1301

Item 1301

Should this page be (see Access) -> OTHER ~ Flight Dynamics - Rotor Hub -

OTHER: Rotor Concept - Constant Velocity Teetering Rotor

Abstract:

A rotorhead that has three blades, is underslung from a constant velocity joint, is stiff in-plane, and utilizes hub springs to provide resistance to out-of-plane motions. (Somewhat similar to the V-22 tilt-rotor)

Features:

Proposed Application: See below [Application of this Constant Velocity Teetering Rotor to the Dragonfly's General Arraignment:]

THE CURRENT METHOD UNDER DEVELOPMENT IS THE 'CONCENTRIC DOUBLE UNIVERSAL JOINT' VERSION.

Drawing A:

Drawing B: Improvement of elastomeric over Drawing A.

Torque Elastomeric in Drawing B:

The elastomerics are located around the centrally located plane of rotation and misalignment. The are positioned between alternating lugs on the Ball and the Ring (or cap).

A ~ Elastomerics when mast plane and tip-path-plane are parallel.

B ~ Elastomerics near maximum flap azimuths when mast plane and tip-path-plane are not parallel.

C ~ Elastomerics near zero flap azimuths when mast plane and tip-path-plane are not parallel.

The volume of all three elastomerics does not vary. The A to B change is a perfect elastomeric motion. The A to C change is not quite, since it involves transferring some of the elastomer vertically between the shims.

Determine if the 12º teetering will want to spread the shims.

Mockup of Drawing B:

Alternative for Negative G Elastomeric:

The rotor will probably be subjected to very little negative G. Therefor the lower elastomerics shown on drawing B (3 req'd) might be replaced by reshaping the top of the ball and changing the top ring to a top plate. A Teflon coating might be located between the top of the ball and the top plate, or grease and a grease fitting might be added, or one small teflon bearing with low viscosity.

This will probably increase the height of the hub slightly, over the above method. The top of the cap could be slightly higher in the middle because the other rotor's blade is not normal to this rotor's mast.

Notes re Rotor Head:

Stiff in-plane.

Ring: (or cap)

Holds the plates for the torque resisting elastomerics.

Yoke:

xx.

Ball:

Consists of an upper ball then a small diameter post (which rocks inside the positive G elastomer) then a screw (who's OD can just slid through the hole at the bottom of the positive G elastomer). This single piece is threaded and locked into the top of the larger diameter mast tube.

Application of this Constant Velocity Teetering Rotor to the SynchroLite's General Arraignment:

Notes:

The SynchroLite has a obliqueness of 12.5º (mast angle of 25º) and a stagger (before shortening the masts) of 27".

Basically, this will consist of replacing the 2-blade teetering rotors with the 3-blade CVTR rotors, plus some additional strengthening, to handle the stronger pitching and rolling moments generated by the CVTR rotors.

Concerns:

It will be difficult, or impossible, to add this to the SynchroLite and keep the total empty weight under 254 lbs.

Application of this Constant Velocity Teetering Rotor to the Dragonfly's General Arraignment:

Sketch of Complete Assembly:
The two final reductions will have a smaller diameter than shown [they are currently a copy of the Dragonfly] and the rotor hubs may be slightly larger. In addition, they will be of a 'conventional' construction.

Locate the gear under the pinion and reduce the rotor to final reduce dimension.

Some provision for resisting excessive flapping near 90º azimuth will probably be added.

Delta3 by control system geometry could be easily implemented, if desirable.

If the SynchroLite's belt & planetary gear is used, it looks like the non-rotating control rods can pass down past this reduction assemply.

Notes re Complete Assembly:

The Dragonfly mast angle is 22º and the stagger is 24". This means that the maximum inward teeter can only be 6º. This does not even allow for the blade's trailing edge to be lower when at high pitch, particularly if the blades have twist. Nor does it consider any reduced clearances at other radiuses close to the hub. At the SynchroLite's specifications, the mast angle is 25º and a stagger is 24", the 6º above becomes 8º and this is probably still to close. To be viable, the hub spring must be very strong and the fixed azimuth spring must be very strong also. This means that the blades rotor and drive train must be very strong.
The fixed azimuth spring should probably be located before the 90º azimuth;

so that it has done its work before the azimuth +/-90º of blade-hub contact.
so that it will also eliminate any clashing of the closing blades at the rear.

