight with a delay different from 90 degrees: that number is valid for a blade hinged on the axis of rotation. For the BO-105 the delay is around 77 degrees, for the UH-60 around 86 degrees. The lower the delay the higher the cross-coupling.

re: Vibration:

What increases is the amplitude. If the blades are well tracked, the rotor acts like a filter, and only lets vibrations get into the fuselage at multiples of N/rev, where N is the number of blades, regardless of the rotor type. E.g., for a four-bladed helicopter, at 4, 8, 12, 16/rev, etc. IIRC the rotor speed for the UH-60 is 4.3Hz, so the 4/rev (which is usually the strongest) is about 17Hz. If the rotor is not well tracked you'll get all the harmonics.

The following needs converting from SynchroLite to UniCopter

G.G. Our gyrocopter has a little 2-per-rev vibration with either a light load or with a full gross wt load. At nominal 2-place loads, rotor shake is almost non-existent. There are other factors in the minimal vibration, but, I also consider this verification of the value of keeping the coning angle minimal.

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]

Re twisting: Model rotors designed for wind tunnels can be built so stiff that they are virtually "stall proof." This degree of stiffness is usually not built into actual helicopter rotors because of the extra weight required. [Source ~ RWP2 p.26]

K.R. http://www.eng.auburn.edu/department/ae/labinfo/AAL/SSAR.html I contacted the team a while back, asking what kind of energy load the controller required, and was told about 5% of the lift HP. Very nice setup, although they don't seem to have done anything with it recently.

Miscellaneous Notes:

Sikorsky S-69 ABC [XH-59A]

Lockheed Model 286 Rigid rotor helicopter

Article ~ The Second Generation Rigid Rotor in VERTIFLITE Fall/Winter 2000

Outside Web Pages:

An Examination of Selected Problems in Rotor Blade Structural Mechanics and Dynamics

http://rotorcraft.arc.nasa.gov/publications/files/Hopkins_AHS03.pdf Have hard copy.

X-wing: http://www.abovetopsecret.com/forum/thread81363/pg1

Initially displayed in 2001 ~ February 16, 2007

The above utility invention is openly and publicly disclosed on the Internet to negate an entity from patenting it, to the exclusion of all others whom may wish to use it. ~ Reference patent law 35 U.S.C. 102 A person shall be entitled to a patent unless - (a) the invention was known ... by others in this country, ..., before the invention thereof by the applicant for patent.