1056

Item 1056

OTHER: Miscellaneous - Thoughtless Idea - De-cone the Pre-cone

Drawing:

This drawing is also showing the presently considered lag-pitch delta2 hinges for torque-collective control.

Idea:

The Preamble:

The hub of a conventional teetering rotor is designed so that the axes of the two spindles (feathering bearings) have a pre-coning angle of 2-3 degrees. The teetering hinge is located an inch or two above these spindles. This undersling on a teetering rotor is so that the CG of the two blades and the teetering hinge are all inline, under mean disk loading. From this mean position, the blades' CGs will rise and fall (cone) in respect to the teetering hinge, as the disk loading changes.

It appears there are two reasons for the precone. One reason was that early blades were very flexible in the out-of plane direction. The precone eliminated any bowing of the blades, when they were operating at a mean thrust. This meant that the teetering hinge must be elevated so that it and the CG of the blades are in an (approximately) common plane. And this is done so that teetering will produce a minimized 2P vibration, due to a reduction of the rotor's center of gravity moving off of the centerline of the mast.

The second reason appears to be that of minimizing the radial loads on the feathering bearings that are used in a conventional helicopter's rotorhead.

The Question:

Why cannot a rotor be produced where;
1/ The teetering hinge has no undersling. It is located inline with the feathering bearings.
2/ The spindles have a small 'downward' coning angle or the blades are manufactured with some anhedral along their full span,.
3/ The blades are constructed with a high amount of stiffness and strength.
Because of the deflection in the blades, under mean disk loading the CGs of the two blades and the teetering hinge are inline, just as they are above. The difference is that now there is no coning angle.

The Perceived Advantages:
1/ Reduction or elimination of flap-back.
2/ Reduction in number and size of the cross-couplings. See portion of Frank Robinson's posting below.

Perceived Disadvantages:
1/ The rotor will be heavier; if made of the same material.
2/ The root of the stronger blades may be thicker and will cause a slight increase in profile drag.
3/ The rotor disk may want to tuck if the disk is lightly loaded during forward flight, which would be a disadvantage in respect to speed stability. ???? Will this be true for a gyrocopter?

Specific to the Helicopter:
1/ The control loads on the feathering bearings will be higher. The gyrocopter does not have feathering bearing, but for better or worse, the flow is passing through the disk in the opposite direction from that of a helicopter (that is not in autorotation).

Specific to the Direct Control Hub (gyrocopter):
1/ The gyrocopter does not provide cyclic control by rotating the pitch of the blades in respect to the rotorhub. It does not need pitch bearings and this means that the pre-cone serves no purpose in respect to reducing the radial loading on non-existing pitch bearings.
2/ The combining of very rigid blades, in the out-of-plane direction, and the removal of any precone, will reduce somewhat the length of the vertical moment arms about the gimbal, BUY ONLY IF the gimbal was located in the center of the teetering hinge.

Application:

1/ For a simple coaxial helicopter that utilizes rotorhub with Direct Control. This 'deconeing' feature may simplify piloting and reduce the necessary gap between the two rotorhubs.

Concerns:

Unloading a teetering rotor reduces its control authority. Perhaps, a negative coning could make this situation even worse.

Rough Working Notes:

1/ [Source ~ RWP1 p.125+]
2/ R-22 ROTOR SYSTEM ~ Portion of posting by Frank Robinson on www,pprune.org ~ Nov. 29, 2000.

"The other undesirable characteristic in rotor systems having 90-degree pitch links is the lateral stick travel required with airspeed changes during forward flight at higher airspeeds. The ideal rotor control system would require only longitudinal stick travel to increase or decrease the airspeed. This is not possible with a 90-degree pitch link system, because the rotor coning angle causes the rotor disc to roll right as the airspeed increases. This occurs because the up-coning angle of the forward blade increases that blade's angle-of-attack with increased airspeed, while the up-coning angle of the aft blade reduces its angle-of-attack. Consequently, the forward blade then climbs while the aft blade dives, thus causing the rotor disc to roll right with increased airspeed. To compensate for this with a 90-degree pitch link rotor, the pilot must apply some left lateral cyclic as the airspeed increases. "

Initially displayed: March 5, 2009 ~ Posted to Rotary Wing Forum: March 7, 2009 ~ Posted on PPRuNe: ---- ~ ~ Last Revised: March 12, 2009

The above utility invention is openly and publicly disclosed on the Internet to negate another from patenting 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.