The strength of the blades will be inversely proportional to the strength of the hub spring(s).
The prototyping blades could be produced for this rotor head, then they could have approximately one foot cut off their tips if required later for the Offset Teetering Rotor.
By having large secondary reducers there will be safety and also may only require simple splash lubrication

Advantages:

When the thrust of a teetering or gimballed rotor is reduced to zero or into a negative 'g' condition the control moment capability will be reduced to zero and helicopter will become uncontrollable. The inclusion of a hub spring allows the rotor to produce a moment under these conditions.

A sufficiently strong hub spring may eliminate the need for droop stops.

I think that the inclusion of a CVJ, and not a universal joint, will overcome the cyclical Corollas effect and thereby the primary cause for needing led/lag hinges.

Simpler and less weight than the Offset Teetering Rotor.

Disadvantages:

Greater angle between masts then Offset Teetering Rotor and ARR.

Blades and rotor hub must be stronger; out-of-plane and especially in-plane.

There will be greater forces on the flight controls. This may require the pilot to apply more force on the cyclic stick.

Thoughts:

To compliment the fixed azimuth hub spring it might be advisable to have some means of limiting the negative blade pitch on the upper rotor, at the same azimuth as the fixed azimuth hub spring

As the stiffness of the hub spring is increased, the rotor will become more and more like an Absolutely Rigid Rotor. This will necessitate stronger blade, rotor and power transmission components. It will also require a reduction of the phase angle.

This idea is moving in the direction of an Absolutely Rigid Rotor therefor why not just produce an ABR

To eliminate any chance of rotor - rotor contact there must be a non-rotating hub at an azimuth near 270º. This extra resistance to flapping will require stronger blades etc. but this disadvantage will be offset, to some degree, by the fact that it is a lateral only resistance and the craft will to roll a lot easier than pitch .

Idea:

Wrap the elastomer and shim as a coil spring ~ to handle the torque.

Fixed Azimuth Hub Spring:

Locate radial bearing and elastomeric 'donut' between rotor hub and upper reducer housing

Concerns:

Will Coriolis be significantly overcome?

The torque will be considerably higher than that in one of the front wheels of a 4-wheel drive vehicle. This is due to the difference in rpm, horsepower and safety requirements. What will this do to weight?

The blades will have a low ground clearance at the sides. A solution might be to apply full opposed lateral cyclic to raise the blades at the side while the craft is sitting on the ground, with the rotors stationary and turning. In addition, the cyclic should probably be locked in the central position. Other possible solutions are outlined on OTHER: Miscellaneous - Safety

Related Pages:

This Site:

OTHER: Mechanical ~ Joint - Constant Velocity (homo-kinetic joint)

OTHER: Flight Dynamics - Rotor Hub - Elastomeric CVJ - Patents

Cyclical Coriolis Effect & Hooks Joint Effect ~ Discusses the Coriolis Effect and thereby the reason for a CVJ

OTHER: Flight Dynamics - Rotor Hub - Hinge Spring (Hub Spring)

OTHER: Mechanical - Bearing - Elastomeric - General

OTHER: Flight Dynamics - General - Lead/Lag

Outside Site:

Thompson Coupling

Lord ~ Rotor Components for Tilt Rotor

Rotorway Elastomeric Rotor Hub

Elastomeric Bearing Upgrade Kit for Rotorway

Notes:

Post on Eng-Tips Forum: Sep 9, 2003

drodger (Aeronautics)

I am currently developing a rotor model for a 4-bladed, gimballed rotor for use in a flight mechanics model. I have been searching publications for a long time looking for models of a gimballed hub. There is much literature relating to teetering rotors, rigid rotors and articulated rotors but I have as yet been unable to find anythin useful on the flapping equations for a 3 or 4-bladed gimballed hub.
I have successfully derived blade element velocities and accelerations for the rotor. However, I am still puzzled how to find the gimbal accelerations. These are defined in a non-rotating frame and the gimbal is treated as a rigid body with aerodynamic forces acting on a pair of torsional springs. I need to be able to find the gimbal accelerations in order to pass into the model for the next time step.
Hence, the question is:-

Does anyone have past experience of N-bladed gimballed rotors and could give me advice or does anyone know of publications which may help me?

Thanks in advance!

Patents, which may be relevant:

OTHER: Flight Dynamics - Rotor Hub - Elastomeric CVJ - Patents

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Initially placed on Internet: December 17, 2003 Last Revised: February 17, 2005

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.